The evolution of digit morphology.

<p>The forelimb and hind limbs of representative taxa illustrate the history of digit morphology in the lineages leading to the taxa compared in this study, the chicken (<i>Gallus gallus</i>), alligator (<i>Alligator mississippiensis</i>) and mouse (<i>Mus musculus</i>). The digits of early birds like <i>Archaeopteryx</i>, are specifically similar to digits 1, 2, and 3 of crocodilians, presenting 2,3 and 4 phalanges on each digit, respectively (node Ar). We arrive at the same conclusion if we compare <i>Archaeopteryx</i> to early dinosaurs, lizards, and early branches of amniotes (such as <i>Captorhinus</i>, <i>Ophiacodon</i>). No comparative morphological evidence has been presented for a 2,3,4 identification of wing digits. Molecular phylogenies confirm the relationships shown in this figure <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003325#pone.0003325-Lee1" target="_blank">[3]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003325#pone.0003325-Meyer1" target="_blank">[33]</a>. Maximally parsimonious inference of morphological history is done following the method in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003325#pone.0003325-Donoghue1" target="_blank">[34]</a>. The nodes of the tree are labeled for corresponding clades: A) Amniota R) Reptilia, D) Diapsida, Ar) Archosauria, T) Theropoda, B) Birds S) Synapsida Th) Therapsida. A geological time scale indicates the approximate time of lineage divergence.