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Dryad

Original CT image stacks of five fossil petrosal bones from Siberia, 3D PDF files of reconstructed endocasts, blood vessels and innervation patterns, 3D PDF instruction file, STL files of the petrosals

Schultz, Julia A.1; Ruf, Irina2; Averianov, Alexander O.3; Schellhorn, Rico1; Lopatin, Alexey V.3; Martin, Thomas1

Published Dec 07, 2021 on Dryad. https://doi.org/10.5061/dryad.t1g1jwt2d

Data files

Abstract

Five partially preserved mammaliaform petrosals recovered from Middle Jurassic sediments of the Berezovsk coal mine (Krasnoyarsk Krai, Russia) show similarities to other early mammaliaforms like the morganucodontan Morganucodon and the docodontan Haldanodon in having an inflated promontorium, and a curved and apically inflated cochlear canal. But they are distinct from dryolestoid and derived mammalian petrosals by the weak coiling of the cochlear duct and the presence of a perilymphatic foramen with an open perilymphatic sulcus. The two larger and robust specimens exhibit striking similarities to docodontan petrosals. Inside the bone an intricate circumpromontorial venous plexus was discovered as recently described for the docodontan Borealestes, confirming this structure is consistently present in basal mammaliaforms. The three smaller and slender petrosals likely belong to haramiyidans and are unique in showing a septum-like structure medially along the cochlear nerve entrance. The protruding perforated bony bar that is preserved in two of the three is interpreted here to be a remnant of a bony septum with multiple foramina for cochlear nerve fibers, representing an autapomorphic feature of Haramiyida. This newly described passageway for nerve fibers shows that the formation of the osteological structure surrounding the nervous pathways of the cochlea is more plastic among the most basal mammaliaforms than previously thought.