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Science 17 April 2009:
Vol. 324. no. 5925, pp. 364 - 367
DOI: 10.1126/science.1167542
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Reports

Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs

Viviane Callier,1* Jennifer A. Clack,1 Per E. Ahlberg2{dagger}

Ichthyostega and Acanthostega are the earliest tetrapods known from multiple near-complete skeletons, with Acanthostega generally considered the more primitive. New material indicates differing ontogenetic trajectories for their forelimbs: In Ichthyostega, the pattern of muscle attachment processes on small humeri (upper arm bones) resembles that in "fish" members of the tetrapod stem group such as Tiktaalik, whereas large humeri approach (but fail to attain) the tetrapod crown-group condition; in Acanthostega, both small and large humeri exhibit the crown-group pattern. We infer that Ichthyostega underwent greater locomotory terrestrialization during ontogeny. The newly recognized primitive characteristics also suggest that Ichthyostega could be phylogenetically more basal than Acanthostega.

1 University Museum of Zoology Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
2 Subdepartment of Evolutionary Organismal Biology, Department of Physiology and Developmental Biology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18A, 752 36 Uppsala, Sweden.

* Present address: Department of Biology, Duke University, Durham, NC 27708, USA.

{dagger} To whom correspondence should be addressed. E-mail: per.ahlberg{at}ebc.uu.se

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Science. ISSN 0036-8075 (print), 1095-9203 (online)