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

AGUSTÍN SCANFERLA; KRISTER SMITH

Quito

Congreso; XI CONGRESO LATINOAMERICANO DE HERPETOLOGÍA; 2017

Macrostomatans (boas, pythons, colubroids and their relatives) constitute by far the most diverse clade ofextant snakes. Their diversity is attested by the astonishing variety of morphologies displayed by the morethan 2800 known species. Around the world, the fossil record of macrostomatan snakes is represented by alarge collection of specimens, the vast majority of which are isolated bones, especially vertebrae. Hence, theinformation offered by complete specimens is necessary to establish a robust background for the evolution ofmacrostomatan snakes. The Messel fossil Konservat-Lagerstätte near Frankfurt am Main (Germany) is anUNESCO World Heritage site renowned for its exceptionally preserved fossil specimens. Systematicexcavations at this site have led to the identification of over 130 vertebrate species that inhabited a small lakebasin, under a warm, humid environment 47 million years ago. Among the squamate specimens recovered inthis fossil site there is a large number of skeletons of different snake species. Thus far, three different speciesof snakes have been described from Messel. The species Messelophis variatus and Rieppelophis ermannorumwere minute boid snakes (SVL 400-800 mm) known from tens of specimens. The skeleton of these smallsnakes clearly indicates different modes of life, from surface-dweling/arboreal (M. variatus) to cryptozoic (R.ermannorum). A remarkable specimen of Messelophis variatus contains skeletal elements of a tiny snakepreserved 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 of development of the preserved bonesand the position inside the host snake suggests that these remains represent an advanced embryo. Inconsequence, this specimen of Messelophis variatus represents the first fossil record of live bearing in snakes.The other species is Palaeopython fischeri, a large snake (adults approximately 2 meters) that was not referredwith certainty to any group that currently conform the family Boidae. However, a complete revision of severalspecimens and new information provided by CT data indicates that P. fischeri belongs to Boinae, thusindicating a more complex biogeographic scenario for this group of macrostomatans. Also, a new juvenilespecimen of Palaeopython fischeri was recently described, in whose stomach is a lizard, in whose stomach isan insect. This is the second known vertebrate fossil containing direct evidence of three trophic levels. Thelizard is identified as Geiseltaliellus maarius, 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 individual suggeststhat this snake could have undergone an ontogenetic dietary shift, as in many extant boines. An undescribedmammal specimen that seems to have been regurgitated by a large snake provides support for this hypothesis.Thus, the exquisite preservation and abundance of recovered specimens make Messel snake fauna as aninvaluable source of new insight about the first steps on the evolution of macrostomatan snakes.