Huehuecuetzpalli mixtecus (Reynoso 1998) Middle to Late Albian, Early Cretaceous ~110 mya, ~9.5 cm snout to vent length. Evidently Huehuecuetzpalli was also present in the early Triassic because it nests at the base of all lizards in the analysis by Reynoso (1998). Huehuecuetzpalli was derived from a sister to Lacertulus and Meyasaurus. Huehuecuetzpalli was a phylogenetic predecessor to a new clade of now extinct lizards that included Cosesaurus, Macrocnemus, Jesairosaurus and ultimately, pterosaurs.
Distinct from Meyasaurus, the skull of Huehuecuetzpalli was lower with a posteriorly displaced and enlarged naris. The premaxilla was extended anteriorly. The skull was narrower in dorsal view. The premaxillary ascending process extended nearly to the orbit. The nasal was further split by the posterodorsal expansion of the naris. The pineal foramen was on the frontal/parietal suture. The postfrontal was reduced. It lost contact with the upper temporal fenestra, replaced with lateral extensions of the parietal and medial extensions fo the postorbital. Rather than a broad and solid palate (as in Lacertulus), the pterygoid was shorter, narrower and reduced to just the transverse processes and the quadrate processes. The vomers and palatines were also reduced to struts.
The vertebral transverse processes and chevrons were smaller.Three sacrals were present. The caudals were attenuate. The chevrons paralleled the centra. As in many other lizards, each tail vertebrate included a zone of weakness for breakage in case of attack.
An ossified sternum covered the posterior tip of the T-shaped interclavicle. The scapula and coracoid were fenestrated anteriorly, as in most living lizards. The scapula was very short, but likely made taller by a cartilage extension, as in many living lizards. The radius and ulna were expanded at their ends. The carpus was unossified, even on the adult. This had important ramifications in the lineage of a successor, Cosesaurus in which the two centralia migrated to the anterior (radius side) of the forelimb to become the pteroid and preaxial carpal.
The ilium developed a long posterior process. The fibula was less than half the width of the tibia. The calcaneum was half the size of the astragalus. Metatarsal 5 was shorter and further twisted. Metatarsals 3 and 4 were subequal.
Huehuecuetzpalli could have been an arboreal insectivore considering the extended lengths of its tendril-like toes, the brevity of its torso and the weight reduction in its attenuated palate. The robust hind limb and three sacral vertebrae might suggest a facultative bipedal capability in the manner of similar living lizards and Lacertulus, but the lack of an anterior ilial process argues against that possibility.
A key factor with great ramifications within the Pterosauria, a juvenile Huehuecuetzpalli is known and it is virtually identical in proportion to the adult (Reynoso 1998). So this clade pracitised isometric growth with juveniles and hatchlings virtually identical to adults (i.e. lacking a short snout and large eyes). Mammals, crocs, birds, turtles and other lizards all practice allometric growth in which changes occur with maturity.
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