A vertebrate bone-bed in the Aguja formation (Upper Cretaceous), Big Bend National Park, Texas
Anglen, John Jeffrey
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All known specimens of the giant crocodilian Deinosuchus riograndensis have been recovered from a limited stratigraphic interval within the Terlingua Creek Sandstone and upper shale members of the Upper Cretaceous Aguja Formation in Big Bend National Park. The habitat and behavior of this giant crocodile have long been the subject of speculation. A vertebrate bone-bed near Grapevine Hills that includes remains of D. riograndensis as well as a variety of dinosaurs, turtles, and a mesosuchian crocodile provides an opportunity to study the geology, paleontology, and taphonomy of this stratigraphic interval. The Terlingua Creek Sandstone and upper shale members, as observed in the study area, were divided into 3 related sedimentary facies. Deltaic distributary channel deposits are composed of multi-storied, lenticular bodies of trough cross-bedded sandstone with adjacent, inclined heterolithic strata indicative of channel migration. These channel deposits grade into thin, laterally extensive, coarsening-upward sheet sandstones interpreted as crevasse splay deposits. Crevasse splay deposits contain transported logs and resulted from paleocurrents flowing over a 60-degree arc perpendicular to the distributary channel. The sheet sandstones are separated vertically by layers of bioturbated, carbonaceous mudstone interpreted as interdistributary bay and marsh deposits. An extensive bone-bed within one interdistributary deposit contains a single D. riograndensis individual that died in an interdistributary bay or marsh setting. Overbank flooding scattered and aligned some of the remains and winnowed others away. The same interdistributary bay deposit contains remains of hadrosaur. ceratopsian, and ankylosaurian dinosaurs, as well as turtles and mesosuchian crocodiles in transported and attritional assemblages. Fossil wood containing Teredolites borings indicative of brackish-water conditions is intimately mixed with the bones. Thicknesses of sets of trough cross-beds provide minimum water depths of 1.2 to 2.4 meters for the interdistributary bays. The preferred habitat of Z). riograndensis appears to have been these shallow, brackish-water interdistributary bays. All of the other vertebrates in this environment could have been potential prey for D. riograndensis. Although there is no conclusive evidence, such as abundant bite marks on bones, to conclude that D. riograndensis was responsible for the attritional assemblage of vertebrate remains here, this possibility is as likely as others.