ON CHALIMINIA MUSTELOIDES (EUCYNODONTIA: TRITHELEDONTIDAE) FROM THE
LATE TRIASSIC OF ARGENTINA, AND A PHYLOGENY OF ICTIDOSAURIA
Agustín G. Martinelli and Guillermo W. Rougier, Journal of Vertebrate
Paleontology, 2007, 27(2):442–460
A new specimen of Chaliminia musteloides Bonaparte is described from the upper section of the Los Colorados Formation (Late Carnian–Early Norian) of Argentina. A cladistic analysis of ictidosaurians supports its monophyly. Chaliminia and Elliotherium, grouped in the new subfamily Chalimininae, were placed as sister taxa to the new subfamily Pachygenelinae, which includes Pachygenelus, Diarthrognathus, and Tritheledon. Irajatherium is basal to both subfamilies as a stem Tritheledontidae. Riograndia represents the most basal non-tritheledontid ictidosaurian. The resulting tree topology agrees with the age of the fossil record of each ictidosaurian species. Although our analysis does not test the relationship of ictidosaurians with regard to Mammaliaformes, it is clear from our tree that ictidosaurians are closely related to them. Several derived characters seem to have been acquired convergently in both lineages and relate to food processing, development of primary chewing musculature and ancillary structures. In other aspects, ictidosaurians remained very conservative, or reverted to ancestral morphologies, such as the presence of interpterygoidal-vacuities. Analysis of the vacuities indicates that they represent an ancestral condition that facilitated the development of the mammalian skull base constituted by the reduced pterygoids, the basisphenoid, and posteriorly the development of the presphenoid by the ventral extension of the braincase. Possibly, the main factors in the remodeling of this area are the development of the sphenoid sinus and the forward and ventral expansion of the cranial cavity. The re-acquisition of interpterygoid vacuities among ictidosaurians could be the result of a paedomorphic (neotenic or progenetic) alteration of the developmental pattern.