Late Triassic Metoposaurid Amphibian Skull Allometry: Comparison of the Lamy, New Mexico, Population to Four Other Populations

Abstract

Metoposaurs (Temnospondyli: Metoposauridae) were large, ubiquitous amphibians of the Late Triassic. They were important members of the Upper Triassic Chinle Group fauna of New Mexico and are found in these strata across the state. One of the most important metoposaur bonebeds in the world occurs in the Adamanian (upper Carnian) Garita Creek Formation near Lamy, NM, where the fossils of hundreds of individuals were hydraulically concentrated on a Late Triassic floodplain. In nearly all metoposaur populations, worldwide, many skull features show shape change throughout ontogenetic growth (allometry). Some workers have believed these allometric growth trajectories to be of taxonomic value. Here, using the largest populations available (for maximum statistical sample size), we investigate the taxonomic utility of these shape changes. Skull allometry in three populations of the metoposaur Koskinonodon perfectum (Lamy, NM; Rotten Hill, TX; Popo Agie Formation of WY) was compared to establish intra-specific variation. Then, three populations representing different genera ( Dutuitosaurus ouazzoui from northern Africa; Koskinonodon perfectum from western North America; Metoposaurus diagnosticus from eastern Europe) were compared to show variation at the generic level. Anteroposterior movement of the orbits and relative skull width at three positions (snout tip at the anterior nares, basal snout at the anterior orbits, greatest width across the quadratojugals) with respect to midline length throughout ontogeny were determined. Relative orbit position moved anteriorly in D. ouazzoui and K. perfectum (Rotten Hill), posteriorly in K. perfectum (Lamy) and K. perfectum (WY), and remained approximately fixed in M. diagnosticus . Skull width allometries were of mixed values, but with rare exceptions, the snout grew wider with respect to length throughout ontogeny, whereas the posterior skull became relatively narrower. Thus, the skulls of juveniles were somewhat triangular, and they grew to be more parallel-sided with a wider, blunter snout in adults. Comparison of the calculated allometric constants from the various populations yielded a surprising result: intra-specific variation in growth patterns was as great as or greater than inter-generic variation. The three K. perfectum populations, including the Lamy metoposaurs, were not significantly more similar to each other than they were to different genera. The salient point of this preliminary work is that allometric skull growth trajectories in metoposaurs probably have little or no taxonomic value, and metoposaur taxa should probably not be erected based solely on variations in growth patterns.