New Mexico Geological Society Annual Spring Meeting — Abstracts

New Mexico Museum of Natural History and Science (NMMNH) locality L-3845 in the Petrified Forest Fonnation of the Chinle Group (Rio Arriba County, NM) yields fossils representing a rich fauna of vertebrates (fish and reptiles) and invertebrates (mostly conchostracans) of early-mid Norian (~218-210 Ma) age. We discuss here a probable decapod crustacean (NMMNH P-29041) found in the spoil pile at the site. The specimen is in matrix of slightly sandy mudstone that contains poorly sorted, texturally mature sand-to gravel-size clasts of very fine grained mudstone. Location of the specimen and rock color indicate that it probably originated in one of several fming-upward sequences between 0.3
and 1 m above the main bone-bearing layer.

The decapod specimen is dorsoventrally compressed, preservation is only fair, and, due to its fragility, preparation is proceeding slowly. The specimen measures 48 mm long by 19 mm wide exclusive of legs and antennae. No clear indication of in seen theI present state of preparation, however, two pairs of antennae are present; one pair is apparently very long. Of five pairs of legs, the first pair are slender and apparently bear small chelae. Limb pairs two, three, and four are robust uniramous pereiopods, and the legs of pair five terminate in small paddle-like structures. The wide, flat body, reduced abdomen, and lack of a caudal fan indicate the animal is probably of the brachyurous
(short-tailed) type, which includes the crabs. The type and sequence of thoracic limbs is typical of swimming crabs. However, only the ventral surface is exposed, so the carapace, whose morphology is particularly diagnostic of the Malacostraca, is not visible. However, based primarily on the presence of five pairs of thoracic limbs, we tentatively assign it to the Decopoda.

Decapod crustaceans first appear in the Late Devonian. Ten families of decapods are known from the Triassic; four of these are found in North America, and two are known from the Chinle Fonnation. Therefore, this specimen is particularly important because it sheds light on a rarely preserved aspect of the Chinle fauna and thus provides insight into the freshwater ecosystems of the Upper Triassic.