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The transformation from a long reptilian tail to a shortened tail ending in a pygostyle and accompanied by aerodynamic fanning rectrices is one of the most remarkable adaptations of early avian evolution. However, no fossils directly capture this transition, and information regarding the structural morphology and the early evolution of the pygostyle in Mesozoic birds and their integuments is relatively limited. Here we provide a review of the pygostyle morphology of Early Cretaceous birds with comparison to the structure in living birds. This study emphasizes the convergent evolution of distally co-ossified caudal vertebrae in non-avian maniraptorans and early birds. There further exist distinct differences in pygostyle morphology between Sapeornithiformes, Confuciusornithiformes, Enantiornithes, and Ornithuromorpha. The morphology of the pygostyle and rectrices in early ornithuromorphs appear similar to that of extant birds, whereas the pygostyle in more primitive birds does not appear morphologically capable of supporting the rectricial bulbs and musculature necessary to control an aerodynamic fan-shaped tail. The rectricial bulbs and rectricial fan appear to have coevolved with the plough-shaped pygostyle early in the evolution of the Ornithuromorpha. This study also shows that the confuciusornithiform pygostyle was more similar to that of enantiornithines than previously recognized, consistent with the presence of nearly identical ornamental tail feathers in both groups.
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55卷 第3
201710
古 脊 椎 动 物 学 报
VERTEBRATA PALASIATICA
Morphological coevolution of the pygostyle and tail
feathers in Early Cretaceous birds
WANG Wei1,2 Jingmai K. OCONNOR1
(1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate
Paleontology and Paleoanthropology, Chinese Academy of Sciences Beijing 100044 wangwei2014@ivpp.ac.cn)
(2 University of Chinese Academy of Sciences Beijing 100049)
Abstract The transformation from a long reptilian tail to a shortened tail ending in a pygostyle
and accompanied by aerodynamic fanning rectrices is one of the most remarkable adaptations
of early avian evolution. However, no fossils directly capture this transition, and information
regarding the structural morphology and the early evolution of the pygostyle in Mesozoic
birds and their integuments is relatively limited. Here we provide a review of the pygostyle
morphology of Early Cretaceous birds with comparison to the structure in living birds. This
study emphasizes the convergent evolution of distally co-ossied caudal vertebrae in non-avian
maniraptorans and early birds. There further exist distinct differences in pygostyle morphology
between Sapeornithiformes, Confuciusornithiformes, Enantiornithes, and Ornithuromorpha.
The morphology of the pygostyle and rectrices in early ornithuromorphs appear similar to that
of extant birds, whereas the pygostyle in more primitive birds does not appear morphologically
capable of supporting the rectricial bulbs and musculature necessary to control an aerodynamic
fan-shaped tail. The rectricial bulbs and rectricial fan appear to have coevolved with the plough-
shaped pygostyle early in the evolution of the Ornithuromorpha. This study also shows that the
confuciusornithiform pygostyle was more similar to that of enantiornithines than previously
recognized, consistent with the presence of nearly identical ornamental tail feathers in both
groups.
Key words Jehol Biota, Aves, rectricial bulb, rectrices
Citation Wang W and J K. O’Connor, 2017. Morphological coevolution of the pygostyle and tail
feathers in Early Cretaceous birds. Vertebrata PalAsiatica, 55(3):
1 Introduction
Avian origins have been studied for over a century but the hypothesis that “birds are
maniraptoran theropod dinosaurs” only became universally accepted in the past two decades
following the discovery of abundant feathered maniraptorans in northeastern China (both non-
avian and avian) (Xu et al., 2014). To be concise, we will hereinafter use “theropod” to refer to
国家自然科学基金项目(批准号:91514302)及中国科学院战略性先导科技专项(编号:XDB18030501)资助
收稿日期:2016-04-15
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non-avian theropod dinosaurs and “maniraptoran” for non-avian maniraptoran theropods. To
date, hundreds of specimens of derived maniraptorans and birds have been collected from the
Late Jurassic Tiaojishan Formation and the Early Cretaceous Jehol Group (Zhou, 2004, 2014;
Wang et al., 2015). The Jehol Group, consisting of the 125 Ma Yixian Formation and the 120
Ma Jiufotang Formation, has also produced thousands of early bird specimens representing
half the known Mesozoic taxonomic diversity. Together with the Protopteryx-horizon of the
Huajiying Formation, the fossils from the Jehol Group form the spectacularly diverse Jehol
Biota (Pan et al., 2013). Together, these faunas preserve a wealth of paleobiological evidence
(such as stomach contents, soft tissue structures, and behavior) that greatly elucidate the
dinosaur-bird transition and the early evolution of birds (Xu et al., 2014).
Despite this wealth of new data, there still exist significant gaps in our understanding
of the structural transformation of birds from the theropod condition. One such evolutionary
transformation is the reduction of the long boney tail (present in theropods and the basalmost
birds Archaeopteryx and Jeholornis) into an abbreviated tail ending in a pygostyle, a
compound element formed through fusion of the distalmost caudal vertebrae. All birds with
a pygostyle form a monophyletic clade, the Pygostylia (Chiappe, 2002), which excludes only
the long bony-tailed birds, Archaeopteryx and the Jeholornithiformes (Jeholornis and kin).
The earliest record of the pygostyle is in the 130.7 Ma Protopteryx-horizon of the Huajiying
Formation, the earliest stage of the Jehol Biota, with the rst appearance of three pygostylian
lineages, the Confuciusornithiformes, Enantiornithes, and Ornithuromorpha (Zhang and Zhou,
2000; Zhang et al., 2008a; Wang et al., 2015). Unfortunately, no intermediate morphotypes
are known. The only specimen that may potentially elucidate this important transition is the
holotype of Zhongornis haoae from the Jiufotang Formation (Gao et al., 2008). Although
initially described as having an intermediate tail morphology, the only known specimen of
this taxon is a juvenile and poor preservation obscures interpretations. Descriptions of the tail
vary from 14 to 20 vertebrae (Gao et al., 2008; O’Connor and Sullivan, 2014) and thus the
transmutation of the tail during early avian evolution remains poorly understood. Historically,
the pygostyle has been considered a uniquely avian feature (Haeckel, 1883). However, more
recently pygostyle-like structures have been discovered in three clades of maniraptorans:
Therizinosauroidea, Oviraptorosauria, and Scansoriopterygidae (Xu et al., 2003; Barsbold et
al., 2000a, b; Zhang et al, 2008b).
In Neornithes (the clade that includes all living birds) the tail complex consists of two
parts: the tail feathers and the uropygium, the latter consisting of the caudal vertebrae and
associated soft tissue (Fisher, 1959; Baumel, 1988; Baumel et al., 1990; Gatesy and Dial, 1993,
1996a, b; Balmford et al., 1993; Thomas, 1997). The aerodynamic tail feathers, called rectrices,
are stiff and asymmetrical and overlapped dorsally and ventrally by rows of short tail coverts
(called tectrices or deck feathers). There are usually 5–6 free caudal vertebrae with expanded
transverse processes followed by a small mediolaterally compressed pygostyle (Baumel and
Witmer, 1993). The blade-like dorsal portion, the pygostyle lamina (lamina pygostyli), is
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inferred to form through the fusion of the neural spines and arches. The cone shaped pygostyle
base (basis pygostyli) is formed through the fusion of the vertebral bodies (centra) (Baumel et
al., 1990; Baumel and Witmer, 1993). The pygostyle base has ventral processes that are weakly
excavated for muscle attachment. Paired broadipose tissue structures called rectricial bulbs lie
on both sides of the pygostyle and predominantly attach to the pygostyle lamina; the calami of
the rectrices are imbedded in this tissue, except for the medial pair which attach directly onto
the dorsodistal end of the pygostyle (Baumel, 1988). Spiraling around the surface of each bulb
is a striated muscle, the rectricial bulb muscle (m. bulbi rectricium), primarily responsible for
controlling the spread of the tail feathers (Baumel, 1988; Baumel and Witmer, 1993; Gatesy
and Dial, 1996a, b). In some birds (e.g., in Piciformes and Coliiformes) the ventral processes
are laterally expanded forming the ventrally concave pygostyle disc (discus pygostyli) that
provides an expanded area for the attachment of the well-developed caudal depressor muscle (m.
depressor caudae) present in these species; the disc is inferred to be derived from rudimentary
transverse processes (Baumel, 1988; Baumel and Witmer, 1993).
Except for the ightless paleognathus birds (the “ratites”), all modern birds share nearly
the same tail complex formed by pygostyle, rectricial bulbs and rectrices. Although in many
taxa the rectrices are modified for display, most birds have an aerodynamic fan consisting
of 10–12 feathers (Thomas and Balmford, 1995). This sophisticated tail complex plays a
significant role in avian flight. The rectricial fan produces lift to supplement the wings, the
force of which can be adjusted by controlling the surface area of the rectricial fan by adjusting
the spread of the feathers (Thomas and Balmford, 1995; Thomas 1997). This supplemental
lift is particularly important during takeoff, landing (for decelerating and braking) and slow
ight (Pennycuick, 1968; Spedding et al., 1984; Tucker, 1992; Gatesy and Dial, 1993). Tail
fanning is also used when turning, acting as a paddle or rudder (Baumel et al., 1990; Thomas,
1993, 1997), and the presence of a tail helps to maintain stability and balance, and streamline
the body (Thomas, 1993). Because the tail complex functions as an integrated whole, the
uropygium and integument are morphologically correlated and one can be used to predict the
other (Felice and O’Connor, 2014). Furthermore, pygostyle morphology can be used to predict
ight or foraging style in both extinct and extant birds (Felice, 2014). These results support the
hypothesis that pygostyle and rectricial morphology co-evolve (Clark et al., 2006).
Despite the large number of specimens uncovered from the Jehol Biota over the past
several decades (Zhou, 2004, 2014; Xu et al., 2014), the early evolution of the avian tail
complex is poorly understood. Several short discussions have been published regarding
specic taxa: Jeholornis (O’Connor et al., 2012, 2013), Eopengornis (Wang X et al., 2014),
Shanweiniao (O’Connor et al., 2009), Yixianornis (Clarke et al., 2006) and Iteravis (Zhou et
al., 2014). However, the only research focused on the morphological changes that occur in the
derived maniraptoran tail during early avian evolution predates most important discoveries
(Gatesy and Dial, 1993, 1996a, b). The pygostyle and tail feathers, when preserved, are
typically only briefly described (Chiappe et al., 1999; Clarke et al., 2006; O’Connor et al.,
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2009; Hu et al., 2015), and very little information is available regarding the soft tissue (rectricial
bulbs and muscle), although potential traces are reportedly preserved in the ornithuromorph
Iteravis (Zhou et al., 2014) and the enantiornithine Feitianius (O’Connor et al., 2015).
Extensive interspecific comparison is lacking and the relationship between skeletal features
and integument has only been explored in the pengornithid enantiornithines (Hu et al., 2015;
O’Connor et al., 2016). Here we describe in detail the morphological characteristics of the
pygostyle and tail feathers in Mesozoic birds and closely related theropods. We discuss
morphological differences between clades, potential function, and place this information in the
context of recent phylogenetic analyses in order to test hypotheses regarding the origin of the
extant avian tail fanning complex.
Institutional abbreviations BMNH, Beijing Natural History Museum, Beijing, China;
CAGS, Chinese Academy of Geological Sciences, Beijing, China; CNU, Capital Normal
University, Beijing, China; DHNM, Dalian Natural History Museum, Dalian, China; IVPP,
Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences,
Beijing, China; LPM, Liaoning Paleontology Museum, Shenyang, China; STM, Tianyu
Natural History Museum of Shandong, Pingyi, China.
2 Materials and methods
This study mainly utilized the collections of the IVPP, supplemented by data from the
collections of the STM. Only taxonomically identifiable specimens with pygostyle or tail
integument preserved were included in this study. Because the morphology of the caudal
skeleton is affected by post-hatching development (Heers et al., 2014; Zhou et al., 2013),
juvenile specimens (such as the holotype of Zhongornis haoae) were excluded from this study.
Anatomical nomenclature primarily follows the “Handbook of Avian Anatomy” using English
equivalents for the Latin terminology (Baumel and Witmer, 1993).
Preserved in nely laminated lacustrine sediments, almost all specimens from the Jehol
Biota are compressed, preserved nearly two-dimensionally in slabs. This preservation limits
the information available from individual elements to a single exposed surface (e.g., dorsal,
lateral, ventral or dorsolateral etc.) preventing the use of geometric morphometric analysis
methods (Zelditch et al., 2012). Thus, three simple but reliable measurements were chosen: the
total length of the pygostyle (TLP), the mid-length width of the pygostyle (MLW, for dorsally/
ventrally preserved specimens), and the mid-length height of the pygostyle (MLH, for laterally
preserved specimens). In order to reduce the effects of body size, we normalized the TLP using
the total length of the humerus (TLH) and the total length of femurs (TLF), two most reliable
and widely used proxies for body size and mass in most birds (Liu et al., 2012; Hone, 2012).
Most of the data (27 specimens) were collected directly from specimens measured using
stainless hardened digital calipers; measurements from four specimens were taken from the
literature (Sapeornis CAGS-03-07-08 (Yuan, 2005), Zhouornis (Zhang et al., 2013), Sulcavis
(O’Connor et al., 2013), Shanweiniao (O’Connor et al., 2009)).
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3 Description and analysis
3.1 Description of pygostyles and tail feathers
Non-avian theropoda Non-maniraptoran theropods have typical reptilian tails usually
consisting of dozens of free caudal vertebrae, e.g., about 35 in Tyrannosaurus (Persons and
Currie, 2011a) and 42 in Carnotaurus (Persons and Currie, 2011b). The proximal vertebrae
have short centra with neural arches and transverse processes present. These processes are
completely reduced in the distal vertebrae, which have elongate centra with the length more
than four times the centra height/width in some taxa (e.g., Sinosauropteryx, Ornitholestes)
(Pittman et al., 2013). The fusion of distal caudal vertebrae is unknown in non-maniraptoran
theropods, and rare in non-avian maniraptoran theropods. The most primitive (Fig. 1) known
pygostyle-like structure is present in the basal therizinosaur Beipiaosaurus (IVPP V 11559) (Xu
et al., 2003). The complete tail of Beipiaosaurus (V 11559) consists of 30 vertebrae. Preserved
Fig. 1 Phylogeny of Maniraptora and
related Theropoda dinosaurs, modied
from Brusatte et al., 2014
in lateral view, the last five caudals are well co-ossified
into a rod-like element with a slightly convex dorsal
margin formed by the prezygopophyses. The distal sixth
and seventh caudal centra are completely fused to each
other but not to the other five fused vertebrae. The tail
plumage consists of lamentous (without a fused rachis)
“proto-feathers” with no proximodistal differentiation (Xu
et al., 2003). No composite laments are observed on the
tail of Beipiaosaurus, which falls outside Pennaraptora
(the derived clade of maniraptorans in which pennaceous
feathers are present) (Brusatte et al., 2014).
Some members of the Oviraptorosauria (basal most
clade in the Pennaraptora) also possess fused terminal
caudals. Some members of this bird-like clade have at one
time been considered secondarily ightless birds although
most similarities are derived cranial features and probably
related to the presence of herbivory (Dyke and Norell, 2005). In one specimen of the primitive
taxon Similicaudipteryx (IVPP V 12056) the distal most caudal vertebrae (at least two) are co-
ossied into a dagger-like mass (He et al., 2008). This morphology is also observed in derived
oviraptorosaurs, such as Nomingia, Citipati and Conchoraptor (Barsbold et al., 2000a, b;
Persons et al., 2013). In the very primitive Caudipteryx the distal ve caudal vertebrae articulate
tightly through the presence of well-developed prezygopophyses and the last three of these are
ankylosed but remain unfused (IVPP V 22606). These vertebrae lack neural spines and transverse
processes, which are present in the proximal caudal vertebrae. Tail feathers are preserved in at
least three primitive oviraptorosaurs: Protarchaeopteryx, Caudipteryx and Similicaudipteryx. The
holotype of Caudipteryx has an estimated eleven pairs of tail feathers attached to the caudalmost
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ve or six vertebrae, which form the incipient pygostyle. Although the tail feathers in all these
three taxa are pennaceous (vaned), no specimen preserves evidence of hooklets on the barbules (Ji
et al., 1998). In Caudipteryx V 22606 two layers of tail feathers are evident: one shorter layer of
rachis-less body feathers and another longer layer of pennaceous rectrices (Fig. 2).
One member of the Scansoriopterygidae, Epidexipteryx, has a reduced tail consisting
of 16 vertebrae ending in a rod-like structure approximately formed by the distal ten caudal
vertebrae which are unfused. The distal ten vertebrae are reduced to simple centra without
processes. Two pairs of elongate feathers project from the pygostyle in Epidexipteryx;
described as “ribbon-like” (Zhang et al., 2008b), these tail feathers resemble the poorly
preserved “rachis-dominated” feathers present in some enantiornithines. Pennaceous tail
feathers are otherwise unknown in the Scansoriopterygidae.
Basalmost birds With the exceptions of Archaeopteryx and Jeholornis, all Mesozoic
birds have a pygostyle. Archaeopteryx is the basalmost bird with a tail composed of 21–22
articulated caudal vertebrae (Elzanowski, 2002; Foth et al., 2014) suggesting that the
Fig. 2 Photographs and camera lucida
drawings of distal caudal vertebrae of
Caudipteryx sp. (IVPP V 22606)
Abbreviations: ch. chevron人字骨;
cv. caudal vertebra尾椎; . laments
丝状羽毛; pi. pinnules羽枝;
poz. postzygopophysis后关节突;
prz. prezygopophysis前关节突
Scale bar=5mm
plesiomorphic avian tail was composed of no fewer than
20 caudal vertebrae. Pennaceous tail feathers, presumably
attached through ligamental connections, line the lateral
surfaces of the tail, overlapping to form a frond-like
morphology (Gatesy and Dial, 1996b; Mayr et al., 2007).
The 11th specimen of Archaeopteryx preserves a notch
in the caudal margin of the tail frond. The proximal
rectrices are asymmetric and shorter than the symmetrical
overlapping distal rectrices (Feduccia and Tordoff, 1979;
Foth et al., 2014). The slightly more derived Jeholornis
has 27 free caudal vertebrae (IVPP V 13350) forming
a tail longer than that of Archaeopteryx (Zhou and
Zhang, 2002a, 2003a). Jeholornis possesses a unique tail
plumage with two functionally discriminate rectricial
pterylae (O’Connor et al., 2012, 2013). One consists of a
short fan-shaped array of rectrices presumably embedded
in the soft tissue dorsal to the short proximal caudal
vertebrae, and the second is distally located, consisting
of laterally oriented cranially curved rectrices forming a
frond-like arrangement without an extensive aerodynamic
surface. Vane symmetry is equivocal due to preservation.
Although the lift generated by the tails in these two taxa
are insignicant when compared with that of the extant pigeon, the tail complex in Jeholornis is
relatively more efcient than that of Archaeopteryx, producing similar lift while at the same time
incurring much less estimated drag (O’Connor et al., 2013).
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The Sapeornithiformes and the Confuciusornithiformes are the basalmost known avian
lineages with a pygostyle (Zhou and Zhang, 2002b; Zhang et al., 2008a). Despite published
diversity, the Sapeornithiformes is considered a monospecific clade with all taxa referable to
Sapeornis chaoyangensis (Pu et al., 2013; Gao et al., 2012). The pygostyle is best preserved in
two specimens, IVPP V 13275 and V 13276, both preserving this element in left dorsolateral
view. In the cranial three vertebrae of the pygostyle, neural spines and transverse processes can
be faintly distinguished; the subsequent vertebrae are more completely fused and individual
processes cannot be identified. The dorsal surface is interpreted as forming a weak pygostyle
lamina (Fig. 3). On each lateral surface of the lamina there is a laterally projecting ridge inferred
to form through fusion of the transverse processes. More than ten additional specimens of
sapeornithiforms preserving the pygostyle were examined at the STM. In roughly a third of these
specimens fusion of the pygostyle is incomplete, suggesting that the pygostyle fully co-ossies
late during ontogeny. Tail feathers are rarely preserved, present in only one published specimen
(STM 16-18); this specimen preserves multiple elongate rectrices preserved in lateral view,
interpreted as forming a long, strongly graded tail-fan (Zheng, et al., 2013; Wang X et al., 2014;
Xu et al., 2014). Two other specimens (STM 15-18,16-5) preserve this feature in dorsoventral
view revealing a tail consisting of 8-10 narrow, symmetrical, non-overlapping feathers.
Distinct from that of sapeornithiforms, the pygostyle in confuciusornithiforms is more
strongly co-ossified, proportionately longer and more robust (Martin et al., 1998; Chiappe
et al., 1999). This detailed study of the confuciusornithiform pygostyle reveals a more
Fig. 3 Photographs and camera lucida drawings of pygostyles of Sapeornithiformes
A. Sapeornis, IVPP V 13276, in dorsal view; B. Sapeornis, IVPP V 13275, in dorsal-lateral view
Abbreviations: dr. dorsal ridge背侧脊; llr. left lateral ridge左侧脊; ns. neural spine棘突;
rlr. right lateral ridge右侧脊; tp. transverse process横突. Scale bars=3 mm
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complex morphology than previously recognized (Martin et al., 1998; Chiappe et al., 1999).
The pygostyle has a pair of dorsolateral ridges (Fig. 4A) and another pair of proximoventral
processes (Fig. 4B). The proximoventral processes project cranially beyond the proximal
articular surface (Fig. 4A, B) and are proximally restricted whereas the dorsolateral processes
extend the entire length dorsally dening a deep concavity. The ventral surface is keeled; the
height of the keel is obscured by compression but it appears that it forms a majority of the
pygostyle corpus (Fig. 4B). The caudal margin is rounded and slightly laterally expanded
(Fig. 4C). In lateral view, foramina are present in some specimens (e.g., IVPP V 16066, STM
13-58). The number and position of these foramina are variable, and we suggest these may
represent a subadult feature, subject to ontogenetic change and lost with maturity. Numerous
confuciusornithiform specimens preserve a single pair of rectrices that are elongate, rachis-
dominated and only distally pennaceous (racket-plumes). Other specimens preserve only
short rachis-less body feathers surrounding the pygostyle (Fig. 4A, C). These racket-plumes
are interpreted as sexually dimorphic ornaments present only in males, evolved under sexual
selection (Chiappe et al., 2008).
Enantiornithes Enantiornithes is the most diverse group of Cretaceous birds. Although
their pygostyle and tail feathers do not vary to the extent observed in extant birds, accordingly
they show a greater morphological diversity than other known Cretaceous clades. The caudal
vertebrae are well fused; sutures and individual processes cannot be recognized in adults. The
three-dimensionally preserved pygostyle of Halimornis (Chiappe et al., 2002) clearly reveals
that a typical enantiornithine pygostyle consists of paired dorsolateral ridges demarcating a
deeply incised dorsal surface (in the case of Halimornis, incised forming a deep V-shaped
excavation); ventrally, the body is very narrow, with a pair of ventrolateral processes. The
dorsolateral ridges project cranially beyond the proximal articular surface forming the dorsal
fork described by Chiappe et al. (2002). Together with the smaller ventrolateral ridges, the
pygostyle appears X-shaped in proximal view (Fig. 5). The distal end of the pygostyle is
tapered, formed by a constriction in the mediolateral width of the dorsal surface and the fact
the ventrolateral processes do not extend the entire length of the pygostyle.
Although most if not all known enantiornithine taxa share this unique general
morphology, appreciable variation within this template is observed. The basal Pengornithidae
(Zhou et al., 2008; Wang X et al., 2014; Hu et al., 2015) possess all the characteristic
enantiornithine features but their pygostyle is proportionately shorter and broader and the distal
end is untapered: the ventrolateral processes are proximally restricted and the dorsal surface
maintains an even width (Pengornis IVPP V 15336; Fig. 5A). The dorsal surface is broadly
concave, rather than deeply incised as in Halimornis. The caudal margin of the pygostyle has
a medial cleft, unique to this family of enantiornithines. Compared to the typical condition,
the bohaiornithid pygostyle (Fig. 5B, C) is more slender and the ventrolateral processes
taper gently rather than end abruptly as in most enantiornithines (Wang M et al., 2014).
The Longipterygidae, the family that includes Longipteryx, Rapaxavis, and Shanweiniao,
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Fig. 4 Photographs and camera lucida drawings of pygostyles of Confuciusornithiformes
A. Confuciusornis, IVPP V 11640, in right doesolateral view; B. Jinzhouornis, IVPP V 12352, in ventral view;
C. Confuciusornis, IVPP V 13156, in dorsal view
Abbreviations: ldlr. left dorsolateral ridge左背侧脊; lpvp. left proximoventral process左腹侧前突;
rdlr. right dorsolateral ridge右背侧脊; rpvp. right proximoventral process右腹侧前突; vk. ventral keel腹侧脊
Scale bars=3 mm
is characterized by a pygostyle that is proportionately larger and more robust than other
enantiornithines (O’Connor et al., 2011). In Rapaxavis the distal end of the pygostyle caudal
to the ventrolateral processes forms a spade-like expansion that is distally tapered (Fig. 5D).
Other simpler morphologies have been reported (e.g., Vescornis) (Zhang et al., 2004) but in the
absence of additional evidence these are interpreted as due to poor or incomplete preservation.
In some enantiornithine specimens (e.g., Eoenantiornis IVPP V 11537, Longipteryx IVPP
V 12325) rectrices are distinctly absent, with only rachis-less body feathers present, similar
to some confuciusornithiform specimens. Many specimens preserve a single pair of elongate
rachis-dominated tail feathers (e.g., Protopteryx IVPP V 11665, Dapingfangornis LPM 00039,
Bohaiornis LPM B00167, Parapengornis IVPP V 18687) (Zhang and Zhou, 2000; Li et al.,
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2006; Hu et al., 2011; Hu et al., 2015). Paraprotopteryx (STM V 001) preserves two pairs of
feathers (Zheng et al., 2007). These feathers vary with regards to the extent of the pennaceous
vane, being fully pennaceous in Parapengornis and Eopengornis and distally restricted in
all other specimens in which preservation is clear (Dapingfangornis, GSGM-07-CM-001,
Fig. 5 Photographs and camera lucida drawings of pygostyles of enantiornithines
A. Pengornis, IVPP V 15336, in left dorsolateral view; B. Parabohaiornis, IVPP V 18690, in left laterodorsal
view; C. Sulcavis, BMNH-Ph 00805, in left lateral view; D. Rapaxavis, DNHM D 2522, in ventral view
Abbreviations: ldlr. left dorsolateral ridge左背侧脊; lpdp. left proximodorsal process; lpvp. left proximoventral
process左腹侧前突; lvlr. left ventrolateral ridge左腹侧脊; rdlr. right dorsolateral ridge右背侧脊;
rpdp. right proximodorsal process; rvlr. right ventrolateral ridge右腹侧脊. Scale bars=3 mm
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Paraprotopteryx STM V 001). The shape of the distally restricted pennaceous portion varies
between taxa being distinctly racket-shaped in Dapingfangornis and Paraprotopteryx whereas
the distal expansion is more gradual in other specimens (GSGM-07-CM-001) (O’Connor et al.,
2012).
Ornithuromorpha The Ornithuromorpha contains all living birds (Neornithes) and
extinct taxa more closely related to living birds than to the Enantiornithes (Chiappe, 2002;
O’Connor and Zhou, 2013; Jetz et al., 2012; Jarvis et al., 2014; Wang et al., 2015). Several
Early Cretaceous fossil ornithuromorphs preserve evidences of an advanced tail complex.
The pygostyle is most commonly preserved in lateral view (e.g., holotype of Yixianornis
grabaui IVPP V 12631 (Zhou and Zhang, 2001), holotype of Piscivoravis lii IVPP V 17078
(Zhou et al., 2013), holotype of Iteravis huchzermeyeri IVPP V 18958 (Zhou et al., 2014)
and Gansus yumenensis CAGS-IG-04-CM-002 (You et al., 2006)), revealing a ploughshare-
shape approximately like that observed in extant birds and proportionately smaller than in
other Early Cretaceous avian taxa (Fig. 6). The ratio of dorsoventral MLH compared to TLP
in Jehol ornithuromorphs is larger (varies from 0.37 in Yixianornis to 0.60 in Hongshanornis)
than in other Cretaceous clades (Confuciusornis 0.25, enantiornithine Longipteryx 0.17)
with the exception of Sapeornis (0.45). Because all available Early Cretaceous basal
ornithuromorph specimens preserve the pygostyle in lateral view, their width cannot be
successfully measured. We suggest the predominance of lateral preservation implies that
the pygostyle is mediolaterally thin, as in neornithines. Generally, the pygostyle in Early
Cretaceous ornithuromorphs consists of a pygostyle lamina and a narrow pygostyle base, with
the transverse processes forming uneven low ridges on the lateral surface (Fig. 6). In lateral
view the pygostyle is upturned (Fig. 6) so that it is angled relative to the distal free caudals;
this morphology is also observed in doves, falcons and many other living birds (Baumel et al.,
1988). Although somewhat similar to sapeornithiforms in overall shape, the ornithuromorph
pygostyle is mediolaterally thinner and proportionately shorter than in this taxon and
signicantly differs from the robust morphology in other Mesozoic avian lineages. Non-feather
dark traces preserved surrounding the pygostyle in Yixianornis V 12631 and Iteravis V 18958
have been interpreted as the carbonaceous remains of the rectricial bulbs (Clarke et al., 2006;
Zhou et al., 2014).
The basalmost ornithuromorph preserving tail feathers is the holotype of Schizooura lii
(IVPP V 16861), which preserves a forked tail, considered by aerodynamic models to be the
most efcient tail shape (Zhou et al., 2012, Thomas et al., 1993). All other ornithuromorph
specimens with caudal integument preserve the impression of an aerodynamic tail fan
formed by multiple (at least 6-10) rectrices. Fan-shaped arrays of rectrices are present in
Hongshanornithidae (Hongshanornis V 14533, DNHM D2945/6; Tianyuornis STM 7-53;
Archaeornithura STM 7-145), Songlingornithidae (Yixianornis: V 12631; Yanornis: STM 9-51)
and Piscivoravis (V 17078) (Chiappe et al., 2014; Zheng et al., 2014; Zhou et al., 2013; Wang
et al., 2015). The rachis and individual barbs are discernible in the rectrices of some exquisitely
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Fig. 6 Photographs and camera lucida drawings of pygostyles of ornithuromorphs
A. Yixianornis, IVPP V 12631, in left lateral view; B. Piscivoravis, IVPP V 17078, in left lateral view;
C. Iteravis, IVPP V 18958, in left lateral view
Abbreviations: pb. pygostyle base尾综骨基部; pl. pygostyle lamina尾综骨板. Scale bars=3 mm
preserved specimens (e.g., Yixianornis: V 12631, Piscivoravis: V 17078). At least eight
rectrices are preserved in the holotype specimen of Yixianornis grabaui (Clarke et al., 2006);
a minimum of six feathers were reported in the holotype specimen of Piscivoravis lii (Zhou
et al., 2013); and more than ten are present in a referred specimen of Hongshanornis DNHM
D2945 (Chiappe et al., 2014). The caudal margins of these tail fans are weakly graduated.
Although the exact number of rectrices cannot be determined in any specimen, more than two
rectrices are present in all known basal ornithuromorphs with the possible exception of the
poorly preserved tail in the holotype of Iteravis huchzermeyeri (Zhou et al., 2014).
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3.2 Data analysis
Pygostyle data was collected from 31 specimens belonging to 20 genera and representing
at least 8 families (Table 1). Almost all ornithuromorph pygostyles are preserved laterally and
do not provide data on the MLW. The width was measured from the holotype of Schizooura lii,
but lateral compression of the pygostyle in this specimen and specimens of Sapeornis (IVPP V
13275 and V 13276) prevents accurate measurements. Selected linear pygostyle measurements
were compared to determine if clades can be differentiated on the basis of pygostyle absolute
sizes and relative proportions.
The Mesozoic birds from Jehol can be divided into four groups: Sapeornithiformes,
Confuciusornithiformes, Enantiornithes and Ornithuromorpha, based on the phylogeny. We
choose TLH and TLF, which are commonly used, both as the appropriate proxies of avian
body size (Liu et al., 2012; Hone, 2012). The result (Table 1) indicates that TLP/TLH values
in sapeornithiforms (0.11–0.17) and ornithuromorphs (0.15–0.27) are distinctly lower than
those of confuciusornithiforms (0.45–0.54) and most of enantiornithines (0.34–0.66, exclude
those of Pengornithidae). The result can be visible in Fig. 7A: scatter points belonging
to sapeornithiforms and ornithuromorphs are more closed to TLH-axis, while those of
confuciusornithiforms and enantiornithines are near TLP-axis. In the enantiornithines, low
TLP/TLH values can also be found in the Pengornithidae (0.18–0.30). Consistent result also
can be conrmed when the proxy is changed into TLF (see TLP/TLF in Table. 1; Fig. 7B). The
two aspects of results both suggest proportionally shorter pygostyles relative to body size in
sapeornithiforms and ornithuromorphs, especially in the latter.
Clades cannot be easily distinguished by MLW/TLP values (Table 1; Fig. 7C). Because
the data of sapeornithiforms and ornithuromorphs are too questionable and limited (only one
datum of each) to be considered, and the ratio of MLW/TLP present in confuciusornithiforms
Fig. 7 Scatter diagram of metrical data (Table 1) of fossil pygostyles
A. TLP-TLH; B.TLP-TLF; C. TLP-MLW; D. TLP-MLH. In accord with the phylogenetic result, all the data
can also be separated into four parts by each parameter-couple in rectangular plane coordinate systems
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Table 1 Measurements of pygostyles, humeri and femurs of Mesozoic avian fossils (mm )
Genera Specimen TLP MLW MLH TLH TLF TLP/
TLH
TLP/
TLF
MLW/
TLP
MLH/
TLP
Sapeornithidae
(the only family of
Sapeornithiformes)
Sapeornis
IVPP V 13275
21.3 7.5 122.9 74.1 0.17 0.29 0.35
IVPP V 13276
13.7 6.1 120.4 72.1 0.11 0.19 0.45
CAGS-03-07-08
20.0 130.0 0.15
Confuciusornithidae
(the only family of
Confuciusornithi-
-formes)
Confuciusornis
IVPP V 13175
30.4 7.6 67.2 55.3 0.45 0.55 0.25
IVPP V 16066
27.5 7.2 54.0 44.5 0.51 0.62 0.26
IVPP V 10932A
25.7 4.6 48.7 36.3 0.53 0.71 0.18
IVPP V 13172
28.9 3.6 53.1 45.8 0.54 0.63 0.12
IVPP V 11640
32.6 8.4 64.5 0.51 0.26
IVPP V 13156
30.5 4.1 60.4 47.1 0.50 0.65 0.13
IVPP V 13171
30.7 6.3 62.3 55.5 0.49 0.55 0.21
Jinzhouornis
IVPP V 12352
14.8 5.9 53.3 30.5 0.47 0.49 0.24
Protopterygidae Protopteryx
IVPP V 11665
11.2 1.8 28.0 23.7 0.40 0.47 0.16
Pengornithidae Pengornis
IVPP V 15336
19.3 5.3 63.5 48.1 0.30 0.40 0.27
IVPP V 18632
11.7 3.2 45.8 34.6 0.26 0.34 0.27
Parapengornis
IVPP V 18687
9.4 5.0 51.7 39.8 0.18 0.24 0.53
Bohaiornithidae Longusunguis
IVPP V 17964
22.7 3.5 41.8 35.8 0.54 0.63 0.15
Parabohaiornis
IVPP V 18691
17.8 3.0 43.9 36.0 0.41 0.49 0.17
IVPP V 18690
21.9 2.8 47.3 37.5 0.46 0.58
Bohaiornis
IVPP V 17963
20.2 4.4 51.7 42.6 0.39 0.47 0.22
Zhouornis
CNUVB-0903
17.3 50.6 44.5 0.34 0.39
Sulcavis
BMNH-Ph 00805
19.6 46.5 41.3 0.42 0.47
Longipterygidae Longipteryx
IVPP V 12325
22.5 4.7 44.6 28.1 0.50 0.80 0.21
IVPP V 12552
26.3 4.4 40.1 0.66 0.17
Longirostravis
IVPP V 11309
13.4 2.6 2.2 23.1 19.5 0.58 0.69 0.19 0.16
Shanweiniao
DNHM 1878
12.4 22.4 17.6 0.55 0.70
Hongshanornithidae Hongshanornis
IVPP V 14533
5.2 3.1 25.6 24.9 0.20 0.21 0.60
Songlingornithidae Yixianornis
IVPP V 12631
11.1 4.1 49.4 41.0 0.22 0.27 0.37
Yanornis
IVPP V 13278
10.0 64.8 0.15
Piscivoravis
IVPP V 17078
14.8 6.0 80.6 56.0 0.18 0.26 0.41
(undetermined) Iteravis
IVPP V 18958
6.8 3.1 45.7 35.2 0.15 0.19 0.46
Schizooura
IVPP V 16861
15.1 5.0 56.8 43.6 0.27 0.35 0.33?
Citations: Sapeornis CAGS-03-07-08 (Yuan, 2005), Zhouornis (Zhang et al., 2013), Sulcavis (O’Connor
et al., 2013), Shanweiniao (O’Connor et al., 2009).
(0.12–0.24) and enantiornithines (0.15–0.53) are indistinguishable. The single datum point
representing the ornithuromorphs, taken from the holotype of Schizooura lii, is not reliable
due to deformation. However, we hypothesize that wide spread lateral preservation in this
clade precisely reflects their very low width values. The presence of robust ventrolateral
and/or dorsolateral processes may contribute to the width in confuciusornithiforms and
enantiornithines (Figs. 4, 5). Higher MLH/TLP values are interpreted from sapeornithiforms
(0.45) and ornithuromorphs (0.37–0.60), indicating proportionately taller and shorter
pygostyles compared to confuciusornithiforms (MLH/TLP value is 0.21–0.26) and
enantiornithines (MLH/TLP value is 0.16–0.22) (Table 1; Fig. 7D). This greater dorsoventral
depth is produced by the prominent neural spines in sapeornithiforms and the dominant
pygostyle lamina in ornithuromorphs, which both are dorsally projecting (Figs. 3, 6).
Four groups (TLP-TLH, TLP-TLF, TLP-MLW and TLP-MLH) of visibly separated
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clusters of data represent the pygostyle measurements of four investigated clades. Despite
of the ratio values discussed above, cluster distributions of four clades can be more or less
separated as their borders with slight overlaps show (Fig. 7). Therefore, four avian clades can
be, to some extent, differed according to the absolute sizes.
4 Discussion
4.1 Convergent evolution of the “pygostyle”
Fusion or partial fusion of the terminal caudal vertebrae in maniraptorans is observed
in the Therizinosauroidea, Oviraptorosauria and potentially also the Scansoriopterygidae (Xu
et al., 2003; Persons and Currie, 2011a, b; Persons et al., 2013; Gao et al., 2008). However,
morphological differences between these phylogenetically separated taxa indicate these
co-ossified structures cannot be considered equivalent to the avian pygostyle. Outside the
Ornithuromorpha, no group preserves evidence of a tail complex. The rod-like fused caudals
in the primitive Beipiaosaurus completely lack rectrices. Although pennaceous tail feathers
are present in oviraptorosaurs, the absence of a pygostyle lamina or a similar dorsal ridge
makes the development of rectricial bulbs or equivalent structures in this clade unlikely.
Co-ossification, when present, only occurs between the centra and prezygopophyses;
other processes, such as the transverse processes inferred to fuse into the pygostyle base
in ornithuromorphs, are absent in the distal caudal vertebrae of oviraptorosaurs. In the
Oviraptorosauria, the presence of extremely deep hemal arches (chevrons) suggests a massive
caudofemoralis muscle (Persons et al., 2013). Hemal arches are remarkably shortened in
almost all primitive pygostylians and even lost in most living birds (Baumel and Witmer,
1993).
Anatomical differences between the tightly associated caudal vertebrae in oviraptorosaurs
and the pygostyle in birds suggest the decrease and fusion of caudal vertebrae in maniraptorans
occurred through different genetic pathways, indicating fusion of the distal most caudals
evolved independently in different maniraptoran lineages (Rashid et al., 2014). To avoid
confusion (Lü and Hou, 2005), fusion in the distalmost caudals of non-avian theropods should
be described as pygostyle-like and a true pygostyle should be regarded as a synapomorphy of
the Pygostylia (Aves).
4.2 Evolution of the tail complex
In the last major study of the evolutionary transition of the avian tail, researchers
considered the pygostyle in the basal bird Confuciusornis to be roughly equivalent to that
of modern birds (Gatesy and Dial, 1996a, b). However, in light of the numerous specimens
uncovered over the past two decades, representing a diverse aviary and a wealth of
morphological data, signicant differences are now clearly observable between the pygostyle
and associated tail feathers of modern birds and that of basal clades (Sapeornithiformes,
Confuciusornithiformes, Enantiornithes and Ornithuromorpha).
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The sapeornithiform pygostyle is relatively smaller and poorly co-ossified compared
to most other Early Cretaceous pygostylians. Although forming a single element, individual
processes can still be observed in the cranial half of the pygostyle in most specimens; the
adult condition is exemplied in the adult or near adult specimens IVPP V 13275 and V 13276
(Fig. 3) (Zhou and Zhang, 2003b). As a result of the increased caudal fusion, the pygostyle in
confuciusornithiforms have comparatively smoother margins. Compared to sapeornithiforms,
the pygostyle is proportionately longer in confuciusornithiforms and enantiornithines (only
exceptions of the Pengornithidae discussed later). Notably, the bilateral dorsolateral ridges
present in confuciusornithiforms and enantiornithines define an excavated dorsal platform
whereas in ornithuromorphs (including extant birds) the neural spines are fused forming the
blade-like pygostyle lamina so that the surface area of the dorsal margin is negligible. In the
blade-like ornithuromorph pygostyle, the centra fuse forming the narrow pygostyle base,
which is obscured in other pygostylians by the presence of lateral ridges and ventrolateral
processes. The transverse processes are highly reduced in primitive ornithuromorphs, forming
only a series of uneven low ridges on the lateral surface (Fig. 6). However, in some taxa of
living birds (like pheasant and raptors) the transverse processes are inferred to secondarily
form the ventral process, and expanded in wood peckers to form the pygostyle disc (Baumel,
1988) (Fig. 8). These specialized transverse processes are restricted to the cranioventral end
of the pygostyle where present in living birds, differing strongly from the fused transverse
processes observed forming a ridge on the lateral surfaces of the sapeornithiform pygostyle or
the unfused processes on the lateral surface in ornithuromorphs (Fig. 3).
Differences in pygostyle morphology between these four groups co
rrespond to consistent
differences in tail plumage. Sapeornis has been interpreted as having a strongly graded fan
consisting of approximately eight pennaceous feathers (Zheng et al., 2013; Wang X et al.,
2014; Xu et al., 2014), but this taxon has not been studied directly with regards to its rectricial
morphology and only one published specimen preserves this feature in lateral view. Additional
specimens preserved in dorsal or ventral view confirm that a fan-shaped array of rectrices
is present, but they also suggest that the rectrices did not overlap and thus would not have
formed an aerodynamic surface (O’Connor et al., 2016). However, until these specimens can
be further studied tail function remains inconclusive in sapeornithiforms. Tail feathers are not
present in all confuciusornithiform specimens, some of which preserve only rachis-less body
feathers like those found on the neck and other parts of the body, which are also present in
sapeornithiform, enantiornithine and ornithuromorph fossils also preserving rectrices. Other
confuciusornithiform specimens additionally preserve a pair of elongate rachis-dominated
rectrices interpreted as male-specific ornaments (Chiappe et al., 2008). Consistent with
similarities in pygostyle morphology, similar ornamental rachis-dominated rectrices are also
present in some enantiornithines (e.g., Protopteryx, Paraprotopteryx, Dapingfangornis) (Zhang
and Zhou, 2000; Zheng et al., 2007; Li et al., 2006). The only exception is the pengornithid
Chiappeavis (STM 29-11), which has recently been described with a “tail fan” consisting of
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approximately eight overlapping rectrices (O’Connor et al., 2016). This fan is proportionately
shorter than that observed in Jehol ornithuromorphs. Rectricial fans in early ornithuromorphs
apparently consisted of eight to more than ten overlapping rectrices, comparable to living birds.
Differences in the pygostyle between clades of Early Cretaceous birds, supported by
differences in rectricial morphology, suggest that the complete modern tail complex only
evolved in the ornithuromorph lineage. In neornithines the rectricial bulbs rest on sockets on
either side of the pygostyle, separated medially by the pygostyle lamina (Fig. 8). These bulbs
root the calami of the rectrices forming the tail fan, allowing their position to be precisely
controlled by the rectricial bulb muscle (Baumel, 1988; Gatesy and Dial, 1996b). The nearly
modern morphology of the pygostyle in all Early Cretaceous ornithuromorphs and the common
preservation of a rectricial fan strongly suggest the complete tail complex evolved very early
in this lineage. A few specimens even preserve carbonized imprints surrounding the pygostyle
interpreted as soft tissue traces of the rectricial bulbs (Zhou et al., 2014), supporting inferences
this feature was present. The presence of a fan-shaped array of rectrices and a possible low
pygostyle lamina in sapeornithiforms might suggest the presence of rudimentary rectricial bulbs
in this lineage as well. However, the proportionately greater length (and width) of the pygostyle
indicates that the rectricial bulbs, if present, would have differed from that in ornithuromorphs.
The tail complex can be considered absent in the confuciusornithiforms based on the absence of
a fan-shaped tail in all the hundreds of known specimens; this is supported through differences
in pygostyle morphology, notably the absence of a pygostyle lamina.
A pygostyle lamina is also totally absent in enantiornithines. We consider short body
feathers (apparently rachis-less) were present on the tail in all enantiornithine groups regardless
of rectricial morphology. A majority of enantiornithine specimens preserve a pair of rachis-
Fig. 8 Illustration of extant avian pygostyles in both dorsal view (A–D) and left-lateral view (a–d)
A, a. Cynus cynus, IVPP OV 1721; B, b. Lophura nycthemera, IVPP OV 1863;
C, c. Streptopelia orientalis, IVPP OV 1883; D, d. Buteo buteo, IVPP OV 1888
Light blue: pygostyle lamina, its lateral surfaces are the major attachments for rectricial bulbs and rectricial
bulb muscle, anteriorly up-lateral and dorsal surfaces are attachments for caudal levator muscle;
purple: pygostyle base, its anteriorly low-lateral and ventral surfaces are attachments for caudal depressor
muscle; peach: ventral expansion of pygostyle base for well-developed caudal depressor muscle;
light yellow: dorsal expansion of pygostyle lamina for well-developed caudal levator muscle. Scale bar=10 mm
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dominated ornamental long rectrices, resembling those in confuciusornithiforms. The “fan
tail” from which longiptergyid Shanweiniao gets its name (O’Connor et al., 2009) has been
reinterpreted as non-aerodynamic, consisting of at least two pairs of rachis-dominated feathers,
similar to the tail in Paraprotopteryx (O’Connor et al., 2016). This argument is partially based
on pygostyle morphology, which agrees with the detailed observations from this study. The tail
fan in pengornithid Chiappeavis (STM 29-11) greatly differs from the rectricial morphology in
other pengornithids, all other members of which possess a pair of rachis-dominated streamer
rectrices (Parapengornis, Eopengornis). However, the pygostyle of Chiappeavis is typical of
pengornithids, although it is proportionately shorter and broader than in Parapengornis. Given
the observed coevolution between pygostyle and tail feathers, similar pygostyle morphologies
are expected to share similar rectrices both within Cretaceous primitive birds and living taxa
(Clarke et al., 2006; Felice and O’Connor, 2014; Felice, 2014), making the two apparently
functionally disparate tail morphologies observed in the Pengornithidae difcult to interpret.
Even if Chiappeavis possessed some structural equivalent to rectricial bulbs, their morphology
would be expected to differ greatly from that of modern birds given the different shape of
the pygostyle in pengornithids. As a result, there may also exist differences in tail function
and ability between the two groups. Disparity in the tail plumage of Chiappeavis compared
to other pengornithids, and differences in pygostyle morphology between Chiappeavis
and ornithuromorphs suggests that if a tail complex was present in Chiappeavis, it evolved
independently in parallel to ornithuromorphs. We suggest that this tail fan even may have been
capable of generating lift but did not have the muscular control of the fan shape associated
with the ornithuromorph tail complex. The tail complex consisting of a blade-like pygostyle,
rectricial bulbs, and aerodynamic rectrices appears to be unique to the Ornithuromorpha as
originally hypothesized by Clarke et al. (2006).
4.3 Coevolution of tail feathers and the pygostyle
Modern birds exhibit a huge diversity of tail feather morphologies. To a lesser extent the
shape of the pygostyle also shows quite a bit of variation. Bird tails can have both signicant
motor and/or display functions. The diversity of tail morphologies in extant birds are
hypothesized to be a product of natural and/or sexual selection (Balmfold et al., 1993; Thomas,
1997; Hedenström, 2002). Recent work has documented the coevolution of the tail feathers
with the pygostyle in living birds (Felice and O’Connor, 2014; Felice, 2014), indicating that
rectrices with specialized functions require structurally modied pygostyles.
In addition to the rectricial bulbs and their muscle, the caudal depressor and caudal
levator (m. levator caudae) muscles also form connections with the pygostyle (Baumel, 1988).
Fleshy part of the pars profunda of the caudal depressor muscle adhere to the ventrolateral
portion of the pygostyle base and the tendon of this muscle attaches along the anterior-ventral
margin of the pygostyle. The tendon of the distal part of the caudal levator muscle (pars
distalis) attaches on the craniodorsal portion of the lateral surface of the pygostyle (Baumel,
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1988). In certain extant raptor clades, the birds descend on their prey from great height at
great speeds. Such taxa must be capable of generating large braking forces after seizing prey
near the ground. This maneuvering requires a robust caudal depressor muscle. In order to
provide an expanded area for the attachment of this expanded muscle, falconids possess an
expanded tubercle, or in some cases paired accessory sesamoid akelets (Richardson, 1972),
on or near the cranioventral margin of the pygostyle probably (Fig. 8D). Even greater ventral
expansions, in the form of the pygostyle disc (Baumel and Witmer, 1993), are developed in
woodpeckers, and some piciforms. Woodpeckers have evolved stiffened feathers in order
to support their bodies against gravity as an adaptation for vertical climbing (Manegold and
Töpfer, 2013). This posture also requires a strong caudal depressor muscle as the expanded
pygostyle disc suggests. The enantiornithine Parapengornis has been described possessing a
pygostyle that is homomorphic to that of woodpeckers, indicating a similar vertical climbing
behavior in some Mesozoic enantiornithines (Hu et al., 2015). However, in the holotype
of Parapengornis eurycaudatus (IVPP V 18687) the pygostyle is preserved in dorsal view,
preventing observation of any expanded ventral surface (also preserved in dorsal view in the
referred specimen IVPP V 18632). Instead, V 18687 displays a broad, concave dorsal platform
dened by lateral ridges generally similar to those of other pengornithids. The dorsal expansion
present in the pygostyle of Parapengornis and other pengornithids (Hu et al., 2015; Wang X et
al., 2014) is in fact opposite the condition observed in the woodpecker pygostyle, in which it
is the ventral surface that is expanded. The dorsal platform present in pengornithids and other
enantiornithines is more consistent with the presence of an expanded caudal levator muscle,
which attaches dorsally in living birds, rather than an expanded caudal depressor muscle as
in woodpeckers and falconids (Fig. 9). Furthermore, in woodpeckers the specialized stiffened
rectrices of the tail are not especially elongated and are additionally supported by a layer
of tectrices. The tail feathers are proportionately longer in Parapengornis and pennaceous
tectrices appear to be absent. Together, it seems unlikely that these rachis-dominated rectrices
could serve as a prop.
In extant phasianids with ornamental tails, the craniodorsal margin of the pygostyle is
laterally expanded to provide additional surface for the attachment of the enlarged caudal
levator muscle, which acts to elevate the elongated ornamental deck feathers (Gatesy and
Dial, 1993). This craniodorsal expansion is obviously more strongly developed in males,
though it presents in both genders (Fig. 8B). This dorsal platform is reminiscent of that in
confuciusornithiforms and enantiornithines (Figs. 4, 5). The pygostyle of the male peafowl
(Pavo) represents an extreme case: to lift its huge tail (coverts and rectrices), the narrow
body of the pygostyle separates enlarged dorsal and ventral surfaces abnormally expanded
for muscular attachments (“”-shaped in cranial view) (Fig. 9). Male birds living in
wooded environments, like that inferred for the Jehol (Zhou, 2006), often have extravagant
ornaments; the high degree of clutter and reduced visibility in wooded environments allows
the evolution of such features under sexual selection (Endler and Thery, 1996). Paired elongate
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Fig. 9 Reconstruction of caudal fusion/pygostyle from maniraptorans to birds
Line drawing near the group name shows left lateral view of distalest caudal vertebrae of each clade. Line
drawing in the box shows proximal/cranial view of pygostyle and soft tissue attachments: bone in white, articular
surface is gray dot, caudal levator muscle in red, caudal depressor muscle in blue, rectricial bulb in yellow
A. Sapeornithiformes; B. Confuciusornithiformes; C. Pengornis; D. Parabohaiornis; E. Rapaxavis;
F. Yixianornis; G. Piscivoravis; H. Iteravis; I. Gansus; J. Cynus cynus; K. Lophura nycthemera;
L. Pavo cristatus; M. Streptopelia orientalis; N. Buteo buteo; O. Dendrocopos major; P. Passer montanus. (not scaled)
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tail feathers are common in enantiornithines and have typically been regarded as ornamental
and sexually dimorphic, as in Confuciusornis. The tail morphology in Parapengornis and
other enantiornithine species is reminiscent of those found in male individuals of many
extant arboreal birds, such as paradise-flycatchers (Terpsiphone). One Early Cretaceous
enantiornithine, the holotype of Feitianius paradisi, preserves a robust pygostyle of the typical
enantiornithine morphology and a tail morphology consisting of at least three distinct feather
morphologies that strongly resembles tail displays present in living sexually dimorphic birds
(O’Connor et al., 2016). This level of extravagance is only observed in extant polygamous
birds. This study suggests that in addition to possessing such sexually specic ornaments (Fig.
10), some Cretaceous birds also engaged in display behaviors as common in extant polygamous
birds, including the birds of paradise. Soft tissue surrounding the pygostyle in the holotype of
Feitianius paradisi was previously identified as including rectricial bulbs (O’Connor et al.,
2016), but we consider that this may not be correct – instead this may include expanded levator
musculature, as hypothesized here (Fig. 9). We conclude that the dorsal platform present in
the confuciusornithiforms and enantiornithine pygostyle may have accommodated a fortied
caudal levator muscle, like that in phasianids. This strongly suggests these basal birds raised
the paired long rachis-dominated deck feathers in some form of display, although whether to
attract females or fend of competitors is unknown.
Fig. 10 Reconstruction of Feinianius paradise by Michael Rothman showing the tail relaxed and with the
levator musculature exed, engaged in a display
A. Perched and non-displaying; B. Perched and displaying
Acknowledgement We thank ZHOU Zhonghe, Ni Xijun, LI Zhiheng and WANG Min for
discussion, ZHENG Xiaoting for accession to collections in STM, ZHOU Shuang and HU Han
for providing photos. We thank M. Rothman for use of his reconstruction of Feitianius. This
research was supported by the National Natural Science Foundation of China (91514302) and
the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB18030501).
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22
早白垩世鸟类尾综骨与尾羽之间的形态协同演化
王 维1,2 邹晶梅1
(1 中国科学院古脊椎动物与古人类研究所,中国科学院脊椎动物演化与人类起源重点实验室 北京 100044)
(2 中国科学院大学 北京 100049)
摘要:从兽脚类恐龙中爬行类的骨质长尾,到以尾综骨为终端,并附着具有空气动力学功
能的扇状尾羽的短巧尾部,是早期鸟类演化中最显著的适应性转变之一。但能直接反映这
一转变的化石记录匮乏,而且对中生代鸟类尾部形态结构,以及尾综骨和尾羽早期演化的
认知也相对不足。在此对早白垩世鸟类的尾综骨形态予以概述并将其与现生鸟类尾部结构
类比。本研究强调了非鸟手盗龙类中尾椎的联合骨化与早期鸟类的尾综骨实属趋同演化。
本研究表明,会鸟形类、孔子鸟形类、反鸟类和今鸟型类的尾综骨结构存在明显差异。今
鸟型类尾综骨和尾羽(舵羽)与现代鸟类的相似,而相对更原始的鸟类的尾综骨,从形态来
看,并不能支持舵羽球状膨大和必要的肌肉附着来操控具有空气动力学功能的扇状尾羽。
由此可见,舵羽球状膨大、舵羽扇面与犁铧状的尾综骨是在今鸟型类演化早期相伴相生
的。相对于从前的认知,本研究还发现孔子鸟类的尾综骨与反鸟类的有更多相似之处,与
二者都具有的几乎相同的装饰性尾羽相符合。
关键词:热河生物群,鸟类,舵羽球状膨大,舵羽
中图法分类号Q 915.865 文献标识码A 文章编号1000−3118(2017)03−0001−26
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Advanced online publication
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