PALEOLIMNOLOGY OF TWO DINOSAUR WATERING HOLES IN WEST TEXAS

Hundreds of crayfish burrows are present in dinosaur-bearing strata in Big Bend National Park, Texas and in nearby Moon Valley, Terlingua. Burrows vary from two to 15 centimeters in diameter, and some exceed one meter in length. Burrows mostly have a single limb although some branch and a few have greatly enlarged chamber portions. Most have characteristic external “mud ball” texture. No body fossils or gastroliths have been discovered. Burrows in Big Bend are mostly oblique to horizontal in orientation and are found in maroon-colored siltstone directly below a thin lumpy bed of lacustrine limestone. The burrows are six meters above a juvenile alamosaur bone bed in charophyte-bearing mudstone. Burrows in Moon Valley exist in great profusion as dense as three per square meter in gray to purple-brown siltstone. Most are vertically oriented. An adult alamosaur femur is present in pebbly concretion approximately 3.5 m above the burrows.

Burrows in both locations are associated with large viviparid gastropods. The presence of these gastropods in conjunction with the crayfish burrows indicates that semi-permanent, non-marine water bodies of undetermined size existed in both locations. The abundance of burrows in Moon Valley is best interpreted as representing an extended period of little deposition rather than the existence of numerous specimens. Crayfish are omnivorous, even cannibalistic, and are recognized predators on snails. The absence of both fish fossils and of small snails perhaps reflects the recognized predatory efficiency of crayfish in small ponds. It remains unknown if crayfish were actually dining on dinosaur remains as is suggested by the bits of bone that are present adjacent burrows in both locations.

Crayfish and gastropods assist in reconstruction of local Upper Cretaceous limnology as each group responds to specific physical and chemical parameters. Crayfish and viviparid gastropods are considered to be hearty, as they exist in a wide range of temperatures although optimum crayfish breeding occurs at approximately 20ºC. Crayfish thrive only in clean, well-oxygenated, calcium rich, alkaline water (pH of 7 to 9). Viviparid gastropods function well within these parameters. One might assume that extant dinosaurs found these semi-permanent ponds to be suitable sources for water.