COPROLITES ACROSS THE CRETACEOUS/TERTIARY BOUNDARY, SAN JUAN BASIN, NEW MEXICO

We document Late Cretaceous (Kirtlandian, Edmontonian) through early Cenozoic (Puercan, Torrejonian, Wasatchian) vertebrate coprolites from the San Juan Basin in San Juan and Sandoval Counties, New Mexico. A minimum of four morphotypes are present (spindle shaped, elongate cylinders, cylindrical segments, conglomerated masses) that exhibit various surface textures (worn, smooth, pitted, striated) and other features (constriction marks). This includes one specimen assignable to Alococopros triassicus, extending the chronological distribution of the ichnotaxon into the Late Cretaceous. Most of these coprolites can be confidently assigned to carnivorous producers based on morphological, compositional (phosphatic) and inclusional evidence. Many have inclusions (such as bones or gar scales) that are easily identifiable. With plant fossils and herbivorous vertebrate body fossils some of the most prevalent found in the Late Cretaceous-Eocene strata of the San Juan Basin, an explanation is warranted for the lack of plant-bearing coprolites from the area. The fact that the more durable, phosphatic fecal remains of carnivores are more readily fossilized than the feces of herbivores due to accelerated bacterial mineralization seems a likely explanation. Another possible explanation for the lack of coprolites produced by herbivores lies in the relatively small number of coprolites documented from the San Juan Basin, despite collection and study of vertebrate assemblages in the area for over 100 years. A similar issue arises when we consider the relatively small size (<10 cm long) of all coprolite specimens (and assumed producers) from the Late Cretaceous Fruitland, Kirtland, and Ojo Alamo formations in the San Juan Basin, despite the relatively large size of many of the known vertebrates. It is difficult to determine with certainty if this is due to preservational or collection bias. Despite the inherent biases and uncertainties, we find that coprolite morphologies (size, shape, surface texture) and inclusions (bone fragments, fish scales, and gas bubbles) do not change significantly across the K/T boundary in the San Juan Basin. This suggests that either none of the preserved coprolites are dinosaurian, or that dinosaurian coprolites are homeomorphic with those of some other vertebrates, such as crocodiles.