ISOPOD AND TANAIDACEAN CRUSTACEANS (PERACARIDA) FROM THE CRETACEOUS (ALBIAN) OF CHIAPAS, SOUTHERN MEXICO: SYSTEMATIC AND PALEOECOLOGICAL IMPLICATIONS

Two new species of isopod (Sphaeromatidae and Cirolanidae) and one tanaidacean (Antharcocarididae) are part of the crustacean assemblage from a new fossil-bearing locality in southeastern Mexico. Dolomites at the base of the Sierra Madre Formation of probable Albian age, crop out in a series of quarries near the town of El Espinal. The new sphaeromatid species is represented by 150 specimens, most of them molts, preserved as dorsal, ventral and lateral (rolled) views, without appendages. Some specimens presenting a ventral aspect show pereopods, mouth parts, antennae, uropods and coxal plates. A few specimens are noticeable larger, and the size difference may represent sexual dimorphism. There is also a small number of juvenile specimens with the same morphological characters. The cirolanid species is represented by 63 individuals, most of them being more entire and better preserved than the sphaeromatids; nearly all of the individuals include pereopods, mouth parts, pleopods, uropods and antennae. Cirolanids are observed in dorsal, ventral and lateral (rolled) views. As with the sphaeromatids, cirolanids are also represented by juveniles and adults. SEM images revealed macro and micro cuticular structure which in sphaeromatids appears as a honeycomb pattern; the cuticular structure is not clearly visible in the cirolanids. The flora and fauna associated with this assemblage, as well as sedimentological features, suggest a paralic depositional environment, probably an estuary or shallow lagoon. The faunistic marine elements are poorly represented by a lobster exuvia and some echinoderm plates. Microfossils are absent except for some unidentified ostracods. The sphaeromatid fossil record is the best represented among isopods, although usually only one isopod species is reported from each fossil localities. This report documents the coexistence of two species from different families, which most closely resembles that of extant genera. Tanaidaceans are the most abundant group in the study locality. The occur in discrete horizons in what is interpreted as mass mortality events. Hundreds of specimens have preserved chelae, cephalothorax, pereon, pleon, telson and pereopods. According to chelae size, two types of specimens are found, as some present chelae larger than cephalothorax.