LIMB ALLOMETRY AND LATERAL LINE GROOVE DEVELOPMENT INDICATES TERRESTRIAL-TO-AQUATIC LIFESTYLE TRANSITION IN METOPOSAURIDAE (AMPHIBIA:TEMNOSPONDLYI)

Metoposaurs are generally large temnospondyls known from Upper Triassic strata across much of Pangea. Based on limb size, previous workers proposed that metoposaurs became increasingly aquatic throughout their lives. An allometric study agrees, showing strongly negative allometry (α = 0.76) in the mid-shaft femoral diameter of Buettneria perfecta, which defines limb bone strength because it controls cross-sectional area, and must increase in positive allometry (α = 1.5) to maintain constant stress on the bones during growth. Stress-strength calculations based on the limb allometry suggest that adult metoposaurs were water-bound, while juveniles may have been terrestrial, implying ecological separation of adults and juveniles and explaining the absence of small juveniles in all major metoposaur death assemblages (Scurry County and Rotten Hill, TX, Lamy, NM, Argana, Morocco, Krasiejow, Poland; N >> 150 individuals, all “adults”). It also offers a means by which these animals could disperse throughout Pangea.

Lateral line systems are arrays of water motion sensing organs. Where they overlie dermal bone in metoposaurs, they are housed in well-defined grooves. Most adult metoposaurs (e.g., Buettneria, Dutuitosaurus, Metoposaurus) have deep, wide, lateral line grooves in their skulls, but small juvenile skulls have extremely poorly developed lateral line grooves. We consider this additional evidence of terrestriallity in juveniles and propose that the grooves developed later in ontogeny as the animal became fully aquatic.

Lateral line grooves are also poorly developed on skulls of the small adult metoposaur Apachesaurus; these grooves are reduced to a barely-discernable string of slightly larger pits in the coarsely pitted skull texture. Apachesaurus has been considered a more terrestrial metoposaur because of its elongate vertebrae with robust rib facets. Poorly developed lateral line grooves in Apachesaurus substantiate this hypothesis. Thus, the extent of lateral line groove development is an additional measure of the terrestriality of metoposaurs and supports the idea of terrestrial juveniles and aquatic adults in most metoposaurs, and of terrestriality in adult Apachesaurus. A heterochronic agent (progenesis) may have facilitated speciation of Apachesaurus from larger metoposaurid genera.