A SMALL ARCHOSAUROMORPH WITH ARCHOSAUR-LIKE FEATURES FROM THE MIDDLE-LATE TRIASSIC OF KYRGYZSTAN (CENTRAL ASIA)

BUCHWITZ, M.; EZCURRA, M. D.

Berlin

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

Reptile fossils from the lacustrine shales of the late Middle to early Late TriassicMadygen Formation of Kyrgyzstan, Central Asia, are famous for soft tissue preservation(e.g., Longisquama, Kyrgyzsaurus), but bones are often severely crushed and hard tointerpret. Here we report an almost undeformed, articulated pelvis and hindlimbexcavated in fluvial deposits of this unit. The new specimen with a femoral length of 84mm possesses an anterior trochanter on the femur, a prominent cnemial crest on the tibia,and an ankle with a crurotarsan-like appearance. The articulation between astragalus andcalcaneum is convex-concave (concavity on astragalus) and the calcaneum possesses along, posteriorly-deflected calcaneal tuber and a ball-shaped fibular facet. However,several features suggest an assignment to non-archosauriform archosauromorphs, namelythe short and plate-like pubis and ischium that together define a thyroid fenestra, aperforating foramen between astragalus and calcaneum, a large medial centrale that doesnot contact the tibia, and at least three distal tarsals.A phylogenetic analysis including 14 archosauromorphs and 30 further amniote taxafinds the new reptile fossil nested within Archosauromorpha as the sister-taxon of a cladeincluding rhynchosaurs, Trilophosaurus, Prolacerta and archosauriforms, but notprotorosaurs. In addition, the most comprehensive available data matrix focused on basalarchosauriforms finds the new specimen among its outgroup taxa and outsideArchosauria. The affinities of the new specimen with the allegedly arboreal Madygenreptile taxa Longisquama or Kyrgyzsaurus cannot be properly determined because bothgenera are only known from anterior skeletons and their systematic position withindiapsids is problematic.Morphological disparity analyses find that multiple archosauromorph groups invadeda similar morphospace during the Middle-Late Triassic, including the new Madygenarchosauromorph, early avemetatarsalians, and poposauroids. The convergently acquiredarchosaur-like features of the Madygen specimen seem to be functionally related to asemi-erect to erect posture, suggesting that it was a ground-dwelling reptile. The newMadygen archosauromorph clearly departs from the morphospace of other non-archosaurarchosauromorphs and possibly represents a member of a previously unknown group ofTriassic reptiles.