Endocranial anatomy of the early prozostrodonts (Eucynodontia: Probainognathia) and the neurosensory evolution of the mammalian forerunners

Prozostrodon brasiliensis and Therioherpeton cargnini are non-mammaliaform cynodonts that lived similar to 233 million years ago (late Carnian, Late Triassic) in western Gondwana. They represent some of the earliest divergent members of the clade Prozostrodontia, which includes tritheledontids, tritylodontids, brasilodontids, and mammaliaforms (including Mammalia as crown group). Here, we studied the endocranial anatomy (cranial endocast, nerves, vessels, ducts, ear region, and nasal cavity) of these two species. Our findings suggest that during the Carnian, early prozostrodonts had a brain with well-developed olfactory bulbs, expanded cerebral hemispheres divided by the interhemispheric sulcus, and absence of an unossified zone and pineal body. The morphology of the maxillary canal represents the necessary condition for the presence of facial vibrissae. A slight decrease in encephalization is observed at the origin of the clade Prozostrodontia. This new anatomical information provides evidence for the evolution of endocranial traits of the first prozotrodonts, a Late Triassic lineage that culminated in the origin of mammals.

Automated summary

In this study, the endocranial anatomy of Prozostrodon brasiliensis and Therioherpeton cargnini, two non-mammaliaform cynodonts from the Late Triassic period, was investigated. The findings indicate that early prozostrodonts had well-developed olfactory bulbs, expanded cerebral hemispheres, and the absence of unossified zone and pineal body during the Carnian. This new anatomical information provides evidence for the evolution of endocranial traits in the first prozostrodonts, which eventually led to the emergence of mammals.