Journal of Paleontology, 91(5), 2017, p. 1001–1024 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.22
Paleocene echinoid faunas of the eastern United States
Louis G. Zachos Department of Geology and Geological Engineering, 118G Carrier Hall, University ofMississippi, University,MS 38677 ⟨
lgzachos@olemiss.edu⟩
Abstract.—Twenty-one species of echinoid are reported from Paleocene rocks of the eastern United States along the outcrop belt from Texas to New Jersey. Six of these are strictly early Paleocene age (Danian), nine strictly late Paleocene age (Thanetian), five range throughout the Paleocene, and one previously reported Paleocene species (Diplodetus moscovensis) is most likely a Cretaceous species. Two new species are described, Salenia palmyra n. sp., from the Danian Clayton Formation in Alabama and Georgia, and Ova rancoca n. sp., from the Thanetian Vincentown Formation in New Jersey. One new species from the Clayton Formation in Mississippi, Arbaciella? sp., is left in open nomenclature.
Introduction
Sedimentary rocks of Paleocene age are exposed in the eastern United States in the Atlantic Coastal Plain from New Jersey to South Carolina, and the Gulf of Mexico Coastal Plain from Georgia through Texas (Fig. 1). Unfortunately, the spottiness of unweathered Paleocene outcrops throughout this region, the wide range in lithologies (and associated paleoecologies), and the relatively poor collection record have resulted in the lack of comprehensive studies of the macrofaunas of these deposits. This is particularly true of the echinoid faunas. The Paleocene was a period of transition in echinoid faunas,
with retention of a distinctly Cretaceous character and only a few tantalizing glimpses of the genera that came to dominate the remaining Cenozoic. Only the echinoid fauna of the late Paleocene (Thanetian) Vincentown Formation of New Jersey (Fig. 2) has been fairly well known until now, and it has long been recognized as more closely resembling the faunas of the Late Cretaceous than those of the post-Paleocene Cenozoic (Cooke, 1959). This study, following new collection, curation, and taxonomic re-evaluation of Paleocene echinoids from the Gulf of Mexico Coastal Plain as well as additional material from the Vincentown Formation, is a comprehensive look at their diversity and general biogeography within the eastern United States. The Paleocene echinoids of the Gulf Coast are a more
diverse group than previously documented, with cidaroids, salenioids, temnopleuroids, phymosomatoids, arbacioids, holasteroids, cassiduloids, and spatangoids. No evidence of clypeasteroids has been found in any Gulf Coast sediments older than Ypresian (early Eocene). The 21 species of echinoids reported in this study are distributed unequally in the Paleocene- age rocks of the eastern United States (Fig. 3). Six of these are strictly early Paleocene age (Danian), nine strictly late Paleocene age (Thanetian), five range throughout the Paleocene (one of these, Plagiochasma cruciferum, possibly ranging from the Late Cretaceous of Delaware), and one previously reported Paleocene species (Diplodetus moscovensis) is most likely a strictly Cretaceous species.
None of the 21 listed species ranges into sediments younger
than Paleocene. Only four nominal species of early Eocene (Ypresian) echinoids are known from the eastern United States. These include an unidentified hemiasterid from the Nanjemoy Formation, Evergreen, Prince George County, Virginia (Zachos and Levin, 2010), Linthia hollandi Barry (in Barry and Le Blanc, 1942) from the Sabinetown Formation, Pendleton Bluff, Sabine County, Texas, Eurhodia elbana Cooke, 1942, and Fibularia texana (Twitchell, in Clark and Twitchell, 1915), both from the Bashi Formation, below the dam south of Elba on the Pea River, Coffee County, Alabama. This abrupt loss of diversity and turnover in the echinoid genera and species coincides with the Paleocene/Eocene Thermal Maximum (Zachos, et al., 2001). Echinoid diversity increased during the middle and late Eocene (Lutetian, Bartonian, and Priabonian) in the region, marking the recovery of echinoid faunas following this significant climatic event and the loss of many of the holdover genera of the Cretaceous.
Geologic setting
Paleocene sediments are exposed in a narrow belt from New Jersey into northern Virginia (Fig. 1), but with only a few scat- tered outcrops inNorth and South Carolina. In the Gulf ofMexico coastal plain there is a nearly continuous outcrop belt of Paleo- cene deposits stretching 2000 km. from southwestern Georgia to the Rio Grande in Texas (and continuing into Mexico). The stratigraphic correlation of Paleocene formations along this outcrop belt is shown in Figure 2. The Atlantic Coastal Plain deposits are primarily siliciclastic and glauconite rich, although the echinoid-bearing portion of the Vincentown Formation is calcareous. The lowermost Paleocene deposits in the eastern Gulf of Mexico Coastal Plain are calcareous, but clays are predominant in the upper portion. The calcareous beds thin considerably into the Mississippi Embayment region but are found on both the Mississippi and Arkansas sides of the embayment, grading northward into siliciclastic marine beds that extend into southern Illinois (Cope et al., 2005). Except for
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