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Previously known only from isolated teeth and lower jaw fragments recovered from the Cretaceous and Palaeogene of the Southern Hemisphere, the Gondwanatheria constitute the most poorly known of all major mammaliaform radiations. Here we report the discovery of the first skull material of a gondwanatherian, a complete and well-preserved cranium from Upper Cretaceous strata in Madagascar that we assign to a new genus and species. Phylogenetic analysis strongly supports its placement within Gondwanatheria, which are recognized as monophyletic and closely related to multituberculates, an evolutionarily successful clade of Mesozoic mammals known almost exclusively from the Northern Hemisphere. The new taxon is the largest known mammaliaform from the Mesozoic of Gondwana. Its craniofacial anatomy reveals that it was herbivorous, large-eyed and agile, with well-developed high-frequency hearing and a keen sense of smell. The cranium exhibits a mosaic of primitive and derived features, the disparity of which is extreme and probably reflective of a long evolutionary history in geographic isolation. The gondwanatherians were mammals known only from teeth and some jaw fragments that lived in the southern continents alongside dinosaurs; here the entire cranium of a bizarre and badger-sized fossil mammal from the Cretaceous of Madagascar shows that gondwanatherians were related to the better-known multituberculates, a long-lived and successful group of now-extinct rodent-like mammals. Anatomy of a Gondwana mammal The gondwanatheres were mammals that lived the southern continents alongside the dinosaurs during the Late Cretaceous and early Paleocene. Known only from a few teeth and some jaw fragments, their appearance and evolutionary relationships remained obscure. The entire skull of a bizarre and badger-sized fossil mammal from the Cretaceous of Madagascar changes all that. Although almost certainly highly derived — as one would expect from a member of the unique endemic island fauna of Madagascar at that time — Vintana is clearly a gondwanathere. The anatomy of the herbivorous, large-eyed and agile creature shows that gondwanatheres were related to the better-known multituberculates, a long-lived and successful group of (now also extinct) rodent-like mammals.

Many recent studies have detailed the morphology of archosaurian endocrania. However, the outgroup to Archosauria, Phytosauria, has yet to be studied with modern techniques that would allow reconstruction of their internal anatomy. Pseudopalatus mccauleyi is a derived phytosaur from the Upper Triassic Chinle Formation, Arizona. A skull of P. mccauleyi, USNM 15839, was imaged using computed tomography in order to create the first high-quality, digitally reconstructed phytosaur endocast. Pseudopalatus mccauleyi exhibits overall endocast morphology that is similar to that of an extant crocodylian. These clades, phytosaurs and extant crocodylians, exhibit convergent Baupläne and similar inferred ecologies. A notable difference between the endocasts of the two clades is a considerable dural expansion in P. mccauleyi that denotes a large pineal body. This expansion, and the overall morphology of the endocast, is consistent with the historic endocranial reconstructions of the phytosaurs Pseudopalatus buceros, Smilosuchus gregorii, and Parasuchus hislopi. A comparison with phylogenetically diverse archosaurian endocasts reveals that endocast morphologies are highly conserved within Pseudosuchia, regardless of Bauplan or ecology. This conservatism is in contrast to the diversity of endocast morphology observed within Theropoda and Sauropodomorpha, or between members of those clades and Pseudosuchia. The most pronounced variability in pseudosuchian endocast morphology is a trend in size reduction of the pineal region, from a large basal condition to a reduced derived condition wherein the pineal region is indistinguishable from the rest of the endocast. A similar trend in pineal reduction is also seen in theropods and sauropods. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline/UJVP

The Gondwanatheria are an enigmatic clade of Cretaceous and Paleogene mammals known from South America, Africa, Madagascar, India, and the Antarctic Peninsula. The eight valid species—each belonging to a monotypic genus and the first of which was described only 30 years ago—are represented almost exclusively by isolated teeth, in addition to fragmentary dentaries attributed to Sudamerica, Gondwanatherium, Ferugliotherium, and an unnamed taxon from Tanzania. No cranial (skull exclusive of lower jaw) or postcranial material has heretofore been assigned to the Gondwanatheria, a severe limitation that has precluded a comprehensive assessment of phylogenetic affinities. Here we describe, in detail, the first cranial specimen of a gondwanatherian mammal. This material consists of a complete and wellpreserved cranium of the sudamericid Vintana sertichi, recovered from the Upper Cretaceous (Maastrichtian) Maevarano Formation in the Mahajanga Basin of northwestern Madagascar. Salient features of the cranium include elongate, scimitarlike jugal flanges, huge orbits, strong klinorhynchy, and a vaulted nuchal region. Micro-computed tomography greatly facilitated the delineation of sutures and the description of internal morphology. The cranial features of Vintana are compared with those of a broad range of synapsids, with particular concentration on other Mesozoic mammaliaforms. The cranium of Vintana exhibits a mosaic of extremely primitive and extremely derived features. It is the second largest known for a Mesozoic mammaliaform, superseded only by that of the eutriconodontan Repenomamus giganticus from the Early Cretaceous of China. Vintana is the largest known Late Cretaceous mammaliaform; it is also the largest known Mesozoic mammaliaform from Gondwana.

Crurotarsi are a clade of archosauromorphs ranging in age from the Middle Triassic to Recent that includes two semiaquatic, faunivorous subclades: Crocodylia and the predominantly late-Triassic Phytosauria. Phytosaurs and crocodylians exhibit generally similar overall body morphology, and each exhibits a range of narrow to broad rostral cranium morphotypes. These morphological similarities lead to the commonly adopted hypothesis that the two clades exhibited a number of ecological and behavioral similarities. One such hypothesized ecological similarity is that phytosaurs utilized a range of food item types that was the same as that utilized by extant crocodylians. In particular, phytosaurs possessing slender rostra with a high aspect ratio were previously hypothesized to have been strictly or primarily piscivorous, much like extant crocodylians with slender, high aspect ratio rostra that have been described as piscivorous. However, a review of available literature reporting on direct observations and other dietary data in extant crocodylians revealed that no extant crocodylian taxa are either strictly piscivorous or lacking at least one population that consumes teleosts as a primary food source. Instead of being correlated with consumption of a specific food type, then, rostrum morphology in extant crocodylians appears to be correlated with the relative size of food items that can be consumed by a given individual. Cranium shape, jaw musculature, and biomechanical performance were assessed in both phytosaurs and extant crocodylians to test hypotheses of morphological and functional similarities of the cranium between these two clades. Results of these analyses were interpreted in the context of prey:predator size ratios correlating with rostrum morphology in extant crocodylians in order to better constrain inferred diet ranges and variation among phytosaur taxa. In general, phytosaurs were more similar to crocodylians with very high aspect ratio rostra in most facets of cranium shape than crocodylians with lower aspect ratio rostra. This trend supports the interpretation that the diets of phytosaurs were probably most like those of extant crocodylians with high aspect ratio rostra. However, no overlap in any tested aspect of cranium shape was found between phytosaurs and crocodylians, precluding an inference of phytosaur diet as being the same as any sampled crocodylian taxon. The topology, individual origin and insertion area size proportions, and individual muscle force proportions of phytosaur and extant crocodylian jaw musculature all exhibited a great deal of consistency, regardless of variation in rostrum morphology among the sampled taxa. Clear indicators of greater jaw musculature similarity among particular taxa that would support inferences of dietary similarities between those taxa were thus not found. Biomechanical modeling revealed that bite forces produced by phytosaurs generally surpassed those of extant crocodylians, though the crania of most phytosaurs performed worse, from a structural perspective, than most extant crocodylians. These results, though somewhat conflicting, indicate that phytosaurs were typically able to produce higher bite forces than similarly sized extant crocodylians but were less capable of withstanding the resultant force experienced by the cranium during performance of a maximally powerful bite or when biting a hard object. These diverse results, synthesized into a single conclusion, indicate that smaller phytosaurs with relatively narrow, gracile rostra were probably restricted to utilizing food items even smaller, relative to their own size, than do any extant crocodylians. Larger phytosaurs and those with somewhat more robust, if also narrow, rostra were probably capable of utilizing food items of a prey:predator size ratio similar to or exceeding those utilized by extant crocodylians with relatively low rostrum aspect ratios. These new inferences highlight the complexity of interactions between cranium shape and jaw muscle performance that result in differences in trophic niche occupation. Better appreciation for these interactions will allow for a greater understanding of the mechanisms that lead to phenomena such as niche partitioning. Furthermore, more accurately constrained reconstructions of phytosaur ecology will enable further investigations into the complex ecosystem structure and changes during the Late Triassic.

The Gondwanatheria are an enigmatic clade of Cretaceous and Paleogene mammals known from South America, Africa, Madagascar, India, and the Antarctic Peninsula. The eight valid species—each belonging to a monotypic genus and the first of which was described only 30 years ago—are represented almost exclusively by isolated teeth, in addition to fragmentary dentaries attributed to Sudamerica, Gondwanatherium, Ferugliotherium, and an unnamed taxon from Tanzania. No cranial (skull exclusive of lower jaw) or postcranial material has heretofore been assigned to the Gondwanatheria, a severe limitation that has precluded a comprehensive assessment of phylogenetic affinities. Here we describe, in detail, the first cranial specimen of a gondwanatherian mammal. This material consists of a complete and well-preserved cranium of the sudamericid Vintana sertichi, recovered from the Upper Cretaceous (Maastrichtian) Maevarano Formation in the Mahajanga Basin of northwestern Madagascar. Salient features of the cranium include elongate, scimitar-like jugal flanges, huge orbits, strong klinorhynchy, and a vaulted nuchal region. Micro-computed tomography greatly facilitated the delineation of sutures and the description of internal morphology. The cranial features of Vintana are compared with those of a broad range of synapsids, with particular concentration on other Mesozoic mammaliaforms. The cranium of Vintana exhibits a mosaic of extremely primitive and extremely derived features. It is the second largest known for a Mesozoic mammaliaform, superseded only by that of the eutriconodontan Repenomamus giganticus from the Early Cretaceous of China. Vintana is the largest known Late Cretaceous mammaliaform; it is also the largest known Mesozoic mammaliaform from Gondwana.