A walking fish trace fossil from the early Permian Robledo Mountains Formation of South-Central New Mexico, USA

Abstract

ABSTRACT A new ichnotaxon, Ambulopisces voigti (ichnogen. et ichnosp. nov.), from the early Permian (Wolfcampian) Robledo Mountains Formation (Hueco Group) of south-central New Mexico, is described and shown to represent a fish walking on a muddy substrate. A. voigti is a compound ichnotaxon comprising alternating left and right pectoral fin impressions that show the walking behaviour plus a superimposed sinusoidal caudal fin drag that can be assigned to Undichna. The pectoral fin impressions are surrounded by expulsion rims, which show that the fins were weight-bearing; these are larger posteriorly where sediment was pushed up by the propulsive motion of the fin. The shape of the expulsion rims thus indicates the direction of travel. Propulsion was accomplished by alternately pressing the pectoral fins into the substrate and flexing the body to bring the opposing pectoral fin and the body forward. The identity of the tracemaker is uncertain, but it was a small, low-profile fish whose pectoral fins spanned only ~ 60 mm. The proximity of the pectoral fin bases to the centerline of the trace does not support a lobe-finned fish as the tracemaker.