Elkobatrachus brocki, a new pelobatid (Amphibia : anura) from the Eocene Elko Formation of Nevada

摘要

Elkobatrachus brocki is a new, small pelobatid anuran that is represented by a small collection of fairly well-preserved and for the most part articulated to closely associated fossils recovered from the middle Eocene Elko Formation near Elko, Nevada. The Elko Formation is divided into three informal members, lower, middle, and upper, and was deposited primarily under warm, temperate conditions in a fluviolacustrine system of a broad, shallow basin extending over a large area of present-day northeastern Nevada. The fossils were preserved in a sandy limestone unit near the base of the middle member, which lies about 100 in above a unit yielding a radiometric date of 46.1 Ma. A phylogenetic analysis suggests that E., brocki is the most primitive pelobatid Currently known and differs from all other pelobatids in the following autapomorphies: 1) alary process of premaxilla is broad-based and forms a laterally-deflected, straight blade whose transverse axis is oriented anteroposteriorly; 2) pars acromialis of scapula is triangular with anteriorly directed apex positioned at the level of the dorsal rim of the glenoid fossa; and 3) urostyle length exceeds that of the vertebral column. Elkobatrachus brocki is the oldest known pelobatid that exhibits burrowing specializations in its skeleton. Thus, like extant pelobatids, it very likely could avoid high daytime temperatures and periods of dryness by constructing a burrow in which it estivated. The ability of early Tertiary pelobatids to presumably avoid drought by estivating in burrows is thought to be a preadaptation for hibernation in burrows to Survive subfreezing temperatures resulting from global cooling that began in the middle Eocene.