VERTEBRATES FROM A MIDDLE EOCENE ESTUARINE MANGROVE COMMUNITY IN THE RIO GRANDE EMBAYMENT

A diverse mangrove-dwelling vertebrate community comes from the middle Eocene Laredo Formation, Claiborne Group, of Webb County, Texas. Fifteen tons of estuarine oyster-shell “hash” excavated at Laredo were screen-washed and treated with acetic acid. Non-mammalian vertebrate remains provide evidence of a diverse suite of species which indicate local salinity, water depth and climatic conditions. These include the sharks, sawfish and rays Striatolamia macrota, Carcharhinus, Galeocerdo eaglesomei, Pristis, Rhinobatis, Dasyatis, Myliobatis and Rhinoptera. Bony fishes include Lepisosteus, a megalopid (earliest record in North America), cf. Paralbula marylandica, cf. Arius, Diaphyodus wilsoni, Jefitchia and Cylindracanthus. Crocodilians include Pristichampsus vorax, Allognathosuchus sp., and a caiman. Turtles cf. Baptemys, cf. Allaeochelys, Trionyx, an emydid and Hadrianus. The presence of Pterosphus schucherti marks its first record in middle Eocene strata of North America. Taxa including cf. Allaeochelys, Galeocerdo eaglesomei, Diaphyodus, Pterosphenus schcherti and the megalopid indicate a Tethyan influence. A glyptosaurine lizard, three birds and an amphibian were also recovered. Paleoecological analyses of vertebrate, invertebrate, and floral remains indicate that the community lived in or near a tropical, Nypa mangrove-estuarine complex. Thirty-two mammal species include four new omomyid primates, marsupials, insectivores, a bat, a mesonychid, carnivores, a horse, two rhinos, a titanothere, six artiodactyls, a sirenian and at least eight rodent species. Epihippus gracilis, Amynodon advenus, Mytonomys, Protoreodon petersoni and Leptoreodon leptolophus indicate that the middle Laredo Formation is Uinta C in age. Turritella cortezi allows correlation of the Laredo Formation with the Hurricane Lentil, Cook Mountain Formation of east Texas, dated at 41.6 Ma. and lower Bartonian in age.