Nasal Anatomy of the Non‐mammaliaform Cynodont Brasilitherium riograndensis (Eucynodontia, Therapsida) Reveals New Insight into Mammalian Evolution

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 mCT 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 This article includes AR WOW Videos, which can be viewed at

Role of ortho‐retronasal olfaction in mammalian cortical evolution

“…Equivocal evidence from the fossil record (below) hints at partial ossification of nasal capsule elements prior to the origin of mammals (Kielan-Jaworowska et al, 2004; Ruf et al, 2014). However, full ossification of the ethmoid and its turbinals arose in the last common ancestor of crown Mammalia (Rowe, 1988), potentially accommodating expression of the full complement of ~1200 OR genes believed present ancestrally (Rowe et al, 2011).…”

The postcranial anatomy of Brasilodon quadrangularis and the acquisition of mammaliaform traits among non-mammaliaform cynodonts

“…Some of its appendicular bones (humerus, ulna, radius, femur and tibia) were figured but briefly described or not at all [27]. The skull of UFRGS-PV-1043-T was also used for the reconstruction of the inner ear, brain and nasal cavities [28–30]. Most postcranial elements are well-preserved, including the epiphysis in long bones.…”

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