1022


Journal of Paleontology 91(5):1001–1024


Figure 13. Comparison of the junctions of the peripetalous and lateral fascioles (outlined) in (1) Linthia alabamensis Clark, 1915, MMNS 7164, from the McBryde Member, Clayton Formation, Palmyra Tract, Lowndes County, AL and (2) Ova rancoca n. sp., holotype, USNM 540894, from the Vincentown Formation, Vincentown, Burlington County, NJ. Scale bars are 5mm.


migration of echinoid genera from east to west across the Atlantic Ocean, but rarely in the other direction. The expectation is that while theremaybegeneracommontoboththe easternandwestern hemispheres, it is probable that genetic drift would rapidly split any immigrant species in North America into separate clades. The expanding Atlantic Ocean basin was likely a significant biotic barrier at the beginning of the Paleocene, becoming more so over time. Nearly all of the echinoid occurrences in the eastern North


American Paleocene are in carbonate rocks. These are generally coarse-grained, and in the case of the Vincentown Formation (Thanetian) in New Jersey and the Clayton Formation (Danian) exposed in western Georgia and eastern Alabama comprise bryozoan and coralline algal debris. The fault-bound exposure of Salt Mountain Limestone (Thanetian) in Alabama represents a coral reef buildup, although the echinoids (mostly spines and test fragments) are found in the detrital fore-reef deposits. What paleoecological differentiation there is between the southern fau- nas of the Gulf Coast (mostly Danian) and the northern faunas of the Atlantic Coast (Thanetian) is obscured by the differences in age. Furthermore, the lack of any definite Selandian echinoid occurrences in eastern North America inhibits any study of the transition between these two main faunal groups of echinoids. Finally, although echinoids are common at a very few local sites, these occurrences are generally monospecific, and echinoids are rare overall fromthese Paleocene rocks and not often found in situ. This has proven a serious impediment to paleoecologic studies. What is clear is that theDanian in theGulfCoast region represents a period of faunal recovery following the catastrophic end of the Cretaceous, an effect perhaps enhanced by proximity to the Chicxlub impact, which is emphasized by the low diversity and paucity of the echinoid fauna. One aspect of the diversity indica- tive of recovery is that the majority of the species are epibenthic (or semi-infaunal) forms, either regular urchins, or irregular urchins, lacking fascioles, whose modern representatives are epibenthic. Paleocene carbonate platforms were dominated by


bryozoan and coralgal communities (Scheibner and Speijer, 2008), and Kroh (2001, 2003) noted a preference of epibenthic forms from the Danian of Austria for coralline-algae dominated carbonate environments and a similar preference could be at play in the faunas studied here.


Acknowledgments


I am indebted to G. Phillips, Mississippi Museum of Natural Science, Jackson, MS, for loan of material, arrangement and logistic support of collecting trips, and for many discussions regarding the geology and paleontology of the Gulf Coast. I would like to thank K. Lucker, Resource Management Services, Greenville, AL for access to Palmyra property; C. Gardner, Waste Management of Mississippi, Houston, MS for access to the Prairie Bluff Landfill; C. Swann and G. Martin for donation of specimens; C. Garvie for loan of specimens. I also thank R. Portell, Florida Museum of Natural History; A. Molineux, Non-Vertebrate Paleontology Laboratory, University of Texas; and D. Levin, M. Florence, and K. Hollis, U. S. National Museum of Natural History for their help with and access to collections. P. Calloman, Academy of Natural Sciences of Drexel University, Philadelphia for information on types. M. Hopkins and B. Hussaini, American Museum of Natural History, supplied the photographs of the lectotype of Linthia tumidula. Finally, I would like to thank A. Kroh and an anonymous reviewer for many comments that substantially improved the manuscript.


References


Agassiz, L., 1838–1842, Monographies d’Échinodermes Vivans et Fossiles: Neuchâtel, aux frais de l’auteur, Petitpierre, 621 p.


Agassiz, L., 1840, Catalogus systematicus ectyporum echinodermatum fossilium Musei Neocomensis, secundum ordinem zoologicum dispositus; adjectis synonymis recentioribus, nec non stratis et locis in quibus reperiuntur. Sequuntur characteres diagnostici generum novorum vel minus cognitorum: Solothurn, Oliv. Petitpierre, 20 p.


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