PRELIMINARY ANALYSIS OF GROWTH AND AGE STRUCTURE OF BUETTNERIA (AMPHIBIA: METOPOSAURIDAE) ASSEMBLAGES FROM THE UPPER TRIASSIC OF WEST TEXAS AND NEW MEXICO

Abstract

Two mass death assemblages of the Upper Triassic temnospondyl amphibian Buettneria perfecta Case, the “Lamy bonebed” from the Garita Creek Formation in central New Mexico, and “Rotten Hill” from the Tecovas Formation of West Texas, yield tens to hundreds of individuals. We used a statistical approach to resolve size classes (= age groups) in clavicles and interclavicles from which we generated a growth curve and age distribution for Buettneria . Comparison of these data to extant salamander outgroups (e.g., Andrias, Cryptobranchus, Chioglossa , others) and other amphibians showed that growth was indeterminate and that only sexually mature (marked by size, slow linear growth, and age distribution shape) adults were present in the fossil assemblages. They lived, on average, 10 or 11 years past sexual maturity. Linear size (measured by skull and femur length) increased by a factor of ~1.9 between sexual maturity and death, similar to the outgroups. Juvenile Buettneria are recognizable at very small sizes elsewhere in the Chinle Group, but are not present in these assemblages and are very rare in the fossil record even though population dynamics dictates that they must greatly outnumber adults. Where were the juveniles? Analysis of the Rotten Hill population showed that the diameter of Buettneria’s limb bones grew in strong negative allometry; e.g., the allometric constant for femur length versus midshaft diameter = 0.78, where a constant of 1.5 is required to maintain