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Evolution and history of digital mobility in synapsids

Ben Creisler

New in PLoS ONE:

Susanna B. Kümmell & Eberhard Frey (2014)
Range of Movement in Ray I of Manus and Pes and the Prehensility of
the Autopodia in the Early Permian to Late Cretaceous Non-Anomodont
PLoS ONE 9(12): e113911.

The mobility of ray I was analysed in seventy-eight Early Permian to
Late Cretaceous specimens of non-mammalian Synapsida and one extant
mammal. In all non-mammaliamorph Synapsida investigated, ray I formed
a digital arcade. The first phalanx was maximally extendable to the
zero position in the metapodiophalangeal joint I. Metapodiale I was
the functional equivalent to a basal phalanx of digits II–V. In
contrast, there was no digital arcade in ray I in Mesozoic
Mammaliamorpha. Phalanx 1 I was dorsally extendable and metapodiale I
was functionally part of the metapodium. During the propulsion phase,
autopodial rotation occurred in the majority of Synapsida with
abducted limb posture. Regarding ray I, the reduction of autopodial
rotation can be estimated, e.g., from the decrease of lateral rotation
and medial abduction of the first phalanx in the metapodiophalangeal
joint I. Autopodial rotation was high in Titanophoneus and reduced in
derived Cynodontia. In Mammaliamorpha the mobility of the first ray
suggests autopodial rolling in an approximately anterior direction.
Most non-mammaliamorph Therapsida and probably some Mesozoic
Mammaliamorpha had prehensile autopodia with an opposable ray I. In
forms with a pronounced relief of the respective joints, ray I could
be opposed to 90° against ray III. A strong transverse arch in the row
of distalia supported the opposition movement of ray I and resulted in
a convergence of the claws of digits II–V just by flexing those
digits. A tight articular coherence in the digital joints of digits
II–V during strong flexion supported a firm grip capacity. Usually the
grip capacity was more pronounced in the manus than in the pes.
Prehensile autopodia of carnivorous Therapsida may have been utilized
to hold prey while biting, thus helping to avoid fractures of the
laterally compressed fangs.