The Late Cretaceous shark, Ptychodus rugosus, (Ptychodontidae) in the Western Interior Sea Shawn a. hamm

1030 Grimes Drive, Forney, Texas 75126 – sahamm@sbcglobal.net

Specimens from the Smoky Hill Chalk Member of the Niobrara Chalk in western Kansas previously identified as the Late Cretaceous shark Ptychodus anonymus Williston 1900 are re-diagnosed as P. rugosus Dixon 1850 based on comparisons with the type speci-men and other associated tooth sets from the English Chalk. Within the Western Interior Seaway, P. rugosus occurs in Late Coniacian deposits in Kansas and from Santonian deposits in Alabama and Texas. Specimens from the Smoky Hill Chalk are represented by relatively complete tooth sets from individuals of varying ontogenetic stages.

Key words. Niobrara Chalk, Kansas, elasmobranch, paleoecology, biostratigraphy

TranSacTionS of The KanSaS academy of Science

Vol. 113, no. 1/2 p. 44-55 (2010)

inTroducTion

Ptychodontid sharks are an enigmatic group of Cretaceous selachians that posses a variety of distinctive and varied tooth morphologies. The genus is known primarily by isolated teeth from nearly every continent (Cappetta, 1987); however, a number of associated tooth sets from various species are known (Williston, 1900; Dibley, 1911; Woodward, 1911; Canavari, 1916; MacLeod, 1982; Shimada, et al., 2009). MacLeod (1982) described an articulated, associated upper and lower dentitions, isolated vertebral centra, and denticles of Ptychodus rugosus (SMU 69001-69018) from the Austin Chalk in Dallas, Texas. Until now, Macleod’s (1982) report documented the only specimen of P. rugosus from the Western Interior Seaway. This specimen is important as it illustrates the dental pattern, arrangement, and the articulation pattern of each tooth file within the upper and lower dentitions.

Institutional abbreviations: AMNH - American Museum of Natural History; BMNH - British Museum of Natural History, London, England; CSU - Columbus State University, Columbus, Georgia; FHSM - Fort Hays State University, Sternberg Museum of Natural History, Hays, Kansas; SMU - Southern Methodist University, Shuler Museum of Paleontology, Dallas, Texas.

SySTemaTic PaleonTology

Class Chondrichthyes Huxley, 1880Subclass Elasmobranchii Bonaparte, 1838

Cohort EuselachiiOrder incertae sedis

Family Ptychodontidae Jaekel, 1898Genus Ptychodus Agassiz, 1835Ptychodus rugosus Dixon 1850

(Figs. 1-7)

Type specimen. BMNH 39793 from the Zone of Micraster cor-testidinarium (Late Coniacian) of the English Chalk (Dibley, 1911; Woodward, 1911).

Diagnosis. Lower medial file teeth quadrate to rectangular in shape with a highly elevated and rounded tooth cusp; apex of the cusp crossed by no more than six wavy, discontinuous transverse ridges; ridges terminate sharply at a shelf-like marginal area; margin covered with concentrically oriented and rugose, enameloid ridges. Lateral file teeth distally inclined, less elevated with greatly rounded tooth cusps exhibiting identical marginal ornamentation in the medial file; posterior teeth nearly flat with crenulated ornamentation. Material. AMNH 19953 (partial upper and lower dentitions); CSU-K-07-05, 1, 2 (2 isolated teeth); FHSM VP-368 (single tooth);

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FHSM VP-2073 (associated tooth set); FHSM VP-2222 (upper dentition); FHSM VP-2223 (lower dentition); FHSM VP-14854 (associated teeth and denticles); KUVP 55248 (single tooth); SMU 69001 (lower dentition); SMU 69002 (upper dentition); SMU 69003 (denticles); SMU 69007-69014 (vertebrae). Taxonomic noTeS

Dixon (1850, p. 362, Pl. XXI, fig. 5) based Ptychodus rugosus on an isolated tooth from the County of Kent England; however he did not refer this tooth to a specific catalog number. Woodward (1911) also noted the type was based on a single tooth and then described an associated tooth set (BMNH 39793). A recent search in the old records of the BMNH collections yielded a single tooth labeled 39793 (Fig. 1, A-D) that is identical to the tooth figured by Dixon (1850). Included with this tooth is the associated tooth set described by Woodward (1911). However, none of the teeth figured by Woodward (1911) included the type specimen. Therefore the single tooth described and figured by Dixon (1850) is the type for the species and Woodward’s (1911) material with the same catalog number are here considered paratypes (Fig. 1, E). Teeth previously assigned to “P. anonymus” (Williston, 1900; Stewart, 1990; Everhart and Caggiano, 2004; Everhart, et al., 2003) from the Niobrara Chalk are morphologically identical to teeth in the type and paratype specimens of P. rugosus (BMNH 39793) from the English Chalk, and are also comparable to SMU 69001(lower dentition) and 69002 (upper dentition) of P. rugosus from the Austin Chalk in Dallas, Texas.

Williston (1900, p. 241) indicates that his type specimens of P. anonymous come from “Walnut Creek”… “The horizon is probably Benton” (e.g. the Fort Benton, including the Greenhorn and Carlile formations). Everhart and Caggiano (2004, p. 132) speculated that the Walnut Creek locality was in north central

Kansas, but another Walnut Creek is located in northern Rush County, Kansas and exposes the upper Greenhorn Formation and Fairport Chalk Member of the Carlile Shale (Middle Cenomanian-Middle Turonian). In either case, Williston’s type specimens of P. anonymus are not from the Niobrara, although he noted (p. 241) that “Other specimens agreeing in these characters are from the Niobrara” and reports have since reflected his observation (Stewart, 1990; Everhart, 2005; Everhart, et al., 2003; Shimada and Fielitz, 2006).

Ptychodus anonymus is still a valid taxon and differs greatly from P. rugosus in tooth morphology and stratigraphic range. Ptychodus anonymus is found in Middle Cenomanian through Middle Turonian deposits, whereas P. rugosus is known only from Late Coniacian through Late Santonian deposits. Ptychodus anonymus can easily be distinguished from P. rugosus by 1) having clearly defined and spaced transverse ridges that extend across the entire length of the crown, 2) greater number of transverse ridges in teeth from distal tooth files, 3) finer and granular marginal area, 4) having a much smaller tooth crown height to tooth height ratios and 5) smaller crown width to tooth width ratios, (see Hamm, 2008). Based on these criteria, I herein reassign the material from the Niobrara Chalk in Kansas to P. rugosus.

Dibley (1911, p. 72) noted that “teeth of the same form [P. rugosus] from the Niobrara Chalk of the western states of North America were named P. whipplei” by Marcou 1858. Lucas and Johnson (2002, p. 49) and Lucas (2006, p. 157, fig. 1) figured the type specimen of P. whipplei (BMNH P. 15705) and reported that the type stratum is within the El Vado Member of the Mancos Shale (Early Coniacian). Ptychodus whipplei is distinguished from P. rugosus by having a narrower cusp with distinct transverse ridges, a shorter marginal area, and lateral tooth files maintaining a narrow, elevated and distally inclined tooth crowns.

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Case et al., (1990) reported the occurrence of juvenile or embryonic anterolateral teeth of P. rugosus from the Niobrara Formation in Saskatchewan, Canada. However, that record is based on three very small teeth (2 x 3mm in size; SMNH-P496.12, P- 496.11, and P- 496.39) that do not exhibit the specific characteristics of P. rugosus. This material is also Cenomanian in age and not temporally equivalent to the Niobrara. This record is considered invalid.

generalized deScriPTion

The specimens of Ptychodus rugosus analyzed for the purposes of this work include: AMNH 19553, FHSM VP-2222-2223 and SMU 69001-69002, all of which include whole or partial upper and lower dentitions. There are eight to ten individual antero-posterior oriented tooth files on the right and left sides of each jaw, not including the medial file. Individual teeth possess specialized structures for interlocking with adjacent teeth in the same file and with teeth in neighboring tooth

Figure 1. Type specimen, Ptychodus rugosus (BMNH 39973). A, occlusal view; B, labial view; C, lingual view; D, posterior view; E, Para-types, associated teeth. Zone of Micraster cor-testidinarium (Early Coniacian) of the English Chalk.

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files (Macleod, 1982). The posterior tooth side bears a sulcus or groove that articulates with a labial protuberance of the succeeding tooth. Each tooth in a single file is interlocked in the same fashion. Except for the central-most teeth, all other files have teeth in which the distal tooth edges become progressively angled for interlocking between the mesial edges of two teeth of the previous file. Within the same file, some teeth may have a straight distal tooth edge. Throughout the dentition the mesial tooth edge is tucked underneath the distal tooth edge in the proceeding file providing a strong but flexible tooth battery when feeding. The medial file of the upper dentition is much smaller than the first adjacent tooth file (R1/L1) (or para-medial file sensu Patterson, [1966]). They are comprised of small symmetrical teeth that are square in outline and have a small, centrally located “bump” of dentine located in the middle of a flat occlusal

surface. Depending on the ontogenetic stage of a specimen the occlusal bump can be well developed, raised, and surrounded by a margin with ornamentation identical to the other tooth files. Alternatively, it can be poorly developed with a completely smooth margin. The para-medial (R1/L1) files are the largest and most symmetrical teeth in the upper dentition. There are between three and five transverse ridges in the anterolateral tooth files, which become progressively elongate labio-lingually along the commissure of the jaw, with the crown height progressively decreasing laterally and overall tooth shape continuing to extend mesio-distally. The posterior files are differentiated from the anterolateral files by being symmetrical and rectangular in shape with a slightly inflated to completely flat occlusal surface and lacking specific crown features. Except for the teeth of the medial file, which are the largest teeth within the dentition,

Figure 2. Ptychodus rugosus. A, SMU 69002, upper dentition; B, SMU 69001, lower dentition. Austin Chalk (Santonian), Dallas, TX.

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the tooth morphology in the lower dentition is identical to those in the upper dentition. Medial teeth are square in outline (in occlusal view), with a high tooth cusp that is rounded at the apex and bears three to six irregular, wavy and discontinuous transverse ridges. The ridges extend down the mesial and distal edges of the cusp, which terminates sharply at a 90° angle at the margin. The tooth margin is covered with concentric, parallel to wavy enameloid ridges that are oriented perpendicular to the transverse ridges. The labial edge has a pronounced protuberance that overhangs the tooth root and bears very fine, non distinct ornamentation.

The dentitions of Ptychodus rugosus are narrow, elongated and exhibit a row locking tooth configuration and dignathic heterodonty in both upper and lower jaws (Welton and Farish, 1993). The length of each dentition and the tooth size of the lower medial tooth give an indication of ontogenetic stage. Both dentitions in SMU 69001(lower dentition, Fig.2A) and 69002 (upper dentition, Fig. 2B) measure 64 cm in length. FHSM VP-2222 (upper dentition,

Fig. 3A) measures 63.5 cm in length while FHSM VP-2223 (lower dentition, Fig. 3B) measures 64 cm in length. These specimens contain the largest tooth size and dentition lengths observed worldwide and are interpreted as fully mature individuals. Compared to the number of teeth present within each tooth file in these dentitions, the number of preserved teeth in the upper and lower dentitions of AMNH 19553 is incomplete. Although they are glued to a piece of cardboard (Fig. 4); comparison of the tooth file configuration with the other dentitions observed, there are enough teeth preserved with AMNH 19553 to extrapolate the length of each dentition. Based on tooth size and length of preserved teeth within each tooth file, the estimated length for each dentition to be 21.5 cm and is interpreted as a juvenile.

The implication for having narrow, elongated and pavement-like dentitions is that Ptychodus rugosus had an elongate and narrow snout that may have utilized inertial suction feeding for prey capture (Shimada et al., 2009). Studies

Figure 3. Ptychodus rugosus. A, FHSM VP-2222, upper dentition; B, FHSM VP-2223, lower dentition. Smoky Hill Chalk (Late Coniacian), Gove Co., KS.

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on modern elasmobranchs utilizing suction feeding indicate that dental changes through ontogeny also indicate change in habitat, food sources, and how prey items are acquired (Lowry and Motta, 2010). This suggests that Ptychodus may have had similar behavioral interactions through ontogeny, as dynamic predators feeding on fishes and ammonites in the open marine realm (Hamm, 2008).

anaTomical noTeS

Postcranial remains are associated with AMNH 19953 and SMU 69001-69018, including four types of scales and vertebral centra. The scales are labeled Type A, Type B, Type C, and Type D; a convention employed by

MacLeod (1982) and Shimada (1994). Type A scales are interpreted as pharyngeal denticles. They are hourglass shaped and have a smooth polygonal crown and root which possesses several large, oval foramina at the base (Fig. 5A). Type B scales have thin and broad crowns bearing posteriorly pointed projections and an expanded and rhombic shaped root (Fig. 5B). There are five to six raised parallel ridges that run from the anterior to posterior edges of the crown. This type of scale is interpreted as a drag reducing scale like those found on the ventral and dorsal sides of the body (Bechert et al., 1986; Raschi and Tabit, 1992) of fast-swimming sharks. These scales have a close morphology similar to the Type B

Figure 4. Ptychodus rugosus. A, AMNH 19953, upper dentition; B, AMNH 19553, lower dentition. Smoky Hill Chalk (Late Coniacian), Gove Co., KS.

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scales of Cretoxyrhina (Shimada, 1994, p. 64, fig. 9) and to the extant requiem shark genus Carcharhinus (Raschi and Tabit, 1992), suggesting that P. rugosus was capable of fast swimming. Type C scales have a large, broad and smooth crown, with a slightly rounded anterior projection (Fig. 5C). The crown tapers down to an elongated root that bears numerous foramina on all sides. Reif (1978) noted that this type of scale, located on the snout and anterior fin edge are always smooth in fast-swimming sharks. Type D scales are similar to Type C scales except they have an extremely elongate and pointed anterior projection (Fig. 5D).

Vertebrae. Two specimens of P. rugosus from the Western Interior have vertebral centra associated with dental remains: SMU- 69007-69014 with seven centra (Fig. 6A) and AMNH 19553 preserves five 5 articulated centra (Fig. 6B). Unfortunately, the vertebral columns are incomplete and the total number of centra for the species is unknown. It is uncertain as to where within what the column the vertebrae were located. The centra in AMNH 19553 are taxonomically distorted; however of the centra associated with SMU 69001-69002 are not. Centra are circular (not dorso-ventrally compressed); and have insertions for neural and haemal arches. The

Figure 5. Ptychodus rugosus dermal denticles. A. Type A denticles; B. Type B denticles; C. Type C denticles; D. Type D denticles. AMNH 19953 from the Smoky Hill Chalk (Late Coniacian), Gove Co., KS; SMU 69003 from the Austin Chalk (Santonian), Dallas, TX.

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Figure 6. Ptychodus rugosus vertebral centra. A, SMU 69004-69017, Austin Chalk (Santonian), Dallas, TX; B, AMNH 19953, Smoky Hill Chalk (Late Coniacian), Gove Co., KS.

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vertebral morphology in these specimens is similar to the Cretaceous lamniform sharks Cretoxyrhina and Squalicorax (Shimada, 1994; Shimada and Cicimurri, 2005), but differs from these lamniforms in having numerous parallel lamellae vertebrae oriented perpendicular to the concentric lamellae. The presence of circular vertebra with in P. rugosus confirms a spindle shaped body. Extant pelagic sharks with fusiform bodies like Carcharodon, Isurus and Carcharhinus have circular centra, whereas true benthic sharks like Squatina and Myliobatis, have dorso-ventrally flattened centra (Applegate, 1967; Compagno, 1977). STraTigraPhic and geograPhical diSTribuTion

In North America, Ptychodus rugosus is known from the Late Coniacian in Kansas and the Early Santonian in Texas and the Late Santonian/Early Campanian in Alabama (Western Interior Seaway and Mississippi Embayment). Current data indicates that these records and are contemporaneous with records from the English Chalk (Dibley, 1911; Woodward, 1911).

According to locality data, all of the Kansas specimens (FHSM VP-2222-2223, FHSM VP-2073, FHSM VP-14854 and AMNH 19553) are from the lower part of the Smoky Hill Chalk (Late Coniacian) which contains Hattin’s (1982) Marker Units 1-4 and Stewart’s (1990) zone of Protosphyraena perniciosa. Unfortunately, no matrix is associated with these specimens to process for foraminifera or coccolithophores in order to determine more precise stratigraphic occurrences. Watkins et al. (1990) noted the coccolith Micula decussata at the base of the Fort Hays Limestone and upper 10 m of the Smoky Hill Chalk. Blair and Watkins (2009) took samples of nannofossils at 10 cm intervals, for 5.0 m above and below the Coniacian/Santonian boundary from the Smoky Hill Chalk. This is outside the stratigraphic range for the specimens discussed herein; however, they also note the presence

of M. decussate in the Late Coniacian of the Smoky Hill Chalk. Additional matrix samples are needed from the localities of the specimens collected and processed for nannofossil data for better stratigraphic resolution with the Smoky Hill Chalk. Only one specimen of P. rugosus (SMU 69001-69018) has been reported from Texas (MacLeod, 1982). Planktonic foraminifera samples recovered with this specimen place it within the Marginotruncana concavata subzone (Early Santonian) of the Globotruncana builloides Zone (Pessagno, 1967) of the Austin Chalk in Dallas, Texas (MacLeod, 1982). Two isolated teeth of P. rugosus (CSU-K-07-05, 1, 2) were recently recovered from a site on private property along a small branch of Hatchecubbee Creek in Russell County, Alabama (Fig. 7). They were found within a lag deposit at the base of the Blufftown Formation (Late Santonian-Early Campanian) and were collected between Sissingh’s (1977) Zones 17 and 18, which contains the foraminifera Calculites ovalis and C. obscuris, respectively. Other material collected within this zone includes Exogyra ponderosa, Squalicorax yangaensis, Scapanorhynchus texanus, numerous small Enchodus sp. palatines, and Anomoeodus phaseolus (D. Schwimmer pers. comm., 2010). These two teeth represent the first occurrence of P. rugosus in Alabama.

concluSionS

Specimens of Ptychodus anonymus from the Smoky Hill Chalk Member of the Niobrara Chalk are re-diagnosed as P. rugosus based on comparisons with the type specimen English Chalk. They are represented by relatively complete tooth sets exhibiting specific dental morphologies between the upper and lower dentitions from individuals of varying ontogenetic stages. Post cranial remains indicate that P. rugosus had a spindle shaped

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body and may have been capable of high mobility. Each specimen of P. rugosus has been found within precise stratigraphic boundaries from Late Coniacian deposits in Kansas and from Santonian deposits in Alabama and Texas, which are contemporaneous with its occurrence in European deposits.

acKnowledgmenTS

This paper was part of my M.S. thesis presented to the Department of Geoscience graduate faculty of the University of Texas at Dallas. I would like to thank the following

for allowing specimen loans to specimens in their care: Ivy Rutzky and John Maisey (AMNH); R.J. Zakrzewski (FHSM) and Dale Winkler (SMU). The Alabama teeth were collected by G. Dent Williams, and I thank David Schwimmer (CSU) for allowing me to examine and photograph those specimens. I also thank Zerina Johanson (BMNH) for specific information and photographs of the type specimen of P. rugosus. The comments and suggestions of D. J. Cicimurri (Clemson University) and M. J. Everhart (Sternberg Museum) are greatly appreciated and improved an earlier version of this manuscript.

Figure 7. Ptychodus rugosus. A, B. CSU K-07-05 1, lower medial/upper para-medial tooth; A. apical view, B. distal view. C, D. CSU K-07-05 2, para-medial (R1); C. apical view; D. distal view. Teeth from the Blufftown Formation (Late Santonian-Early Campanian), Russell Co., Alabama.

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