New Mexico Geological Society Annual Spring Meeting — Abstracts

Abundant and diverse crustaceans occur in upper Turonian shallow marine sandstones of the Fite Ranch Member, Tres Hermanos Formation, near Carthage, Socorro County. Present collections comprise more than 400 identified specimens, including about 90 carapaces, representing 10 species, of which 9 are believed to be new. Decapod assemblages of this magnitude are very uncommon in the Late Cretaceous of North America, and the Fite Ranch collections vastly augment previously known Late Cretaceous decapods from New Mexico (limited to 6 specimens, 3 species).

Brachyuran (6 species; 32% of specimens), anomuran (3 species; 67% of specimens) and palinuran (1 species; 1% of specimens) are present in the Fite Ranch assemblage. Of the brachyurans (true crabs), Pseudonecrocarcinus ovalis (Stenzel) (44 carapaces), Raninella n. sp. (36 carapaces), Necrocarcinus (Necrocarcinus) n. sp. (2 carapaces), N. (Cenomanocarcinus) n. sp. (2 carapaces), Tetracarcinus n. sp. (5 carapaces), and Xanthias? n. sp. (42 chelipeds) have been identified thus far. Anomurans (mostly mud shrimp) include two new species of Protocallianassa (known from more than 200 cheliped elements, several abdomens, and one poorly preserved cephalothorax), and Paguristes n. sp. (27 cheliped elements). Palinurans (lobsters) are rare (5 chelipod elements), and all appear to be Archaeocarabus? n. sp. These decapods mat be considered a Protocallianassa-Pseudonecrocarcinus-Raninella assemblage, after its dominant constituents, which total more than 75% of the specimens.

The discovery of this decapod assemblage in the Fite Ranch Member is important for several reasons: (1) it is by far the most diverse and abundant Late Cretaceous decapod assemblage known from the southern Western Interior; (2) most of the species appear to be new, and thus will add significantly to knowledge of North American Late Cretaceous decapod diversity; (3) the occurrence of these genera in upper Turonian strata in central New Mexico represents stratigraphic and geographic range extensions for most of them; and (4) several of the species are represented by numerous specimens, allowing study of intraspecific variability; this abundance is unusual for a group (Late Cretaceous decapods) in which many of the described North American species are known from but a single specimen.

Although abundant, these decapod remains are typically small (less than 15 mm) and easily overlooked, which probably explains why they have not been reported previously. They occur in a transgressive, nearshore sandstone body, together with large numbers of more conspicuous shallow marine molluscs, including the bivalves Lopha belliplicata novamexicana, Pinna petrina, Inoceramus dimidius and Homomya sp., and the ammonite Coilopoceras colletti. Careful examination of similar depositional environments in other parts of the New Mexico Late Cretaceous sequence may well produce additional decapod assemblages.