CRETACEOUS ELASMOBRANCHS OF THE GREENHORN FORMATION (MIDDLE CENOMANIAN-MIDDLE TURONIAN), WESTERN SOUTH DAKOTA

DAVID J. CICIMURRI

Bob Campbell Geology Museum, Clemson, SC, 29634

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Abstract— Cooperation between the U.S. Forest Service, Bureau of Recreation, and the South Dakota School of Mines and Technology has resulted in the accumulation of a significant collection of elasmobranch fossils from the Greenhorn Formation of western South Dakota. Thirteen taxa were recovered from late Cenomanian rocks of the lower portion of the formation, whereas 14 taxa were collected from early middle Turonian rocks at the very top of the formation. Both of these assemblages are dominated by pelagic taxa. A few of the sharks and rays within the lower and middle portions of the formation are consistently found in rocks representing shallow-water environments, indicating that several regressive episodes occurred during the transgressive phase of the Greenhorn Cyclothem. The elasmobranch assemblages from the Greenhorn Formation of South Dakota are similar (or nearly identical) to those from temporally equivalent rocks of Texas, Kansas, and Arizona. The absence of these taxa in the Atlantic and West Coast regions is due to the lack of appropriate-aged strata in these areas.

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INTRODUCTION

The Greenhorn Formation is exposed around the Precambrian core of the Black Hills of South Dakota and Wyoming. Despite this fact, the formation has largely been neglected with regard to its paleontological resources. Recently, the South Dakota School of Mines and Technology, in conjunction with the U.S.F.S. and U.S.B.R., has been conducting a systematic study of the formation within South Dakota. Museum personnel have discovered a diverse plesiosaur assemblage in the formation, including the first occurrence of the large pliosaur Polyptychodon interruptus in the Western Interior of North America (VonLoh and Bell 1998).

Squamates from the Greenhorn include the enigmatic dolichosaur Coniasaurus crassidens, as well as a basal mosasauroid and another poorly understood taxon (Bell 1993; Bell and VonLoh 1998). Most unexpectedly, a pterosaur wing metacarpal was collected from the middle portion of the formation, and a neonatal chelonioid costal was found in the uppermost part of the formation (VonLoh and Bell 1998).

Cobban (1951) noted the presence of teleosts in the base of the formation at Orman Lake, but stated that remains were too fragmentary for a generic identification. Later, Foell (1982) reported the large teleost Xiphactinus audax. In addition to this taxon, SDSM has collected Enchodus, Protosphyraena, Pachyrhizodus, and Micropycnodon. Prior to this report, elasmobranch distributions within the Greenhorn Formation of South Dakota were virtually unknown. Cobban (1951) reported Isurus appendiculata (= Cretolamna appendiculata), Isurus desorii (= Cretoxyrhina mantelli), Squalicorax falcatus, and Ptychodus whipplei, and Martin et al. (1996) noted the presence of odontaspidids.

ABBREVIATIONS

Institutional - AMM, Adams Memorial Museum, Deadwood, SD; SDSM, Museum of Geology, South Dakota School of Mines and Technology, Rapid City.

Other - AR, Angostura Reservoir, Fall River County, SD; BFR, Belle Fourche Reservoir, Butte County, SD; E, Edgemont, Fall River County, SD; OLM, Orman Lake Limestone Member, Greenhorn Formation; PCM, Pool Creek Member, Carlile Shale.

GEOLOGIC SETTING

The Late Cretaceous Greenhorn Formation was deposited in a north-south trending inland seaway between the cordilleran mobile belt to the west and the stable North American craton to the east (Macdonald and Byers 1988). Exposures of the formation are extensive in Colorado, Kansas, Wyoming, Nebraska, South Dakota, and northern New Mexico (Hattin 1975). In the Black Hills region of South Dakota, the Greenhorn Formation is exposed around the Precambrian core of the Black Hills Uplift (Figure 1). This area is also the site of a facies change within the formation from carbonate-rich facies to the south and east, and clay-rich facies to the west ( Hattin 1975; Macdonald and Byers 1988). Greenhorn carbonate rocks are thought to have been deposited far from the nearest shorelines in relatively deep, mostly quiet-water environments where pelagic sedimentation prevailed. There is considerable debate as to water depth of the Western Interior Seaway during deposition of the Greenhorn Formation. Using microinvertebrate and geological structure data, Hattin (1975) and Kauffman (1969) suggested that deposition occurred in middle to outer shelf depths (100- 200 m maximum) except during regression when the depth was shallower. Based on reconstructed slope measurements and microinvertebrates, Eicher (1967) suggested that water depth during maximum transgression was 915m.
Exposures of the Greenhorn Formation around the Black Hills
Figure 1. Exposures of the Greenhorn Formation around the Black Hills. BFR (Belle Fourche Reservoir), AR (Angostura Reservoir), and Edgemont (E) are the primary localities discussed in the text. SDSM localities for each location are indicated (i.e. V9536 is at BFR). pC = Precambrian core of the Black Hills Uplift

In western South Dakota the Greenhorn consists predominately of marl and calcareous shale, with abundant limestone concretions, thin to moderately thick limestone beds, and bentonites. West of this area the unit grades into non-calcareous shale of the Marias River, Frontier, and Mancos formations (Williamson et al. 1993). There is a lithologic difference in the Greenhorn Formation between the northern and southern limits of the Black Hills, which is probably the result of diachronous deposition within the basin, but could also be due to unconformities. Cobban (1951) described the Greenhorn Formation in the northern Black Hills using four informal lithologic units, but only the basal member was named. This basal unit was named the Orman Lake Member for exposures around Orman Lake (Belle Fourche Reservoir) near Belle Fourche, South Dakota. The formation reaches a maximum thickness of 110 m in Crook County, Wyoming, but in the study area the maximum measured thickness is 63 m (southern Black Hills at Angustora Reservoir). Maximum thickness in the northern Black Hills has not been determined because of a lack of continuous outcrops.

The Greenhorn Formation overlies non-calcareous shale of the Belle Fourche Shale, and underlies black non-calcareous shale of the PCM. In the southern Black Hills, the Belle Fourche and Greenhorn are separated by a thick bentonite that Cobban (1951) has called the "X" bentonite (Figure 2). This bentonite appears to be missing in the northern Black Hills, where basal beds of the Orman Lake Member unconformably overlie the Belle Fourche Shale (Figure 3). The Cenomanian-Turonian boundary is located near the middle of the Greenhorn Formation, not far above the "B" bentonite of Elder (1986). This distinctive bentonite is exposed both in the northern and southern portions of the Black Hills of South Dakota (Figure 4). There may be an unconformity at the top of the formation in the northern Black Hills, indicated by vertebrate lag beds and an abrupt change from light colored limestone and marl to dark non-calcareous shale (Figure 5). In the southern Black Hills, the top of the formation is placed at the contact between a gradational interval of 18 m of calcarenites and black non-calcareous shale of the PCM.

 

Stratigraphic section of the entire OLM, Greenhorn formation
Figure 2. Stratigraphic section of the entire OLM, Greenhorn Formation, as exposed at the southern portion of AR, Fall River Co., SD. The contact with the Belle Fourche Shale is just below the base of the "X" bentonite.

 

Stratigraphic section of the Belle Fourche Shale/Greenhorn Formation
Figure 3. Stratigraphic section of the Belle Fourche Shale/Greenhorn Formation contact as exposed at the southeast end of BFR, Butte Co., SD. OLM = Orman Lake Member.

 

Stratigraphic section of the middle portion of the Greenhorn Formation found at the southern end of the AR

Figure 4. Stratigraphic section of the middle portion of the Greenhorn Formation found at the southern end of AR. The Cenomanian (Cen)-Turonian (Tur) boundary is extrapolated from the Hot Springs, Fall River Co., SD site of Elder (1986). This section is within the Sciponoceras gracile Biozone (late late Cenomanian age).

The Greenhorn Formation of western South Dakota shares many ammonite biozones with the principal reference section in southern Colorado near Pueblo. In the northern Black Hills the basal bed of the formation is in the Dunveganoceras pondi Biozone that is early late Cenomaniain in age, whereas the uppermost beds are in the Collignoniceras woollgari Biozone of early middle Turonian age (Obradovich 1993). In the southern Black Hills the basal portion of the OLM is found in the Pleisiacanthoceras wyomingense Biozone, indicating a late middle Cenomanian age (Obradovich 1993). In the northern Black Hills, this ammonite is found in the Belle Fourche Shale, 5 m below the basal Greenhorn beds. The base of the Greenhorn Formation is therefore older in the southern Black Hills than it is to the north. In the southern Black Hills the top of the formation is also located in the Collignoniceras woollgari Biozone. Based on this ammonite zonation, the age of the Greenhorn Formation in Western South Dakota is considered to be late middle Cenomanian to early middle Turonian.

Although elasmobranch fossils were collected from various locations in the Black Hills region of South Dakota and Wyoming, this report focuses on three particularly important areas (Figure 1). The Greenhorn Formation is exposed around nearly the entire perimeter of BFR in Butte County. Excellent exposures of the base of the OLM are found at the southern end , whereas the Greenhorn/Carlile Shale contact is located in the northern end. Nearly the entire Greenhorn Formation is exposed in the southern portion of AR, as well as areas around Edgemont, both in Fall River County. Precise geographic information is on file at SDSM.

Stratigraphi section of the Greenhorn Formation/Carlile Shale contact exposed in the northwest portion of BFR
Figure 5. Stratigraphic section of the Greenhorn Formation/Carlile Shale contact exposed in the northwest portion of BFR. Though not clearly defined, an unconformity may exist between calcareous rocks of the Greenhorn Formation and non-calcareous rocks of the Pool Creek Member (PCM).

METHODS

Fossiliferous rocks were disaggregated using a 10% acetic acid solution. The concentrate was soaked in water to remove calcium acetate residue, then screened with 0.3 mm sieves. The remaining material was dried and sorted with a binocular microscope. Macrovertebrates found as surface float were collected after their position was noted. Stratigraphic sections were measured using combinations of Jacob's staff, tape measure, and Brunton compass. The tooth illustrations were drawn freehand or traced from photographs. Ptychodus teeth were cast in plaster before being photographed. The teeth of P. whipplei and P. sp. described in this report were not available for illustration, so similar teeth from the late middle Turonian Pool Creek and late Turonian Turner Sandy members of the Carlile Shale have been substituted as representative specimens. Systematic paleontology and tooth terminology generally follows Cappetta (1987) and Welton and Farish (1993).

SYSTEMATIC PALEONTOLOGY

Superfamily Hybodontoidea Zangerl, 1981

Family Ptychodontidae Jaekel, 1898

Genus Ptychodus Agassiz, 1835

Ptychodus anonymus Williston, 1900

(Figure 6, A-B)

Referred specimens _ From SDSM V942: SDSM 34932, two medial teeth, SDSM 34933, two lateral teeth, SDSM 34934, posterior tooth, SDSM 35297, anterolateral tooth, SDSM 35309, medial tooth, all found as float derived from the base of the OLM; SDSM 35312, lower medial tooth, SDSM 35313, 11 teeth, all found as float 2 m above the base of the OLM. From SDSM V943: SDSM 35301, two posterior teeth from basal OLM. From SDSM V944: SDSM 34922, two anterior teeth, SDSM 35239, high-crowned variant, SDSM 35240, three teeth, all float derived from basal OLM; SDSM 31256, 19 teeth from 0.15 m below the "A" bentonite of Elder (1986); SDSM 35247, distal lateral tooth from basal OLM; SDSM 35249, 23 teeth found as float 2.5m above base of OLM; SDSM 35252, two teeth from basal OLM; SDSM 35259, two teeth as float from base of OLM; SDSM 35282, 15 teeth as float from one meter above base of OLM; SDSM 35278, 22 teeth found as float 2 m above base of OLM; SDSM 35289, tooth as float 0.3 m below third thick bentonite within the formation; SDSM 35294, eight teeth as float from within basal meter of OLM; SDSM 35295, posterior tooth as float 12 m above base of Greenhorn Formation; SDSM 35539, two anterior teeth, SDSM 35540, two posterior teeth, SDSM 35544, two anterior teeth, SDSM 35545, two posterior teeth, all as float just below "A" bentonite of Elder (1986). From SDSM V945: SDSM 35257, three teeth, SDSM 35262, two distal lateral teeth, all from 12 m below ledge-forming limestones of upper Greenhorn Formation. From SDSM V948: SDSM 35268, 13 teeth found as float from basal OLM; SDSM 35270, three teeth as float within basal 2.5 m of OLM. From SDSM V9415: SDSM 35272, four teeth, SDSM 35274, lateral tooth, all from calcarenite 0.7 m above "X" bentonite. From SDSM V9533: SDSM 34923, two anterior teeth, SDSM 35594, 34 posterior teeth, SDSM 35601, anterior tooth, all as float from basal OLM. From SDSM V9534, SDSM 35315, two teeth as float from upper Greenhorn Formation. SDSM V9536: SDSM 35322, five teeth, SDSM 35323, one tooth, SDSM 35637, 18 teeth, all as float from basal OLM. From SDSM V9537: SDSM 35326, eight teeth from calcarenite 1.5 m above "X" bentonite. From SDSM V9538: SDSM 35602, seven teeth as float from basal OLM. From SDSM V9624: SDSM 35577, two anterior teeth from calcarenites one meter below contact with Carlile Shale.

Description _ Anterior teeth average 1 cm in greatest width and have a high, rounded cusp. The transition from cusp to a marginal shelf is smooth and steeply sloping. Eight to twelve transverse ridges extend from the base of the cusp across the apex. These may be straight or sinuous, and curl, bifurcate, and anastomose at their distal ends. The marginal area bulges anteriorly and is covered with a granular pattern or very fine, discontinuous concentric ridges.

Lateral teeth have a weakly developed cusp that is offset distally, whereas the crowns of posterior teeth are flat or nearly so. Transverse ridges may be straight, concentric, bifurcating, or any combination, and generally extend from one end of the crown to the other.

Discussion _ P. anonymus is found in Cenomanian to Turonian rocks throughout the Western Inerior of the U.S. (Evetts 1979; Shimada and Martin 1993; Welton and Farish 1993; Wolberg 1985). Meyer (1974) reported a late Cenomanian to early Coniacian range for the species in the Gulf Coast area.

The tooth morphology of P. anonymus varies greatly, reflecting monognathic and probably dignathic heterodonty. Meyer (1974) erected what he thought were two stratigraphically distinct subspecies, P. anonymus anonymus and P. a. blechroconus (p. 48 Fig. 12, a-d; p. 52 Fig.13, a-d). Though they may be stratigraphically distinct elsewhere, both types can be found in basal rocks of the OLM, and we believe these represent morphological variation within a single taxon.

Ptychodus decurrens Agassiz, 1843

(Figure 6, C-D)

Referred specimens _ From SDSM V945: SDSM 35255, incomplete tooth 11 m below top of ledge-forming limestone of upper Greenhorn. From SDSM V9532: SDSM 34935, anterior tooth found at base of ledge-forming limestone of upper Greenhorn Formation.

Description _ The anterior tooth is large and mesiodistally elongate. The crown is weakly inflated but with no development of a cusp. Eleven coarse, straight to sinuous transverse ridges cross the medial area of the crown and are associated with rugose ornamentation. The ridges abruptly become much finer on the marginal area and they bifurcate distally, reaching the crown perimeter. Anterior and posterior margins bear fine bifurcating longitudinal ridges. As in all Ptychodus teeth, the crown overhangs the root.

Discussion _ Teeth of P. decurrens are flat or weakly inflated, which distinguishes them from high-crowned species such as P. mammilaris, P. anonymus, and P. occidentalis. The transverse ridges are more robust than in P. occidentalis, and there is also an abrupt decrease in ridge size at the beginning of the marginal area. Teeth of P. decurrens differ from another low-crowned taxon, P. polygyrus, in that the transverse ridges do not have a concentric pattern.

Ptychodus occidentalis Leidy, 1868

(Figure 6, E-G)

Referred Specimens _ From SDSM V942: SDSM 35314, five teeth, SDSM 34922, lateral tooth, all float 2 m above the base of the OLM. From SDSM V943: SDSM 35299, juvenile upper medial tooth, basal calcarenite of OLM. From SDSM V944: SDSM 31263, anterior tooth, SDSM 34925, three teeth, SDSM 32520, two posterior teeth, all as float from basal 3 m of OLM; SDSM 35260, posterior tooth as float in lower 3 m of OLM; SDSM 35283, five teeth as float 1 m above base of OLM; SDSM 35286, two teeth as float 2 m above base of OLM; SDSM 35293, four teeth as float in basal meter of OLM; SDSM 35296, incomplete tooth as float 12 m above base of Greenhorn Formation. From SDSM V948: SDSM 35264, anterolateral tooth, SDSM 35265, two posterior teeth, SDSM 35266, two upper medial teeth, SDSM 35267, lower medial tooth, all as float from basal calcarenites of OLM; SDSM 35269, medial tooth as float 2.5 m above base of OLM. From SDSM V9415: SDSM 35273, two teeth, SDSM 35276, medial tooth, all from calcarenite 0.7 m above "X" bentonite. From SDSM V9536: SDSM 3521, medial tooth and incomplete lateral tooth, all as float from basal OLM.

Description _ Lower medial teeth are anterioposteriorly elongate, mesiodistally narrow, with a nearly flat crown in which the anterior margin greatly overhangs the root (Figure 6, F). Ornamentation consists of very fine transverse ridges that curl at their distal ends, and the marginal area has even finer, discontinuous ridges. Upper medial teeth can be quite large, reaching 4.5 cm in length and 4 cm in total height (Figure 6, E). The crown is symmetrical and very high, with little or no development of a cusp. The transition from margin to crown apex forms nearly a straight, steep line. Five to 13 fine, sinuous, often discontinuous ridges cross the cusp apex. These ridges bifurcate and anastomose distally, becoming finer until they reach the crown perimeter. Ornamentation on the anterior and posterior portions of the teeth consists of very fine longitudinal ridges that bifurcate and anastomose distally.

Lateral teeth become lower crowned, and the inflation also becomes offset distally (Figure 6, G). Ornamentation is similar to that of anterior teeth, but the number of transverse ridges is reduced to less than ten.

Posterior teeth have a rhombic occlusal outline, and the crown is flat. Ornamentation is as in anterior and lateral teeth, but the number of transverse ridges is reduced to seven or less.

Discussion _ Variations in the thickness of the crown and development of a marginal shelf in some medial and anterior teeth may reflect dignathic heterodonty, although the possibility of sexual dimorphism cannot be ruled out.

Although crown ornamentation of P. occidentalis is similar to P. decurrens, upper medial and anterior teeth of P. occidentalis are much more inflated. Asymmetrically inflated lateral teeth are also characteristic of P. occidentalis. Lateral and posterior teeth of P. decurrens can be distinguished from posterior teeth of P. occidentalis in having more robust transverse ridges.

Ptychodus whipplei Marcou, 1858

(Figure 6, H-J)

Referred specimens _ From SDSM V9624: SDSM 35583, lateral tooth, SDSM 35590, anterior tooth, both from uppermost Greenhorn Formation, within one meter below the Carlile Shale contact.

Description _ The anterior tooth measures 1 cm in width and is nearly symmetrical. The crown possesses a tall, laterally compressed central cusp that meets the marginal area at nearly a right angle. Five coarse transverse ridges are restricted to the cusp. Extremely fine ridges form a concentric pattern on the marginal area. Conspicuous wear facets are located on the cusp and the mesial and distal edges of the marginal area. The crown overhangs the root, especially at the anterior end.

The lateral tooth is highly asymmetrical with a distally placed, distally inclined cusp. The cusp is tall, laterally compressed with four coarse transverse ridges that do not enter the marginal area. The transition from cusp to marginal area is steeply sloping, and the mesial marginal area is longer than the distal marginal area. Marginal ornamentation consists of very fine, discontinuous concentric ridges.

Discussion _ Anterior teeth of P. whipplei can be distinguished from P. anonymus in that the cusp is laterally compressed and has a smaller diameter. In lateral teeth, the cusp is narrower than in P. anonymus, and the anterior crown margin is often irregular, not flat. The number of transverse ridges is reduced, with only two to nine as opposed to eight to twelve. Also, these ridges do not curl around the base of the cusp as in P. anonymus.

P. whipplei is often a common constituent of Turonian rocks of the Western Interior (Cappetta 1973; Edwards 1976; Meyer 1974; Williamson et al. 1993; Wolberg 1985). However, its distribution appears to have been restricted to this area. Williamson et al. (1993) indicated that this taxon preferred deep-water environments, but in South Dakota it is only known from shallow-water rocks of the Greenhorn Formation and Carlile Shale (see Cappetta 1973; Cicimurri 1998). This suggests that P. whipplei was able to inhabit a wide range of habitats, so its absence from areas outside the Western Interior of North America is difficult to explain.

Ptychodus sp.

(Figure 6, K-N)

Referred specimens _ From SDSM V9624: SDSM 35587, eight teeth from uppermost Greenhorn, within one meter below the contact with the Carlile Shale.

Discussion _ The teeth are small, less than 1 cm in greatest dimension. Anterior teeth have a high, conical crown that is slightly laterally compressed. There is no clear transition between a cusp and marginal area in anterior and anterolateral teeth. Four to seven straight or chevron-shaped transverse ridges cross the apex, but are often restricted to the upper third to half of the crown. These ridges may coalesce at the apex, resembling a radiating pattern. Very fine longitudinal ridges are found on the anterior and posterior faces of the crown. Ornamentation on the marginal area consists of very fine, short, interconnected ridges or granules that have a concentric pattern.

Lateral and posterior teeth have a lower crown that becomes more distally inclined towards the back of the jaw. Ornamentation is similar to that of anterior teeth.

Discussion _ Teeth with this morphology also occur in the PCM of South Dakota and the Boquillas Formation of Texas (Cicimurri and Bell 1996; Cicimurri 1998). The teeth appear to be close to P. mortoni in having a high, conical cusp, with the occasional development of radiating ridges on the crown. These characteristics are unlike all other Ptychodus species (see Welton and Farish 1993), and we believe this form represents a new taxon that is closely related to P. mortoni. The taxon questionably occurs in the OLM (based on abraded specimens).
Ptychodus anonymus
Figure 6. A-B, Ptychodus anonymus, SDSM 34923, anterior tooth; A, occlusal view, anterior at top, B, anterior view. C-D, P. decurrens, SDSM 34935, medial tooth; C, occlusal view, anterior at top, D, anterior view. E-G, P. occidentalis; E, SDSM 35233, upper medial tooth, lateral view, anterior at left; F, SDSM 35234, lower medial tooth, lateral view, anterior at left; G, SDSM 35236, lateral tooth, anterior view. H-J, P. whipplei, H, SDSM 3xxxx, anterior tooth, occlusal view, anterior at top, I, lateral view, anterior at right; J, SDSM 35583, lateral tooth, anterior view. K-N, Ptychodus sp., K, SDSM 34982, anterior tooth, occlusal view, anterior at top; L, lateral view, anterior at right; M, SDSM 34983, anterior tooth, occlusal view, anterior at top; N, lateral view, anterior at right. Scale bars = 1 cm for all specimens.

Order Lamniformes Berg, 1958

Family Anacoracidae Casier, 1947

Genus Squalicorax Whitley, 1939

Squalicorax curvatus Williston, 1900

(Figure 7, A)



Referred specimens _ From SDSM V942: SDSM 35310, three teeth as float from basal OLM. From SDSM V944: SDSM 35241, 43 teeth, SDSM 35244, six teeth, all from 0.15 m below the "A" bentonite of Elder (1986); SDSM 31262, incomplete tooth as float 3.5 m above the base of the OLM; SDSM 35284, three teeth as float from basal meter of OLM; SDSM 35290, two teeth as float from 0.3 m below the third thick bentonite of the Greenhorn Formation; SDSM 35291, two teeth within basal meter of OLM. From SDSM V946: SDSM 35304, 16 teeth as float from near "A" bentonite of Elder (1986). From SDSM V948: SDSM 35271, incomplete tooth as float 2.5 m above base of Greenhorn Formation. From SDSM V9415: SDSM 35273, four teeth from calcarenite 0.7 m above "X" bentonite. From SDSM V9533: SDSM 35562, 15 teeth from basal calcarenites of OLM. From SDSM V9536: SDSM 35546, 19 teeth, SDSM 35639, seven teeth, all from basal calcarenites of OLM. From SDSM V9537: SDSM 34926, two teeth, SDSM 35557, 46 teeth, all from calcarenite 1.5 m above "X" bentonite. From SDSM V9538, SDSM 35553, anterolateral tooth from basal OLM calcarenites.

Description _ The teeth are rather small (8 mm or less) with a sharply distally inclined cusp. The crown is thick due to a weakly to moderately convex labial face and a strongly convex lingual face. The mesial cutting edge is finely serrated, long, and may be straight, convex, or sinuous. The distal cutting edge is also serrated, but less than half as long as the mesial edge. A distal blade is low, finely serrated, and intersects the distal cutting edge at a sharp angle. The root is low when viewed lingually and consists of two short, diverging, rounded lobes.

Discussion _ These specimens display a high degree of variability, and Cenomanian anacoracid teeth have variously been referred to S. baharijensis and S. curvatus (Welton and Farish 1993; Cappetta and Case 1999). We identify the specimens described above as S. curvatus based on the convex labial crown face, which follows the original description of the species by Williston (1900).

Squalicorax falcatus Agassiz, 1843

(Figure 7, B)

Referred specimens _ From SDSM V9623: SDSM 35582, ten teeth from uppermost Greenhorn Formation, within one m of the Carlile Shale contact. From SDSM V9624: SDSM 35593, two teeth from same horizon as previous specimens.

Description _ Teeth reach up to 1.5 cm in greatest dimension. Anterior teeth are high-crowned with a distally inclined cusp. The labial crown face is flat, whereas the lingual face is convex. The serrated mesial cutting edge is straight or weakly convex, but the distal cutting edge is shorter, convex and finely serrated. A distal blade is low, finely serrated, and meets the base of the distal cutting edge at an acute angle (obtuse in some anterior teeth). The root is moderately high when viewed lingually, with short, diverging, somewhat rounded to rectangular lobes (as seen in lateral teeth of Cretolamna).

Lateral and posterior teeth have a lower crown and more distally inclined cusp. The mesial cutting edge is often sinuous. The root is also lower with thin, rounded lobes.

Discussion _ Teeth attributed to S. falcatus exhibit a high degree of variation, and the taxon has been reported as having a Cenomanian to Campanian range (Welton and Farish 1993; Martin et al. 1998). In recent years there have been suggestions and attempts to split S. falcatus into separate, time-restricted species (Cappetta and Case 1999). We advise against this splitting because without numerous complete dentitions, it is uncertain that the variation within S. falcatus represents monognathic, dignathic, ontogenetic or sexual heterodonty, or if more than one species is actually represented. However, we believe that the teeth referred to S. falcatus by Martin et al. (1998) actually represent lateral teeth of S. kaupi (i.e. early forms of S. kaupi retain an S. falcatus morphology in lateral and posterior row groups).

Squalicorax volgensis

(Gluckman in Gluckman and Shvazhaite), 1971

(Figure 7, C)

Referred specimens _ From SDSM V946: SDSM 35303, two teeth as float from near "A" bentonite of Elder (1986). From SDSM V9533: SDSM 35563, one tooth from basal OLM. From SDSM V9536: SDSM 35320, two teeth from basal OLM. From SDSM V9537: SDSM 35558, two teeth from calcarenite 1.5 m above "X" bentonite. From SDSM V9623: SDSM 35574, 37 teeth from uppermost Greenhorn Formation, within 1 m of the contact with the Carlile Shale. From SDSM V9624: SDSM 35588, six teeth from same stratigraphic horizon as SDSM 35574.

Description _ Symphyseal teeth are nearly symmetrical with a tall, narrow cusp and one pair of low, rounded "cusplets" that are not detached from the cusp. The lingual crown face is very convex and smooth, whereas the labial face is convex near the apex, but becomes flat or concave near the crown foot. Mesial and distal blades are smooth, sharp, but do not reach the apex. The root is narrow with short, diverging lobes.

Anterior teeth are mesiodistally narrow with a tall, distally inclined cusp. The mesial cutting edge is long and may be convex, straight, or slightly sinuous. The distal cutting edge is half the length of the mesial edge and may be convex, straight, or even concave. A distal blade is short, very convex, and separated from the distal cutting edge by a notch in the enameloid (sometimes nearly forming a distinct cusplet). The cutting edges are often smooth, but fine serrations may be found on the lower portion of the mesial edge. The root is bilobate with short, sub-rectangular lobes.

Lateral and posterior teeth are broader than anteriors, but lower crowned and sharply distally inclined. The mesial cutting edge is long and convex (especially evident in posterior teeth), whereas the distal edge is short and convex. The distal blade varies in length, but it is convex and separated from the distal cutting edge by a deep notch (sometimes resembling a lateral cusplet). Cutting edges are generally unserrated, but the mesial edge may be weakly serrated on the lower portion. The root is bilobate, with the mesial lobe often longer than the distal lobe.

Discussion _ This taxon appears to be less derived than other temporally equivalent or younger species of Squalicorax in the lack of or weak development of serrations. Morphological variations such as the shape of the mesial cutting edge, degree of distal inclination of the cusp, and variation of the serration pattern indicate both monognathic and dignathic heterodonty within an individual, as has been noted by Welton and Farish (1993). Siverson (1996) even suggested the possibility of ontogenetic heterodonty, indicating that the "S. volgensis" morphology represented juvenile teeth of S. curvatus/S. falcatus. We believe S. volgensis is a distinct taxon because even the smallest teeth of S. curvatus and S. falcatus are serrated, whereas even large teeth of S. volgensis can be unserrated.

Family Cretoxyrhinidae Gluckman, 1958

Genus Cretodus Sokolov, 1965

Cretodus crassidens (Dixon), 1850

(Figure 7, D-E)

Referred specimen _ From SDSM V9624: SDSM 35591, incomplete anterolateral tooth as float from uppermost part of Greenhorn Formation, within one meter below the contact with the Carlile Shale.

Description _ The tooth is large, measuring 3cm in preserved height. The crown is tall, broad-based, and slightly distally recurved. The labial crown face is flat and smooth, whereas the lingual crown face is highly convex. The root is incomplete, but the lingual boss is massive and shelf-like.

Discussion _ Unfortunately the specimen is highly weathered. However, the massive lingual boss distinguishes this tooth from Cretoxyrhina. The tooth is larger and more massive than teeth of Cretoxyrhina and Cretolamna from the same deposits. The lack of labial and lingual longitudinal ridges separate SDSM 35591 from C. semiplicatus.

This specimen represents the earliest occurrence of C. crassidens in the Black Hills Region. It is relatively common in the Pool Creek and Turner Sandy members of the Carlile Shale of South Dakota (Cappetta 1973; Cicimurri 1998).

Cretodus semiplicatus (Munster) in Agassiz, 1843

(Figure 7, F-G)

Referred specimen _ AMM98.1.2, incomplete anterior tooth from basal calcarenites of OLM, near Fruitdale, Butte Co., SD.

Description _ The tooth is large, measuring nearly 3cm in preserved height. It is symmetrical with a tall, rather narrow, sharply pointed cusp. The labial crown face is slightly convex, with several very short longitudinal ridges located at the crown foot. The lingual face is highly convex, with conspicuous longitudinal ridges extending more than half the length of the crown. The root appears to have been symmetrical as well, with large, diverging lobes and a large lingual boss.

Discussion _ Although incomplete, this specimen can be distinguished from C. crassidens by longer longitudinal ridges on the lingual crown face. This type of ornamentation is not seen in Cretolamna or Cretoxyrhina.

AMM98.1.2 represents the youngest occurrence of C. semiplicatus in the Black Hills region. It appears to be somewhat more common in older rocks of the Mowry Shale and Belle Fourche Shale (Cicimurri 1998; Cicimurri in press).

Genus Cretolamna Gluckman, 1958

Cretolamna appendiculata Agassiz, 1843

(Figure 7, H)

Referred specimens _ From SDSM V 942: SDSM 35308, lateral tooth as float from basal OLM. From SDSM V944: SDSM 35280, lateral tooth as float one meter above base of OLM. SDSM V9533: SDSM 35597, three incomplete teeth as float from basal OLM. From SDSM V9535: SDSM 35317, lateral tooth as float 30m below "B" bentonite of Elder (1986). From SDSM V9536: SDSM 35324, two lateral teeth from basal OLM calcarenites. From SDSM V9537: SDSM 35330, lateral tooth from calcarenite 1.5 m above "X" bentonite. From SDSM V9623: SDSM 35578, lateral tooth from uppermost Greenhorn Formation, within one meter below the Carlile Shale contact. From SDSM V9624: SDSM 35592, incomplete tooth from same horizon as previous specimen.

Description _ The teeth are of moderate size, not larger than 2.5 cm in total height. Anterior teeth are symmetrical with a tall, erect, slightly sigmoidal crown. The labial crown face is flat, whereas the lingual face is convex. There is no crown ornamentation, and one pair of large, diverging lateral cusplets straddles the central cusp. The cutting edge is sharp, smooth and continuous across the central cusp. The root is bilobate with rather short, rounded lobes and a rounded lingual boss.

Lateral and posterior teeth are lower crowned, with a broader, recurved cusp. Lateral cusplets are not as divergent as in anterior teeth, but are much broader. Root lobes are short, rectangular, and labio-lingually thin.

Discussion _ The taxon Cretolamna (especially C. appendiculata) has an unusually long temporal range, having been reported from early Cretaceous-middle Paleogene (Eocene) rocks from around the world (Cappetta 1987; Kent 1999). Teeth of this shark can be distinguished from the cretoxyrhinids Cretoxyrhina and Paraisurus in having large lateral cusplets, and from Leptostyrax, Cretodus semiplicatus, and some Protolamna in lacking crown ornamentation (Welton and Farish 1993, Cappetta and Case 1999).

Genus Cretoxyrhina Gluckman, 1958

Cretoxyrhina mantelli (Agassiz), 1843

(Figure 7, I-K)

Referred specimens _ From SDSM V942: SDSM 35298, incomplete tooth from basal OLM; SDSM 35311, five incomplete teeth as float two meters above the base of the OLM. From SDSM V944: SDSM 34942, posterior tooth from basal OLM; SDSM 35238, lateral tooth as float near base of OLM; SDSM 35241, 24 teeth, including juveniles, SDSM 35242, 29 teeth, all from 0.15 m below "A" bentonite of Elder (1986). From SDSM V945: SDSM 35256, three incomplete teeth from 11 m below top of ledge-forming limestone at top of Greenhorn Formation; SDSM 35258, juvenile tooth from 12 m below top of ledge-forming limestone of upper Greenhorn; SDSM 35261, three incomplete teeth as float 11 meters below ledge-forming limestone of upper Greenhorn; SDSM 35263, tooth in matrix from 1.5 m below top of Greenhorn caprock. From SDSM V946: SDSM 35302, seven teeth, SDSM 35306, three teeth, all as float near "A" bentonite of Elder (1986); SDSM 31261, incomplete tooth as float from ledge-forming limestone of upper Greenhorn Formation; SDSM 35248, eight teeth, SDSM 35253, 11 incomplete teeth, all as float 2 m above base of OLM; SDSM 35251, three teeth as float 0.61 m above base of OLM; SDSM 35281, six incomplete teeth, SDSM 35292, three incomplete teeth, all as float one meter above base of OLM; SDSM 35285, ten teeth as float 2 m above base of OLM; SDSM 35288, two incomplete teeth as float 0.3 m below third thick bentonite of basal Greenhorn Formation; SDSM 35541, incomplete tooth, SDSM 35543, anterior tooth, all from near "A" bentonite of Elder (1986). From SDSM V9415: SDSM 35278, three teeth from calcarenite 0.7 m above "X" bentonite. From SDSM V9533: SDSM 35598, eight anterior teeth, SDSM 35599, three lateral teeth, SDSM 35600, five posterior teeth, all float from basal calcarenites of OLM. From SDSM V9534: SDSM 35316, three anterior teeth as float from upper Greenhorn. From SDSM V9535: SDSM 34941, anterior tooth as float from basal OLM; SDSM 35318, three teeth as float 30 m below "A" bentonite of Elder (1986). From SDSM V9536: SDSM 35319, 11 teeth, two incomplete teeth, all float from basal calcarenites of OLM. From SDSM V9537: SDSM 35325, four teeth, SDSM 35328, three juvenile teeth, all from calcarenite 1.5 m above "X" bentonite. From SDSM V9538: SDSM 35603, five teeth from basal OLM. From SDSM V9623: SDSM 35580, six teeth from uppermost Greenhorn Formation, within one meter below contact with Carlile Shale. From SDSM V9624: SDSM 35573, nine teeth from same horizon as SDSM 35580.

Description _ Teeth can attain 3 cm in total height. Anterior teeth have a tall, narrow, slightly sigmoidal crown that may be distally inclined. The labial face is flat, whereas the lingual face is convex. There is no ornamentation, and cutting edges are smooth and sharp across the entire crown. The root is massive and often asymmetrical with a longer mesial lobe.

Lateral teeth are broader and lower crowned than anterior teeth. A small pair of weakly developed lateral cusplets may be found on some teeth. Root lobes are short and rectangular. Posterior teeth are small, low crowned, with a pair of lateral cusplets. Root lobes are short and rounded.

Juvenile teeth are small and gracile, with a well-developed pair of lateral cusplets. The root is delicate, with long and narrow lobes.

Discussion _ Teeth of Cretoxyrhina are very common in the Greenhorn Formation of South Dakota. Indeed, the taxon had a worldwide distribution during the late Cretaceous (Cappetta 1987; Siverson 1996). The teeth of this shark differ from Cretolamna in its more robust proportions and the lack of lateral cusplets on anterior and most lateral teeth. The taxon can be distinguished from Cretodus in the lack of longitudinal ridges on the cusp.

This taxon is uncommon in the overlying PCM of the Carlile Shale (Cicimurri 1998), and it is absent altogether in the Turner Sandy Member (Cappetta 1973; Evetts 1979). It is thought that Cretoxyrhina inhabited mostly deep-water environments (Williamson et al. 1993; Shimada 1994; Schwimmer 2000). As noted above, we document this shark in shallow-water rocks of the Greenhorn Formation and PCM. The rocks of the Turner Sandy Member were deposited in an upper shoreface environment dominated by wave-generated and tidal currents (Sawyer 1990). The absence of Cretoxyrhina in these rocks suggests the water was too shallow for this shark, and its niche seems to have been occupied by Cretodus crassidens (see Cappetta 1973).

Family Mitsukurinidae Jordan, 1898

Genus Scapanorhynchus Woodward, 1889

Scapanorhynchus raphiodon (Agassiz), 1844

(Figure 7, L)

Referred specimens _ From SDSM V9623: SDSM 35579, 26 teeth from uppermost Greenhorn Formation, within one m below the contact with the Carlile Shale. From SDSM V 9624: SDSM 35589, four incomplete teeth from same horizon as previous specimens.

Description _ Teeth are of moderate size, up to 2 cm in total height. Lower anterior teeth have a tall, slender cusp with a sigmoidal profile. The labial crown face is flat and smooth, and the enameloid extends basally onto the root lobes. The lingual face is convex and bears fine longitudinal striations that do not reach the cusp apex. Lateral cusplets are generally absent, but a single pair of very small cusplets was found on some teeth. The root is bilobate with long, thin lobes. A large lingual boss is bisected by a deep nutritive groove.

Lateral teeth are wide basally and distally inclined. The cusp is labio-lingually compressed with a cutting edge that is continuous along the entire cusp. The labial face is smooth, whereas the lingual face may have very fine longitudinal striations. At least one pair of lateral cusplets is always present. The lingual root boss is reduced, but the nutritive groove remains. The root itself becomes labio-lingually flattened, and the lobes are sub-rectangular.

Discussion _ The specimens described here represent the earliest occurrence of the taxon in the Black Hills. This shark was also common in the Pool Creek and Turner Sandy members of the Carlile Shale (Cappetta 1973; Cicimurri 1998), as well as temporally equivalent rocks throughout the Western Interior of North America (Edwards 1976; Wolberg 1985; Williamson et al. 1993; Cicimurri and Bell 1996).

Family Odontaspididae Muller and Henle, 1839

Genus Carcharias Rafinesque, 1810

Carcharias aff. amonensis (Cappetta and Case), 1975

(Figure 7, M)

Referred specimen _ From SDSM V9533: SDSM 35566, one tooth from basal calcarenite of OLM.

Description _ The tooth is small, measuring only 4 mm in total height. It is slightly asymmetrical with a distally inclined cusp. The cusp is broad-based but tapers to a sharp point. The labial face is flat and the lingual face is convex, with both faces being unornamented. A single pair of low, broad, sharply pointed cusplets flanks the central cusp. The root is broadly bilobate with short, thin, rounded lobes. A lingual nutritive groove bisects the root.

Discussion _ Though SDSM 35566 is abraded, there is no indication of crown ornamentation, and the lateral cusplets are low and broadly triangular. For these reasons we tentatively refer this tooth to C. amonensis, rather than C. saskatchewanensis or C. tenuiplicatus (see Fig. 7, M-O). A tooth referred to this taxon by Cicimurri (1998) from the uppermost Greenhorn has been reassigned to Carcharias sp. (of this report). C. amonensis has an early to late Cenomanian range in South Dakota, and it is especially abundant in middle Cenomanian rocks of the Belle Fourche Shale (Cicimurri 2001). This shark is also commonly found in Cenomanian rocks of Kansas and Texas (Shimada and Martin 1993; Welton and Farish 1993; Cappetta and Case 1999).

Carcharias saskatchewanensis Case et al., 1990

(Figure 7, N)

Referred specimens _ From SDSM V944: SDSM 35246, 77 teeth from 0.15 m below the "A" bentonite of Elder (1986); SDSM 35254, one tooth as float 0.6 m above base of the OLM. From SDSM V946: SDSM 35307, four teeth as float near "A" bentonite of Elder (1986). From SDSM V9415: SDSM 35279, 31 teeth from calcarenite 0.7 m above "X" bentonite. From SDSM V9533: SDSM 35564, 69 teeth from basal OLM calcarenites. From SDSM V9536: SDSM 34927, two anterior teeth, SDSM 34928, two lateral teeth, SDSM 35547 nine teeth, SDSM 35550, 28 teeth, all from basal calcarenites of OLM. From SDSM V9537: SDSM 35559, 47 teeth from calcarenite 1.5 m above "X" bentonite. From SDSM V9538: SDSM 35555, three teeth from basal OLM.

Description _ Teeth are very small, measuring 4 mm or less in total height. Anterior teeth have a tall, sharply pointed central cusp that is flanked by a pair of needle-like lateral cusplets. The cusplets are closely connected to the central cusp. Labial and lingual crown faces are convex (lingual face more so) and unornamented. The cutting edge is smooth, sharp, and continuous across the cusp and lateral cusplets. Root lobes are short, rounded, and divergent, overhung by a prominent labial shelf at the base of the crown.

Lateral and posterior teeth become low crowned and more distally inclined. The labial and lingual crown faces are convex, and the labial face often bears a few short longitudinal ridges. Generally only a single pair of sharp lateral cusplets are tightly connected to the central cusp. The root has very short, widely separated lobes of unequal length. A deep nutritive groove bisects the lingual boss of all teeth.

Discussion _ This species was reported by Case et al. (1990) from Saskatchewan, Canada. Teeth have also been collected from the Lincoln Limestone Member, Greenhorn Limestone of Kansas (Shimada and Martin 1993), and Welton and Farish (1993:91) illustrated similar teeth from Cenomanian rocks of Texas that represent shallow-water environments. This species is not known to occur in Turonian rocks.

Carcharias tenuiplicatus (Cappetta and Case), 1975

(Figure 7, O-P)

Referred specimens _ From SDSM V944: SDSM 35245, six teeth from 0.15 m below the "A" bentonite of Elder (1986). From SDSM V9533: SDSM 35565, six teeth from basal calcarenites of the OLM. From SDSM V9536: SDSM 35551, two teeth from basal OLM. From SDSM V9537: SDSM 34920, two teeth, SDSM 34930, three anterior teeth, SDSM 35560, nine teeth, all from calcarenite 1.5 m above the "X" bentonite.

Description _ The teeth are very small, less than 5 mm in total height. Anterior teeth have an erect, sharply pointed central cusp that is flanked by at least one pair of needle-like lateral cusplets. The labial crown face is nearly flat, whereas the lingual face is convex. Coarse longitudinal ridges are found at the crown foot of the lingual face, but extend nearly to the cusp apex on the labial face. A broad labial basal shelf overhangs the root. The root is bilobate with short, widely separated, rounded lobes.

Lateral and posterior teeth are lower crowned, distally inclined, often with more pronounced labial ornamentation. Two pairs of lateral cusplets are closely associated with the central cusp (the second pair is much smaller than the first). The cutting edge is smooth and continuous across the cusp and lateral cusplets of all teeth, and the root is bisected by a deep nutritive groove.

Discussion - The tooth morphology of this taxon is similar to that of C. saskatchewanensis, except that all teeth bear coarse longitudinal striations on the labial face, as well as shorter ridges on the lingual face. In addition, there are generally two pairs of lateral cusplets.

This taxon is also common in Kansas and Texas, where it occurs in Cenomanian rocks representing nearshore, shallow-water environments (Shimada and Martin 1993; Welton and Farish 1993). The shark has not been reported from Turonian rocks, and this lead Cappetta and Case (1999) to rename the taxon Cenocarcharias tenuiplicatus. We do not agree with this reassignment and choose to retain Carcharias, especially because early Turonian marine rocks outside of Texas have not been adequately sampled.

Carcharias sp.

(Figure 7, Q-T)

Referred specimens _ From SDSM V9623: SDSM 35584, 40 teeth from uppermost Greenhorn Formation, within one meter below the contact with the Carlile Shale.

Description _ The teeth are small, not reaching 1 cm in total height. Anterior teeth have a tall, slender, slightly sigmoidal cusp. The labial face is smooth and flat, whereas the lingual face is convex and smooth. The cutting edge is sharp and continuous across the central cusp. A single pair of short, narrow, divergent lateral cusplets flanks the cusp. These are nearly detached from the central cusp when viewed labially. The root is bilobate with relatively long, narrow, rounded lobes. The lingual boss is bisected by a nutritive groove.

Lateral and posterior teeth have a broad-based, sharply pointed, recurved cusp. Teeth are unornamented with flat labial and convex lingual faces. The central cusp is flanked by a pair of low, broadly triangular, divergent lateral cusplets (a second smaller pair was found on some teeth). The root is broad but thin, bilobate with sub-rectangular lobes and a deep lingual nutritive groove.

Discussion _ This tooth type also occurs in late middle Turonian rocks of the PCM of South Dakota (Cicimurri 1998). They are similar to teeth of ?Odontaspis subulata described by Meyer (1974) from Turonian and Coniacian rocks of Texas.

This tooth type differs from C. amonensis in that lateral teeth generally have only one pair of lateral cusplets, and the distal recurvature is greater than in C. amonensis.

Genus Johnlongia Siverson, 1996

Johnlongia parvidens (Cappetta), 1973

(Figure 7, U-W)

Referred specimens _ From SDSM V9623: SDSM 34937, anterior tooth, SDSM 34938, lateral tooth, SDSM 35575, ten teeth, all from uppermost Greenhorn Formation, within one meter below base of Carlile Shale. From SDSM V9624: SDSM 35586, one tooth from same horizon as previous specimens.

Description _ The teeth are small, less than 1 cm in total height. Anterior teeth have a tall, slender, sigmoidal cusp. Cutting edges are sharp but restricted to the upper portion of the cusp. The crown faces are smooth, with weakly convex labial and strongly convex lingual faces. A pair of small lateral cusplets are closely connected to the central cusp. The root is unusual in having a massive lingual boss that is bisected by a large nutritive groove. Root lobes are short and rounded, asymmetrical with a longer distal lobe. Anterior teeth of juvenile individuals are identical to those of adults, except for conspicuous labial longitudinal ridges.

The lateral tooth has a broad-based cusp that quickly tapers apically. It is labio-lingually thin with a sharp, continuous cutting edge. The labial crown face bears longitudinal ridges that are restricted to the lower half of the cusp. A pair of tall, needle-like lateral cusplets are closely connected
to the central cusp. In labial view, the enameloid extends nearly to the base of the root lobes. Root lobes are thin with a sub-rectangular outline.

Discussion _ Cappetta (1973) originally reported Odontaspis parvidens from the Turner Sandy Member of the Carlile Shale, South Dakota. Siverson (1996) erected the genus Johnlongia to include odontaspid teeth having this unusual morphology. To date, the taxon is only known from Cenomanian rocks of Australia (Siverson 1996) and middle to late Turonian rocks of South Dakota (Cappetta 1973; Cicimurri 1998).

Order Rajiformes Berg, 1940

Family Rhinobatidae Muller and Henle, 1838

Genus Rhinobatos Link, 1790

Rhinobatos incertus Cappetta, 1973

(Figure 7, X)

Referred specimens _ From SDSM V944: SDSM 35243, one tooth from 1.5 m below the "A" bentonite of Elder (1986). From SDSM V9531: SDSM 35640, six teeth from calcarenites near "B" bentonite of Elder (1986). From SDSM V9533: SDSM 35595, two female teeth from basal OLM. From SDSM V9537: SDSM 35329, three teeth from 1.5 m above the "X" bentonite. From SDSM V9623: SDSM 35585, one tooth from uppermost Greenhorn Formation, within one meter below the contact with the Carlile Shale.

Description _ The teeth are microscopic, no larger than 1 mm in greatest dimension. The crown is mesiodistally wide and unornamented. The labial face forms a flat to weakly convex surface, and the crown foot overhangs the root. An elongate, rounded lingual protuberance is located at the center of the crown that is flanked by a pair of shorter protuberances. Each protuberance is separated by a deep sulcus. A central cusp grades from absent to pronounced (if present it is lingually inclined). The cutting edge, if present, is restricted to the central area of the crown. The root is low with a flat to convex attachment surface. It is often divided into two triangular lobes by a deep nutritive groove. A large foramen located just below the central lingual protuberance opens into the groove. Smaller foramina may be visible on each side of the central protuberance.

Discussion _ Rhinobatos exhibits sexual dimorphism in tooth morphology with females having low-crowned teeth, whereas male teeth possess a tall, conspicuous cusp. This characteristic probably inhibits intraspecific competition _ males of a given species may have different diets than the females. Recent species of Rhinobatos inhabit relatively shallow coastal waters of depths less than 100 m (Ticas et al. 1997).

Elasmobranchii incertae cedis

Genus Cretomanta Case et al., 1990

Cretomanta canadensis Case et al., 1990

(Figure 7, Y)

Referred specimens _ From SDSM V944: SDSM 35542, three teeth from calcarenite below "A" bentonite of Elder (1986). From SDSM V946: SDSM 35305, one tooth from near "A" bentonite of Elder. From SDSM V9415: SDSM 35277, two teeth from calcarenite 0.7 m above "X" bentonite. From SDSM V9533: SDSM 35596, eight teeth from basal OLM. From SDSM V9537: SDSM 35327, 21 teeth from calcarenite 1.5 m above "X" bentonite. From SDSM V9623: SDSM V35581, four teeth from uppermost Greenhorn Formation, within one meter below the contact with the Carlile Shale.

Description _ The teeth are microscopic, less than 1 mm in total height. The crown consists of a low, lingually inclined or recurved cusp that is often distally inclined as well. Labial and lingual crown faces are smooth and convex, and the labial face may bulge at the crown foot. The cusp may be conical or possess mesial and distal cutting edges. The root is globular with numerous foramina. The attachment surface is convex.

Discussion _ Cretomanta is an enigmatic and problematic taxon. It was originally placed in Mobulidae by Case et al. (1990) based on the superficial resemblance to the Recent devil ray, Manta. Welton and Farish (1993) referred similar teeth to Rhincodontidae based on the superficial resemblance to the whale shark, Rhincodon. Because this taxon is based solely on isolated teeth, Cappetta and Case (1999) hesitated to refine its systematic classification, although they suggested that the animal fed primarily on plankton.
teeth drawings

Figure 7. A, Squalicorax curvatus, SDSM 35562, anterolateral tooth, lingual view; B, S. falcatus, SDSM 35582, anterior tooth, lingual view; C, S. volgensis, SDSM 35562, anterior tooth, lingual view; D-E, Cretodus crassidens, SDSM 35591, anterolateral tooth, D, lingual view, E, labial view; F-G, C. semiplicatus, AMM 98.1.2, anterior tooth, F, lingual view, G, labial view; H, Cretolamna appendiculata, SDSM 35578, lateral tooth, lingual view; I-J, Cretoxyrhina mantelli, SDSM 35598, anterior tooth, I, lingual view, J, labial view; K, C. mantelli, SDSM 35599, lateral tooth, lingual view; L, Scapanorhynchus raphiodon, SDSM 34940, lower anterior tooth, lingual view; M, Carcharias aff. C. amonensis, SDSM 35566, anterior tooth, labial view; N, C. saskatchewanensis, SDSM 3555, anterior tooth, labial view; O, C. tenuiplicatus, SDSM 34930, anterior tooth, labial view; P, C. tenuiplicatus, SDSM 34920, lateral tooth, labial view; Q-T, Carcharias sp., SDSM 35584, Q-R, anterior tooth, Q, lingual view, R, labial view, S-T, lateral tooth, S, lingual view, T, labial view; U-V, SDSM 34979, Johnlongia parvidens, U, lingual view, V, mesial view; W, SDSM 34937, J. parvidens, lateral tooth, labial view; X, Rhinobatos incertus, SDSM 35640, occlusal view; Y, Cretomanta canadensis, SDSM 35596, lateral view. Scale bars are indicated in millimeters.

DISCUSSION

The Greenhorn Formation of western South Dakota has yielded a wide variety of vertebrates that includes bony fish, elasmobranchs, chelonioids, mosasauroids, pliosauroids and plesiosauroids, and a pterosaur. The greatest diversity and abundance of these taxa occurs within basal rocks of the OLM, and within calcarenites at the very top of the formation. Thirteen elasmobranch taxa were recovered from the OLM, whereas 14 were collected from the top of the formation (Table 1).

The oldest Greenhorn Formation elasmobranchs were collected from the base of the OLM at AR and Edgemont in the southern Black Hills. These rocks, as well as associated selachians such as Carcharias and Rhinobatos, indicate deposition took place in relatively shallow water (above wave base). The remains were associated with the ammonite Plesiacanthoceras wyomingense and are of late middle Cenomanian age (Obradovich 1993). At BFR in the northern Black Hills, this ammonite occurs within non-calcareous black shale of the Belle Fourche Shale, 5 m below the base of the Greenhorn. This illustrates nicely the diachronous nature of Greenhorn deposition, as the Greenhorn Seaway (during the late middle Cenomanian) was shallower in the southern portion of present-day South Dakota than it was 128 km to the north.

Elasmobranch fossils collected from the base of the OLM at BFR are associated with the ammonite Dunveganoceras pondi, indicating an early late Cenomanian age (Obradovich 1993). Although slightly younger, this assemblage is identical to that from AR. At BFR, two calcarenites are exposed at the base of the OLM, one unconformably overlying the Belle Fourche Shale, the other separated from the first by a bentonite (the calcarenites contain identical vertebrate taxa). At AR, the base of the OLM is separated from the Belle Fourche Shale by the "X" bentonite. A highly irregular surface is preserved at the top of the Belle Fourche Shale at BFR, indicating the "X" bentonite was eroded from this area prior to initial deposition of the Greenhorn Formation.

The elasmobranch assemblages from the Greenhorn Formation in South Dakota are very similar to temporally equivalent assemblages in the Western Interior of the U.S. (see Table 1 for a detailed comparison). This should not be surprising, as sharks and rays are mobile organisms that are able to radiate into areas with desirable environmental conditions.

Few selachian taxa have been recovered from the latest Cenomanian portion of the Greenhorn Formation, and no elasmobranchs were collected from the early Turonian portion. This is in part due to the lack of exposures and/or fossiliferous rocks. The fossils that we collected from the middle portion of the formation often occurred in thin, discontinuous calcarenites located just below the Cenomanian/Turonian boundary within the Sciponoceras gracile Biozone. The rocks and associated fossils are similar to those at the base of the OLM, indicating regression of the sea and a recurrence of shallow-water conditions.

An extinction event occurred at or near the Cenomanian/Turonian boundary, indicated by faunal turnover in inoceramid and foraminiferan species (Kauffman 1984; Elder 1986). In South Dakota, the sharks Carcharias saskatchewanensis and C. tenuiplicatus have only been found in Cenomanian rocks. Their apparent absence in younger rocks throughout the Western Interior suggests that at least some vertebrate taxa were affected as well.

The Greenhorn Formation/Carlile Shale contact is located in the northern part of BFR, and elasmobranch fossils were collected from an interval within 0.5 m below the contact. The fossils were associated with the ammonite Collignoniceras woollgari, indicating an early middle Turonian age (Obradovich 1993). Species diversity is comparable to that of the basal OLM, though more than half the taxa are different (see Table 1). Rocks within this interval consist of gray calcareous shale and discontinuous calcarenites, apparently grading into non-calcareous black shale. We believe the calcarenites represent wave-influenced shoals or offshore bars related to regression of the Greenhorn Seaway during the middle Turonian (Foster 1990; VonLoh and Bell 1998). It was thought that these rocks represented transgressive lags in the basal portion of the PCM of the Carlile Shale (Cicimurri 1998), but their calcareous nature leads us to place them within the Greenhorn Formation, below non-calcareous shale of the PCM.

Uppermost Greenhorn rocks exposed at BFR are much different than those at AR. Whereas rocks in the northern Black Hills consist of calcareous shale, marl, and calcarenite, those in the southern region consist predominantly of thick limestone beds. Again, this reflects the diachronous nature of Greenhorn deposition within the Western Interior Basin. Only Creoxyrhina mantelli, Squalicorax falcatus and large teeth of Ptychodus decurrens have been observed in the upper Greenhorn at AR.

The selachian assemblages of the Greenhorn Formation of South Dakota are dominated by pelagic taxa (with the exception of Rhinobatos and possibly Ptychodus). The lack of benthic taxa such as sclerorhynchids and orectolobids is puzzling, especially when one considers the abundant food supply that was available (evidenced by the abundance and high diversity of teleost taxa, as well as some invertebrates). Temporally equivalent rocks of Texas and Arizona contain a variety of sclerorhynchids and orectolobids (Welton and Farish 1993; Williamson et al. 1993; Cicimurri and Bell 1996).

The majority of the taxa described in this report are rare or unknown from the Atlantic and Pacific coastal areas of North America. For example, only a single occurrence of Ptychodus has been documented from the Pacific Coast (Stewart 1988), and only one occurrence of Cretoxyrhina has been reported from the Atlantic Coast (Cappetta and Case 1975). We agree with Schwimmer (2000) that this is primarily related to a lack of temporally equivalent strata in these areas.

DISTRIBUTION
Taxon
1
2
3
4
5
Ptychodus anonymus ** * *    
P. decurrens *? * * * *
P. occidentalis *   *    
P.whipplei * * *    
Ptychodus sp. * *      
Squalicorax curvatus *   *    
S. falcatus ** * *    
S. volgensis **   *    
Scapanorhynchus raphiodon * * *    
Carcharias amonensis *? * *    
C. saskatchewanensis * * *    
C. tenuiplicatus *   *    
Carcharias sp. * *      
Johnlongia parvidens *        
Cretodus crassidens * *      
C. semiplicatus * * * *  
Cretolamna appendiculata ** * * *  
Cretoxyrhina mantelli ** * * *  
Rhinobatos incertus ** *? * *  
Cretomanta canadensis ** * *    

Table 1. Geographic distribution of elasmobranch taxa described in this report. 1, OLM of the Greenhorn Formation , South Dakota; 2, uppermost Greenhorn Fm. of South Dakota; 3, Lincoln Limestone Member, Greenhorn limestone of Kansas; 4, late Cenomanian-middle Turonian rocks of Texas; 5, Mancos Shale of Arizona. Occurences for 3-5 are based on Shimada and Martin 1993, Welton and Farish 1993, Williamson et al. 1993, Cicimurri and Bell 1996, and Cappettta and Case 1999. ? indicates uncertain occurence of the species.

CONCLUSIONS

Based on ammonite occurrences, the Greenhorn Formation of South Dakota is of late middle Cenomanian to early middle Turonian age. Twenty elasmobranch taxa have been identified from vertebrate assemblages within four ammonite biozones: Plesiacanthoceras wyomingense (late middle Cenomanian), Dunveganoceras pondi (early late Cenomanian), Scipponoceras gracile (late late Cenomanian), and Collignoniceras woollgari (early middle Turonian). Though slightly older, the assemblage from the base of the formation in the southern Black Hills is identical to that from the base of the formation in the northern Black Hills. These assemblages are also very similar to temporally equivalent rocks of Kansas. Taken as a whole, the elasmobranch assemblage of the Greenhorn Fm. of South Dakota compares closely with that of the Cenomanian and Turonian of Texas.

The Greenhorn assemblage is dominated by pelagic taxa and, in North America, most of the species are known only from the Western Interior and Gulf Coast areas. Rhinobatos and species of Carcharias are found in several horizons within the Cenomanian portion of the formation, indicating that several minor regressive events took place during the transgressive phase of the Greenhorn Cyclothem.

ACKNOWLEDGEMENTS

This report stems from M.S. thesis research conducted by DJC, and guidance from committee members Gorden Bell, James Fox, James Martin, and Andrew Rogerson is greatly appreciated. We are indebted to Gorden Bell, Bill Schurmann, Dan Varner, Christian Cicimurri, Marcia Bell, and Bruce Schumacher for all of their help in the field. Barbara Beasely and Kimball Banks helped make Federal land accessible to SDSM, and Willliam Cobban confirmed the identity of the ammonites mentioned in this report. Carrie Herbel was kind enough to arrange the loan of the specimens to DJC. Gorden Bell also provided the funding for us to conduct fieldwork. Sincere thanks go to Marjorie DeCocker for drawing the teeth in Figure 7. We would also like to thank Vincent Santucci for passing the manuscript along to the reviewers. Finally, thanks to Gary Johnson, Lindsay McClelland, and Barbara Stahl. Their editorial comments improved the original version of this manuscript.

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DISTRIBUTION

Taxon 1 2 3 4 5

Ptychodus anonymus · · · ·

P. decurrens ·? · · · ·

P. occidentalis · ·

P. whipplei · · ·

Ptychodus sp. · ·

Squalicorax curvatus · ·

S. falcatus · · · ·

S. volgensis · · ·

Scapanorhynchus raphiodon · · ·

Carcharias amonensis ·? · ·

C. saskatchewanensis · · ·

C. tenuiplicatus · ·

Carcharias sp. · ·

Johnlongia parvidens ·

Cretodus crassidens · ·

C. semiplicatus · · · ·

Cretolamna appendiculata · · · · ·

Cretoxyrhina mantelli · · · · ·

Rhinobatos incertus · · ·? · ·?

Cretomanta canadensis · · · ·

Table 1. Geographic distribution of elasmobranch taxa described in this report. 1, OLM of the Greenhorn Formation, South Dakota; 2, uppermost Greenhorn Fm. of South Dakota; 3, Lincoln Limestone Member, Greenhorn Limestone of Kansas; 4, late Cenomanian-middle Turonian rocks of Texas; 5, Mancos Shale of Arizona. Occurrences for 3-5 are based on Shimada and Martin 1993, Welton and Farish 1993, Williamson et al. 1993, Cicimurri and Bell 1996, and Cappetta and Case 1999. ? indicates uncertain occurrence of the species.