Trimerorhachis (Amphibia: Temnospondyli) from the Lower Permian of Texas and New Mexico: cranial osteology, taxonomy and biostratigraphy

Abstract

The morphology and taxonomy of the Early Permian temnospondyl amphibian genus Trimerorhachis are revised. Using previously undescribed material from Texas, numerous details of the cranial osteology of T. insignis Cope, 1878 are redescribed. Emphasis is placed on the structure of the palate, the sutures of the skull roof, and on the variation of some phylogenetically important character-states. The taxonomic revision, which makes particular use of morphometric characters, supports the four species previously considered to be valid, and adds a fifth species. T. sandovalensis Berman & reisz, 1980 from the Cutler Formation of New Mexico is probably the most primitive species and does not share any unambiguous derived character-states with the other species. T. insignis – the type species – is the only one known from the Nocona and Petrolia formations of Texas. In the later formations up to the Middle Clear Fork Group, there are two lineages diverging from the T. insignis morphology in opposite directions. One, here designated T. greggi sp. nov. , has larger orbits and a shorter snout and occurs in the Lower, and probably also the Middle, Clear Fork. The other is T. mesops Cope, 1896, which has smaller orbits and a longer snout and occurs in the Lower and Middle Clear Fork and possibly the earlier Waggoner Ranch Formation of Texas. T. rogersi olson, 1955 is a possible late population of T. mesops type that occurs in the late Middle Clear Fork. The simplest model is that there was one short-snouted Trimerorhachis lineage from the base of the Permian to the top of the Petrolia Formation (contemporaneous with the long-snouted Neldasaurus in Archer City Formation beds), followed by two diverging lineages showing some character displacement from the Waggoner Ranch Formation upwards, but possibly incipiently present in the Nocona/Petrolia populations.