LATE TRIASSIC (ADAMANIAN) VERTEBRATE MICROFOSSIL ASSEMBLAGE FROM THE BLUE MESA MEMBER OF ARIZONA REVEALS COPROLITES WITH SCALE, BONE, AND TOOTH FRAGMENTS

The Blue Hills, in Apache County Arizona, USA, are famous for their Late Triassic (Adamanian) vertebrate fossils. These badlands, southeast of the Petrified Forest National Park and northeast of St. Johns, Arizona, expose part of the lower Chinle Group, including both the Bluewater Creek and Petrified Forest Formations. The Blue Mesa Member of the Petrified Formation here is primarily mudstones representing floodplains including overbank and pond deposits. The first microvertebrate collections from the Blue Hills came from the “meal pot” localities found by C.L. Camp of the University of California Museum of Paleontology (UCMP) in the 1920s. The New Mexico Museum of Natural History (NMMNH) began collecting vertebrates there in 1997. In addition to microvertebrates, the Blue Hills also yield larger metoposaurs, aetosaurs, phytosaurs, and other tetrapods as well as numerous coprolites. In the lower Chinle Group, coprolites comprise the vast majority of the vertebrate trace fossils, as almost all Chinle track sites are in the upper portion of the unit.

Recent discoveries from NMMNH locality L-4127, about 4 meters above C.L. Camp’s microvertebrate localities in the lower Blue Mesa Member of the Petrified Forest Formation, have yielded microvertebrate-sized coprolites, some of which include scales, bones, and tooth fragments. The coprolites are most commonly subrounded, but some are pellet-shaped or cylindrical. They are generally white, beige, or gray with black and/or red spots, and range from 1–5 mm in size. The bone, scale, and possible tooth fragments range from 0.3–3 mm in size. Of the total coprolites currently collected, those with visible fossils comprise 4.5% of the total. Scales are uncommon, whereas bone fragments are more commonly found. Some investigation of smaller (≤0.5 mm diameter) fossiliferous matrix from L-4127 has revealed similar, but less frequent (~1.5%), coprolites with bones, teeth, and scales. Teeth are far less common, whereas scales are much more common in this size fraction. Although the tooth fragments may pertain the coprolite perpetrator, bone and scales obviously represent food items. The fossil record contains more coprolites from carnivores than from herbivores, which can provide information and insight on patterns of predation and on digestive efficiency.