PALEOECOLOGY OF COASTAL ENVIRONMENTS USING INVERTEBRATE ASSEMBLAGES OF THE CRETACEOUS WOODBINE FORMATION OF NORTH CENTRAL TEXAS

The Woodbine Formation (Cenomanian: 94-98 Mya) of North Central Texas is known for its unique fossil assemblage from a little studied stage in Earth history. Research at a productive fossil locality in Tarrant County called the Arlington Archosaur Site (AAS), has determined that the paleoenvironmental setting is that of an ancient coastline with an intertidal to brackish water system. The coastal system has a “White Zone.” A section of coastline associated with a coastal environment where fresh water deposits and salty marine deposits maintain equilibrium within the tide cycles which is supported by invertebrate taxa biomarkers and trophic structure. The sediments were deposited on a low lying coastal plain along the Rudradian peninsula by delta channel bifurcation and avulsion. The coastal system was part of the Greenhorn cycle interior seaway that ran through North America during the high stand of the mid-Cretaceous. The Woodbine exposures preserve a diverse ecosystem that was well suited for a delta plain including: turtles, crocodyliforms, dinosaurs (theropod and ornithopod). However, an under addressed issue with the Woodbine Formation is the understanding of the invertebrate ecology of these sediments. Various invertebrate groups such as bivalves, decapods, gastropods, and cephalopods existed in the Woodbine waters and had lifestyles that allowed them to adapt to varying coastal environments. Invertebrate taxa recovered from the Woodbine Formation sediments include Brachidontes, Anchura, Linuparus, and Acanthoceras. All of these taxa play intricate roles in each of the environments and influenced by these intertidal systems. Biostratigraphically, Anchura, Linuparus, and Acanthoceras are restricted to the Lewisville Member of the Woodbine, and were used as markers to assign the AAS to the Lewisville Member with an approximate age range of 96-98 MYA. Many of the invertebrates occur within the lower strata and represent changes in environments from an intertidal to brackish waters. This data demonstrates the proximity of the Woodbine members to the Cretaceous coastline and development of near shore marine trophic structures.