Macrocnemus bassanii (Nopcsa 1931) Ladinian, Middle Triassic ~220mya, is known from several specimens ranging in size and varying in morphology. Romer (1970) considered Macrocnemus a lepidosaur. Carroll (1988) placed it with Protorosaurus and Prolacerta, which is where most current workers consider it, but that would be an error. Here, following a larger phylogenetic analysis, Macrocnemus is derived from a sister to the basal lizard, Huehuecuetzpalli. The smallest Macrocnemus, BES SC 111, is also the most primitive. Larger species are considered below. As in Huehuecuetzpalli, this clade practices isometric growth, so all morphological differences are indeed valid traits, not signs of immaturity within a single genus or species.
BES SC 111 (Renesto & Avanzini 2002) ~30 cm, was previously described as a juvenile, but has a tooth count a third higher than in the larger T4822. Phylogenetically it appears at the base of a clade that includes all the larger specimens of Macrocnemus, which in turn lead to Tanystropheus, Dinocephalosaurusand Langobardisaurus+ Tanytrachelos. BES SC 111 also leads to Jesairosaurus and Cosesaurus.
Distinct from Huehuecuetzpalli, the rostrum was further elongated, the postorbital portion was shortened and the jugal produced a quadratojugal process. The teeth were all small, numerous and in contact with one another. The maxilla was nearly as tall as the orbit.
The cervicals series of eight was robust and elongated, clearly distinct from the dorsals. All neural spines were short. Only two sacrals were present, but adjoining vertebrae had broad transverse processes. The chevrons were further reduced and many were appressed to their centrum. Gastralia were present.
The coracoid and scapula had no fenestrations and the short scapula had a posterior lean. The two centrale appear to have migrated to the medial carpus but remain posterior to distal carpal I. Manual digit V was shorter than metacarpal IV.
The ilium was taller with a smaller posterior process. The femur and tibia were subequal. The astragalus and calcaneum were subequal in size creating a mesotarsal ankle joint. The metacarpals were less disparate in size but IV was longer than III. Pedal digit V was reduced, subequal to metatarsal IV. Renesto and Avanzini (2002) demonstrated a match between the pes of BES SC III and sprawling but digitigrade Rhynchosauroides ichnites.
An extradermal frill extends sagittally from the nasals to the anterior caudals. Hairs or strands appear on the tail. The dewlap was deeper than the skull. Scaled membranes extended medial to the tibia and on either side of the anterior caudals.
BES SC III appears to be arboreal, but in a different fashion than the drepanosaurids. Long, tendril-like extremities suggest active basilisk-like climbing rather than slow chameleon-like grasping. A further reduction in caudofemoral retractor anchors continues a trend towards reliance only on pelvis anchors to rotate, rather than retract the hind limbs. A basilisk-like adornment and other similarities suggests a possible similar lifestyle and behavior regimen.
All known specimens of Macrocnemus had a relatively short pedal digit five, so they are distinct from all other huehuecuetzpalians. However, several characters appear to bridge gaps between Huehuecuetzpalli and Cosesaurus. Strangely, many of the nested descendants of Macrocnemus retained or regained the elongated digit V. This is a mystery that needs more data and more study.