Huehuecuetzpalli

Huehuecuetzpalli Temporal range: Middle to Late Aptian,
Artist's reconstruction
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Genus:	†Huehuecuetzpalli Reynoso, 1998
Type species
†Huehuecuetzpalli mixtecus Reynoso 1998

Huehuecuetzpalli mixtecus is an extinct lizard from the Early Cretaceous (specifically the middle to late Aptian stage) Tlayúa Formation in Tepexi de Rodríguez, Central Mexico. Although it is not the oldest known lizard, Huehuecuetzpalli may be the most basal member of Squamata (the group that includes lizards and snakes), making it an important taxon in understanding the origins of squamates. It may or may not be a basal member of Iguania, a large clade of lizards that traditionally includes the iguanas and their close relatives, chameleons, and agamids: if it is an iguanian, H. mixtecus represents the earliest major offshoot of the squamate evolutionary tree.

The generic name comes from the Nahuatl words huehuetl ("the ancient") and cuetzpalli ("lizard"), while the specific name refers to the La Mixteca region.

Description

Unique characteristics (autapomorphies) of Huehuecuetzpalli include a long pair of premaxilla bones at the tip of the upper jaw that contributes to an elongated snout and the apparent retraction of the external nares or nostril openings. At the top of the skull, a small rounded postfrontal and a hole called the parietal foramen between the junction of the frontal bone and the parietal bone (the frontoparietal suture) suggest affinities with iguanians, but the retention of divided premaxillae, amphicoelous vertebrae (vertebrae that are concave at both ends), thoracolumbar intercentra (bones between the vertebrae of the back), an entepicondylar foramen in the humerus (upper arm bone), and a second distal tarsal bone in the foot supports the hypothesis that Huehuecuetzpalli is a basal squamate. A recent study suggests that Huehuecuetzpalli was bipedal.

Taxonomy

Huehuecuetzpalli has been suggested to be either a basal squamate or an iguanian. The most parsimonious tree recovered by Reynoso (1998), seen below, recovers it as the outgroup of crown-group squamates:

Squamata	Huehuecuetzpalli Squamata Iguania Agamidae Chamaeleontidae Iguanidae Scleroglossa Scincomorpha Cordylidae Scincidae Gymnophthalmidae Teiidae Lacertidae Xantusiidae Amphisbaenia Dibamidae Gekkota Gekkonidae Pygopodidae Serpentes Anguimorpha Anguidae Xenosauridae Helodermatidae Lanthanotus Varanus (crown group)
(total group)