NEW DATA ON THE LATE TRIASSIC Venaticosuchus rusconii (ARCHOSAURIA: PSEUDOSUCHIA) FROM THE ISCHIGUALASTO FORMATION, LA RIOJA, ARGENTINA

VON BACZKO, M. B.; DESOJO, J. B.

San juan

Congreso; ...IV Congreso Latino-Americano de Paleontología de Vertebrados.; 2011

UNSJ

The Family Ornithosuchidae von Huene 1914 is a group of pseudosuchian archosaurs represented by three species, Ornithosuchus longidens (Huxley 1877) from the Elgin Sandstones (early Late Triassic), Scotland; Riojasuchus tenuisceps Bonaparte 1967 from the Los Colorados Formation (late Late Triassic), Argentina; and Venaticosuchus rusconii Bonaparte 1970. The latter species was collected near to Cerro Las Lajas in outcrops of  the Ischigualasto Formation (W of La Rioja Province), from Late Triassic levels that also yielded the archosaurs Trialestes and Pisanosaurus. V. rusconii has been briefly described by Bonaparte in 1970 and is only known by the holotype that consists of an incomplete skull (i.e. part of the palate, antorbital region, without most of the basicranium) and mandible in articulation. In 1971 this author assigned it to Ornithosuchidae based on the following characters: downturned premaxilla, two-tooth diastema between premaxilla and maxilla, and lower jaws shorter than the skull. The jugal of V. rusconii presents a single dorsal process that bifurcates in a Y-shaped fashion, although the posterior ramus is broken. The lower jaws are shorter than the snout but extend further anteriorly than the posterior end of the external naris. The dentary bears two big caniniform teeth followed by very short teeth. The teeth disposition and the jugal morphology of V. rusconii resemble that of R. tenuisceps but differ from the latter in having an antorbital fossa that does not reach the alveolar margin of the maxilla. The anterior extension of the lower jaw of V. rusconii is similar to that of O. longidens. However, in contrast with O. longidens, but as occurs in R. tenuisceps, V. rusconii lacks a very short procumbent first tooth. Nevertheless, the absence of a surangular ridge and the depth of the parabasisphenoid recess sets V. rusconiii apart from the other two genera. The holotype of V. rusconii suffered some kind of pathology on the left side of the skull exhibiting an unusual outgrowth of facial bone. This pathology probably caused the thickened maxilla-jugal region and provoked the loss of the last two alveoli. In the present contribution, the cladistic analysis using a data matrix modified from Nesbitt (2011) supports the inclusion of V. rusconii in the Family Ornithosuchidae based on the following synapomorphies: absence of subnarial foramen between premaxilla and maxilla; palatal process of the maxilla not meeting at the midline; and absence of a foramen on the medial side of the articular. These preliminary results do not resolve the relationships within the ornithosuchids, but agree with Nesbitt (2011) in considering this family as the most basal pseudosuchians. These problems are probably caused by the incompleteness of the specimen, but a more detailed analysis of characters would help to resolve this issue in the future.