Enamel Microstructure of Vintana Sertichi (Mammalia, Gondwanatheria) From the Late Cretaceous of Madagascar
Authors:
von Koenigswald et al
Abstract:
The enamel microstructure of the molariform cheek teeth of Vintana sertichi (Gondwanatheria, Mammalia) from the Late Cretaceous of Madagascar retains many of the plesiomorphic characteristics of mammalian prismatic enamel (e.g., single-layered schmelzmuster; non-decussating, small prisms). However, V. sertichi exhibits a relatively derived condition and resembles Lavanify miolaka (Late Cretaceous, Madagascar), Bharattherium bonapartei (Late Cretaceous, India), and an indeterminate sudamericid (Late Cretaceous, Madagascar) in possessing modified radial enamel, consisting of prisms separated by prominent interrow sheets of interprismatic matrix. A second group of gondwanatherians (Ferugliotherium windhauseni, Gondwanatherium patagonicum, Sudamerica ameghinoi, Greniodon sylvaticus), all from the Late Cretaceous and Paleogene of Argentina, retains relatively primitive, ‘normal’ radial enamel, in which the interprismatic matrix anastomoses around the prisms and does not form interrow sheets. The prisms of gondwanatherian molariforms generally become wider from the enamel-dentine junction to the outer enamel surface, dramatically so in Gondwanatherium. Seams are very rare; tubules are more common. Prism decussation has not been found in any gondwanatherian enamel. Gondwanatherian molariforms lack or have only a thin layer of external prismless enamel, although it is thick in the incisors of Sudamerica. Despite their apparent utility in differentiating among gondwanatherian taxa, or gondwanatherians as a whole from other clades, many of the derived characteristics of gondwanatherian enamel microstructure are found in other mammalian groups and may reflect a high degree of homoplasy.
Dental Morphology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Author:
Krause
Abstract:
The cranium of Vintana sertichi preserves the first associated upper dentition of a gondwanatherian mammal. Gondwanatherians are known almost exclusively from isolated teeth, particularly molariforms. As such, referral of V. sertichi to the Gondwanatheria, differentiation of V. sertichi from other gondwanatherians, and determination of the relationships of V. sertichi to other gondwanatherians must rely heavily on dental morphology. The hypsodont nature, gross morphology, enamel microstructure, and wear pattern of its molariform teeth serve to unequivocally identify the cranium of V. sertichi as that of a sudamericid gondwanatherian. Based on available morphology, V. sertichi appears to have had an upper dental formula of 2.0.1.4. There are two long, curved alveoli in each premaxilla for enlarged, buccolingually compressed, procumbent incisors that were well separated from the cheek teeth by a long diastema. There is no evidence of a canine. The single premolariform tooth appears to have been small and two-rooted, but neither the left nor the right crowns are preserved. Of the eight upper molariform alveoli, four are preserved with teeth in situ, three in the left maxilla (antepenultimate, penultimate, and ultimate) and one in the right (penultimate). The molariform cheek teeth have several salient characteristics: large size, hypsodont crowns, roughly quadrangular outlines, occlusal surfaces worn essentially flat, numerous cementum-filled infundibula, cementum-filled furrows that invaginate from the buccal side but do not extend to the base of the crown, and multiple, short root apices. The relative amounts of wear on the molariform cheek teeth of V. sertichi indicate a mesial-to-distal eruption sequence.
Endocranial and Inner Ear Morphology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Hoffmann et al
Abstract:
We present the first digital reconstruction of the endocranial cavity and endosseous labyrinth of the Late Cretaceous gondwanatherian mammal Vintana sertichi from the Maevarano Formation of Madagascar. The Malagasy specimen is exceptionally well preserved and represents the only described cranium known for Gondwanatheria, an enigmatic clade from the Late Cretaceous and Paleogene of Gondwana. The endocranial cast of Vintana is relatively small for an animal of its estimated body mass. Its encephalization quotient is 0.28–0.56 for a range of body mass estimates, which is similar to that of basal mammaliaforms. The olfactory bulbs are very large, occupying over 14% of the endocranial volume. The cerebral hemispheres are only slightly expanded, more similar to the condition in Morganucodon than to that in multituberculates and monotremes. Unlike the condition in other Mesozoic mammaliaforms, the endocast is greatly flexed at the circular fissure. The osseous labyrinth displays a mixture of derived and primitive features. The cochlear canal is only slightly curved and short compared with that of extant therians. The ratio between total cochlear canal length and maximum cranial length is smaller than in basal mammaliaforms and approximates that of non-mammaliaform cynodonts. By contrast, the presence of both primary and secondary osseous laminae, the tractus foraminosus, and Rosenthal's canal represent derived characteristics of the mammalian inner ear typical of cladotherians. A modern innervation of the cochlea has either evolved independently in Vintana and cladotherians or was already present in the last common ancestor of both clades.
Sensory Anatomy and Sensory Ecology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Kirk et al
Abstract:
Living mammals are distinguished from other extant tetrapods by adaptations for improved senses of hearing, touch, and smell. These adaptations, and concomitant reductions in visual anatomy, evolved during the Mesozoic in the mammalian and therian stem lineages. Here, we present a comparative study of the sensory anatomy of the Late Cretaceous gondwanatherian mammaliaform Vintana sertichi in order to draw inferences regarding its sensory abilities and sensory ecology. Our analyses demonstrate that Vintana has relatively large orbits that may have accommodated large eyes. Vintana also possessed cochlear primary and secondary osseous laminae and a cochlear canal that was relatively longer than in non-mammaliaform cynodonts but shorter than in extant therians. These features suggest that Vintana had some capacity for high-frequency hearing (i.e., graeter than 20 kHz), but that its cochlea may have encoded a more limited range of frequencies than the cochleas of most extant therians. The semicircular canals of Vintana have large radii of curvature and are nearly orthogonal, suggesting high sensitivity to angular head accelerations. These vestibular features may have evolved in order to stabilize large eyes during rapid and/or agile locomotion. Combined with evidence for large olfactory bulbs and a large trigeminal endocast, these data reveal that Vintana possessed a unique suite of sensory adaptations that distinguish it from other Mesozoic mammaliaforms. If these inferences are correct, then Vintana was probably a large-eyed and agile species, with a keen sense of smell and better high-frequency hearing than most other Mesozoic mammaliaforms.
Dental Function and Diet of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Schultz et al
Abstract:
The nearly complete and well-preserved cranium of Vintana sertichi provides an opportunity to investigate its dietary adaptations. We used a combination of comparative morphological and biomechanical analyses to reveal the direction of its power stroke during mastication, reconstruct the positions and relative sizes of its muscles of mastication, and predict its capacity to produce bite forces during incision and molariform occlusion. The orientation of dental wear striations on the upper molariform teeth of Vintana, in combination with the orientation of the enamel islets and synclines and the position of leading and trailing edges, demonstrates that the power stroke was primarily palinal (distally directed), with a significant buccally directed component that is absent in multituberculates, haramiyidans, and other gondwanatherians. The large palinal component of jaw movement was corroborated by biomechanical analyses of the moments generated by the primary jaw adductors around the dentary-squamosal joint axis. Similar analyses also confirmed the previously documented proal (mesially directed) power stroke of the extant rodent Myocastor. Finite element analyses predict that Vintana was capable of producing bite forces that were more than twice as high as the similarly sized Myocastor. Vintana was almost certainly an herbivore, as claimed previously for other sudamericid gondwanatherians. Its size and capacity to generate high bite forces at both the incisors and the molariform teeth suggest that it was a mixed feeder whose diet may have included relatively large, hard food items such as roots, seeds, twigs, or nut-like fruits.
Introduction, Systematic Paleontology, and Geological Context of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Krause et al
Abstract:
Vintana sertichi is a sudamericid gondwanatherian mammal known only from the Upper Cretaceous (Maastrichtian) Maevarano Formation in the Mahajanga Basin of northwestern Madagascar. It is based on a single specimen, a well-preserved and virtually complete cranium discovered in 2010 near Lac Kinkony. The cranium is superficially bizarre and constitutes the only cranial remains of the poorly known, phylogenetically enigmatic Gondwanatheria, which are otherwise known only from isolated teeth and fragmentary dentaries. Gondwanatheria are represented by seven other monotypic genera assigned to two families (Ferugliotheriidae and Sudamericidae). Historically, this clade was assigned to Xenarthra, Paratheria (as a sister group to Xenarthra), Multituberculata, Allotheria (as a sister group to Multituberculata), Mammalia incertae sedis, and, most recently, back to Multituberculata or a close relative of Multituberculata. The craniodental evidence provided by Vintana supports inclusion in Sudamericidae, the monophyly of Gondwanatheria, and the position of Gondwanatheria as nested within or sister to Multituberculata. In addition to briefly reviewing the taxonomic composition and phylogenetic history of Gondwanatheria, this introductory chapter sets the stage for the other chapters in the volume by (1) briefly summarizing the inferred life habits of gondwanatherians; (2) reviewing the systematic paleontology of V. sertichi; and (3) providing overviews of the discovery of the holotypic specimen, its preservation, its preparation, and the imaging and measurement techniques used to study it. The chapter closes with an overview of the geological context of V. sertichi, which indicates that the species lived in a coastal floodplain environment and in a highly seasonal, semiarid climate.
First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism
Authors:
Krause et al
Abstract:
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.
