VERTEBRATE BURROWS AND ASSOCIATED TERRESTRIAL TRACE FOSSILS, LATE EOCENE STRATA, MCCARTY MOUNTAIN, SOUTHWESTERN MONTANA: BURROWS, NOT CONCRETIONS

Results from our study-in-progress on the ichnopedofacies of late Eocene, early Chadronian strata demonstrate that the mammal-fossil-bearing “nodules” or “concretions” at McCarty Mountain, southwest Montana, are actually burrow complexes in paleosols. Many of these paleosols have pedogenic features characteristic of Inceptisols but there are also some with more mature features that are classified as Alfisols. The burrows occur in a massive to blocky mudstone that also contain rhizoliths, rhizocretions, rare rhizohaloes, a variety of dung beetle traces including Coprinisphaera, wasp and beetle cocoons assigned to Fictovichnus, and soil bee cells assigned to Celliforma.

The burrows represent at least three morphotypes based on their architectural morphology. Morphotype I is a helical burrow with a near-circular diameter of 10–14 cm, forming at least four helices over a depth of ~1 m; this form is assigned to Daimonelix. Morphotypes II and III are burrow complexes are similar in their architecture, composed of multiple meters of mostly horizontal to subhorizontal, multiple, irregularly branched T- and Y-shaped tunnels, and chambers, but differ greatly in their diameters. Complexes with smaller circular to elliptical tunnels (II) range from 20–35 cm in diameter with chambers up to 70 cm long and 45 cm wide. Complexes with larger circular to elliptical tunnels (III) range from 40–60 cm in diameter with chambers up to 50 cm long and 80 cm wide.

Several potential tracemakers may have produced these burrows based on the fossil mammals and tortoises found at McCarty Mountain over the past 125 years. Tortoises vary widely in size based on their age and are abundant in Chadronian strata. Mammalian carnivores recovered from similar-aged strata at or not far from McCarty Mountain, like Ursids (Parictis), Amphicyonids (Brachyrhynchocyon), Canids (Hesperocyon), and Creodonts (Hyaenodon) are also a possibility. Rodents and rabbits appear to be too small to have made the burrows, but small artiodactyls may be possible. We excavated a burrow within late Eocene basin-fill strata in southwestern Montana that contained a fully articulated oreodont, partial remains of the canid, Hesperocyon, and rabbit teeth––all which are part of the McCarty Mountain faunal assemblage, and, thus, oreodonts are potential burrowers at this locale.