Lacertulus bipes (Carroll and Thompson 1982) ~ Late Permian 255 mya, ~ 5 cm snout/vent length, was originally described as a facultatively bipedal primitive lizard or eosuchian representing a distinct lineage of one or the other. Here Lacertulus was derived from a sister to Gephyrosaurus and Homoeosaurus. It was a phylogenetic predecessor to the squamates including Meyasaurus, Iguana and Liushusaurus.
Overall smaller than and distinct from Gephyrosaurus, the skull of Lacertulus had a wider premaxilla, like that of Homoeosaurus. The nasal was notched above the naris. The maxilla formed the majority of the ventral margin of the naris. The orbit was probably large. The maxilla was relatively short.The vomers were wider. Thepterygoids extended anteriorly as far as the palatines did. The lateral process of the palatine was set further posteriorly. The mandible was relatively short (in height) as well. Distinct from most lizards, the teeth were subthecodont in implantation.
There were ~24 presacral vertebrae, perhaps one fewer than in Gephyrosaurus. The neural arches were low and not fused to the centra. The neural spines were low to absent. None of the tail vertebrae show any sign of caudal autotomy, which in several living lizards permits the tail to breakoff during an attack and regrow later.
The forelimbs were much shorter than the hind limbs, which suggests the ability to run bipedally, as do 19 living lizards. The radius and ulna display little to no expansion of their ends. The olecranon is ossified in a pattern distinct from living lizards. The carpals are tiny but ossified and rounded, rather than locked in a tight mosaic as in many living lizards. The loss of ossification in the carpus in Huehuecuetzpalli likely signalled the shifting of carpals seen in Cosesaurus, a descendant. The metacarpals and fingers seem to be similar to those in Huehuecuetzpalli.
The pelvis was taller than long, as in Gephyrosaurus, with a distinct anterior process on the ilium, but much smaller than in Cosesaurus. Such a process in living lizards capable of bipedal locomotion helps raise the front of the body off the ground (Snyder 1954). The pubis was narrower, as in Huehuecuetzpalli, producing a larger thyroid fenestra. The femur was longer than half the glenoid-acetabulum length. The tibia was more robust than the fibula. The astragalus, calcaneum and centrale remained unfused, as in descendant taxa and unlike most lizards. The pes was twice the length of the manus. Metacarpals 1-4 were bound together. The foot was large with a tendril-like digit 4 ideal for arboreal (tree branch) locomotion. |
