A new and large short-necked chelid turtle from the Loncoche Formation (Late Campanian-Early Maastrichtian) Mendoza Province, Argentina: Macro, microanatomy, and preliminary phylogenetic relationships

MARCELO S. DE LA FUENTE; IGNACIO J. MANIEL; JUAN M. JANNELLO; JULIANA STERLI; BERNARDO GONZÁLEZ RIGA; FERNANDO NOVAS

Rio de Janeiro

Simposio; 5th Turtle Evolution Symposium; 2015

The lower section of the Loncoche Formation (Early Campanian- Upper Maastrichtian) outcropping at Ranquil-Có locality (Mendoza province, Argentina) has yielded a rich assemblage of vertebrates (comprise fish, amphibians and reptiles) that was studied in 1989 by Gonzalez Riga. According to this author these vertebrate remains were found in a tidal paleoenvironment, resulting from transportation, mixing and deposition of continental remains mixed with those from near costal environments. Among the turtles recovered in this locality only chelids and meiolaniforms were recognized. Posteriorly to these findings Jose F. Bonaparte recovered in 1990 a large turtle from the same locality and horizon. This turtle is represented by one almost complete specimen that has been housed in Museo Argentino de Ciencias Naturales ?Bernardino Rivadavia? (MACN). Remains of basicranium (basioccipital and basisphenoid), fragments of  both quadrates, left ophistotic,  a lower jaw, cervical vertebrae, one caudal vertebra, appendicular skeleton, a partial carapace and one almost complete plastron is well preserved in MACN Pv M2, and representing one of the most complete cretaceous short-necked chelid known from South America. To star exploring the phylogenetic relationships of MACN Pv M2 it was included in a data matriz built up by 48 characters and 17 taxa. The preliminary phylogenetic analysis suggests that MACN Pv M2 is nested on a clade that includes Phrynops hilarii + Mesoclemmys nasuta + long-necked chelids. The pelvic girdle attached by suture to the carapace and plastron in MACN Pv M2, and the retention of plesiomorphic  and synapomorphic characters such as the pattern of the cervical vertebrae (e.g., third or fourth opisthocoelous, fifth biconvex, sixth procoelous), cervical scale, splenial bone in the lower jaw, ilium extends over eight costal and anterior margin of the suprapigal bone allow us to assign this specimen to Pleurodira Chelidae. The assemblage of plesiomorphies (such as both rami of the lower jaw fused, lateral mesoplastra present, area articularis mandibularis concave, a short midline epiplastral suture, an anterior peripherals bones shorter than posterior ones) and apomorphies (such as extremely wide anterior plastral lobe, wider than posterior lobe, and a slightly epiplastral notch) recognized as autopomorphies within this analysis allow to assigned MACN Pv M2 as a new short-necked chelid taxa. The microanatomy of MACN Pv M2 shows a diploe structure. The internal cortex is equal or slowly thinner than the external one, and the cancellous bone occupy the main proportion (50-60%). The external cortex is composed of structural fibre bundles that extend parallel to the external surface, and oriented longitudinally and transversally to the progression of the elements. The cancellous bone is mostly well developed. The internal cortex consists of parallel-fibred bone that locally can grade into lamelar bone. The histology of MACN Pv M2 shows features that suggest adaptation to the aquatic lifestyle (i.e., well vascularized external cortex, the vascularization of the internal cortex composed of scattered vascular canals and primary osteon of longitudinally orientation).