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A new clade of putative plankton-feeding sharks from
the Upper Cretaceous of Russia and the United States
Kenshu Shimadaab, Evgeny V. Popovc, Mikael Siverssond, Bruce J. Weltone & Douglas J. Longfg
a Department of Environmental Science and Studies and Department of Biological Sciences,
DePaul University, 2325 North Clifton Avenue, Chicago, Illinois 60614, U.S.A.,
b Sternberg Museum of Natural History, Fort Hays State University, Hays, Kansas 67601,
U.S.A.;
c Department of Paleontology, Saratov State University, 83 Astrakhanskaya Street, Saratov
410012, Russia,
d Department of Earth and Planetary Sciences, Western Australian Museum, 49 Kew Street,
Welshpool, Western Australia 6106,
e New Mexico Museum of Natural History and Science, 1801 Mountain Road NW,
Albuquerque, New Mexico 87104, U.S.A.,
f Department of Biology, St. Mary's College, 1928 Saint Mary's Road, Moraga, California
94575, U.S.A.,
g Department of Ichthyology, Institute for Biodiversity Science and Sustainability, California
Academy of Sciences, 55 Music Concourse Drive, Golden Gate Park, San Francisco, California
94118, U.S.A.
Published online: 28 Aug 2015.
To cite this article: Kenshu Shimada, Evgeny V. Popov, Mikael Siversson, Bruce J. Welton & Douglas J. Long (2015): A new
clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States, Journal of Vertebrate
Paleontology, DOI: 10.1080/02724634.2015.981335
To link to this article: http://dx.doi.org/10.1080/02724634.2015.981335
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ARTICLE
A NEW CLADE OF PUTATIVE PLANKTON-FEEDING SHARKS FROM THE UPPER
CRETACEOUS OF RUSSIA AND THE UNITED STATES
KENSHU SHIMADA,*
,1,2
EVGENY V. POPOV,
3
MIKAEL SIVERSSON,
4
BRUCE J. WELTON,
5
and DOUGLAS J. LONG
6,7
1
Department of Environmental Science and Studies and Department of Biological Sciences, DePaul University, 2325 North Clifton
Avenue, Chicago, Illinois 60614, U.S.A., kshimada@depaul.edu;
2
Sternberg Museum of Natural History, Fort Hays State University, Hays, Kansas 67601, U.S.A.;
3
Department of Paleontology, Saratov State University, 83 Astrakhanskaya Street, Saratov 410012, Russia, popovev@bmail.ru;
4
Department of Earth and Planetary Sciences, Western Australian Museum, 49 Kew Street, Welshpool, Western Australia 6106,
mikael.siversson@museum.wa.gov.au;
5
New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104, U.S.A.,
weltonbj@comcast.net;
6
Department of Biology, St. Mary’s College, 1928 Saint Mary’s Road, Moraga, California 94575, U.S.A., dlong@stmarys-ca.edu;
7
Department of Ichthyology, Institute for Biodiversity Science and Sustainability, California Academy of Sciences, 55 Music
Concourse Drive, Golden Gate Park, San Francisco, California 94118, U.S.A.
ABSTRACT—Eorhincodon casei from Russia and Megachasma comanchensis from the United States are two Cretaceous
taxa initially described as putative planktivorous elasmobranchs, but the type specimens of these two taxa were subsequently
reinterpreted to represent taphonomically abraded teeth of an odontaspidid, Johnlongia Siverson (Lamniformes:
Odontaspididae). Here, we redescribe the type materials of ‘E. casei’ and ‘M. comanchensis’ and describe additional
specimens of these species from other Late Cretaceous localities in Russia and the United States. These specimens
demonstrate that (1) the two fossil taxa are valid species; (2) they warrant the establishment of a new genus of presumed
planktivorous sharks, Pseudomegachasma, gen. nov., to accommodate the two species; and (3) the new genus is sister to
Johnlongia and together constitute a new subfamily Johnlonginae, subfam. nov., tentatively placed in the family
Odontaspididae sensu stricto. This taxonomic placement indicates that the putative planktivorous clade was derived from a
presumed piscivorous form (Johnlongia), with an implication that Pseudomegachasma, gen. nov., evolved a plankton-eating
habit independent of the four known planktivorous elasmobranch clades (Rhincodontidae, Megachasmidae, Cetorhinidae,
and Mobulidae). It also indicates that planktivorous diets evolved independently at least three times in the order
Lamniformes (i.e., Megachasmidae, Cetorhinidae, and Odontaspididae), and more significantly, Pseudomegachasma, gen.
nov., would represent the oldest known plankton-feeding elasmobranch in the fossil record. The present fossil record suggests
that Pseudomegachasma, gen. nov., evolved in a relatively shallow-water environment in Russia in the early Cenomanian or
earlier and subsequently migrated to the North American Western Interior Seaway by the mid-Cenomanian.
http://zoobank.org/urn:lsid:zoobank.org:pub:D5D0400FD438-4A95-8301-DD47991572F6
SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP
INTRODUCTION
Megachasma pelagios Taylor, Compagno, and Struhsaker,
1983 (‘megamouth shark’; Lamniformes: Megachasmidae), is a
large (up to ca. 5.5 m total length [TL]) planktivorous shark
(Compagno, 2001). Since the discovery of the extant M. pelagios,
megachasmid teeth have been known from the Cenozoic fossil
record (Cappetta, 2012), although the origin of megachasmids
was suggested to have been rooted sometime in the Mesozoic
(Shirai, 1996; Martin et al., 2002). Subsequently, Shimada (2007)
reported a fossil shark from the Cretaceous of Colorado, U.S.A.,
which he attributed to a new megachasmid, M. comanchensis
Shimada, 2007. However, because of the stratigraphic gap of fos-
sil megachasmids between the mid-Cenomanian and late Paleo-
gene, some workers expressed their skepticism of this claim (e.g.,
De Schutter, 2009; Maisey, 2012). Recently, Cappetta (2012:201,
252) explicitly dismissed Shimada’s (2007) proposition by
suggesting that the type specimens of M. comanchensis, along
with allegedly the oldest whale shark, Eorhincodon casei Nessov,
1999, from the early Cenomanian of Russia, represent “rolled [D
taphonomically abraded] teeth” of the odontaspidid shark John-
longia Siverson, 1996.
Here, we redescribe ‘Eorhincodon casei’ and ‘Megachasma
comanchensis’ by reexamining previously described materials,
including their type specimens, as well as examining newly col-
lected specimens from Upper Cretaceous deposits in Russia
(Fig. 1) and the United States to demonstrate that the two fossil
taxa are valid species. However, we introduce a new genus to
accommodate the two species (and replace the junior homonym
Eorhincodon) with an interpretation that this megachasmid-like
taxon does not have a direct phylogenetic link to Megachasmi-
dae, represented by the single genus Megachasma. Rather, we
consider the new taxon to represent a separate putative planktiv-
orous shark clade that followed an earlier evolutionary pathway,
convergent on the later evolution of the Megachasmidae. We
also erect a new subfamily to accommodate the planktivorous
clade along with its proposed sister taxon, Johnlongia.
*Corresponding author.
Journal of Vertebrate Paleontology e981335 (13 pages)
Óby the Society of Vertebrate Paleontology
DOI: 10.1080/02724634.2015.981335
Downloaded by [Douglas Long] at 08:07 30 August 2015
Institutional Abbreviations—CNIGRM—Chernyshev’s Cen-
tral Museum of Geological Exploration, St. Petersburg, Russia;
FHSM, Fort Hays State University, Sternberg Museum of Natu-
ral History, Hays, Kansas, U.S.A.; NMMNH, New Mexico
Museum of Natural History and Science, Albuquerque, New
Mexico, U.S.A.; SSU, Saratov State University, Saratov, Russia;
ZIN PC, paleontological collection, Zoological Institute of Rus-
sian Academy of Sciences, St. Petersburg, Russia.
SYSTEMATIC PALEONTOLOGY
Order LAMNIFORMES Berg, 1958
Family ODONTASPIDIDAE M€
uller and Henle, 1839
Subfamily JOHNLONGINAE, subfam. nov.
Type Genus—Johnlongia Siverson, 1996.
Included Genera—Johnlongia Siverson, 1996 (Fig. 2), and
Pseudomegachasma, gen. nov. (Figs. 3, 4).
Etymology—Named for the genus Johnlongia that typifies this
group.
Diagnosis—Lamnoids possessing anterior teeth with strongly
lingually curved cusp; extremely tall lingual protuberance of root;
exceptionally prominent nutritive groove that deeply bisects root;
and one or more enlarged, lateral root foramina immediately lin-
gual to mesial and distal extremities of tooth neck.
Remarks—Johnlongia and Pseudomegachasma, gen. nov., dif-
fer significantly from all other described Odontaspis-like genera
by (1) their Megachasma-like anterior teeth (particularly evident
in Pseudomegachasma, gen. nov.) with an extremely large lingual
root protuberance; (2) the lack of one or both lateral cusplets in
some large anterior teeth (presumably from adult individuals) of
species in both genera (Siverson 1996:pl. 5, fig. 1; Fig. 4Y); and
(3) the greatly enlarged lateral foramina on the lingual side of
the root (Figs. 2K, 3AB) even in commissural teeth (Siverson
1996:pl. 5, fig. 14). Although Cenocarcharias Cappetta and Case,
1999, exhibits similar foramina (Cappetta, 2012:fig. 184E), they
are not as large as those on most teeth of Johnlongia and Pseudo-
megachasma, gen. nov. The unique dental features of Johnlongia
and Pseudomegachasma, gen. nov., warrant taxonomic distinc-
tion and justify the establishment of a new suprageneric taxon.
The decision to establish a new subfamily rather than a new fam-
ily was influenced by the lack of more complete remains (e.g.,
associated teeth and/or skeletal remains).
The family Odontaspididae sensu lato traditionally comprises
two extant genera, Carcharias Rafinesque, 1810, and Odontaspis
Agassiz, 1838 (e.g., Compagno, 1984), that are often collectively
referred to as ‘sandtiger sharks.’ However, recent morphological
(Compagno, 1990; Shimada, 2005) and molecular (Martin et al.,
2002; Heinicke et al., 2009; V
elez-Zuazo and Agnarsson, 2011;
Naylor et al., 2012) studies have suggested that ‘Odontaspididae’
is likely non-monophyletic; thus, the genera Carcharias and
Odontaspis may be better assigned to two separate families, the
Carchariidae M€
uller and Henle, 1838, and Odontaspididae sensu
stricto, respectively. There are also numerous tooth-based fossil
taxa classified into Odontaspididae sensu lato (e.g., Cappetta,
2012, recognizes 16 extinct odontaspidid genera), but given the
difficulties in deciphering even the phylogeny of extant lamni-
forms where whole specimens and molecular samples are at
hand, it is probably naive to assume that teeth of all Cretaceous
‘sandtiger sharks’ are referable to this single family in a strict
sense. It is quite possible that many of the early ‘sandtiger’ forms,
including Johnlongia and Pseudomegachasma, gen. nov., are
related rather distantly to the Carchariidae and/or Odontaspidi-
dae sensu stricto. We tentatively assign Johnlonginae, subfam.
nov., to the Odontaspididae sensu stricto rather than to the
Carchariidae based on the comparatively tall and upright main
cusp in distally situated teeth of Johnlongia (e.g., Siverson 1996:
pl. 5, figs. 14, 15; for dentition of extant Carcharias and
Odontaspis, see Compagno, 1984:217, 220, 221) and the large rel-
ative size of the lateral cusplets in teeth from young individuals
of this extinct genus (e.g., Siverson, 1996:pl. 5, fig. 4; note that
lateral cusplets are highly reduced or absent in very young free-
swimming extant Carcharias: see Bass et al., 1975). In extant
Carcharias, distally located teeth typically have a very low,
FIGURE 1. Geographic and stratigraphic positions of ‘odontaspidid’
shark remains from Russia described in this paper. A, Cenomanian
paleogeographic map (after Sobolevskaya, 1951) showing land masses
(dotted area) and sea (white space) with discussed fossil localities (1,
Lebedinskij quarry locality; 2, Melovatka-5 locality; 3, Bezobrazovka-1
locality; 4, Bagaevka locality; 5, Saratov-1 locality; 6, Saratov-2 locality;
7, Kikino locality; 8, Mochaleika locality; see Supplementary Data
Appendix S1 for detailed stratigraphic information); B, stratigraphic sec-
tions at selected localities indicating odontaspidid-bearing horizons
(Fig. 1 A).
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mesiodistally elongated crown and are densely spaced to collec-
tively form a pavement-like surface (e.g., Cunningham, 2000:pl.
10, fig. 1). There is no indication that Johnlongia and Pseudome-
gachasma, gen. nov., had crushing-type teeth. In the absence of
associated skeletal and dental remains, we take a conservative
approach by not establishing a new taxon above subfamily level
for Johnlongia and Pseudomegachasma, gen. nov.
Genus PSEUDOMEGACHASMA, gen. nov.
Type Species and Type Locality—Eorhincodon casei Nessov,
1999, from Polpino Formation (lower Cenomanian) of Lebedin-
skij quarry, Belgorod Province, Russia (see below for detail; ‘1’
in Fig. 1).
Other Included Species—Megachasma comanchensis Shimada,
2007, from basal Greenhorn Limestone (middle Cenomanian) in
southeastern Colorado, U.S.A. (see Shimada et al., 2006; Gallardo
et al., 2013) (see below for detail).
Etymology—Named for its superficial resemblance to the
megamouth shark in tooth morphology: pseudo (Greek ‘pseudes’)
Dfalse; Megachasma Dgenus name of megamouth shark.
Diagnosis—Small lamnoid teeth (known specimens no more
than 9 mm in total tooth height) consisting of tall crown (erect
or inclined), well-marked tooth neck on lingual side, and massive
root, along with the following combination of characters; crown
with sharply pointed apex and sprawling base; crown apex
strongly directed lingually, giving hook-like appearance, but
crown apex may be flexed occlusally; mesial and distal cutting
edges on crown weak and blunt, and may be absent from crown
base to mid-portion of crown; crown surfaces smooth; lingual
crown face strongly convex and labial face moderately convex;
basal margin of crown gently convex or concave on labial face
and broadly and deeply concave on lingual face; labiolingual
length and mesiodistal length at crown base about equal; short
sharp or blunt lateral cusplet, or mesiodistally oriented low heel,
may be present at mesial and distal extremities, or on one side,
of crown base; root massive and apicobasally short, with deep
nutritive groove that bisects pronounced lingual protuberance
and may possess one or more prominent nutritive pores near
center of groove; apical face of lingual protuberance flat or
weakly convex; one or more lateral root foramina immediately
lingual to mesial and distal extremities of tooth neck; and labio-
mesial corner of root labially pointing and labiodistal corner less
pointy than labiomesial corner.
Remarks—The genus name Eorhincodon was preoccupied by
another shark taxon erected by Li (1995) in a separate paleonto-
logical context. Li’s (1995) specimen of Eorhincodon is now
interpreted to be a carcharhiniform tooth (Cappetta, 2006:303,
2012:299), but the genus name remains unavailable. Thus, a new
genus name (Pseudomegachasma, gen. nov.) is needed to accom-
modate Nessov’s (1999) ‘E. casei’ and Shimada’s (2007) ‘M.
comanchensis.’
PSEUDOMEGACHASMA CASEI (Nessov, 1999), comb.
nov.
(Fig. 3)
Rhincodontidae: Nessov, Mertinene, Golovneva, Potapova,
Sablin, Abramov, Bugaenko, Nalbandyan, and Nazarkin,
1988:126, fig. 1.10.
Eorhincodon casei: Nessov, 1997:pl. 1, fig. 14a, b (nomen
nudum).
Eorhincodon casei, sp. nov.: Nessov, 1999:101, figs. a–c (original
description).
Type Specimen—CNIGRM 10/12292 (holotype), tooth
(Fig. 3A–I; erroneously referred to as ‘30/12292’ under an old
institutional acronym ‘TSNIGM’ in Nessov, 1999).
Referred Specimens—ZIN PC15/30, tooth (Fig. 3J–O); PC16/
30, tooth (Fig. 3P–T); PC17/30, tooth; PC18/30, tooth; PC19/30,
tooth; PC20/30, tooth; SSU 155/80, tooth (Fig. 3U–X); 155/81,
tooth; 155/82, tooth; 155/83, tooth; 155/84, tooth (Fig. 3Y–AD);
155/85, tooth; 155/86, tooth (Fig. 3AE–AK); 155/87, tooth
(Fig. 3AL–AO); 155/88, tooth; 155/89, tooth (Fig. 3AP–AT);
155/90, tooth; 155/91, tooth (this study).
Ages and Localities—‘Level GLE 20’ (Nessov, 1999) within
Polpino Formation (early Cenomanian), Lebedinskij quarry,
Gubkin town, Belgorod Province, Russia, for type specimen and
ZIN PC15/30–PC20/30 (‘1’ in Fig. 1); lower member (early Cen-
omanian) of Melovatka Formation, ‘Melovatka-5’ locality, Vol-
gograd Province, Russia, for SSU 155/80–155/85 (‘2’ in Fig. 1);
upper member (late Cenomanian) of Melovatka Formation,
‘Bezobrazovka-1’ locality, Kalininsk District, Saratov Province,
Russia, for SSU 155/87 (‘3’ in Fig. 1); basal horizon (middle
Turonian but specimen likely reworked from Cenomanian) of
Bannovka Formation, ‘Bagaevka’ locality, Bagaevka, Saratov
FIGURE 2. Teeth of Johnlongia allocotodon Siverson, 1996, from upper
Cenomanian Melovatka Formation in Saratov Province (‘5’ in Fig. 1),
Russia. A–G, SSU 155/93 in labial (A), lingual (B), mesial (C), distal (D),
apical (E), and basal (F) views plus close-up view of prominent lateral
root foramina on mesial root surface (G; cf. Fig. 2C); H–L, SSU 155/94 in
labial (H), lingual (I), mesial (J), distal (K), and basal (L) views; M–P,
SSU 155/95 (distal cusplet broken) in labial (M), lingual (N), mesial (O),
and apical (P) views. Scale bars equal 5 mm (A–Pexcept for G) and
1mm(G).
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District, Saratov Province, Russia for SSU 155/89 (‘4’ in Fig. 1);
upper member (late Cenomanian) of Melovatka Formation,
‘Saratov-1’ locality, Saratov District, Saratov Province, Russia,
for SSU 155/86 (‘5’ in Fig. 1); upper member (late Cenomanian)
of Melovatka Formation, ‘Saratov-2’ locality, Saratov District,
Saratov Province, Russia, for SSU 155/88 (‘6’ in Fig. 1); basal
horizon (early Santonian but specimen likely reworked from
Cenomanian) of Kirsanov Formation, ‘Kikino’ locality,
Kamenka District, Penza Province, Russia, for SSU 155/90 (‘7’
in Fig. 1); basal horizon (early Santonian but specimen likely
reworked from Cenomanian) of Kirsanov Formation,
‘Mochaleika’ locality, Kamenka District, Penza Province, Rus-
sia, for SSU 155/91 (‘8’ in Fig. 1).
Diagnosis—As for the genus Pseudomegachasma, gen. nov.,
with following combination of characters, including distinguish-
ing characters from P. comanchensis, comb. nov. (see below):
weak mesial and distal cutting edges on crown more prominent
than in P. comanchensis, comb. nov.; smooth labial crown face at
crown base more convex than in P. comanchensis, comb. nov.,
and shows low, blunt longitudinal rise at center of crown base;
and lateral cusplets generally present and sharply pointed.
Description—Specimens of Pseudomegachasma, comb. nov.,
from Russia are represented by 19 teeth from eight different
localities, of which the largest number of specimens (seven
teeth including the holotype) comes from the type locality
(Supplementary Data, Appendix S2). Whereas most speci-
mens come from either early Cenomanian or late Cenoma-
nian deposits, three teeth come from post-Cenomanian
(middle Turonian and early Santonian) deposits. However,
those three teeth show signs of taphonomically induced
rounding and are interpreted to represent reworked fossils
from underlying Cenomanian rocks.
All teeth are small; the tallest tooth is SSU 155/89, measur-
ing 8.1 mm in total tooth height and the crown height is
7.6 mm. The crown is apicobasally high (generally about twice
the crown width). The average crown height is 5.8 mm (range:
4.4–7.6 mm) and average crown width is 2.6 mm (range: 2.0–
3.7 mm) (n D19; Supplementary Data, Appendix S2). The
crown base is moderately broad mesiodistally and narrows
rapidly just above the base, developing apically into a sharp,
narrow cusp. The lateral extensions of the crown base gener-
ally come with one pair of lateral cusplets that varies from
narrow, needle-like, lingually recurved forms to blunt, conical
forms, although at least five teeth (and possibly as many as
nine teeth) in the sample lack the distal lateral cusplet and
one tooth lacks the mesial lateral cusplet (Supplementary
Data, Appendix S2). Where a lateral cusplet is absent, the
crown base forms a short rounded shoulder extending basally
onto the root. Where lateral cusplets are present, they may be
either symmetrical or asymmetrical. Individual cusplets are
apicobasally short, and although they are usually distinct, lat-
eral cusplets are not well separated from the base of the main
cusp. Smooth mesial and distal cutting edges of the main cusp
are usually present that extend across the apex and basally,
usually terminating at a point where the crown base flares into
lateral cusplets, but cutting edges are usually not developed
on mesial and distal sides of lateral cusplets. The main cusp is
strongly flexed lingually and its apex is straight or very slightly
recurved labially. The labial crown face is smooth and moder-
ately convex, and the crown base is gently curved and lacks a
basal ledge or groove. The lingual crown face is smooth and
strongly convex. The tooth neck is well developed on the lin-
gual face immediately basal to the shoulders of the crown, but
it does not extend to the crown base on the labial side. The
crowns are symmetrical to strongly asymmetrical with varying
degrees of distal inclination of the main cusp.
The root is proportionally massive in relation to the crown,
although crown length is always greater than root length.
The average root length is 4.2 mm (range: 2.9–6.0 mm), and
the average root width is 3.1 mm (range: 2.0–5.5 mm) (n D
19; Supplementary Data, Appendix S2). Only two specimens
(SSU 155/88 and 155/90) have a root width that exceeds its
root length (Supplementary Data, Appendix S2). Roots are
bilobate, but each lobe is usually exceptionally short. The
mesial and distal root lobes are of nearly equal length in
teeth with an erect main cusp, whereas the mesial root lobe
is longer than the distal root lobe in teeth with a greater
inclination of the main cusp. The lingual root face is devel-
oped into a massive protuberance that also constitutes the
basal surface of the root. The basal surface is rounded to flat
and is generally bisected by a deep nutritive groove that may
be variable in width from a wide prominent groove to a slit-
like narrow groove. The bisection may extend as much as
one-fourth of the way into the root labially from the tip of
the lingual protuberance. Whereas many minute, scattered
foramina are present throughout the root surface, one to a
few prominent lateral root foramina are generally present
immediately lingual to mesial and distal lateral cusplets or
shoulders of the crown on both mesial and distal surfaces of
the root (Fig. 3I).
Remarks—Nessov (1999) erected ‘Eorhincodon casei’ based
primarily on 38 isolated teeth from the lower Cenomanian. Most
of the 38 teeth came from the lower part (0.5–3.5 m from the bot-
tom) of ‘Level GLE 20,’ but at least one tooth came from the
uppermost part of ‘Level GLE 20’ that is 6–7 m above the basal
phosphatic layer of the Polpino Formation and 2–3 m below the
overlying middle Turonian deposit (indicated by thickened por-
tions of ‘Level GLE 20’ in Fig. 1B). Nessov (1999:101) also
reported an additional tooth of ‘E. casei’ from the Cenomanian
of Volgograd Province. The whereabouts of all teeth described
by Nessov (1999) are unknown except the holotype, and they are
considered to be lost.
Nessov (1999) also documented four large complete and three
fragmentary isolated vertebrae from Belgorod Province which
he attributed to his ‘Eorhincodon casei.’ He noted (p. 101) that
the four complete vertebrae, one of which measured more that
9 cm in diameter, came from the lower Cenomanian (level GLE-
20), whereas the three fragmentary vertebrae came from the
FIGURE 3. Teeth of Pseudomegachasma casei, comb. nov. (Nessov, 1999), from Upper Cretaceous of Russia. A–I, CNIGRM 10/12292 (holotype)
from lower Cenomanian Polpino Formation in Belgorod Province (‘1’ in Fig. 1 ) in labial (A), lingual (B), mesial (C), distal (D), basal (E), apical (F),
and oblique (G,H) views plus close-up view of lateral root foramina on mesial root surface (I; cf. Fig. 3H); J–O, ZIN PC15/30 from lower Cenomanian
Polpino Formation in Belgorod Province (‘1’ in Fig. 1) in labial (J), lingual (K), mesial (L), distal (M), basal (N), and apical (O) views; P–T, ZIN
PC16/30 from lower Cenomanian Polpino Formation in Belgorod Province (‘1’ in Fig. 1) in labial (P), mesial (Q), distal (R), basal (S), and apical (T)
views; U–X, SSU 155/80 from lower Cenomanian Melovatka Formation in Volgograd Province (‘2’ in Fig. 1) in labial (U), mesial (V), apical (W), and
basal (X) views; Y–AD, SSU 155/84 from lower Cenomanian Melovatka Formation in Volgograd Province (‘2’ in Fig. 1) in labial (Y), lingual (Z),
mesial (AA), distal (AB), basal (AC), and apical (AD) views; AE–AK, SSU 155/86 from upper Cenomanian Melovatka Formation in Saratov Prov-
ince (‘5’ in Fig. 1) in oblique (AE), labial (AF), lingual (AG), mesial (AH), distal (AI), basal (AJ), and apical (AK) views; AL–AO, SSU 155/87 from
upper Cenomanian Melovatka Formation in Saratov Province (‘3’ in Fig. 1) in labial (AL), mesial (AM), basal (AN), and apical (AO) views; AP–
AT, SSU 155/89 from the middle Turonian Bannovka Formation (reworked from Cenomanian) in Saratov Province (‘4’ in Fig. 1) in labial (AP), lin-
gual (AQ), mesial (AR), basal (AS), and apical (AT) views. Scale bars equal 5 mm (A–AT except for I) and 1 mm (I).
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upper Albian. Like most tooth specimens, the whereabouts of all
Nessov’s (1999) vertebral specimens are unknown, and they are
considered to be lost.
PSEUDOMEGACHASMA COMANCHENSIS (Shimada,
2007), comb. nov.
(Fig. 4)
cf. Johnlongia sp.: Shimada, Schumacher, Parkin, and Palermo,
2006:13, fig. 9.8.
Megachasma comanchensis: Shimada, 2007:513–514, fig. 1 (origi-
nal description).
Megachasma comanchensis Shimada, 2007: Cumbaa, Shimada,
and Cook, 2010: table 1, fig. 4G.
Megachasma comanchensis Shimada, 2007: Gallardo, Shimada,
and Schumacher, 2013:fig. 3K, L.
Type Specimens—FHSM VP-15095 (holotype), tooth
(Fig. 4A–E); VP-15176 (paratype), tooth (Fig. 4F–K) (as
described by Shimada, 2007).
Referred Specimens—FHSM VP-15177, two tooth fragments
(Shimada, 2007); VP-17628, tooth (Fig. 4L–P); VP-17629, tooth
(Gallardo et al., 2013; Fig. 4Q–V); NMMNH P-67453, tooth
(this study; Fig. 4W–AD).
Ages and Localities—Lincoln Limestone Member (middle
Cenomanian) of Greenhorn Limestone, ‘Tobe locality’ (Shimada
et al., 2006) and ‘Table Mesa locality’ (Gallardo et al., 2013),
southeastern Colorado, U.S.A.; Bouldin Member (late
FIGURE 4. Teeth of Pseudomegachasma comanchensis, comb. nov. (Shimada, 2007), from Upper Cretaceous of the United States. A–H, FHSM VP-
15095 (holotype) from basal Lincoln Limestone (late middle–early late Cenomanian) of Greenhorn Limestone in Colorado (see Shimada, 2007) in
labial (A), lingual (B), mesial (C), basal (D), and apical (E) views; F–K, FHSM VP-15176 (paratype) from basal Lincoln Limestone (late middle–early
late Cenomanian) of Greenhorn Limestone in Colorado (see Shimada, 2007) in labial (F), lingual (G), mesial (H), distal (I), basal (J), and apical (K)
views; L–P, FHSM VP-17628 from basal Lincoln Limestone (late middle–early late Cenomanian) of Greenhorn Limestone in Colorado (see Gallardo
et al., 2013) in labial (L), lingual (M), mesial (N), basal (O), and apical (P) views; Q–V, FHSM VP-17629 from basal Lincoln Limestone (late middle–
early late Cenomanian) of Greenhorn Limestone in Colorado (see Gallardo et al., 2013) in labial (Q), lingual (R), mesial (S), distal (T), basal (U),
and apical (V) views; W–AD, NMMNH P-67453 from Bouldin Flags Member of Eagle Ford Formation in Texas in oblique (W), labial (Y), lingual
(Z), mesial (AA), distal (AB), basal (AC), and apical (AD) views plus close-up view of lateral root foramen on mesial root surface (X; cf. Fig. 4W).
Scale bars equal 5 mm (A–AD except X) and 1 mm (X).
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Cenomanian or possibly early Turonian; Kennedy, 1988) of
Eagle Ford Formation, Austin, Texas, U.S.A. (this paper).
Diagnosis—As for the genus Pseudomegachasma,withthe
following combination of characters, including distinguishing
characters from P. casei, comb. nov. (see above): weak mesial
and distal cutting edges on crown even less prominent than
P. casei, comb. nov., and may even be absent at mid-portion
of crown giving circular outline in transverse section; smooth
labial crown face at crown base flatter than P. casei, comb.
nov.; and lateral cusplets generally absent, and if present, less
prominent and less sharply pointed than in P. casei, comb.
nov.
Description—Specimens of Pseudomegachasma comanchen-
sis, comb. nov., from the United States include five complete or
nearly complete teeth from three different localities (note: two
fragmentary teeth listed in Shimada, 2007, are not included
here). Four teeth are from two mid-Cenomanian localities in
southeastern Colorado, including the holotype and paratype
(Shimada, 2007; Gallardo et al., 2013). An additional, previously
undescribed specimen (NMMNH P-67453) comes from the
Bouldin Flags Member (upper Cenomanian or possibly lower
Turonian) of the Eagle Ford Formation of Texas (NMMNH
locality 8855; Fig. 4W–AD) and represents the geologically
youngest specimen of P. comanchensis, comb. nov. It also repre-
sents the largest and most complete specimen of the species.
All five teeth are small, where the tallest tooth (NMMNH P-
67453) measures 8.6 mm in total tooth height and 7.6 mm in
crown height. The crown is apicobasally high (generally about
twice crown width); the average crown height is 4.0 mm (range:
2.0–7.6 mm) and average crown width 3.4 mm (range: 0.7–
4.3 mm) (n D5; Supplementary Data, Appendix S2). The crown
base is moderately broad mesiodistally and narrows rapidly just
above the base, developing apically into a sharp, narrow cusp.
The lateral extensions of the crown base generally come with no
definite lateral cusplets but instead short rounded shoulders
extend basally onto the root, although the labiomesial corner of
the crown base in NMMNH P-67453 shows a low blunt tubercle
that likely represents a vestigial lateral cusplet (e.g., Fig. 4Y).
Based on specimens that preserve the cusp apex, smooth mesial
and distal cutting edges of main cusp are usually present but they
are restricted to its apical one-third. The main cusp is strongly
curved lingually where its apex is straight or is very slightly
recurved labially. The labial crown face is smooth and moder-
ately convex, and the crown base is gently curved and lacks a
basal ledge or groove. The lingual crown face is smooth and
strongly convex. The tooth neck is well developed on the lingual
face particularly immediately basal to the crown shoulders, and
it extends faintly to the crown base on the labial side. The crowns
are symmetrical to moderately asymmetrical with varying
degrees of distal inclination of the main cusp.
The root is proportionally massive in relation to the crown
where the crown length is equal to or greater than the root
length. The average root length is 3.5 mm (range: 2.0–6.5 mm)
and the average root width is 2.5 mm (range: 1.3–5.2 mm);
root length always exceeds root width (n D5; Supplementary
Data, Appendix S2). Roots are bilobate, but each lobe is
exceptionally short, especially the distal one. The lingual root
face is developed into a massive protuberance that also consti-
tutes the basal surface of the root. The basal surface is
rounded to flat and is bisected by a deep nutritive groove that
may vary in width and may show one or two large foramina
within it. The bisection extends about one-fifth to one-fourth
of the way into the root labially from the tip of the lingual
protuberance. Whereas many minute scattered foramina are
present throughout the root surface, one lateral root foramen
is generally present immediately lingual to mesial and distal
shoulders of the crown on both mesial and distal root surfaces
(e.g., Fig. 4X).
DISCUSSION
Teeth of Pseudomegachasma casei, comb. nov., and P. coman-
chensis, comb. nov., are strikingly similar to each other (Figs. 3,
4) and are thus interpreted to be congeneric where both species
are considered taxonomically distinct. We reject Cappetta’s
(2012) proposition that P. comanchensis, comb. nov., and P.
casei, comb. nov., represent abraded and reshaped teeth of John-
longia for the following two reasons. First, the holotype and
many other specimens of P. casei, comb. nov., as well as
NMMNH P-67453 from Texas and at least one additional speci-
men of P. comanchensis, comb. nov., from Colorado (Fig. 4Q–
V) preserve a delicate cusp apex and root surface with no signs
of major pre- or post-depositional sedimentary abrasion. The
interpretation of taphonomic alteration in this instance is incon-
sistent with laboratory simulations and observations of other in
situ instances of sedimentary abrasion on elasmobranch teeth
(Irmis and Elliott, 2006; Becker and Chamberlain, 2012; Boesse-
necker et al., 2014). We also note that a number of specimens of
the Oligocene–Miocene megachasmid (Megachasma applegatei
Shimada, Welton, and Long, 2014), including its holotype, show
slight taphonomic modification from sedimentary abrasion (Shi-
mada et al., 2014: figs. 3–5). The ‘rounding effect’ due to sedi-
mentary abrasion is expressed as slight erosion throughout
different features of each of those teeth (i.e., crown apex, lateral
cusplets, and root lobes all alike), not just confined to root lobes,
and is consistent with similar observations of overall mild abra-
sive wear by Pyenson et al. (2009). Second, Johnlongia teeth
have not been recovered from the Lebedinskij quarry and Melo-
vatka-5 localities (‘1’ and ‘2’ in Fig. 1 and Supplementary Data,
Appendix S2) where P. casei, comb. nov., is represented by mul-
tiple specimens. In addition, the possibility of such megachas-
mid-like teeth representing teeth of Johnlongia from different
tooth positions can be dismissed given that such a tooth form
(i.e., lobeless root) is not known from localities with reasonably
large sample sizes of Johnlongia (e.g., Siverson, 1996; Cappetta,
2012). Therefore, we consider Pseudomegachasma, gen. nov., to
be a valid taxon, and not congeneric with Johnlongia.
The separation between Johnlongia and Pseudomegachasma,
gen. nov., is further substantiated on the basis of quantitative
analysis using dental variables, such as crown height (CH), crown
width (CW), root length (RL), and root width (RW) (Fig. 5A).
Figure 5B shows the relationships between CH/CW ratios and
RL/RW ratios among J. allocotodon (n D8), P. casei, comb. nov.
(n D19), and P. comanchensis, comb. nov. (n D5). Whereas the
plots of P. comanchensis, comb. nov., completely overlap with
those of P. casei, comb. nov., for both ratios, they are noticeably
different from plots of J. allocotodon, although there is partial
overlap in data. Figure 5D and E are box plots showing the dif-
ference in each type of ratio. They indicate that the two species
of Pseudomegachasma, gen. nov. tend to have slender crowns
with narrower roots (i.e., likely due to reduced lateral cusplets)
relative to the teeth of Johnlongia. It should be noted that the
two species of Pseudomegachasma, gen. nov., can be separated
from each other on the basis of CH/RL ratios (Fig. 5C, F). Teeth
of P. comanchensis, comb. nov., tend to have a prominent root
(primarily due to their robust lingual root protuberance) relative
to the crown compared to teeth of P. casei, comb. nov., which
are quantitatively more similar to teeth of Johnlongia despite
the reduced root lobes in P. casei, comb. nov. In summary, the
two genera, Johnlongia and Pseudomegachasma, gen. nov., are
distinguishable morphologically and quantitatively (Fig. 5D, E).
Likewise, our analysis shows that P. casei, comb. nov., and P.
comanchensis, comb. nov., are not only noticeably different on a
morphological basis (see descriptions above) but also on a quan-
titative basis (Fig. 5F).
Nessov (1999) described Pseudomegachasma casei, comb.
nov., under his new genus Eorhincodon, meaning ‘the first whale
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shark,’ because he interpreted the taxon as a putative planktivo-
rous shark within the whale shark family Rhincodontidae (Orec-
tolobiformes). Nessov’s (1999) proposition that his new taxon
represented a planktivore is reasonable, because teeth of all four
known planktivorous elasmobranchs (Rhincodontidae, Mega-
chasmidae, Cetorhinidae, and Mobulidae) display a trend toward
secondary homodonty as a result of reduced tooth sizes, simpli-
fied conical to hook-like main cusp with vestigial or no lateral
cusplets, and reduced root lobes often making the root bulbous
(e.g., see Cappetta, 2012). Whereas P. comanchensis, comb. nov.,
has also been proposed to be a planktivorous shark (Shimada,
2007), Nessov’s (1999) Eorhincodon, however, cannot be placed
in the whale shark clade because teeth of the genus more closely
resemble those of Megachasma pelagios (e.g., see Yabumoto
et al., 1997) than any known rhincodontids (note a pronounced
overhang of the labial crown base and constriction at the tooth
neck in rhincodontid teeth, unlike M. pelagios; e.g., Herman
et al., 1992:pl. 27; Cappetta, 2012:fig. 165). We consider Pseudo-
megachasma, gen. nov., to have no direct phylogenetic affinity
with Rhincodontidae on this basis.
Teeth of Pseudomegachasma, gen. nov., are reminiscent of
those of Megachasma pelagios in exhibiting a hook-like crown,
well-defined tooth neck on the lingual face, and a lobeless root.
Moreover, the morphological variability seen in teeth of P. casei,
comb. nov., and P. comanchensis, comb. nov. (e.g., Figs. 3, 4),
indicates that, like M. pelagios, the dentition of both species of
Pseudomegachasma, gen. nov., can be interpreted to exhibit a
weak monognathic heterodonty. However, Pseudomegachasma,
gen. nov., is distinguished from M. pelagios by an exceptionally
prominent nutritive groove and the presence of lateral root
foramina, much like a Cretaceous, putative odontaspidid, John-
longia (Shimada, 2007; cf. Fig. 2 vs. Figs. 3, 4). Despite the
resemblance between Pseudomegachasma, gen. nov., and M.
pelagios, the similarity is here interpreted to be superficial (i.e.,
convergence; see further discussion below) because the recently
described Oligocene–Miocene megachasmid from the western
United States, M. applegatei, is now considered to be the sister
taxon to M. pelagios that diverged no later than the earliest late
Miocene (Shimada et al., 2014). In addition, there is a 70-Ma gap
between Pseudomegachasma, gen. nov. (mid-Late Cretaceous),
and Megachasma (late Paleogene; Shimada et al., 2014) that has
no fossil record of shark teeth with the megachasmid tooth
pattern.
Teeth of Pseudomegachasma,gen.nov.,andJohnlongia are
similar in possessing a well-defined tooth neck on the lingual
face and an enormous lingual protuberance of the root
marked by an exceptionally deep nutritive groove (Cappetta,
2012; this study). The close morphological resemblance
between the two taxa is exemplified by the fact that Cappetta
(2012) considered ‘Megachasma comanchensis’and
‘Eorhincodon casei’ to be taphonomically altered Johnlongia.
Whereas Johnlongia has been placed in the family Odontaspi-
didae (Siverson, 1996; Cappetta, 2012), possibly incorrectly in
a strict sense (see above for discussion of the family), the close
resemblance between Pseudomegachasma, gen. nov., and
Johnlongia is considered here as evidence that these two
FIGURE 5. Dental measurements and comparisons among teeth of Johnlongia allocotodon from Russia (n D8), teeth of Pseudomegachasma casei,
comb. nov., from Russia (n D19), and teeth of P. comanchensis, comb. nov., from the United States (n D5). A, measured variables using specimen
SSU 155/84 as an example (top, profile view; bottom, apical view; see Supplementary Data Appendix S2 for measurements of each tooth); B, scatter
plots comparing CH/CW ratios and RL/RW ratios among the three taxa; C, scatter plots comparing CH/CW ratios and CH/RL ratios among the three
taxa; D–F, box plot representations of CH/CW ratios (D), RL/RW ratios (E), and CH/RL ratios (F) of the three taxa (box, interquartile range, i.e.,
central 50% data; horizontal line in box, mean value; vertical whiskers, total range of data); asterisks, maximum and minimum outliers. Abbreviations:
CH, crown height; CW, crown width; RL, root length; RW, root width.
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genera shared an immediate common ancestry that forms the
basis of Johnlonginae, subfam. nov. (see above; Fig. 6). This
Pseudomegachasma-Johnlongia sister relationship concomi-
tantly implies that Pseudomegachasma, gen. nov., also belongs
to the Odontaspididae. This taxonomic placement is signifi-
cant because it makes Pseudomegachasma, gen. nov., an odon-
taspidid that putatively evolved a plankton-feeding adaptation
independent of the four known planktivorous elasmobranch
clades (i.e., Rhincodontidae, Megachasmidae, Cetorhinidae,
and Mobulidae). It also indicates that plankton-feeding
evolved at least three times independently in Lamniformes
(i.e., Megachasmidae, Cetorhinidae, and Odontaspididae), and
more significantly, Pseudomegachasma, gen. nov., would rep-
resent the oldest known planktivorous elasmobranch in the
fossil record (Fig. 7; see below for further discussion).
Evidence at hand suggests that the extant Megachasma pela-
gios shares an immediate commonancestrywiththeOligo-
cene–Miocene M. applegatei that is characterized by a crown
with a large, narrow main cusp, well-developed lateral cusp-
lets, and a strongly bilobed root with a prominent lingual pro-
tuberance (Shimada et al., 2014). The fossil record shows that
Odontaspididae sensu lato emerged by 130 Ma (Hauterivian;
Cappetta, 2012), and Johnlongia was in existence by 105 Ma
(early late Albian Toolebuc Formation in Queensland, Aus-
tralia; M.S., personal observation of specimen housed in Kro-
nosaurus Korner Museum, Richmond, Queensland). The
Johnlongia-Pseudomegachasma sister relationship is intriguing
because it suggests that Pseudomegachasma, gen. nov., with
the planktivorous tooth pattern was also derived from the
piscivorous tooth pattern as seen in Johnlongia through reduc-
tion in root lobes, reduction or loss of lateral cusplets, and
morphological simplification of the main cusp. Thus, one can
argue for the convergence of the planktivorous tooth pattern
between M. pelagios and Pseudomegachasma,gen.nov.,in
which the planktivorous tooth pattern evolved from the pisciv-
orous tooth pattern independently in a parallel manner in the
two lamniform clades, Megachasmidae and Odontaspididae,
albeit at different times (Cretaceous vs. Miocene) (Fig. 7). It
should be noted that our notion of ‘planktivory’ here is a
robust one, as demonstrated by the fact that certain extant
‘planktivorous elasmobranchs’ (e.g., Rhincodontidae) can also
feed on sizable nektonic organisms, such as bony fishes and
squid (Compagno, 2001). In fact, it is quite possible that Pseu-
domegachasma, gen. nov., could have been a facultative plank-
tivore where it could have still fed on small nektonic fishes.
The crown apex of some teeth of the taxon (e.g., Fig. 3J, P)
shows slight wear that appears to support this idea.
Shirai (1996:fig. 4) hypothesized that the megachasmid lineage
emerged during the Mesozoic despite the lack of corroborating
fossil evidence, but subsequent molecular studies have placed
the estimated origination time for the megachasmid clade in the
104–90 Ma range, supporting its Mesozoic origin (Martin et al.,
2002:fig. 5; Heinicke et al., 2009:fig. 2, table 1, which also lists
the total range of ‘confidence/credibility interval’ of 139–73 Ma
based on the most inclusive data set). Hence, Shimada et al.’s
(2014) proposition that their Oligocene–Miocene Megachasma
applegatei represents the oldest known species of Megachasma
appears to be incongruent with the molecular-based origination
time estimates for Megachasmidae. However, we here offer a
possible explanation for the perceived discrepancy. Based on the
fact that teeth of M. applegatei are reminiscent of odontaspidid
teeth (Shimada et al., 2014), it is likely that the ancestor of mega-
chasmids possessed teeth much like typical odontaspidid teeth.
Whereas the oldest odontaspidid sharks are known from the
Hauterivian, there are many known mid-Cretaceous species
attributed to the family ‘Odontaspididae’ (Cappetta, 2012).
Because Megachasmidae appears to have close phylogenetic
affinity with Odontaspididae sensu stricto (and Pseudocarcharii-
dae) based on molecular data (Naylor et al., 2012), one possibil-
ity is that the megachasmid ancestor (and the pseudocarchariid
ancestor) with no direct phylogenetic ties to Johnlonginae, sub-
fam. nov., may be nested unrecognized within these mid-Creta-
ceous taxa with the ‘odontaspidid tooth pattern.’ In other words,
where geologically younger M. applegatei is more odontaspidid-
like (i.e., ‘more archaic’) than Pseudomegachasma, gen. nov.,
there is nothing to presuppose that the ‘molecularly defined
Megachasmidae’ in deep time must, or ought to, have had the
‘megachasmid tooth pattern.’ As Maisey et al. (2004:45) stated,
“the absence of apomorphic characters in teeth [i.e., the lack of
the ‘megachasmid tooth pattern’ in this case], rather than the
absence of teeth themselves [i.e., the lack of undisputed mega-
chasmid teeth in the Cretaceous], may prevent us from recogniz-
ing a taxon’s presence [i.e., Megachasmidae] in the fossil record
and lead us to underestimate its first occurrence [i.e., mid-Cen-
ozoic].” Therefore, we contend that the ‘megachasmid tooth
pattern’ present in Pseudomegachasma, gen. nov., has no direct
phylogenetic attribution to the megachasmid clade.
Both Pseudomegachasma casei, comb. nov., and P.
comanchensis, comb. nov., are highly derived forms in achiev-
ing a planktivorous tooth pattern (Fig. 7). However, P. casei,
comb. nov., is slightly more Plesiomorphic than P. coman-
chensis, comb. nov., because the former generally possesses
prominent lateral cusplets, unlike the latter. The Plesiomor-
phic morphology of P. casei, comb. nov., is consistent with
the fact that the earliest material of P. casei, comb. nov.,
occurs in rocks slightly older (early Cenomanian) than rocks
that yield the earliest specimens of P. comanchensis, comb.
nov. (late middle Cenomanian). The fossil record at present
indicates that Johnlongia gave rise initially to P. casei, comb.
nov., at least by the early Cenomanian, and then P. casei,
comb.nov.gaverisetoP. comanchensis, comb. nov., at least
by the late middle Cenomanian (Fig. 6).
FIGURE 6. Stratigraphic ranges of the two species of Pseudomega-
chasma and their phylogenetic relationships with Johnlongia within the
proposed new subfamily Johnlonginae, subfam. nov. Sources of illus-
trated teeth (left, labial view; right, profile view; not to scale): Johnlongia
(SSU 155/93 in this study; cf. Fig. 2 A, C); P. casei, comb. nov. (CNIGRM
10/12292 in this study; cf. Fig. 3A, D); and P. comanchensis, comb. nov.
(Shimada, 2007:fig. 1B, C; see also Fig. 4A, E in this study).
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Pseudomegachasma, gen. nov., is so far represented at 11 Late
Cretaceous localities, eight in Russia and three in the United
States (Supplementary Data, Appendix S2). Their geographic
distribution shows that they are all restricted to the mid-latitudi-
nal zone of the Northern Hemisphere (between 30N and 55N).
More intriguingly, all the North American and Russian localities
are located well into respective epicontinental seas, the sea-cov-
ered Russian Plate and the Western Interior Seaway (Sobolev-
skaya, 1951; Kauffman and Caldwell, 1993). These
epicontinental seas were still relatively shallow in the early and
middle Cenomanian, respectively (e.g., Haq et al., 1987), with
highly productive ecosystems (e.g., Shimada et al., 2006; Kholo-
dov et al., 2007). The fossil record suggests that Pseudomega-
chasma, gen. nov., and more specifically P. casei, comb. nov.,
evolved in the early Cenomanian in a shallow-water environ-
ment in Russia. Whereas the estimated water-depth range in the
Russian region is 100–150 m during the late Albian (Barabosh-
kin and Nikul’shin, 2006), it is estimated to be no more than
50 m at least in parts of the Volga River Basin during the early
Cenomanian, and the shallow-water condition (e.g., 70–80 m; no
more than 200 m) continued through the end of the late Ceno-
manian (Zozyrev, 2006). Pseudomegachasma, gen. nov., eventu-
ally migrated to North America, likely following the warm,
westward circum-global ocean currents through the Tethys Sea-
way and Atlantic Ocean (e.g., see Jacobs et al., 2005). It is note-
worthy that at least some teeth of P. casei, comb. nov., found, or
presumably derived, from Cenomanian deposits (e.g., SSU 155/
91; Fig. 3AP–AT) are somewhat reminiscent of teeth of P.
comanchensis, comb. nov., in exhibiting more rounded cutting
edges and less prominent lateral cusplets relative to teeth of P.
casei, comb. nov., from the early Cenomanian deposits. This
observation suggests that the evolution of ‘comanchensis-grade’
morphology appears to have begun in Russia within the clade of
P. casei, comb. nov.
Pseudomegachasma, gen. nov., represents a putative planktiv-
orous elasmobranch based solely on dental morphology. There
FIGURE 7. Stratigraphic distributions of elasmobranch families that contain planktivorous forms and highlighting Johnlongia and Pseudomega-
chasma, gen. nov., as well as Megachasma applegatei and M. pelagios to show parallel evolution and convergence of ‘megachasmid tooth pattern’ from
‘odontaspidid tooth pattern’ in each clade (*, excludes other odontaspidid taxa outside of this particular clade, i.e., Johnlonginae, subfam. nov.; strati-
graphic data based on Friedman et al., 2010:fig. 3; Cappetta, 2012; Shimada et al., 2014). Sources of illustrated teeth (left, labial view; right, profile
view; not to scale): Johnlongia (SSU 155/93 in this study; cf. Fig. 2 A, C); Pseudomegachasma, gen. nov. (Shimada, 2007:fig. 1B, C; see also Fig. 4A,
E in this study); Megachasma applegatei (Shimada et al., 2014:fig. 3B, C); and M. pelagios (Taylor et al., 1983:fig. 8A, C).
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are some other elasmobranch taxa that exhibit tooth morpholo-
gies indicative of planktivorous diet (see above for dental charac-
terizations of planktivorous elasmobranchs). Suggested
examples include Archaeomanta Herman, 1979, from the Paleo-
gene, Cretomanta Case, Tokaryk, and Baird, 1990, from the
Upper Cretaceous, Nanocetorhinus Underwood and Schl€
ogl,
2013, from the Miocene, and Pseudocetorhinus Duffin, 1998,
from the Upper Triassic (see Cappetta, 2012, and Underwood
and Schl€
ogl, 2013, for illustrations of their teeth; note that
Ginter, 2008, described ‘filter-feeding sharks’ from the Devo-
nian, but his notion of ‘filter-feeding’ is quite different from
planktivory because he stated that those sharks possibly used
their delicate multicuspid teeth like “sieve, preventing minute
organisms from escaping from the buccal cavity before
swallowing” [p. 147]). However, whereas Pseudocetorhinus may
not be a planktivore because many of its teeth are actually broad
mesiodistally with little resemblance to teeth of extant planktivo-
rous elasmobranchs (e.g., see Cappetta, 2012:fig. 320), the exact
systematic positions of these tooth-based taxa are uncertain
(Adnet et al., 2012; Cappetta, 2012; Underwood and Schl€
ogl,
2013). We contend Pseudomegachasma, gen. nov., to be the
oldest planktivorous elasmobranch clade in the fossil record
with the assumption that Pseudocetorhinus is likely not a plankti-
vore (see above) and the fact that the only other possible plank-
tivorous elasmobranch in the pre-Cenozoic fossil record is
Cretomanta, with an oldest known occurrence from the mid-
Cenomanian (e.g., Shimada et al., 2006; Underwood and
Cumbaa, 2010). Nevertheless, the presence of other possible
planktivorous taxa indicates that the evolutionary history of
plankton-feeding elasmobranchs may have been more complex
than our current understanding suggests.
One may wonder as to why the fossil record of Pseudomega-
chasma, gen. nov., is limited, but its scarcity is not necessarily
surprising given that it took over two decades for the discovery
of the first example of Johnlongia outside of North America
(Cappetta, 2012:201) and, more remarkably, over 130 years to
recover the first specimen of Johnlongia from the well-studied
Niobrara Chalk of Kansas (Shimada et al., 2004; Shimada and
Fielitz, 2006). The preferred habitat of Pseudomegachasma, gen.
nov., appears to have been quite specific (middle of epicontinen-
tal seas), and it was geologically relatively short-lived (Cenoma-
nian–?Turonian). The absence of definite early Turonian records
of Pseudomegachasma, gen. nov., is possibly related to the
extreme rarity or absence of preserved shallow-water deposits of
this age (e.g., see Smith et al., 2001). Whereas it is possible that
specimens of Pseudomegachasma, gen. nov., may have been
overlooked or misidentified in existing paleontological collec-
tions, it is likewise reasonable to assert that the genus was a rare
component of marine communities during the Cretaceous.
ACKNOWLEDGMENTS
We thank the late J. H. McLellan who collected NMMNH P-
67453 in 1985, S. G. Lucas and J. Spielman (NMMNH) who
curated the specimen for the purpose of this study, and R. W.
Boessenecker for providing us with useful information. We also
thank A. V. Lapkin (Saratov-Moscow) for kindly donating speci-
mens SSU 155/86 and 155/88 and other relevant samples to the
SSU collection, E. M. Pervushov (SSU) for stratigraphic discus-
sion of Russian localities, A. V. Birukov, F. K. Timirchev (SSU),
and SSU geology students for field assistance at the Melovatka-5
locality during 2011–2012, and A. O. Averianov (ZIN) for pro-
viding access to Albian–Cenomanian shark materials from the
Belgorod Province (Lebedinskij and Stoilenskij quarries),
including the collection of the late L. Nessov. The second
author’s (E.V.P.) portion of this study was supported by the Rus-
sian Fund for Basic Researches (RFBR grant 14-05-00828) and
by the SSU (internal fund granted by vice-rector A. V.
Stalmakhov). We thank G. Guinot, an anonymous reviewer, and
the editors for their constructive comments on the drafts of the
manuscript, which significantly improved this paper.
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Submitted June 26, 2014; revisions received October 2, 2014; accepted
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Handling editor: Charlie Underwood.
Citation for this article: Shimada, K., E. V. Popov, M. Siversson, B. J.
Welton, and D. J. Long. 2015. A new clade of putative plankton-feeding
sharks from the Upper Cretaceous of Russia and the United States. Jour-
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