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Akidolestes (Cretaceous symmetrodont mammal) postcranial skeleton



From: Ben Creisler
bcreisler@gmail.com

A new paper about a Mesozoic mammal (think "theropod food"):

Meng Chen and Zhe-Xi Luo (2012)
Postcranial Skeleton of the Cretaceous Mammal Akidolestes cifellii and
Its Locomotor Adaptations.
Journal of Mammalian Evolution (advance online publication)
2012, DOI: 10.1007/s10914-012-9199-9
http://www.springerlink.com/content/23252m78k072p834/



Spalacotheroid “symmetrodontans” are a group of extinct Mesozoic
mammals. They are basal taxa in the trechnotherian clade that includes
modern marsupials and placentals. Therefore, fossils of
spalacotheroids can provide information on the ancestral condition
from which marsupials and placentals likely have evolved. Here, we
describe the postcranial skeleton of Akidolestes cifellii, a
spalacotheroid species from the Lower Cretaceous Yixian Formation of
northeastern China. Our comparison of the skeletal features of
Akidolestes and the closely related Zhangheotherium and Maotherium
indicates some major morphological and functional differences in the
postcranium among these spalacotheroid mammals. Akidolestes shows
characters for terrestrial habitat preference. Overall it appears to
be a generalized terrestrial mammal. Akidolestes differs from
Zhangheotherium and Maotherium in some characteristics of the scapula,
the pelvis, and the hind limb, some of which can be directly
correlated with different locomotor capabilities, and possibly also
habitat preferences. This suggests that a greater ecomorphological
differentiation occurred in these stem therian mammals than previously
thought and that ecological differentiation is a major pattern in
early therian mammal evolution.



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Also another recent paper about synapsid locomotion:

Susanna B. Kümmell and Eberhard Frey (2012)
Digital arcade in the autopodia of Synapsida: standard position of the
digits and dorsoventral excursion angle of digital joints in the rays
II–V.
Palaeobiodiversity and Palaeoenvironments (advance online publication)
2012, DOI: 10.1007/s12549-012-0076-6
http://www.springerlink.com/content/h557442r3h913450/

In most extant Mammalia, the digits II–V are flexed to a digital
arcade in the middle joint during most of the propulsion phase. An
extension occurs only at the end of the propulsion phase, but the
digits never extend dorsally in the middle joint. In most extant
Reptilia, however, the digits do not show any arcade in the middle
joint and are extended dorsally during the propulsion phase. The
mechanical effect of a digital arcade is a reduction of the lever arm
of the ground reaction force and an increase of that of the profound
flexor tendon. This saves muscular force and increases the pressure
application force of terminal sole pads or claws. In autopodia with a
digital arcade, a Z-configuration is formed by the middle and basal
phalanges as well as the metapodialia, forming a spring that
economises the rolling movement. Fossil Synapsida between Early
Permian and Late Cretaceous were analysed to reconstruct the posture
of their digits. The middle joints of the digits II–V in most
Therapsida including Mesozoic Mammaliamorpha formed a digital arcade
and possessed terminal and basal sole pads similar to extant Mammalia.
The claws did not necessarily add to friction during the rolling mode.
This is reconstructed from the possible excursion angles of the
digital joints and their mechanical constraints, from ichnofossils and
from autopodia preserved in situ. The digital arcade must have evolved
within the non-therapsid Synapsida or perhaps earlier. The digital
arcade, however, was reduced or absent in Gobiconodon, Cistecephalus
and the graviportal Kannemeyeriiformes.