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

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.