OPHTHALMOSAURIA (ICHTHYOSAURIA) FOREFIN FROM THE AALENIAN-BAJOCIAN BOUNDARY OF

FERNANDEZ, M.

JOURNAL OF VERTEBRATE PALEONTOLOGY

Año: 2003 vol. 23 p. 691 - 694

0272-4634

Limb design of ichthyosaurs shows unique modifications, even in most basal forms. The evolution of the forefin can be followed through the stratigraphic record in different clades of ichthyosaurs. Among the most significant modifications are the loss of digit I and the shortening of limb bones including radius and ulna. Both transformations occurred during the Late Triassic. In ‘‘post-Triassic forms’’ the major modification is the acquisition of an extrazeugopodial element anterior to the radius and at least one digit distal to it. Until now, this transformation was known in Callovian and younger ichthyosaurs and defines the clade Ophthalmosauria. Contact between the zeugopodium and the humerus is modified to involve three, rather than two, elements.The only forefin known from the Aalenian–Bathonian gap is that of the holotype of Chacaicosaurus cayi from the Early Bajocian of Patagonia. The aim of the present paper is to describe a fragment of an ichthyosaur forefin from the Aalenian–Bajocian boundary of Argentina. This forefin, although incomplete, preserves the humerus-zeugopodium contact that allows its inclusion within the clade Ophthalmosauria. The new material represents the oldest record of the group and demonstrates that modifications of the humerus-zeugopodium contact occurred much earlier than previously supposed.Chacaicosaurus cayi from the Early Bajocian of Patagonia. The aim of the present paper is to describe a fragment of an ichthyosaur forefin from the Aalenian–Bajocian boundary of Argentina. This forefin, although incomplete, preserves the humerus-zeugopodium contact that allows its inclusion within the clade Ophthalmosauria. The new material represents the oldest record of the group and demonstrates that modifications of the humerus-zeugopodium contact occurred much earlier than previously supposed.