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Mandibular and dental characteristics of the Late Jurassic mammal Henkelotherium guimarotae (Paurodontidae, Dryolestida)
Abstract
Henkelotherium guimarotae Krebs 1991 is an important Jurassic mammal for understanding therian evolution. We are presenting a new study of extensive, previously undescribed, mandibles and dentitions. The revised dental formula is: I4? or 5?/i4, C1/c1, P4/p4, M6/m7. The canine and premolars show an alternate replacement that ends with M4/m4 eruption, and is followed by a late sequential eruption of the last three lower (m5-7) and last two upper (M5-6) molars. The lower premolars erupted in the following order: p1 → p3 → p2 → p4, and the canine erupted most probably shortly before p4. The timing of the premolar replacement before the late molar eruption is similar to that of Dryolestes leiriensis, and is a characteristic of dryolestidans. Henkelotherium lower molars have subequal roots, a plesiomorphy of non-dryolestidan mammals, and the upper molars are supported by a strong, curved lingual root; a derived character. In the upper molars, the postvallum wear surface is contiguous to the parastyle wear surface of the succeeding molar, which differs from dryolestids. The parastylar lobe of the succeeding molar, and the postvallum of the preceding molar, are imbricated, and can develop strong, continuous wear surfaces, matching the prevallid crest of the lower molar. Henkelotherium differs from dryolestids in having an inflected, shelf-like mandibular angular process with a foramen. This large sample of Henkelotherium shows a significant variation gradient along the molar series, with the strongest wear occurring only in two to three consecutive molars. The extraordinarily long molar row is correlated with the late growth of jaws; and the jaw with late addition of molars sustained an effective mastication, much longer in older adults of dryolestidans than in other Mesozoic stem therians. The late eruption of several more molars after completion of antemolar replacement suggests that dryolestidans had either a longer-lived life, or slower life-history traits, or a combination of both, than crown therians.
... This family is primarily known from cranial elements such as the petrosals, the lower jaw, and dental fossils, but a limited number of postcranial elements have also been described (Hughes et al., 2015;Luo et al., 2012;Martin, 2013). New and well-preserved fossil specimens, such as the specimen of Dryolestes described here and Henkelotherium (Krebs, 1991;Jäger et al., 2019;Luo & Martin, 2023) provide new data to further test the phylogenetic relationships of the Dryolestidae. ...
... Dryolestids are characterized by several distinctive morphological features. They exhibit high mandibular molar counts of 7-9 molar loci in fully grown adults, more than other Mesozoic mammals and even more than closely related genera formerly assigned to "paurodontids" (Luo & Martin, 2023;Martin, 1997Martin, , 1999. The lower molars also have a unique talonid "heel" formed by a single cusp (Davis, 2011;Luo et al., 2002;Martin, 1997Martin, , 1999Patterson, 1956;Prothero, 1981;Simpson, 1929). ...
... Dryolestids also exhibit a highly derived root structure in their two-rooted lower molars, whereby the mesial root is larger and mesio-distally expanded relative to the smaller cylindrical distal root (Figs. 2, 3), a size and shape disparity far beyond that seen in other dryolestoideans (Krebs, 1971;Lasseron et al., 2022;Luo & Martin, 2023;Martin, 1997Martin, , 1999Martin et al., 2021;Prothero, 1981;Simpson, 1929). To date, the evolution and functional morphology of molar crowns have been examined extensively for early mammals, including dryolestids (Crompton 1971;Crompton & Jenkins, 1968;Schultz & Martin 2014). ...
... Others 32 proposed that the 'primitive' sequence of replacement in ancestral eutherians comprised the eruption of all molars before the premolar tooth replacement was completed (a similar pattern to fast-growing, small-bodied modern placentals). However, in more ancient Mesozoic mammaliaforms (a more inclusive clade of fossil predecessors to crown Mammalia, of which eutherians are a subclade), new and rapidly expanding evidence shows the addition of posterior molars and continued growth of the dentary in docodontans and morganucodontans, indicating that their skulls experienced prolonged growth in later life stages 31,[36][37][38] . The late addition of molars in a dentary that continues to grow is now also known in stem therians, such as spalacotheroids and dryolestoideans 38 . ...
... However, in more ancient Mesozoic mammaliaforms (a more inclusive clade of fossil predecessors to crown Mammalia, of which eutherians are a subclade), new and rapidly expanding evidence shows the addition of posterior molars and continued growth of the dentary in docodontans and morganucodontans, indicating that their skulls experienced prolonged growth in later life stages 31,[36][37][38] . The late addition of molars in a dentary that continues to grow is now also known in stem therians, such as spalacotheroids and dryolestoideans 38 . This suggests that prolonged growth is the ancestral condition in early diverging mammaliaforms and some mammals. ...
Living mammal groups exhibit rapid juvenile growth with a cessation of growth in adulthood¹. Understanding the emergence of this pattern in the earliest mammaliaforms (mammals and their closest extinct relatives) is hindered by a paucity of fossils representing juvenile individuals. We report exceptionally complete juvenile and adult specimens of the Middle Jurassic docodontan Krusatodon, providing anatomical data and insights into the life history of early diverging mammaliaforms. We used synchrotron X-ray micro-computed tomography imaging of cementum growth increments in the teeth2–4 to provide evidence of pace of life in a Mesozoic mammaliaform. The adult was about 7 years and the juvenile 7 to 24 months of age at death and in the process of replacing its deciduous dentition with its final, adult generation. When analysed against a dataset of life history parameters for extant mammals⁵, the relative sequence of adult tooth eruption was already established in Krusatodon and in the range observed in extant mammals but this development was prolonged, taking place during a longer period as part of a significantly longer maximum lifespan than extant mammals of comparable adult body mass (156 g or less). Our findings suggest that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals and the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.
... It differs from NLMH 110017 by a much less pronounced size difference between cusps a and b (b is relatively larger in NLMH 110019), stronger crests a-c and a-g, and slightly larger overall size of the preserved trigonid (cusp b-a-c) area. These differences can be attributed to the molar position, which exhibit a gradient of size and morphology along the tooth row as observed in a series of mandibles of Haldanodon exspectatus from the Guimarota coal mine (Luo and Martin 2023). In H. exspectatus, the size difference between cusp a and g decreases in distal direction; i.e. in posterior molar positions (m4-5) cusp g becomes relatively higher (m6, if present, is vestigial with reduced morphological detail). ...
... 3,4,6). A considerable morphological gradient within the molar row is common in Mesozoic mammals as described by Martin (1999) and Luo and Martin (2023) for dryolestidans. Accordingly, specimen NLMH 110017 is interpreted to be an anterior and NLMH 110018 is interpreted to be a middle Docodon sp. ...
Docodon hercynicus sp. nov. from the Upper Jurassic (upper Kimmeridgian) Süntel Formation of the Langenberg
Quarry in Lower Saxony is the first docodontan recorded from Central Europe. The two lower molars available are
characterised by vertical enamel ridges at the distal flank of cusp a, which are typical for Docodon. This is only
the second record of Docodon from Europe with the genus otherwise restricted to the Kimmeridgian-Tithonian of
the Morrison Formation in western North America. A distal fragment of a much smaller lower molar from the same
strata exhibits similar vertical enamel ridges and indicates the possible presence of a second Docodon species in
the Langenberg Quarry. Docodon hercynicus n. sp. is further evidence for the close terrestrial faunal interrelationships
between Europe and North America. It represents only the third record of a docodontan from Late Jurassicearliest
Cretaceous strata of Europe and supports a relic occurrence and late survival of mammaliaforms on
a palaeo-island in the Lower Saxony Basin within the European Archipelago.
... A similar pattern is also present in Maotherium sinense and O. lii (Plogschties and Martin 2020;Mao et al. 2020). This posteriorly decreasing size (length) gradient of molar roots is also present in cladotherian mammals, and is likely because the teeth erupting earlier in development had more time for growth and lengthening the roots, than younger teeth that erupted later in molar row (Luo and Martin 2023). ...
3º volume do Tylostoma, o boletim informativo da Sociedade Portuguesa de Paleontologia.
The 3rd volume of Tylostoma, the Portuguese Society of Paleontology newsletter.
The Ksar Metlili Formation, from the Jurassic – Cretaceous transition (Tithonian – Berriasian) of eastern High Atlas Mountains, Morocco, has yielded one of the richest microvertebrate assemblages from the Mesozoic of Gondwana. With at least 19 species, mammaliaforms are particularly diverse. ‘Dryolestoidea’ are the most abundant and the most diverse; nevertheless, only one species, Donodon perscriptoris Sigogneau-Russell, 1991a, of the monotypic Donodontidae, had been described so far. Here, we describe four new species and three new genera of ‘dryolestoids’ from the Ksar Metlili deposits: Donodon minor sp. nov., Stylodens amerrukensis gen. et sp. nov., Anoualestes incidens gen. et sp. nov., and Amazighodon orbis gen. et sp. nov, all of which are included in the Donodontidae based on their dental morphology. We present the first phylogenetic analysis that incorporates the five donodontid species from the Ksar Metlili site into the cladotherian phylogeny. Our new analysis provides evidence for the paraphyly of the ‘Dryolestoidea’: Dryolestidae, ‘Paurodontidae’, Donodontidae, and Zatheria form a clade to the exclusion of the South American Meridiolestida. Donodontids are found to be closer to prototribosphenidans and zatherians than to any other ‘dryolestoid’ family and are more derived than meridiolestidans. This topology refutes a previous hypothesis that meridiolestidans are phylogenetically rooted among donodontids. Our phylogenetic analysis also supports Donodontidae as an endemic African monophyletic group. The close relationships of the donodontids and zatherians suggest that zatherians had a geographical origin possibly from Africa or Gondwana instead of Laurasia.
The new spalacotheriid “symmetrodontan” Cifellitherium suderlandicum gen. et sp. nov. from the Barremian–Aptian
of the Balve locality in northwestern Germany is the first record of spalacotheriids in Central Europe. The new taxon is
based on one lower and two upper molars. Cifellitherium is similar to Spalacotherium, but differs by smaller stylocone
and larger paraststyle on the upper molars, and a labially interrupted cingulid on the lower molar. The new dryolestid
Minutolestes submersus gen. et sp. nov. is based on one lower and two upper molars of small size. Phylogenetic anal�ysis revealed it as sister taxon of the clade comprising Laolestes, Krebsotherium, Dryolestes, and Guimarotodus. A
dryolestid mandible with unevenly rooted molars and extremely worn down teeth cannot be attributed to Minutolestes
submersus gen. et sp. nov. due to possessing molars twice as large in size. The mandible is assigned to a new dryolestid
taxon, Beckumia sinemeckelia gen. et sp. nov., and has the dental formula 3i, 1c, 4p, 8m and a fully reduced Meckel’s
groove. The mandible lacks any trace of a coronoid or splenial. In the phylogenetic analysis, it appears as sister taxon
of a clade comprising Achyrodon, Phascolestes, Crusafontia, and Hercynodon. The new mammals are the stratigraph�ically youngest European representatives of their clades. The late survival of Beckumia sinemeckelia gen. et sp. nov.
and Minutolestes submersus gen. et sp. nov. is possibly the result of isolated evolution in an Early Cretaceous island
environment. This finding of new spalacotheriid and dryolestid mammals from Central Europe adds to an emerging
paleobiogeographic pattern that Europe was distinct from Asia in the constituents of mammalian faunas during the
Barremian–Aptian.
CT visualization of the mandible and dentition of Hadrocodium wui, a stem mammaliaform from the Lower Jurassic
Lower Lufeng Formation of Yunnan, China has revealed new features not accessible by previous microscopic study of
the fossil. Its mandible shows a postdentary trough with an overhanging medial ridge and a short Meckel’s sulcus. An
incomplete part of the ectotympanic and possibly a remnant of Meckel’s element are preserved in the postdentary trough.
Thus, Hadrocodium is similar to other mammaliaforms in retaining a mandibular middle ear, contrary to our earlier inter�pretation. The mandible exhibits a large postcanine diastema from shedding of anterior premolars without replacement,
an age-dependent feature better developed in older adults. Another adult feature is the alignment of the ultimate molar
to the coronoid process. This is consistent with age-dependent changes in other mammaliaforms where the last molars
of the toothrow shift from medial of the coronoid process in the juvenile, to a position in front of the coronoid process
in the adult. The mandible has a short mobile symphysis. The dentition consists of I5, C1 (two-rooted), P3 (including
P1 position) and M2 (M2 with confluent roots), and i4, c1 (partially two-rooted), p3, and m2 (m2 with partially con�fluent roots). The two-rooted upper canines are more derived than other Early Jurassic mammaliaforms from the same
fauna, although similar to docodontans. Hadrocodium is unique in that the lower m2 cusp a occludes in the embrasure
between upper M1–M2, but the posterior part of m2 shows between-cusp occlusion with upper M2 main cusp A. M2 is
half the size of the lower m2, and occludes only with the distal half of m2. The upper postcanines show a steep gradient
of posteriorly decreasing tooth size, more so than other mammaliaforms. The CT examination corroborates that there
are no unerupted teeth in the upper or lower jaws, and the holotype of H. wui is dentally and osteologically mature and
capable of independent feeding.
Here we report on a new Early Cretaceous eutherian represented by a partial skeleton from the Jiufotang Formation at Sihedang site, Lingyuan City, Liaoning Province that fills a crucial gap between the earliest eutherians from the Yixian Formation and later Cretaceous eutherians. The new specimen reveals, to our knowledge for the first time in eutherians, that the Meckelian cartilage was ossified but reduced in size, confirming a complete detachment of the middle ear from the lower jaw. Seven hyoid elements, including paired stylohyals, epihyals and thyrohyals and the single basihyal are preserved. For the inner ear the ossified primary lamina, base of the secondary lamina, ossified cochlear ganglion and secondary crus commune are present and the cochlear canal is coiled through 360°. In addition, plesiomorphic features of the dentition include weak conules, lack of pre- and post-cingula and less expanded protocones on the upper molars and height differential between the trigonid and talonid, a large protoconid and a small paraconid on the lower molars. The new taxon displays an alternating pattern of tooth replacement with P3 being the last upper premolar to erupt similar to the basal eutherian Juramaia . Parsimony analysis places the new taxon with Montanalestes , Sinodelphys and Ambolestes as a sister group to other eutherians.
This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.
We describe the first maxillae and additional new specimens of Reigitherium bunodontum, a small meridiolestidan from the Late Cretaceous La Colonia Formation, Patagonia, Argentina. The new material supports a dental formula of I?, C1, P4, M3, resolves postcanine positional uncertainty and corrects previous interpretations. Our phylogeny recovers Reigitherium as a meridiolestidan allied to other bunodont Mesungulatoidea, as the sister group of the Paleocene Peligrotherium. Posterior premolars/molars of Reigitherium, and to a smaller degree Peligrotherium, are dominated by an incomplete transverse ridge running between the protoconid-metaconid in the lowers and the paracone-stylocone in the uppers, semi-symmetrical basins developing mesially and distally from these central ridges. The trigonid-derived single transverse crest results from a mesial shift of the robust metaconid, an enhancement of the basin crest stretching from the protoconid/metaconid, and a shallower trigonid basin. The mesungulatoid condition, with its complete absence of talonid, contrasts sharply with that of therians with lophs, or transverse ridges, which involved at least one talonid-derived loph resulting in two transverse crests per tooth. Mesungulatoid meridiolestidans achieved complex tooth-on-tooth occlusion with a predicted increase in herbivory/omnivory, departing from the traditional sharp-cusp insectivores plesiomorphic for meridiolestidans and Mesozoic mammals in general. Reigitherium’s dramatic remodeling of the primitive meridiolestidan molar morphology, extensive continuous occlusal surface, accessory cuspules, and highly textured crenulated enamel illustrates one of most distinctive adaptations to herbivory among Mesozoic mammals.
In the last decades, several discoveries have uncovered the complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and jaws. Here, we describe a new meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium , and the early Paleocene Peligrotherium . The in loco tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids may include more than one species. Tooth eruption and replacement show that molar eruption in mesungulatids is heterochronically delayed with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Meridiolestidans seem endemic to Patagonia, but given their diversity and abundance, and the similarity of vertebrate faunas in other regions of Gondwana, they may yet be discovered in other continents.
Synopsis
The evolutionary origins of the modern mammalian ear structures can be traced back into their phylogenetic predecessors in the Mesozoic Era. This evolutionary history shows a step-wise acquisition of middle- and inner-ear structures along separate Mesozoic mammal lineages that led to convergence of several derived characters correlated with distinct hearing functions in extant mammals.
A functional analysis of a well-preserved snout of the early-diverging mammaliaform Morganucodon watsoni, with matching upper and lower dentitions, and of the holotype of Megazostrodon rudnerae, showed that both taxa had a primarily orthal occlusal path. In Morganucodon, the direction was individually variable and either strictly orthal or slightly distally or mesially inclined. An analysis with the Occlusal Fingerprint Analyser (OFA) software confirmed an earlier hypothesis that the main cusp A of the upper molars occluded between cusps b and a of the lower antagonists. According to the OFA analysis, there was more extensive contact between cusp a and the preceding anterior upper molar than previously assumed, showing some similarities to the two-on-one pattern described for Megazostrodon. According to our analyses, the molars of Morganucodon and Megazostrodon had an adaptation to piercing, as well as shear-cutting. ‘Shearing flanks,’ which were the focus of previous studies, seem to be a result of attrition, rather than functional areas in themselves. The posterior upper molars in Morganucodon were rotated along their longitudinal axis and lingually inclined within the tooth row, resulting in a triangle between M1 and M2 into which the large m2 can occlude. Together, this suggests a predetermined tooth placement and that contrary to previous hypothesis Morganucodon did not rely on extensive wear in order to form a precise occlusion.
The Middle Jurassic docodont Borealestes serendipitus was the first Mesozoic mammal found in Scotland over 40 years ago. Its affinities and morphology have remained poorly understood. Although multiple dentary fragments and isolated teeth have been recovered from Scotland and England, they have not yet been described in sufficient detail. We report new, more complete specimens collected during recent field work on Skye, Scotland, combined with previously collected material. This includes upper and lower dentition and an almost complete right dentary. We present an updated description and diagnosis of the genus Borealestes, based on high-resolution micro-computed tomography (micro-CT) and synchrotron scans. We identify seven key features that distinguish Borealestes from other docodonts, including a pronounced a–c crest, absence of the a–g crest on cusp a, an anterior fovea at the buccolingual midpoint of the upper molar, and the convergence of the Meckel’s groove with the ventral margin of the mandible. We also present a revised diagnosis for the second species, B. mussettae. Our phylogenetic analysis supports a clade formed by Borealestes, Haldanodon, Docofossor, and Docodon. Ontogenetic variation in the mandibular morphology of Borealestes is similar to that seen in Docodon and Haldanodon, with the delayed emergence of the ultimate lower molar, the shift of the last molar to the front of the coronoid process, and a posterior shift of the Meckel’s sulcus in successively older individuals. This supports a distinctive growth pattern in the clade including Borealestes and Docodon, one that may be present in Docodonta as a whole.
Early suckler?
One trait that is unique to mammals is milk suckling. Suckling requires the presence of stability and motion in the throat, both of which require a complex hyoid apparatus. Zhou et al. describe a mammaliform docodontan fossil from the Jurassic that was preserved with a nearly intact hyoid (see the Perspective by Hoffmann and Krause). The structure is complex and saddle shaped, like that seen in modern mammals, suggesting that a muscularized throat was present before the development of mammals.
Science , this issue p. 276 ; see also p. 222
In mammals, the infraorbital canal provides a passage for the infraorbital ramus of the maxillary branch of the trigeminal nerve. The infraorbital nerve ensures tactile sensitivity of the upper teeth and face between the eye and upper lip and, more significantly, the innervation of mystacial vibrissae (whiskers). In contrast, most non-mammalian synapsids display a more “reptilian-like” ancestral condition in which a long and ramified maxillary canal completely enclosed the infraorbital nerve along with other branches of the trigeminal nerve. The phylogenetic transition from the ancestral “reptilian-like” to the derived “mammal-like” condition has been hypothesized to occur at the base of the Probainognathia clade. Using μCT and synchrotron scanning, this study aims to document this transition in detail by focusing on a sample of non-mammalian probainognathian cynodonts and early mammaliaforms. We find that the mammalian condition is the result of a gradual shortening of the maxillary canal, which enabled the infraorbital nerve to ramify within the soft tissues of the face. Mobile whiskers became possible only after the mammalian infraorbital nerve had evolved, which suggest that these structures appeared in Probainognathus and more derived cynodonts. Finally a foramen located on the ventral margin of the lacrimal bone, which has been often homologized with the infraorbital foramen of derived Probainognathia and early Mammaliaformes, is most probably homologous to the mammalian zygomaticofacial foramen.
X-ray computed tomography of the cladotherian Henkelotherium guimarotae from the Kimmeridgian Guimarota coal mine (Portugal) adds new information on its postcranial skeleton. Contrary to earlier description, the humerus of Henkelotherium shows a plesiomorphic cladotherian condition similar to that seen in Dryolestes leiriensis in having individual radial and ulnar condyles on the anterior aspect of the distal joint, whereas the posterior aspect appears to be trochlear. The scapula is also more plesiomorphic than previously assumed in having a laterally flared crest on the inferior margin. In this regard, it is similar to the scapula of spalacotherioid “symmetrodontans” such as Zhangheotherium but more derived than the latter, in having a larger supraspinous fossa. The hind limb is similar to that of small extant therians, which is interpreted to be assocated with an upright limb posture. The previously proposed hypothesis of an arboreal lifestyle for Henkelotherium is corroborated by a number of skeletal features such as large humeral entepicondyles, prominent digital flexor tubercles, elongated and slender tail vertebrae, as well as a high pedal phalangeal index of 153%.
We describe two partial dentaries of mammals from the Middle Jurassic of Scotland. They belong to the early cladotherian Palaeoxonodon ooliticus. These dentaries comprise the first specimen of P. ooliticus ever found—although its significance was initially unrecognised so it remained undescribed until now—and the most recently discovered specimen, found during fieldwork in 2017. The new specimen preserves part of the coronoid process of the dentary, previously unknown for P. ooliticus, demonstrating the presence of a deep masseteric fossa, with a prominent crest enclosing the fossa anteriorly, and a masseteric foramen, located in the masseteric fossa on the buccal surface of the dentary. On the lingual surface, the mandibular foramen is offset from the Meckel’s sulcus, and positioned below the alveolar plane. These morphologies allow an updated analysis of the phylogenetic position of P. ooliticus, confirming a sister-taxa relationship between Palaeoxonodon and Amphitherium. The position of the mandibular foramen, and the slight extension of the masseteric fossa into the body of the dentary are new autapomorphies for Palaeoxonodon.
Among the collections made during the 1896–1899 Princeton Expeditions to Patagonia are three specimens with partial crania and postcrania of Necrolestes patagonensis Ameghino, 1891, from Santa Cruz Province, Argentina. Necrolestes has anatomical specializations found in extant subterranean mammals, and other features identifying Necrolestes as a late Miocene member of the South American Meridiolestida, a non-therian clade that otherwise is limited to the Cretaceous and Paleocene. Anatomical aspects of the three Princeton specimens have been reported previously but they are more fully described here after additional preparation. The most novel discoveries concern specializations of the snout that help identify Necrolestes as a head-lift digger, as are the extant African golden moles and Australian marsupial moles. As in other meridiolestidans and most non-therian mammals, Necrolestes has a septomaxilla at the external nasal aperture. However, the septomaxilla of Necrolestes has structures that buttress an internarial bar and what we interpret as ossified external nasal cartilages, a remarkable adaptation for digging not known in any extant subterranean therians. The upturned snout of Necrolestes produces a shovel effect and also is known in two lineages of subterranean placentals, namely Oligocene palaeanodonts and Miocene proscalopid soricomorphs. The braincase, ear region, and basicranial vasculature are built on a pattern like that in other non-therian mammals, but additional spaces in the squamosal and exoccipital bones increase the volume of the middle ear, which in extant therians is associated with enhancement of low-frequency hearing, another subterranean adaptation. Our findings reinforce the meridiolestidan affinities for Necrolestes and reconstruction of its subterranean habit.
We investigated dental homologies, development, and growth in living and fossil hyracoids and tested if hyracoids and other mammals show correlations between eruption patterns, gestation time, and age at maturity. Unlike living species, fossil hyracoids simultaneously possess replaced P1 and canine teeth. Fossil species also have shorter crowns, an I3/i3, I2, and a hypoconulid on m3. Prenatal specimens of the living Procavia capensis and Heterohyrax brucei show up to three tooth buds posterior to dI1 and anterior to the seven upper cheek teeth that consistently erupt; erupted teeth include an anterior premolar but not a canine. Most lower cheek teeth finish eruption during growth in hyracoids, not after growth as in most other afrotherians. All hyracoids show the m1 at (lower) or near (upper) the beginning of eruption of permanent teeth; M3/m3 is the last permanent tooth to erupt. The living P. capensis erupts most lower antemolar loci before m2. In contrast, fossil hyraxes erupt lower antemolars after m2. Although the early eruption of antemolars correlates with increased gestation time and age at maturity in primates and Tupaia (i.e., ‘Schultz's rule’), and although modern hyraxes resemble some anthropoid primates in exhibiting long gestation and eruption of antemolars at or before molars, eruption patterns do not significantly covary with either life history parameter among afrotherians sampled so far. However, we do observe a shift in eruption timing and crown height in Procavia relative to fossil hyracoids, mirroring observations recently made for other ungulate-grade mammals.
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Citation for this article: Asher, R. J., G. F. Gunnell, E. R. Seiffert, D. Pattinson, R. Tabuce, L. Hautier, and H. M. Sallam. 2017. Dental eruption and growth in Hyracoidea (Mammalia, Afrotheria). Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1317638.
Theria comprises all but three living mammalian genera and is one of the most ecologically pervasive clades on Earth. Yet, the origin and early history of therians and their close relatives (i.e., cladotherians) remains surprisingly enigmatic. A critical biological function that can be compared among early mammal groups is mastication. Morphometrics and modeling analyses of the jaws of Mesozoic mammals indicate that cladotherians evolved musculoskeletal anatomies that increase mechanical advantage during jaw rotation around a dorsoventrally-oriented axis (i.e., yaw) while decreasing the mechanical advantage of jaw rotation around a mediolaterally-oriented axis (i.e., pitch). These changes parallel molar transformations in early cladotherians that indicate their chewing cycles included significant transverse movement, likely produced via yaw rotation. Thus, I hypothesize that cladotherian molar morphologies and musculoskeletal jaw anatomies evolved concurrently with increased yaw rotation of the jaw during chewing cycles. The increased transverse movement resulting from yaw rotation may have been a crucial evolutionary prerequisite for the functionally versatile tribosphenic molar morphology, which underlies the molars of all therians and is retained by many extant clades.
Multiple mammalian lineages independently evolved a definitive mammalian middle ear (DMME) through breakdown of Meckel's cartilage (MC). However, the cellular and molecular drivers of this evolutionary transition remain unknown for most mammal groups. Here, we identify such drivers in the living marsupial opossum Monodelphis domestica, whose MC transformation during development anatomically mirrors the evolutionary transformation observed in fossils. Specifically, we link increases in cellular apoptosis and TGF-BR2 signalling to MC breakdown in opossums. We demonstrate that a simple change in TGF-β signalling is sufficient to inhibit MC breakdown during opossum development, indicating that changes in TGF-β signalling might be key during mammalian evolution. Furthermore, the apoptosis that we observe during opossum MC breakdown does not seemingly occur in mouse, consistent with homoplastic DMME evolution in the marsupial and placental lineages.
The sequence of eruption of the second generation of teeth varies across taxa, is highly functional, and is strongly influenced by genetic effects. We assessed postcanine dental eruption sequence across artiodactyls in order to test two hypotheses: 1) dental eruption sequence is a good phylogenetic character for artiodactyls; and, 2) eruption sequence is adaptive and associated with life history variables like postnatal growth and longevity in artiodactyls (Schultz’s Rule). We examined postcanine eruption sequence in 81 genera (100 species) spanning ten families of Artiodactyla. Our ancestral state reconstruction supports the interpretation that the third molar erupted last in the ancestor of Artiodactyla, and that the fourth premolar erupted after the third molar in the ancestor of Ruminantia. Our results indicate that eruption of the third molar last evolved secondarily in the caprines, likely sometime in the Miocene. Overall, our results support the hypothesis that dental eruption sequence is phylogenetically conserved in artiodactyls. Caprines occupy high elevation habitats, and we hypothesize that evolution of their unique dental eruption sequence may be associated with limited resource availability in high elevation mountain systems and the necessity to process a wide range of vegetation types.
The evolution of the mammalian jaw during the transition from non‐mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more‐efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont–mammaliaform transition have been debated for nearly a century.
We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography ( CT ) and digital reconstruction techniques to create three‐dimensional models of the jaw adductor musculature in a number of representative non‐mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution.
As an emerging consensus, current evidence suggests that the mammal‐like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in E ucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal M ammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri . Consequently, transformation of the jaw adductor musculature from the ancestral (‘reptilian’) to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint.
Background
The minute, finely-tuned ear ossicles of mammals arose through a spectacular evolutionary transformation from their origins as a load-bearing jaw joint. This involved detachment from the postdentary trough of the mandible, and final separation from the dentary through resorption of Meckel’s cartilage. Recent parsimony analyses of modern and fossil mammals imply up to seven independent postdentary trough losses or even reversals, which is unexpected given the complexity of these transformations. Here we employ the first model-based, probabilistic analysis of the evolution of the definitive mammalian middle ear, supported by virtual 3D erosion simulations to assess for potential fossil preservation artifacts. ResultsOur results support a simple, biologically plausible scenario without reversals. The middle ear bones detach from the postdentary trough only twice among mammals, once each in the ancestors of therians and monotremes. Disappearance of Meckel’s cartilage occurred independently in numerous lineages from the Late Jurassic to the Late Cretaceous. This final separation is recapitulated during early development of extant mammals, while the earlier-occurring disappearance of a postdentary trough is not. Conclusions
Our results therefore suggest a developmentally congruent and directional two-step scenario, in which the parallel uncoupling of the auditory and feeding systems in northern and southern hemisphere mammals underpinned further specialization in both lineages. Until ~168 Ma, all known mammals retained attached middle ear bones, yet all groups that diversified from ~163 Ma onwards had lost the postdentary trough, emphasizing the adaptive significance of this transformation.
Rich, T.H., Hopson, J.A., Gill, P.G., Trusler, P., Rogers-Davidson, S., Morton, S., Cifelli, R.L., Pickering, D., Kool, L., Siu, K., Burgmann, F.A., Senden, T., Evans, A.R., Wagstaff, B.E., Seegets-Villiers, D., Corfe, I.J., Flannery, T.F., Walker, K., Musser, A.M., Archer, M., Pian, R. & Vickers-Rich, P., June 2016. The mandible and dentition of the Early Cretaceous monotreme Teinolophos trusleri. Alcheringa 40, xx–xx. ISSN 0311-5518.The monotreme Teinolophos trusleri Rich, Vickers-Rich, Constantine, Flannery, Kool & van Klaveren, 199939.
Rich, T.H., Vickers-Rich, P., Constantine, A., Flannery, T.F., Kool, L. & van Klaveren, N., 1999. Early Cretaceous mammals from Flat Rocks, Victoria, Australia. Records of the Queen Victoria Museum and Art Gallery 106, 1–34.View all references from the Early Cretaceous of Australia is redescribed and reinterpreted here in light of additional specimens of that species and compared with the exquisitely preserved Early Cretaceous mammals from Liaoning Province, China. Together, this material indicates that although T. trusleri lacked a rod of postdentary bones contacting the dentary, as occurs in non-mammalian cynodonts and basal mammaliaforms, it did not share the condition present in all living mammals, including monotremes, of having the three auditory ossicles, which directly connect the tympanic membrane to the fenestra ovalis, being freely suspended within the middle ear cavity. Rather, T. trusleri appears to have had an intermediate condition, present in some Early Cretaceous mammals from Liaoning, in which the postdentary bones cum ear ossicles retained a connection to a persisting Meckel’s cartilage although not to the dentary. Teinolophos thus indicates that the condition of freely suspended auditory ossicles was acquired independently in monotremes and therian mammals. Much of the anterior region of the lower jaw of Teinolophos is now known, along with an isolated upper ultimate premolar. The previously unknown anterior region of the jaw is elongated and delicate as in extant monotremes, but differs in having at least seven antemolar teeth, which are separated by distinct diastemata. The dental formula of the lower jaw of Teinolophos trusleri as now known is i2 c1 p4 m5. Both the deep lower jaw and the long-rooted upper premolar indicate that Teinolophos, unlike undoubted ornithorhynchids (including the extinct Obdurodon), lacked a bill.Thomas H. Rich [trich@museum.vic.gov.au], Sally Rogers-Davidson [srogers@museum.vic.gov.au], David Pickering [dpick@museum.vic.gov.au], Timothy F. Flannery [tim.flannery@textpublishing.com.au], Ken Walker [kwalker@museum.vic.gov.au], Museum Victoria, PO Box 666, Melbourne, Victoria 3001, Australia; James A. Hopson [jhopson@uchicago.edu], Department of Organismal Biology & Anatomy, University of Chicago,1025 East 57th Street, Chicago, IL 60637, USA; Pamela G. Gill [pam.gill@bristol.ac.uk], School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, U.K. and Earth Science Department, The Natural History Museum, Cromwell Road, London SW7 5BD, UK; Peter Trusler [peter@petertrusler.com.au], Lesley Kool [koollesley@gmail.com], Doris Seegets-Villiers [doris.seegets-villiers@monash.edu], Patricia Vickers-Rich [pat.rich@monash.edu], School of Earth, Atmosphere and Environment, Monash University, Victoria 3800, Australia; Steve Morton [steve.morton@monash.edu], Karen Siu [karen.siu@monash.edu], School of Physics and Astronomy, Monash University, Victoria 3800, Australia; Richard L. Cifelli [rlc@ou.edu] Sam Noble Oklahoma Museum of Natural History, University of Oklahoma, Norman, OK 73072, USA; Flame A. Burgmann [flame.burgmann@monash.edu], Monash Centre for Electron Microscopy, 10 Innovation Walk, Monash University, Clayton, Victoria 3800, Australia; Tim Senden [Tim.Senden@anu.edu.au], Department of Applied Mathematics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australian Capital Territory 0200, Australia; Alistair R. Evans [alistair.evans@monash.edu], School of Biological Sciences, Monash University, Victoria 3800, Australia; Barbara E. Wagstaff [wagstaff@unimelb.edu.au], School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia; Ian J. Corfe [ian.corfe@helsinki.fi], Institute of Biotechnology, Viikinkaari 9, 00014, University of Helsinki, Finland; Anne M. Musser [anne.musser@austmus.gov.au], Australian Museum, 1 College Street, Sydney NSW 2010 Australia; Michael Archer [m.archer@unsw.edu.au], School of Biological, Earth, and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia; Rebecca Pian [rpian@amnh.org], Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, USA. Received 7.4.2016; accepted 14.4.2016.
We describe a new spalacotheriid (acute-angled) "symmetrodont" (Mammalia, Trechnotheria), Symmetrolestes parvus gen. et sp. nov., from the Lower Cretaceous, likely Barremian, Kitadani Formation of the Tetori Group, central Japan. The specimen consists of a fragmentary right lower jaw with first incisor and five preserved postcanine teeth (interpreted as p5-m4). Symmetrolestes has acute-angled molariforms with complete shearing surfaces on the para- and protocristids, and relatively tall crowns, features that are referable to Spalacotheriidae. Symmetrolestes is more derived than zhangheotheriids in having complete shearing surfaces, taller crowns, and more complete cingulids. It differs from other spalacotheriids in having fewer molariforms (m1-4), higher number of premolariforms (p1-5), and gradual transition between premolariforms and molariforms. Our cladistic analysis of 29 characters shows Symmetrolestes as the sister group of the remaining Spalacotheriidae. This node is supported by only one character (Bremer support: 1) and therefore not particularly stable. The remaining spalacotheriids are arranged in a fully pectinated tree conforming to the topology of the previous researchers, in which Spalacolestinae occupy an apical position. The combination of the occurrences of a primitive spalacotheriid, Symmetrolestes, in Japan and of Zhangheotheriidae, which is the sister taxon of Spalacotheriidae, in China suggests a possibility for an East Asian origin of Spalacotheriidae, although it implies long ghost lineages for the latest Jurassic to Early Cretaceous East Asian "symmetrodonts".
The Upper Cretaceous (Cenomanian) Mata Amarilla Formation in western Central Santa Cruz Province of Argentina has yielded fragmentary teeth of a large ?docodontan, an australosphenidan, a meridiolestidan (Amarillodon meridionalis gen. et sp. nov.), and a stem dryolestid (Treslagosodon shehuensis gen. et sp. nov.). These represent the first possible records for docodontans, Cretaceous australosphenidans, and stem dryolestids in South America. Both ?docodontan tooth fragments are unusually large and exhibit potential durophagous adaptation. The Amarillodon gen. nov. lower (?deciduous) posterior premolar has a trigonid angulation of 100° and is autapomorphic by a large and exoedaenodont distolabial accessory cusp on the distal cingulid. Both the mesial and distal cingulid are shelf-like. The mesiolabial upper molar fragment of the ausktribosphenid australosphenidan is similar to an upper M1 of aff. ?Bishops from the lower Albian (Lower Cretaceous) of Australia by its large stylar cusp ?C and breached paracone. The lower molar of the new stem dryolestid is characterized by a large talonid cusp d and a mesio-distally strongly compressed mesial root that is weaker than the distal one. The australosphenidan, if corroborated, suggests faunal interrelationships between Australia and South America by the late Early/early Late Cretaceous.
In non-mammalian synapsids and early mammals, evolutionary transformations in the feeding and hearing apparatuses are posited to have been prerequisites for the radiation of extant mammals. Unlike most vertebrates, including many early synapsids, mammals have precise dental occlusion, a lower jaw composed of one bone, and middle ear ossicles derived from ancestral jaw bones. We illuminate a related functional transition: therian mammals (eutherians and metatherians) evolved anteriorly directed chewing strokes, which are absent in other synapsid lineages. Anteriorly directed jaw movement during occlusion necessitates anteriorly directed muscle force vectors, and we posit that a shift in muscle orientation is reflected in the fossil record by the evolutionary appearance of a posteriorly positioned angular process in cladotherians (therians and their close kin). Anteriorly directed occlusion might have been absent in earlier synapsids because of the presence of attached middle ear elements in the posterior region of the jaw that prohibited the posterior insertion of jaw musculature. These changes to the masticatory apparatus in cladotherians likely permitted the evolution of novel masticatory movements, including grinding in both the anterior and medial directions (e.g., rodents and ungulates, respectively). Thus, this evolutionary transition may have been a critical prerequisite for the dietary diversification of therians.
This book summarizes the most relevant published paleontological information, supplemented by our own original work, on the record of Mesozoic mammals’ evolution, their close ancestors and their immediate descendants. Mammals evolved in a systematically diverse world, amidst a dynamic geography that is at the root of the 6,500 species living today. Fossils of Mesozoic mammals, while rare and often incomplete, are key to understanding how mammals have evolved over more than 200 million years. Mesozoic mammals and their close relatives occur in a few dozen localities from Argentina, Brazil, Chile, Bolivia, and Peru spanning from the Mid- Triassic to the Late Cretaceous, with some lineages surviving the cataclysmic end of the Cretaceous period, into the Cenozoic of Argentina. There are roughly 25 recognized mammalian species distributed in several distinctive lineages, including australosphenidans, multituberculates, gondwanatherians, eutriconodonts, amphilestids and dryolestoids, among others. With its focus on diversity, systematics, phylogeny, and their impact on the evolution of mammals, there is no similar book currently available.
Docodonta are one of the earliest diverging groups of mammaliaforms, and their morphology provides key information on the transition between non-mammalian cynodonts and Mammalia. We describe the partial skulls of two docodontans Borealestes serendipitus and Borealestes cuillinensis sp. nov. from the Kilmaluag Formation (Middle Jurassic: Bathonian), Isle of Skye, Scotland. We visualize their cranial anatomy using laboratory and synchrotron X-ray micro-CT. The skulls belong to two partial skeletons, currently comprising the most complete Mesozoic mammal fossils reported from the British Isles. The associated upper and lower dentitions show that the lower dentition of Borealestes is not diagnostic to species level. We establish, B. cuillinensis, based on upper molar characters, and re-identify upper molars previously assigned to ‘Borealestes’ mussettae as belonging to B. cuillinensis. ‘Borealestes’ mussettae, based on distinctive lower molars, is found to be morphologically and phylogenetically distinct from Borealestes, necessitating assignment to a new genus, Dobunnodon gen. nov. The skulls of Borealestes retain many plesiomorphic features seen in Morganucodon but absent in more crownward mammaliaforms. Our study highlights that generic and species taxonomy of docodontans are more reliable when based on both upper and lower teeth, while lower molar morphology may underrepresent the true diversity of Mesozoic mammaliaforms.
Making a mammalian ear
Mammals have keen hearing owing to their complex inner ear. In our vertebrate ancestors, as in extant reptiles, the three bones that make up the inner ear were instead part of the jaw. Understanding the functional transition of these bones is challenging given their small and delicate nature. Mao et al. describe a new genus and species of stem therian mammal represented by six well-preserved specimens, seemingly caught as they slept huddled together (see the Perspective by Schultz). The unprecedented preservation reveals a clear transitional stage between the two very different functions of the bones.
Science , this issue p. 305 ; see also p. 244
Schultz’s rule predicts early eruption of replacement teeth (incisors, canines, and premolars) relative to molars as growth slows and life history events take place over a greater span of time. Here, we investigate if the opposite trend might occur during the domestication process as a consequence of an accelerated life-history and driven by increased energetic needs. We provide new data on tooth eruption in four mammalian species and their domesticated forms: wolf and dog, polecat and ferret, bezoar and goat, wild boar and pig. Our results show some variation in eruption sequences between wild and domestic forms, but none that is consistent and reliably distinct from intraspecific variation. There may be variation in the absolute timing of dental eruption, but despite well documented changes across life history variables, which distinguish wild from domestic forms, eruption sequences remained constant in each wild and domestic version of the species we examined. A conserved eruption sequence is in accordance with many earlier studies, which found no evidence for Schultz’s rule in some wild clades of mammals. Phylogenetic conservation and functional factors likely play an important role in constraining patterns of growth and tooth eruption in these mammals. Furthermore, we suggest that the domestication processes started too recently for fundamental changes of tooth eruption sequences to occur.
Objectives:
Although a great deal is known about the biology of tooth development and eruption, there remains disagreement about the factors driving the evolution of dental eruption sequence. We assessed postcanine eruption sequence across a large sample of primates to test two hypotheses: (1) Dental eruption sequence is significantly correlated with life history and body size variables that capture postnatal growth and longevity (Schultz's Rule), and (2) Dental eruption sequence is conserved phylogenetically.
Materials and methods:
We assessed postcanine dental eruption sequence for 194 individuals representing 21 primate genera spanning eight families. With the inclusion of an additional 29 primate genera from the literature, this is the most comprehensive report on dental eruption sequence in primates to date. We used a series of phylogenetic analyses to statistically compare dental eruption sequence to life history and body size and test for phylogenetic signal in these traits.
Results:
Dental eruption sequence is conserved phylogenetically in primates, and body and brain size are both significantly associated with dental eruption sequence. Ancestral state reconstruction supports the hypothesis that the third molar erupted before one or more of the premolars in the ancestor of primates and derived clades within primates evolved an eruption sequence in which the third molar erupts after the premolars.
Discussion:
Schultz's Rule, as it is currently written and applied, is not supported by this extended data set. Our results demonstrate that dental eruption sequence is a far better predictor of phylogeny and will likely prove useful in phylogenetic hypotheses about relationships between extinct and extant mammalian taxa. The evolution of dental eruption sequence is likely driven by factors that significantly influence body size and mandibular symphyseal fusion.
We investigated dental homologies, development, and growth in living and fossil hyracoids, and tested if hyracoids and other mammals show correlations between eruption patterns, gestation time, and age at maturity. Unlike living species, fossil hyracoids simultaneously possess replaced P1 and canine teeth. Fossil species also have shorter crowns, an upper and lower I3 locus, an upper I2, and a hypoconulid on m3. Prenatal specimens of the living and show up to three tooth buds posterior to upper dI1 and anterior to the seven upper cheek teeth that consistently erupt; these include an anterior premolar but not a canine. Most lower cheek teeth finish eruption during growth in hyracoids, not after growth as in most other afrotherians. All hyracoids show the m1 at (lower) or near (upper) the beginning of eruption of permanent teeth; M3/m3 is the last permanent tooth to erupt. The living P. capensis erupts most lower antemolar loci before m2. In contrast, fossil hyraxes erupt lower antemolars after m2. While the early eruption of antemolars correlates with increased gestation time and age at maturity in primates and (i.e., "Schultz's Rule"), and while modern hyraxes resemble some anthropoid primates in exhibiting long gestation and eruption of antemolars at or before molars, eruption patterns do not significantly co-vary with either life history parameter among afrotherians sampled so far. However, we do observe a shift in eruption timing and crown height in relative to fossil hyracoids, mirroring observations recently made for other ungulate-grade mammals.
The size of the infraorbital foramen (IOF) is correlated with the size of the infraorbital nerve and number of mystacial vibrissae in mammals. Accordingly, IOF cross-sectional area has been used to infer both the rostral mechanoreceptive acuity and phylogenetic relationships of extinct crown primates and plesiadapiforms. Among living mammals, extant primates, scandentians, and dermopterans (Euarchonta) exhibit smaller IOF cross-sectional areas than most other mammals. Here we assess whether fossil adapoids, omomyoids, and plesiadapiforms show a reduction in relative IOF area similar to that characterizing extant euarchontans. The IOFs of 12 adapoid, 7 omomyoid, 15 plesiadapiform, and 3 fossil gliran species were measured and compared to a diverse extant mammalian sample. These data demonstrate that adapoids and omomyoids have IOFs that are similar in relative size to those of extant euarchontans. Conversely, IOFs of plesiadapiforms are on average about twice as large as those of extant euarchontans and are more comparable in size to those of extant non-euarchontan mammals. These results indicate that crown primates share a derived reduction in relative IOF size with treeshrews and colugos. Accordingly, a decreased reliance on the muzzle and an increased reliance on the hands for environmental exploration may have first evolved in the euarchontan stem lineage. However, the relatively large IOFs of plesiadapiforms imply a continued reliance on the muzzle for close exploration of objects. This finding may indicate that either parallel evolutionary decreases in IOF size occurred within Euarchonta or that plesiadapiforms lie outside the euarchontan crown group.
One of the major evolutionary transitions of the mammaliaform lineage was the origin of a typically mammalian pattern of growth. This is characterized by rapid juvenile growth followed by abrupt cessation of growth at adult size and may be linked with other important mammaliaform apomorphies of dental replacement and morphology. Investigation of growth patterns in the tritylodontid cynodont
Oligokyphus
and the basal mammaliaform
Morganucodon
provides insight into this crucial transition. We collected mandibular depth measurements from large samples of
Morganucodon
and
Oligokyphus
and constructed distributions of mandibular depth versus frequency for each species. These were compared with distributions from species from three different growth classes of extant amniote: testudines + crocodilians, mammals + birds, and lepidosaurs. Discriminant function analysis was used to differentiate between known growth classes by using different combinations of three measures of mandibular depth distribution shape (skew, kurtosis, and coefficient of variation) as proxies for different juvenile and adult growth patterns. Classification of the fossil species showed that
Morganucodon
closely resembled extant placental mammals in having rapid juvenile growth followed by truncated, determinate adult growth.
Oligokyphus
showed intermediate growth patterns, with more extended adult growth patterns than
Morganucodon
and slightly slower juvenile growth. This suggests a gradual evolution of mammalian growth patterns across the cynodont to mammaliaform transition, possibly with the origin of rapid juvenile growth preceding that of truncated, determinate adult growth. In turn, acquisition of both these aspects of mammalian growth was likely necessary for the evolution of diphyodont tooth replacement in the mammaliaform lineage.
The genus Morganucodon is found in Yunnan, China, in normal (non-karstic) sedimentary deposits (Lufeng beds) of probable Rhaetian age; and in Wales in karstic deposits in the Carboniferous Limestone. These latter deposits cannot be younger than Sinnemurian or older than Rhaetic. A new suborder-Morganucodonta-of the Triconodonta is created for Morganucodon and its allies. Morganucodon and Eozostrodon are not synonyms. The lower jaw of Morganucodon resembles closely that of an advanced cynodont, except for the presence of a squamosal-dentary joint in the former. There was no reduction in the functional importance of the reptilian (quadrate-articular) jaw-joint in passing from the cynodont condition to that of Morganucodon. The mechanism of shearing is discussed. The action of the cheek-teeth is pure shear. Primarily, the function of the squamosal-dentary articulation was to resist couples produced by the shearing, thus primitively, the presence of efficient shearing cheek-teeth is associated with a squamosal-dentary articulation. The skull of Morganucodon will be described in a later paper.
Ontogenetic analysis of the dentition in a wide range of marsupials and eutherians provides valuable criteria for assessing tooth homologies among extant therian mammals. In addition, these developmental data offer evidence for the loss of teeth during mammalian evolution. Such studies can distinguish the deciduous or successional nature of individual teeth, even when replacement does not occur postnatally. Ontogenetic analyses also provide evidence for the formation of vestigial deciduous teeth that do not erupt; these rudiments serve as valuable clues to the homologies of some tooth positions that have been lost during mammalian phylogeny. The continued accumulation of developmental data on the pattern of early budding and differentiation of deciduous tooth germs offers useful criteria for identifying tooth class homologies of highly modified teeth, in addition to their bony and occlusal relationships.
The dryolestoid mammal Dorsetodon haysomi gen. et sp. nov. is described from the Purbeck Limestone Group (Lower Cretaceous) of southern England, on the basis of lower molars. Dorsetodon is assigned to the Paurodontidae, a family of Theria previously known only from North America. The distinction between Paurodontidae and Henkelotheriidae (from the Upper Jurassic of Portugal), although maintained for lack of solid contrary data, is argued to have been based on variable or subjective characters. A further small mammal, Chunnelodon alopekodes gen. et sp. nov., representing an undetermined cladothenan family, is also described from lower molar teeth. The non-procumbent paraconid on the lower molar places Chunnelodon as a sister-taxon to the Laurasian Dryolestoidea.
Three new genera of dryolestoid mammals from the Late Cretaceous of Patagonia are based on isolated molars. Two of them plus Casamiquelia BONAPARTE are referred to the Casamiqueliidae n. fam. The current state of knowledge of the Los Alamitos Theria and the relationships of "Barberenia" with dryolestoids are discussed. The Los Alamitos fauna is the only assemblage of mammals recorded in the Late Cretaceous of Gondwana. There is no current evidence to consider that this fauna evolved in isolation.