The caudal intercentrum system (CIS) of fossil and living snakes revealed by a new specimen of Dinlysia patagonica

FERNANDO FABIO GARBEROGLIO; RAÚL ORENCIO GÓMEZ; TIAGO R. SIMOES; MICHAEL WAYNE CALDWELL; SEBASTIAN APESTEGUIA

Calgary

Congreso; Society of Vertebrate Paleontology 77th annual meeting; 2017

Society of Vertebrate Paleontology

Snakes are an extremely derived and long-lived clade of lizards that have either lost or highly modified many of the synapomorphies that would clearly link them to their closest lizards sistergroup. This is one of the main reasons behind the seemingly intractable conflicts around the origin of snakes, as the identification of homologous characters that are shared with their lizard relatives is problematic. We focus here on one morphological complex, the caudal intercentrum system (CIS), otherwise known as haemal arches (or chevron bones), and their usually associated haemapophyses (or peduncles). Interpretations of a CIS have been reported for three fossil snake taxa: Eupodophis descouenssi, Haasiophis terrasanctus, Wonambi naracoortensis. For Eupodophis and Haasiophis, conflicting interpretations range from neomorphic structures with no homologies to the CIS, to haemal arches and haemapophyses homologized with those of other lizards. The isolated vertebra referred to Wonambi is more problematic. Wonambi and several other madtsoiids show posteroventral processes (i.e. haemapophyses) on the caudal vertebrae, leading previous authors to suggest the presence of haemal arches, although the arches are not preserved. A similar case occurs with Najash rionegrina; although the presence of haemal arches was not considered before for this taxon, a CIS is strongly suggested by distinct articulatory surfaces on the distal end of the caudal pedicels. Dinilysia patagonica is one of the best preserved and most well-known Cretaceous snakes, but its caudal region remained unknown. Here we describe the first known caudal series of Dinilysia based on MACN-RN-1016, which comes from the Bajo de la Carpa Formation of the Neuquén Group (Santonian, Upper Cretaceous), Río Negro Province, Argentina. It comprises a string of 13 articulated vertebrae, from posterior-most precloacals to the first caudals, displaying unequivocal evidence of a CIS: haemapophyses and unfused haemal arches. These new data provides important insights on the presence of a CIS in other fossil snakes. It also challenges previous concepts on the homologies of the caudal region in snakes, such as the idea of ?unpaired fused chevrons? in derived snakes. The new data from Dinilysia indicates it is more likely for the ventral projections in the caudals of extant snakes to represent elongated haemapophyses and the caudal intercentra (haemal arches) to be lost.