TAXONOMIC AND MORPHOLOGIC CHANGES IN CORALS AND RUDISTS FROM APTIAN AND ALBIAN STRATA, TEXAS AND PUERTO RICO

Changes in the environment have modified the ecological role of reef builders through geologic time, and major extinctions have reset evolutionary patterns. In Cretaceous reefs, the relative ecologic role of scleractinian corals and rudist bivalves is investigated. During the middle Cretaceous in the Caribbean region, rudist bivalves took over the role as dominant skeletal organisms, and corals co-dominated or were fewer in number than rudists, but numbers supporting this hypothesis are lacking. The change in faunal composition occurred during the Aptian to middle Albian in the Caribbean region but occurred during later intervals of time in the Mediterranean Province. In this work, I investigate if taxonomic and morphologic changes occurred in both corals and rudists at the critical mid-Cretaceous interval of faunal turnover in the Caribbean region. Paleolatitudinal trends from the core of the tropics (Puerto Rico) to its northern margin (Texas) are examined, thereby tracing an environmental gradient in the tropical ecosystem. Temporal comparisons are made from the Aptian through Albian, bracketing the critical ecological interval that crosses a mass extinction boundary. Taxonomic diversity and morphologies are presented for corals and rudists from Albian strata from Puerto Rico (Barrancas Formation, Rio Maton Limestone, Aguas Buenas Limestone) and Texas (Mural Limestone, Glen Rose, Rodessa Formation, Edwards Limestone, Stuart City, and Devil's River Formation). Taxonomic names and morphologies from the Albian are compared to those from the older Aptian rocks occurring in these same regions. The existing hypothesis of morphologic change from massive and laminar corals in pre-middle Albian times, to massive/laminar forms in the middle Albian, to branching corals dominating Lower Cenomanian rocks is presented and investigated. Gaps in the data base occur and are highlighted. Sub-stage resolution is possible only for the Texas region; stage-level resolution is presented for Puerto Rican strata.