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Temnospondyl trackway from Triassic shows forelimbs as main propulsion



Ben Creisler
bcreisler@gmail.com


New in PLoS ONE:

Claudia A. Marsicano, Jeffrey A. Wilson & Roger M. H. Smith (2014)
A Temnospondyl Trackway from the Early Mesozoic of Western Gondwana
and Its Implications for Basal Tetrapod Locomotion.
PLoS ONE 9(8): e103255.
doi:10.1371/journal.pone.0103255
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0103255



Background

Temnospondyls are one of the earliest radiations of limbed
vertebrates. Skeletal remains of more than 190 genera have been
identified from late Paleozoic and early Mesozoic rocks. Paleozoic
temnospondyls comprise mainly small to medium sized forms of diverse
habits ranging from fully aquatic to fully terrestrial. Accordingly,
their ichnological record includes tracks described from many
Laurasian localities. Mesozoic temnospondyls, in contrast, include
mostly medium to large aquatic or semi-aquatic forms. Exceedingly few
fossil tracks or trackways have been attributed to Mesozoic
temnospondyls, and as a consequence very little is known of their
locomotor capabilities on land.

Methodology/Principal Findings

We report a ca. 200 Ma trackway, Episcopopus ventrosus, from Lesotho,
southern Africa that was made by a 3.5 m-long animal. This relatively
long trackway records the trackmaker dragging its body along a wet
substrate using only the tips of its digits, which in the manus left
characteristic drag marks. Based on detailed mapping, casting, and
laser scanning of the best-preserved part of the trackway, we
identified synapomorphies (e.g., tetradactyl manus, pentadactyl pes)
and symplesiomorphies (e.g., absence of claws) in the Episcopopus
trackway that indicate a temnospondyl trackmaker.

Conclusions/Significance

Our analysis shows that the Episcopopus trackmaker progressed with a
sprawling posture, using a lateral-sequence walk. Its forelimbs were
the major propulsive elements and there was little lateral bending of
the trunk. We suggest this locomotor style, which differs dramatically
from the hindlimb-driven locomotion of salamanders and other extant
terrestrial tetrapods can be explained by the forwardly shifted center
of mass resulting from the relatively large heads and heavily pectoral
girdles of temnospondyls.