323-6 Window into Post-Extinction Dynamics: Survival, Diversification, Deep Water, and (Regional) Endemicity Documented in the Early Paleocene Wills Point Formation of Texas (USA)

Abstract:

The early Paleocene Wills Point Formation (Midway Group) has a very poorly known marine vertebrate fauna stratigraphically positioned (~62-64 Ma) after the end Cretaceous mass extinction and within the so-called Patterson Gap. However, recent efforts are uncovering a unique fauna demonstrating that this time period is key to understanding aspects of the Mesozoic to Cenozoic marine transition. The bony fish record (Osteichthyes) comprises vertebral and other skeletal fragments, but is mostly represented by a growing diversity of otolith taxa. Some of these otolith species (Serranus? caribbaeus, Ampheristus americanus, and Hoplostethus stringeri) are known extinction survivors restricted to Texas and Arkansas. Their records in the younger Wills Point Formation extends their known survival in Texas for millions of years after the mass extinction. This survivorship of regional endemics relatively close to the impact in the Yucatan Peninsula in the western part of the gulf is unexpected. Much of the otolith diversity is centered among the percomorphs (members of Ophidiiformes, Scrombiformes?, Acropomatiformes) and helps support the Danian as a key interval of the post-extinction radiation of this group that is dominant today. The shark fauna (represented by isolated teeth) has a lower diversity, but comprises representatives of different clades including Carcharhiniformes (Triakidae) and Squaliformes (Squalidae and Etmopteridae?). The triakid appears to be a species of Palaeogaleus, but fragments of other tiny teeth with thin central cusps point to the possibility of a lamniform or even another squaliform lineage. The squaliform fossils of a species of Squalus and an Etmopterus-like taxon likely are the first records of this ordinal clade in the Danian of the Gulf Coast, and they support the interpretation of parts of the formation as being deposited in deep waters. Squalus is a mass extinction survivor, but at present Etmopterus and its closest relatives are known only later in the Cenozoic. The Wills Point fauna helps to demonstrate that the later part of the Danian was a key time in marine vertebrate evolution with: 1. a unique mixture of Cretaceous surviving species and lineages coincident with early records of post-extinction diversifying clades (such as the percomorphs); 2. the long-term survival of some regionally endemic fish otolith taxa; and 3. insights into deep water faunas on the Gulf Coast.

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