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Exaeretodon (Late Triassic cynodont ) monotypic association in Brazil + synapsid hearing

Ben Creisler

New online papers:

Rodrigo Temp Müller, Hermínio Ismael de Araújo-Júnior, Alex Sandro
Schiller Aires, Lúcio Roberto da Silva & Sérgio Dias-da-Silva (2015)
Biogenic control on the origin of a vertebrate monotypic accumulation
from the Late Triassic of southern Brazil.
Geobios (advance online publication)

This study is a taphonomic investigation of a new Late Triassic
monotypic association from the Hyperodapedon Assemblage Zone (Santa
Maria Formation, southern Brazil). All skeletal elements belong to the
traversodontid cynodont Exaeretodon, representing a minimum of four
individuals from different ontogenetic stages. Biogenic traces are
identifiable on some elements, such as invertebrate scavenging traces,
vertebrate bite marks, and evidence of trampling which was probably
responsible for accelerating the disarticulation of upper postcanine
teeth and for fracturing a skull and a lower jaw. We classify this
accumulation as generated by extrinsic biogenic action
(predation/necrophagy and possibly trampling) due to the following
reasons: (i) random spatial orientation of the elements enclosed into
the matrix; (ii) absence of hydraulic equivalence among the specimens
and presence of mudstone lenses in close association with the fossils;
(iii) presence of nearly all Voorhies’ groups; (iv) association of a
large number of cranial elements presenting different stages of
disarticulation; and (v) presence of biogenic traces. Both
disarticulation patterns and invertebrate scavenging traces indicate
that the bones remained exposed for some time before burial. During
this time of exposure, carnivore vertebrates also scavenged on some
nutritive and transportable postcranial elements. We suggest an
ecteniniid cynodont as a potential modifier agent
(predation/necrophagy) of this particular monotypic accumulation of


Michael Laass (2015)
The origins of the cochlea and impedance matching hearing in synapsids.
Acta Palaeontologica Polonica (in press)

The origin of tympanic hearing in early synapsids is still
controversial, because little is known about their inner ear and the
function of their sound conducting apparatus. Here I describe probably
the earliest tympanic ear in the synapsid lineage, the ear of
Pristerodon (Therapsida, Anomodontia) from the Late Permian of South
Africa, which was virtually reconstructed from neutron tomographic
data. Although Pristerodon is not a direct ancestor of mammals, its
inner ear with distinctive cochlear cavity represents a connecting
link between the primitive therapsid inner ear and the mammalian inner
ear. The anatomy of the sound conducting apparatus of Pristerodon and
the increased sound pressure transformer ratio points to a sensitivity
to airborne sound. Furthermore, the origins of the cochlea and
impedance matching hearing in synapsids coincided with the loss of
contact between head and substrate, which already took place at least
in Late Permian therapsids even before the postdentary bones became
detached from the mandible.