Cainozoic Research, 8(1-2), pp. 13-28, December2011 Chondrichthyans from the Clayton Limestone Unit of the Midway Group (Paleogene: Paleocene) of Hot Spring County, Arkansas, USA ¹, ² Martin+A. Becker Lauren+C. Smith¹ & John+A. Chamberlain+Jr. 1 Department ofEnvironmentalScience, William Paterson University, Wayne, New Jersey 07470; e-mail: [email protected] 2 Department ofGeology, Brooklyn College andDoctoralProgram in Earth and EnvironmentalSciences, City University ofNew YorkGraduate Center, New York 10016; email:[email protected] Received 7 September 2010; revised version accepted 26 June 2011 LimestoneUnit of The Clayton the Midway Group (Paleocene) in southwestern Arkansas preserves one ofthe oldest chondrichthyan Cenozoic from the Gulf Coastal Plainofthe United assemblages yet reported States. Present are at least eight taxa, including; Odontaspis winkleriLeriche, Carcharias cf. whitei Carcharias Anomotodon 1905; (Arambourg, 1952); sp.; novus (Winkler, 1874); Cretalamnasp.; Otodus obliquus Agassiz, 1843; Hypolophodon sylvestris (White, 1931); Myliobatis dixoni Agassiz, 1843; and a chimaeridofindeterminate affiliation.Also present are lamnoid-type and carcharhinoid-type chondrichthyan vertebral centra. The Clayton chondrichthyan assem- blage derives from an outcrop locatedonly a few kilometersfrom a site exposing an assemblage ofMaastrichtianchondrichthyans from Because and the upper Arkadelphia Marl. these assemblages are closely spaced stratigraphically geographically, they provide data on chondrichthyan taxonomic turnover across the Cretaceous/Paleoceneboundary in this regionof the GulfCoastal Plain. The evolutionary bottleneck in chondrichthyan diversity associatedwiththe end-Cretaceousmass extinction event that hasbeendocumented fromother parts ofthe world also appears to be strongly expressed inthe Arkansas region ofthe GulfCoastal Plain. KEY WORDS:: Clayton LimestoneUnit,southwest Arkansas, chondrichthyans, Paleocene,K/P boundary. Introduction formes (Kent, 1994; 1999a). To date, only a few reports offossil chondrichthyans have For over one and a quarter centuries, the occurrence of been documentedfrom Arkansas, none ofwhich occur in Cenozoic chondrichthyan fossils has been well- consecutive formations.These reports include: 1)two Mis- documentedin North America. Earliest reports are those sissippian teeth from the Pitkin Formation and a single offamous vertebrate such Edward paleontologists as Cope, Paleocene tooth from the Midway Group (Freeman, Othniel Marsh and who Joseph Leidy published numerous 1966); 2) sixteen species from the uppermost San- and identifications of Cenozoic chondrich- tonian-lowermost descriptions Campanian, including one new genus from the Atlantic Coastal Plain Marsh, and from the thyans (ie.g. 1869; two new species Brownstown Marl (Meyer, Cope, 1875a; Leidy, 1877). To date, most states inundated 1974); and, 3) at least seventeen species from the by sea levelrise during the Cenozoic contain some record Maastrichtian Arkadelphia Formation (Becker et al, ofchondrichthyan fossils (e.g. Fowler, 1911; Thurmond 2006). This is surprising considering that the stratigraphic & Jones, 1981;Wroblewski, 2004); and in some instances record in the southwesternpartofthe state consists largely their in formations occurrence consecutive (e.g. Ward & of marginal and shallow marine limestone, sand, marland Weist, 1990). Such occurrences in consecutive forma- chalk. It is also interesting to note thatthe shoreline ofthe tions have provided an important basis for classifying the western GulfCoastal Plain ran through this region during appearance of new species as well as documenting faunal much of the late Cretaceous and early Cenozoic (Ken- turnover and extinction of other species. This is particularly nedy etal., 1998). This shorelinereconstruction indicates evident throughout the Chesapeake Bay Region where that entrance to the Western Interior Seaway from the subtle changes in tooth morphology are stratigraphically east would have taken place across southwestern Arkansas chronicled in major chondrichthyan orders such as Lamni- and nearthe base the Ouachita Mountains. -14- Figure 1. 1. Hot Figure Location maps ofClayton LimestoneUnit(Paleocene) ofthe MidwayGroup andArkadelphia Formation(Maastrichtian), Spring County, Arkansas. 1- late Maastrichtian - early Paleocene paleogeographicreconstruction oftheAtlantic and GulfCoastal plainsand Western InteriorSeaway (redrawnfrom Kennedy eta!., 1998; Smith eta/., 1994). Dots indicatethe location ofPaleocene chondrichthyansites discussed in thisstudy: A,New Jersey, (Fowler, 1911; Case, 1996); B, ChesapeakeBay Region, (Ward & Weist, 1990;Kent, 1994); C, SouthCarolina, (Purdy, 1998); D, Mississippi, (Case, 1994); E, Arkansas, (Becker etal, 2006; this study); F, Texas, (Stidham & Janus, 2008); G-H, South and North Dakota, (Cvancara & Hoganson, 1993); I, Wyoming, (Wroblewski, 2004); J, Montana, (Bryant, 1989). 2- Geologic map of Midway and Wilcox Groups (Paleogene) in the southwestern Arkansas study area (modified from Haley etal., 1993). Location ofClayton LimestoneUnit chondrichthyan site (this study) indicated by of Becker al. indicated 3- Detailed locator of Limestone Unit (X) and ArkadelphiaFormationsite et (2006) by (Y). map Clayton (X) and ArkadelphiaFormation (Y) chondrichthyan sites discussed in this study. -15- Figure 2. Fossil site. 1- Main collection Arrows indicate of white to chondrichthyan area. position light gray, highly fossiliferous limestonebeds andthin beds oflight gray marl and sandy marl ofthe Clayton LimestoneUnit. The Wilcox Group is disconform- able with the ofthe upper contact Clayton Limestone Unit and consists of light brown sands and limonite-stainedquartz gravel. 2— Closeup ofCretalamnasp. tootheroding directly out ofClayton LimestoneUnit. 3-Closeup ofCarcharias cf. whitei (Arambourg, 1952)tooth eroding directly out of Clayton Limestone Unit. -16- 1 2 Arkansas. 1-White Figure 3.Closeup ofthe Clayton Limestone Unit ofthe Midway Group, Hot Spring County, to light gray, highlyfossilifer- 2- V in ous limestone bed with multiplesteinkerns of gastropodsand oyster shells; alve oftypical large oyster seen light gray marl and sandy marl. shoreline disconformities Such broad distribution of contemporaneous dense vegetative overgrowth. Regional marine and shallow seaway across the Atlantic and GulfCoastal separate the Midway Group from the underlying Formation and Plains as well as the Western InteriorSeaway may account Arkadelphia (Maastrichtian) overlying similarities and in for the overall in genera species seen non-marineWilcox Group (Eocene) (Haley etal., 1993). which is North America chondrichthyan assemblages. Historically, the Midway Group over 180 metres has not been dividedinto distinct geologic units. However, Commission have In this paper, we describe a new chondrichthyan assem- recent studies by the Arkansas Geologic blage recovered from the Paleocene Clayton Limestone identifiedexpansive clays withinthe Midway Group to be Unitof the in Arkan- of construction Midway Group Hot Spring County, an area concern to practices (McFarland, resulted in localized divisionof sas (Fig. 1). Although no new generaor species arereported 1998). Such concerns have faunais be- into the Limestone Unit that here, the Paleocene chondrichthyan significant the Midway Group Clayton re- cause it occurs stratigraphically directly above, and withina sides directly belowthePorters CreekClay Unit(McFarland, few of of The LimestoneUnit consists of kilometers, an outcrop exposing an assemblage 1998). Clayton primarily Maastrichtian from the limestone chondrichthyans upperArkadelphia highly fossiliferous, light-colored separated by Marl(Becker etal., 2006). The Arkadelphia and Clayton thin beds of clay and sandy intervals while the Porters Limestone assemblages provide a unique opportunity to Creek Clay Unit consists of highly expansive, dark-colored study chondrichthyan taxonomic turnover across the Creta- calcareous clay with minimalfossil evidence. ceous/Paleogene boundary and within close geographic Our Clayton LimestoneUnit site is approximately 3 kilometers of proximity. Large scale, global studies ofKriwet & Ben- northwest ofMalvernand outcrops along both sides an commercial ton (2004) have previously demonstratedthe devastating access road to a shopping facility (Figs. 1- consist of white to effects of the K/P mass extinction event on these highly 2). Outcrop exposures light gray, fossiliferous limestone beds to 0.5 meters thick mobile marinepredators. highly up that contain multiple steinkerns ofmollusks, particularly gastropods and oyster shells (Fig. 3). Interbeddedwith the Materials & Methods limestone are thin beds of light gray marl and sandy marl typically less than 10 centimeterswithoccasional disarticu- less Regional Geology, Location and Age lated oyster shells. Smallinvertebrate fossils, typically abundant the site and The Midway Group is a fossiliferous marginal marine deposit than2 centimeters, are throughout Gulf include and skeletal structures ofmollusks, of Paleocene age that can be found throughout the shells, spines Coastal Plain and several states ofthe Mississippi Embay- brachiopods, arthropods, echinoids, bryozoans and corals. ment. In Arkansas, the Midway Group occurs in the Teethand vertebralcentra from chondrichthyans up to 3.5 centimeters also abundant and efforts southwestern part of the state along a northeastern trend are our collecting that runs from Texarkana to Little Rock. Intermittent out- recovered a few teeth belonging