New Mexico Geological Society Annual Spring Meeting — Abstracts

Several workers have proposed that Late Triassic metoposaurid amphibians became increasingly aquatic during their ontogenetic development. Our data document moderately strong negative allometry (allometric constant = 0.76) in the femur diameter of Buettneria perfecta from the Lamy (NM) Amphibian Quarry. Femur diameter is a measure of limb bone strength and must increase in positive allometry (allometric constant ~ 1.5) to maintain constant stress on the bones throughout growth. We conclude that large, adult Buettneria were water-bound, whereas the juveniles were probably much more terrestrial. This provided ecological separation of adults and juveniles, and explains the absence of small juveniles in all major water-deposited metoposaur death assemblages.

Lateral line systems are arrays of water motion sensing organs present in fishes and aquatic amphibians. Where lateral line organs are present over dermal bone, they are housed in well-defined grooves in the bone. Adult Buettneria from Lamy have deep, wide, lateral line grooves in their skulls in an established pattern. In contrast, a small juvenile Buettneria skull from St. Johns, AZ, shows extremely poorly developed lateral line grooves. We propose that because lateral line organs are useless except in water, the undeveloped nature of the grooves represents additional evidence of terrestriality in this juvenile metoposaur and that the grooves would become fully developed as the animal became fully aquatic.

We also note poorly developed lateral line grooves on a partial Apachesaurus skull from Apache Canyon, NM. The grooves here are reduced to a barely-discernable string of slightly larger pits in the already pitted skull texture. Apachesaurus has been considered to be a somewhat terrestrial metoposaur because of its more elongate vertebrae that show robust rib facets. The poorly developed lateral line grooves in Apachesaurus seem to substantiate the more terrestrial habitat of this metoposaur. Thus, the extent of lateral line groove development is an additional measure of the terrestriality of these animals. Reduced lateral line groove development strengthens the idea of a terrestrial juvenile and an aquatic adult Buettneria, and of terrestriality in Apachesaurus.