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Skeletal Morphology and Systematics of the Cretaceous Euenantiornithes (Ornithothoraces: Enantiornithes)

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Chiappe and Walker Euenantiornithes 559
CHAPTER 11
Skeletal Morphology and Systematics of the Cretaceous Euenantiornithes
(Ornithothoraces: Enantiornithes)
Luis M. Chiappe and Cyril A. Walker
Chiappe and Walker Euenantiornithes 560
INTRODUCTION
Between 1974 and 1976, J. F. Bonaparte (then of the Universidad Nacional de Tucumán,
Argentina) quarried a site within the continental deposits of the Maastrichtian Lecho Formation
(Bonaparte et al., 1977; Bonaparte and Powell, 1980), on lands of the Estancia El Brete, in the
northwestern Argentine province of Salta. These excavations resulted in a collection of different
dinosaurian taxa, including the armored titanosaurid sauropod Saltasaurus loricatus (Bonaparte
and Powell, 1980). While collecting and preparing the remains of Saltasaurus, a series of small
and mostly disarticulated bones was found. These bones, originally identified as avian by J. F.
Bonaparte (Bonaparte et al., 1977; Bonaparte and Powell, 1980), formed the collection that led
Walker (1981) to recognize a monophyletic ensemble of early avians: Enantiornithes.
Subsequent studies of Mesozoic birds demonstrated that other Cretaceous taxa discovered
earlier could be allocated within the supraspecific taxon that Walker (1981) had recognized.
Walker (1981) pointed out characters shared by the bones from El Brete and those of Alexornis
antecedens from the Late Cretaceous of Mexico (Brodkorb, 1976). Alexornis was consequently
assigned to Enantiornithes by Martin (1983), who also included within this taxon the Late
Cretaceous Gobipteryx minuta (Elzanowski, 1974) from the Mongolian Gobi Desert. Later
discoveries, primarily in the 1990s, showed that Enantiornithes embodied a diverse monophyletic
group of birds evolving throughout the Cretaceous (Chiappe, 1991, 1993, 1995a, 1997; Chiappe
and Calvo, 1994; Zhou et al., 1992; Zhou, 1995a; Martin, 1995; Sanz et al., 1995, 1996, 1997;
Feduccia, 1996; Chatterjee, 1997; Padian and Chiappe, 1997, 1998) (Fig. 11.1).
Place Figure 11.1 near here
Although the taxon name Enantiornithes was erected in 1981, Sereno (1998) was the first
to publish a phylogenetic definition for this speciose clade. Sereno (1998) defined Enantiornithes
as all taxa closer to Sinornis
than to Neornithes. Under this phylogenetic definition, most
attributes previously used to diagnose Enantiornithes (e.g., Chiappe, 1992, 1996; Chiappe and
Calvo, 1994; Zhou, 1995; Sanz et al., 1995, 1996, 1997), were left to support the monophyly of a
clade that is only a subset of Sereno’s newly defined Enantiornithes. As discussed elsewhere
(Sereno, 2000; Sereno, Rao, and Li, this volume; Chiappe and Lacasa-Ruiz, this volume;
Chiappe, Chapter 20 in this volume), the Spanish Early Cretaceous
Iberomesornis and
Noguerornis may be primitive members of the Sinornis lineage and thus basal Enantiornithes. All
the remaining and more advanced enantiornithines form a monophyletic taxon, Euenantiornithes,
defined phylogenetically as all taxa closer to Sinornis than to Iberomesornis (Chiappe, Chapter
20 in this volume).
Several chapters in this book (e.g., Zhou and Hou, this volume; Sereno, Rao, and Li, this
volume; Sanz, Pérez-Moreno, Chiappe, and Buscalioni, this volume; Chiappe and Lacasa-Ruiz,
this volume; Chinsamy, this volume) provide in-depth coverage of anatomical, ecological, and
physiological aspects of Enantiornithes (see the Preface of this volume for a justification for so
many chapters on this one avian clade). The anatomical treatment of these, however, is specific
to a few taxa (e.g., Iberomesornis, Concornis, and Eoalulavis by J. Sanz et al.; Noguerornis by L.
Chiappe and A. Lacasa-Ruiz; Sinornis by P. Sereno et al.). In this article we provide a summary
of the osteology and historical relationships of Euenantiornithes, the clade that contains most
enantiornithine species, followed by a brief discussion of the biological attributes that can be
inferred from their morphology and fossil record.
Institutional Abbreviations: IVPP, Institute of Vertebrate Paleontology and Paleoanthropology,
Beijing (China); MUCPv, Museo de Ciencias Naturales, Universidad del Comahue, Neuquén
Chiappe and Walker Euenantiornithes 561
(Argentina); PVL, Instituto Miguel Lillo, Tucuman (Argentina); PU, Princeton University
(currently at Yale-Peabody Museum, New Haven), Princeton (USA); YPM, Yale-Peabody
Museum, New Haven (USA).
GEOLOGICAL SETTING
Euenantiornithine birds have been found in a variety of Cretaceous strata worldwide
(Martin, 1983; Molnar, 1986; Chiappe, 1993, 1996a; Chiappe and Calvo, 1994; Dashzeveg et al.,
1995; Varricchio and Chiappe, 1995; Kurochkin, 1995, 1996; Sanz, Pérez-Moreno, Chiappe, and
Buscalioni, this volume; Zhou and Hou, this volume) (Fig. 11.1; Table 11.1). Although
insufficient space prevents a discussion of the paleoenvironment, associated fauna, and
chronology of each site, a brief summary is given below (see Table 11.1 for specifics on the
chronology).
Place Table 11.1 near here
Most euenantiornithine birds are from non-marine deposits. Early Cretaceous
euenantiornithines are known exclusively from Europe, Asia, and Australia (Fig. 11.1). The
European (e.g., Concornis lacustris, Eoalulavis hoyasi) and Asian (e.g., Sinornis santensis,
Otogornis genshisi, Boluochia zenghi) species come from lacustrine deposits (e.g., Las Hoyas
and the Montsec range in Spain, Liaoning Province and Inner Mongolia in China) indicating a
subtropical environment with a variety of plants, arthropods, fishes, amphibians, and reptiles
(Fregenal-Martínez, 1991; Martínez Delclòs, 1995; Meléndez, 1995; Sanz, Pérez-Moreno,
Chiappe, and Buscalioni, this volume; Zhou and Hou, this volume). The Australian
Nanantius
eos constitutes the only Early Cretaceous species known from marine deposits (Molnar, 1986;
Kurochkin and Molnar, 1998), although the occurrence of abundant fossil logs within these beds
suggests that they were deposited not far from the coast; this evidence also points to a forested
shoreline (Dettmann et al., 1992). Late Cretaceous enantiornithines are known from North and
South America, Asia, and Madagascar (Fig. 11.1). Most taxa (e.g., Soroavisaurus australis,
Neuquenornis australis, Avisaurus archibaldi, Alexornis antecedens) occur in fluvial deposits
(e.g., western North American, Argentine, and Malagasy localities; see Table 11.1) indicating
fluvial plains or systems of braided rivers. These species are usually associated with the typical
dinosaurs, mammals, lizards, and other reptiles of the Late Cretaceous of western North America
(Lillegraven and McKenna, 1986; Bryant, 1989; Currie and Padian, 1997) and Gondwana
(Bonaparte, 1996; Krause et al., in press). Some of them (e.g.,
Gobipteryx minuta), however,
come from deposits indicating arid or semi-arid environments sometimes containing vast fields
of dunes (e.g., Khulsan; Loope et al., 1998). These same deposits entombed much of the rich
Late Cretaceous fauna of central Asia (Jerzykiewicz and Russell, 1991; Dashzeveg et al., 1995).
The only known marine Late Cretaceous enantiornithine is Halimornis thompsoni from the
Mooreville Formation of western Alabama (Lamb et al., 1993; Chiappe et al., in press). Chiappe
et al. (in press) estimated that the depositional site of Halimornis was as far as 50 km offshore in
the North American Western Interior.
SYSTEMATIC PALEONTOLOGY
Taxonomic Hierarchy
Aves Linnaeus, 1758
Ornithothoraces Chiappe, 1996
Enantiornithes Walker, 1981
Chiappe and Walker Euenantiornithes — 1
Table 11.1 Geographical andstratigraphical distribution of euenatiornithine birds
TAXON
DISTRIBUTION
HORIZON/AGE
REFERENCE
EARLY CRETACEOUS
Euenantiornithes indet.
La Pedrera, Lleida, Spain
La Pedrera de Rúbies Fm.,
Berriasian-Barremian
Sanz et al., 1997
Concornis lacustris
Las Hoyas, Cuenca, Spain
“Calizas de La Huerguina” Fm.,
Barremian
Sanz and Buscalioni, 1992; see Sanz et al., 1995
Eoalulavis hoyasi
Las Hoyas, Cuenca, Spain “Calizas de La Huerguina” Fm.,
Barremian
Sanz et al., 1996
Eoenantiornis buhleri
Sihetun, Liaoning, China
Yixian Fm., Early Cretaceous
Hou et al., 1999
Liaoxiornis delicatus*
1
Dawangzhangzi, Lingyuan, Liaoning, China
Yixian Fm., Early Cretaceous
Hou and Chen, 1999; Ji and Ji, 1999
Longchengornis
sanyanensis*
Hou, 1997
Cuspirostrisis houi*
Chaoyang, Liaoning, China
Jiufotang Fm., Early Cretaceous
Hou, 1997
Largirostrornis sexdentoris*
Chaoyang, Liaoning, China
Jiufotang Fm., Early Cretaceous
Hou, 1997
Sinornis santensis
2
Chaoyang, Liaoning, China
Jiufotang Fm., Early Cretaceous
Sereno and Rao, 1992; Zhou et al., 1992
Boluochia zhengi
Chaoyang, Liaoning, China
Jiufotang Fm., Early Cretaceous
Zhou, 1995
Otogornis genghisi
Chaibu-Sumi, Inner Mongolia, China
Yijinhluo Fm., Early Cretaceous
Hou, 1994
Nanantius eos
Warra Station and Canary, Queensland
Toolebuc Fm., Albian
Molnar, 1986; Chiappe, 1996; Molnar & Kurochkin,
1998
LATE CRETACEOUS
Kizylkumavis cretacea*
Dzhyrakuduk, Kizylkum Desert, Uzbekistan
Bissekty Fm., Coniacian
Nessov, 1984
Sazavis prisca*
Dzhyrakuduk, Kizylkum Desert, Uzbekistan
Bissekty Fm., Coniacian
Nessov in Nessov and Jarkov, 1989
Enantiornis martini*
Dzhyrakuduk, Kizylkum Desert, Uzbekistan
Bissekty Fm., Coniacian
Nessov and Panteleyev, 1993
Enantiornis walkeri*
Dzhyrakuduk, Kizylkum Desert, Uzbekistan
Bissekty Fm., Coniacian
Nessov and Panteleyev, 1993
“Ichthyornis” minusculus*
Dzhyrakuduk, Kizylkum Desert, Uzbekistan
Bissekty Fm., Coniacian
Nessov, 1990
Neuquenornis volans
Neuquén City, Neuquén; Puesto Tripailao, Río
Negro, Argentina
Río Colorado Fm.,
Campanian
Chiappe and Calvo, 1994; Chiappe, 1996
Alexornis antecedens
El Rosario, Baja California, Mexico
Bocana Roja Fm., Campanian
Brodkorb, 1976; see Martin, 1983
Gobipteryx minuta
3
Khulsan, Khermeen Tsav, and Ukhaa Tolgod,
South Gobi Aimak, Mongolia
Barun Goyot Fm. and Djadokhta
Fm. , Campanian
Elzanowski, 1974; Martin, 1983; Kurochkin, 1996;
this paper
Chiappe and Walker Euenantiornithes — 2
Euenantiornithes indet. Tugrugeen Shireh, South Gobi Aimak,
Mongolia
Djadokhta Fm. Campanian Suzuki et al., 1999
Euenantiornithes indet.
New Mexico, USA
Indet. Fm., Campanian
Martin, 1983
Avisauridae, nov. sp.
southern Utah, USA
Kaiparowits Fm., Campanian
Hutchison, 1993
Halimornis thompsoni
Greene County, Alabama, USA
Mooreville Fm., Campanian
Lamb et al., 1993; Chiappe et al., in press
Avisaurus gloriae
Glacier County, Montana, USA
Two Medicine Fm., Campanian
Varricchio and Chiappe, 1995
Euenantiornithes indet. Mahajanga, Madagascar Maevarano Fm., Campanian-
Maastrichtian
Forster et al., 1996, 1998
Euenantiornithes indet.
Massecaps, Hérault, France
Indet. Fm., Campanian-
Maastrichtian
Buffetaut, 1998
Gurilynia nessovi
Gurilyn Tsav, South Gobi Aimak, Mongolia
Nemegt Fm., Campanian-
Maastrichtian
Kurochkin, 1999
Avisaurus archbaldi
Garfield County, Montana, USA
Hell Creek Fm., Maastrichtian
Brett-Surman & Paul, 1985; see Chiappe, 1993
Euenantiornithes indet.
Niobrara County, Wyoming, USA
Lance Fm., Maastrichtian
This paper
Enantiornis leali
El Brete, Salta, Argentina
Lecho Fm., Maastrichtian
Walker, 1981; Chiappe, 1996
Soroavisaurus australis
El Brete, Salta, Argentina
Lecho Fm., Maastrichtian
Walker, 1981; Chiappe, 1993
Lectavis bretincola
El Brete, Salta, Argentina
Lecho Fm., Maastrichtian
Walker, 1981; Chiappe, 1993
Yungavolucris brevipedalis
El Brete, Salta, Argentina
Lecho Fm., Maastrichtian
Walker, 1981; Chiappe, 1993
Euenantiornithes indet.
El Brete, Salta, Argentina
Lecho Fm., Maastrichtian
Walker, 1981; Chiappe, 1996
* Species based on very fragmentary material; its validity needs further examination.
1- Including its junior synonym, Lingyuanornis parvus (Ji and Ji, 1999).
2- Including its junior synonym, Cathayornis yandica (Zhou et al., 1992).
3- Including its junior synonym, Nanantius valifanovi (Kurochkin, 1986).
Chiappe and Walker Euenantiornithes 562
Definition—Following Sereno’s (1998) phylogenetic definition, Enantiornithes is
defined as all taxa closer to Sinornis santensis than to Neornithes.
Included taxa—Enantiornithes includes Iberomesornis romerali (Sanz and Bonaparte,
1992; Sanz, Pérez-Moreno, Chiappe, and Buscalioni, this volume), Noguerornis gonzalezi
(Lacasa-Ruiz, 1989; Chiappe and Lacasa-Ruiz, this volume), and a large variety of
Euenantiornithes. One of us (C. W.) thinks that Wyleyia valdensis (Harrison and Walker, 1973)
may be an Enantiornithes.
Euenantiornithes Chiappe, this volume
DefinitionFollowing the phylogenetic analysis of Chiappe (Chapter 20 in this volume),
Euenantiornithes is stem-based, phylogenetically defined as all taxa closer to Sinornis santensis
than to Iberomesornis romerali.
Included speciesSeventeen valid species of euenantiornithine birds have been
described from Cretaceous beds worldwide (Table 11.1). These are: Enantiornis leali [this
species was listed on a table legend by Walker (1981), and properly diagnosed by Chiappe
(1996a)],
Gobipteryx minuta [Elzanowski, 1974; “Nanantius valifanovi” (Kurochkin, 1996) is
here regarded as a junior synonym of Gobipteryx minuta], Alexornis antecedens (Brodkorb,
1976), Avisaurus archibaldi (Brett-Surman and Paul, 1985), Nanantius eos (Molnar, 1986),
Sinornis santensis [Sereno and Rao, 1992; following Sereno, Rao, and Li (this volume),
“Cathayornis yandica” (Zhou et al., 1992) is regarded as a junior synonym of Sinornis santensis],
Concornis lacustris (Sanz and Buscalioni, 1992), Lectavis bretincola (Chiappe, 1993),
Yungavolucris brevipedalis (Chiappe, 1993), Soroavisaurus australis (Chiappe, 1993),
Neuquenornis volans (Chiappe and Calvo, 1994), Otogornis genghisi (Hou, 1994), Avisaurus
gloriae (Varricchio and Chiappe, 1995), Boluochia zenghi (Zhou, 1995b), Eoalulavis hoyasi
(Sanz et al., 1996), Eoenantiornis buhleri (Hou et al., 1999), and Halimornis thompsoni (Chiappe
et al., in press). Although unquestionably euenantiornithines, the validity of the fragmentary
Kizylkumavis cretacea (Nessov, 1984), Sazavis prisca (Nessov and Jarkov, 1989), Enantiornis
martini (Nessov and Panteleyev, 1993), and Enantiornis walkeri (Nessov and Panteleyev, 1993)
from the Late Cretaceous of central Asia, remains to be confirmed.
Diagnosis—Based on only partial phylogenetic hypotheses (e.g., Chiappe, 1993;
Varricchio and Chiappe, 1995; Sanz et al., 1995) and derived similarities present in several taxa,
Euenantiornithes (Enantiornithes of many previous authors) was variously diagnosed by different
lists of characters (e.g., Chiappe and Calvo, 1994; Chiappe, 1996b). The present phylogenetic
definition of Euenantiornithes has not altered the traditional concept of this clade (cf. Chiappe,
1993, 1995, 1996b; Sanz et al., 1995, 1996, 1997), but a better understanding of their cladistic
relationships has logically divided the list of characters previously thought to diagnose
Euenantiornithes into synapomorphies of the whole clade and synapomorphies that diagnose
subclades within Euenantiornithes. The relationships among euenantiornithine taxa, however, are
still largely unresolved (see below) and, consequently, it is likely that future studies will
substantially modify this diagnosis.
A cladistic analysis presented below (see Appendix 11.I), provides a diagnosis of
Euenantiornithes based on unambiguous synapomorphies (see Sereno, Rao, and Li, this volume,
for other putative diagnostic characters): parapophyses (costal foveae) located in the central part
of the bodies of dorsal vertebrae (character 1); distinctly convex lateral margin of coracoid
(character 2); supracoracoid nerve foramen opening into an elongate furrow medially and
Chiappe and Walker Euenantiornithes 563
separated from the medial margin of the coracoid by a thick bony bar (character 3); broad, deep
fossa on the dorsal surface of the coracoid (character 4); costal surface of scapular blade with a
prominent, longitudinal furrow (character 6); ventral margin of furcula distinctly wider than the
dorsal margin (character 7); well-developed hypocleideum (character 8); dorsal (superior) margin
of the humeral head concave in its central portion, rising ventrally and dorsally (character 13);
prominent bicipital crest of humerus (character 14); ventral face of the humeral bicipital crest
with a small fossa for muscular attachment (character 15); strongly convex dorsal cotyla of ulna
(character 17), separated from the olecranon by a groove (character 18); shaft of radius with a
long axial groove on its interosseous surface (character 19); minor metacarpal (III) projecting
distally more than major metacarpal (II) (character 20); hypertrophied femoral posterior
trochanter (character 21); very narrow, deep intercondylar sulcus on tibiotarsus that proximally
undercuts condyles (character 23); metatarsal IV significantly thinner than metatarsals II and III
(character 25). Several other characters with an ambiguous optimization are also diagnostic of
Euenantiornithes: lateral process of sternum (character 10); medial process of sternum (character
11); distal end of humerus very compressed craniocaudally (character 16).
Monophyletic status of the El Brete specimens—When erecting Enantiornithes,
Walker (1981) assumed the monophyly of all bird specimens from El Brete, even though very
few had skeletal elements in common. Subsequent discoveries of articulated skeletons from other
fossil sites, sharing derived characters with the El Brete specimens, supported the inclusion of
several isolated bones within a monophyletic group (e.g., Chiappe, 1992). The large number of
articulated enantiornithine skeletons known today has confirmed that nearly all the isolated bones
and incomplete specimens from El Brete belong to a monophyletic group of birds. Perhaps the
only noticeable exception is an isolated mandibular right ramus (PVL-4698; Fig. 11.3).
Considering that more than 50 other specimens collected from El Brete have all been found to be
Euenantiornithes, we tentatively regard this isolated ramus as an euenantiornithine element.
Nevertheless, the reader should be aware that enantiornithine or euenantiornithine
synapomorphies are unknown for mandibular elements.
ANATOMY
Skull and Mandible
Despite the abundance of euenantiornithine material in the Cretaceous deposits around
the world, cranial remains are known only for a handful of taxa. Most of our knowledge of the
cranial anatomy of these birds is based on the skulls of the Early Cretaceous Sinornis santensis
(Zhou et al., 1992; Martin and Zhou, 1997) and Eoenantiornis buhleri (Hou et al., 1999) from
northwestern China (see Zhou and Hou, this volume), an unamed hatchling from the Early
Cretaceous of Catalonia (Spain), and the Late Cretaceous Gobipteryx minuta (Elzanowski, 1974,
1977, 1995; Kurochkin, 1996) from the Mongolian Gobi Desert. In addition, the Chinese Early
Cretaceous Boluochia zhengi (Zhou, 1995b) and the Patagonian Late Cretaceous Neuquenornis
volans (Chiappe and Calvo, 1994) preserve fragmentary skull material. Also, an isolated
mandibular ramus was collected among the enantiornithines from the El Brete (Argentina).
The rostral morphology of Euenantiornithes is quite diverse. Some taxa, such as Sinornis,
Eoenantiornis, Boluochia, and the Catalan hatchling, are toothed (Fig. 11.2), whereas teeth are
completely absent in Gobipteryx. Boluochia (Zhou and Hou, this volume) bears a prominent,
toothless hook at the end of its upper beakonly mandibular teeth are known (Zhou,
1995b)but in Gobipteryx, the beak is broader and without a hook. Enantiornithine teeth (Fig.
Chiappe and Walker Euenantiornithes 564
11.2) are conical, unserrated, and with a constriction between its crown and its base (Martin and
Zhou, 1997). The premaxillae are fused to each other. Their maxillary processes are short (Fig.
11.2); most of the rostrum is formed by the maxilla (Martin and Zhou, 1997; Sanz et al., 1997).
The frontal process of the premaxilla extends near the caudal margin of the naris in the Catalan
hatchling, Eoenantiornis and Sinornis (Fig. 11.2), but in Gobipteryx it appears to reach as far as
the level of the lacrimals. In Sinornis and apparently in Eoenantionis (Hou et al., 1999), paired
nasals dorsally line the caudal half of the snout (Fig. 11.2D) (Martin and Zhou, 1997).
Place Figure 11.2 near here
The nares are subelliptical in Sinornis and the hatchling from Catalonia (Fig. 11.2) and
more subtriangular in Eoenantiornis and Gobipteryx. They are typically smaller than the
antorbital fenestra, which is subtriangular (Figs. 11.2A–C). Martin and Zhou (1997) reported the
presence of maxillary and promaxillary fenestrae within the antorbital fossa of Sinornis, a
conclusion Sereno, Rao, and Li (this volume) appear to endorse for at least the maxillary
fenestra. These fenestrae are absent in Gobipteryx, in which the nasal process (ascending ramus)
of the maxilla is reduced. Their absence was also described for Eoenantiornis (Hou et al., 1999),
although this region is poorly preserved in the only known specimen of this taxon. The jugal
forms the caudoventral angle of the antorbital fenestra (Fig. 11.2B). The shaft of this bone lacks a
dorsal, postorbital process, but caudally it ends in a fork with a caudodorsally directed process.
This process is comparable to that of the jugal of Archaeopteryx, which some authors (e.g.,
Wellnhofer, 1974; Elzanowski and Wellnhofer, 1996; but see Elzanowski, this volume)
reconstructed in direct articulation with the postorbital. As evidenced by the skull of the Catalan
hatchling (Figs. 11.2A, B), however, this jugal process does not join the postorbital but abuts the
lateral surface of the quadrate’s orbital process (Sanz et al., 1997; Sereno, Rao, and Li, this
volume). This condition is most likely the one present in Archaeopteryx, although these bones
are not in articulation in the available specimens. A postorbital bone is present in at least some, if
not all, Euenantiornithes, as the Catalan hatchling has unquestionably documented (Sanz et al.,
1997). This splint-shaped bone does not reach the jugal, leaving a gap between the orbit and the
laterotemporal fenestra. The postorbital articulates with the squamosal, forming the lateral
margin of a small dorsotemporal fenestra (Sanz et al., 1997). The squamosal is thus not
incorporated into the braincase. Martin and Zhou’s (1997) reconstruction of the suspensorial
region of Sinornis without either a postorbital or a “free” squamosal is most likely incorrect. The
configuration of the euenantiornithine temporal region was not far removed from that of a typical
diapsid reptile; the skull of these birds bears a fully enclosed dorsotemporal fenestra and a
laterotemporal fenestra only partially connected to the orbit (Fig. 11.2A). The euenantiornithine
quadrate bears a broad orbital process (contra Martin and Zhou, 1997). This is comparable to the
orbital process (pterygoid ramus) of non-avian theropods, Archaeopteryx, and the alvarezsaurid
Shuvuuia (Chiappe et al., 1996, 1998; Sanz et al., 1997). Distally, the quadrate has two
transversally oriented condyles (Elzanowski, 1977).
The palatal region of Euenantiornithes is known only for Gobipteryx (Elzanowski, 1977,
1995). The pterygoids are long and appear to articulate with the vomer rostrally, preventing the
elongate palatines from contacting each other along the midline (Witmer and Martin, 1987). The
choana is enclosed between the maxilla, vomer, and palatine. An ectopterygoid was reported for
Gobipteryx by Elzanowski (1995). This bone compares well with the hooked ectopterygoid of
Archaeopteryx (Wellnhofer, 1974) and non-avian theropods (Weishampel et al., 1990).
Chiappe and Walker Euenantiornithes 565
Enantiornithines have a vaulted cranial roof (Figs. 11.2, 11.3). The parietals are much
shorter than the frontals but they are also vaulted (Fig. 11.3A). In Neuquenornis, the cranial roof
is separated from the occiput by a shallow transverse nuchal crest (Chiappe and Calvo, 1994).
Neuquenornis has a small occipital condyle and a proportionally much larger foramen magnum
(Fig. 11.3B). Coupled with its short parietals, the presence of a large foramen magnum suggests
that Neuquenornis may have had a fairly “modern” brain organization, although data from either
the inner surface of the braincase or endocranial casts are not available for any enantiornithine.
Place Figure 11.3 near here
The mandibles of the Catalan hatchling (Sanz et al., 1997), Sinornis (Martin and Zhou,
1997), Eoenantiornis (Hou et al., 1999), and Gobipteryx (Elzanowski, 1977) are low and straight
(Fig. 11.2). While in Gobipteryx the two rami fuse to each other at the symphysis, they remain
separated in Sinornis and the Catalan hatchling, although the presence of this condition in the
latter may be related to its early ontogenetic age. The caudal half of the mandible of the Catalan
hatchling is perforated by two elongate mandibular fenestrae (Figs. 11.2A, C). In contrast to
neornithine birds, the rostral mandibular fenestra of the hatchling is not ventrally lined by the
dentary but rather by the angular, an ancestral condition present in non-avian theropods (Sanz et
al., 1997). Elzanowski (1977) described a lateral depression where the dentary of Gobipteryx
bifurcates into caudodorsal and caudoventral prongs; he regarded this depression as the rostral
mandibular fenestra (even though the dentary is not perforated). If this interpretation is correct,
Gobipteryx resembles more derived avians in this respect, a condition that must have evolved
independently. The El Brete mandible, the caudal portion of a right ramus, markedly differs from
those of the Catalan hatchling,
Sinornis, and Gobipteryx in that it is heavily curved in lateral view,
with a strongly concave dorsal margin (Fig. 11.3C). The articular bone of this mandible shows a
depressed retroarticular area with a short, rounded retroarticular process (Fig. 11.3E). A similar, but
larger retroarticular process is present in Gobipteryx (Elzanowski, 1977). As in Gobipteryx, the
mandibular medial process is elongate, but in the El Brete mandible it ends in a hook that projects
slightly rostrally. In the mandibles of the El Brete specimen and Gobipteryx, there is a small
surangular foramen laterally, just rostral to the rim of the articular facets (Fig. 11.3C). Medially, the
El Brete mandible is excavated by a large, elongate, and rostrally pointed fossa auditus (Fig.
11.3D). The rostral margin of this fossa is bounded by a long prearticular (angular of Elzanowski,
1977: fig. 1b).
Axial Skeleton
Several cervical vertebrae are preserved in articulation in the Early Cretaceous Eoalulavis
(Sanz et al., 1996, this volume), Sinornis (Zhou et al., 1992), and the Catalan hatchling (Sanz et
al., 1997). Two isolated mid-cervicals (PVL-4050 and PVL-4057) are part of the El Brete
collection. The neck of the hatchling from Catalonia is composed of nine elements; this number
is concordant with Zhou’s (1995a) estimate of 10 or fewer elements in the cervical series of
Sinornis. Hou et al. (1999) reported 11 cervicals for the neck of Eoenantiornis. The atlas is
known only from the Catalan hatchling. As in Archaeopteryx (Wellnhofer, 1974) and the
alvarezsaurid Shuvuuia (Chiappe, Norell, and Clark, this volume), it is formed by three
elementstwo atlantal arches and the centrum. This condition may be a reflection of the early
ontogenetic age of this specimen, although the presence of unfused atlantal hemiarches is also the
primitive condition for birds (Chiappe, 1992). The remaining cervicals are elongate (in particular
the mid-cervicals from El Brete) and strongly compressed laterally. The spinous processes are
Chiappe and Walker Euenantiornithes 566
low to completely reduced, although they become somewhat taller in the most caudal cervicals.
In the axis and the following two vertebrae of the Catalan hatchling, prominent epipophyses
project farther caudally than the postzygapophyseal facets (Sanz et al., 1997). This condition is
reminiscent of that of the non-avian theropod Deinonychus (Ostrom, 1969). In the mid- and
caudal cervicals, as evidenced by the condition in the Catalan hatchling, the El Brete vertebrae,
and Eoalulavis, the epipophyses are less developed, forming sagittal ridges of varying degree that
do not extend beyond the postzygapophyseal facets. Tapering costal processes fuse to the centra;
in the caudal cervicals of Eoalulavis these are as long as their respective centra (Sanz, Pérez-
Moreno, Chiappe, and Buscalioni, this volume). The centra have sharp, bladelike ventral borders.
In none of the known cervicals is the portion of the centrum caudal to the transverse foramen
perforated by pneumatic foramina. In the El Brete cervicals and in the axis and first four
postaxial elements of the Catalan hatchling, the cranial articular surfaces are heterocoelous, being
wider than tall. The caudal articular surfaces of these vertebrae, however, are taller than wide,
and their degree of heterocoely is incipient, with only a slight convexity transversely and a slight
concavity dorsoventrally. In Eoalulavis, however, both articular surfaces of the caudal cervicals
are not heterocoelous but amphiplatyan (Sanz, Pérez-Moreno, Chiappe, and Buscalioni, this
volume). Because the El Brete cervicals are mid-cervicals, and heterocoely progresses from the
most cranial vertebrae towards the last presacrals (Chiappe, 1996a), the most caudal cervicals of
the taxa represented by the isolated vertebrae from El Brete may have also been non-
heterocoelous. Future findings, however, will have to clarify this issue.
Thoracic vertebrae are preserved in articulation in the El Brete collection, Neuquenornis
(Chiappe and Calvo, 1994), Concornis (Sanz et al., 1995), Eoalulavis (Sanz et al., 1996), and the
Catalan hatchling. Disarticulated thoracic vertebrae are known for Sinornis (Sereno and Rao,
1992; Zhou et al., 1992), and the Late Cretaceous Halimornis from Alabama (Chiappe et al., in
press). The spinous processes of the thoracic vertebrae are laminar and broad (Fig. 11.4). The
caudodorsal tip of the mid- and caudal thoracic vertebrae form a distinct fork; a double fork is
present in the caudalmost thoracic vertebrae of Eoalulavis (Sanz, Pérez-Moreno, Chiappe, and
Buscalioni, this volume). The articular surfaces of the thoracic centra are typically amphiplatyan.
Nevertheless, an isolated cranial thoracic vertebrae from El Brete has an opisthocoelous centrum.
This recalls the condition in the Late Cretaceous Patagonian
Patagopteryx, in which
opisthocoelous cranial thoracic centra make the transition to incipiently heterocoelous caudal
cervicals (Chiappe, 1996b; this volume). The cranial thoracic vertebrae bear ventral processes
(Sereno, Rao, and Li, this volume); these are in some instances very prominent (e.g., Eoalulavis;
see Sanz, Pérez-Moreno, Chiappe, and Buscalioni, this volume). In several articulated thoracic
series from El Brete (e.g., PVL-4051, PVL-4041-2, PVL-4047) and in the cranial thoracic
vertebrae of Eoalulavis, the centra are remarkably compressed. This, however, is not true for
either more recently collected specimens from El Brete or for Halimornis (Chiappe et al., in
press). All enantiornithine thoracic vertebrae exhibit prominent lateral excavations of their centra
(Fig. 11.4), the development of which increases progressively caudally (Chiappe, 1996b). These
can be simple grooves as in Neuquenornis (Chiappe and Calvo, 1994), suboval fossae (e.g.,
Halimornis, Sinornis, Concornis, and the El Brete specimens), or remarkable excavations that
turn the centra into compressed keels as in the cranial thoracic vertebrae of Eoalulavis.
Enantiornithines are unique among avians in that the parapophyses (costal fovea) of the thoracic
vertebrae are situated in a mid-central position, ventral to the transverse process and dorsal to the
lateral excavation (Fig. 11.4).
Chiappe and Walker Euenantiornithes 567
Place Figure 11.4 near here
Synsacral vertebrae are known from several specimens from El Brete, Sinornis (Zhou,
1995a), Concornis (Sanz et al., 1995), and Gobipteryx [“Nanantius valifanovi” (Kurochkin,
1996)]. The euenantiornithine synsacrum is formed by at least eight fused vertebrae. The cranial
articular surface is concave, a condition shared by alvarezsaurids and Patagopteryx. The spinous
processes of the synsacral vertebrae are fused to each other, forming a dorsal crest that decreases
in height caudally. Long, individualized transverse processes project laterally from both the
cranial and caudal sections of the synsacrum. In specimen PVL-4041-4, those of the first two
vertebrae fused together, forming left and right bars parallel to the spinous crest. The following
caudal ones coalesce with the ilium. The ventral surface of the synsacrum bears a shallow, axial
furrow in Gobipteryx (Kurochkin, 1996) and some El Brete specimens (e.g., PVL-4045-2).
Free caudal vertebrae and/or a pygostyle are known for Sinornis (Sereno and Rao, 1992;
Zhou et al., 1992; Sereno, Rao, and Li, this volume), Boluochia (Zhou, 1995b), Concornis (Sanz
et al., 1995), and Halimornis (Chiappe et al., in press). Zhou et al. (1992) estimated the presence
of eight free caudals in Sinornis. The caudal vertebrae are amphiplatyan and they have elongate
transverse processes. The long, enantiornithine pygostyle is formed of at least eight vertebrae
(Lamb et al., 1993; Chiappe et al., in press), although its remarkable elongation in certain taxa
(e.g., Boluochia, in which it is longer than the metatarsals; see Zhou, 1995b; Zhou and Hou, this
volume) suggests that many more elements may have been incorporated in some
enantiornithines. In Halimornis, the pygostyle’s centrum is laterally compressed with paired
laminar processes projecting ventrally; similar ventral processes are present in Sinornis (Sereno,
Rao, and Li, this volume). A proximally forked, axial platform forms the dorsal roof of the
pygostyle in Halimornis (Chiappe et al., in press). Zhou (1995a, b) mentions the presence of a
dorsal crest in the pygostyle of both Boluochia and Sinornis. In Sinornis (IVPP-V-9769A),
however, the roof of the pygostyle bears an overhanging axial platform comparable to that of
Halimornis, and there seems to be no evidence of a dorsal crest.
The long thoracic ribs articulate to short sternal ribs in several taxa (e.g., Sinornis,
Concornis, Neuquenornis). Although several authors pointed out the absence of gastralia among
euenantiornithines (Sereno, Rao, and Li, this volume), a typically reptilian basket of gastralia is
present in
Eoenantiornis. The fact that evidence of gastralia is missing from many well-
articulated specimens of Confuciusornis, whereas gastralia are definitively present in this basal
bird (Chiappe et al., 1999), further cautions against assuming that these elements have been
reduced in a particular taxon when the sample of specimens is small.
Thoracic Girdle and Sternum
Elements of the thoracic girdle and sternum are known for a variety of enantiornithine
taxa, including the Early Cretaceous Sinornis (Sereno and Rao, 1992; Zhou et al., 1992; Zhou,
1995a), Boluochia (Zhou, 1995b), Concornis (Sanz et al., 1995), Eoalulavis (Sanz et al., 1996),
Eoenantiornis (Hou et al., 1999), and the Late Cretaceous Neuquenornis (Chiappe and Calvo,
1994), Alexornis (Brodkorb, 1976), Gobipteryx [“Nanantius valifanovi,” Kurochkin (1996)], and
Halimornis (Chiappe et al., in press). A framentary sternum, several coracoids, and scapulae are
included within the El Brete collection, and portions of the thoracic girdle are known for the
Early Cretaceous hatchling from Catalonia (Sanz et al., 1997) and a yet undescribed avisaurid
from the Late Cretaceous Kaiparowits Formation of Utah (Hutchison, 1993).
FIGURE 11.4. Enantiornithine thoracic vertebrae. A, PVL-4041-2 from the
Late Cretaceous of El Brete (Argentina), right lateral view. B, PVL-4051
from the Late Cretaceous of El Brete (Argentina), left lateral view. C,
Halimornis thompsoni from the Late Cretaceous of Alabama (USA), left
lateral view. D, Sinornis santensis from the Early Cretaceous of Liaoning
(China), left lateral view. Abbreviations: fco, fovea costalis; lec, lateral
excavation of centra; poz, postzygapophysis; spr, spinal process; tp,
transverse process; vptransverse process; vpr, ventral process. C and D not in scale.
Chiappe and Walker Euenantiornithes 568
The enantiornithine coracoid is long and slender; this condition is extreme in Eoalulavis
and Gobipteryx. Its lateral and medial borders are convex and concave, respectively, and a broad
triangular fossa excavates its dorsal surface (Fig. 11.5). Its shoulder end is laterally compressed;
the acrocoracoid process, the humeral articular facet, and the scapular articulation are more or
less aligned (Figs. 11.5A, E). The articular facet for the scapula is located medial and
perpendicular to the humeral articular facet, but the procoracoid process is absent. The scapular
articular facet is characteristically convex (Figs. 11.5C, D). The acrocoracoid process is poorly
developed; the humeral articular facet essentially converges on it. In Enantiornis and other taxa
from El Brete (Fig. 11.5), as well as in a recently discovered enantiornithine from the Late
Cretaceous of France (Buffetaut, 1998), there is a peglike medial process between the
acrocoracoid process and the humeral articular facet (Walker, 1981). Martin (1995) suggested
that this peglike process would have articulated with the furcula, but this seems unlikely given its
position in the coracoid and its topological relationship to the scapula. This process may have
been connected by a ligament to a pit on the shoulder end of the scapula (Chiappe, 1996b),
although a better understanding of its connections must await the discovery of additional,
articulated specimens. This peg-pit complex is absent in Gobipteryx and Halimornis.
Place Figure 11.5 near here
The coracoidal shaft expands to varying degrees below the shoulder end. A large
supracoracoid nerve foramen perforates the shoulder half of the shaft. Its relative position is
variable, but it consistently opens into a medial, longitudinal groove (Fig. 11.5D). In most taxa
(e.g., Enantiornis, Sinornis, Gobipteryx
, the Kaiparowits avisaurid, French enantiornithine), this
foramen opens above the dorsal fossa, but in Neuquenornis, as in PVL-4034 from El Brete, it
opens inside this fossa. The coracoidal sternal end is transversely straight to concave (Fig. 11.5).
It lacks a lateral process. As seen in Neuquenornis and Concornis, the coracoids articulate close
to each other on the sternum (Chiappe and Calvo, 1994; Sanz et al., 1995).
The euenantiornithine scapula is robust and straight, with a well-developed acromion
(Fig. 11.5). The acromion of Eoalulavis and Eoenantiornis is remarkably long (Sanz, Pérez-
Moreno, Chiappe, and Buscalioni, this volume). The scapula of Enantiornis and Halimornis has
an expanded shoulder end. The acromion is wide, bearing a crescentic to semilunate, flat articular
surface (Fig. 11.5). Martin (1995) argued that this facet articulated with the cranial dorsals.
Although possible, this unusual condition has not been observed in any of the euenantiornithine
specimens (e.g.,
Eoalulavis, Neuquenornis) for which the preserved thoracic girdles and
vertebrae retain their topological relationships. Furthermore, the vertebral facet of this alleged
articulation is absent in known enantiornithine thoracic vertebrae. Alternatively, the proximity
between the acromion and the shoulder end of the furcula in Eoalulavis (Sanz, Pérez-Moreno,
Chiappe, and Buscalioni, this volume) suggests that this acromial facet may have articulated with
the furcula. The euenantiornithine acromion is separated from the humeral articular facet by a
thick neck. In Enantiornis this neck is excavated by a deep, circular pit (Chiappe, 1996a; Fig.
11.5K); this pit is absent in Halimornis. The humeral articular facet of the scapula is distinctly
concave (Fig. 11.5). A subtriangular coracoidal facet develops craniomedially and perpendicular
to the latter. The scapular blade is relatively short. Costally, it is scarred by a distinct axial furrow
(Fig. 11.6).
Place Figure 11.6 near here
The enantiornithine furcula has a narrow interclavicular angle (approximately 60° in
Concornis, 45º in Eoalulavis and Eoenantiornis, and 48º in Sinornis). The ascending rami are
Chiappe and Walker Euenantiornithes 569
compressed, with laterally projected ventral borders. As Martin (1995) pointed out, this lateral
convexity of the furcula may have been a place for the origin of flight muscles. The furcula of
Neuquenornis has a short hypocleideum (Fig. 11.6), whereas those of Concornis, Sinornis,
Eoenantiornis, and Eoalulavis bear a much longer one. In Eoalulavis, the hypocleideum is three-
fourths the length of the clavicular rami.
Although a definitive, articulated triosseal foramen has never been reported in any
enantiornithine bird, the connection between the scapula and the coracoid, and the position of the
furcula in some articulated specimens (e.g., Eoalulavis), strongly suggest that this important
avian structure was present in Euenantiornithes. As is typical of neornithine birds, the triosseal
foramen of Euenantiornithes was most likely formed by the coracoid, scapula, and furcula.
The morphology of the sternum of Euenantiornithes is highly variable (Fig. 11.7). Its
ossified portions can be relatively broad as in Concornis or Sinornis, fan-shaped as in
Eoenantiornis, or slender and spear-shaped as in Eoalulavis. The sternum can have a prominent
carina projecting from its cranial margin (e.g., Neuquenornis) (Fig. 11.6), a low keel restricted to
its caudal half (e.g., Concornis, Sinornis), or only a faint ridge (e.g., Eoalulavis) (Fig. 11.7). In
Concornis and Sinornis, the cranial margin is distinctly parabolic. With the exception of
Eoalulavis, the length of the hypocleideum appears to be related to the extension of the sternal
carina. Several enantiornithines have two sternal notches (e.g., Concornis, Sinornis, Boluochia).
In these cases, the lateral process is robust, more than the medial process, and bears a distinct
caudal expansion (e.g., Neuquenornis, Concornis, Sinornis; Figs. 11.7A, C). Eoalulavis lacks any
evidence of sternal processes, although Sanz et al. (1996) argued that cartilaginous portions of
the sternum may have surrounded this central ossified element. The sternum of
Eoalulavis (Fig.
11.7B) differs from all other enantiornithines by the presence of a narrow, cranial notch and an
inverted T-shaped caudal foot; the latter is reminiscent of the condition present in the ornithurine
Liaoningornis (Hou et al., 1996). Sanz et al. (1996) argued that the long hypocleideum of
Eoalulavis could have articulated with this cranial sternal notch, strengthening the thorax. The
sternum of Eoenantiornis also displays a distinct morphology (Hou et al., 1999). It has an overall
round shape with a long, slender caudomedial process. The caudolateral margins of the sternum
of Eoenantiornis bear short caudal processes that lack the distal expansions of other
euenantiornithines. Although Hou et al. (1999) mentioned the presence of a carina in the sternum
of Eoenantiornis, this bone is exposed in dorsal view making impossible to visualize the alleged
ventral keel.
Place Figure 11.7 near here
Thoracic Limb
Nearly complete wings are known for the Early Cretaceous Concornis (Sanz et al., 1995),
Eoalulavis (Sanz et al., 1996), Sinornis (Zhou et al., 1992; Zhou, 1995a), Eoenantiornis (Hou et
al., 1999), and Otogornis (Hou, 1994), and the Late Cretaceous Enantiornis (Walker, 1981;
Chiappe, 1996a) and Neuquenornis (Chiappe and Calvo, 1994). Additional forelimb material is
available for most of the remaining euenantiornithine taxa (Brodkorb, 1976; Sereno and Rao,
1992; Hutchison, 1993; Kurochkin, 1996; Sanz et al., 1997). The El Brete collection includes
several disarticulated wing elements; at least four morphotypes of humerus other than that of
Enantiornis are known from this collection.
The euenantiornithine humerus retains to some extent the primitive torsion of non-avian
theropods. In El Brete specimens PVL-4022 and PVL-4054, the major axes of the proximal and
Chiappe and Walker Euenantiornithes 570
distal ends are at an angle of 42-44°. The deltopectoral crest is flat, lacking any cranial curvature
(Fig. 11.8; Zhou and Hou, this volume). The bicipital crest is robust and prominently projected
cranioventrally. Proximally, this crest is excavated by the transverse ligamental groove; this is a
short and shallow groove in Concornis (Sanz et al., 1995) but longer and deeper in some of the El
Brete humeri (Chiappe, 1996b). On the ventral margin of the bicipital crest’s distal portion there
is a small but distinct fossa, presumably for muscle attachment (Fig. 11.8H). The humeral head is
concave cranially and convex caudally (Figs. 11.8E, I). Its dorsal margin is concave in its middle
area (Fig. 11.8). Caudally, the ventral tubercle is well projected (Fig. 11.8), and in Enantiornis
and other forms from El Brete (PVL-4020, PVL-4043) it is perforated by a proximodistal canal
(Walker, 1981). Although the humerus of several enantiornithines bears a distinct
pneumotricipital fossa, a pneumotricipital foramen is known only for an isolated humerus from
El Brete (PVL-4022; Fig. 11.8C). This is important because the occurrence of such a foramen
suggests the presence of a diverticulum of the clavicular air sac pneumatizing the humerus as in
neornithine birds. Pulmonary air sacs have been inferred for other basal birds (and non-avian
theropods) on the basis of other skeletal features (Britt et al., 1998). The presence of a
pneumotricipital foramen in PVL-4022 provides further support to the idea that at least some
basal birds had already evolved some of the characteristics of the respiratory apparatus of
neornithines. The distal end of the humerus is craniocaudally compressed and transversely
expanded (Fig. 11.8; Zhou and Hou, this volume). In some forms, the ventral epicondyle projects
distally. The dorsal condyle is rectangular and it is commonly horizontally oriented (Fig. 11.8).
The olecranon fossa is wide, and there are no scapulotricipital or humerotricipital grooves (contra
Zhou, 1995a).
Place Figure 11.8 near here
The ulna is longer than or nearly as long as the humerus (Zhou and Hou, this volume).
The dorsal cotyla is elongated; in Enantiornis (Walker, 1981) and Concornis (Sanz et al., 1995),
its surface is distinctively convex (Fig. 11.9). In Enantiornis, a deep groove separates both
cotylae (Figs. 11.9A, D). This groove is shallower, but still distinct, in Concornis. In most forms,
the ulnar shaft lacks any caudal papillae for the insertion of the secondary remigial feathers.
These, however, have been reported in the avisaurid from the Kaiparowits Formation (Hutchison,
1993). Eight secondary feathers are preserved in their natural connection in the ulna of
Eoalulavis (Sanz et al., 1996, this volume). At the distal end of the ulna, the caudal margin of the
dorsal condyle is semilunar (Fig. 11.6). In Enantiornis a large foramen perforates the distal end of
the ulna on its interosseous surface. This foramen is not present in Sinornis, although this taxon
has a fossa in the same position. The radial shaft is much more slender than the ulna; the ratio
between the diameters of the two is approximately 1:2. On its interosseous surface, the radial
shaft is scarred by a longitudinal groove (Fig. 11.6; Sereno, Rao, and Li, this volume; Zhou and
Hou, this volume). Its distal end is spoon-shaped in Neuquenornis and Enantiornis.
Place Figure 11.9 near here
The enantiornithine proximal ends of the metacarpals are fused to the semilunate carpal,
forming a carpometacarpus. Yet, the distal ends of metacarpals II and III are not fused (Figs.
11.6, 11.10). Metacarpal I is subcircular in Neuquenornis and other euenantiornithines, and
subrectangular in Eoenantiornis. This metacarpal lacks a distinct extensor process. Metacarpal III
is longer than metacarpal II, projecting somewhat distally (Figs. 11.6, 11.10). The
intermetacarpal space between these two metacarpals is very narrow.
Place Figure 11.10 near here
FIGURE 11.9. Ulna of Enantiornis leali (PVL-4023) from the Late Cretaceous of El
Brete (Argentina). A, interosseous view. B, caudal view. C, dorsal view. D, ventral
view. Abbreviations: dca, dorsal cotyla; grv, groove dorsal cotyla/olecranon; imb,
impression for m. brachialis; ole, olecranon; vca, ventral cotyla.
FIFIGURE 11.10. Carpometacarpus of Enantiornis leali (PVL-4049) from the Late
Cretaceous of El Brete (Argentina). A, ventral view. B, dorsal view. Abbreviations: alm,
alular metacarpal; ims, intermetacarpal space; mam, major metacarpal; mim, minor
FIGURE 11.10. Carpometacarpus of Enantiornis leali (PVL-4049) from the Late
Cretaceous of El Brete (Argentina). A, ventral view. B, dorsal view. Abbreviations: alm,
alular metacarpal; ims, intermetacarpal space; mam, major metacarpal; mim, minor
metacarpal.
Chiappe and Walker Euenantiornithes 571
Manual digit II is the longest, as in all saurischian dinosaurs. Its proximal phalanx is
longer than the intermediate phalanx. This condition differs from that of other early avians (e.g.,
Archaeopteryx, Confuciusornis, Patagopteryx), and is comparable to that of ornithurine birds
(Chiappe, 1996b). Digit I is well developed but its full length is not longer than metarsals IIIII
as in more primitive birds (e.g., Archaeopteryx, Confuciusornis) and non-avian theropods
(Weishampel et al., 1990). In Eoalulavis, a well-developed alula attaches to the proximal phalanx
of this digit (Sanz et al., 1996). Digit III is typically reduced to a single phalanx; a tiny second
phalanx appears to be present in Sinornis. Manual digits I and II bear distinct claws (e.g.,
Concornis, Eoenantiornis, Sinornis).
Pelvic Girdle
Pelvic remains are known for the Early Cretaceous Sinornis (Sereno and Rao, 1992;
Zhou, 1995a), Concornis (Sanz et al., 1995), Boluochia (Zhou, 1995b), Eoenantiornis (Hou et al.,
1999), and Eoalulavis (Sanz, Pérez-Moreno, Chiappe, and Buscalioni, this volume), and the Late
Cretaceous Gobipteryx [“Nanantius valifanovi,” Kurochkin (1996)], and two isolated pelves
from El Brete (PVL-4041-4, PVL-4032-3).
The pelvic bones of most enantiornithines are fused to each other (Fig. 11.11), although
Zhou et al. (1992) reported that these bones are not fused in Sinornis. The ilium is subtriangular,
with a short, pronglike postacetabular wing (Fig. 11.11A). The slightly longer preacetabular wing
is laterally concave. In PVL-4041-4 the two ilia contact each other in the midline, although this
may be a preservational artifact (Fig. 11.11B). In
Gobipteryx, the ilia are separated from each
other (Kurochkin, 1996). The acetabulum is round and its medial face is completely perforated
(Fig. 11.11), a different condition from other Mesozoic birds in which the floor of the
acetabulum is partially occluded (Martin, 1983, 1987; Chiappe, 1996b). Above the acetabulum,
the dorsal iliac crest expands into a triangular supracetabular tubercle, the apex of which projects
laterally. A prominent antitrochanter is developed on the caudodorsal corner of the acetabulum.
The pubic peduncle is robust and caudoventrally directed (Fig. 11.11A). In PVL-4032-3, the
ventral margin of the ilium is perforated by a small fossa just cranial to the base of the pubic
peduncle. This fossa, presumably part of the origin of the cuppedicus muscle (Rowe, 1986), is
also present in Gobipteryx (Kurochkin, 1996).
Place Figure 11.11 near here
The pubis is opisthopubic, although not to the degree seen in ornithurine birds (Chiappe,
1996a). It is a slender bone with a suboval to straplike cross-section. In most euenantiornithines,
the distal ends of the pubes contact each other, forming a short symphysis (Sanz et al., 1995, this
volume; Kurochkin, 1996; Zhou and Hou, this volume). The distal ends of the pubes are not in
contact in Sinornis and Eoenantiornis, however, even though their tips expand into caudally
projected feet (Fig. 11.16). The distal pubic foot is, however, absent in Gobipteryx (Kurochkin,
1996) and presumably in Concornis (Sanz et al., 1995).
The euenantiornithine ischium is roughly two-thirds to three-fourths the length of the
pubis. It is laterally compressed and much longer than the postacetabular wing of the ilium. The
morphology of the ischium ranges from beltlike in Concornis to knifelike in Sinornis and
Eoenantiornis. In its proximal half, the caudodorsal margin of the ischiadic shaft projects into a
proximodorsal process (Fig. 11.11A; Zhou and Hou, this volume), which in certain cases (e.g.,
PVL-4032-3, Sinornis) abuts against the medial margin of the ilium, near the end of the
postacetabular wing. This contact caudally encloses an ilioischiadic fenestra (Fig. 11.11A)
Chiappe and Walker Euenantiornithes 572
comparable, although not homologous, to that of neognathine birds. Ischiadic proximodorsal
processes are present in a variety of early birds [e.g., Archaeopteryx (Wellnhofer, 1985),
Rahonavis (Forster et al., 1998a, b), Confuciusornis (Chiappe et al., 1999)] and non-avian
theropods [e.g., Unenlagia (Novas and Puerta, 1997)], and it probably represents a primitive
condition for birds. The two ischia approach each other but do not form a distal symphysis in
Concornis (Sanz et al., 1995, this volume). It is unclear whether this is also the case for other
euenantiornithine taxa.
Pelvic Limb
Elements of the pelvic limb are known for almost all species of euenantiornithines.
Several euenantiornithine species (e.g., Avisaurus archibaldi, Soroavisaurus australis,
Yungavolucris brevipedalis) are exclusively known from hindlimb bones.
The femur bears a small, cylindrical head separated from the shaft by a distinct neck (Fig.
11.12). A clear fossa for the capital ligament is present in the El Brete femora (e.g., PVL-4060,
PVL-4273, PVL-4037). The cranial surface of the proximal end bears a low trochanteric crest.
Laterally, the proximal end is excavated by a prominent posterior trochanter (Fig. 11.12). This
structure is present in varying degrees among enantiornithine species (Fig. 11.12). Its consistent
hypertrophy, however, is characteristic of Euenantiornithes among all other birds, suggesting that
important postural or locomotor muscles must have inserted there. Currie and Peng (1993)
postulated that the posterior trochanter received the insertion of the ischiofemoralis muscle,
although it would be conceivable that other muscles also attached to this area. The femoral shaft
is slightly bowed in Concornis, Sinornis, and some of the El Brete femora (Fig. 11.12); in certain
specimens from El Brete (PVL-4038), however, it is completely straight. Distally, the femur
lacks a patellar groove. In Neuquenornis, Concornis, and certain taxa from El Brete (e.g., PVL-
4037, PVL-4038, MACN-S-01), the distal lateral margin of the femur projects caudally, forming
a ridge continuous with the lateral condyle (Fig. 11.12). This ridge is absent in Halimornis and
the Kaiparowits avisaurid.
Place Figure 11.12 near here
The tibia is typically fused to the proximal tarsals, forming a true tibiotarsus (Figs. 11.13,
11.14); a faint suture defining a tall ascending process of the astragalus is preserved in the
holotype of Nanantius eos (Molnar, 1986) and the specimen of Gobipteryx (i.e., “
Nanantius
valifanovi”) described by Kurochkin (1996). The proximal articular end is subcircular in
Nanantius, Concornis, and Soroavisaurus (Fig. 11.13B), but it has a transversely wider axis in
Lectavis (Fig. 11.14C), in which the articular end is more ellipsoidal (Chiappe, 1993). There is
only a single weak cnemial crest on the craniolateral border of the tibiotarsus (Figs. 11.13,
11.14). This is thought to correspond to the ancestral cnemial crest of non-avian theropods
(Chiappe, 1996b). Distally, both the extensor canal and the supratendinal bridge are absent (Figs.
11.13, 11.14). As in other basal birds (e.g., Confuciusornis, Patagopteryx, Vorona), the medial
condyle is wider than the lateral one. In most taxa, these condyles are separated by a narrow
intercondylar notch that proximally undercuts them. In Lectavis, Concornis, Nanantius, and
Soroavisaurus, the medial condyle has a subcylindrical shape, which, along with the lateral
condyle, gives the distal end of the tibiotarsus an hour-glass aspect in cranial view (Figs. 11.13,
11.14). In Gobipteryx (“Nanantius valifanovi”; Kurochkin, 1996), however, the medial condyle
is more suboval and a much wider groove separates the two condyles.
Place Figure 11.13 near here
FIGURE 11.12. A-D, enantiornithine femur
(PVL-4037) from the Late Cretaceous of El
Brete (Argentina) in caudal (A), cranial (B),
medial (C), and lateral (D) views. E-H,
enantiornithine femoral posterior trochanter
(arrow). E, PVL-4037. F, PVL-4060. G,
Neuquenornis volans. H, Sinornis santensis.
Abbreviations: fcl, fossa for capital ligament; Abbreviations: fcl, fossa for capital ligament;
lco, lateral condylus; mco, medial condylus;
pfo, popliteal fossa; ptr, posterior trochanter.
Scale applies to Figures A-D only.
FIGURE 11.13. Tibiotarsus of Soroavisaurus
australis from the Late Cretaceous of El Brete
(Argentina). A, C, cranial view [PVL-4033 (A) and
PVL-4030 (C)]. B, proximal view (PVL-4033). D,
distal view (PVL-4033). Abbreviations: cnc,
cranial cnemial crest; fic, fibular crest; lco, lateral
condylus; mco, medial condylus.FIGURE 11.13.
Tibiotarsus of Soroavisaurus australis from the Tibiotarsus of Soroavisaurus australis from the
Late Cretaceous of El Brete (Argentina). A, C,
cranial view [PVL-4033 (A) and PVL-4030 (C)].
B, proximal view (PVL-4033). D, distal view
(PVL-4033). Abbreviations: cnc, cranial cnemial
crest; fic, fibular crest; lco, lateral condylus; mco,
medial condylus.
Chiappe and Walker Euenantiornithes 573
— Place Figure 11.14 near here
No free tarsals have ever been reported in any euenantiornithine. In consequence, it seems
reasonable to assume that these were fused to the proximal end of the metatarsals (contra Martin,
1983, 1995). In general, metatarsals II–IV are fused only proximally (Figs. 11.6, 11.15; Zhou and
Hou, this volume), but distal fusion of the metatarsals has been reported for Avisaurus gloriae
(Varricchio and Chiappe, 1995). Euenantiornithines retain the ancestral coplanar orientation of
metatarsals IIIV. Metatarsal IV is less prominent than metatarsals II–III (Fig. 11.15; Zhou and
Hou, this volume). Metatarsal II bears a distinct tubercle on its dorsal surface, presumably the
insertion of the tibialis cranialis muscle (Chiappe, 1996b). This tubercle was previously
considered a synapomorphy of enantiornithines, but its occurrence in Confuciusornis (Chiappe et
al., 1999) and dromaeosaurid theropods (Norell and Makovicky, 1997) suggests that it is a
primitive feature. Distally, the trochlea of metatarsal II is broader than the trochleae of
metatarsals III and IV. Despite these common morphological features, the morphological range
of the enantiornithine tarsometatarsi is striking (Fig. 11.15). While Lectavis is characterized by a
long and slender tarsometatarsus, with a keellike hypotarsus, Yungavolucris tarsometatarsus is
short and broad, with a stongly ginglymoid trochlea on metatarsal II and laterally divergent
trochleae of metatarsals III and IV (Chiappe, 1993). Other enantiornithine species (e.g.,
avisaurids, Concornis, Boluochia, Sinornis) have less specialized tarsometatarsi. All
enantiornithines for which the foot is known have a retroverted hallux (Fig. 11.8). Metatarsal I,
however, is not reversed and it retains the primitive articulation on the medial surface of
metatarsal II of non-avian theropods (e.g., Norell and Makovicky, 1997). Unfortunately, the feet
of the highly specialized Lectavis and Yungavolucris are not known.
Place Figure 11.15 near here
Place Figure 11.16 near here
PHYLOGENETIC RELATIONSHIPS
Euenantiornithes is universally accepted as an important basal chapter in the early
evolution of birds and, with rare exceptions [e.g., Elzanowski (1995), who regarded Sinornis
,
Concornis, and the remaining enantiornithines as successive outgroups of ornithurine birds], they
are considered monophyletic. This was not always the case, however. The history of the study of
this Cretaceous avian radiation illustrates a general shift in the perception of the early evolution
of birds. Gobipteryx was the first enantiornithine bird to be properly recognized as a bird at the
time of its description; Elzanowski (1974) identified Gobipteryx as a Cretaceous palaeognath.
This conclusion, however, was criticized by others (e.g., Brodkorb, 1976, 1978), who regarded
Gobipteryx as a non-avian reptile. The discovery of the large assemblage of bones from El Brete
and their recognition as a monophyletic ensemble of birds by Walker (1981) did little to change
this view (J. F. Bonaparte, J. Cracraft, and L. D. Martin are exceptions); the avian status of
Enantiornithes was still not fully accepted (e.g., Steadman, 1983; Kurochkin, 1985; Olson, 1985;
Brett-Surman and Paul, 1985). Interestingly, enantiornithine bones had been found decades
earlier, during explorations of the Cretaceous outcrops of the western United States in the
nineteenth century (Chiappe, 1995a; Padian and Chiappe, 1998)these specimens (Fig. 11.17),
however, were also mistakenly identified as non-avian theropods. In 1892, for example, O. C.
Marsh named a new species of ornithomimid theropod, Ornithomimus minutus, on the basis of
several portions of metatarsals (Marsh, 1892). The holotype is now lost, but one of these
metatarsal fragments (USNM 2909) was of an avisaurid euenantiornithine (Figs. 11.17C, D).
FIGURE 11.14. Tibiotarsus of Lectavis bretincola
(PVL-4021-1) from the Late Cretaceous of El Brete
(Argentina). A, medial view. B, cranial view. C,
proximal view. D, distal view. Abbreviations: lco, lateral
condylus; mco, medial condylus.
FIGURE 11.14. Tibiotarsus of Lectavis bretincola
(PVL-4021-1) from the Late Cretaceous of El Brete
(Argentina).(Argentina). A, medial view. B, cranial view. C,
proximal view. D, distal view. Abbreviations: lco, lateral
condylus; mco, medial condylus.
FIGURE 11.15. Enantiornithine tarsometatarsi. A, B,
Avisaurus archibaldi. C, D, Lectavis bretincola. E, F,
Soroavisaurus australis. G, H, Yungavolucris
brevipedalis. A, C, E, G, dorsal view. B, D, F, H,
plantar view. Abbreviations: IIV, metatarsals IIV.
Modified from Chiappe (1993).
FIGURE 11.16. Eoenantiornis buhleri from the Early
Cretaceous of Liaoning, China.
Chiappe and Walker Euenantiornithes 574
This is also the case for several other “old” specimens such as YPM 865 (Figs. 11.17E, F), which
although its original label reads “bird or Ornithomimus,” the specimen belongs to an
euenantiornithine avisaurid bird. Avisaurus archibaldi is another euenantiornithine that was also
initially regarded as a non-avian theropod (Brett-Surman and Paul, 1985), a taxonomic
conclusion these authors extended to specimens later named Soroavisaurus australis (Chiappe,
1993). These frequent misidentifications evoke the statement commonly used in discussions of
avian origins, namely that the theropod ancestry of birds is illustrated by the fact that specimens
of Archaeopteryx for which feathers are not clearly visible (e.g., Eichstätt) were first
misidentified as non-avian theropods. Undeniably, this has also been the case for enantiornithine
birds. Only with the advent of more complete discoveries (e.g., Chiappe, 1992; Zhou et al., 1992)
came the notion that these primitive and sometimes odd creatures were indeed birds and not non-
avian theropods. Along with this notion came the realization that several early lineages of birds
were vastly different from their extant counterparts.
Place Figure 11.17 near here
The phylogenetic position of Enantiornithes and Euenantiornithes within basal birds has
been a matter of intense debate (see Chiappe, 1995b, 1996b, in press, Chapter 20 in this volume,
for a more extensive discussion). These birds have variously been considered the sister-group of
Archaeopteryx (Martin, 1983), an ornithurine lineage (Cracraft, 1986), or an intermediate lineage
between Archaeopteryx and Ornithurae (Walker, 1981; Thulborn, 1984) (Fig. 11.18). More
recent cladistic analyses with varying data sets and included taxa (e.g., Chiappe, 1991, 1995b,
1996b; Chiappe et al., 1996; Sanz et al., 1995, 1996; Forster et al., 1996, 1998) have consistently
supported Walker’s (1981) early view that Enantiornithes is phylogenetically intermediate
between Archaeopteryx and Neornithes (Fig. 11.18A). These same analyses have rejected the
proposal of Martin (1983, 1991, 1995) and his followers (e.g., Hou et al., 1995, 1996; Feduccia,
1996; Kurochkin, 1996) of a sister-taxon relationship between Archaeopteryx and
Enantiornithes, a paraphyletic group that Martin (1983) called “Sauriurae” (Fig. 11.18B).
Proponents of a close relationship between Enantiornithes and Archaeopteryx have substantiated
their views using primarily non-cladistic methods and, thus, it is hard to evaluate their proposals
in light of cladistic parsimony. The only cladistic analysis supporting the monophyly of
“Sauriurae” is that of Hou et al. (1996; data matrix and character list available from
Science’s
web site). This analysis included only 36 characters, approximately a third or a fifth of the
information used by Chiappe et al. (1996) and Chiappe (Chapter 20 in this volume), respectively.
In Hou et al.’s analysis, “Sauriurae” is supported by five unambiguous synapomorphies, the same
ones proposed by Martin (1983) in his original paper. These characters, however, have very little
(if any) phylogenetic information because they have been shown to be either equivocal or
plesiomorphic (see Chiappe, 1995b). Even Zhou (1995a), one of the co-authors of the analysis,
disregarded the phylogenetic meaning of most of them, concluding that “Cathayornis [here
regarded as a synonym of Sinornis] lies between Archaeopteryx and Ichthyornis in terms of both
evolutionary grade and flight capability.” In Chiappe’s most recent analysis (Chapter 20 in this
volume), Enantiornithes (including Euenantiornithes plus Iberomesornis and Noguerornis) shows
a sister-group relationship with Ornithuromorpha—a relationship is supported by 13
unambiguous synapomorphies. As pointed out by Chiappe (Chapter 20 in this volume), any
cladistic hypothesis supporting the monophyly of “Sauriurae” over those placing Enantiornithes
closer to Neornithes would require a large number of additional steps. At the same time, it would
require accepting that flight was independently fine-tuned in both Enantiornithes and Neornithes,
FIGURE 11.17. Enantiornithine remains collected in the
19th Century. A, B, metatarsal III (PU-17324) of Avisaurus
archibaldi in dorsal (A) and plantar (B) view. C, D,
metatarsal II (YPM-2909) in dorsal (C) and plantar (D)
view. E, F, metatarsal III (YPM-865) in dorsal (E) and
plantar (F) view.
Chiappe and Walker Euenantiornithes 575
because many of the characters that are shared by all these birds have been consistently correlated
to flight enhancement (e.g., carinate sternum, strutlike coracoid, pygostyle, modern proportions
of wing elements, alula).
Place Figure 11.18 near here —
Although the intermediate placement of Enantiornithes and Euenantiornithes between
Archaeopteryx and Neornithes is well supported today, the phylogenetic interrelationships among
different euenantiornithine species have been substantially less explored. To date, only cladistic
analyses including a handful of euenantiornithine species have been conducted (e.g., Chiappe,
1993; Varricchio and Chiappe, 1995; Sanz et al., 1995; Fig. 11.19A). Kurochkin (1996) provided
a hypothesis of relationships including most known enantiornithine species (Fig. 11.19B).
Unfortunately, this study was not framed under cladistic methods, making it harder to evaluate its
merit over previous cladistic hypotheses (e.g., Chiappe, 1993; Sanz et al., 1995; see Sereno, Rao,
and Li, this volume for further discussions on Kurochkin’s work). More problematic is the fact
that the rationale for the proposed clusters of species depicted in his single tree, which is not a
consensus cladogram, was not explained. This is particularly disturbing when we consider that
some of Kurochkin’s (1996) proposed groups cluster taxa for which overlapping skeletal
elements are unknown. For example, the rationale underlying the grouping of Avisaurus,
Soroavisaurus, and Enantiornis in Enantiornithidae (Fig. 11.19B) is unclear when we consider
that while Avisaurus and Soroavisaurus are known by hindlimb elements (Chiappe, 1993),
Enantiornis is known from elements of the forelimb and thoracic girdle (Chiappe, 1996a).
— Place Figure 11.19 near here
For this chapter, a list of 36 characters (one multistate) was scored for 16 valid genera of
euenantiornithine birds (see SYSTEMATIC PALEONTOLOGY) (Appendix 11.I). Because
many of the taxa are highly incomplete, the character matrix includes a high percentage of
missing data. The data matrix was analyzed using Goloboff’s EST computer program [Goloboff
and Farris (1998), in Horovitz (1999)]. This analysis resulted in a “bushlike” consensus
cladogram lacking any kind of resolution. Subsequent analyses of the data matrix excluding taxa
with 90% or more (i.e., Otogornis, Alexornis, and Nanantius) and 80% or more missing data
(i.e., Otogornis, Alexornis, Nanantius, Yungavolucris, Avisaurus, and Lectavis) also produced
consensus trees without resolution. Only when the analyzed matrix was restricted to taxa with
70% or less missing data (i.e., Eoalulavis, Eoenantiornis, Neuquenornis, Enantiornis, Gobipteryx,
Sinornis, and Concornis) did EST produce a consensus tree with some resolution. In this
consensus cladogram, Concornis, Enantiornis, and Neuquenornis formed a group that joined the
remaining four taxa in an unresolved polychotomy (Fig. 11.20). Clearly, an understanding of the
interrelationships within Euenantiornithes has been hampered by the incomplete material
available for several of the species, which often are known by either one bone or another (e.g.,
Yungavolucris, Avisaurus, Nanantius), and the close similarity of many of these birds (Sereno,
Rao, and Li, this volume). Although the result of this new cladistic analysis are largely
incomplete and will certainly show substantial modifications with the addition of new data, it is
interesting to note that this consensus cladogram recovers Sanz et al.’s (1995) proposed
relationship between Concornis and Neuquenornis [Sanz et al. (1995) also included
Soroavisaurus and Avisaurus along with the former two taxa (Fig. 11.19A)]. Deeper comparative
studies of the known euenantiornithine species as well as the discovery of additional material are
necessary for a better understanding of the cladistic relationships among enantiornithine birds,
which remains one of the most challenging issues of basal avian systematics.
Chiappe and Walker Euenantiornithes 576
Place Figure 11.20 near here
PALEOBIOLOGY
Enantiornithes is probably the group of Mesozoic birds for which we are able to provide
the most reliable inferences regarding their general lifestyles, reproductive biology, and growth
strategies. Most enantiornithine fossils occur in non-marine deposits, indicating their
preponderance in the terrestrial avifaunas of the Cretaceous. Nevertheless, as the discovery of the
partial skeleton of Halimornis in marine beds formed as far as 50 km offshore suggests, these
early birds may have played a more important role in the marine and littoral ecosystems than was
earlier understood (Lamb et al., 1993; Chiappe et al., in press).
The morphological disparity seen among the different euenantiornithine species suggests
a diversity of lifestyles. The anisodactyl foot of Sinornis, Concornis, Neuquenornis, and
Soroavisaurus suggests perching capabilities (Sereno and Rao, 1992; Chiappe, 1993; Chiappe
and Calvo, 1994; Sanz et al., 1995), but the pedal morphology of Yungavolucris and Lectavis has
been correlated with aquatic and wading habits, respectively (Walker, 1981; Chiappe, 1993).
Aquatic or near-shore habits have also been proposed for Eoalulavis (Sanz et al., 1996), for
which the presence of exoskeletal remains of aquatic crustaceans inside its digestive tract
indicates that, at the very least, it frequented near-shore environments. Although the presence of
stomach contents in Eoalulavis is the only direct evidence of feeding habits available for
enantiornithine birds, the hooked beak of Boluochia suggests more raptorial habits (Zhou,
1995b). Similar habits have been inferred for Neuquenornis and Soroavisaurus, in which the
hallucial claw is hypertrophied (Chiappe, 1993; Chiappe and Calvo, 1994). Feduccia (1996)
pointed out that the long hypocleideum and caudally restricted sternal keel of some
euenantiornithines (e.g., Sinornis, Concornis) may indicate the existence of a digestive system
with a large crop. He compared this design to that of the folivore hoatzin (Opisthocomus) and
suggested that Sinornis may have had similar feeding habits. Indeed, these birds may have been
folivores, as observed by Feduccia (1996), but the presence of small, sharp teeth in Sinornis
suggests an insectivorous diet more than one based on leaves.
During their evolution throughout the Cretaceous there is a general increment in size and
size range among the euenantiornithine species. All known Early Cretaceous euenantiornithines
are small- to medium-size birds (see comparative measurements in Bochenski, 1996). Sinornis
and Eoalulavis, for example, were the size of a sparrow or finch, and the size of Nanantius and
Concornis was comparable to that of a medium-sized thrush. Late Cretaceous euenantiornithines,
however, are generally larger: Gobipteryx was the size of a jackdaw and Neuquenornis that of a
small falcon. Some of the Late Cretaceous taxa were much larger. Enantiornis had a wingspan of
about 1.2 meters, ranging in size between a skua and a turkey vulture (Chiappe, 1996a), and a
recently discovered enantiornithine from France has been compared in size to a herring gull
(Buffetaut, 1998). Some Late Cretaceous euenantiornithines, however, retained the small size of
their Early Cretaceous relatives; Alexornis was sparrow-sized.
On the basis of the presence of a deep notch in the sternum, Walker (1981) argued that
enantiornithines had a limited capacity for flight. This view, however, was challenged by
Chiappe and Calvo (1994), who, based on the morphology of the wing, sternum, and thoracic
girdle of Neuquenornis, proposed that these birds were probably good fliers. A decisive find was
that of Eoalulavis (Sanz et al., 1996), in which a nearly intact alula was preserved attached to its
manual digit I (Sanz, Pérez-Moreno, Chiappe, and Buscalioni, this volume). The presence of an
Chiappe and Walker Euenantiornithes 577
alula in Eoalulavis, along with derived wing proportions and other flight-correlated structures
(e.g., strutlike coracoid, fused wrist bones, pygostyle) in this and other euenantiornithines,
strongly suggests that these birds had achieved an enhanced flying capacity and control of
maneuverability, including in their repertoire the ability to perform low-speed flight (Sanz et al.,
1996).
Studies of euenantiornithine embryos (Elzanowski, 1981), hatchlings (Sanz et al., 1997),
and egg shell microstructure (Mikhailov, 1991, 1996), as well as of the bone histology of adult
individuals (Chinsamy et al., 1994, 1995), have provided important data on the reproductive
biology and rates of growth of these early birds. Elzanowski (1981) reported on a series of
embryos from the Late Cretaceous of Mongolia, which he tentatively referred to Gobipteryx. The
forelimb bones of these embryos are proportionally longer than those of most neornithine
hatchlings, and their degree of ossification is more advanced than that of hatchlings of extant
precocial birds (e.g., chicken, skua) (Elzanowski, 1981). This precocial development of the
forelimb, along with the differential degree of ossification between the fore- and hindlimbs of
these embryos, led Elzanowski (1981, 1995) to argue that the developmental strategy of
Gobipteryx was superprecocial, that is, juveniles were completely self-sufficient in terms of
finding food and were able to fly soon after hatching. Elzanowski (1995) also argued for male
incubation, because this strategy is generally associated with the high energetic demands
expected of embryos with such a precocial degree of ossification. A precocial developmental
mode is primitive for neornithine birds (Starck, 1993), since phylogenetically basal extant birds
such as ratites and galliforms have this mode of development. Thus, it is likely to be the case for
Euenantiornithes (Chiappe, 1995a). The degree of embryonic ossification, however, appears not
to be necessarily correlated with one or another type of developmental mode (Starck, 1993).
Phylogenetic inference also indicates that biparental care is actually primitive for neornithine
birds (McKitrick, 1992). Given this and the fact that the extreme ossification of the Gobipteryx
embryos may not be indicative of superprecociality, Elzanowski’s (1995) argument for
superprecociality and male incubation in Enantiornithes should be taken with caution.
Elzanowski (1995) also expanded his developmental conclusions to the Patagonian
Neuquenornis, which he regarded as a very young specimen. Feduccia (1996), following
Elzanowski (1995), regarded Neuquenornis as late embryonic. These conclusions, however, are
most likely incorrect. First, the holotype specimen of Neuquenornis (MUCPv-142; Chiappe and
Calvo, 1994) is actually relatively large; the wingspan of this specimen exceeds 40 cm. Second,
many of the long bone ends of MUCPv-142 are completely ossified, including metacarpal I,
which does not ossify before hatching (Elzanowski, 1981). Third, the sternum of this specimen is
fully ossified, including its prominent carina. As Elzanowski (1981) pointed out, most of the
ossification of the sternum of neornithine birds occurs during postembryonic development, and
typically at a slow rate of ossification. None of the structures of MUCPv-142 indicate an
embryonic or near-hatching stage. The poor preservation of some of its bones is nothing more
than a preservational artifact, and it compares well with the preservation shown by other fossils
from the same site.
Data on the growth rates of euenantiornithines were derived from the histological studies
of Chinsamy et al. (1994, 1995). These studies revealed the presence of growth rings (lines of
arrested growth or LAGs) in the cross-sections of two euenantiornithine femora from El Brete.
The presence of growth rings indicates a pause of postnatal growth, presumably a cyclical
interruption of this growth. This pattern of bone deposition is strikingly different from that of
Chiappe and Walker Euenantiornithes 578
neornithine birds, and apparently ornithurine birds in general (Chinsamy, this volume), in which
uninterrupted growth leads to the development of adult size in the range of a single year (Starck,
1993). The presence of growth rings in the compacta of euenantiornithine limb bones suggests
that these birds took more than one year to achieve mature size (Chinsamy et al., 1995).
Furthermore, the microstructure of the euenantiornithine bone was unique, exhibiting virtually no
vascularization and a lamellar, slowly deposited type of bone. The slow rate of growth inferred
from this pattern of bone deposition appears to be consistent with the idea that euenantiornithine
birds were precocial (Chiappe, 1995a), since precocial birds have rates of growth that are slower
than those of altricial birds (Starck, 1993). The presence of this characteristic type of bone tissue
led Chinsamy et al. (1994, 1995) to argue that important physiological differences with respect to
their modern counterparts may have been present in Euenantiornithes. Specifically, they argued
that because growth is strongly correlated with body temperature in living birds (Prosser, 1973),
these bone differences may be indicating that these early birds were not fully endothermic. Other
authors incorrectly used this to claim that euenantiornithine birds were ectothermic (e.g.,
Feduccia and Martin, 1996), but Chinsamy et al. (1994, 1995) simply stated that their thermal
metabolism was probably lower than that of neornithine birds. If so, the metabolic rates of
Euenantiornithes probably fit an intermediate level within the spectrum of ectothermy-
endothermy (Chiappe, 1995a). This argument, although more conjectural than reading growth
rates from the pattern of bone deposition, is interesting since euenantiornithine birds were fully
feathered, and feathers have consistently been correlated to endothermy. More sampling of
euenantiornithine bones as well as other metabolic-indicator studies are necessary before the
meaning of the peculiar pattern of euenantiornithine bone microstructure can be fully understood.
ACKNOWLEDGMENTS
We are grateful to J. Bonaparte, J. Calvo, Chen Pei-Ji, Hou L.-H., Ji Q., Ji S.-A., E.
Kurochkin, M. Norell, H. Osmólska, J. Powell, J. Sanz, D. Varricchio, and Zhou Z. who
facilitated specimens for study and to K. Padian and L. M. Witmer for their constructive reviews.
We also thank S. Orell for her editorial work on the manuscript. S. Reuil provided assistance
with the graphics and illustrations. Figure 11.20 was photographed by L. Meeker. Support for
this research was provided by the Chapman Fund of the American Museum of Natural History,
the Natural History Museum of Los Angeles County, and the National Science Foundation
(DEB-9873705).
Chiappe and Walker Euenantiornithes 579
LITERATURE CITED
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APPENDIX 11.I
Characters and character-states used for cladistic analysis of enantiornithine taxa.
1. Premaxillary teeth: present (0), absent (1).
2. Parapophyses located in the cranial (0), or central (1) part of the bodies of dorsal vertebrae.
3. Cranial end of pygostyle dorsally forked and with a pair of laminar, ventrally projected
processes: absent (0), present (1).
4. Laterally compressed shoulder end of coracoid, with nearly aligned acrocoracoid process,
humeral articular surface, and scapular facet, in dorsal view: absent (0), present (1).
5. Distinctly convex lateral margin of coracoid: absent (0), present (1).
6. Supracoracoid nerve foramen opening into an elongate furrow medially and separated from
the medial margin of the coracoid by a thick, bony bar: absent (0), present (1).
7. Broad, deep fossa on the dorsal surface of the coracoid: absent (0), present (1).
8. Supracoracoid nerve foramen opens above the coracoidal fossa (0) or inside of it (1).
9. Scapular acromion costolaterally wider than deeper: absent (0), present (1).
10. Costal surface of scapular blade with a prominent, longitudinal furrow: absent (0), present
(1).
11. Dorsal and ventral margins of the furcula: subequal in width (0), ventral margin clearly wider
than the dorsal margin (1).
12. Hypocleideum: absent or poorly developed (0), or well developed (1).
13. Ossified sternal keel: absent or incipient (0), present and near to or projecting cranially from
the cranial border of the sternum (1), present and not reaching the cranial border of the sternum
(2).
14. Lateral process on sternum: absent (0), present (1).
15. Lateral process of sternum without (0) or with (1) a subtriangular distal expansion.
16. Medial process of sternum: absent (0), present (1).
17. Dorsal (superior) margin of the humeral head concave in its central portion, rising ventrally
and dorsally: absent (0), present (1).
18. Prominent bicipital crest of humerus, cranioventrally projecting: absent (0), present (1).
19. Ventral face of the humeral bicipital crest with a small fossa for muscular attachment: absent
(0), present (1).
20. Distal end of humerus very compressed craniocaudally: absent (0), present (1).
21. Dorsal cotyla of ulna separated from the olecranon by a groove: absent (0), present (1).
22. Shaft of radius with a long longitudinal groove on its caudoventral surface: absent (0),
present (1).
23. Subcircular extensor process of metatarsal I: absent (0), present (1).
24. Minor metacarpal (III) projecting distally more than major metacarpal (II): absent (0), present
(1).
25. Manual digit II: proximal phalanx shorter (0) or longer (1) than intermediate phalanx.
26. Pubic foot: present (0), absent (1).
27. Femoral posterior trochanter: absent or weakly developed (0), hypertrophied (1).
28. Caudal projection of the lateral border of the distal end of femur: absent (0), present (1).
29. Very narrow, deep intercondylar sulcus on tibiotarsus that proximally undercuts condyles:
absent (0), present (1).
30. Trochlea of metatarsal II much broader than the trochlea of metatarsal III: absent (0), present
(1).
Chiappe and Walker Euenantiornithes 586
31. Metatarsal IV significantly thinner than metatarsals II and III: absent (0), present (1).
32. Medial rim of the trochlea of metatarsal III with a strong plantar projection: absent (0),
present (1).
33. Metatarsal I distinctly J-shaped in medial aspect: absent (0), present (1).
34. Proximal articular surface strongly inclined dorsally: absent (0), present (1).
35. Strong transverse convexity of the dorsal surface of the mid-shaft of metatarsal III: absent (0),
present (1).
36. Distal end of metatarsal II strongly curved medially: absent (0), present (1).
Chiappe and Walker Euenantiornithes 587
Character matrix used for cladistic analysis of enantiornithine taxa. Primitive state: 0; derived states: 1, 2; missing entries: ?.
taxon
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Archaeopteryx lithographica
0
0
0
0
0
0
0
n
0
0
0
0
0
0
n
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Halimornis thompsoni
?
1
1
?
?
?
?
?
1
1
?
?
?
?
?
?
1
1
1
?
?
?
?
?
?
?
?
0
?
?
?
?
?
?
?
?
Concornis lacustris
?
1
?
1
1
1
1
?
1
?
1
1
2
1
1
1
1
1
1
1
1
?
?
?
1
1
?
1
?
1
1
?
0
0
1
0
Sinornis santensis
0
1
1
?
1
1
1
0
?
0
?
1
2
1
1
1
1
1
1
?
?
1
1
1
1
0
1
0
1
?
1
?
0
?
?
0
Gobipteryx minuta
1
?
?
1
?
?
1
0
0
?
1
1
?
?
?
?
?
?
?
?
?
1
?
1
?
1
?
?
?
1
1
?
0
1
0
1
Lectavis bretincola
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
0
?
1
?
?
0
0
?
Soroavisaurus australis
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
1
1
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Yungavolucris brevipedalis
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Avisaurus gloriae
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Enantiornis leali
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Neuquenornis volans
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Boluochia zenghi
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Nanantius eos
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Otogornis genghisi
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Alexornis antecedens
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Eoalulavis hoyasi
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Eoenantiornis buhleri
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Chiappe and Walker Euenantiornithes 588
... This is particularly true for theropod dinosaurs. The Late Cretaceous South American theropod record is taxonomically diverse and comprised of widely divergent clades, including Noasauridae (Bonaparte and Powell, 1980;Bonaparte, 1991), Abelisauridae (Bonaparte, 1985;Bonaparte and Novas, 1985;Aranciaga Rolando et al., 2021), Carcharodontosauridae (Coria and Salgado, 1995;Novas and Pol, 2005;Canale et al., 2014;Canale et al., 2022), Megaraptoridae (Novas, 1998;Porfiri et al., 2014;Coria and Curie, 2016), early diverging coelurosaurs (Kellner, 1999;Novas et al., 2012), alvarezsaurs (Novas et al., 1997), and paravians including avialans (Chiappe and Walker, 2002;Makovicky et al., 2005;Agnolín and Martinelli, 2008;Novas et al., 2008b;Agnolín et al., 2017). To date, the southernmost non-Antarctic record of non-avian theropods comes from the Santa Cruz Province in Argentinean Patagonia, which includes the abelisauroid Austrocheirus isasii and the megaraptorid Orkoraptor burkei (Novas et al., 2008a) both from the Cerro Fortaleza Formation (Campanian-Maastrichtian), and the recently described megaraptorid Maip macrothorax (Aranciaga from the Chorrillo Formation (Campanian-Maastrichtian). ...
... Comparisons -Given the damaged condition of the coracoid, our comparison focuses on the proximal scapula. The presence of a prominent acromion (prior to damage) and concave humeral articular facet indicate that CPAP 4152 belongs to an enantiornithine bird (Walker, 1981;Chiappe and Walker, 2002). The scapula appears unique from other Late Cretaceous South American enantiornithines in the presence of the prominent ridge connecting the acromion and humeral facet, as well as the tapering acromion. ...
... The scapula appears unique from other Late Cretaceous South American enantiornithines in the presence of the prominent ridge connecting the acromion and humeral facet, as well as the tapering acromion. In Enantiornis leali, the humeral articular facet and acromion are not connected in this way, and the acromion branches away from the scapular shaft at more of an angle than what is observed in CPAP 4152 (Walker, 1981;Chiappe and Walker, 2002). The scapula of Martinavis sp. is similar to that of Enantiornis but has a well excavated area between the acromion and humeral articular facet and a broader acromion, which is absent in CPAP 4152 (Walker and Dyke, 2010, Fig. 31). ...
... Nevertheless, these birds have a relatively scarce record in the Upper Cretaceous (Wang et al., 2014;Xu et al., 2021). In South America, however, the opposite is the case, that is, the Lower Cretaceous Enantiornithes are poorly represented whereas their record comes mostly from Late Cretaceous beds (Chiappe and Walker, 2002;Walker and Dyke, 2009;Novas et al., 2010;Candeiro et al., 2012). ...
... Yatenavis can be referred to Enantiornithes on the basis of a transversely expanded and craniocaudally compressed distal end of the humerus (width being three times maximum craniocaudal depth), weakly developed distal condyles, a ventral condyle almost transverse to the humeral shaft, and a very wide olecranal fossa (Sereno, 2000;Chiappe and Walker, 2002;Lawver et al., 2011). Yatenavis shares with select Late Cretaceous enantiornithines (e.g., Alexornis, Kizylkumavis, Martinavis, and the indeterminate enantiornithine from Chubut Province, Argentina e MPEF-PV 2359-, and Madagascar; Brodkorb, 1976;Nessov, 1984;Walker et al., 2007;Walker and Dyke, 2009;O'Connor and Forster, 2010;Lawver et al., 2011) the following features: 1) presence of a small but welldefined dorsal supracondylar process, and 2) presence of a caudal ridge on the ventral condyle distally (Lawver et al., 2011;Fig. ...
... Yatenavis shares morphological features with Late Cretaceous enantiornithines found at diverse localities, including Madagascar, North America, Patagonia, and Central Asia. As previously stated by other authors (Lawver et al., 2011), the morphological similarities between taxa coming from such distant localities may indicate the presence of a Late Cretaceous enantiornithine subclade widely distributed across Gondwana and Laurasia (Chiappe and Walker, 2002;Walker et al., 2007;Dyke and } Osi, 2010), which is in agreement with recently published paleobiogeographical models (Ezcurra and Agnolín, 2012). ...
Article
Enantiornithes were the dominant avialan clade in the Mesozoic. However, their record for the Upper Cretaceous is scarce. In this study, we present and describe Yatenavis ieujensis gen. et sp. nov., one of the youngest occurrence of an enantiornithine bird. The specimen, the distal half of a right humerus, was found in Chorrillo Formation, southern Santa Cruz Province of Argentina, making it also the australmost enantiornithine specimen recorded to date. Yatenavis is unique among enantiornithines for its combination of characters, including a crest on the medial side of the shaft which bears a muscular scar cranially; the presence of a dorsal supracondylar process proximal to the dorsal epicondyle; equally distally projected dorsal and ventral condyles; and a rod-like caudal extension of the ventral condyle bearing a distal sulcus scapulotricipitalis. Several of these features are shared with an unnamed enantiornithine recovered from Upper Cretaceous beds in another fossil site in Patagonia.
... The pectoral girdle, a skeletal structure that connects the forelimb to the trunk, is a key component of the flight apparatus, and its function and evolutionary history have been extensively studied (Baier et al., 2007;Bock, 2013;Novas et al., 2020;Senter, 2006;Burch, 2014;Jasinoski et al., 2006;Ostrom, 1976). The morphology of the pectoral girdle in Late Cretaceous enantiornithine birds is well known from several three-dimensionally preserved specimens (Atterholt et al., 2018;Chiappe and Walker, 2002;Chiappe et al., 2007). However, most Early Cretaceous bird fossils are essentially two-dimensionally preserved as slab specimens, and accordingly do not offer a full anatomical picture of the flight apparatus, a limitation that greatly hinders studies of early flight. ...
... The acrocoracoid process is short and blunt, and extends slightly above the midpoint of the coracoidal glenoid fossa as in Jeholornis and Confuciusornis (Turner et al., 2012Wang and Zhou, 2018bWang and Zhou, 2018bWang et al., 2020a;Zhou and Zhang, 2003b;Zhou and Zhang, 2003b). In enantiornithines (Panteleev, 2018;Chiappe and Walker, 2002), the acrocoracoid process extends slightly above the dorsal margin of the coracoidal glenoid fossa. In most euornithines (e.g., Figure 4D-F), this process is proportionally longer and extends much further beyond the glenoid than in non-euornithine birds. ...
... The pectoral girdle is closely comparable in morphology to those of other enantiornithines (Hu et al., 2015a;Zhang et al., 2014;Chiappe and Walker, 2002). The long and robust acromion of the scapula is separated by a neck from the coracoidal articular surface. ...
Article
Full-text available
The morphology of the pectoral girdle, the skeletal structure connecting the wing to the body, is a key determinant of flight capability, but in some respects is poorly known among stem birds. Here, the pectoral girdles of the Early Cretaceous birds Sapeornis and Piscivorenantiornis are reconstructed for the first time based on computed tomography and three-dimensional visualization, revealing key morphological details that are important for our understanding of early flight evolution. Sapeornis exhibits a double articulation system (widely present in non-enantiornithine pennaraptoran theropods including crown birds) which involves, alongside the main scapula-coracoid joint, a small subsidiary joint, though variation exists with respect to the shape and size of the main and subsidiary articular contacts in non-enantiornithine pennaraptorans. This double articulation system contrasts with Piscivorenantiornis in which a spatially restricted scapula-coracoid joint formed by a single set of opposing articular surfaces, a feature also present in other members of Enantiornithines, a major clade of stem birds known only from the Cretaceous. The unique single articulation system may reflect correspondingly unique flight behavior in enantiornithine birds, but this hypothesis requires further investigation from a functional perspective. Our renderings indicate that both Sapeornis and Piscivorenantiornis had a partially closed triosseal canal (a passage for muscle tendon that plays a key role in raising the wing), and our study suggests that this type of triosseal canal occurred in all known non-euornithine birds except Archaeopteryx , representing a transitional stage in flight apparatus evolution before the appearance of a fully closed bony triosseal canal as in modern birds. Our study reveals additional lineage-specific variations in pectoral girdle anatomy, as well as significant modification of the pectoral girdle along the line to crown birds. These modifications produced diverse pectoral girdle morphologies among Mesozoic birds, which allowed a commensurate range of capability levels and styles to emerge during the early evolution of flight.
... The caudal portion of the meckel's groove is covered by the splenial, which is pierced ventrocaudally by a foramen, presumably homologous to the mylohyal foramen of non-avian theropods (Brochu, 2003) (Fig. 3). The caudal margin of the dentary slants caudoventrally, forming a diagonal contact with the postdentary bones (mostly the surangular) as is typical of other enantiornithines (Chiappe and Walker, 2002) including bohaiornithids (e.g., Bohaiornis, Parabohaiornis, Zhouornis). The length of the surangular is approximately equal to that of the dentary. ...
... There are 10 cervicals including the atlas and axis, a number comparable to that of Bohaiornis and Zhouornis (9e10). Nine to eleven cervical vertebrae are typically preserved in other enantiornithines (e.g., Sinornis, Longipteryx, Eoenantiornis, Pengornis) (Sereno and Rao 1992;Chiappe and Walker, 2002;Zhou et al., 2005Zhou et al., , 2008. The atlas is ring-like in which the atlantal arch has collapsed; the fact that the two halves of the arch are not fused in the midline indicates that the hemi-arches of this vertebra remained individualized at the time of death. ...
... There are 10 thoracic vertebrae based on the number of elements articulated to a long rib. The first three vertebrae are separated by a small gap from the remaining seven (Figs. 6, 7); this raises the possibility that the series may be composed of 11 vertebrae instead of 10, as is common in other enantiornithines (Chiappe and Walker, 2002). The laterally exposed first three thoracic vertebrae show broad spinal processes that fan out dorsally (Fig. 6). ...
Article
Despite the abundant number of enantiornithine fossils from the Jehol Biota, the cranial anatomy of these birds remains only superficially known. Similarly, data on dental replacement within this clade, and among toothed birds in general, is largely lacking. Here we describe a new and exquisitely preserved specimen of a bohaiornithid enantiornithine from the Lower Cretaceous Jiufotang Formation in Liaoning Province, northeastern China. The new specimen provides novel information on the cranial anatomy of these birds and unprecedented data on their tooth resorption, implantation, and replacement pattern, mostly visualized through computed laminography (CL) images. The new information demonstrates the presence of alternating tooth replacement with symmetrical signaling control, a pattern that is possibly shared by other enantiornithines. We also test the monophyly of Bohaiornithidae, one of the most species-rich groups of Jehol enantiornithines, through the addition of the new specimen. While our phylogenetic results support a monophyletic clade composed of several taxa traditionally included in this group (i.e., Bohaiornis, Sulcavis, Zhouornis, Longusunguis, and Parabohaiornis), it excludes Shenqiornis mengi, a species used to phylogenetically defined Bohaiornithidae. The fact that Shenqiornis is not resolved among the above-mentioned clade raises concerns about the usage of Bohaiornthidae as originally defined (i.e., most recent common ancestor of Shenqiornis mengi and Bohaiornis guoi, and all its descendants).
... If the estimated vertebral count for the pygostyle is accurate and PMoL-AB00178 has seven free caudal vertebrae as in C. sanctus 3 , then a total of 18 caudal vertebrae are present in this confuciusornithid (Figs. 1, 4c). Known early pygostylians, except sapeornithids, normally retain 5-8 free caudal vertebrae: there are seven in confuciusornithids 3 , 5-6 in jinguofortisids 30 , 6-8 in enantiornithines 20,39 , and 5-7 in early ornithuromorphs 25,28,40 , similar to the count of 5-6 in crown-group birds 41,42 . Sapeornithids have been reported to have about 12 free caudal vertebrae 43,44 , many more than other early pygostylians. ...
Article
Full-text available
The confuciusornithids are the earliest known beaked birds, and constitute the only species-rich clade of Early Cretaceous pygostylian birds that existed prior to the cladogenesis of Ornithothoraces. Here, we report a new confuciusornithid species from the Lower Cretaceous of western Liaoning, northeastern China. Compared to other confuciusornithids, this new species and the recently reported Yangavis confucii both show evidence of stronger flight capability, although the wings of the two taxa differ from one another in many respects. Our aerodynamic analyses under phylogeny indicate that varying modes of flight adaptation emerged across the diversity of confuciusornithids, and to a lesser degree over the course of their ontogeny, and specifically suggest that both a trend towards improved flight capability and a change in flight strategy occurred in confuciusornithid evolution. The new confuciusornithid differs most saliently from other Mesozoic birds in having an extra cushion-like bone in the first digit of the wing, a highly unusual feature that may have helped to meet the functional demands of flight at a stage when skeletal growth was still incomplete. The new find strikingly exemplifies the morphological, developmental and functional diversity of the first beaked birds.
... An exception are the super-precocial Megapodidae, the so-called mound-builders, which can fly after emerging from the mound and receive no posthatching parental care (Jones and Göth, 2008). The most abundant and successful clade of Mesozoic birds is the Enantiornithes, a group of predominantly arboreal birds that dominated terrestrial avifaunas throughout the Cretaceous (Chiappe and Walker, 2002). All available information suggests these birds were developmentally highly precocial, hatching fully independent and flight capable (Xing et al., 2017;Zhou and Zhang, 2004). ...
Conference Paper
In 2019, a plaster (number PN 19-1) containing some indications of the presence of a hybodont shark was removed from Phu Noi Site, located in Kalasin Province and belonging to the Phu Kradung Formation. Because of the large size of the plaster (1.65 X 0.95 m), it has not yet been possible to CT-scan it, but careful manual preparation allowed showing that the plaster displays part of the head of the animal, including right and left meckel’s cartilage, part of the right palatoquadrate, two labial cartilages and perhaps the anterior part of the neurocranium, one scapulocoracoid with a part of a propterygium, one dorsal fin spine possibly associated with some neural arches as well as some patches of dermal denticles and scattered teeth. The specimen appears to have been somewhat disarticulated but all the elements belong to a single specimen. The dermal denticles, with a triangular crown are quite unusual and were so far only recovered from the Khlong Min Formation and Phu Noi site(Cuny et al., 2009; Cuny et al., 2014). They are here associated with teeth that were described from the Khlong Min Formation as Hybodus sp (Cuny et al., 2009). The dorsal fin spine also displays a rather unusual ornamentation made mostly of well-developed costae, but with the presence of some star-ridged tubercles at the base of the crown, reminding those of Strophodus or Asteracanthus (Stumpf et al., 2021). The association of these dermal denticles, teeth and dorsal fin spine demonstrate that PN 19-1 belongs to a new species and probably to a new genus different from Hybodus based on the dorsal fin spine and dermal denticles. This new species is present both in the Khlong Min Formation in the Southern part of Thailand and the Phu Kradung Formation in Isan, which strongly support the hypothesis that most of the Phu Kradung Formation is Jurassic in age.
... Non-neornithine avialans thrived throughout the Cretaceous and remained diverse through the Maastrichtian, before suddenly disappearing at the K-Pg boundary [73]. Until this point, Enantiornithes were the dominant Mesozoic avialan clade with more than 60 known species and a worldwide distribution [471][472][473]. Why did they become extinct, while neornithines survived? ...
Article
Full-text available
The extant diversity of the avian clade Palaeognathae is composed of the iconic flightless ratites (ostriches, rheas, kiwi, emus, and cassowaries), and the volant tinamous of Central and South America. Palaeognaths were once considered a classic illustration of diversification driven by Gondwanan vicariance, but this paradigm has been rejected in light of molecular phylogenetic and divergence time results from the last two decades that indicate that palaeognaths underwent multiple relatively recent transitions to flightlessness and large body size, reinvigorating research into their evolutionary origins and historical biogeography. This revised perspective on palaeognath macroevolution has highlighted lingering gaps in our understanding of how, when, and where extant palaeognath diversity arose. Towards resolving those questions, we aim to comprehensively review the known fossil record of palaeognath skeletal remains, and to summarize the current state of knowledge of their evolutionary history. Total clade palaeognaths appear to be one of a small handful of crown bird lineages that crossed the Cretaceous-Paleogene (K-Pg) boundary, but gaps in their Paleogene fossil record and a lack of Cretaceous fossils preclude a detailed understanding of their multiple transitions to flightlessness and large body size, and recognizable members of extant subclades generally do not appear until the Neogene. Despite these knowledge gaps, we combine what is known from the fossil record of palaeognaths with plausible divergence time estimates, suggesting a relatively rapid pace of diversification and phenotypic evolution in the early Cenozoic. In line with some recent authors, we surmise that the most recent common ancestor of palaeognaths was likely a relatively small-bodied, ground-feeding bird, features that may have facilitated total-clade palaeognath survivorship through the K-Pg mass extinction, and which may bear on the ecological habits of the ancestral crown bird.
... 5PeQ). This tooth might be a juvenile representative of Maniraptora indet. 2 or perhaps belongs to a different group of theropods, since unserrated teeth occur in a variety of theropod clades, including unenlagiine dromaeosaurs (Buitreraptor Makovicky et al., 2005), ornithomimosaurs (Pelecanimimus P erez-Moreno et al., 1994), and enantiornithine birds (Chiappe and Walker, 2002). ...
Article
Full-text available
The TuronianeConiacian continental fossil record in Europe is scarce. Here we present a new fossil assemblage of early Coniacian age that was systematically collected from the coal-bearing Gosau Group of the Tiefengraben locality near St. Wolfgang, Austria. The diverse assemblage is composed of at least 60 taxa including sporomorphs and Normapolles-related pollen, seeds and leaves of angiosperms and gymnosperms, charophytes, gastropods, bivalves, ostracods, termites, fishes, crocodiles and dinosaurs. Concerning charophytes, ostracods, gastropods, crocodiles and dinosaurs, the discovered specimens either extend the temporal and spatial range of specific groups (in some cases as possible relict forms) or suggest the occurrence of new taxa. The discovered remains of algae, molluscs, ostracods, calcareous nannofossils and lepisosteid fish represent a mixed faunal assemblage from different palaeohabitats, from marginal marine to low salinity and freshwater or terrestrial environments. As Normapolles-related angiosperm plants dominate the flora with a relatively high number of dentate leaves, a slightly cooler microenvironment compared to other Turonian-Coniacian Central European localities is indicated. The characteristically grooved crocodylian teeth of Tiefengraben differ from the previously known Upper Cretaceous European crocodyliform teeth and suggest a more diverse crocodyliform fauna in the region. Dinosaurs are represented by teeth of at least three different theropods, the largest of which is referred here to as basal tetanurans. The fossil assemblage of this early Gosau Group occurrence is of great importance for our understanding of the continental floristic and faunistic composition of the western Tethyan archipelago during the CenomanianeCampanian gap.
Chapter
Our understanding of the early evolution of birds has advanced over the past 2 decades, thanks to an ever-improving fossil record. Extraordinary fossils have revealed new details about the evolution of the avian brain, respiratory system, digestive tract, and reproductive system. Many of the traits most strongly associated with birds first arose in nonavian theropod dinosaurs. Theropods evolved pennaceous feathers, incipient wings, and gliding flight long before modern birds appeared. Birds likewise inherited features such as an expanded forebrain, gizzard, dorsally immobile lung, pigmented eggs, and paternal brooding system from their theropod ancestors. Yet, the earliest birds also retained primitive traits such as teeth, clawed hands, long bony tails, partially buried nests, and slower growth. The evolution of birds was profoundly influence by the Cretaceous–Paleogene mass extinction, which wiped out the previously dominant Enantiornithines (“opposite birds”). This sets the stage for modern birds to radiate into the most diverse major clade of tetrapods.
Article
Full-text available
The occurrence of lateral openings and pleurocoels (lateral fossae) in the corpus of the thoracic vertebrae of extant and fossil neornithine birds is reviewed, with both features having been identified as osteological correlates of the avian pulmonary system. Openings mainly occur in larger species with a high overall bone pneumatization but do not seem to serve for the passage of lung or air sac diverticula. Pleurocoels, on the other hand, are not directly related to pneumatic features and constitute a plesiomorphic trait that was widespread in Mesozoic non-neornithine birds. It is noted that an inverse correlation exists between the occurrence of pleurocoels and the pneumatization of the humerus, with pleurocoels being mainly found in extant and fossil taxa, in which the humerus is not pneumatized by diverticula of the clavicular air sac. Here it is hypothesized that pleurocoels primarily serve to increase the structural resistance of the vertebral body and were reduced multiple times in neornithine birds. In some taxa, their reduction may be related to the development of the furcula, which assists ventilation of the clavicular and cervical air sacs and may thereby contribute to the pneumatization of both, the humerus and the thoracic vertebrae. If so, Mesozoic non-neornithine birds, which had a rigid furcula with massive shafts as well as non-pneumatic humeri and pronounced pleurocoels, are likely to have differed in functional aspects of their air sac system from extant birds.
Article
Full-text available
Classification of fossil eggshells of amniotic vertebrates. Acta Palaeont. Polonica, 36, 2, 193-238. Fossil avian and reptilian eggs and eggshells, from the Cretaceous of Mongolia and USSR (Kazakhstan, Zaisan basin) as well as samples of dinosaurian and the Eocene avian eggshells from USA, China, France and Argentina were studied. Methodological, terminological and biomineralization aspects of eggshell structure are discussed. Considered are different classifications of eggshell according to the structural levels of eggshell matter organization (texture. general histostructure, superficial morphology). Basic types, morphotypes, types of pore system and types of surface ornamentation are the main structural categories employed i n the systematic description of fossil material. About 18 groups of fossil eggshells referred to turtles, geckoes, crocodiles, and to 14 "families" or dinosaur and bird oological remains are described. Their com-position, occurence, paleobiology and systematics are shortly presented. K e y w o r d s: fossil and Recent eggs, eggshells, ~ e p t i l i a , Aves para tax on om^, classification, paleobiology.