On the evolution and homologies of ichthyopterygian forefins

ABSTRACT

The evolution of forefins within the Ichthyopterygia is re-evaluated based on the largest set of data available up to present, including recently reported Early Triassic forms. Three humeral morphotypes are recognized; one is plesiomorphic, and the other two characterize two of the major clades. The forefin elements of various ichthyopterygians are identified, based on osteogenetic sequences and topological arrangements. Both lines of evidence, ontogenetic and morphological, support a single conclusion that Stenopterygius, and all other ichthyopterygians belonging to the clade that survived into the Jurassic, lack digit I of the forelimb. Digits I and II are probably absent in Shastasaurus and closely related forms. The topology along the primary axis and digital arch is conservative among all ichthyopterygians examined, as expected from the underlying mechanism of limb development.