Trilobites From a Cambrian Extinction Interval at the Base of the Steptoean, Riley Formation, Central Texas
Abstract
The Riley Formation of central Texas provides the only record of Steptoean trilobites in the southern mid-continent region. The objective of this thesis is to document the faunal assemblages associated with the globally documented extinction event at the base of the Steptoean (Saltzman et al., 2000) and its aftermath. A positive carbon isotope excursion (SPICE; e.g., Saltzman et al., 2000) that has been recognized on several continents but not previously in the southern mid-continent of Laurentia, is also documented. Peak SPICE δ13C values have been recorded up to +5‰; the Riley Formation records the climbing limb of the SPICE and a peak δ13C value of +3‰. The SPICE is also associated with marine transgression and deposition of siliclastic sediments, which is characteristic of the Cap Mountain and Lion Mountain members of the Riley Formation.
Palmer (1954) recognized two faunal assemblage in the Riley Formation, the Aphelaspis Zone and the overlying post-Aphelaspis Zone. Here the zones are revised into species-based assemblages: the Coosella perplexa Zone, Aphelaspis spinosa Zone, Aphelaspis longifrons Zone, and Blandicephalus texanus Zone. The most recent study of the trilobites in the Riley Formation is over 60 years old (Palmer, 1954) so here select taxa is revised from the Cap Mountain Limestone and the Lion Mountain Sandstone. The genus Blountia is of particular interest because it is one of the few genera to survive the extinction at the base of the Steptoean. Pre- and post-extinction species from across Laurentia are revised and a preliminary phylogenetic analysis investigates the relationship between the species, and the relationship with two relatives, Maryvillia and Blountina, which some authors (e.g., Pratt, 1992; Rasetti, 1956) consider to be synonymous with Blountia. Aphelaspis, Blandicephalus, Cheilocephalus, Coosella, Coosina, Dunderbergia, Glaphyraspis, Llanoaspis, Labiostria, and a new genus related to Dunderbergia are also looked at. New species include Blountia morgancreekensis, Blountia nevadensis, Blountia angela, Blountia newfoundlandensis, Glaphyraspis dianae, and Glaphyraspis richardi.
Collections
- OU - Theses [2090]