Plioplatecarpus Primaevus (Mosasauridae) from the Bearpaw Formation (Campanian, Upper Cretaceous) of...

Plioplatecarpus Primaevus (Mosasauridae) from the Bearpaw Formation (Campanian, UpperCretaceous) of the North American Western Interior SeawayAuthor(s): Robert HolmesSource: Journal of Vertebrate Paleontology, Vol. 16, No. 4 (Dec. 31, 1996), pp. 673-687Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4523766 .

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Journal of Vertebrate Paleontology 16(4):673-687, December 1996 C 1996 by the Society of Vertebrate Paleontology

PLIOPLATECARPUS PRIMAEVUS (MOSASAURIDAE) FROM THE BEARPAW FORMATION (CAMPANIAN, UPPER CRETACEOUS) OF THE NORTH AMERICAN

WESTERN INTERIOR SEAWAY

ROBERT HOLMES Canadian Museum of Nature, P.O. Box 3443, Station D,

Ottawa, Canada KIP 6P4

ABSTRACT-One almost complete mosasaur skeleton and much additional material from the Upper Cretaceous Bear- paw Formation of South Central Saskatchewan permits the first adequate description and diagnosis of Plioplatecarpus primaevus Russell. It possesses the following unique (within mosasaurs) characters: 11 maxillary teeth; a large shield- shaped septomaxilla forming a median internarial septum; a posterodorsally directed iliac process; and no obturator foramen. It can be distinguished from Plioplatecarpus marshi in the possession of a modest sized coracoid foramen, and both P. houzeaui and P. marshi in the possession of an unreduced presacral column of 30 vertebrae, all bearing functional zygapophyses, indicating that P. primaevus is the primitive sistergroup to these two taxa. Although extremely similar to UNO 8611-2, an unnamed specimen from Alabama, the lack of an eminence on the posterior surface of the quadrate shaft in the latter establishes UNO 8611-2 as the primitive sister group of P. primaevus + [P. houzeaui + P. marshi]. A quadrate eminence, long considered to be diagnostic of the genus, shows considerable variability in size and shape in P. primaevus, suggesting that it is not suitable as a specific diagnostic character. Other species of Plio- platecarpus (P. crassartus, P. depressus, "Platecarpus" somenensis, "Mosasaurus" scanicus, and undescribed species from Scabby Butte, Alberta and Anderson River, North West Territories), have not been sufficiently characterized to include in a phylogenetic analysis.

INTRODUCTION

The mosasaur species Plioplatecarpus has long been known from relatively incomplete material from Maastrichtian deposits of Europe (Dollo, 1882, 1885, 1889, 1890, 1892, 1904). Spec- imens pertaining to the genus have more recently been identified from North America (Russell, 1967a; Burnham, 1991; To- karyk, 1993), but most material is very fragmentary, precluding comprehensive description and taxonomic assignment.

In 1959, a joint field party from the Canadian Museum of Nature (then National Museums of Canada) and the Royal Sas- katchewan Museum (formerly the Saskatchewan Museum of Natural History) collected marine vertebrates from the deposits of the Bearpaw Formation exposed along the South Saskatch- ewan River in South Central Saskatchewan. Subsequently, much of this area has been inundated after construction of a reservoir dam. The collection includes, in addition to plesiosaur elements and a partial skeleton of Mosasaur conodon, one almost complete skeleton and a second partial skeleton of Plio- platecarpus primaevus, permitting a complete description of the latter for the first time. Subsequent collecting from the area indicates that this taxon was the most common mosasaur in the Bearpaw Sea of South Central Saskatchewan.

GEOLOGY

The type of Plioplatecarpus primaevus (USNM 18254) was collected near Pierre, Hughes County, South Dakota from the DeGrey Member of the Pierre Shale (Crandell, 1958:fig. 8). The age of the DeGrey Member has been estimated using ammonite range zones (Gill and Cobban, 1965, 1966) at between 75 and 73.5 million years (Izett et al., 1971), although mosasaur material is reported only in the upper portion of the member (Cald- well, 1968).

There is some uncertainty surrounding the precise prove- nance of the Saskatchewan specimens described here. The three principle specimens (NMC 11835, 11840, and P 1756.1) were found within several kilometers of the Cruikshank Member type section (northeast portion of section 35-19-11 W3, Caldwell,

1968), which contains not only the entire Cruikshank Member, but also underlying beds of the Snakebite and overlying Aquad- ell Members (Caldwell, 1968). As the Cruikshank Member is not known to be fossiliferous, Tokaryk (1993) hypothesized that P 1756.1, and by inference the other two specimens, came from either Aquadell or Snakebite aged beds. However, it is not possible to be more precise. Ammonite range zone data (Caldwell, 1968) suggests that the age of the Snakebite Member ranges from 72.5 to slightly more than 71 million years, establishing the maximum range of the specimens. The thin Cruikshank Member straddles the 71 million year mark, and the Aquadell Member ranges between slightly less than 71 and 70 million years respectively (Izett et al., 1971). Only the basal layers of the Aquadell Member are exposed in the region of the Cruik- shank type section, indicating that the specimens could not be younger than 70.5 million years. The Saskatchewan specimens are therefore younger than the type material, but older than the European Plioplatecarpus houzeaui and P. marshi, which have been dated at between 67 and 68 million years before present (Russell, 1967).

MATERIALS AND METHODS

All specimens are preserved in a heterogeneous matrix ranging from a soft clay-like material to an extremely resistant, black substance. Using x-ray powder diffraction, the latter was identified as largely apatite, precluding chemical preparation. The specimens were therefore mechanically prepared. The crumbly nature of the matrix has resulted in many of the more delicate parts of the skeleton being damaged or lost during collection or subsequent preparation. Gypsum encrustation and in- filtration have also disrupted some bone surfaces, obscuring many fine details. Although the bones of most specimens, including NMC 11835, 11840, and P 1756.1 were clearly associated, in all cases they were disarticulated and badly disturbed during burial.

The following institutional abbreviations are used to identify the specimens discussed in the text: IRSNB, Institut Royal des

673



674 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 16, NO. 4, 1996

Sciences Naturelles de Belgique; NMC, Canadian Museum of Nature (formerly the National Museums of Canada); P, Royal Saskatchewan Museum (formerly the Saskatchewan Museum of Natural History); TMP, Tyrrell Museum of Paleontology; UNO, University of New Orleans; USNM, Smithsonian Insti- tute.

The most complete specimens appear to be subadult, with snout-vent lengths of approximately two meters. In addition, there are numerous partial skeletons and isolated elements. Al- though generally not diagnostic to species, all have comparable geographic and stratigraphic occurrences, and other than size, are indistinguishable from the more complete specimens. Con- sequently, they are tentatively assigned to P. primaevus. In addition to many fragments that pertain to subadult individuals, there are isolated elements of much smaller and larger individuals. For example, an isolated quadrate (P 1793.1) is approximately 50 percent taller than, and an isolated cervical vertebra (P 1795.1) is one half the anteroposterior length of, the equivalent elements in NMC 11835. Fragments of an even smaller individual (TMP 83.126.1) are about 40 percent of the linear dimensions of the equivalent elements in NMC 11835.

Specimens examined (Unless otherwise stated, specimens are from the South Saskatchewan locality, Bearpaw Formation):

NMC 11835. Plioplatecarpus primaevus. Associated remains comprising most of skull including partial braincase, quadrates and mandible fragments; Complete presacral column and pygal series; Many rib fragments, but few complete elements; Endo- chondral pectoral girdle, humeri, epipodials, and incomplete, scattered carpals, metacarpals, and phalanges; Both pelvic girdles, femora, epipodials, and incomplete, scattered tarsals, met- atarsals, and phalanges.

NMC 11840. Plioplatecarpus primaevus. Well preserved braincase and much of posterior portion of dermatocranium and quadrates; cervical vertebral column; right scapulocoracoid.

NMC 11394. Plioplatecarpus sp. Isolated quadrate of small individual from Anderson River (Mason River Formation), North West Territories.

NMC 34441. Plioplatecarpus sp. Isolated quadrate. NMC 21853. Plioplatecarpus sp. from Scabby Butte (St.

Mary River Formation), Alberta. Two complete, articulated anterior paddles; posterior dorsal vertebral series; fifteen pygals and partial caudal series; ribs; limb and girdle fragments and dermal ossifications. Locality roughly coeval with the South Saskatchewan River site (Langston, 1975).

P 628.1. Plioplatecarpus primaevus. Articulated, complete posterior pygal series and most of tail; rear limbs and girdles.

P 1057 Plioplatecarpus sp. Isolated teeth, vertebrae, limb elements.

P 1756.1. Plioplatecarpus primaevus. Complete snout; left postorbitofrontal; parietal fragment; right squamosal; both ju- gals; lower jaw and quadrate; complete anterior column from axis to 11 th thoracic; both scapulocoracoids; and right humerus.

P 1793.1 Plioplatecarpus sp. Left quadrate of very large individual.

P 1795.1 Plioplatecarpus sp. Cervical vertebra of very small individual.

P 2036.1. Plioplatecarpus sp. Premaxilla, maxilla and pterygoid fragments.

TMP 83.64.2. Plioplatecarpus sp. Right dentary with teeth; left postdentary series; partial skull; cervical vertebrae; ribs from Manyberries, Alberta (Bearpaw Formation).

TMP 83.126.1. Articulated posterior dorsal series, pelvic girdle and limb, and most of tail of ?Platecarpus. Associated, in region of the stomach are vertebrae, back of the lower jaw, and other fragments of very small Plioplatecarpus sp found on Red Deer River north of Iddlesleigh, Alberta (Bearpaw Formation).

USNM 18254. Type of Plioplatecarpus primaevus from

Pierre Shale (DeGrey Member), South Dakota. Right quadrate; both scapulocoracoids; three anterior thoracic vertebrae.

SYSTEMATIC PALEONTOLOGY

Family MOSASAURIDAE Gervais, 1853 Subfamily PLIOPLATECARPINAE (Dollo, 1884) Williston,

1897 Genus PLIOPLA TECARPUS Dollo, 1882

Diagnosis (revised from Russell, 1967a and Bell, 1993)- Maximum longitudinal dimension of large oval parietal foramen equivalent to approximately one half of minimum inter- orbital width; frontal shield forms at least half of margin of parietal foramen on dorsal surface of skull; frontal plate widens anterior to orbits, forming roughly rectangular shield with gently convex lateral margins between orbits and external nares; frontal plate forms acute process anteriorly slotting into posterior margin of internarial bar of the premaxilla; otosphenoidal crest of prootic absent; delayed or incomplete ossification of basioccipital exposing canal for basilar artery ventrally; postorbital process of postorbitofrontal extremely short, forming socket of peg-and-socket joint with jugal; quadrate large (maximum height approximately 19 percent of total skull length); tympanic ala greatly expanded to form conch; prominent swelling of variable shape on posterior surface of quadrate shaft (except UNO 8611-2, Burnham, 1991); reduced infrastapedial process; highly convex mandibular condyle mediolaterally compressed and subtriangular in outline; 35-40 vertebrae anterior to chevron bearing caudals; at least 10 pygal vertebrae; Scapula larger than coracoid, mostly as a consequence of posterior expansion of blade; large radius with approximately circular humeral facet; radius with mediolaterally expanded distal end; radius with lateral (anterior) flange on shaft of radius.

The above revised list includes only those characters hypothesized to be derived. Russell (1967a) and Lingham-Soliar (1994a) provide a complete list of characters, some of which are here considered to be plesiomorphic, for the genus Pliopla- tecarpus.

PLIOPLA TECARPUS PRIMAEVUS Russell, 1967a

Holotype-USNM 18254, from the DeGrey Member of the Pierre Shale, South Dakota. Right quadrate, both scapulocoracoids and three anterior thoracic vertebrae.

Diagnosis-Large shield-shaped septomaxilla occupying anterior part of nasal vestibule; medial edge reflected dorsally and with other septomaxilla forms an internarial septum reaching undersurface of internarial bar of premaxilla; 11 maxillary teeth; posterodorsally directed iliac process; obturator foramen absent.

Referred specimens-NMC 11835, NMC 11840, P 628.1, P 1756.1

DESCRIPTIONS AND COMPARISONS

Plioplatecarpus primaevus is a medium sized mosasaur with a short, lightly built skull, robust vertebral column, and large pectoral girdle and paddle (Fig. 1).

The Dermal Skull

The composite reconstruction (Figs. 2, 3) is based on NMC 11835, 11840, and P 1756.1. Where only one side is preserved, bilateral symmetry is assumed. The skull length of the largest individual (NMC 11840) is estimated at 45 cm. The short, blunt antorbital region (about 40 percent of the total skull length as compared with 50 percent in Platecarpus) is unusually broad in dorsal view, and the orbits are relatively large. The deep postorbital region has a maximum height (measured from the



HOLMES-PLIOPLATECARPUS PRIMAEVUS FROM NORTH AMERICA 677

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FIGURE 3. Plioplatecarpus primaevus, skull in occipital view.

teroventrally directed prong that wraps around the curved an- terodorsal edge of the quadrate. The margin of a distinct notch above the latter turns anteriorly toward the supratemporal ar- cade. The dorsolateral surface of the squamosal bears a deep horizontal groove for the supratemporal contribution of the postorbitofrontal.

Palatine-Vomer-Both the palatine-vomers are preserved in P1756.1, and partial left palatines are preserved in both NMC 11835 and 11840. All palatines are incomplete posteriorly. At the posteromedial corner of the choana, the surface of the narrow vomerine process of the palatine rotates to approach the parasagittal plane. At about the level of the fifth maxillary tooth, the vomerine bar is disrupted by what may be the palatine-vomer suture. Anteriorly the vomers fuse to form a median ridge flanked on each side by the convex ventral surface, and more anteriorly gives way to a deep median sulcus.

Septomaxilla-A robust, shield-shaped left septomaxilla is preserved in articulation in P 1756.1. Among mosasaurs, this element is described only in Plotosaurus (Camp, 1942) where it is in the form of a thin, blade-like structure totally unlike that of P 1756.1. It forms, with the maxilla, the floor of the anterior portion of the nasal vestibule. Posteriorly, its surface turns ventrally to form the curved anterior edge of the choana. Medially, its surface is reflected upward as a thin internarial septum that extends posteriorly to at least the level of the fourth maxillary tooth, and appears to have extended dorsally to the underside of the internarial bar of the premaxilla.

Parietal-Anteriorly, the parietal shield forms the posterior margin of the parietal foramen, and appears to extend forward under the frontal to form the median triangular roof of the ce- rebral hemispheres, thus completely enclosing the foramen ventrally (Fig. 4). Laterally, it overlaps the postorbitofrontal extensively. Anteriorly, on each side, its curved lateral edge forms a shelf overhanging the concave lateral surface that drops steeply away toward its articular facet with the prootic. More posteriorly, prominent laterally flaring alae appear to descend to flank the lateral surface of the prootic, but postmortem distortion ob- scures the precise relationship of these elements. Posteriorly, the laterally diverging suspensorial rami arch dorsally. Its dorsoventrally compressed posterior extremity is only loosely at- tached to the supratemporal by a horizontal lap joint as in other mosasaurs. The nature of the supraoccipital-parietal articulation is unclear. However, both NMC 11835 and 11840 show a midventral groove (Fig. 4) that may have received the sagittal crest of the supraoccipital.

Pterygoid-Each pterygoid bears a sigmoidal tract of 13 tooth bases, all with smaller diameters than those of the maxilla. None of the specimens are complete anteriorly, but in P1756.1, the left palatine is flanked medially by a thin splint of bone, most plausibly the pterygoid, that reaches anteriorly to the level of the seventh maxillary tooth, well in front of the posterior

border of the choana. If this interpretation is correct, it reaches further anteriorly than in any other mosasaur. A stout ectopter- ygoid process, incomplete in all specimens, projects laterally from the central area. Behind the posteriorly open pocket for the reception of the basipterygoid process of the basisphenoid, the bone bifurcates into a short medial basisphenoid process and a quadrate process.

Braincase

Basioccipital-Basisphenoid-Immediately anterior to the occipital condyle, the floor of the medullary cavity is pierced by a large circular median canal for the paired basilar arteries that passes ventrally and slightly anteriorly into a deep median recess on the ventral surface of the basioccipital. Rather than emerging as paired foramina on the ventral surface of the basioccipital as in Plioplatecarpus houzeaui (Lingham-Soliar 1994a), the canal turns anteriorly, and is divided into bilateral channels by a strong median carina that arises from the dorsal surface of the basisphenoid to join its descending counterpart from the basioccipital (Fig. 3). The canal is exposed ventrally through a large, irregular median opening with unfinished margins (Fig. 2) in both NMC 11835 and 11840 as in P. marshi (Devillers, 1943; Dollo, 1885).

The basal tubera are much larger and protrude more ventrally than in Clidastes (Russell 1967a:fig. 12), Tylosaurus (personal observation), and exceed even those of Platecarpus (Russell, 1967a:fig. 17) and Prognathodon (Lingham-Soliar and Nolf, 1989) in size. The basisphenoid sheaths the anterior portion of the basioccipital, cradling the anterolateral surface of each basal tuber. Its irregular posterior edge is separated from the basioccipital by a distinct furrow, the upper part of which encloses the posterior opening of the vidian canal (Fig. 6). The very short anterolaterally directed basipterygoid processes are cap- ped by small horizontally oval facets with a much shorter horizontal axis than in Platecarpus.

Prootic-The triradiate prootic sends a laterally compressed, hatchet-shaped anteroventral process to articulate with the alar process of the basisphenoid just dorsal to the sulcus for the vena capitis lateralis. Coossification is advanced in NMC 11840, but the suture is clearly visible in the smaller NMC 11835. Near the edge of its posterodorsally curving ventral margin is the posteriorly facing foramen for the seventh cranial nerve. Its lateral wall is formed by the low, rounded edge of the prootic, but a distinct otosphenoidal crest is lacking. Viewed anteriorly, the prootic is thinnest at midheight, and thickened ventrally and especially dorsally where it forms the lateral half of the otic capsule.

Opisthotic-Exoccipital-Poor preservation anterolateral to the occipital condyle makes interpretation of the coossified opisthotic-exoccipital difficult. However, a deep anteromedially directed groove on the underside of both paroccipital processes clearly marks the roof of the jugular foramen for the common passage of nerves X and XI and the internal jugular and internal carotid. The lateral border of its dorsoventral, elongate external opening is clearly marked by a sharp vertical ridge, but its medial border is indistinctly preserved in eroded bone. The con- dylar (hypoglossal) foramen is not preserved. A shallow sta- pedial sulcus runs on the ventral surface of the paroccipital process parallel to its long axis. The sulcus terminates anteromedially in the fenestra ovalis, a portion of its anterolateral border apparently formed by the prootic. Ventral and slightly anterior to this, the lateral wall of the opisthotic is pierced by an elliptical fenestra rotunda (c.f. Clidastes, Russell, 1967a). The exit of nerve IX, normally seen in this region (Russell, 1967a) could not be identified. More ventrally, the opisthotic forms a triangular lappet that sheaths the anterolateral surface of the basal tuber.




A

10 cm

B

FIGURE 4. Plioplatecarpus primaevus, dorsal dermal skull. NMC 11840 (left), and NMC 11835 (right) in A, dorsal and B, ventral aspects.

Supraoccipital-A prominent sagittal crest, incomplete anteriorly, rises from the posterodorsal surface of the supraoccipital immediately above the foramen magnum and becomes progressively higher anteriorly.

Supratemporal-The supratemporal forms a broad vertical contact with the squamosal laterally, and the quadrate ventrally. Its external surface is divided by a horizontal ridge that sepa- rates the ventral squamosal facet from a more dorsal, anteromedially directed lamina which underlies the suspensorial ra- mus of the parietal. Medially, it forms an extensive contact with the anterolateral surface of the paroccipital process and a suture with the end of the posterodorsal process of the prootic.

Quadrate-As in UNO 8611-2 and P. houzeaui, the quadrate is remarkably large, its height being about 19 percent of

the total skull length as compared with about 13 percent in Platecarpus. A suprastapedial process, typical of plioplatecar- pines in being large and parallel-sided, terminates in a blunt, bevelled point, which, in some specimens almost touches the quadrate shaft. The tympanic ala is incomplete in all specimens, but its thin broken edges indicate that it was delicate. The left quadrate of NMC 11835 bears an ossified tympanum in place. In Platecarpus, the ala produces a thickened, bladelike, posterodorsally projecting infrastapedial process that nearly meets the suprastapedial process posterior to the tympanic notch. How- ever, in P. marshi and P. houzeaui, the ala skirts posteriorly across the lateral rim of the mandibular condyle to form a low flange (the "apophyse infracolumellaire" of Dollo, 1904) that projects directly posterior from the posterior surface of the




TABLE 1. Selected measurements of the first 40 vertebrae of Pliopla- tecarpus primaevus (NMC 11835). A, maximum centrum length. B, maximum condyle width. C, maximum condyle height. D, maximum interprezygapophyseal width.

Vertebra A B C C/B D

1 15 39 34 0.87 - 2 47 29.5 22 0.75 - 3 46 31 22 0.71 40 4 49.5 34 27 0.79 37 5 47 30 23 0.77 35 6 48 32 27 0.84 30 7 50 35 26.5 0.76 44 8 52 37 27.5 0.74 36 9 53.5 39 30.5 0.78 42

10 56 38 31 0.82 38 11 63 43 29.5 0.69 48 12 65 43 30.5 0.71 49 13 66 43 33 0.77 46 14 66.5 43 33 0.77 48 15 67 46 35.5 0.77 42 16 55.5 38 29 0.76 40 17 57 39 31 0.79 40 18 56 42.5 30 0.71 39 19 57.5 40 33 0.83 37 20 57 44 34.5 0.78 40 21 54 41.5 33.5 0.81 32 22 56 44 36 0.82 37 23 56 42 34 0.81 37 24 58 44 37 0.84 43 25 63 43 37 0.86 37 26 61 43 39.5 0.92 38 27 59 42.5 36.5 0.86 32 28 58 39 34.5 0.88 32 29 60 46 42 0.91 32 30 57 38.5 40 1.04 28 31 60.5 45 41 0.91 - 32 54 44 39 0.89 33 61 47 42 0.89 - 34 60.5 46 42 0.91 - 35 58 45 36 0.80 - 36 52 46 40 0.87 37 54 47 41 0.87 - 38 53 46 43 0.93 - 39 50 47 41 0.87 40 52 42.5 43 1.01 -

Immediately ventral to the postzygapophyseal buttress, the lateral surface of the neural arch of all dorsal vertebrae bears an elliptical depression. It is largest in vertebra seven, and becomes progressively smaller anteriorly and posteriorly. The size and curvature of each depression correspond to the prezyga- pophysis of the next posterior vertebra, suggesting that they may have accommodated the anterior edges of these prezyga- pophyses during extreme lateral undulations of the trunk.

In anterior dorsal vertebrae, anteroposteriorly compressed synapophyses project laterally and slightly posteriorly, termi- nating in an extensive, dorsoventrally elongate facet. The dorsoventral dimension of this process and facet expands for the first five vertebrae to become equivalent to the centrum height, after which it rapidly becomes reduced. The processes become more circular in cross section, elongate, and gradually assume a dorsolateral orientation by vertebra 12, after which the processes begin to rotate to a ventrolateral orientation, although their shape remains unchanged. Throughout the column, their bases shift gradually to a slightly more anterior position.

The centrum length gradually increases to the eight dorsal (Table 1), posterior to which the centra become shorter again. Posterior to a second "peak" at about the eighteenth dorsal, centrum length consistently decreases to the sacrum. This pat- tern is in marked contrast to that seen in UNO 8611-2, in which

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FIGURE 10. Plioplatecarpus primaevus, dorsal vertebrae in left lateral, anterior, and posterior views. A, fifth; B, twelfth; C, sixteenth.

the centrum continues to elongate until the thirteenth dorsal, and remains long until the sacrum (Burnham, 1991).

The centrum articulations of the first 20 dorsal vertebrae are oval to heart-shaped, posterior to which the vertical dimension gradually increases, becoming slightly greater than the horizontal dimension in the last dorsal vertebra (Table 1). Consequent- ly, the surface area of the facet increases throughout the series. The first five dorsals bear broad midventral carinae, with the first bearing a boss less well-developed than, but otherwise comparable to, that on the last cervical.

A count of twenty-three dorsals is comparable to that of Pla- tecarpus (22) and Tylosaurus (22-23), but is considerably higher than the minimum count of 18 attributed to UNO 8611-2 (Burnham, 1991) and 15 in P. houzeaui (Lingham-Soliar 1994a). The incomplete presacral column of the holotype of P. marshi comprises 18 vertebrae, but the total number might have been as high as 23 (Lingham-Soliar, 1992a), which, assuming seven cervicals, gives a total of 16 dorsal vertebrae.

Ten disarticulated pygal vertebrae are preserved in NMC 11835; some may have been lost. The most anterior (putative "sacral") pygal (Fig. 1lb) bears a tall neural spine. Anterov- entrally, the neural arch forms an anteriorly projecting hood over the neural canal as in the posterior dorsal vertebrae. The centra, bearing roughly circular articular facets, decrease slightly in length posteriorly (Table 1). A dorsoventrally compressed, blade-like transverse process ("synapophysis") projects from the midlateral surface of the centrum laterally and slightly forward, and ventrally at an angle of about 60 from the horizontal.




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FIGURE 11. Plioplatecarpus primaevus, vertebrae in left lateral, anterior, and posterior views. A, last presacral; B, first pygal.

This is much less than that described (Russell, 1967a) for Cli- dastes and Mosasaurus maximus (400), M. conodon, Platecar- pus and Ectenosaurus (300) and Tylosaurus (200), but comparable to the condition in Amphekepubis (100). It gradually as- sumes a slightly more ventral orientation posteriorly, forming an angle of about 150 with the frontal plane in the last pygal.

All remaining caudal vertebrae bear unfinished haemapo- physes from which the unfused haemal arches have become disarticulated (Fig. 12). Neural arches bear tall spines that tilt only slightly posteriorly. The anterior hood over the neural canal seen in posterior dorsals and anterior pygals is absent. Cen- trum diameter increases slightly in the first several vertebrae posterior to the pygal region, after which the diameter undergoes a gradual reduction. Centrum length undergoes a more rapid reduction posteriorly. Neural spines become shorter and more posteriorly inclined.

Ribs

Thirteen rib sections in NMC 11835 bear holocephalous heads. In a few cases, their positions on the column can be approximated by matching the rib head with the synapophysis. Each bears a shallow sulcus on the posterior surface of its angle. More distally, the shaft generally becomes circular in cross section, and in a few cases increases slightly in diameter, but there is no evidence of the marked "inflation" reported in UNO 8611-2 (Burnham, 1991).

Appendicular Skeleton

Pectoral Girdle-The outline of the scapula (Fig. 13) is closely comparable with that of Platecarpus, the only difference being the greater development of the posterior part of the scapular blade. As in P. marshi and P. houzeaui (Dollo, 1882; Lingham-Soliar, 1992a) it is anteroposteriorly elongate and dis- tinctly larger than the coracoid. The broadly convex dorsal scapular border is directly above the glenoid as in Plotosaurus (Camp, 1942), but the elements are otherwise dissimilar. The scapula of P. primaevus can be distinguished from that of P. marshi in being shorter anteroposteriorly, but it is very similar to that of P. houzeaui. As in P. houzeaui and Platecarpus, it

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FIGURE 12. Plioplatecarpus primaevus, caudal vertebrae in left lateral, anterior, and posterior views. A, twelfth (second postpygal); B, sixteenth (sixth postpygal); C, ?twenty-first (eleventh postpygal).

bears a distinct embayment, variable in shape (compare right and left elements in Fig. 14), on its ventral margin immediately posterior to the glenoid.

The thickened, straight anterior edge of the coracoid is much longer than the sharp, strongly concave posterior edge, placing

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FIGURE 13. Plioplatecarpus primaevus, left scapulocoracoid, external view. Both elements are drawn in the same plane for maximum exposure.




10 cm

FIGURE 14. Plioplatecarpus primaevus, left and right scapulocoracoids of the type (USNM 18254) in external view.

most of the coracoid anterior to the level of the glenoid fossa (Fig. 13). The coracoid foramen is comparable in size to that of P. houzeaui but much smaller than that of P. marshi. An- teromedial to the foramen and approximately paralleling the anterolateral margin is a very narrow, deeply incised emargination, unlike the wide emargination of P. marshi or P. houzeaui (Lingham-Soliar, 1992a, 1994a).

Pectoral Limb-Although the humerus (Fig. 15) is massive,

A Bpgl dell pr

eenf ect

ulf 5cm

c 0 pgl

Ppect eect

pect pect

E F

ref if U

FIGURE 15. Plioplatecarpus primaevus, propodial and epipodial of left anterior paddle based on NMC 11835. A, dorsal view; B, ventral view; C and D, humerus, proximal and distal aspects; E, radius, proximal and distal aspects; F, ulna, proximal and distal aspects.

it is not particularly long, being about 1.36 times the average dorsal vertebral length as compared with ratios of 1.25:1.00 for Clidastes liodontus and Mosasaurus conodon, and 1.80:1.00 in Platecarpus, Ectenosaurus, Prognathodon and Tylosaurus. Un- like mosasaurines or tylosaurines, but like other plioplatecar- pines, the combined width of the radial and ulnar facets on its greatly expanded distal head exceeds the total horizontal width of its proximal head. Its morphology is closely comparable to that of Plioplatecarpus marshi (Dollo, 1882; Lingham-Soliar and Nolf, 1989). The shape of the gently convex, terminal glenoid condyle suggests considerable freedom of motion, the only apparent limitation being a restricted anterior excursion. Both the ectepicondyle and entepicondyle are prominent, but incompletely ossified, and do not form discrete processes as in Mo- sasaurus conodon or Clidastes (Russell, 1967a).

Both radius and ulna are stout (Fig. 15). The proximal facet of the radius is approximately circular in outline as in P. marshi (Lingham-Soliar, 1992a). The anterior edge of the dorsoventrally compressed central shaft bears a prominent dorsally (laterally) convex keel, which is more extensively developed than in P. marshi or any other mosasaur with the possible exception of Mosasaurus conodon (Russell, 1967a:fig. 51).

The roughly hourglass-shaped ulna is dissimilar to the pe- culiar bowed element seen in P. marshi (Dollo, 1882; Lingham- Soliar, 1992b). Posterior to the extensive, oval humeral facet, is a stout posterior crest. The proximodistal surface of the ulna bears a low, rounded olecranon. Distally, it bears distinct facets for the ulnare and intermedium.

The disarticulated, incomplete forelimbs of NMC 11835 have been reconstructed (Fig. 1) with the help of two undisturbed, virtually complete pectoral paddles (NMC 21853) from the roughly coeval St. Mary River Formation (Caldwell, Cumbaa and Holmes, in prep). Although somewhat smaller than those of NMC 11835, the humeri and radii of USNM 21853 are otherwise indistinguishable, indicating that they form an adequate basis for reconstruction of the missing parts of the pectoral limb of NMC 11835.




A B

5cm

FIGURE 16. Plioplatecarpus primaevus, left pelvic girdle based on NMC 11835.

Pelvic Girdle-The pelvic girdle of P 628.1 was preserved in loose articulation, permitting confident identification of the scattered elements in NMC 11835 (Fig. 16). Although the three elements do not fit together precisely, it is clear that the shaft of the club-shaped ilium curved posterodorsally in contrast to all other mosasaurs, in which the shaft passes anterodorsally towards the vertebral column. Its morphology is very similar on both sides of NMC 11835 and P 628.1, although the dorsal process is more strongly curved and a tuberosity immediately dorsal to the acetabulum is more prominent in the latter.

The pubis is remarkable in the absence of an obturator foramen, a condition unknown in other mosasaurs. A distinct pu- bic process is absent, but appears to be represented by a rugose swelling that forms the anterior margin of its head. The posterior margin of the head is drawn out into a laterally reflected flange. Its long shaft curves both anteroventrally and mediov- entrally to a slightly expanded symphysial region.

The ischium is much shorter than the pubis, and must have had a large cartilaginous extension in order to have reached the symphysis. Posteriorly, it bear a large, rectangular ischiadic tu- bercle.

Pelvic Limb (Fig. 17)-The well ossified femur, although only slightly shorter than the humerus (1.2 times the length of an average dorsal vertebra), is much slenderer. Its gently convex proximal articular surface is circular in outline rather than horizontally oval as in other mosasaurs (Russell, 1967a). The prominent fourth trochanter is separated from a stout internal trochanter by a shallow intertrochanteric fossa. The adductor ridge is absent. The distal head is more compressed dorsoventrally than the proximal head.

The massive tibia bears a broadly oval femoral articular facet and a distinct cnemial crest. Its short shaft, almost circular in cross section, is drawn out posteriorly into a flange whose edge forms part of the crural foramen. Its distal articular surface is indistinctly divided into a flat facet for the intermedium and a larger, strongly convex facet for the tibiale.

The fibula, although not conspicuously compressed dorsoventrally, is considerably less massive than the tibia. The slightly expanded proximal head bears an approximately circular, convex terminal facet. The distal head is more expanded mediolaterally than in Platecarpus. No other elements of the pelvic limb could be identified.

DISCUSSION

In all essential features, the Saskatchewan Bearpaw Pliopla- tecarpus material is closely comparable to the type material of Plioplatecarpus primaevus; the hypothesis that they are con- specific cannot be falsified. However, it is possible that the very incomplete type material (USNM 18254) is not diagnostic. In

A B t -4th

ti' tr

5cm

iC 4th fif

inttr Inttr

o E ifte

fe if f

FIGURE 17. Plioplatecarpus primaevus, propodial and epipodial of right hind paddle based on NMC 11835. A, dorsal; B, ventral; C, femur in proximal and distal aspects; D, fibula in proximal and distal aspects; E, tibia in proximal and distal aspects.

the original diagnosis (Russell, 1967a) it was suggested that Plioplatecarpus primaevus could be distinguished from P. marshi in the possession of a larger quadrate eminence and a smaller coracoid foramen, and from P. houzeaui in the possession of functional zygapophyses in the anterior trunk region. How- ever, it is now clear that the shape and size of the quadrate eminence is variable (see below), and the other characters occur in many mosasaurs, and are therefore probably plesiomorphic. Geographic and temporal proximity, although suggestive, are insufficient in themselves to support specific associations of fossil taxa. Nevertheless, the type material cannot pertain to any other known mosasaur except the Saskatchewan material described here. Although declaring Plioplatecarpus primaevus, a name well established in the literature, a nomen dubium and erecting a new name for the Saskatchewan Plioplatecarpus is arguably the more correct option, this would add yet another name to the literature and create unnecessary confusion. There- fore, USNM 18254 is accepted as the type of Plioplatecarpus primaevus, with the Saskatchewan material, primarily NMC 11835, 11840, and P 1756.1, supplying the diagnostic features.

Diagnosis of the Genus Plioplatecarpus

Plioplatecarpus primaevus, being the most complete species of the genus, provides an opportunity to revise the generic diagnosis. Eighteen diagnostic characters have been proposed. Most are unique within mosasaurs, but since their states have yet to be established in species of Plioplatecarpus known from less complete material, some may prove to be restricted to P. primaevus. Although a few (size and position of the parietal foramen, size of the quadrate conch, and quadrate condyle shape) have been supported by formal phylogenetic analysis (Bell, 1993), others (e.g. vertebral counts) should be viewed as provisional, subject to confirmation in other species of Pliopla- tecarpus and subsequent inclusion in a phylogenetic analysis.

Interrelationships within the Genus Plioplatecarpus

The genus Plioplatecarpus was erected on the basis of Plio- platecarpus marshi from the Maastrichtian "Craie Grossierea Silex Gris" of Belgium (Dollo, 1882). The initial diagnosis was




subsequently modified and expanded (Dollo, 1885, 1889, 1890, 1892, 1904) largely on the basis of the more complete P. houzeaui from the slightly older "Craie Brun Phosphatee de Cipley" of the same region. Although Plioplatecarpus material has been known in North America for some time (Cope, 1869-1870; Crandell, 1958), none was identified as Plioplatecarpus until recently (e.g., P. depressus, Russell, 1967a; Plioplatecarpus sp., Russell, 1988; P. crassartus, Lingham-Soliar and Nolf, 1989), possibly because it is difficult to ascertain the affinities of this incomplete material from Dollo's perfunctory descriptions. More recent discoveries of Plioplatecarpus in North America (Langston, 1975; Burnham, 1991; Tokaryk, 1993) suggest that members of the genus were common throughout the Western Interior Seaway, ranging from present day Alabama (Burnham, 1991) to the Northwest Territories (Russell, 1967b). However, most of these taxa are represented by fragmentary material and remain undescribed and undiagnosed, and provided very few characters with which to resolve interspecific relationships. Consequently, past attempts (Burnham, 1991; Bell, 1993) have been necessarily incomplete. Recent redescriptions of P. marshi and P. houzeaui (Lingham-Soliar, 1994a), a description of a reasonably complete specimen from Alabama (Burnham, 1991), and Plioplatecarpus primaevus in this paper provide new data with which to examine interrelationships of Plioplatecarpus.

For the purposes of this discussion, the relatively well known Platecarpus ictericus (Russell, 1967a) is used as the outgroup to polarize characters within Plioplatecarpus. A sistergroup relationship between these two genera has been demonstrated by a number of authors (e.g., Russell, 1967a; Burnham, 1991; deBraga and Carroll, 1993; Bell, 1993) despite the use of dif- ferent phylogenetic techniques and incompletely overlapping character lists, and can be considered well established.

Many of the characters previously used to assess the interspecific relationships of Plioplatecarpus (Russell, 1967a; Burn- ham, 1991; Bell, 1993) require comment.

A large median opening in the ventral wall of the basilar artery canal (Dollo 1895) has generally been considered diagnostic of the genus (e.g., Russell, 1967a). However, it is absent in P. houzeaui, (represented rather by small, paired openings, Lingham-Soliar, 1994a), is very small and asymmetric in UNO 8611-2 (Burnham, 1991), and is completely absent in at least two North American specimens (Bell, 1993), prompting the latter to suggest that at best, the character may diagnose a clade of non-North American Plioplatecarpus. However, its occur- rence in both NMC 11835 and 11840 effectively reopens the question. These specimens confirm that it is not a foramen or fenestra, but an unossified portion of the basioccipital, and therefore not a "character" in the usual sense. Although this feature is not included in the present phylogenetic analysis (Fig. 18), such a persistent delay of ossification presumably has a genetic basis, and therefore taxonomic significance at some level.

Both P. primaevus and UNO 8611-2 have at least 10 pygal vertebrae, the Plioplatecarpus specimen from Scabby Butte (NMC 21853) has at least 12, P. marshi (IRSNB 3673) has 13 (Lingham-Soliar, 1992a), and one specimen of P. houzeaui has 15 (R3130, Russell, pers. comm.), all of which greatly exceed the plausibly primitive count of five in Platecarpus. However, in Mosasaurus lemonnieri, the count can vary from 12 to 22 (Lingham-Soliar, 1992b), suggesting that although the presence of an elongate pygal region is probably a valid character state, specific counts are of little use.

A prominent quadrate eminence is a conspicuous feature of Plioplatecarpus. However, it may be lacking in UNO 8611-2 (although the bone is poorly preserved in the critical area, Burn- ham, 1991) and an incipient eminence is present in some Pla- tecarpus specimens (personal observations, Lingham-Soliar 1994b). This suggests that the Platecarpus-Plioplatecarpus

,4C,

8,9

7

1-6

FIGURE 18. Hypothesis of relationship of Plioplatecarpus species. Fragmentary or undescribed taxa are not included. Characters: 1, large parietal foramen formed in part by frontals; 2, short postorbital process of postorbitofrontal bearing a socket to receive ascending process of jugal; 3, rectangular preorbital region of frontal; 4, disproportionately large quadrate with large conch; 5, scapula larger than coracoid; 6, at least 10 pygal vertebrae; 7, quadrate eminence consistently well developed; 8, functional zygapophyses restricted to anterior trunk; 9, reduced presacral count.

clade had a propensity to develop a thickening on the posterior surface of the quadrate shaft, but only consistently and promi- nently within Plioplatecarpus, presumably above the level of UNO 8611-2. Furthermore, the eminence is quite variable in size and shape in Plioplatecarpus marshi and P. houzeaui (Lingham-Soliar 1994a) and P. primaevus. Although its presence is a useful character at the generic level, continuous vari- ation in size and shape renders it unsuitable for resolving species level taxonomy.

Functional zygapophyses occur throughout the "presacral" vertebral column and into the anterior pygal region in Plate- carpus and are retained in Plioplatecarpus primaevus, the Scab- by Butte specimen (NMC 21853), and UNO 8611-2. However, both P. houzeaui and P. marshi are derived in that prezyga- pophyses disappear posterior to the fifth and third thoracic vertebrae respectively (Lingham-Soliar, 1994a).

A prepygal vertebral count of 30 in P. primaevus is very close to that of Platecarpus (29). Much lower counts have been reported in other species, but in no specimen is it certain that the entire column is preserved. UNO 8611-2 has 25, but this is considered a minimum count (Burnham, 1991). The incomplete column of the type of P. marshi comprises 18 prepygals, with a total count estimated to be not more than 23 (Lingham-Soliar, 1992b). In P. houzeaui, an estimate of 22 vertebrae based on associated, partially disarticulated material (Lingham-Soliar 1994a:fig. 16) can only be accepted as a minimum count.

Although new data have made it possible to hypothesize a scheme of relationships of Plioplatecarpus for the first time (Fig. 18), there are still many difficulties. Except for somewhat pachyostotic ribs and a lack of a quadrate eminence in UNO 8611-2 (and poor preservation makes the latter condition equiv- ocal), there is little to distinguish it from Plioplatecarpus primaevus. Unless discovery of new material provides more dis- tinguishing features, it may be necessary to synonymize these two taxa. Some or all of the remaining species that have been included by various authors within the genus (P. crassartus, P. depressus, "Platecarpus" somenensis, "Mosasaurus" scanicus and Plioplatecarpus sp. from Scabby Butte and Anderson Riv-




er) may also be synonymous with one of the diagnosable species, but there is presently insufficient evidence to make that determination.

ACKNOWLEDGMENTS

I would like to thank D. A. Russell, who encouraged me to undertake this project. Thanks also to G. Fitzgerald, C. Ken- nedy, and K. Shepherd of the Canadian Museum of Nature for access to the collections, and working space, and R. Day and R. Waller of the same institution for help in photographing specimens and mineralogical analysis of the matrix respectively. R. Purdy of the United States National Museum kindly arranged a loan of the type material of Plioplatecarpus primaevus, and H. Sues arranged for its transport to Canada. I would also like to thank T. Tokaryk of the Royal Saskatchewan Mu- seum for access to collections under his care, loan of material, and hospitality during my visit to Regina. I extend my thanks to A. Newman of the Royal Tyrrell Museum of Palaeontology for access to their collections, and S. Godfrey for his hospitality during my stay in Drumheller. I very much appreciate information provided by G. Caldwell on the stratigraphy of the Bear- paw and Pierre Shales, and comments on growth in mosasaurs from Amy Sheldon. Special thanks to M. Caldwell, T. Lingham- Soliar, H. Sues, and M. Poulin for reading early versions of this paper, and M. Caldwell and T. Lingham-Soliar, both of whom reviewed this manuscript and have offered many helpful sug- gestions. This work was made possible through access to the facilities of the Canadian Museum of Nature and monetary support from the Fonds pour la formation de chercheurs et l'aide

a la recherche (FCAR), Government of Quebec. Publication costs were subsidized by the Canadian Museum of Nature.

LITERATURE CITED

Bell, G. L. Jr. 1993. A phylogenetic revision of Mosasauroidea (Squa- mata). University of Texas, Austin, 293 pp.

Burnham, D. 1991. A new mosasaur from the Upper Demopolis For- mation of Sumter County, Alabama. MSc. dissertation, University of New Orleans, New Orleans, 63 pp.

Caldwell, W. 1968. The late Creataceous Bearpaw Formation in the South Saskatchewan River Valley. Saskatchewan Research Coun- cil, Geology Division. Report no. 8, ix + 86 pp.

Camp, C. L. 1942. California mosasaurs. University of California Memoirs 13:1-68.

Cope, E. D. 1869-1870. Synopsis of the extinct Batrachia, Reptilia, and Aves of North America. Part 11:106-235. Transactions of the American Philosophical Society, new series.

Crandell, D. 1958. Geology of the Pierre Area, South Dakota. U.S. Geological Survey Professional Paper 307, 83 pp.

deBraga, M., and R. L. Carroll. 1993. The origin of mosasaurs as a model of macroevolutionary patterns and processes. Evolutionary Biology 27:245-322.

Devillers, C. 1943. Nerfs criniens et circulations c6phaliques de Plio- platecarpus marshi. Annales de Paldontologie 30:47-59.

Dollo, L. 1882. Note sur l'ostuologie

des Mosasauridae. Bulletin du Musde Royal D'Histoire Naturelle de Belgique 1:55-80.

- 1884. Le mosasaure. Revue des Questions Scientifiques. 16: 648-653.

1885. Notes d'ostdologique herp6tologique. Annales de la So-

ci6t6 Scientifique de Bruxelles 9:309-338.

1889. Premiere note sur les mosasauriens de Mesvin. Bulletin de la Soci6t6 Belge de Geologie, de Paldontologie, et d'Hydrologie, Bruxelles 3:271-304.

1890. Premibre note sur l'ostdologie des mosasauriens de Maestricht. Bulletin de la Socidt6 Belge de G6ologie, de Paldon- tologie, et d'Hydrologie, Bruxelles 4:151-169.

1892. Nouvelle note sur l'ostdologie des mosasauriens. Bul- letin de la Soci6t6 Belge de Gdologie, de Paldontologie, et d'Hy- drologie, Bruxelles 6:219-259.

1904. Les mosasauriens de la Belgique. Bulletin de la Soci6te Belge de Gdologie, de Paldontologie, et d'Hydrologie, Bruxelles 18:207-216.

Gervais, P. 1853. Observations relatives aux reptiles fossiles de France. Academie des Sciences, Paris, Comptes-rendus 36:374-377, 470- 474.

Gill, J. R., and W. A. Cobban. 1965. Stratigraphy of the Pierre Shale, Valley City and Pembina Mountains areas North Dakota. U.S. Geo- logical Survey Professional Paper 392-A, iii + 20 p., 7 figs.

and 1966. The Red Bird section of the Upper Creta- ceous Pierre Shale in Wyoming. U.S. Geological Survey Profes- sional Paper 393-A, iv + 73 p., 17 figs., 12 plates.

Gilmore, C. 1928. Fossil lizards of North America. National Academy of Sciences 22, Memoir 3, 169 pp.

Izett, G. A., W. A. Cobban., and J. R. Gill. 1971. The Pierre Shale near Kemmling, Colorado, and its correlation to the East and West. U.S. Geological Survey Professional Paper 684-A, iii + 19 pp., 2 tables, 11 figs., 1 plate.

Langston, W. 1975. The ceratopsian dinosaurs and associated lower vertebrates from St. Mary River Formation (Maestrichtian) at Scab- by Butte, southern Alberta. Canadian Journal of Earth Sciences 12: 1576-1608.

Lingham-Soliar, T. 1992a. A new mode of locomotion in mosasaurs- subaqueous flying in Plioplatecarpus marshi. Journal of Vertebrate Paleontology 12:405-421.

1992b. The tylosaurine mosasaurs (Reptilia, Mosasauridae) from the Upper Cretaceous of Europe and Africa. Bulletin de l'Institut Royal des Sciences Naturelles de Belgique. Sciences de la Terre 62:171-194.

1994a. The mosasaur Plioplatecarpus (Reptilia, Mosasauridae) from the Upper Cretaceous of Europe. Bulletin de l'Institut Royal des Science Naturelles de Belgique. Sciences de la Terre 64:177- 211.

1994b. The mosasaur "Angolasaurus" bocagei (Reptilia: Mo- sasauridae) From the Turonian of Angola reinterpreted as the ear- liest member of the genus Platecarpus. Paliontologische Zeitschrift 68:267-282.

Lingham-Soliar, T, and D. Nolf. 1989. The mosasaur Prognathodon (Reptilia, Mosasauridae) from the Upper Cretaceous of Belgium. Bulletin de l'Institut Royal des Sciences Naturelles de Belgique. Sciences de la Terre 59:137-190.

Russell, D. 1967a. Systematics and morphology of American mosasaurs (Reptilia, Sauria). Bulletin of the Peabody Museum of Natural History 23, viii + 241 pp.

Russell, D. A. 1967b. Cretaceous vertebrates from the Anderson River, Northwest Territories. Canadian Journal of Earth Sciences 4:21-38.

Russell, D. 1988. A checklist of North American marine Cretaceous vertebrates including fresh water fishes. Occasional Papers of the Tyrrell Museum of Paleontology no. 4, 58 pp.

Tokaryk, T 1993. A plioplatecarpine mosasaur from the Bearpaw shale (Upper Cretaceous) of Saskatchewan, Canada. Modern Geology 18:503-508.

Williston, S. W. 1897. Range and distribution of the mosasaurs. Kansas University Quarterly 6:177-189.

Received 21 June 1995; accepted 21 November 1995.



Plioplatecarpus Primaevus (Mosasauridae) from the Bearpaw Formation (Campanian, Upper Cretaceous) of...

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