A new dinosauriform specimen from the Chañares Formation of north-western Argentina.

 

(A) preserved bones of CRILAR-Pv 552 in approximate anatomical arrangement. (B) skeletal reconstruction of Lewisuchus admixtus. From Ezcurra et. al, 2019.

Formed during the breakup of Gondwana, the Chañares Formation is part of the Ischigualasto-Villa Unión Basin, and represents one of the most continuous continental Triassic succesions in South America. Volcanism have played an important role in the generation and preservation of the Chañares Formation’s exceptional tetrapod fossil record. The diverse and well-preserved tetrapod assemblage includes proterochampsids, pseudosuchians, ornithodirans, large dicynodonts and smaller cynodonts. Almost all dinosauromorphs are preserved in diagenetic concretions that erode out of a thick siltstone interval 15–20 m above the base of the formation, and include Lagosuchus talampayensis, Marasuchus lilloensis, Lewisuchus admixtus, and Pseudolagosuchus major. 

Unfortunatelly, our knowledge about Lewisuchus admixtus, and Pseudolagosuchus major are based on partial skeletons that has generated a contentious debate during the last 20 years about the synonymy between two of these species. The discovery of a new dinosauriform partial skeleton (CRILAR‐Pv 552) allows comparisons for the first time with both Lewisuchus admixtus and Pseudolagosuchus major.

Geological map of the Chañares–Gualo area in Talampaya National Park (From Marsicano et al., 2015)

The new specimen was found in 2013, and includes fragments of both premaxillae and maxillae, partial right jugal, right quadrate, fragment of right pterygoid, supraoccipital, both prootics, parabasisphenoid, right dentary lacking posterior end, anterior end of left dentary, partial right retroarticular complex, an isolated tooth, three or four anteriormiddle cervical vertebrae, distal half of a posterior cervical or dorsal neural spine, two middleposterior dorsal vertebrae, two sacral vertebrae, two consecutive anterior caudal vertebrae, eight to ten middle-distal caudal vertebrae, two haemal arches, base of the left scapular blade, partial left coracoid, distal half of right humerus, proximal region and distal end of left ulna, distal end of left radius, proximal region of a metacarpal, partial ilia, proximal end of left pubis, distal end of both pubes, partial left ischium, both partial femora, almost complete left tibia, distal end of right tibia, partial left half of fibula, partial fibular shaft, and some possible metatarsal shaft fragments. Based on the unique combination of cranial and postcranial characters, CRILAR-Pv 552 can be referred to Lewisuchus admixtus, and supports the hypothesis that Pseudolagosuchus major is a subjective junior synonym of Lewisuchus admixtus.

 

References:

Ezcurra, M. D., Nesbitt, S. J., Fiorelli, L. E., & Desojo, J. B. (2019). New specimen sheds light on the anatomy and taxonomy of the early Late Triassic dinosauriforms from the Chañares Formation, NW Argentina. The Anatomical Record. doi:10.1002/ar.24243

Marsicano, C. A.Irmis, R. B.Mancuso, A. C.Mundil, R. & Chemale, F., The precise temporal calibration of dinosaur origins, Proc. Natl Acad. Sci. USAhttp://dx.doi.org/10.1073/pnas.1512541112 (2015).

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