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

GARBEROGLIO, FERNANDO F.; GÓMEZ, RAÚL; SIMÕES, TIAGO R.; CALDWELL, MICHAEL W.; APESTEGUÍA, SEBASTIÁN

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 synapomol]lhies that would clearly link them to their closest lizards sistergroup. Tliis is one of the niain reasons beliind the seemingly intractable conflicts arotmd the origin of snakes, as the identificati011 of homologous characters that are shared with their lizard relatives is problematic. We focus here 011 one lllOl]lhological cowplex, the caudal intercentnun system (CIS), otherwise kno"Wn as haemal arches (or cl1evron bones), and their usually associated haeniapophyses (or pedtmcles).Interpretations of a CIS have been reported for three fossil s1iake taxa.: Eupodophis descouenssi, Haasiophis te1rasanctus, Wonambi naracoonensis. For Eupodophis and Haasiophis, conflicting intel]lretations range from neomol]lliic structures with no h0111ologies to the CIS,to haemal arcltes and haeniapophyses homologized with those of other lizards. T11e isolated vertebra referred to Wonambi is more probleniatic. Wonambiand severa!other niadtsoiids show posteroventral processes (i.e.haeniapophyses) 011 the caudal vertebrae, leading previous authors to suggest the presence of liaenial arcl1es, althoug11 the arcltes are not preserved. A similar case o=us with Najash rionegrina; althoug11the presence of haenial arches was not consideredbefore for this taxon, a CIS is str011gly suggested by distinct articulatory surfaces 011 the distal end of the caudal pedicels. Dini(¡1sia patagonica is one of the best preserved and most well-known Cretaceous s1iakes, but its caudal region reniained tmkno\\l. Here we describe the first known caudal series of Dinilysia based 011 MACN-RN-1016, wliiclt comes from the Bajo de la Cal]la Formation of the Neuquén Group (Santonian, Upper Cretaceous), Río Negro Provú1ce, Argentiria. It comprises a stringof 13 articulated vertebrae,from posterior-most precloacals to the first caudals, displayi.ng unequivocal evidence of a CIS: haeniapophyses and tmfused haenial arcl1es.These new elata provides iniportant insigl1tson the presence of a CIS in other fossil snakes. It also challenges previous concepts 011 the homologies of the caudal region in snakes, sucl1 as the idea of '\mpaired fused cltevTons"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 el011gated haeniapophyses and the caudal intercentra (haemal arcl1es) to be lost.