Journal of Vertebrate Paleontology, Program and Abstracts

Rodents are the most diverse group of mammals,
inhabiting almost every region of the world with
a variety of lifestyles. In particular, extant
species of Eusciurida (Sciuridae, Gliridae,
Aplodontia rufa) are characterized by different
body plans, varied locomotor behaviours and
habitat use. However, assigning locomotion
modes to extinct taxa of rodents is very difficult.
Usually, fossil remnants of small mammals are
mostly represented by isolated teeth or jaw
fragments. In this study, we focus on the ear
region, which is responsible for detecting sound
(middle ear ossicles and cochlea of the inner ear)
and balance (semicircular canals, utricle and
saccule of the inner ear). Our aim is to identify
the relationships between ear morphology and
locomotor behaviour in extant rodents, in order
to reconstruct the ecology of fossil taxa. For the
extant sample, we used a previously published
data set, which comprises 45 extant specimens
of Eusciurida (n = 40, including 33 Sciuridae
and seven Gliridae) and selected further
mammals (n = 5), to elucidate the locomotion
mode of nine fossil representatives of rodents.
Based on multivariate analyses of the three-
dimensional linear and angular measurements of
the vestibular system and the cochlea, as well as
the volume of the different parts of the inner and
middle ear, we reconstructed the palaeobiology
of sciurids. The high cross-sectional diameter of
the semicircular canals and the more acute angle
between the anterior and posterior canals
represent the main anatomical features to
distinguish fossorial taxa from arboreal and
gliding taxa. In contrast to previous studies
focussing on mammals in general, locomotion
types in the investigated rodents cannot be
distinguished based on semicircular canal height
or width. Among our fossil sample, we propose
that the most basal taxa were fossorial
(†Ischyromys), before evolving into arboreal
species (†Palaeosciurus) with only one
occurence of ‘leaving’ trees as gliding taxa
(Pteromyini).