TAPHONOMY OF THE EARLY PERMIAN CARDILLO QUARRY, A TETRAPOD BONEBED IN THE CHAMA BASIN, NORTH-CENTRAL NEW MEXICO

The Early Permian Cardillo quarry is located near Arroyo del Agua, in the Chama Basin in north-central New Mexico. The quarry is stratigraphically low in the Cutler Formation and is Wolfcampian in age. During excavations in 1979, 1980 and 2002, the remains of the labyrinthodont amphibian Eryops, the diadectamorph Diadectes, a captorhinid, a varanopseid pelycosaur, and the pelycosaurs Sphenacodon and Ophiacodon were recovered from this locality. Numerically, the pelycosaurs dominate the bonebed. Taphonomic data collected during the 2002 excavations reveal that the Cardillo quarry fossil assemblage is an attritional accumulation. The bones lie within a series of three distinct, pedogenically-modified conglomerates that also include calcrete nodules, chert, quartzite and other siliceous pebbles. The skeletal material is mostly disarticulated, though a partially articulated pelycosaur skeleton has been recovered from the overbank sediments above the uppermost conglomerate. Isolated skeletal elements and bone fragments are in various stages of weathering and show differing degrees of abrasion. The assemblage does not appear to have been hydraulically sorted because all three Voorhies groups are well represented. The Cardillo quarry assemblage was formed by a series of crevasse splays, and the formation of each splay deposit followed the same sequence of events. First, basement material (siliceous pebbles) was incorporated into lag deposits in a stream, together with individual bones and bone fragments from upstream. This lag of pebbles and bones was then reworked onto the floodplain by crevasse splays during flooding of the stream, and probably incorporated additional bone material that was lying on the floodplain. The mixture of bones and pebbles was subsequently buried under fine silts during overbank deposition and pedogenically modified. Thus, the Cardillo quarry is a classic example of a time-averaged vertebrate fossil assemblage.