Small bird and mammal tracks from a mid-Cenozoic volcanic province in Southern Colorado: implications for palaeobiology

Abstract

ABSTRACT The surprise discovery of two small assemblages of well-preserved, small bird and small mammal tracks in a thin unit of reworked tuff in the mid-Cenozoic volcanic field of southwestern Colorado represents the first significant report of tetrapod tracks in the region. The bird tracks are represented by a new ichnospecies of Avipeda (A. circumontis ichnosp. nov.), which differs from other Avipeda ichnospecies in representing a trackmaker with a narrower foot and comparatively robust digit traces and digital pads. The mammal tracks which are also well preserved are small (< 1.5 cm long and wide) tetradactyl-pentadactyl and rodent like. They are arguably unique and among the best-preserved small mammal tracks presently known from the Cenozoic, and are here assigned to the new ichnotaxon Musvestigium minutus ichnogen et ichnosp nov. The track-bearing surface represents a very thin unit of reworked tuff sandwiched between massive ignimbrite flows, and therefore represents a localised window into a brief period of biotic activity in local phase of quiescence in a large, active volcanic province.