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Article From an Arkadelphia Formation—Midway Group Lag Deposit (Maastrichtian—), Hot Spring County, , USA

Martin A. Becker 1,*, Harry M. Maisch, IV 2 and John A. Chamberlain, Jr. 2,3

1 Department of Environmental Science, William Paterson University, 300 Pompton Road, Wayne, NJ 07470, USA 2 Doctoral Program in Earth and Environmental Science, City University of New York Graduate Center, 365 Fifth Ave, New York, NY 10016, USA; [email protected] (H.M.M.); [email protected] (J.A.C.) 3 Department of Earth and Environmental Sciences, Brooklyn College, City University of New York, 2900 Bedford Avenue, Brooklyn, NY 11210, USA * Correspondence: [email protected]; Tel.: +1-973-7200-2721

Academic Editor: Jesus Martinez-Frias Received: 23 June 2016; Accepted: 6 September 2016; Published: 14 September 2016

Abstract: The Arkadelphia Formation—Midway Group (Maastrichtian—Paleocene) contact near Malvern, Arkansas preserves a K-Pg boundary assemblage of species consisting of skull, shell, and non-shell postcranial skeletal elements. The Malvern turtles are preserved within a coquina lag deposit that comprises the basalmost Midway Group and also contains an abundance of other reptiles, as well as chondrichthyans, osteichthyans, and invertebrates. This coquina lag deposit records a complex taphonomic history of exhumation and reburial of vertebrate skeletal elements along a dynamic ancestral shoreline in southwestern Arkansas during the late -early Paleocene. Based on stratigraphic occurrence, the Malvern turtle assemblage indicates that these marine reptiles were living at or near the time of the K-Pg mass and represent some of the latest Cretaceous turtles yet recovered from the Gulf Coastal Plain of the .

Keywords: turtles; K-Pg boundary section; Arkansas; Arkadelphia Formation; Midway Group

1. Introduction To date, only a few published reports identify the occurrence of turtles from the of Arkansas. The earliest of these reports was written over half a century ago [1], and described a new species of turtle from the Brownstone Formation which Schmidt named Podocnemis barberi. Four later, Schmidt [2] described two additional turtle species, Phyllemys barberi and Catapleura arkansasensis from the Marlbrook Marl. All of Schmidt’s species identifications were based on nearly complete carapaces and plastra. The present report identifies an assemblage of skull, shell, and non-shell postcranial turtle skeletal elements of uncertain taxonomic status. This turtle assemblage consists of nineteen specimens preserved in matrix and was recovered from a coquina lag deposit composing the basalmost Midway Group along the Ouachita in Hot Spring County, Arkansas (Figures1 and2).

Geosciences 2016, 6, 41; doi:10.3390/geosciences6030041 www.mdpi.com/journal/geosciences Geosciences 2016, 6, 41 2 of 12 Geosciences 2016, 6, 41 2 of 12

FigureFigure 1. LocationLocation maps maps for for the the Arkadelphia Arkadelphia Form Formation—Midwayation—Midway Group Group (late (late Maastrichtian— Maastrichtian— Paleocene)Paleocene) turtles turtles from from Hot Hot Spring Spring County, County, Arkansas Arkansas (modified (modified from from BeckerBecker et al. [[3]).3]). (A)) Late Late MaastrichtianMaastrichtian ( (JeletzkytesJeletzkytes nebrascensis nebrascensis Zone)Zone) paleogeographic paleogeographic reconstruction reconstruction of of the the Atlantic Atlantic and and Gulf CoastalCoastal plains plains and and Western Western Interior Interior Seaway; Seaway; (B ()B physiographic) physiographic provinces provinces in Arkansas in Arkansas with with turtle turtle site discussedsite discussed in this in thisstudy study indicated indicated by by(X); (X); (C ()C geologic) geologic map map and and stratigraphic stratigraphic column column of of Upper Upper CretaceousCretaceous and and formations formations in in the study area of southwestern Arkansas and Arkadelphia Formation—MidwayFormation—Midway Group turtle site discussed in this study identifiedidentified by arrows.

Prior studies indicate that this lag deposit is part of a Cretaceous–Paleocene boundary section in Prior studies indicate that this lag deposit is part of a Cretaceous–Paleocene boundary section in the region [3–5] and provides excellent time control for the of these turtle specimens. While the the region [3–5] and provides excellent time control for the age of these turtle specimens. While the exact origin of the Malvern turtle lag may be the result of impact generated tsunamis, third order sea exact origin of the Malvern turtle lag may be the result of impact generated tsunamis, third order level cyclicity or some combination of both these processes, the stratigraphic occurrence of this turtle sea level cyclicity or some combination of both these processes, the stratigraphic occurrence of this assemblage indicates that these marine reptiles were living at or near the time of the K-Pg mass turtle assemblage indicates that these marine reptiles were living at or near the time of the K-Pg mass extinction, and represent some of the latest Cretaceous turtles yet recovered from the Gulf Coastal extinction, and represent some of the latest Cretaceous turtles yet recovered from the Gulf Coastal Plain of the United States. Plain of the United States.

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Figure 2. Outcrop exposure along the Ouachita River of the upper Arkadelphia Formation and Figure 2. Outcrop exposure along the Ouachita River of the upper Arkadelphia Formation and basalmost Midway Group in Hot Spring County, Arkansas. Note strike and dip and steep inclination basalmost Midway Group in Hot Spring County, Arkansas. Note strike and dip and steep inclination of the bedding relative to river water flow. Small arrow indicates the stratigraphic position of the lag of thedeposit bedding from relative which the to riverturtle waterfossils flow.in this Small study arrowwere recovered. indicates Pick the axe stratigraphic is approximately position 1 m of and the lag depositthe fromperson which is 1.8 m the (Modifie turtle fossilsd from inBecker this et study al. [4]). were recovered. Pick axe is approximately 1 m and the person is 1.8 m (Modified from Becker et al. [4]). 2. Age of the Arkadelphia Formation—Midway Group Contact 2. Age ofMultiple the Arkadelphia lines of evidence Formation—Midway support a Maastrichtian—Paleocene Group Contact age assignment for the MultipleArkadelphia lines Formation—Midway of evidence support Group contact a Maastrichtian—Paleocene near Malvern, Arkansas. This age evidence assignment includes: for the [6]; calcareous nannofossils [7]; palynology [8]; dinoflagellates[9]; ostracods [10]; Arkadelphia Formation—Midway Group contact near Malvern, Arkansas. This evidence includes: ammonites [7]; chondrichthyans [3,11]; osteichthyans [4,12]; plesiosaurs [5]; magnetostratigraphy [13]; foraminifera [6]; calcareous nannofossils [7]; palynology [8]; dinoflagellates [9]; ostracods [10]; and geologic mapping [14,15]; Additional discussion on the lithology and age of the Arkadelphia ammonitesFormation [7]; chondrichthyansand Midway Group [3 ,11in ];southwestern osteichthyans Arkansas [4,12]; plesiosaurscan be found [5 ];in magnetostratigraphy the geologic map of [13]; and geologicArkansas mappingby Haley et [ 14al., 15[14]]; and Additional in the accompanying discussion stratigraphic on the lithology summary and by age McFarland of the Arkadelphia [16]. Formation and Midway Group in southwestern Arkansas can be found in the geologic map of Arkansas by Haley3. Geologic et al. [ 14Setting] and in the accompanying stratigraphic summary by McFarland [16]. The coquina lag deposit from which the Malvern turtles were recovered was first described in a 3. Geologic Setting study documenting Late Cretaceous chondrichthyans [3] and in subsequent studies of Theosteichthyans coquina lag[4] and deposit plesiosaurs from which [5]. The the best Malvern outcrop turtles exposure were of this recovered coquina was lag firstknown described to date in a studyoccurs documenting on a small Late island Cretaceous in the Ouachita chondrichthyans River (Figure [2),3] totally and in covered subsequent by water studies except of during osteichthyans times [4] of low water flow. This island outcrop reveals a disconformable contact where individual beds of the and plesiosaurs [5]. The best outcrop exposure of this coquina lag known to date occurs on a small uppermost Arkadelphia Formation, the lag deposit comprising the basalmost Midway Group, and island in the Ouachita River (Figure2), totally covered by water except during times of low water beds of the lower Midway Group above the lag strike to the North and dip at a steep angle to the flow.West. This islandThese outcropbeds represent reveals a disconformableloss of original contacthorizontality where individualduring syn-depositional beds of the uppermost and Arkadelphiapost-depositional Formation, deformation the lag deposit(Figure 2). comprising the basalmost Midway Group, and beds of the lower MidwayAt the Groupoutcrop above depicted the in lag Figure strike 2, tothe the uppermost North and Arkadelphia dip at a steep Formation angle is to a thedark West. gray, Thesethinly beds representbedded loss micaceous of original marl horizontality with occasional during lenses syn-depositional of glauconitic sands, and bivalve post-depositional shells and infrequent deformation (Figure2).

At the outcrop depicted in Figure2, the uppermost Arkadelphia Formation is a dark gray, thinly bedded micaceous marl with occasional lenses of glauconitic sands, bivalve shells and infrequent Geosciences 20162016,, 66,, 4141 4 of 12

chondrichthyan and osteichthyan teeth. The contact of the coquina lag deposit with the underlying chondrichthyanArkadelphia Formation and osteichthyan is sharp and teeth. undulatory, The contact an ofd can the coquinabe recognized lag deposit by a withprominent the underlying lithology Arkadelphiachange from Formationthe dark-colored, is sharp micaceous and undulatory, marl of and the can Arkadelphia be recognized Formation by a prominent to a thick-bedded lithology changecoquina from lag thewith dark-colored, interbedded micaceous micrite, and marl occasional of the Arkadelphia lenses of Formation fine-grained, to a thick-beddedcross-bedded coquina quartz lagsand with and interbedded rip-up clasts micrite, of Arkadelphia and occasional composition. lenses of fine-grained,The coquina cross-beddedlag comprises quartz the basalmost sand and rip-upMidway clasts Group, of Arkadelphia is laterally discontinuous, composition. The and coquina varies lagin thickness comprises from the basalmosta few centimeters Midway to Group, over is30 laterally cm thick discontinuous, where it outcrops. and varies The uppermost in thickness surf fromace a fewof the centimeters coquina lag to overcan 30be cmidentified thick where by the it outcrops.numerous, The well-rounded uppermost quartz surface and of the phosphate coquina pebbles, lag can be as identified well as chondrichthyan by the numerous, and well-rounded osteichthyan quartzteeth and and plesiosaurs phosphate remains pebbles, [3–5]. as well The turtle as chondrichthyan assemblage described and osteichthyan in this report teeth was and also plesiosaurs recovered remainsfrom this [ 3surface.–5]. The turtle assemblage described in this report was also recovered from this surface. Directly above the coquina lag, interbedded sandy, graygray carbonaceous marls and dense, micritic limestonelimestone typical of the Midway Group are exposed and continue in stratigraphic order to the West along thethe highhigh angleangle ofof dip.dip. OneOne of of these these micrites, micrites, lying lying immediately immediately above above the the lag, lag, is clearlyis clearly visible visible in Figurein Figure2. Directly2. Directly across across the the interstate interstate and and adjacent adjacent to to the the field field site, site, Paleocene Paleocene chondrichthyan chondrichthyan and and osteichthyan teeth teeth belonging belonging to to the the Midway Midway Group Group occur occur in inrecent recent highway highway excavations excavations [17] [ 17and] and are arealso also exposed exposed approximately approximately 3 km 3 km to tothe the northeast northeast along along strike strike in in bluffs bluffs adjacent adjacent to to a shoppingshopping plaza [[11,12].11,12].

4. Field Field Methods Methods All nineteen turtle specimens identifiedidentified in this paper were collected during the last decade of fieldworkfieldwork at the Malvern, Arkansas site along with other vertebrates as reportedreported inin [[3–5].3–5]. The turtle material was recovered directly from thethe basalmostbasalmost Midway Group lag deposit during times of low waterwater flowflow andand byby snorkelingsnorkeling and scuba diving the riverriver bedbed (Figures(Figures2 2and and3). 3). Most Most skull, skull, limb, limb, carapace and plastral elements are preserved on slabsslabs of coquina lag collected from the river bed, and required aa minorminor amountamount ofof laboratorylaboratory cleaningcleaning and preparation to expose additional bone surfacessurfaces and suture edges.edges.

Figure 3. Scuba diving recovery of turtle peripherals (NJSM GP23413) described in this study. The Figure 3. Scuba diving recovery of turtle peripherals (NJSM GP23413) described in this study. The arrow arrow below the diver points to large coquina blocks of the lag deposit eroded and transported below the diver points to large coquina blocks of the lag deposit eroded and transported directly directly downstream from the adjacent outcrop exposure depicted in Figure 2. Scale Bar = 20 cm. downstream from the adjacent outcrop exposure depicted in Figure2. Scale Bar = 20 cm.

5. Repository 5. Repository Turtle specimens have been reposited in the fossil vertebrate collections of the New Jersey State Turtle specimens have been reposited in the fossil vertebrate collections of the New Jersey Museum and have the catalogue numbers NJSM GP 23395; 23400–23404; 23406; 23408; 23409; State Museum and have the catalogue numbers NJSM GP 23395; 23400–23404; 23406; 23408; 23409; 23411–232418; 23627–23628 (Figures 4–8). 23411–232418; 23627–23628 (Figures4–8).

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Figure 4. Turtle neurals and peripherals from the basalmost Midway Group lag deposit, Hot Spring Figure 4. Turtle neurals and peripherals from the basalmost Midway Group lag deposit, Hot Spring County, Arkansas. (A–C) neural (NJSM GP23416) in exterior (A), interior (B), and cross-section (C) County,Figure Arkansas. 4. Turtle neurals (A–C) neuraland peripherals (NJSMGP23416) from the basa in exteriorlmost Midway (A), interior Group lag (B ),deposit, and cross-section Hot Spring (C) views; (D) neural (NJSM GP23408) in exterior view; (E) peripheral (NJSM GP23402) in interior view; County, Arkansas. (A–C) neural (NJSM GP23416) in exterior (A), interior (B), and cross-section (C) views;(F) ( Dperipheral) neural (NJSM(NJSM GP23404) GP23408) in in interior exterior view; view; (G– (IE) )peripherals peripheral (NJSM (NJSM GP23413) GP23402) in ininterior interior (G), view; views; (D) neural (NJSM GP23408) in exterior view; (E) peripheral (NJSM GP23402) in interior view; (F) peripheralexterior edge (NJSM (H), and GP23404) cross-section in interior (I) views. view; Scale (G bars–I) in peripherals A–F; I = 1 cm; (NJSM G–H GP23413)= 5 cm. Abbreviations: in interior (G), (F) peripheral (NJSM GP23404) in interior view; (G–I) peripherals (NJSM GP23413) in interior (G), exterioraf: articular edge (H foramen,), and cross-section cb: cancellous (I )bone, views. crb: Scale cortical bars bone, in A p1–p3:–F; I = perpherials, 1 cm; G–H pt:= pycnodont 5 cm. Abbreviations: tooth, exterior edge (H), and cross-section (I) views. Scale bars in A–F; I = 1 cm; G–H = 5 cm. Abbreviations: af: articularpx: pelecypod foramen, steinkern, cb: cancellous pxs: peripheral bone, crb:cross-section, cortical bone, se: sutural p1–p3: edge, perpherials, sg: sulcus pt: groove, pycnodont af: articular foramen, cb: cancellous bone, crb: cortical bone, p1–p3: perpherials, pt: pycnodont tooth, tooth,vk: px: ventral pelecypod keel. steinkern, pxs: peripheral cross-section, se: sutural edge, sg: sulcus groove, px: pelecypod steinkern, pxs: peripheral cross-section, se: sutural edge, sg: sulcus groove, vk: ventralvk: ventral keel. keel.

Figure 5. Cont. Geosciences 2016, 6, 41 6 of 12 Geosciences 20162016,, 66,, 4141 66 ofof 1212 Geosciences 2016, 6, 41 6 of 12

FigureFigureFigure 5. Turtle 5.5. TurtleTurtle costals costalscostals from fromfrom the thethe basalmost basalmostbasalmost MidwayMidway GroupGroup Group laglag lag depositdeposit deposit HotHot Hot SpringSpring Spring County,County, County, Arkansas.Arkansas. Arkansas. ((FigureA)) CostalCostal 5. Turtle (NJSM(NJSM costals GP23411)GP23411) from inthein exteriorexterior basalmost view;view; Midway ((B)) costalcostal Group (NJSM(NJSM lag deposit GP23403)GP23403) Hot inin Spring interiorinterior County, view;view; Arkansas.((C)) costalcostal (A) Costal (NJSM GP23411) in exterior view; (B) costal (NJSM GP23403) in interior view; (C) costal (NJSM(NJSM(A) Costal GP23401)GP23401) (NJSM ininGP23411) interiorinterior inview;view; exterior ((D)) view;costalcostal ((NJSMB(NJSM) costal GP23395)GP23395) (NJSM GP23403) inin exteriorexterior in view;interiorview; ((EE view;)) costalcostal (C )(NJSM(NJSM costal (NJSM GP23401) in interior view; (D) costal (NJSM GP23395) in exterior view; (E) costal (NJSM GP23406)(NJSM GP23401) in exterior in view.interior Scale view; bars (D in) costalA,, C–– EE(NJSM == 55 cm;cm; GP23395) B == 11 cm.cm. in Abbreviations:Abbreviations: exterior view; cs:cs: (E Cylindracanthus) costal (NJSM GP23406) in exterior view. Scale bars in A, C–E = 5 cm; B = 1 cm. Abbreviations: cs: Cylindracanthus rostrum,rostrum,GP23406) drp:drp:in exterior distaldistal view.ribrib projection, projection,Scale bars ingx:gx: A , gastropodgastropodC–E = 5 cm; cross-section,cross-section, B = 1 cm. Abbreviations: se:se: suturalsutural cs:edge,edge, Cylindracanthus ss:ss: surfacesurface rostrum,sculpturing.sculpturing.rostrum, drp: distaldrp: distal rib projection, rib projection, gx: gastropod gx: gastropod cross-section, cross-section, se: sutural se: sutural edge, ss:edge, surface ss: sculpturing.surface sculpturing.

FigureFigureFigure 6. Turtle 6.6. TurtleTurtle costals costalscostals from fromfrom the thethe basalmost basalmostbasalmost MidwayMidway GroupGroup Group laglag lag depositdeposit deposit HotHot Hot SpringSpring Spring County,County, County, Arkansas.Arkansas. Arkansas. ((FigureA)) CostalCostal 6. Turtle (NJSM(NJSM costals GP23414)GP23414) from inin the interiorinterior basalmost view;view; Midway ((B)) costalcostal Group (NJSM(NJSM lag GP23628)GP23628) deposit inHotin exteriorexterior Spring view;view; County, andand Arkansas. ((C)) costalcostal (A) Costal (NJSM GP23414) in interior view; (B) costal (NJSM GP23628) in exterior view; and (C) costal (NJSM(NJSM(A) Costal GP23415)GP23415) (NJSM in inGP23414) exteriorexterior in view.view. interior ScaleScale view; barsbars ( Binin) Acostal––C == (NJSM 55 cm.cm. Abbreviations: Abbreviations:GP23628) in exterior drp:drp: distaldistal view; ribrib and projection,projection, (C) costal (NJSM GP23415) in exterior view. Scale bars in A–C = 5 cm. Abbreviations: drp: distal rib projection, rh:rh:(NJSM ribrib head,head, GP23415) se:se: suturalsutural in exterior edge,edge, view. sg:sg: sulcussulcus Scale groove,groove,bars in Ass:ss:– Csurfacesurface = 5 cm. sculpturing,sculpturing, Abbreviations: st:st: sharkshark drp: tooth.tooth.distal rib projection, rh: ribrh: head, rib head, se: suturalse: sutural edge, edge, sg: sg: sulcus sulcus groove, groove, ss: surface surface sculpturing, sculpturing, st: st: shark tooth. tooth.

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Figure 7. Turtle plastral elements from the basalmost Midway Group lag deposit Hot Spring County, Figure 7. Turtle plastral elements from the basalmos basalmostt Midway Group lag deposit deposit Hot Hot Spring Spring County, County, Arkansas. (A) Partial plastron (NJSM GP23417) in exterior view; (B) partial plastron (NJSM GP23627) Arkansas. (A) Partial plastron (NJSM GP23417) in exterior view; (B) partial plastron (NJSM GP23627) Arkansas.in exterior (A) Partial view; ( plastronC) partial (NJSM plastron GP23417) (NJSM GP23418) in exterior in interiorview; ( Bview;) partial (D) partialplastron plastron (NJSM (NJSM GP23627) in exterior view; (C) partial plastron (NJSM GP23418) in interior view; (D) partial plastron (NJSM in exteriorGP23412) view; in interior (C) partial view. plastronScale bars (NJSM in A–D GP23418) = 5 cm. Abbreviations: in interior view;ace: acetabulum, (D) partial ax: plastron ammonite (NJSM GP23412)cross-section, in interior de: view.distal edge, ScaleScale et: barsbars inin AA–– tooth,DD == 55 pe: cm.cm. proximal Abbreviations:Abbreviations: edge, se: sutural ace:ace: acetabulum,acetabulum, edge. ax:ax: ammonite cross-section, de:de: distal edge, et: Enchodus tooth, pe: proximal edge, se: sutural edge.

Figure 8. Turtle mandible and shoulder girdle from the basalmost Midway Group lag deposit Hot Spring County, Arkansas. (A) Mandible (NJSM GP23409) in interior view; (B) shoulder girdle (NJSM GP23400) in interior view. Scale bars in A and B = 1 cm. Abbreviations: an: angular, ar: articular, Figurebx: 8. bivalve Turtle cross-section, mandible and c: coracoid, shoulder d: girdle dentary, from fs: thefish basalmostbasalmostscale, g: glenoid, Midway Mg: Group Meckelian lag depositgroove, Hot Springmlr: County, midline Arkansas.Arkansas. ridge, pa: prearticular, (A)) MandibleMandible s: scapula. (NJSM(NJSM GP23409)GP23409) inin interiorinterior view;view; (B) shoulder girdle (NJSM GP23400) in interior view. Scale bars in AA andand BB == 11 cm.cm. Abbreviations:Abbreviations: an: angular, ar: articular, 6. Turtle Assemblage bx: bivalve cross-section,cross-section, c: coracoid, d: dentary, fs: fishfish scale, g: glenoid, Mg: Meckelian groove, mlr: midlineThe nineteen ridge, turtle pa: prearticular, specimens occurrings:s: scapula.scapula. in the Malvern lag deposit represent single bones from multiple individuals, the majority of which are preserved on separate coquina blocks. The 6. Turtle Turtleassemblage Assemblage Assemblage consists of common carapace and plastral elements (Figures 4–7), and a nearly complete mandible and partial shoulder girdle (Figure 8). The nineteenAlthough turtlethe bones specimens are fragmentary, occurring the in overall the Malvern size, shape lag depositand thickness, represent sutural single edges, bones and from multiplesurface individuals,individuals, sculpturing the theare majority distinctmajorityof to which turtles,of which are and, preserved arebased preserved on on these separate features,on coquinaseparate represent blocks.coquina different The blocks. assemblage turtle The consistsassemblagespecies. of common consistsHowever, carapaceof we common refrain and fromcarapace plastral any taxonomic elementsand plastral (Figuresiden elementstification4–7), due and(Figures to a nearlythe 4–7), lack complete ofand multiple a nearly mandible skeletal complete and partialmandibleelements shoulder and thatpartial girdle can shoulder (Figurebe attributed8 girdle). to(Figure the same 8). individual and to the fragmentary nature of Althoughindividual bones. the bones are fragmentary, thethe overalloverall size,size, shapeshape andand thickness,thickness, suturalsutural edges, and surface sculpturingsculpturing are are distinct distinct to turtles,to turtles, and, and, based based on these on these features, features, represent represent different different turtle species. turtle However,species. However, we refrain we from refrain any from taxonomic any taxonomic identification identification due to the due lack to of the multiple lack of skeletalmultiple elements skeletal thatelements can be that attributed can be to theattributed same individual to the same and toindividual the fragmentary and to nature the offragmentary individual bones.nature of individual bones.

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The two neural bones and five peripherals show distinct sutural edges for articulation to adjoining peripherals; a sulcus groove from overlaying scutes; a ventral keel for vertebral articulation, and a foramen for articulation to the distal rib projection (Figure4). The eight costals show distinct sutural edges for articulation to adjacent costals and neurals, surface sculpturing, distal projections for articulation to adjoining peripherals, a sulcus groove from overlaying scutes, and a partial rib head for vertebral articulation (Figures5 and6). The four plastral elements show sutural edges, proximal and distal projections and a partial acetabulum (Figure7). The near complete mandible in interior view shows the dentary with mid-line cusp, as well as the Meckelian groove, prearticular, angular and articular regions (Figure8). The partial shoulder girdle shown in Figure8 includes the scapula, coracoid, and glenoid. In general, the Malvern turtle assemblage resembles late Cretaceous and early Paleocene turtle genera found in overlapping but different habitats including open marine environments, the littoral zone, estuaries and (e.g., [18–24]). Some examples of these contemporaneous turtle genera described from more complete and articulated specimens in the Atlantic and Gulf Coastal Plains of North America include: Taphrospys, Catapleura, , Adocus and Trionyx.

7. Discussion

7.1. Origin of the Turtle Lag Deposit The Malvern lag deposit containing the turtle assemblage described in this report occurs within a coquina lag directly above a disconformity that marks the contact between the uppermost Arkadelphia Formation and basalmost Midway Group. The turtle within this lag deposit all show different degrees of taphonomic wear, with partial erosion of cortical bone and exposure of cancellous bone in some specimens (Figure4E,F, Figures6A and7B,D), and occur with chondrichthyans and osteichthyans as well as ammonites, gastropods and bivalves (Figure4G, Figure5A,C,D, Figure6B, Figure7C,D and Figure8B). Because this lag deposit also occurs at the boundary between the Maastrichtian and Paleocene, the Malvern turtles described in this report, along with other vertebrates were living at or near the time of the K-Pg mass extinction. Accordingly, the Malvern lag deposit is significant in that the concentration of turtle fossils and other vertebrates may record the effects of: (1) impact generated tsunamis as previously identified along the K-Pg boundary in the Gulf Coastal Plain; (2) third order eustatic sea level change in the latest Cretaceous; or (3) combined impact generated tsunamis and sea level cyclicity. Below, we discuss these three scenarios but note that current evidence does not yet allow us to distinguish among these alternative origins for the Malvern turtle lag.

7.2. Impact Generated Tsunamis for Lag Deposit Formation In the Gulf of Mexico, the current view is that K-Pg tsunami deposits are the result of a bolide impact on the edge of the Yucatan Peninsula (e.g., [7,25–33]). Some of the key characteristics identified by these researchers for recognizing these deposits in outcrop that are also seen in the Malvern lag include: (1) a strong erosional base associated with scouring; (2) pebble conglomerates, reworked blocks and coquina accumulations; (3) mixing of fossils coming from different environments; and (4) rapid lateral changes in facies. At a site near the outcrop section depicted in Figure2, Larina et al. [ 7] recently compiled a nine meter measured section utilizing ammonites, dinoflagellates and calcareous nannofossils to trace the continuity of the K-Pg boundary across the Embayment and Gulf Coastal Plain. Their measured section also identified a prominent disconformity and lag deposit at the top of the Arkadelphia Formation and basalmost Midway Group that they interpret to represent impact generated tsunamis [7]. This interpretation is also consistent with the observation that macrofossils within the Malvern section belong to groups such as ammonites and plesiosaurs that are generally thought to go extinct by the end of the Maastrichtian [4,5,7]. Geosciences 2016, 6, 41 9 of 12

7.3. Third Order Sea Level Generated Lag Deposits Prior models of lag deposit formation during the Late Cretaceous and of North America indicate that many concentrations of vertebrate fossils occur at the boundary between third order sea level cycles (e.g., [34–43]). These sea level cycles can be recognized in outcrop by disconformities and lag deposits that occur along geologic contacts between clay-dominated and sand or dominated lithologies (e.g., [44–46]). Vertebrate fossils concentrated in these lags can differ on the order of several million years and represent organisms with different habitat preferences (e.g., [37,46,47]). Eustatic sea level cycles of Haq [48] identify a total of five minor to medium amplitude regressive-transgressive events during the Maastrichtian. Sea level fluctuation during these five events ranged between 25 and 75 m, with the largest event KMa5, occurring just below the K-Pg boundary [48]. Similar sea level cycles are also identified in the northern Gulf Coastal Plain by Mancini et al. [49] and Mancini and Puckett [50]. During the early late Maastrichtian in the northwestern Gulf Coastal Plain, their research documents a regressive sea level interval followed by an erosional hiatus leading into the Paleocene. Duration of Maastrichtian sea level cycles identified by Haq [48] averages 1–2 million years. It is also noteworthy that Haq [48] identified condensed section deposits that contain shell and bone beds similar to the Malvern turtle lag described in this report as key features for identifying transgressive facies, stratigraphic correlation, and construction of third order sea level cycles for the Cretaceous. In this regard, it is possible that a similar third-order sea level event accounts for the Malvern turtle lag.

7.4. Combined Impact Generated Tsunamis and Third Order Sea Level Cyclicity It is also possible that impact generated tsunamis and regional sea level events could act in combination to produce the Malvern turtle lag deposit identified in this report. A case in point is a microspherulitic lag that occurs at the base of the in southeastern Missouri. This lag was originally interpreted by Campbell et al. [31] and Oboh-Ikeunobe et al. [33] as the result of an end-Cretaceous tsunami. However, dinoflagellates preserved in these beds show that the lag is actually the product of an early Danian sea level cyclicity event in which erosion exhumed and re-deposited both Maastrichtian macrofossils and end-Cretaceous impact spherules [51]. Of all the tsunamites yet examined in the Gulf Coastal Plain, this one is geographically closest to Malvern.

8. Future Research Determining the exact origin of the Malvern turtle lag relies on detailed timing of biostratigraphically significant fossils through the section. To date, no vertebrate fossils known exclusively from the Paleocene are present in the Malvern turtle lag or in the beds that reside directly above. However, chondrichthyans and osteichthyans from the Paleocene have been identified and described in nearby sections of the Midway Group approximately 3 km to the northeast along strike from the Malvern locality by Becker et al. [11] and Becker and Chamberlain [12]. While a latest Maastrichtian age assignment for the Malvern section is provided by Larina et al. [7] based on ammonites and calcareous nannofossils, no vertebrates or Danian microfossils are identified in their study. However, Larina et al. [7] indicated that the lag comprising the basalmost Midway Group at the Malvern site contains abundant impact spherules. A critical, but as yet unperformed, line of research that potentially could resolve a single origin or combined origin scenario presented above is the distribution of dinoflagellates and impact spherules through the Malvern turtle lag section. We are currently processing these in order to refine the timeframe recorded by the Malvern turtle lag. A rapid or prolonged origin for the formation of this lag may favor one of the three scenarios presented above. Nevertheless, stratigraphic evidence indicates the turtle assemblage found in the Malvern lag contains the remains of individuals that were living at or near the time of the K-Pg mass extinction and represent some of the latest Cretaceous turtles yet recovered from the Gulf Coastal Plain of the United States. Geosciences 2016, 6, 41 10 of 12

Acknowledgments: We thank David Parris and Rodrigo Pellegrini of the New Jersey State Museum for assistance with specimen identification and establishing a repository for the turtle specimens in this report. Ralph Scimeca of William Paterson University provided assistance with figure preparation. James Sloan of the Arkansas Highway and Transportation Department assisted with access to the Midway Group outcrop exposure adjacent to the nearby interstate. Four anonymous reviewers improved an earlier version of this manuscript. This research was supported in part by Assigned Release Time and Center for Research Grants from William Paterson University to Martin A. Becker, and by Professional Staff Congress-City University of New York awards to John A. Chamberlain. Author Contributions: Martin A. Becker and Harry M. Maisch performed the field work; Martin A. Becker identified the turtle specimens; and Martin A. Becker, John A. Chamberlain, and Harry M. Maisch wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. Funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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