''Huehuecuetzpalli mixtecus'' is an extinct lizard from the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

(specifically the middle to late

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

stage)

Tlayúa Formation The Tlayúa Formation is an Early Cretaceous (late Albian) geological formation near Tepexi de Rodríguez (municipality), Tepexi de Rodríguez, Puebla. Paleobiota The formation contains a diverse array of vertebrate and invertebrate fossils. Ab ...

in Tepexi de Rodríguez, Central

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. Although it is not the oldest known lizard, ''Huehuecuetzpalli'' may be amongst the most basal members of

Squamata Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, ...

(the group that includes lizards and snakes), making it an important taxon in understanding the origins of squamates. The generic name comes from the

Nahuatl Nahuatl (; ), Aztec, or Mexicano is a language or, by some definitions, a group of languages of the Uto-Aztecan language family. Varieties of Nahuatl are spoken by about Nahua peoples, most of whom live mainly in Central Mexico and have smaller ...

words ''huehuetl'' ("the ancient") and ''cuetzpalli'' ("lizard"), while the specific name refers to the

La Mixteca La Mixteca is a cultural, economic and political region in Western Oaxaca and neighboring portions of Puebla, Guerrero in south-central Mexico, which refers to the home of the Mixtec people. In their languages, the region is called either Ñuu Djau ...

region.

Description

Unique characteristics (

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...

) of ''Huehuecuetzpalli'' include a long pair of

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

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 The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

and a hole called the parietal foramen between the junction of the

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.''Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, par ...

and the

parietal bone The parietal bones () are two bones in the Human skull, skull which, when joined at a fibrous joint, form the sides and roof of the Human skull, cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, an ...

(the frontoparietal suture) suggest affinities with iguanians, but the retention of divided premaxillae, amphicoelous vertebrae (vertebrae that are concave at both ends),

thoracolumbar The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates ...

intercentra (bones between the vertebrae of the back), an

entepicondylar foramen The entepicondylar foramen is an opening in the distal (far) end of the humerus (upper arm bone) present in some mammals. It is often present in primitive placentals, such as the enigmatic Madagascan ''Plesiorycteropus''. In most Neotominae and al ...

in the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

(upper arm bone), and a second distal tarsal bone in the foot supports the hypothesis that ''Huehuecuetzpalli'' is a basal squamate. A 2021 study suggested 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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

squamates Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, i ...

: Its position as a stem-group squamate has been supported by subsequent studies.''''

References

Cretaceous lizards Reptilia incertae sedis Reptiles described in 1998 Fossil taxa described in 1998 Fossils of Mexico Prehistoric reptile genera {{paleo-lizard-stub