MICROVERTEBRATE PALEOECOLOGY OF THE CRETACEOUS WOODBINE FORMATION (CENOMANIAN) AT THE ARLINGTON ARCHOSAUR SITE, NORTH-CENTRAL TEXAS

Cenomanian terrestrial vertebrate communities are poorly known in North America. The mid-Cenomanian Woodbine Formation in north-central Texas was deposited in a coastal setting on the southeastern margin of the Cretaceous intercontinental seaway. Sedimentological, paleobotanical, and microvertebrate evidence indicates the Arlington Archosaur Site (AAS) represents an intermittently inundated delta plain environment. The AAS preserves, in ascending order: a peat bed representing a low growth understory; a vertebrate-rich mudstone; a paleosol (histic gleysol) with a heavily rooted zone containing calcareous concretions associated with burned logs and tree stumps, indicating seasonal dryness and wildfires; and a fossil rich siderite sand transgressive lag.

Collection activities at the AAS initially focused on quarrying macrovertebrates, i.e. crocodilians and the ornithopod dinosaur ?Protohadros. The complete plastron and carapace of a large indeterminate turtle have been collected, as well as the disarticulated remains of adult and juvenile crocodilians and ?Protohadros. Numerous crocodilian teeth (Woodbinesuchus et al.) are present throughout the section and are more common in the bone bearing basal peat layer. Ornithopod teeth attributed to ?Protohadros are present in the bone producing mudstone-paleosol horizon, but are rare in the basal peat bed and overlying lag deposits.

Intensive surface collecting and a program of screen washing initiated in 2009 have revealed a diverse ichthyofauna, including chondrichthyans (an indeterminant hybodont, Cretodus, Pseudohypolophus, and Onchopristis), the semionotiform Lepidotes, an indeterminate pycnodont, the tetraodontiform Stephanodus, and the lungfish Ceratodus sp. nov. Theropod dinosaur teeth and salamanders are also present. The fauna suggests changing environmental conditions at the AAS. Many chondrichthyans are tolerant of fresh water, yet the ray Pseudohypolophus is brackish to marine. Although the presence of disarticulated dinosaur remains in marine sediments can be explained by the ‘bloat and float’ model, the permeable skin of salamanders requires that they inhabit fresh water. The stratigraphy and fauna suggests a terrestrial setting followed by a transgressive lag and more dominant marine conditions.