Skull Roof Research Articles

An Eocene conger eel (Teleostei, Anguilliformes) from the Lillebælt Clay Formation, Denmark

A conger eel (Anguilliformes, Congridae) is described from the lower Lutetian concretionary nodules of the Lillebælt Clay Formation exposed at Trelde Næs, eastern Jutland, based on two partially complete articulated cranial skeletons. One of the cranial specimens exhibits an otolith void from which a cast was taken, used by Schwarzhans (2007) to describe the extinct Pseudoxenomystax treldeensis, which is placed herein within the new genus Smithconger gen. nov. Smithconger treldeensis (Schwarzhans, 2007) is characterized by well-developed lateral processes on the frontals, supraoccipital crest absent, sphenotic spine rather large, anteriorly pointed and exposed on the flattened surface of the skull roof, otic bullae considerably reduced, maxilla almost straight and distally pointed, maxillary and dentary teeth numerous and arranged in multiple rows, dentary with slightly convex ventral profile, opercle with smooth posterior margin and subopercle short. The otoliths of Smithconger treldeensis show high dorsal rim, broad and deep dorsal depression, no ventral furrow, sulcus straight, shallow, centrally positioned with anteriorly reduced colliculum, and ostial channel at anterior tip of colliculum short, not reaching the predorsal rim. The otolith-based species Bathycongrus waihaoensis Schwarzhans, 2019 from the Kaiatan (Bartonian/Priabonian) of New Zealand is also assigned to the genus Smithconger. Smithconger is tentatively referred to the congrid subfamily Congrinae due to the lack of hypohyals in the hyoid bar. This new Eocene genus of conger eel shows a certain degree of similarity with the extant Bassanago. The diversity and relationships of other Eocene congrids is also briefly discussed.

An Eocene conger eel (Teleostei, Anguilliformes) from the Lillebælt Clay Formation, Denmark

A conger eel (Anguilliformes, Congridae) is described from the lower Lutetian concretionary nodules of the Lillebælt Clay Formation exposed at Trelde Næs, eastern Jutland, based on two partially complete articulated cranial skeletons. One of the cranial specimens exhibits an otolith void from which a cast was taken, used by Schwarzhans (2007) to describe the extinct Pseudoxenomystax treldeensis, which is placed herein within the new genus Smithconger gen. nov. Smithconger treldeensis (Schwarzhans, 2007) is characterized by well-developed lateral processes on the frontals, supraoccipital crest absent, sphenotic spine rather large, anteriorly pointed and exposed on the flattened surface of the skull roof, otic bullae considerably reduced, maxilla almost straight and distally pointed, maxillary and dentary teeth numerous and arranged in multiple rows, dentary with slightly convex ventral profile, opercle with smooth posterior margin and subopercle short. The otoliths of Smithconger treldeensis show high dorsal rim, broad and deep dorsal depression, no ventral furrow, sulcus straight, shallow, centrally positioned with anteriorly reduced colliculum, and ostial channel at anterior tip of colliculum short, not reaching the predorsal rim. The otolith-based species Bathycongrus waihaoensis Schwarzhans, 2019 from the Kaiatan (Bartonian/Priabonian) of New Zealand is also assigned to the genus Smithconger. Smithconger is tentatively referred to the congrid subfamily Congrinae due to the lack of hypohyals in the hyoid bar. This new Eocene genus of conger eel shows a certain degree of similarity with the extant Bassanago. The diversity and relationships of other Eocene congrids is also briefly discussed.

The dissorophoid temnospondyl Parioxys ferricolus from the early Permian (Cisuralian) of Texas

AbstractParioxys ferricolus Cope, 1878 is a long-neglected taxon of dissorophid temnospondyls from the early Permian (Cisuralian) of Texas. Reexamination of the original material and preparation of a previously undescribed specimen shed some light on the ontogeny and reveal numerous eucacopine features. P. ferricolus is characterized by: (1) a preorbital region 1.5 times as long as the posterior skull table in juveniles and twice as long in adults, (2) a parietal only two-thirds the length of the frontal, (3) interpterygoid vacuities shorter than half the skull length, (4) choanae posteriorly extended, and (5) vomer with enlarged, V-shaped median depression. P. ferricolus is found nested with Kamacops acervalis Gubin, 1980, with which it shares synapomorphies in the palate, within the dissorophid clade Eucacopinae.

Skeletal Transformations and the Origin of Baleen Whales (Mammalia, Cetacea, Mysticeti): A Study on Evolutionary Patterns

A review of the morphological patterns exhibited by all the main radiations of mysticete (baleen whale) cetaceans provided a broad assessment of the fundamental morphological transformations that occurred in the transition to the Mysticeti clade. Skull and postcranial characters were illustrated, described and compared, and their distribution was mapped on a combined phylogeny in the search for morphological support for the principal mysticete clades (i.e., Mysticeti, Chaeomysticeti and Balaenomorpha). In particular, characters of the skull (rostrum, vertex, temporal fossa, tympanic bulla and dentary) and the postcranial appendicular skeleton (scapula, humerus, radius and ulna) were all involved at different degrees in the process of morphological transformations leading to the modern-day mysticetes. Apart from a few typical characteristics of the rostrum that were already present in the earliest-diverging mysticetes (presence of lateral process of the maxilla, presence of multiple dorsal infraorbital foramina, thin lateral border of maxilla and presence of mesorostral groove), most of the other anatomical districts were unaffected by the transition so the earliest mysticetes show a number of archaeocete characters in the tympanic bulla, dentary and skull roof. The analysis of the whole dataset supported the hypothesis that the origin and evolution of mysticetes constituted a step-wise process and that the bauplan of the modern-day mysticetes was fully assembled at the level of the common ancestor of all Balaenomorpha.

Nusaviichthys nerivelai gen. et sp. nov., an Albian crossognathiform fish from the Tlayúa lagerstätte, Mexico

The fish Nusaviichthys nerivelai gen. et sp. nov. is described here based on well-preserved fossils from the Albian deposits of the Tlayúa quarry, near Tepexi de Rodríguez, Puebla, central Mexico. The occurrence of this new fish from Tlayúa expands the taxonomical and morphological diversity of the crossognathiformes of the Caribbean region of America during the Early Cretaceous. The inclusion of the osteological features of this species into comparative and phylogenetic analyses reveal its belonging to the order Crossognathiformes, suborder Pachyrhizodontoidei, and family Notelopidae; some of the most significant of these features include the presence of a large extrascapula bone; circumorbital bones forming a completely closed ring; medioparietal condition of the skull; posterior infraorbitals hypertrophied, covering the entire cheek, and extended up to the anterior part of the opercular bones; the relative short uroneural 1 anteriorly restricted to the preural 1; and the cycloid scales with a crenulated posterior edge. This new species differs from its relatives in the presence of numerous and small conic teeth in both jaws and vomer; the large triangular gular plate, and the numerous radiating grooves ridges present in the supraorbital, dermosphenotic, skull roof bones, opercular bones, and jaws; and the almost horizontal position of the neural and hemal spines of the last caudal centra. Despite the weak support, the naturalness of this extinct teleost order is recovered in the phylogenetic analysis performed here, which includes this new Mexican fossil fish as well as other crossognathiform genera not included in previous studies.

New Record of Capybara (Rodentia: Caviidae: Hydrochoerinae) from the Pleistocene of San Diego County, California with Remarks on Their Biogeography and Dispersal in the Pleistocene of Western North America

We describe a new species of capybara from late Pleistocene deposits (Rancholabrean NALMA) in northern San Diego County, California, USA which tentatively dates to Marine Isotope Stage (MIS) 5 interglacial (~130 ka to 80 ka). The specimen represents a new species of Hydrochoerus based on morphological characters of the upper incisor (I1) and the upper (maxillary) third molar (M3). Hydrochoerus hesperotiganites sp. nov. differs from other described species of Hydrochoerus in its larger size, wider skull roof, more robust zygomatic process of the maxilla and descending zygomatic process of the lacrimal and in details of the otic region. The new species is the only confirmed record of fossil Hydrochoerus in North America and is the northwestern-most record of any capybara in North America. All previous records of fossil capybara from North America represent one of two extinct genera, Neochoerus or Phugatherium. Northward dispersal of capybaras from central and southern México probably occurred along the coasts of Sinaloa and Sonora, entering the north or northeast flowing drainages which entered the Gulf of California, then further north into the San Simon drainage to the Gila River and ultimately into the Colorado River, or directly northward along the coast of Sonora to the mouth of the Colorado River.lsid: urn:lsid:zoobank.org:pub:BD2DA9EB-32EF-4761-8551-E585D6AF317C

Ontogenetic variation in the skull of Stenopterygius quadriscissus with an emphasis on prenatal development

The availability of a large sample size from a range of ontogenetic stages makes Stenopterygius quadriscissus a good model to study ontogenetic variation in a fossil sauropsid. We qualitatively examined pre- and postnatal ontogenetic changes in the cranium of S. quadriscissus. The prenatal ossification sequence is similar to other diapsids, exhibiting delayed chondrocranial ossification compared to the dermatocranium. In the dermatocranium, the circumorbital area is more ossified earlier in development relative to other elements, especially those of the skull roof where ossification is comparatively weaker across prenatal stages. Perinatally all cranial elements are ossified, and many scarf and step joints are already closed. We propose four prenatal and three postnatal stages in S. quadriscissus on the basis of relative ossification, size and qualitative cranial characters pertaining to the jugal, parietal, frontal, pterygoid and surangular. These will provide a basis for determining ontogenetic stages in other ichthyosaurs. Moreover, our postnatal observations aid in refining ontogenetic characters for phylogenetic studies. Lastly, we observed that the antimeric sutures of the midline of the skull roof are open perinatally and that fusion of the midline only appears in the adult stage. We hypothesize that the loose connection of the midline functions as a fontanelle, limiting potential damage during birth.

ANATOMICAL VARIATIONS OF THE PARIETAL FORAMEN AND ITS RELATIONS TO THE CALVARIAL LANDMARKS: A CROSS-SECTIONAL CADAVERIC STUDY.

The aim: Estimate the prevalence of the parietal foramen in the adult human skulls of Ukrainian origin, and study its morphology and relationships to main anatomical landmarks of the skull. Materials and methods: A cross-sectional observational study of PF was conducted with 42 random cadaveric adult human skull roofs (calvaria) collected from the laboratory and museum of Human Anatomy Department, Kharkiv National Medical University, Ukraine. The patency and the length of the PF canal were determined, and PF external/internal diameters and the distance to the calvarial landmarks from PF were measured using the caliper. Mean and standard deviation were calculated to compare with the existing data. Results: In the present study 85.7% (n = 36) of the calvaria had the PF, 54.8% (n = 23) had bilateral location of PF, 30.9% (n = 13) had unilateral presence of PF (right side: 23.8%, n=10 and left side: 7.1%, n=3), and 14.3 % (n = 6) demonstrated bilateral absence of PF. Conclusions: An anatomical variation in parietal foramen is not uncommon, and the differences can be based on multiple factors like geography and race. It is important to have detailed information on anatomical variations in different population groups to facilitate surgical and radiological interventions.

A compsemydid turtle from the Upper Cretaceous of Var, southern France

Calissounemys matheroni gen. et sp. nov. (Testudines) is described on the basis of a skull and shell elements from the Upper Cretaceous of Var, southern France. This new taxon is assigned to the family Compsemydidae and characterized by a thick-boned, robust skull, a shallow temporal emargination, a crista supraoccipitalis not extending beyond the posterior edge of the skull roof, large nasals meeting along the midline for their full length; frontals retracted from the orbital margin, absence of a cheek emargination, a large jugal forming a substantial part of the orbital margin, absence of a secondary palate and an uneven upper triturating surface; and the shell with vertebral 1 clearly wider than vertebrals 2-3, with the lateral margins strongly divergent towards the anterior border and wider than long vertebrals 2-3. This find increases the diversity of the Late Cretaceous turtle fauna from southern France, and fills a stratigraphical gap in the fossil record of Compsemydidae between the Early Cretaceous and the Paleocene in Europe.

Scalp Angiosarcoma with Skull Roof Infiltration and Metastasis to the Lung.

Scalp Angiosarcoma with Skull Roof Infiltration and Metastasis to the Lung.

The contribution of ontogenetic growth trajectories on the divergent evolution of the crocodylian skull table.

To explore shape variability among crocodylian skull tables, an analysis using geometric morphometric methods is conducted with the inclusion of extant and fossil taxa. Skull tables are variable and the differences likely play a role in hydrodynamics, species recognition, and biomechanical adaptations. Comparisons of allometric change within taxa are explored revealing that adults significantly diverge from juvenile skull table morphologies in most species and these changes happen in a stereotyped way. In all analyses, adults of the smallest extant taxa plot alongside the juveniles of related taxa and heterochrony may explain the maintenance of these morphologies into adulthood. When landmarks representing the supratemporal fenestrae are included, longirostrine taxa are broadly separated from one another due to variation in the size of the supratemporal fenestrae. The hypotheses of previous studies suggesting that the size of the supratemporal fenestrae is influenced by snout length-with longer snouts corresponding to larger fenestrae-must be re-evaluated. Although species of the crocodyloids Tomistoma and Euthecodon approach or exceed the length of the snout in gavialoids, their supratemporal fenestrae are proportionally smaller-this suggests a phylogenetic constraint in crocodyloids regardless of snout length.

Developmental origins of the crocodylian skull table and platyrostral face.

The dorsoventrally flattened skull typifies extant Crocodylia perhaps more than any other anatomical feature and is generally considered an adaptation for semi-aquatic feeding. Although the evolutionary origins of caniofacial flattening have been extensively studied, the developmental origins have yet to be explored. To understand how the skull table and platyrostral snout develop, we quantified embryonic development and post-hatching growth (ontogeny) of the crocodylian skull in lateral view using geometric morphometrics. Our dataset (n=103) includes all but one extant genus and all of the major ecomorphs, including the extremely slender-snouted Gavialis and Tomistoma. Our analysis reveals that the embryonic development of the flattened skull is remarkably similar across ecomorphs, including the presence of a conserved initial embryonic skull shape, similar to prior analysis of dorsal snout shape. Although differences during posthatching ontogeny are recovered among ecomorphs, embryonic patterns are not distinct, revealing an important shift in developmental rate near hatching. In particular, the flattened skull table is achieved by the end of embryonic development with no changes after hatching. Further, the rotation of skull roof and facial bones during development is critical for the stereotypical flatness of the crocodylian skull. Our results suggest selection on hatchling performance and constraints on embryonic skull shape may have been important in this pattern of developmental conservation. The appearance of aspects of cranial flatness among Jurassic stem crocodylians suggests key aspects of these cranial developmental patterns may have been conserved for over 200 million years.

Revised Description of the Early Permian Recumbirostran “Microsaur” Nannaroter mckinziei Based on New Fossil Material and Computed Tomographic Data

The early Permian Richards Spur locality of Oklahoma has produced abundant material of numerous terrestrial fossil tetrapods, including various “microsaurs,” several of which are considered to belong to the clade Recumbirostra. We present a new partial skull of the recumbirostran “microsaur”Nannaroter mckinziei; through computed tomography (CT) analysis of both this new specimen and the holotype, we provide an updated description of the taxon. This new description provides novel information regarding several regions that could not be examined previously due to either being absent in the holotype or difficult to access. This includes missing and obscured aspects of the skull roof, braincase, lower jaw, and the palatal region. Furthermore, the new information obtained from this description was used to update phylogenetic character codings ofNannaroter, and a revised phylogenetic analysis was conducted. The results of this updated analysis are congruent with those of other recent phylogenetic analyses of recumbirostran “microsaurs.” This new information adds to the ever-growing body of early tetrapod CT data, which has been, and will continue to be, important in revealing details regarding early tetrapod anatomy, interrelationships, paleoecology, and evolution.

Morphological disparity in the evolution of the ophthalmosaurid forefin: new clues from the Upper Jurassic of Argentina

AbstractJurassic and Cretaceous marine deposits worldwide show that ophthalmosaurid ichthyosaurs are major components of marine tetrapod communities for almost 76 myr. One of the major features characterizing this clade is the complexity and diversity of their stylopodium–zeugopodium morphology. Upper Jurassic deposits of the Vaca Muerta Formation in north‐west Patagonia (Argentina) have yielded the richest Tithonian ophthalmosaurid records from Gondwana. Here, we present a new ophthalmosaurid from this lithostratigraphic unit, Sumapalla argentina gen. et sp. nov., recognized based on several unique features of the skull roof, forefin, scapular, and pelvic girdles. On phylogenetic analysis it was recovered as a basal member of Platypterygiinae. The peculiar morphology of the forefin of the new taxon indicates a previously unnoticed morphological diversity in the ophthalmosaurid appendicular skeleton and provides an excellent opportunity to test its range of variation. For this purpose, we implemented a holistic approach, using 2D geometric morphometric analysis focused on the humerus, and disparity analysis of the zeugopodium. Our results reveal a long duration of one humeral morphotype characterizing Arthropterygius spp. and closely related taxa. Furthermore, we found a peak of zeugopodial disparity during the Kimmeridgian, followed by a decrease by the end of the Jurassic, and then a recovery by the Early Cretaceous.

First definitive abelisaurid theropod from the Late Cretaceous of Northwestern Argentina

ABSTRACT The fossil record of abelisaurid theropods in South America is mostly limited to Brazil and Argentina. In Argentina, abelisaurids are generally known from Patagonia, where their record is relatively abundant and includes well-known and complete specimens. However, for North-western Argentina, abelisaurids are represented by incomplete and isolated bones and teeth that remain largely unpublished. The aim of this contribution is to report a nearly complete abelisaurid braincase from the Late Cretaceous Los Blanquitos Formation (Campanian), Amblayo Valley, Salta province, Argentina. The specimen shows plesiomorphic features for abelisaurids, including a thin skull roof, absence of skull projections like horns or bulges, and low and narrow parietal eminence that lie at the same level as the sagittal crest. Furthermore, the specimen possesses some autapomorphies that support its status as a new taxon and its small size allows it to be assigned as one of the smallest abelisaurids recorded up to date. The finding of this specimen constitutes the first unequivocal occurrence of an abelisaurid in Northwestern Argentina and brings new evidence concerning the geographic distribution of the clade during Late Cretaceous times in South America.

On the “cartilaginous rider” in the endocasts of turtle brain cavities

Abstract In recent years, paleoneurology became a very popular research field and hundreds of brain-endocasts were described. The interpretation of a dorsal protuberance of the brain-endocast puzzled researchers for a long time, the so-called (cartilaginous) rider. This is mainly because of technical limitations in the past and due to non-accessibility of comparative material. Using turtles as a case-study, we conducted a literature review and studied embryological data in addition to fossil and extant species’ endocasts. We assessed three hypotheses on the origin of the rider as relating to 1) the pineal gland, to 2) the blood vessel system, and to 3) skull roof elements. Based on our integrated anatomical observations, we refute the pineal gland hypothesis (1) and an exclusive blood vessel explanation (2). However, we show that, in most cases, the cartilaginous origin applies (3). The related cartilages, mainly the anterior process of the chondrocranial tectum synoticum, can persist until adulthood. Its diversity is interpreted in regard to the mechanical support for the temporal skull region, the shape of which has been shown to be in turn related to neck retraction and jaw mechanics. Finally, we highlight the value of embryological data to provide profound hypotheses for evolutionary research despite its low quantitative evaluability. We argue that it should be studied in conjunction with modern computer-aided data acquisition whenever possible.

A NEW CRANIUM OFCROCODYLUS ANTHROPOPHAGUS FROM OLDUVAI GORGE, NORTHERN TANZANIA

Olduvai Gorge (northern Tanzania) is one of the best known and most iconic palaeontological and archaeological sites in the world. In more than a century of research it has yielded an impressive record of fossils and stone tools which stands as a compendium of human evolution in the context of environmental changes of East Africa in the last 2 Ma. Recent field work in the DK site at Olduvai lead to the retrieval of a partial crocodile cranium nicknamed Black Sun because it was discovered during an annular solar eclipse. The specimen is here described and compared with extinct and extant African crocodylids. The new cranium can be referred to Crocodylus anthropophagus, a Pleistocene species hitherto found only in Olduvai Gorge. Thanks to the good preservation of the skull table, its morphology is here characterised for the first time. Black Sun represents to date the earliest (ca. 1.9–1.85 Ma) and the most informative cranium of C. anthropophagus in the fossil record. Our phylogenetic analysis supports a strict relationship between C. anthropophagus and Crocodylus thorbjarnarsoni, a large species from the Plio-Pleistocene of the Turkana Basin (Kenya). These two sister taxa share a combination of characters which places them at the base of Crocodylus, providing an intriguing element to the debate on the African or extra-African origin of this genus.

Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications.

Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 was described on the basis of a single fossil excavated near Besano (Italy) nearly three decades ago. Here, we re-examine its cranial osteology and assign five additional specimens to B. leptorhynchus, four of which were so far undescribed. All of the referred specimens were collected from the Middle Triassic outcrops of the Monte San Giorgio area (Italy/Switzerland) and are housed in various museum collections in Europe. The revised diagnosis of the taxon includes the following combination of cranial characters: extreme longirostry; an elongate frontal not participating in the supratemporal fenestra; a prominent ‘triangular process’ of the quadrate; a caudoventral exposure of the postorbital on the skull roof; a prominent coronoid (preglenoid) process of the surangular; tiny conical teeth with coarsely-striated crown surfaces and deeply-grooved roots; mesial maxillary teeth set in sockets; distal maxillary teeth set in a short groove. All these characters are shared with the holotype of Mikadocephalus gracilirostris Maisch & Matzke, 1997, which we consider as a junior synonym of B. leptorhynchus. An updated phylogenetic analysis, which includes revised scores for B. leptorhynchus and several other shastasaurids, recovers B. leptorhynchus as a basal merriamosaurian, but it is unclear if Shastasauridae form a clade, or represent a paraphyletic group. The inferred body length of the examined specimens ranges from 1 m to about 8 m. The extreme longirostry suggests that B. leptorhynchus primarily fed on small and elusive prey, feeding lower in the food web than an apex predator: a novel ecological specialisation never reported before the Anisian in a large diapsid. This specialization might have triggered an increase of body size and helped to maintain low competition among the diverse ichthyosaur fauna of the Besano Formation.

Braincase anatomy of the Paleocene crocodyliform Rhabdognathus revealed through high resolution computed tomography.

Dyrosaurids were highly specialized, largely marine, relatives of living crocodylians, and one of the few archosaur lineages to survive the K-Pg extinction. Dyrosaurids lived during the Cretaceous to the Eocene and represent a unique combination of morphology and ecology not seen in living crocodylians. Little is known about their endocranial anatomy, leaving many questions about their neurosensory adaptations unaddressed. Recently, µCT (micro-computed tomography) scans were made of a well-preserved skull of Rhabdognathus, a Paleocene dyrosaurid from Mali. This marks the first time the braincase and neurosensory features of a dyrosaurid have been examined using CT. We focus our attention to three specific internal structures: the cranial endocast; the inner ear; and the paratympanic sinuses. The cranial endocast of Rhabdognathus revealed novel features including a unique conformation of its paratympanic system, a prominent dorsal venous system that communicates with the external skull table, extremely enlarged tympanic vestibules that meet at the midline of the endocranium, a prominent spherical cerebrum, and elongate olfactory tracts accounting for half the total endocast length. The bizarre laterally facing lateral Eustachian foramen of dyrosaurids is now understood to be a complex fossa including both a ventrally directed lateral Eustachian foramen and a laterally directed foramen for the basioccipital diverticulum. A novel median pterygopharyngeal canal was discovered connecting the pharynx to the adductor chamber. These revelations require a reinterpretation of the associated external foramina visible on the posterior of the skull in dyrosaurids and potentially their close relatives the pholidosaurids. The olfactory tract terminates in an enlarged olfactory region possessing complex bony projections—a unique morphology perhaps serving to increase surface area for olfaction. The inner ear of Rhabdognathus exhibits characteristics seen in both Pelagosaurus and Gavialis. The vestibule is spherical, as in Gavialis, but is significantly expanded. The semicircular canals are enlarged but pyramidal in shape as in the thalattosuchian Pelagosaurus. The proportion of the cochlear length to total endosseous labyrinth height is roughly 0.5 in Rhabdognathus implying that the hearing capabilities resemble that of thalattosuchians. A suite of expanded sense organs (e.g., bony olfactory lamina; hypertrophied vestibule of the inner ear), and the clear expansion of the cerebrum to a more symmetrical and spherical shape suggest that dyrosaurids possess neuroanatomical modifications facilitating an agile predatory near-shore ecology.

Embryonic origins of the flattened skull table and snout in Crocodylia

Major groups of vertebrates, such as mammals, birds, and crocodylians, are recognizably distinct due to unique anatomical, physiological, and behavioral characteristics. The origins and ecological consequences of these features have been the focus of intense research, including more recently the developmental mechanisms facilitating their evolution. The dorsoventrally flattened snout and skull table typify extant Crocodylia perhaps more than any other anatomical feature. The origins and ecological consequences of the crocodylian skull have been the focus of extensive study. While it is generally recognized as an adaptation for feeding at the water's surface, the developmental origins of craniofacial flattening have yet to be explored. To understand how the skull table and platyrostral snout develop, we quantified the embryonic development and post-hatching growth (ontogeny) of the crocodylian skull in lateral view using geometric morphometrics. Our dataset (n=103) includes all but one extant genus and all of the major ecomorph groups, including the rare and extremely slender-snouted Gavialis and Tomistoma. Our ontogenetic morphospace captures similar variation in snout length as have been previously found for the ontogeny of dorsal snout shape, including the presence of a conserved embryonic skull shape for all but the two most extremely slender-snouted forms. Comparisons of ontogenetic trajectories revealed that by the end of embryonic development the adult condition of a dorsally flat skull table is achieved in all species and that ecomorphs did not differ in their pattern of skull roof ontogeny. The ontogenetic flattening of the snout did show post-hatching differences among ecomorphs, likely related to differences in the length and depth of the snout, but not during embryonic development. These anatomical changes correlate with recently described molecular and cellular processes that pattern the temporal and facial form, allowing the identification of developmental mechanisms which potentially facilitated the origin of these apomorphic features. These findings demonstrate that the iconic flattened skull table and platyrostral face of Crocodylia are the result of a conserved pattern of shape changes during embryonic development. Considering that these features appear by the Early Jurassic among extinct stem crocodylians, it is likely that these aspects of cranial development have been conserved for over 200 million years.