The bony labyrinth of late Permian Biarmosuchia: palaeobiology and diversity in non-mammalian Therapsida
Date
2017-07
Authors
Benoit, Julien
Manger, Paul R
Fernandez, Vincent
Rubidge, Bruce S
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Biarmosuchia, as the basalmost group of Therapsida (the stem group of mammals), are important for understanding mammalian
origins and evolution. Unlike other therapsid groups, the bony labyrinth of biarmosuchians has not yet been studied, despite insightful
clues that bony labyrinth morphology can provide to address palaeobiology and phylogeny of extinct animals. Here, using CT
scanning, surface reconstruction and a 3D geometric-morphometric protocol of 60 semi-landmarks on the bony labyrinth of
30 therapsids (including three Mammaliaformes), it is demonstrated that bony labyrinth morphology of biarmosuchians is very
distinctive compared to that of other therapsids. Despite the primitive nature of their cranial morphology, biarmosuchians display
highly derived traits in the structure of the bony labyrinth. The most noticeable are the presence of a long and slender canal linking the
vestibule to the fenestra vestibuli, an enlarged and dorsally expanded anterior canal, and the absence of a secondary common crus
(except for one specimen), which sets them apart from other non-mammalian therapsids. These characters provide additional support
for the monophyly of Biarmosuchia, the most recently recognized major therapsid subclade. Although implications of the derived
morphology of the biarmosuchian bony labyrinth are discussed, definitive interpretations are dependent on the discovery of
well-preserved postcranial material. It nevertheless sheds light on a previously overlooked diversity of bony labyrinth morphology
in non-mammalian therapsids.
Description
Keywords
Biarmosuchia, Therapsida, bony labyrinth, inner-ear, geometric morphometrics