Huehuecuetzpalli

Huehuecuetzpalli mixtecus is a small lizard from the Albian Tlayúa Formation of Puebla, Mexico. Its name comes from Nahuatl words for “ancient lizard.” A nearly complete adult skeleton and a juvenile give researchers an unusual chance to compare growth stages in an early squamate. The skull and limbs combine traits that look closer to living squamates with features retained from older reptile lineages. Its position near the base of Squamata is a result of anatomical analysis, not evidence that it was a direct ancestor of modern iguanas or any other living family. It joins other early forms in the ancient lizard and snake catalogue.

Quick facts

A fossil from the Tlayúa quarry

Víctor-Hugo Reynoso described the genus and species in 1998 from fossils collected at Tlayúa, near Tepexi de Rodríguez in central Mexico. The adult holotype, IGM 7389, preserves most of the skeleton. A second specimen, IGM 4185, is smaller and was interpreted as a juvenile. Both came from the middle member of the Tlayúa Formation, a deposit famous for exceptionally preserved marine fossils as well as land animals washed into the basin.

The adult and juvenile are useful together because a feature that changes during growth should not automatically be treated as a species-level character. Their difference in size is direct evidence of at least two age stages, although the sample is too small to chart growth rates, sexual variation or the maximum size of the species.

A long snout and a mixed skull

The skull has an elongated premaxilla, the bone at the front of the upper jaw. This arrangement lengthens the snout and makes the external nostrils appear set farther back. The skull roof also combines features that were used to compare the animal with iguanians with a set of more primitive conditions, including paired premaxillae. A parietal opening lies near the boundary between the frontal and parietal regions.

The vertebrae are amphicoelous, concave at both ends, and small intercentra occur along the trunk. The wrist and ankle retain elements that are reduced or absent in many living lizards. These are observations from the skeleton. They do not by themselves establish a single simple position on the family tree.

Why its classification has been debated

Reynoso’s original analysis placed Huehuecuetzpalli outside the crown group formed by living lizards and snakes, but close to it. A small postfrontal and the parietal opening suggested iguanian affinities, while other features supported a more basal placement. Reynoso also considered whether some apparently primitive traits might reflect paedomorphosis, the retention of juvenile features into adulthood.

That proposal is a possible explanation, not a direct observation of development. Later phylogenetic matrices can change the position of an extinct taxon when new fossils, characters or alternative scorings are added. It is therefore safer to describe the animal as an early squamate with a mosaic anatomy than to call it an “ancestor” of a modern group. The much older Triassic Megachirella and the Jurassic Bellairsia provide different anatomical snapshots rather than steps in a proven family line.

Could it run on two legs?

The hind limbs are longer than the forelimbs, and the animal had a long tail. Those proportions prompted a suggestion that it could briefly lift the front of its body and run bipedally, as some modern lizards do. This would be a facultative gait used under particular conditions, not a permanently upright posture.

No trackway has been attributed to the species, and no preserved joint sequence records a running stride. The proposal is a functional inference from limb proportions. Ordinary movement may have been quadrupedal, while short bursts on the hind limbs remain plausible but unconfirmed.

Life around the Tlayúa lagoon

During the Albian, the Tlayúa area included a shallow marine carbonate setting. Fine limestone layers preserve fishes, invertebrates and terrestrial vertebrates carried from nearby land. A fossil’s burial in a lagoon does not make the animal aquatic. The developed limbs and body proportions are consistent with a small terrestrial lizard, while the exact habitat it preferred is unknown.

Small teeth are compatible with feeding on invertebrates, but neither specimen preserves gut contents. No eggs, nest, footprints or bite marks identify its prey or reproductive behaviour. The Tlayúa fossils reveal the place where the animal was buried, not every place where it lived.

What the two skeletons can tell us

Many early squamates are known from jaws or isolated vertebrae. The paired Tlayúa specimens are more informative because they preserve the skull, trunk and limbs in the same individuals. They support a small-bodied lizard with a distinctive snout and a combination of ancestral and derived skeletal traits. They do not resolve every branch near the origin of living lizards, and the suggested running gait remains a testable interpretation rather than a fossilised behaviour.