FIRST APPROACH TO A RECONSTRUCTION OF THE ADDUCTOR CHAMBER OF RIOJASUCHUS TENUISCEPS (ARCHOSAURIA: PSEUDOSUCHIA) FROM THE LOS COLORADOS FORMATION, LATE TRIASSIC OF ARGENTINA

VON BACZKO, M. B.; TABORDA, J.R.A.; DESOJO, J. B.

Berlín

Congreso; Society of Vertebrate Paleontology 74th Annual Meeting; 2014

Riojasuchus tenuisceps is a terrestrial quadrupedal ornithosuchid archosaur from the Late Triassic of Argentina. A muscular reconstruction of the adductor chamber of Riojasuchus was made based on the information known for other extant species of reptiles: Caiman latirostris, Alligator mississippiensis, Iguana iguana, and Sphenodon punctatus. The M. adductor mandibulae externus (MAME) originates in the supratemporal fenestra attaching to the postorbital, squamosal, and parietal, and inserts on the dorsal surface of the surangular. Unlike Caiman and Alligator, the quadrate of Riojasuchus seems to be excluded from the MAME attachment area because of its anterodorsal orientation and the squamosal and quadratojugal overlapping its anterior surface. The M. adductor mandibulae posterior (MAMP) is dorsoventrally oriented, originating on the ventral region of the quadrate and inserting on the dorsal area of the angular and articular, filling the posterior half of the internal mandibular fenestra as in recent crocodiles. The M. pseudotemporalis (MPst) attaches to the laterosphenoid, but in Riojasuchus, as well as in Sphenodon and Iguana, the laterosphenoid has no bridge as that recognized on crocodiles. The MPst of Riojasuchus is dorsoventrally oriented unlike that of Caiman, Alligator, and Iguana which is posterodorsally oriented. The MPst would insert on the cartilago transiliens, connecting with the M. intramandibularis (MI) that inserts in the dorsal surface of the angular and medial surface of the dentary and splenial. The M. depresor mandibulae (MDM) is more dorsally oriented in Riojasuchus (65°) than in Caiman (45°), with an insertion on the squamosal more alike to that of theropod dinosaurs (Tyrannosaurus, Allosaurus). This is because of posteroventrally oriented process of the squamosal which takes MDM the attachment area backwards and closer to the articular. The M. pterygoideus dorsalis (MPtd) covers the dorsal region of the palate but the attachement area is not clearly defined in Riojasuchus; it inserts on the medial region of the angular and articular. Conversely, the M. pterygoideus ventralis (MPtv) attaches on the posterodorsal region of the pterygoid and inserts on the posterolateral side of the angular and articular covering the posterior region of the mandible as seen in recent reptiles. This reconstruction of the adductor chamber of Riojasuchus is the first step that will allow us in a near future to make biomechanical analysis on the skull of this reptile.