Habemus excrementum: large herbivorous tetrapod coprolites from the Cretaceous of Patagonia, Argentina.

APESTEGUÍA, S., MAKOVICKY, P.J., GALLINA, P. OTERO, A., ŐSI, A. Y SMITH, N.

ushuaia

Jornada; V Reunión Argentina de Icnología y III Reunión de Icnología del Mercosur; 2007

CADIC

The Huincul Formation has provided an abundant tetrapod record, in which dinosaurs are dominant, including herbivores of large (i.e. titanosaurian and rebbachisaurid sauropods) and medium-size (i.e. iguanodontian ornithischians), as well as carnivorous dinosaurs (i.e. abelisauroids, carcharodontosaurids, dromaeosaurids). Strata cropping out in Río Negro Province, some 10 to 15 km from El Chocón, contain not only bones but also abundant remains of fossil wood, including a partial petrified forest at the same stratigraphic level. A recent paleontological expedition to the area discovered abundant coprolites containing plant remains. The material is silicified and natural breaks reveal plant ultrastructures. Most of the coprolites are dome-shaped, around 12 to 15 cm in diameter and less than 10 cm in height. Most remains have been concentrated as an erosional lag. No specimens were found in situ, but some coprolites are still surrounded by a matrix devoid of plant remains, showing always a well defined coprolite boundary. The plant material is arranged in a roughly circular pattern, with larger elements bent at the edges. Several different plant taxa are represented by a variety of organs, but twigs and shoots (ca. 1 cm in diameter) from a single morphotype characterized by peculiar knobs are predominant. The twigs show straight cuts with frayed surfaces that are approximately perpendicular to their main axes. Rarer plant remains include stems and possible reproductive structures of sphenophytes, and other taxa yet to be determined. The presence of abundant seeds in a few specimens indicates the occasional ingestion of possible angiosperm fruits. Additionally, the coprolites represent a unique taphonomic window into the Huincul Formation, preserving plant organs that do not occur elsewhere. Coprolite dimensions suggest that the producers were mid-sized herbivorous dinosaurs, such as ornithopod ornitischians or sauropod juveniles. Detailed analyses of the cut surfaces of the plant matter coupled with comparative anatomy of the jaws and teeth of possible consumers may allow for more exclusive referral to a particular group. Ornithischian buccal morphology has been interpreted as indicative of extensive oral processing, so the poor oral processing exhibited in these coprolites suggest an assignment to sauropods. Future taxonomic identification of the twigs and their elevation in the cannopy may provide an estimate of the size of the consumer and its feeding strategy. The exquisite preservation and diversity of plant structure in the Huincul Formation coprolites promises to yield novel paleofloral and paleoenvironmental insights, as well as important new data on plant-animal interactions. This new material further underscores the importance of coprolites as a unique source of paleobiological information.