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Chapter 9: “A new polycotylid plesiosaur (Reptilia; Sauropterygia) from the Greenhorn Limestone (Upper Cretaceous; lower upper Cenomanian), Black Hills, South Dakota” (Schumacher), in Martin, J.E., and Parris, D.C., eds., The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas: Geological Society of America Special Paper 427. This PDF file is subject to the following conditions and restrictions: Copyright © 2007, The Geological Society of America, Inc. (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. For any other use, contact Copyright Permissions, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, fax 303-357-1073, editing@geosociety.org. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. This file may not be posted on the Internet.

133

The Geological Society of AmericaSpecial Paper 427

2007

A new polycotylid plesiosaur (Reptilia; Sauropterygia) from the Greenhorn Limestone (Upper Cretaceous;

lower upper Cenomanian), Black Hills, South Dakota

Bruce A. SchumacherSternberg Museum of Natural History, Fort Hays State University, 3000 Sternberg Drive, Hays, Kansas 67601, USA

ABSTRACT

A plesiosaur skeleton from the base of the Greenhorn Limestone (lower upperCenomanian), western South Dakota, represents a new taxon tentatively referred to thePolycotylidae. The specimen possesses a number of features in common with the Poly-cotylidae, including an exceptionally elongate muzzle and mandibular symphysis, lackof a parietal foramen, homodont dentition, and faint striae only on the medial side ofteeth. However, the palate and limb morphologies differ with respect to Polycotylidaeas currently known. The pterygoids are united along their midline anterior to theparasphenoid, lacking an anterior interpterygoid vacuity. The preserved parasphenoidis relatively robust, possesses a strong ventral keel, and is sutured to the dorsal surfaceof the pterygoids. The paddles possess many plesiomorphic features, including onlyrelatively minor postero-distal expansion of the propodials, epipodials that are longerthan wide, a distinct antebrachial foramen, and relatively elongate phalanges. Derivedfeatures make it a plausible representative sister taxon to Polycotylidae, a prospect thatis complemented by its stratigraphic position. This hypothesis is hampered by the palatemorphology, which suggests a separate lineage of short-necked plesiosaurs far removedfrom genera classically assigned to Polycotylidae (Dolichorhynchops, Polycotylus, Tri-nacromerum). Nevertheless, the specimen represents the persistence of plesiomorphiclimb traits and possession of a closed palate within an early polycotylid lineage until atleast the early late Cenomanian in the Midcontinent of North America.

Keywords: Greenhorn, Cenomanian, Cretaceous, polycotylid, plesiosaur.

Schumacher, B.A., 2007, A new polycotylid plesiosaur (Reptilia; Sauropterygia) from the Greenhorn Limestone (Upper Cretaceous; lower upper Cenomanian), BlackHills, South Dakota, in Martin, J.E., and Parris, D.C., eds., The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas: Geological Societyof America Special Paper 427, p. 133–146, doi: 10.1130/2007.2427(09). For permission to copy, contact editing@geosociety.org. © 2007 The Geological Society ofAmerica. All rights reserved.

INTRODUCTION

In 1934, Charles C. Haas and his son, Arthur, collected thepartial skeleton of a plesiosaur from the base of the GreenhornLimestone in the northern Black Hills area of South Dakota (Fig. 1). They prepared major parts of the skeleton, including the skull, cervical vertebrae, and the left front and rear paddles.Mr. Haas consulted all of the scientific literature of the day anddelved deeply into learning about and interpreting his find. He

pronounced the skeleton “a curious mix of fish, reptile, and bird,”and put together a detailed documentary but failed to find anAmerican museum willing to purchase the specimen. A few yearslater he donated the specimen to the Adams Memorial Museumin Deadwood, South Dakota. The specimen was first identified by S.C. Simms, director of the Field Museum (Chicago) at thattime, as Trinacromerum bentonianum. Upon first examining thespecimen I determined it to be a new taxon based purely uponplesiomorphic features of the paddles (Schumacher, 1997). The

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unexpected nature of the palate was not known until later prepa-ration of the skull’s ventral surface (Schumacher, 1999). In thelate 1990s the skull, anterior cervicals, and left paddles underwentfurther preparation for removal of aged shellac, and the remain-der of the axial skeleton was removed from the original sugar sackand paste field wrappings employed by Mr. Haas. In recent yearsthe Adams Memorial Museum has built an interpretive displayaround the fully prepared specimen (Fig. 2), and the skeleton iseasily accessible for research purposes.

Institutional Abbreviations

AMM:Adams Memorial Museum, Deadwood, South Dakota;AMNH: American Museum of Natural History, New York;FHSM: Sternberg Museum of Natural History, Fort Hays StateUniversity, Hays, Kansas; FMNH: Field Museum of NaturalHistory, Chicago, Illinois; KUVP: Museum of Natural History,University of Kansas, Lawrence; BMNH: The Natural HistoryMuseum, London; SDSM: Museum of Geology, South DakotaSchool of Mines and Technology, Rapid City, South Dakota;USNM: U.S. National Museum, Washington DC; YPM: YalePeabody Museum, New Haven, Connecticut.

Anatomical Abbreviations

ac, acoustic chamber; bo, basioccipital; bs, basisphenoid; cf,carotid foramina; d, dentary; ec, ectopterygoid; fr, frontal; mx,maxilla; nc, infilling of internal narial chamber; oc, occipitalcondyle; oe, opisthotic-exoccipital; pa, parietal; pl, palatine; pm,premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; ps,parasphenoid; pt, pterygoid; ptf, pterygoid facet of basioccipital;so, supraoccipital; v, vomer; vf, vomeronasal fenestra.

Measurements

The following criteria are used for data presented in Tables 1and 2. Snout length is quantified as the distance from the tip of thepremaxilla to the anterior end of the parasphenoid, divided by totalskull length. Measurements of limb elements are expressed asmaximum length divided by maximum width. Direct comparisonof similar phalanges from different individuals is usually not pos-sible, because few articulated and undisturbed paddles exist. Thus,phalangeal proportions are reported as an average of proximal pha-langes in digits 2 through 4 from an individual paddle. It is note-worthy that overall phalanx length increases slightly and regularlyfrom digits 1 to 5. The leading and trailing digits of the paddles(digits 1 and 5) contain phalanges with the shortest and the longestlength/width ratios, respectively, and were excluded. Phalanxlength is a significant and useful character in that gross proportionremains the same in individual specimens. For instance, if theproximal phalanges are relatively short, so are the more distalones. These observations hold true in forelimbs and hind limbs.

SYSTEMATIC PALEONTOLOGY

Plesiosauria de Blainville, 1835Polycotylidae Williston, 1908

Pahasapasaurus, gen. nov.

Genotype. Pahasapasaurus haasi.

Type species. Pahasapasaurus haasi, sp. nov.

Stratigraphic occurrence. Field notes of C. Haas state that the spec-imen originated in the “sharkstooth sandstone, Graneros Formationalong Maloney Creek, Butte County, South Dakota.” Cobban(1951) reassigned this bed as the base of the Greenhorn Formation,Gries and Martin (1985) referred the unit to the Greenhorn Lime-stone, and VonLoh and Bell (1998) assigned this as the basal OrmanLake Member of the Greenhorn Formation. At the time Mr. Haascollected the specimen, he had the foresight to retain associated ma-terials from the excavation pit. In the Black Hills and throughout theWestern Interior the base of the Greenhorn is marked by character-istic calcarenite (as defined by Folk, 1974) layers, which are rich inshark teeth and other particulate vertebrate material (Hattin, 1975).One slab of calcarenite matrix retained with the specimen bears animpression of the ammonite Dunveganoceras pondi (early lateCenomanian). Numerous teeth of sharks and bony fish were en-countered during preparation of the unopened blocks, includingSqualicorax falcatus, Cretodus sp., Cretolamna appendiculata,Ptychodus occidentalis, Pachyrhizodus sp., Protosphyraena sp.,and Enchodus sp. Examination of the enclosing matrix, identifica-tion of associated fossil materials, and photographs taken at the ex-cavation site in 1934 (Fig. 3) conclusively demonstrate that thespecimen originated at the base of the Greenhorn Limestone.

Age. Late Cretaceous, early late Cenomanian.

Figure 1. Location map of Pahasapasaurus haasi excavationsite in western South Dakota.

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Figure 2. Pahasapasaurus haasi skeleton during exhibit preparation. Left paddles are displayed on right side of specimen, and both paddles are posi-tioned too far forward in order to fit within display case. Vertebral column terminates (left) with poorly preserved posterior trunk vertebrae. Note thevisibly elongate neck, even though the front paddle is positioned next to posterior cervicals.

Type locality. Along Maloney Creek, just south of the BelleFourche River, roughly 6 mi (8 km) southeast of Fruitdale, T. 8N., R. 4 E., Butte County, South Dakota.

Etymology. Paha Sapa, Sioux Indian words meaning Black Hills.

Diagnosis. As for type and only species.Pahasapasaurus haasi, sp. nov.

(Figs. 2–7)

Holotype. AMM 98.1.1, Pahasapasaurus haasi, gen. et sp. nov.,including a partial skull, portions of left front and rear paddles,poorly preserved vertebral column and rib portions. Casts of theskull and front paddle are housed at SDSM.

Etymology. Named in honor of Charles C. Haas, who discovered,collected, and initially prepared and researched the specimen.

Diagnosis. A relatively large (�6 m in length), “long-beaked”plesiosaur possessing derived cranial features characteristic ofpolycotylids, a palate configuration unique among polycotylids,and plesiomorphic paddle morphologies. Diagnostic charactersinclude anterior interpterygoid vacuity absent; pterygoids unitedalong midline anterior to parasphenoid; parasphenoid relatively

robust element, possessing prominent ventral keel and sutured todorsal surface of pterygoids; pineal foramen absent; temporal fen-estra elongate (as in Trinacromerum); symphysis elongate andincluding the splenial, encompassing the first 12 dentary teeth;teeth faintly striate on medial face; homodont dentition; teethrelatively robust; humerus sigmoidally curved; propodials bear-ing three distal facets and only a minor degree of posterodistalexpansion; epipodials longer than wide, bearing distinct ante-brachial foramen; phalanges rounded and elongate.

Description. The specimen includes a partial skull (Fig. 4) lack-ing the posterior portion of the suspensorium and palate. Thesquamosals, quadrates, jugals, and posterior portion of the ptery-goids are the chief components missing; however, most of thebasicranium is present. The posterior halves of the lower jaw areaffixed to the skull by a hard, concretionary matrix and were leftin this position for fear that their removal would jeopardize theintegrity of attached pieces. In the future, it would be advisable toremove the lower jaw portions, allowing for a more accurate toothcount and an examination of the midpoint of the palate. The muz-zle and symphyseal portion of the lower jaw are separate from thecranium and mandible along a vertical fracture. The sun-bleachedcondition of the basioccipital and first five cervical vertebrae

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TABLE 1. RELATIVE PROPORTIONS OF SHORT-NECKED PLESIOSAUR CRANIA

a b c d e f

Pahasapasaurus haasiAMM 98.1.1 33.3 ~90 37% 62.5 ~87 72%

Liopleurodon feroxR. 3536 30.5 154.0 20% – – –R. 2680 – – – 67.2 109.6 61%

Brachauchenius lucasiFHSM VP321 41.0 172.5 24% – – –USNM 4989 – – – 56.3 90.0 63%

Peloneustes philarcusR. 3318 21.4 67.6 32% – – –R. 3803 20.4 70.3 29% 51.5 76.2 68%

Trinacromerum bentonianum KUVP 5070 33.5 74.5 45% – – –YPM 1129 25.2 67.2 37.5% – – –

Dolichorhynchops osborniKUVP 1300 23.7 59.0 40% 39.4 53.5 74%FHSM VP404 26.1 56.6 46% 37.5 51.0 73%KUVP 40001 46.6 94.2 49% 67 90 74%AMNH 5834 32.1 74.3 43% 54.3 69.3 78%

Polycotylus latipinnisSDSM 23020 46.5 98.0 47% 63.5 87.0 73%

Note: Length measurements are in centimeters; a—symphyseallength; b—lower jaw length; c—% length symphysis (a/b); d—snoutlength; e—skull length; f—% length snout (d/e).

indicates that these elements were exposed upon discovery.Details of the vertebral column are difficult to discern owing todeterioration and crushing prior to fossilization. Minimally, 21cervical centra are present, but the posterior portion of the cervi-cal series is heavily damaged, and a more accurate count is notpossible. Most of the trunk vertebrae are present but in a very poor

state of preservation. Small girdle fragments and numerous sec-tions of ribs are better preserved but poorly represented. Most ofthe left front paddle is preserved, but it lacks the fifth digit anddistal phalanges. Although the proximal podials and phalangesare articulated correctly, some mixing of phalanges is likelywithin the digits. Large portions of the left femur and proximalpodials of the left paddle are well preserved but shattered and aremissing numerous pieces. The overall length from the tip of thesnout to the end of the preserved trunk series (likely extending toabout the sacral area) is �4.5 m, so the overall length of the ani-mal was likely �6 m. The maximum length of the humerus (45cm) also illustrates the relatively large size of P. haasi, and in thisrespect is more similar to Polycotylus and Trinacromerum versusthe smaller Dolichorhynchops (O’Keefe, 2004). P. haasi is onto-genetically mature, as indicated by its relatively large size and thelack of visible neuro-central sutures on the vertebrae.

SKULL: The skull is 75 cm in length from the tip of the pre-maxillae to the posterior edge of the occipital condyle. The pre-served cranium is not crushed but is otherwise distorted, with therostrum and lower jaws bent to the right, likely owing to post-burial deformation. The rostrum and anterior half of the lowerjaws are well preserved, although lacking most of the teeth. Ascan be seen in Table 1, the snout and symphyseal lengths com-pare well with those of other polycotylid plesiosaurs. Severalsmall teeth are preserved in the upper and lower jaws beneath theorbits. Of the larger teeth, only the anterior-most tooth of the leftdentary is relatively complete, although portions of several oth-ers are present. The teeth exhibit distinct striae only on theirmedial sides. The dentition is homodont, with the size of alveoligradually decreasing posteriorly. The premaxillae bear 6 teeth,the first 5 of which produce subtle swells on the lateral sides ofthe bones. The number of maxillary teeth is questionable becauseof poor preservation. The right maxilla bears 5 visible aveolianteriorly and a series of 8 progressively smaller teeth extendingto a point at the posterior edge of the orbit. Tooth spacing sug-gests room for 6 or 7 additional teeth for a total of 19 to 20 teeth

TABLE 2. LENGTH OVER WIDTH RATIOS OF LIMB ELEMENTS IN CRETACEOUS SHORT-NECKED PLESIOSAURS

Humerus Femur Radius Ulna Tibia Fibula Phalanges

Pahasapasaurus haasiAMM 98.1.1 1.96 2.17 1.07 1.08 0.96 ? 1.85

Trinacromerum bentonianumFHSM VP12059 1.90 1.94 0.82 0.74 0.70 0.87 1.76USNM 10945 0.82 0.83 0.74 0.86 1.75

Dolichorhynchops osborniKUVP 1300 1.84 2.12 0.62 0.72 0.64 0.75 1.41FHSM VP404 1.78 1.83 – – 0.58 0.60 1.39

“Trinacromerum bonneri”KUVP 40002 1.65 1.8 0.59 0.78 0.68 0.75 1.32

Polycotylus latipinnisSDSM 23020 1.63 ? 0.63 0.82 0.60 0.76 1.27AMNH 1735 – – – – – – 1.29KUVP 5916 – 1.79 – – 0.66 0.73 1.22

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in the right maxilla. Preservation of the lower jaw also limits theaccuracy of a tooth count to �25 in each dentary. Both the pre-maxillae and dentaries bear a high number of foramina on theirlateral surfaces, with an especially high density of foramina at theanterior tips of both elements.

SKULL ROOF: The dorsal surface of the skull is preservednearly to the posterior edge of the parietal and largely lacks theposterior and lateral borders of the temporal opening (squamosals,quadrates, and jugals). The midpoint of the skull roof is missing,and the configuration of various elements here is difficult to dis-cern. Small portions of the right and left postfrontals are attachedto the parietal, indicating the posterior border of the orbits. Theright postorbital bar is largely missing, but the left has been flat-tened on the left maxilla and palatine. The orbital openings arelarge, rounded, and ovoid with the height less than the length. Themissing region of skull roof reveals two well-defined masses offerruginous marl that filled the internal narial chambers. The infill-ings are oblate spheroids possessing a botryoidal surface that werebound dorsally by missing portions of the premaxillae, maxillae,and possibly the frontals. The preserved prefrontals likely con-tacted the frontals posteriorly, and contact the maxillae anteriorly.The antero-dorsal margin of the orbit is solidly constructed, unlikethe delicate construction of this area in other polycotylids (Carpen-ter, 1996; O’Keefe, 2004), and there is no preservation of the exter-nal nares. The premaxillae are preserved to just anterior to thenarial infillings, and the contacts of the premaxillae with the max-illae are traceable to this point. The anterior-most portion of theparietal is missing and reveals the frontals beneath. Posteriorly, theparietal is preserved to the point at which the element deflects lat-erally to define the posterior walls of the temporal fenestrae.Although the parietal crest is broken along its length, most of theelement is preserved and bears no evidence of a foramen.

PALATE: The vomer is slender, elongate, and extends anteri-orly to between the first and second premaxillary teeth. Theposterior termination of the vomer, where it presumably contactsthe pterygoids and palatines at the internal nares, is masked by themidpoint of the attached lower jaw. Both of the palatines are bro-ken and shifted medially and broadly overlap the pterygoids. Theanterior half of the left palatine lies along the midline next to theright palatine, obscuring the anterior processes of the pterygoids.Ectopterygoids either are not preserved or not discernible owing to the displacement of the palatines. Unlike typical polycotylids(Carpenter, 1996; O’Keefe 2001, 2004), the pterygoids are tightlyunited along the midline, and an anterior interpterygoid vacuity isabsent. The pterygoids do not trend anteriorly to slender roundedbars as in typical polycotylids, and the entire visible ventral surfaceanterior to the parasphenoid is a sheet of solid bone. On their dor-sal surfaces, each pterygoid bears a ridge along the midline. Thisfeature is not reflected on the ventral surfaces, which are subtlyconcave along the midline, and thus the pterygoids have the formof flattened triangles in cross section. The pterygoids are preservedto a position just beyond the posterior interpterygoid vacuity. Thekidney-shaped central plates of the pterygoids that lie lateral to the posterior interpterygoid vacuity in typical polycotylids (as de-scribed by O’Keefe, 2004) are not preserved with the exception ofsmall pieces affixed to the ventral surface of the basioccipital. Thepreserved parasphenoid is a relatively robust element possessing athick ventral keel, unlike the flattened thin rod that is typical ofmost polycotylids. The parasphenoid is sutured to the dorsal sur-face of the pterygoids and exposed ventrally in a rounded notchwhere the united pterygoids separate posteriorly into the posteriorinterpterygoid vacuity. A break through the basicranium revealspaired canals interpreted as the carotid foramina. There is no appar-ent delineation between the basisphenoid and parasphenoid.

Figure 3. Arthur Haas, son of Charles,posing at the Pahasapasaurus dig site in1934. At left lies the skull on its dorsalsurface, with the vertebral column trend-ing to beneath Arthur’s feet.

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Figure 4. (continued on the next page) Skull, AMM 98.1.1. (A) Right lateral view. (B) Outline drawing in right lateral view. (C) Dorsal view. (D)Outline drawing in dorsal view.

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Figure 4. (continued) (E) Ventral view; note that anterior half of lower jaw has been removed. Skull, AMM 98.1.1. (F) Outline drawing in ventralview. (G) Symphyseal region of lower jaw in dorsal view.

MANDIBLE: The majority of both dentaries are present,although only the symphyseal portion is free from the craniumand well preserved. The mid-portion of each jaw remains inocclusion with the cranium attached by an ironstone matrix, andthey are heavily weathered on their ventral surface. The posteriorrami of both jaws are lacking, thus little or none of the articular,

prearticular, coronoid, or surangular is preserved. Portions of theangulars and splenials are likely preserved, although poor preser-vation prevents distinguishing them from the dentary. The sym-physis is enormously elongate (38 cm), extending nearly half theestimated total jaw length, and encompassing minimally the first12 dentary teeth. In ventral view, a gap exists for a distance of

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�5 cm beyond the fused dentaries before the point where the den-taries are inflected laterally at a greater angle. This is presumablythe region occupied by an anterior projection of the splenials asfigured by Williston (1908, Fig. 10) and O’Keefe (2004, Fig. 6).The continued anterior projection of the splenial into the whollyfused portion of the symphysis is not evident. Even the delinea-tion between the right and left dentaries cannot be traced through-out their union along the ventral margin, likely owing to the latestage ontogeny of the individual. In dorsal view, what may be theanterior projections of the splenial extend to the eighth tooth posi-tion of the dentary.

RECONSTRUCTED SKULL: To assess the overall propor-tions of the skull in comparison with other polycotylids, and inparticular to provide an accurate estimate as to the size and shapeof the temporal openings, a model was sculpted of the completeskull using a plaster cast of the original as a base (Fig. 5). Teethattributable to Polycotylus latipinnis in the collections of SDSMwere used to complete the dentition, as large alveoli and thicktooth roots within the jaws of Pahasapasaurus suggest a robusttooth form. The postorbital bars extend from the existing portionsof the postfrontals, and from there are sculpted to complete asmoothly rounded orbital area. The suspensorium is sculpted ver-tically, ending posteriorly at a position even with the occipitalcondyle, which coincides well with the posterior portion of thepreserved parietal. The major realization from the model is thesize of the temporal fenestrae, which are longer than wide as inTrinacromerum (Carpenter, 1996; O’Keefe, 2004). Were the sus-pensorium reconstructed with an anterior tilt and jaw articulationextending beyond the occipital condyle, as in Trinacromerum(O’Keefe, 2004), the lateral temporal bars would be longer still.On the basis of the long temporal fenestrae, the parietal crest isreconstructed as a low subtle ridge, as in Trinacromerum (Carpen-ter, 1996) and Polycotylus (O’Keefe, 2004), versus the high,arched crest of Dolichorhynchops, which has shorter temporalfenestrae. The sculpted skull is on display in the AMM along withthe type specimen.

VERTEBRAE: Of the vertebrae present, only the anteriorcervicals are relatively well preserved, although they are flattened

(Fig. 6). Distinguishing the various components of the atlanto-axial complex is hampered by poor preservation, and whether ornot the atlas centrum is exposed on the lateral sides, as in crypto-cleidoids (O’Keefe, 2004), is not discernible. Despite being dis-torted, it can be noted that the anterior cervical vertebral centrawere more or less equidimensional or perhaps slightly wider thanlong, but not extremely shortened as in Polycotylus (Storrs, 1999).The lateral walls of the anterior cervical centra are slightly con-cave. The articular facets are well scalloped, and the ventral sur-faces bear the subcentralia foramina common to all plesiosaurs.Smaller paired foramina are also on the dorsal surface of the cen-tra within the neural canal. The proximal heads of some cervicalribs remain attached to the vertebrae and are single headed. Theremainder of the preserved presacral series is severely crushedand obscured by partial decomposition prior to fossilization, andaccurate assessment of vertebral centra from this portion of theseries is not possible.

LIMB ELEMENTS: The paddles of P. haasi are uniqueamong polycotylids, as they retain many primitive characteristics(Fig. 7). The humerus measures 45 cm in maximum length, withonly a relatively minor degree of postero-distal expansion (seeTable 2). Three distinct facets are on the distal margin of thehumerus for articulation of the epipodials, but the endochondralmargin of the articular area continues around the postero-distalcorner. The humerus displays the sigmoid curvature discussed byO’Keefe (2004), as weakly expressed in Trinacromerum andDolichorhynchops and strongly present in Polycotylus. However,the upper portion of the shaft is crushed and appears to haveskewed the angle of the humeral head posteriorly. The radius andulna contrast markedly with those of other polycotylids, as theyare relatively large elements compared to overall paddle size, arelonger than wide, and enclose an elongate antebrachial foramen.The shafts of both the radius and ulna are entirely surrounded byperichondral bone, whereas typical polycotylids have endochon-dral bone surrounding these elements on all but the leading edgeof the radius. Although not articulated with the paddle as origi-nally on display, a supernumerary epipodial was assembled fromloose fragments and seems likely to have originated from the

Figure 5. Reconstructed skull of Pahas-apasaurus haasi.

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Figure 6. From left to right: basioccipital,atlas-axis complex, and cervical verte-brae 3 through 5 in ventro-lateral view.

front paddle as shown in Figures 7a,c. The phalanges are long incomparison with those of other polycotylids and are rounded in cross section, with the shafts enveloped in perichondral bone.In all of these features, P. haasi retains primitive traits in its pad-dle morphology.

Metacarpal II of P. haasi also differs from that of other poly-cotylids. As seen in the type specimens of T. bentonianum(USNM 10945), D. osborni (KUVP 1300), and P. latipinnis(AMNH 1735), metapodial II in both the forelimb and hind limbis an element easily distinguishable from any other podial. The

proximal border exhibits a convex v-shaped articulation that isshared between distal podial 1 and fused distal podial 2–3.Metacarpal II in P. haasi possesses only a very small articular areaon its antero-proximal border for contact with distal carpal 1.

The hind limb has been shattered and is thus less well pre-served than the forelimb. The tibia is roughly equidimensional,and the fibula appears to be longer than wide but is incompletelypreserved. As with the radius and ulna, the tibia and fibula enclosea long epipodial foramen. The femur is 49 cm in maximumlength, which is slightly longer than the humerus. This is typical

Figure 7. (continued on the next two pages) Left paddles, AMM 98.1.1, in ventral view. (A) Forelimb. (B) Outline drawing of forelimb.

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Figure 7. (continued) (C) Epipodials and mesopodials of forelimb. (D) Hind limb.

for polycotylids, as the femora are slightly longer but slightly lessposterdistally expanded than the humeri (Table 2).

GASTROLITHS: Along with associated objects recoveredfrom the P. haasi excavation pit were a number of silicic stonespurported to be gastroliths. Recent literature has addressed thepossibility of distinguishing true gastroliths from other polishedstones based upon micro-texture and lithology (Johnston et al.,1994; Whittle and Onorato, 2000; Wings, 2004). Above all else,context is the most definitive aspect in the case for gastroliths.Stones intimately associated with fossil skeletons and occurring

in offshore marine environments, where abiotic input of pebblesize materials is highly unlikely, can be more confidently arguedto be gastroliths. Only two of the stones originally on display withP. haasi are likely gastroliths. They are both �3 cm in maximumdimension and are the only two stones coated with the shellac var-nish used in the initial preparation of the skeleton. Moreover, onlythese two stones exhibit a rusty ferruginous rind from the enclos-ing bedrock. Four other larger stones are irregular in shape and arebrightly colored and highly polished. It is likely that these stoneswere simply added to the original two gastroliths over the years to

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Figure 7. (continued) (E) Outline drawing of hind limb.

enhance the museum display. While preparing a small block con-taining portions of dorsal vertebrae, a third silicic stone similar tothe other two was noted intimately embedded within the matrix.

This is not the first reported case of suspect gastroliths occur-ring with the skeleton of a short-necked plesiosaur. Other accountsinclude stones with the pliosaur Peloneustes (Andrews, 1913) andpolycotylids (Martin and Kennedy, 1988; Sato and Storrs, 2000).In 1994 the author collected an association of cf. polycotylid verte-brae with small silicic stones from the Belle Fourche Shale (lowermiddle Cenomanian) in Rapid City, South Dakota. Althoughdocumentation exists to substantiate the use of stones by pliosauridand polycotylid taxa, the number and sizes of the stones are much smaller than those commonly reported in association withelasmosaur skeletons (Williston, 1893; Riggs, 1939; Welles andBump, 1949; Darby and Ojakangas, 1980; Storrs, 1993; Everhart,2000). Also, there are significantly more polycotylid skeletons forwhich no association of gastroliths exists in comparison with elas-mosaurid specimens. Whether the employment of gastroliths inplesiosaurs was for buoyancy (Williston, 1893) and/or as a mech-anism to aid in digestion (Brown, 1904; Everhart, 2004; McHenryet al., 2005), the ecology of long-necked forms demanded thepractice to a higher degree than the short-necked forms.

CLADISTIC ANALYSIS

Pahasapasaurus was scored and included in the data matrixof O’Keefe (2004) using PAUP*4.0 (Swofford, 2001) and thesame outgroup, parsimony parameters, and branch and boundalgorithm employed by O’Keefe. Some characters listed asunknown for Polycotylus in the original matrix were scored on thebasis of a nearly complete skull and skeleton (SDSM 23020). Theanalysis yielded seven most parsimonious trees having a treelength of 172, a consistency index excluding uninformative char-acters of 0.650, and a rescaled consistency index of 0.485. A strict

consensus tree yields a single unresolved polytomy for the Poly-cotylidae (Fig. 8), and all other relationships below the Polycotyl-idae are similar to those derived by O’Keefe. Thus, inclusion ofP. haasi within the Polycotylidae is supported by the O’Keefe(2004) data matrix, but all resolution within the group is lost. An Adams consensus of this tree places Edgarosaurus and Pahas-apasaurus as a clade of primitive taxa below the level of thepolytomy including Dolichorhynchops, Polycotylus, and Tri-nacromerum. In this respect these results differ from those of

Figure 8. Cladogram of the Cryptocleidoidea (adapted from O’Keefe,2004).

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O’Keefe (2004), whose analysis consistently depicted Polycoty-lus as a sister taxon to other polycotylids.

DISCUSSION

Systematic Paleontology

Although in some respects Pahasapasaurus seems to be wellplaced within the modern definition of Polycotylidae (Carpenter,1996; O’Keefe, 2004), it possesses characters that clash with theclassical view of the clade. Historical definitions of Polycotylidaeconsistently included the presence of an interpterygoid vacuityand short epipodials (Williston, 1908; Welles, 1962; Carpenter,1996). Brown (1981) noted a chronological trend for a shorten-ing of the epipodials in Plesiosauria. Sato and Storrs (2000) andAlbright et al. (2007)1, discussed early polycotylids from Japanand Utah, respectively, and suggested that long epipodials of thisspecimen represent the retention of a primitive character withinthe Polycotylidae. The only portion of the polycotylid definitionlisted by O’Keefe (2004) that can be compared in P. haasi is theinclusion of the splenials within the symphysis. Although thischaracter is not directly preserved in P. haasi, the space necessaryto accommodate anterior extensions of the splenials between thedentaries is present. The O’Keefe definition does not mentionshort epipodials and also does not consider the nature of the ptery-goids anterior to the parasphenoid. Although crucial areas of theP. haasi cranium are not preserved, certain fundamental charac-ters are markedly different from any previously described poly-cotylid. The complete lack of an interpterygoid vacuity seems illplaced within the Polycotylidae. If one accepts the palate trans-formational series proposed by O’Keefe (2001, Fig. 21), the sec-ondary closing of the interpterygoid vacuity occurred not justwithin the Elasmosauridae but potentially within a branch of thePolycotylidae as well. The parasphenoid of P. haasi is also not atall similar to polycotylids, having a more robust construction witha strong ventral keel, and terminating by suturing atop the coa-lesced pterygoids. Overall, the visible construction of the P. haasipalate looks more similar to that observed in pliosaurids such asPeloneustes and Pliosaurus (Linder, 1913). However, elas-mosaurs like Libonectes morgani and Callawayasaurus colombi-ensis also lack an anterior interpterygoid vacuity, and they have aparasphenoid with a gracile ventral keel (Welles, 1962; Carpen-ter, 1997, 1999).

Tooth counts in P. haasi are suspect owing to preservation,and tooth counts overall seem to be a variable character within aparticular taxon (Carpenter, 1996). But the presence of 6 premax-illary teeth is notable in comparison with the more usually

reported number of 5 in polycotylids. Although neck length can-not be quantified, the value of minimally 21 cervical vertebrae is likely not missing a large number of the series; thus the com-plete cervical count of P. haasi is not significantly different thanthat of other polycotylids. Visibly (Fig. 2), the overall preservedneck length seems appreciably longer than the polycotylidDolichorhynchops (Williston, 1903, plate 1), perhaps owing toless anteroposterior shortening of the cervical centra.

Several authors have pointed out that limb modifications,such as shortening and broadening of the epipodials and increasedhyperphalangy, are evolutionary trends common to all plesiosaurlineages (Welles, 1943; Brown, 1981; Carpenter, 1997). In lightof this, the appendicular morphology of polycotylids containsvaluable phylogenetic markers for the clade. Polycotylid limbelements exhibit several key trends from the late Cenomanian tothe middle Campanian in the Western Interior (Irwin and Schu-macher, 2003). The paddles become more tightly knit throughtime, and individual elements more closely conform to each other.Podials evolve to shorter, wider elements, that unchecked wouldhave resulted in shorter, wider appendages. The resultant loss inpaddle length from podial shortening was likely accommodatedby continued hyperphalangy, although testing of this hypothesisis hampered by relatively few fully articulated paddles. The fore-shortening of bony elements applies throughout the paddle. Forexample, the podials and phalanges of Trinacromerum are consis-tently longer than those of Dolichorhynchops or Polycotylus.Likewise, the limb elements of Pahasapasaurus as a whole areproportionally longer than those of other polycotylids. Further-more, this character lends some credence to the scrappy typespecimen of Polycotylus latipinnis, which includes short, blockyphalanges (see Table 2). Postero-distal expansion of the propodi-als likely accommodates at least one supernumerary epipodial inPahasapasaurus. More than one supernumerary in the epipodialrow shows ontogenetic variability, but contrary to Carpenter (1996),I find the character to have taxonomic utility. No specimen of Tri-nacromerum that I am aware of exhibits a fourth epipodial, al-though there is room for a small such ossification. Specimens ofDolichorhynchops usually possess a small fourth epipodial in oneor more of the limbs (Carpenter, 1996). Specimens of Polycoty-lus possess large, well-developed fourth metapodials (Williston,1906), with a distinct facet for the ossification evident on thepropodials (O’Keefe, 2004). These observations are apparent incomparing the length versus width ratios of propodials from thevarious taxa, where Polycotylus diplays the highest degree ofpostero-distal expansion (Table 2).

Phylogenetic Analysis

To this point much uncertainty remains in polycotylid phylo-geny (Sato and Storrs, 2000; Druckenmiller, 2002; O’Keefe, 2004;Buchy et al., 2005; Albright et al., 2007). In just the last severalyears, seven new polycotylid taxa have been described, includingP. haasi (Druckenmiller, 2002; Bardet et al., 2003; Buchy et al.,2005; Sato, 2005; Albright et al., 2007). All the newly erected taxa

1As the present work was going to press, Albright et al.’s (2007) paper was pub-lished containing information highly pertinent to the content of this manuscript.Although their work could not be fully incorporated, it is noted that both Paha-sapasaurus haasi and Palmula quadratus share the retention of primitive limbcharacters as outlined herein.

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are monospecific and known from incomplete skeletons, greatlyhampering the cladistic method by an abundance of unknown oruncertain character states. Given this, and after extensive testingof published and independent cladistic matrices, I concur withBuchy et al. (2005), who noted that the inclusion or removal ofone or more taxa (and/or characters) can greatly modify the topol-ogy of the cladogram. I speculate that the lack of resolution withinthe Polycotylidae is due not only to the nature of available mate-rial but also in part to the dubious inclusion of Edgarosaurus muddi(Druckenmiller, 2002) within the group. This assertion is sup-ported by the cladistic analysis of Albright et al. (2007). E. muddipossesses a suite of characters not in kind with the Polycotylidae,including 6 to 7 premaxillary teeth; constricted, scooplike snoutand lower jaw; distinct parietal foramen; short symphysis; canini-form teeth; posterior extension of premaxilla not in contact withparietal; relatively short muzzle; suspensorium extending far pos-terior of the occipital condyle; and apomorphic paddle morpholo-gies. Most of these listed characters are more commonly associatedwith members of the Pliosauridae.

In his phylogenetic analysis of the Cryptocleidoidea, O’Keefe(2004) included Edgarosaurus within his Polycotylidae, resultingin a polytomy with Polycotylus and other polycotylid plesiosaurs.O’Keefe (2001, 2004) further concluded that Polycotylus is aplausible representative sister taxon to other polycotylids basedupon the presence of several primitive characters, includingrobust and heavily striated teeth; long and low sagittal crest; ptery-goids narrowing lateral to the anterior interpterygoid vacuity; anda relatively large number of cervical vertebrae. The long and lowsagittal crest is shared by Trinacromerum and possibly Pahas-apasaurus. A narrowing of the pterygoid bars lateral to the ante-rior interpterygoid vacuity is a character that is shared byTrinacromerum and Dolichorhynchops, and differences in Poly-cotylus observed by O’Keefe (2004) are based upon an erodedpterygoid portion (FMNH 187). The apparent primitive conditionof a higher cervical count could represent a reversal. Polycotylusshares other characters, although more derived, with Trinacro-merum and/or Dolichorhynchops, including propodials (markedpostero-distal expansion, sigmoid curvature of humerus); paddles(shorter podials, tightly knit phalanges with square cross sections,large supernumerary epipodials, patchy loss of perichondral bonearound the margins of podials); vertebrae (short, deeply com-pressed lateral and ventral walls of centra); and pelvis (highlyelongate ischia). In their review of plesiosaurs from the “Fort Ben-ton” interval of Kansas, Schumacher and Everhart (2005) foundthat the only identifiable polycotylid known in pre-Coniaciandeposits of North America is Trinacromerum and an unnamedpolycotylid of Cenomanian age (P. haasi). Thus, the known mor-phology and stratigraphic occurrence (early Campanian) of Poly-cotylus do not support the hypothesis that it is the sister taxon tomore derived polycotylids. If Pahasapasaurus is accepted as apolycotylid, then the classic polycotylid palate construction andshort epipodials of Trinacromerum, Dolichorhynchops, and Poly-cotylus indicate that they are all more closely related to each otherthan to Pahasapasaurus.

CONCLUSIONS

Pahasapasaurus haasi constitutes a new plesiosaur taxon ofthe Late Cretaceous Western Interior Seaway. Although mostclosely resembling a member of the Polycotylidae, it bears a num-ber of features markedly different from classically known poly-cotylid morphology, including long epipodials and phalanges, aclosed palate, and a robust parasphenoid bearing a strong ventralkeel. The addition of Pahasapasaurus to existing cladistic analy-ses supports its inclusion within the Polycotylidae but destroysany resolution within the clade. Edgarosaurus muddi is a dubiousmember of the Polycotylidae owing to a suite of characters notheld in kind with the group. Five new monospecific polycotylidtaxa have been named in only the last few years, indicating that a great deal is still to be learned about their variation and diver-sity. Of North American genera ascribed to Polycotylidae, onlytwo monospecific type specimens are from Cenomanian and olderdeposits (Edgarosaurus, Pahasapasaurus). The three classicallytreated polycotylids of Turonian and younger age (Dolichorhyn-chops, Polycotylus, Trinacromerum) are better represented andsomewhat better known. An improved understanding of poly-cotylid phylogeny and systematics will require new discoveriesof poorly known pre-Turonian forms. Nevertheless, Pahasapa-saurus indicates that a lineage of short-necked plesiosaurs withaffinities to the Polycotylidae, but possessing a closed palate andplesiomorphic limbs, persisted until at least the early late Ceno-manian in the Midcontinent of North America.

ACKNOWLEDGMENTS

Thanks go to Mary Kopco, Adams Memorial Museum direc-tor, for realizing the need to better care for this specimen and thusdiscovering its importance. The Deadwood Historic PreservationCommission and the Charles Haas family provided the means tocompletely prepare the skeleton and to construct the exhibit case.Verne and Don Haas, son and grandson of Charles, providedimportant historical and archival information, including originalfield notes and photographs. P.R. Bjork provided advice and en-couragement during the preparation process and construction ofthe skull model. G.L. Bell Jr. and J. VonLoh are strongly acknowl-edged for assistance with this project and many lunchtime discus-sions of plesiosaurs. Critical reviews of this article were providedby K. Carpenter, J.E. Martin, D.C. Parris, T. Sato, and G. Storrs.

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