Tetrapodophis amplectus is not a snake: re-assessment of the
osteology, phylogeny and functional morphology of an Early Cretaceous
dolichosaurid lizard

Abstract

The origin of snakes remains one of the most contentious evolutionary
transitions in vertebrate evolution. The discovery of snake fossils
with well-formed hind limbs provided new insights into the
phylogenetic and ecological origin of snakes. In 2015, a fossil from
the Early Cretaceous Crato Formation of Brazil was described as the
first known snake with fore- and hind limbs (Tetrapodophis amplectus),
and was proposed to be fossorial, to exhibit large gape feeding
adaptations (macrostomy) and to possess morphologies suggesting
constriction behaviours. First-hand examination of T. amplectus,
including its undescribed counterpart, provides new evidence refuting
it as a snake. We find: a long rostrum; straight mandible; teeth not
hooked zygosphenes/zygantra absent; neural arch and spines present and
tall with apical epiphyses; rib heads not tubercular; synapophyses
simple; and lymphapophyses absent. Claimed traits not preserved
include: braincase/descensus parietalis; 'L'-shaped nasals;
intramandibular joint; replacement tooth crowns; haemal keels;
tracheal rings; and large ventral scales. New observations include:
elongate retroarticular process; apex of splenial terminating below
posterior extent of tooth row; >10 cervicals with hypapophyses and
articulating intercentra; haemapophyses with articulating arches;
reduced articular surfaces on appendicular elements; rows of small
body scales; and reduced mesopodial ossification. The axial skeleton
is uniquely elongate and the tail with >100 vertebrae is not short as
previously claimed, although overall the animal is small (~195 mm
total length). We assessed the relationships of Tetrapodophis using a
revised version of the original morphological dataset, an independent
morphological dataset, and these two datasets combined with molecular
data. All four were analysed under parsimony and Bayesian inference
and unambiguously recover Tetrapodophis as a dolichosaur. We find that
Tetrapodophis shows aquatic adaptations and there is no evidence to
support constricting behaviour or macrostomy.

Open access:
https://www.tandfonline.com/doi/full/10.1080/14772019.2021.1983044