Brasilitherium

Abstract: Nasal regions of the non-mammalian cynodonts Massetognathus, Probainognathus, and Elliotherium were reconstructed from micro-computed tomography scans and compared with scans and published accounts of more derived forms, including Brasilitherium, Morganucodon, Haldanodon, and extant mammals. The basic structure of the modern mammalian nose, already present in non-mammalian cynodonts of the Early Triassic, underwent little modification during the Triassic. A respiratory chamber opened into a nasopharyngeal passage through an enlarged primary choana bordered posteriorly by a transverse lamina that formed the floor to a more posterior olfactory chamber. Cartilaginous respiratory turbinals initially provided a surface for evaporative cooling during periods of increased activity in the exceptionally high ambient temperatures of the Triassic. A similar mechanism for heat loss is present in extant crocodilians, squamates, and mammals. In the Late Triassic and Early Jurassic non-mammaliaform cynodonts (El...

Abstract: The mammalian nasal cavity is characterized by a unique anatomy with complex internal features. The evolution of turbinals was correlated with endothermic and macrosmatic adaptations in therapsids and in early mammals, which is still apparent in their twofold function (warming and moistening of air, olfaction). Fossil evidence for the transformation from the nonmammalian to the mammalian nasal cavity pattern has been poor and inadequate. Ossification of the cartilaginous nasal capsule and turbinals seems to be a feature that occurred only very late in synapsid evolution but delicate ethmoidal bones are rarely preserved. Here we provide the first µCT investigation of the nasal cavity of the advanced non-mammaliaform cynodont Brasilitherium riograndensis from the Late Triassic of Southern Brazil, a member of the sister-group of mammaliaforms, in order to elucidate a critical anatomical transition in early mammalian evolution. Brasilitherium riograndensis already had at least partially ossified turbinals as remnants of the nasoturbinal and the first ethmoturbinal are preserved. The posterior nasal septum is partly ossified and contributes to a mesethmoid. The nasal cavity is posteriorly expanded and forms a distinctive pars posterior (ethmoidal recess) that is ventrally separated from the nasopharyngeal duct by a distinct lamina terminalis. Thus, our observations clearly demonstrate that principal features of the mammalian nasal cavity were already present in the sister-group of mammaliaforms. Anat Rec, 297:2018–2030, 2014. © 2014 Wiley Periodicals, Inc.

Abstract: A digital cranial endocast of the specimen UFRGS-PV-1043-T, Brasilitherium riograndensis, was obtained from high-resolution computed tomography (μCT) scan images. This taxon is a small cynodont from the Late Triassic of Brazil, and has been used as the sister-group of the mammaliaforms in cladistic analyses. The digital endocast of UFRGS-PV-1043-T is mostly complete, allowing the description and collection of accurate linear and volumetric measurements, which were taken and compared with other non-mammaliaform cynodonts. Impressions of vessels were observed in the inner walls of the braincase. Despite the lack of a cribiform plate and the presence of a wide orbital vacuity, the endocast of Brasilitherium shows olfactory bulb casts that are relatively larger than in other non-mammaliaform cynodonts, suggesting a pattern of gradual increase in size and improvement of the olfactory sense for these structures toward the mammalian condition. The cerebral hemispheres are elongated and clearly divided by a median sulcus. The parafloccular casts are well defined, and their position corresponds to the maximum width of the endocast. In the ventral view, a large hypophyseal cast and a wide opening for the cavum epiptericum are evident. The encephalization quotient (EQ) calculated for Brasilitherium is greater than the range of EQs reported for most non-mammaliaform cynodonts (although it may be similar to that of some taxa, according to the equation used to estimate their body masses), but it is smaller than that of the mammaliaforms and mammals. A slighter increase in the brain size of Brasilitherium compared with other non-mammaliaform cynodonts was observed, along with a more significant increase in the size of the olfactory bulbs. This study supports the proposition of an early evolution of the mammalian brain associated with selective pressures for better sensorial acuity, especially regarding improved olfaction, which began with small Triassic mammaliamorphs.

Abstract: The Prozostrodontia includes a group of Late Triassic-Early Cretaceous eucynodonts plus the clade Mammaliaformes, in which Mammalia is nested. Analysing their growth patterns is thus important for understanding the evolution of mammalian life histories. Obtaining material for osteohistological analysis is difficult due to the rare and delicate nature of most of the prozostrodontian taxa, much of which comprises mostly of crania or sometimes even only teeth. Here we present a rare opportunity to observe the osteohistology of several postcranial elements of the basal prozostrodontid Prozostrodon brasiliensis, the tritheledontid Irajatherium hernandezi, and the brasilodontids Brasilodon quadrangularis and Brasilitherium riograndensis from the Late Triassic of Brazil (Santa Maria Supersequence). Prozostrodon and Irajatherium reveal similar growth patterns of rapid early growth with annual interruptions later in ontogeny. These interruptions are associated with wide zones of slow growing bone tissue. Brasilodon and Brasilitherium exhibit a mixture of woven-fibered bone tissue and slower growing parallel-fibered and lamellar bone. The slower growing bone tissues are present even during early ontogeny. The relatively slower growth in Brasilodon and Brasilitherium may be related to their small body size compared to Prozostrodon and Irajatherium. These brasilodontids also exhibit osteohistological similarities with the Late Triassic/Early Jurassic mammaliaform Morganucodon and the Late Cretaceous multituberculate mammals Kryptobaatar and Nemegtbaatar. This may be due to similar small body sizes, but may also reflect their close phylogenetic affinities as Brasilodon and Brasilitherium are the closest relatives to Mammaliaformes. However, when compared with similar-sized extant placental mammals, they may have grown more slowly to adult size as their osteohistology shows it took more than one year for growth to attenuate. Thus, although they exhibit rapid juvenile growth, the small derived, brasilodontid prozostrodontians still exhibit an extended growth period compared to similar-sized extant mammals.