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Brasilitherium (Triassic cynodont), digital reconstruction of inner ear



From: Ben Creisler
bcreisler@gmail.com


A new online paper not yet mentioned:

Pablo Gusmão Rodrigues, Irina Ruf & Cesar Leandro Schultz (2013)
Digital Reconstruction of the Otic Region and Inner Ear of the
Non-Mammalian Cynodont Brasilitherium riograndensis (Late Triassic,
Brazil) and Its Relevance to the Evolution of the Mammalian Ear.
Journal of Mammalian Evolution (advance online publication)
doi: 10.1007/s10914-012-9221-2
http://link.springer.com/article/10.1007/s10914-012-9221-2


The external anatomy of the petrosal, the bony labyrinth of the inner
ear, and the stapes of Brasilitherium riograndensis (specimen
UFRGS-PV-1043-T) were investigated by digital 3D reconstructions based
on μCT scan images. Brasilitherium is the most basal taxon bearing a
distinct promontorium, although less inflated than that of
Morganucodon and still lacking a flat medial facet. A bony wall formed
by the petrosal separates the cochlear canal and the vestibule from
the brain cavity, with an internal acoustic meatus bearing distinct
foramina for the facial nerve (VII) and vestibulocochlear nerve
(VIII). The semicircular canals are irregular in shape, the anterior
canal being the largest and the lateral one the smallest.
Brasilitherium has an elongated but straight cochlear canal. The
stapes resembles the morphology of derived non-mammaliaform cynodonts,
such as Probainognathus and Pachygenelus, and differs from
Thrinaxodon. By the allometric relationship of the cochlear canal and
the estimated body mass, Brasilitherium can be grouped with Yunnanodon
and Morganucodon in a regression line, which is below the line of
mammals and above the line of non-avian reptiles. Brasilitherium fits
in a sequence of gradual elongation of the cochlear canal associated
with the enhancement in the capacity to hear higher frequencies. Among
the constraints that might have triggered these transformations in
small, insectivorous, and possibly nocturnal Mesozoic cynodont taxa is
the improvement of detecting acoustically active insects.