The Late Cretaceous Shark, Ptychodus rugosus, (Ptychodontidae) in the Western Interior Sea

Abstract

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 specimen 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.

Full text

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 identied 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 le 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, 1880
Subclass Elasmobranchii Bonaparte, 1838
Cohort Euselachii
Order incertae sedis
Family Ptychodontidae Jaekel, 1898
Genus Ptychodus Agassiz, 1835
Ptychodus 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 le 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 le teeth distally inclined, less
elevated with greatly rounded tooth cusps
exhibiting identical marginal ornamentation in
the medial le; posterior teeth nearly at 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);

Transactions of the Kansas Academy of Science 113(1/2), 2010 45
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, g. 5) based
Ptychodus rugosus on an isolated tooth from
the County of Kent England; however he did
not refer this tooth to a specic 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
gured by Dixon (1850). Included with this
tooth is the associated tooth set described by
Woodward (1911). However, none of the teeth
gured by Woodward (1911) included the type
specimen. Therefore the single tooth described
and gured 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 reected 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 dened and
spaced transverse ridges that extend across the
entire length of the crown, 2) greater number
of transverse ridges in teeth from distal tooth
les, 3) ner 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, g. 1) gured 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 les
maintaining a narrow, elevated and distally
inclined tooth crowns.

46 Hamm
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 specic
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 les on the right and left sides
of each jaw, not including the medial le.
Individual teeth possess specialized structures
for interlocking with adjacent teeth in the
same le 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.

Transactions of the Kansas Academy of Science 113(1/2), 2010 47
les (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 le is interlocked in
the same fashion. Except for the central-most
teeth, all other les have teeth in which the
distal tooth edges become progressively angled
for interlocking between the mesial edges of
two teeth of the previous le. Within the same
le, 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 le providing a strong but
exible tooth battery when feeding.
The medial le of the upper dentition is much
smaller than the rst adjacent tooth le (R1/
L1) (or para-medial le 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 at 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
les. Alternatively, it can be poorly developed
with a completely smooth margin. The para-
medial (R1/L1) les are the largest and most
symmetrical teeth in the upper dentition. There
are between three and ve transverse ridges
in the anterolateral tooth les, 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 les are differentiated
from the anterolateral les by being
symmetrical and rectangular in shape with a
slightly inated to completely at occlusal
surface and lacking specic crown features.
Except for the teeth of the medial le, 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.

48 Hamm
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 ne, non distinct ornamentation.
The dentitions of Ptychodus rugosus are
narrow, elongated and exhibit a row locking
tooth conguration 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 le
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 le conguration
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 le, 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.

Transactions of the Kansas Academy of Science 113(1/2), 2010 49
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 shes 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 ve 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.

50 Hamm
scales of Cretoxyrhina (Shimada, 1994, p.
64, g. 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
n 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 ve 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.

Transactions of the Kansas Academy of Science 113(1/2), 2010 51
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.

52 Hamm
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 conrms
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 attened
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 rst 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 specic
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

Transactions of the Kansas Academy of Science 113(1/2), 2010 53
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
specic 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.

54 Hamm
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