New Mexico Geological Society Annual Spring Meeting — Abstracts

A new tracksite in the Abo Formation, New Mexico Museum of Natural History (NMMNH) locality 4394, is 2.4 km south of the type section at Abo Pass. The tracks are found in a thinly bedded, fine-grained sandstone unit 141 m above basal contact Abo Formation, so it is presumably middle Wolfcampian in age. The track-brearing stratum is a 0.3-1.3-m-thick sandstone with extensive ripple laminae that we interpret to represent a shallow water, floodplain sandflat. Six vertebrate ichnotaxa are present: Limnopus vagus Marsh, Dromopus agilis Marsh, Batracgichnus delicatulus Lull, Dimetropus sp., Gilmoreichnus hermitanus Gilmore, and Hyloidichnus bifurcatus Gilmore. The vertebrate tracks show a distinct lack of tail drags, and there is no significant invertebrate ichnofauna at this site. The Abo Pass tracksite is older than the Robledo Mountain mega-tracksite (NMMNH locality 846) in south-central New Mexico, but has a similar tetrapod ichnofaunal diversity. However, the Abo Pass tracksite shows a distinct difference in the abundance of Limnopus, the track of a large temnospondyl amphibian. Limnopus is 1 % of the census at NMMNH locality 846, whereas Limnopus is 50% of the census at the Abo Pass tracksite. Factors that could account for this abundance differnce of Limnopus include facies and age differences. Limnopus is rare at other sites of facies similar to the Abo Pass tracksite, such at those near Villanueva in northern NM. The Abo Pass tracksite thus suggests that large amphibians were more common in inland environments during the middle part of the Wolfcampian than in coastal environments during the late Wolfcampian.