The Eocene Messel snake fauna: a window into the early evolution of macrostomatan snakes

KRISTER SMITH; AGUSTÍN SCANFERLA

Quito

Congreso; XI Congreso Latinoamericano de Herpetología; 2017

Macrostomatans(boas, pythons, colubroids and their relatives) constitute by far the mostdiverse clade of extant snakes. Their diversity is attested by the astonishingvariety of morphologies displayed by the more than 2800 known species. Aroundthe world, the fossil record of macrostomatan snakes is represented by a largecollection of specimens, the vast majority of which are isolated bones,especially vertebrae. Hence, the information offered by complete specimens isreally necessary to establish a robust background for the evolution ofmacrostomatan snakes. The Messel fossil Konservat-Lagerstätte near Frankfurt amMain (Germany) is an UNESCO World Heritage site renowned for its exceptionallypreserved fossil specimens. Systematic excavations at this site have led to theidentification of over 130 vertebrate species that inhabited a small lakebasin, under a warm, humid environment 47 million years ago. Among the squamatespecimens recovered in this fossil site there is a large number of skeletons ofdifferent snake species. Thus far, three different species of snakes have beendescribed from Messel. The species Messelophisvariatus and Rieppelophis ermannorum were minute boid snakes (SVL 400-800 mm)known from tens of specimens. The skeleton of these small snakes clearlyindicates different modes of life, from surface-dweling/arboreal (M. variatus) to cryptozoic (R. ermannorum). A remarkable specimen of Messelophis variatuscontains skeletal elements of a tiny snake preserved within the body cavity.These bones are positioned in the posterior third of the snake skeleton, nearto the vent and far of the inferred position of the stomach. The grade ofdevelopment of the preserved bones and the position inside the host snake suggeststhat these remains represent an advanced embryo. In consequence, this specimenof Messelophis variatus representsthe first fossil record of live bearing in snakes. The other species is Palaeopython fischeri, a large snake (adults approximately 2 meters) that wasnot referred with certainty to any group that currently conform the familyBoidae. However, a complete revision of several specimens and new informationprovided by CT data indicates that P.fischeri belongs to Boinae, thus indicating a more complex biogeographic scenariofor this group of macrostomatans. Also, a new juvenile specimen of Palaeopython fischeri was recentlydescribed, in whose stomach is a lizard, in whose stomach is an insect. This isthe second known vertebrate fossil containing direct evidence of three trophiclevels. The lizard is identified as Geiseltaliellusmaarius, a stem basilisk. A general picture of the trophic ecology of P. fischeri is not yet possible,although the presence of a lizard in the stomach of a juvenile individualsuggests that this snake could have undergone an ontogenetic dietary shift, asin many extant boines. An undescribed mammal specimen that seems to have beenregurgitated by a large snake provides support for this hypothesis. Thus, theexquisite preservation and abundance of recovered specimens make Messel snakefauna as an invaluable source of new insight about the first steps on theevolution of macrostomatan snakes.