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7-2003 Current Taxonomic Status of the Plesiosaur Pantasaurus Striatus from the Upper , Wyoming F. Robin O’Keefe Marshall University, [email protected]

William Wahl Jr.

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Recommended Citation O'Keefe, F. R., and W. Wahl, Jr. 2003. Current taxonomic status of the plesiosaur Pantasaurus striatus from the Upper Jurassic Sundance Formation, Wyoming. Paludicola 4(2):37–46.

This Article is brought to you for free and open access by the Biological Sciences at Marshall Digital Scholar. It has been accepted for inclusion in Biological Sciences Faculty Research by an authorized administrator of Marshall Digital Scholar. For more information, please contact [email protected], [email protected]. Paludicola 4(2):37-46 July 2003 @ 2003 by the Rochester Institute of Vertebrate

CURRENT TAXONOMIC STATUS OF THE PLESIOSAURPANTOSAURUS STRIATUS FROM THE UPPER JURASSIC SUNDANCE FORMATION, WYOMING

F. Robin O'K.eefe1and William Wahl, JR.2

!-Department of Anatomy, New York College of Osteopathic Medicine, Old Westbury, New York I 1568 2-Wyoming Center, 110 Carter Ranch Road, Thermopolis, WY 82443

..... ~.... .

ABSTRACT Plesiosaur material has been known from the Redwater member of the Sundance Formation (JuniSSic: ) of Wyoming for over I 00 years, but has received little research attention. Here we report on the taxonomic status of a long-necked cryptocleidoid plesiosaur from the Redwater Shale, the correct identity of which is striatus Marsh 1893. The taxon MuraenQSOUrus reedii Mebl 1912 is shown to be a junior synonym of Pantowurus striatus. Pantosaurus is described on the basis of the holotype and referred specimens, and found to be a cryptocleidoid plesiosaur possessing between 35 and 40 cervical vertebrae. These vertebrae are very similar in proportion and anatomical detail to those of Muraerw:saurus leedsii from the of England. However, the forelimb of Pantosaurus is diagnostic and differs from that of MuraenQSOUrus in several particulars, the most important being the relatively large size of the radius and its corresponding humeral articulation. Although no cranial material is available at this time, we believe that Panlosaurus striatus is a valid taxon.

INTRODUCTION Sundance is the Redwater Shale member identified by Pipiringos, who describes it as: "Redwater shale This paper concerns the current taxonomic status member: 94' Greenish-black calcareous shale and of a long-necked cryptocleidoid plesiosaur from the siltstone; contains thin sandy limestone interbeds and Upper Jurassic Sundance formation of Wyoming. smooth dense septarian limestone concretions; is Jurassic plesiosaur material from Wyoming has been conspicuously glauconitic, oolitic and fossiliferous known since the 19th century, but has received throughout; contains Cardioceras sp. at the base. comparatively little research attention. Recent field Pachyteuthis "densus" is common, especially in the work by teams from the Tate Museum in Casper, lower half. Makes ledgy slope." (Pipiringos 1957 p. Wyoming has succeeded in gathering significant new l I). The Redwater shale member is believed to be plesiosaur material from this formation that sheds light Oxfordian in age, and is therefore slightly younger than on the and relationships of Sundance the mostly Oxford Clay. The Redwater Shale plesiosaurs. Analysis of this material supports Mehl's member yields disarticulated elements and articulated (1912) conclusion that there are two small skeletons (in concretions) of marine , and was cryptocleidoid taxa in the Sundance Formation: originally termed the ' beds' by Marsh Pantosaurus striatus Marsh I 893, and a new taxon that (1891 , 1893). is described in a following paper. (O'Keefe and Wahl Taxonomic History-Plesiosaur material 2003). This report will review the taxonomjc history of (Reptilia: ) from the Redwater Shale the plesiosaur Pantosaurus striatus, establish that Member of the Sundance Formation was first noted by reedii is a junior synonym of 0 . C. Mai-sh in 1891 _ Marsh first mentioned a Pantosaurus striatus, discuss new material referable to plesiosaur specuneri- during his discussion of the this taxon, and offer observations on its possible Baptanodon (Marsh 1891), where he relationships. devoted a single sentence to a small plesiosaur from the Stratigraphic Context-The Sundance Formation 'Baptanodon beds' and bestowed the name is a heterogeneous group of Upper Jurassic marine "Parasaurus" on the specimen. Marsh later realized sediments found widely in Wyoming, Montana, and this name was preoccupied, however, and suggested South Dakota. The Sundance Formation is underlain by the name Pantosaurus for the specimen in 1983 (Marsh the Lower Jurassic Nugget Sandstone Formation, and 1893). Io 1895 Marsh described and figured his overlain by the Morrison Formation (Pipiringos 1957). Wyoming plesiosaur for the first time: "In 1885, a new The most fossiliferous of the seven members of the outcrop of the Baptanodon beds was found in the

37 38 PALUDJCOLA, VOL. 4, NO.2, 2003

Freeze Out mountains in Wyoming, and here several figure any of it, we are forced to accept Mehl's skeletons of Baptanodon were obtained, all in conclusion that shirleyensis is a nomen concretions of limestone, as in the original locality. dubium. Also, if we hypothesize that this specimen One concretion from this new locality enclosed the represents the longer-necked of the two Sundance skeleton of a small Plesiosaur of much interest, being cryptocleidoids, 'P/esiosaurus shirleyensis' would be a the first Jurassic form observed from this country. This junior synonym of Marsh's Pantosaurus striatus in any specimen was named by the writer Parasaurus striatus, case. the specific term denoting a characteristic feature of the Mehl (1912) bad access to additional plesiosaur vertebrae, which are all strongly grooved, as indicated material from the Freeze Outs (UW 5544, locality UW in the one shown in Figure 4, below. This generic name V -1800 1), and this material still resides in the also proved preoccupied, and was replaced by collections of the University of Wyoming. Mehl Pantosaurus, the name of the now being concurred with Knight in believing that there were Pantosaurus striatus...... _... probably two valid taxa of small plesiosaurs present in The skull in this was provided with teeth. the Sundance Formation (for information oo the second The neck was long and slender. The vertebrae taxon see O'Keefe and Wahl 2003). Mehl based this preserved, resemble most nearly in form and size those conclusion primarily on humerus morphology (Figures ofPlesiosaurus plicatus Phillips." (Marsh 1895). l ,2); one of Mehl's new specimens was a partial Marsh's holotype of Pantosaurus striatus still exists forelimb possessing humerus, epi- and metapodials, and resides in the collections of the Peabody Museum and some phalanges, while Mehl accepted the forelimb at Yale (specimen YPM 543); however, no trace of of Knight's holotype material of 'Cimoliosaurus' teeth or other cranial material is currently with the laramiensis as diagnostic. Both forelimbs were derived specimen. and showed undoubted affinities to Oxford Clay taxa The next worker to deal with Sundance such as Muraenosaurus, , and Tric/eidus. plesiosaurs was W. C. Knight, who described the large Mehl's specimen still exists in the collections of the pliosaur Mega/neusaurus rex from the Sundance University of Wyoming, and was examined in the Formation in 1898. This taxon remains the only large course of this study (UW 5544). Tbjs specimen was Jurassic pliosaur known from North America, and has given the name Muraenosauros reedii by Mehl, the use received almost no research attention since its original of the genus name Muraenosaurus indicating description despite its potential relevance to the similarities between this humerus and that of anatomy and biogeography of the well-known large Muraenosaurus leedsii. Mehl also provided a line pliosaurs from the Jurassic Oxford Clay of England drawing of a partial propodial from the Yale (Andrews 1913). The holotype material of Pantosaurus holotype taken from a photograph, and Megalneusaurus rex still resides at the University of maintained that the humeri were not similar. However, Wyoming (UW 4602). In a later publication, Knight the similarities between this humerus and that of ( 1900) also briefly described two other taxa from the Muraenosaurus are questionable, and comparison with same formation, naming one 'Plesiosaurus shirleyensis' the Pantosaurus material was enlightening. and the other 'Cimoliosaurus' /aramiensis. In summary, the taxa erected on potentially The matter rested here until 1912, when M. G. diagnostic material and in need of clarification here Mebl attempted to rationalize the Wyoming material are: Pantosaurus striatus Marsh 1893, bolotype YPM with regard to the then-recent publication of Andrew's 543; Muraenosaurus reedii Mebl 1912, holotype UW seminal first volume on Oxford Clay plesiosaurs 5544; and ? Laramiensis Knight (Mehl (Andrews 1910). Mebl (1912) determined that the 1912), holotype lost, but figured in Knight (1900) and taxon 'Piesiosaurus shirleyensis' Knight was founded Mehl (1912). Pantosaurus striatus, and its probable on fragmentary and non-diagnostic material, and was junior synonym Muraenosaurus reedii, are handled therefore not useful- a nomen dubium in modem below, while Tricjeidus? Laramiensis is discussed in parlance. However, Knight's original description the following paper (O'Keefe and Wahl2003). (1900) mentions fragments of a lower jaw, as well as ( - teeth that were "large, numerous, incurved, elliptical in MATERIALS cross-section, [and with] interior surface of teeth covered with numerous very fine striae." Mebl All currently known plesiosaur material from the apparently did not have access to this cranial material Redwater Shale was surveyed in the course of this at the time of his redescription and does not mention il study. The relevant material for the present paper are Knight also states that 'many' cervical vertebrae were the Pantosaurus striatus holotype at Yale (YPM 543, found with the specimen (Knight 1900 p. 115), and that concreted articulated skeleton); an associated vertebral the neck was 'long' (ibid p. 116). Given that none of this material is locatable today, and that Knight did not O'KEEFE AND WAHL-STATUS OF PANTOSAURUS STRJATUS 39

FIGURE I. Forelimbs of Panlo!KltlniS strlabn Marsh from the Redwater Shale Member of the Sundance Formation. A: UW 5544 as drawn in Mchl 1912; B: UW 5544, right forelimb in donal view; C: UW 3, left forelimb in dorsal view (reversed). Abbreviations are: h. humerw; r, radios; sa, supernumerary ossification articulation; u, ulna. Muraenosaurus Cryptoclidus Tricleidus Colymbosaurus ...... 8 C"' N' 0z '¢A ,_j 0 > jA 0u

~

FIGURE 2. Schematic drawings of humeri from relevant ayptocleidoid taxa. Phylogeny modified from O'Keefe 2001, O'Keefe in press. Co/ymhosaunu has not beenincluded in any cladistic analysis 10 dale. First threetaxa after Andrews 1910; Colym/Josavna after Brown 1981; Pol)'cotylw after Williston 1906, Dolichorhynchcps after Williston 1903; Trinocromerum after Williston 1908.

0'¢ O'KEEFE AND WAHL-STATUS OF PANTOSAURUS STRJATUS 41 column and basioccipital (UW 15938, collected as humerus that Tricleidus shares with Colymbosaurus float); and a large collection of isolated float gathered and Polycoty/us; see schematics, Figure 2). from various Redwater Shale localities. All of this This forelimb of Muraenosaurus reedii differs material save the Marsh specimen was collected by the from that of Muraenosaurus /eedsii (Andrews 1910, p. staff and volunteers of the Tate Museum, Casper, 112; Figure 2) in several respects. The humeral shaft is Wyoming, over the last decade, and we gratefully relatively longer and more slender in the Wyoming acknowledge their efforts. The exact location where taxon, and the radial articulation does not extend onto Marsh's specimen was found is not known, the only the anterior edge of the humerus as it does in M recorded information being 'near the Freezeout leedsii. The radius in M leedsii is also similar in size to Mountains' in Carbon County, Wyoming. Mehl (1912) the ulna, unlike the derived, large radius in the states that his and Knight's specimens also come from Wyoming taxon. The Wyoming taxon also lacks the this general area Pipiringos ( 1957) documents a depression in the anterior face of the radius that is section of the Sundance Formation in the northern .... ~- · present in M. leedsii. In summary, Mehl's taxon Freezeout Hills, and the historical plesiosaur specimens Muraenosaurus reedii is probably not congeneric with probably originate from outcrops of the Redwater Muraenosaurus from the Oxford Clay, and displays Shale member in this area. several characters that are potentially diagnostic. The UW 15938 specimen was found at a locality off 33 Mile Road near Roughlock Hill in Natrona PantosaunlS striatus Marsh county, Wyoming, where a large outcrop of Redwater Shale has also yielded several ichthyosaur specimens The type material of Pantosaurus (YPM 543) is a (Massare and Sperber 1999). Most of the float partial skeleton preserved in a hard limestone specimens also originate from this area, including UW concretion, portions of which have been prepared out 3. to yield a distal humerus, four articulated carpals, a fragment of coracoid (Figure 3), and several isolated DESCRIPTION cervical vertebrae (Figure 4). The preservation of all elements is excellent and the bone is not deformed. Muraenosaurus reedii Mehl Many ribs and rib fragments occur on and in the concretion; the ribs are not articulated with the dorsal Mehl's (1912) figure ofthe Muraenosaurus reedii vertebrae and are quite disorganized. The axial forelimb is reproduced here in Figure I, along with a skeleton comprises an articulated string of cervical and recent photograph of the M reedii humerus and radius dorsal vertebrae, broken into several sections. The in articulation (UW 5544). Also illustrated is a well­ anterior end of the cervical series is numbered ' 1' in preserved distal humerus, radius, and ulna (UW 3) orange paint, although it is not an atlas or axis. The from the float collection that is probably referable to column is numbered sequentially through the pectoral the same taxon. Both humeri share several unusual series and into the dorsal series, and the last numbered characteristics that are potentially diagnostic. The vertebra seems to be number '42'. The transition from radial articular facet on the humerus is much longer cervical to pectoral vertebrae occurs around vertebrae than the ulnar articulation. In cryptocleidoids the radial '32' -'34', although the exact transition from cervical to articulation is often the longer of the two facets; pectoral is not prepared. There are therefore about 32 however this condition is much more pronounced in or 33 preserved cervical vertebrae with the specimen. the Wyoming humeri than in any Oxford Clay taxon Most are still in the limestone concretion and the state (Andrews 1910; Brown 1981 p. 333). The radius is also of preparation varies, although one centrum freed from much larger than then ulna, being about twice the matrix is depicted in Figure 4. length and breadth of the later element. The humeral The vertebrae at the anterior end of the cervical shaft is relatively long and rather gracile, more similar column possess very Jow neural spines that are blade~ to Colymbosaurus (Brown 1981) or Polycotylus like, much. as in Cryptoc/idus (Brown 1981) or (Wil)jston 1906) than to Oxford Clay taxa such as Muraenosaurus -('Andrews 191 0). However, the neural Tricleidus, Muraenosaurus, or Cryptoclid:us (Andrews spines in Pantosaurus are angled backward more than 1910; Figure 2). The humerus also bears a clear those of the former taxa. The cervical centra are articulation for one supernumerary ossification in the remarkable in several respects. The centra are rather epipodial row, a feature shared by many long antero-posteriorly, although they are a bit shorter cryptocleidoids (the condition described by O'Keefe than they are wide; in general proportion the centra are 2001 is an oversimplification. More cryptocleidoid taxa quite similar to Muraenosaurus, and longer than in than listed in that paper have this feature [Brown Kimmerosaurus, Tricleidus, or Cryptoclidus (for 1981], and it is the possession of four facets on the analysis and data see Figure 5 and discussion below). 42 PALUDICOLA, VOL. 4, NO. l, 2003

3 em

,·

FIGURE 3. Elements of the holotypc of Pan1osaurus stria/Us Marsh, YPM 543. A, distal end of right humerus. B, fragment of coracoid, probably right. C, articulated group of right carpals. Abbreviations are: ra, radial articulation; ua, ulnar articulation; sa, supernumerary ossification articulation. O'KEEFE AND WAHI.r-STATUS OF PANTOSAURUS STRJATUS 43

3cm

FIGURE 4. Representative cervical vertebra of PanJosourus stria/us Marsh, YPM 543. Top two drawings are M~~roenosaurus leedsii, reproduced from Andrews 1910. 44 PALUDICOLA, VOL. 4, NO. 2, 2003

The centra ace also 'waisted', meaning that the of the third and seventh vertebrae ace identical, and the middle of the centrum has a smaller diameter than other measures for this taxon were size-standardized to either articular face. The foramina subcentralia occur this length. Because Pantosaurus lacks atlas and axis on the ventral surface of the centrum as they do in all the numbers assigned to the cervical vertebrae are plesiosaurs, and ace closely spaced and set at the arbitrary. For the purpose of this analysis the vertebra bottom of small, shallow depressions. The cervical rib labeled '1' was arbitrarily assigned to position 5 on the head articulation is single and carried on a low pedestal column. or boss, is elongate antero-posteriorly, and possesses an Second-order polynomial regressions were fitted excursion on its dorsal margin for a process of the to the plots for each taxon; the regression equations ace cervical nb. The articulation for the neural arch is reported in Figure 5 (size-standardized lengths were closer to the anterior articular face than to the posterior. multiplied by 10 before calculation of the regression The margins of the articuJac faces ace well-ossified and equations to improve accuracy of the X 2 coefficient). In defined, unlike the condition in Tric/eidus 'or Dolichorhynchops, the taxon with the least number of Kimmerosaurus. In all of these features, Pantosaurus is cervical vertebrae ( c'"' 19), the regression is essentially closely comparable to Muraenosaurus (compare Figure linear, the coefficient of the X2 term being .002. As 4). This similarity extends to the fme striae one proceeds caudally down the column the vertebrae ornamenting the ventral pact of the centrum neac both get Iaeger, and the length of successive central articular faces that so impressed Marsh; the only other increases monotonically. However, as the number of taxon possessing this feature is Muraenosaurus cervical vertebrae increases in the series of taxa (Andrews 1910). The vertebral centra of Pantosaurus Tricleidus ( c=25) -Cryptoclidus ( c=31) ace in fact almost identical to those of Muraenosaurus, Muraenosaurus (cz42), the value of the X2 coefficient the only difference being that those of Pantosaurus ace decreases (i.e. -.004, -.007. -.009). This decrease a bit shorter on average in the antero-posterior indicates that the length of each centrum no longer direction. increases monotonically as cervicals ace added to the Analysis of Pantosaurus neck length­ series. In Cryptoclidus the length of the centra stops Unfortunately the neck of Pantosaurus (UW 543) is increasing about c24, but does not decrease, while in not complete; about 33 cervicals ace preserved, but the Muraenosaurus the most posterior centra are actually atlas and axis ace not, and there is no direct indication shorter than those preceding. of the number of missing cervicals. This number is The pattern displayed by Pantosaurus is important because it is highly labile in plesiosaurs and interesting. The coefficient of the X2 term in this varies among all genera (data in O'Keefe 2002), and so regression is -.012, less than any other taxon measured. is useful in diagnosing taxa. The posterior portion of However, the centra defmitely do not decrease in the neck is available, however, and some of the length at the rear of the column, as is the case in vertebrae ace exposed well enough for measurement. Muraenosaurus. There are several difficulties here: the Measurements were made with dial calipers on the actual number of each cervical is not known, there ace best-preserved of these vertebrae to allow comparison few measurements to constrain the regression, and the with other cryptocleidoids. specimen is a juvenile. Given that the lengths of the Plots of the antero-posterior lengths of cervical cervicals do not decrease at the end of the column, we vertebrae ace presented in Figure 5. Measurements tentatively concluded that the neck in Pantosaurus did from five cryptocleidoid taxa ace depicted: not include as many vertebrae as Muraenosaurus. The Dolichorhynchops, Tricleidus, Cryptoclidus, constant length of cervicals at the reac of the column Muraenosaurus, and Pantosaurus. The data for most resembles Cryptoclidus; however, at least 33 Dolichorhynchops (n=2), Cryptoclidus (n=2), and the cervicals ace preserved with the specimen, and two Pantosaurus holotype ace taken from the specimens, more were certainly present (i.e. axis and atlas). This while data for Tric/eidus (n= I) and Muraenosaurus constraint puts tjle minimum number of cervical (n=3) ace taken from Andrews (I 91 0). In order to make vertebrae in Pantosaurus at 35. Given the lack of specimens of different body sizes comparable, the centrum len~ · decrease the actually number is lengths of each vertebra within each column was probably closer to 35 than to the 42 of Muraenosaurus, standardized by dividing each measurement by the and 35 or 36 is probably the most likely estimate of the length of the fifth cervical vertebra in each column. number of cervical vertebrae. More accurate This transformation effectively sets the length of each measurements of the Pantosaurus axial skeleton may fifth cervical equal to one and is a first-order size improve this estimate, but this will involved further approximation. Taxa for which muJtiple specimens preparation. were available ace pooled into single analyses. The Elements of the appendicular skeleton of length of the fifth cervical vertebra in Tricleidus is not Pantosaurus ace illustrated in Figure 3, including the available from Andrews ( 191 0); however, the lengths distal end of a humerus. Examination of the 16 .... C/) 1.5 Q. .... 0 1 4 .t:: .... __...- 0 t / c: 1 3 ::.., .... ·~~ -t of: 12 1 7 0 ...... t:: 1 6 0 ~ ::::: 1.1 0 0 ! C/) 1 5 Q 1 · 2 1.4 ~ CTl .. Y =9.206 + .197 • X+ .002 • X"2; R"2 = .757 ::l .... :A. ~ ~ 1.3 .9 C/) .... ra 0 1.2 0 5 10 15 20 ~ 1.1 1.4 ~ 1.0 ::l~ 1 (.) 1.35 .... ::E .9 1.3 ->< .8 II ~ 1.25 .7 Y = 7. 147 + .567 • X- .009 • X"2; R"2 = .93 ~ .... _/ ro ~ 1.2 '- Q) 0 5 10 15 20 25 30 35 40 45 r .0 ~ Cl) :0 1.15 i:: ~ t ....,; 1.1 en Cl) 0 > 1.05 ..,., .... 0 1 1.5 - Y = 8.845 + .284 • X- .004 • X"2; R"2 = .941 ~ 95 1 4 5 :<: .!: I ~ I I 0> 0 5 10 15 20 25 1 4 ~ -c C/) 1.35 1.6 ::, ~ Cl) 1.3 5 ~ 1.5 Ill 1.25 "C C/) Cl) 1 2 Si N .2 V) C/11.4 c:: 1.1 5 • ::l Ill "C "0 ll.. 1.1 '- :::::1.3 ~ ro 0 1.05 "C 0 1 c Q_1.2 Vj~ .95 v =7 .063 + .613 • x- .012 • x•z: R•z =. 976 ~ ro ~ t u1 .1 1 .... (/) 0 5 10 15 20 -I 25 Cl) ' Vertebra Numb er N 1 ~ Y = 7.938 + .445 • X- .007 • X"2; R"2 = .964 en .9 0 5 10 15 20 25 30 35 Vertebra Number

+>o. FIGURES. Graphs of size-standardized lengths of successive cervical vertebrae, split by taxon. For further discussion see text. V> 46 PALUDICOLA, VOL. 4, NO.2, 2003

Pantosaurus material reveals that this specimen is We believe that Mehl's taxon Muraenosaurus probably a juvenile. The articular facets on the distal reedii is valid, and can be demonstrated to be end of the humerus are poorly defined and ossified, the Pantosaurus striatus Marsh. Mehl himself (1912) carpals are rounded and poorly ossified, and the neural considered and rejected this possibility; however be arches of the cervical vertebrae are not tightly sutured had only a photograph of the Pantosaurus humerus to to the vertebral bodies. All of these features are thought work from, and may have mistaken two of the four to indicate juvenile status in plesiosaurs (Caldwell carpals for epipodials. He also did not consider the 1997; Brown 1981). However, once allowance is made juvenile status of Pantosaurus. Given that the humerus for the ontogenetic lack of ossification, the Muraenosaurus reedii is distinctive-- especially in the Pantosaurus humerus possesses all of the characters of relative size of the radius articulation and the presence Mehl's taxon M. reedii: the shaft appears to have been of one articulation for a supernumerary ossification-­ slender, there is one articulation for a supernumerary and that the humerus of Pantosaurus seems to display ossification in the epipodial row, and most importaritfY; these distinctive characters, we hypothesize that Mehl's the radial articulation is very large relative to the ulnar taxon Muraenosaurus reedii is synonymous with articulation (Figure 3). The poorly-ossified humerus of Pantosaurus striatus Marsh. Marsh's name has priority Pantosaurus is also smaller than either of theM reedii as it was the first published; Muraenosaurus reedii is humeri in Figure 1, which is consistent with therefore a junior synonym of P antosaurus striatus Pantosaurus being a juvenile. The coracoid fragment is Marsh. identifiable as such given the shape of its cross section This study supports the conclusion of Mehl and posterior extension, but displays little diagnostic (1912) that there is a long-necked cryptocleidoid in the morphology. The carpals are poorly ossified and are Sundance Formation, and that this taxon is similar to not diagnostic of themselves. the Oxford Clay taxon Muraenosaurus. We further conclude that the correct name of this taxon is DISCUSSION Pantosaurus striatus Marsh 1893. The holotype of Pantosaurus consists of most of a neck, a distal Mehl ( 1912) contented that there were two small humerus, and other fragmentary girdle bones. This plesiosaur taxa in the Redwater Shale based on taxon possesses cervical vertebrae very similar to those differences he observed in the humeri and epipodials at of Muraenosaurus leedsii; however, the humerus and his disposal. This is somewhat unfortunate as radius are derived and diagnostic. The number of plesiosaur humeri display delayed ossification (Brown cervical vertebrae in the neck is not known with 1981) and are marginally diagnostic at best However, certainty, but was at least 35, and probably less than 40 O'Keefe (2001, in press) identified at least four based on analyses above. No skull material is currently humerus characters that he felt were useful, three of known for this taxon, the only possibility being an which are found in cryptocleidoids. The humerus in isolated basioccipital collected as float (UW 15938); cryptocleidoids varies more, and is more diagnostic, in however it is not clear that this element belongs with this group than in most other plesiosaurs (Figure 2), the rest of the material as the specimen was not although caution must still be used because the number articulated when found. of diagnostic characters is small and ontogenetic variation is a confounding factor. Given Mehl's SYSTEMATIC PALEONTOLOGY designation of the taxon Muraenosaurus reedii on the basis of a forelimb (holotype UW 5544), however, we Suborder de Blainville, 1835 cannot avoid the task of evaluating whether this taxon Genus Pantosaurus 1893 is valid based on the type material, and furthermore we must determine if the diagnostic characters (if any) can Type Species-Pantosaurus striatus, by mono- be identified in other Redwater Shale specimens. typy. - Another issue is the identification of all three Diagnosis--As for species. propodials in this paper as forelimbs rather than ( - hindlimbs; this assignment seems safe given that in Pantosaurus striatus Marsh 1891 cryptocleidoids (i.e. Cryptoclidus, Andrews 191 0) (Figures 2, 3, 4) there is a distinct angle between the midline of the humerus and the midline of the distal forelimb. This Synonymy-Muraenosaurus reedii Mehl 1912 angle is much less pronounced in the hindlimb. The Holotype-YPM 543, a partial skeleton compri­ three limbs shown in Figures 2 and 3 display the sing axial skeleton, ribs, pectoral girdle and limb pronounced angle observed in other cryptocleidoid fragments. forelimbs. Referred Material-OW 5544 (holotype of 'Muraenosaurus reedii}, UW 15938, UW 3 O'KEEFE AND W AHL-STATIJS OF P ANTOSAURUS STRIATUS 47

Occurrence--Redwater Shale member of the d'Amphibiologie. Nouvelles Annales du Museum Sundance Formation, (Oxfordian); (National) d'History Naturaelle, Paris 4:233-296. Natrona and Carbon Counties, Wyomjng. J