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Amphibetulimus (stem therian mammal, Middle Jurassic, Siberia) redescribed



From: Ben Creisler
bcreisler@gmail.com


A new online paper:


Alexander Averianov, Thomas Martin, Alexey Lopatin & Sergei Krasnolutskii (2013)
Stem therian mammal Amphibetulimus from the Middle Jurassic of Siberia.
Paläontologische Zeitschrift (advance online publication)
DOI: 10.1007/s12542-013-0217-x
http://link.springer.com/article/10.1007/s12542-013-0217-x

Amphibetulimus krasnolutskii is known from the Middle Jurassic
(Bathonian) Itat Formation of Krasnoyarsk Territory, West Siberia,
Russia, by several edentulous and three dentigerous dental fragments,
preserving p1, antepenultimate, and ultimate lower molars, and by an
upper molar. It is unique among stem therians by widely open trigonids
on the posterior lower molars, paraconids that are higher than the
metaconids and have keeled mesiolingual vertical crests, pronounced
unilateral hypsodonty of the lower molars and correlated unequal
alveolar borders of the dentary ramus, and a linear Meckelian groove
that is not connected to the mandibular foramen and extends along the
pterygoid ridge. Amphibetulimus differs from more derived stem
therians by a simple unicuspid talonid without an incipient talonid
basin and a distinct labial cingulum on the upper molars. The lack of
an ectotympanic facet and the long linear Meckelian groove extending
onto the pterygoid ridge suggest that Amphibetulimus had a derived
state of the transitional mammalian middle ear, where the ear ossicles
were connected to the dentary not by a thick “ossified” Meckelian
cartilage, but by a thin Meckelian cartilage, as in prenatal and early
postnatal stages of some modern therians.