Biodiversity and Extinction Patterns of Chondrichthyes at the Cretaceous-Paleogene Boundary, Central Texas
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The Cretaceous-Paleogene (KP) mass extinction is the second largest mass extinction in the history of the world with over 70% of known marine and terrestrial species suffering extinction in that event. One of the most well studied KP sections is located in the Brazos River in Falls/Milam County and has been extensively studied for its sedimentology, isotopic properties, and latest Cretaceous microfossils, but the vertebrate fauna has not been studied until now. The Brazos River sites are composed of hummocky cross-bedded sandstone beds with intercalated bone beds between the hummocks. I sorted hundreds of teeth from sharks, batoids, and bony fish, as well as thousands of bone fragments from the processed sediments. At present, I have identified at least 15 genera and 18 species of elasmobranchs including members of the genera Rhinobatos, Rhombodus, Squalicorax, Carcharias, and Pararhincodon. The chondrichthyian fauna iv from the Brazos sites inhabited many different niches ranging from benthic to pelagic forms. Modern members of many of the chondrichthyian forms inhabit shallow, warm to temperate waters, with some forms that are found in deeper waters. Of the identified genera 73% are found after the KP Boundary, but none of the identified species are known to occur in the Paleogene. The Brazos River sites show a high degree of chondrichthyian biodiversity right before the KP Boundary. The Brazos River sites were compared to four other late Maastrichtian North American sites using Simpson’s Faunal Similarity index. The Kemp Clay in Texas shared 86.67% of its genera with the Brazos sites; the Arkadelphia Formation in Arkansas shared 66.67% of its genera with the Brazos sites; the Peedee Formation in North Carolina shared 37.5% of its genera with the Brazos sites; and the New Egypt Formation in New Jersey shared 46.67% of its genera with the Brazos sites. As expected, geographically closer regions had a higher faunal similarity index than regions separated by greater distances.
Janus, Tracey (2011). Biodiversity and Extinction Patterns of Chondrichthyes at the Cretaceous-Paleogene Boundary, Central Texas. Available electronically from