Week of Vintana: Did the Modern Mammalian Ear Evolve Before the Allotherian/Cladotherian Mammal Split?

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.