A nearly complete right mandibular tooth plate of Ischyodus bifurcatus Case (Holocephali: Chimaeroidei) is reported from the Point Loma Formation (upper Campanian) of the Upper Cretaceous Rosario Group in southern California, USA. The individual is estimated to have measured nearly 1 m in total body length. Remains of I. bifurcatus have been reported from marine rocks deposited in epicontinental seas and continental shelf paleoenvironments of temperate latitudes in the Northern Hemisphere. Previous records of the species consist of specimens from Santonian to Maastrichtian strata of the US (Delaware, New Jersey, North Carolina, Georgia, Alabama, Arkansas, Montana, and Wyoming), Sweden, and European Russia. The tooth plate described herein is the first verifiable record of I. bifurcatus from California, and, more significantly, represents the only known definite Mesozoic record of Ischyodus from the entire North Pacific region.
Introduction
Chimaeriformes (so-called ‘ratfishes’ or ‘ghostsharks’) includes holocephalian cartilaginous fishes with a durophagous lifestyle (Stahl, 1999). Extant taxa in this order are represented by three families, six genera, and approximately 50 species (Didier et al., 2012; Weigmann, 2016), including forms that inhabit both deep water (Rhinochimaeridae and Chimaeridae) and nearshore marine environments (Callorhinchidae). In the fossil record, principal anatomical elements used for chimaeroid taxonomy and phylogenetic research are tooth plates, which occur as two pairs in the upper jaw (vomerine and palatine tooth plates) and one pair in the lower jaw (mandibular tooth plates). The oldest chimaeroid fossils occur in upper Triassic strata (Stahl, 1999). The clade reached its maximum diversity in the mid-Cretaceous (Newton, 1878; Popov and Machalski, 2014). The most common chimaeroid genus in the Cretaceous, as well as in the entire Mesozoic, is Ischyodus Egerton, 1843, a taxon ranging from the mid-Jurassic to Miocene and consisting of 39 nominal species (Stahl, 1999; Hoganson and Erickson, 2005). Although exceptionally rare in the fossil record, complete and nearly complete skeletons of Ischyodus (e.g. Stahl, 1999, fig. 136A, B) show that the extinct genus included relatively large forms (at least 40—150 cm in estimated total body length) and was morphologically similar to the extant genus Callorhinchus or ‘elephantfish’ (Popov et al., 2013).
The Late Cretaceous record of Ischyodus consists of eight nominal species (Stahl, 1999; Hoganson and Erickson, 2005). Some species of Ischyodus possess a bifurcated median tritor on their mandibular tooth plates, like I. bifurcatus Case, 1978, from the Santonian to Maastrichtian, and I. rayhaasi Hoganson and Erickson, 2005, and I. yanshini Averianov, 1991, from the Campanian to Maastrichtian (see Appendix 1). The species with a bifurcated median tritor constitute a Late Cretaceous ‘bifurcatus-group’ that is distinct from a group of Ischyodus with a simple non-forked mandibular median tritor, such as I. lonzeensis, I. gubkini, and I. thurmanni from the Late Cretaceous, as well as I. dolloi from the Paleocene. Ischyodus latus, Ischyodus sp., and I. aff. bifurcatus from the Albian to Cenomanian described by Newton (1878), Nessov (1997), and Popov and Machalski (2014), respectively, exhibit a prominent crushing dentition and even stronger ‘bifurcation’ on their mandibular median tritor than that of the ‘bifurcatus-group.’ However, the taxonomic status of these three species awaits reexamination (Popov and Machalski, 2014).
Herein, we report the first verified occurrence of Ischyodus bifurcatus from the Campanian of southern California, USA (Figure 1A). The specimen is a tooth plate housed at the San Diego Natural History Museum (SDNHM) and is catalogued as SDNHM 25417. This specimen is noteworthy because it represents the first record of I. bifurcatus from western North America as well as the first and only known definite Mesozoic record of Ischyodus from the entire North Pacific region.
Geological setting and associated fossils
The chimaeroid tooth plate, SDNHM 25417, was collected from a blue-gray, massive, sandy marine mudstone stratum in the lower part of the Point Loma Formation exposed in Carlsbad, San Diego County (Figure 1B). At Carlsbad, the Point Loma Formation consists of a relatively thin (~23 m) sequence of thickly to thinly bedded, massive to locally flaggy mudstones and siltstones with occasional interbeds and lenses of fine- to mediumgrained, locally cemented sandstone (Sliter, 1968). Generally, the Point Loma Formation in the Carlsbad area unconformably overlies Upper Cretaceous fanglomerates of the Lusardi Formation. However, the formation locally rests nonconformably on Upper Jurassic to Lower Cretaceous metavolcanic rocks of the Santiago Peak Volcanics (Coombs and Deméré, 1996). A high-relief unconformity separates the Point Loma Formation from overlying estuarine strata of the middle to upper Eocene Santiago Formation. Bukry (1994) described nannofossil floras from the Carlsbad section of the Point Loma Formation and reported that the section contains the Zone CC21 to CC22 boundary within the late Campanian (~75 Ma). Sliter (1968) described planktonic and benthonic foraminifers from the Carlsbad Cretaceous section and assigned them to the Globotruncana rosetta Zone, also of late Campanian age. Sliter (1968) also suggested that deposition of the Carlsbad strata occurred on the continental shelf.
Widely dispersed and well preserved fossils of marine invertebrates occur in the Carlsbad section of the Point Loma Formation, including remains of crustaceans (Icriocarcinus xestos), echinoids, cephalopods (Anapachydiscus peninsularis, Pachydiscus catarinae, Anglonautilus catarinae, and Baculites sp.), gastropods (Anchura gibbera and Bernaya sp.), and bivalves (Indogrammatodon sp., Pinna sp., Spondylus rugosus, and Crassatella sp.) (Bishop, 1988; Loch, 1989; Groves, 1990; Squires and Saul, 2001). Vertebrate fossils from the Carlsbad section consist of teeth, otoliths, and axial skeletal elements of actinopterygians, a peripheral plate of a chelonian (SDNHM 31366), and skeletal remains of ornithischian dinosaurs, including an ankylosaurid ankylosaur Aletopelta coombsi (Coombs and Deméré, 1996; Ford and Kirkland, 2001) and unidentified hadrosaurs (Hilton, 2003).
Figure 1.
Geographic location and stratigraphic position of chimaeroid tooth plate described in the present study. A, map of California, USA, showing fossil chimaeroid locality (star symbol); B, simplified stratigraphic column of San Diego area (southern California) showing formations of Upper Cretaceous Rosario Group (Point Loma Formation where fossil chimaeroid was collected highlighted in gray), that overlies metavolcanic Upper Jurassic—Lower Cretaceous Santiago Peak Volcanics, and that unconformably underlies estuarine strata of middle—upper Eocene Santiago Formation.
Systematic paleontology
Besides SDNHM, specimens in the following two institutions are referred to in this paper: the Regional Museum of Physical Geography, Saratov State University (SSU), Russia, and the University of Montpellier (UM), France. Descriptive terms and measurements of chimaeroid tooth plates used in this study (Figure 2) follow Popov (1999, 2003), Popov and Efimov (2012), and Popov and Machalski (2014) by building on the schemes presented by Newton (1878), Ørvig (1985), Patterson (1992), and Stahl (1999). Higher-level taxonomy (superfamily and above) follows Nelson et al. (2012). The family ‘Edaphodontidae’ Owen, 1846, is here considered to represent a collective taxonomic unit in need of revision (see Popov and Beznosov, 2006; Popov and Machalski, 2014). Its composition and relationships with other chimaeroid families require phylogenetic analysis, which is beyond the scope of the present study.
Figure 2.
Right mandibular tooth plate of Ischyodus bifurcatus (SDNHM 25417) from upper Campanian of southern California, USA. A, basal view; B, occlusal view; C, line drawing interpretation of tooth plate in occlusal view (cf. Figure 2B) with identified tritors and type of pleromin fill (dashed lines = restored parts); D, mesial (symphysial) view. Anatomical abbreviations: aot, anterior outer tritor; ibmt, inner branch of median tritor; it, inner tritor(s); mt, median tritor; obmt, outer branch of median tritor; pot, posterior outer tritor; st, symphysial tritor. Measurement abbreviations: L, maximum mesio-distal length of tooth plate; Km, distance between symphysial margin and anterior outer tritor (‘reference width’ of Popov and Machalski, 2014). Symbols for tritor infilling: dotted or solid black, vascular pleromin; horizontal parallel lines, laminated pleromin. Scale bar = 1 cm.
Class Chondrichthyes Huxley, 1880
Subclass Holocephali Bonaparte, 1832
Superorder Holocephalimorpha Nelson, 2006
Order Chimaeriformes Obruchev, 1953
Suborder Chimaeroidei Patterson, 1965
Superfamily Callorhinchoidea Garman, 1901
Family ‘Edaphodontidae’ Owen, 1846
Genus
Ischyodus
Egerton, 1843
Type species.—Chimaera townsendi Buckland, 1835; Upper Jurassic (Tithonian), southern England.
Ischyodus bifurcatus
Case, 1978
Figure 2
Ischyodus brevirostris Egerton. Davis, 1890, p. 414, pl. XLII, figs. 12, 13, 15 (non fig. 14 = Amylodon sp.).
Edaphodon sp. Case, 1967, p. 8, fig. 33.
Ischyodus bifurcatus Case, 1978, p. 21, pl. 1, figs. 1—4; Case, 1979, p. 226, pl. 2, figs. 1—3; Case and Schwimmer, 1992, p. 347, fig. 2; Popov and Ivanov, 1996, p. 56, fig. 2; Stahl, 1999, p. 132; Panteleyev et al., 2004, p. 117; Averianov and Popov, 2014, p. 327; Cicimurri and Ebersole, 2014, p. 10, fig. 8A—D, non fig. 9A—C (Edaphodon sp.); Cicimurri and Ebersole, 2015, p. 21, fig. 11(4); Zverkov et al., 2017, p. 2.
Ischyodus cf. bifurcatus Case. Robb, 1989, p. 81, fig. 13A, B, ?C; Averianov and Popov, 1995, p. 661; Nessov and Averianov, 1996, p. 16.
Ischyodus bifurcatus? Case. Nessov and Averianov, 1996, p. 16.
Ischyodus sp. Yarkov, 2001, p. 57.
Material.—SDNHM 25417, a nearly complete right mandibular tooth plate, collected in February 1984 by R. A. Cerutti.
Locality.—SDNHM Locality 3162B: Carlsbad, San Diego County, California. The locality was discovered during the development of the Carlsbad Research Center in a trench now covered by Faraday Avenue.
Stratigraphic horizon.—A mudstone stratum in the lower part of the Point Loma Formation (additional stratigraphic data are on file at SDNHM). Bukry (1994) assigned this stratigraphic section to coccolith zones CC21 and CC22, late Campanian.
Description.—SDNHM 25417 measures 47 mm in maximum mesiodistal length (‘L' in Figure 2C) and 11 mm between the symphysial margin and anterior outer tritor (‘Km’ in Figure 2C; = ‘reference width’ of Popov and Machalski, 2014). The specimen is a medium-sized, moderately robust but flat mandibular plate with a moderately developed mandibular beak. The distribution of tritors on the occlusal side is typical of Ischyodus, comprising the (1) symphysial tritor, (2) anterior outer tritor, (3) posterior outer tritor, (4) median tritor, and (5) double inner tritors. The symphysial tritor is narrow and low in cross section, and primarily consists of laminated pleromin, but is laterally flanked by vascular pleromin (up to one-fourth of the tritor width). The pleromin body of the symphysial tritor is visible in basal view due to taphonomic abrasion in which traces of descending lamina are missing because of the abraded state (Figure 2A). All the remaining non-symphysial tritors show infilling of vascular (tubular) pleromin. The anterior and posterior outer tritors are mesiodistally elongated, with the posterior tritor being slightly wider and situated distally compared to the anterior tritor. The three labially positioned tritors (i.e., symphysial, anterior outer, and posterior outer tritors) protrude along the labial margin, defining two prominent concavities along the plate margin. The large, centrally placed median tritor is ‘compound' in cross section (sensu Popov and Machalski, 2014) and is bifurcated anteriorly to form equal-sized, moderately developed, well separated inner and outer branches. The inner branch is well separated from the nearly straight symphysial margin of the tooth plate and is located slightly distally with respect to the anterior tip of the outer branch. The two inner tritors are small and parallel one another along the symphysial margin of the plate, and their vascular pleromin is compact without visible tubes. The inner tritor is positioned at the same level as the mesial extremity of the anterior outer tritor located laterally.
Taxonomic remarks.—SDNHM 25417 has a relatively large median tritor that constitutes a major crushing component of the mandibular plate. The position of the median tritor with respect to other tritors and the anterior bifurcation of the median tritor are consistent with those of the holotype of Ischyodus bifurcatus (UM 334) described by Case (1978). Previous studies have suggested that the median tritor of I. bifurcatus represents fusion of the medial and ‘internal posterior’ ( = posterior inner) tritors (e.g. Case, 1978, 1979; Case and Schwimmer, 1992; Stahl, 1999). However, some Jurassic and mid-Cretaceous species of Ischyodus show varying degrees of development of the inner branch of the median tritor (e.g. Popov and Machalski, 2014, fig. 4C) in which ‘bifurcation’ without any tritor fusion (seen in I. thurmanni) is interpreted to be the plesiomorphic condition. Because of the wide morphological variation of the median tritor, with more or less prominent bifurcation in multiple Jurassic and Cretaceous species of Ischyodus (e.g. Ischyodus egertoni, I. emarginatus, and I. latus: Popov and Machalski, 2014), bifurcation could have evolved independently in multiple Mesozoic clades of Ischyodus besides the species categorized into the ‘bifurcatus-group’ (see Introduction and Discussion herein).
The holotype of Ischyodus bifurcatus from the early or middle Maastrichtian has an additional small median tritor in the distal part of the plate between the median and posterior outer tritors (Case, 1978, pl. 1, fig. a), although this character is not specifically described in Case's (1978) original species diagnosis. Russian specimens (SSU collection: EVP, personal observation) also show variation in the total number of additional small median tritors (up to 5), although additional tritors appear to be more common in specimens from younger (e.g. Maastrichtian: typically 1 to 5 additional median tritors) deposits than older (e.g. Campanian: typically 0 to 3 additional median tritors) deposits. SDNHM 25417 from the Campanian lacks such additional median tritors, which is consistent with the temporal trend noted above.
Davis (1890) described and figured several tooth plates of Ischyodus bifurcatus from the Campanian of Sweden under the name I. brevirostris Newton (Agassiz, 1843). These specimens show clear bifurcation of the mandibular median tritor (Davis, 1890, pl. XLII, figs. 12, 13, 15) and do not differ significantly from tooth plates commonly referred to I. bifurcatus including SDNHM 25417. All the Swedish specimens illustrated by Davis (1890), except an incomplete vomerine plate (Davis, 1890, pl. XLII, fig. 14), are identified here as I. bifurcatus. The vomerine plate is similar to previously undescribed vomerine specimens (SSU 154/628) of a rhinochimaerid, Amylodon karamysh Averianov and Popov, 1995, from the Campanian of the Saratov region of Russia, and we therefore assign the Swedish vomerine plate to Amylodon sp.
Discussion
The holotype of Ischyodus bifurcatus is a large mandibular plate (L = 82 mm, Km = 17 mm), that is nearly twice the size of SDNHM 25417 (L= 47 mm, Km = 11 mm). Isolated mandibular plates (n = 40+) of this species from the Campanian Rybushka Formation in the Saratov and Volgograd regions of Russia, also show a considerably wide size range (e.g. L = ca. 36 to ca. 75 mm, Km = 6 to 25 mm; see Figure 2C), although their tritor pattern is relatively consistent (SSU collection: EVP, personal observation). Examination of two illustrated nearly complete skeletons of Ischyodus (I. quenstedti and I. avitus) by Stahl (1999, fig. 136A, B) reveals that the ratio between the L value and total body length (TL, that includes the elongate rostrum and tail) is 1:21. Whether or not the L:TL relationship remains isometric through ontogeny requires testing, but the ratio can be used to extrapolate rough TL measurements from L values of isolated mandibular tooth plates. For example, the L value of the holotype and SDNHM 25417 would suggest that the fish when alive measured about 172 and 99 cm TL, respectively, and the smallest and largest individuals represented by isolated mandibular plates of I. bifurcatus in the SSU collection suggest a TL range of 76 to 156 cm. Even if a conservative L:TL ratio as low as, for example, 1:18 is used, the holotype and SDNHM 25417 would have roughly measured at least about 1.5 and 0.8 m TL, respectively (see also Popov et al., 2013).
Figure 3.
Campanian paleogeography (after Smith et al., 1994, map 11) showing localities of Ischyodus bifurcatus and related species of the ‘bifurcatus-group’ (plots and locality numbers based on Appendix 1). Symbols: star, I. bifurcatus from California described in the present study (SDNHM 25417: Figure 2); circles, all other I. bifurcatus records; squares, all other Late Cretaceous records of species of the ‘bifurcatus-group.’
In his original description of Ischyodus bifurcatus, Case (1978) listed occurrences of the species from Santonian to Maastrichtian deposits of the USA, including Delaware, New Jersey, and Arkansas (Figure 3; Appendix 1). Subsequently, additional North American occurrences were reported from Montana, Wyoming, Alabama, Georgia, and North Carolina (Case, 1979; Robb, 1989; Case and Schwimmer, 1992; Cicimurri and Ebersole, 2014). Elsewhere, I. bifurcatus is also known from the Campanian of Sweden (Davis, 1890; see above) and the Campanian–Maastrichtian of European Russia (e.g. Averianov and Popov, 1995, 2014; Nessov and Averianov, 1996; Popov, 2004; Appendix 1). Case (1978) and Case and Schwimmer (1992) also listed occurrences of I. bifurcatus from the lower Campanian of Chico, California, the middle Campanian of New Jersey, the upper Campanian of Belgium, the upper Campanian to lower Maastrichtian of New Mexico, and the upper Campanian of Belgium (Appendix 1). These records, however, are unverifiable because they are based on specimens in private collections and their taxonomic identity cannot be confirmed due to the lack of illustrations. Verified records of I. bifurcatus indicate that the species is confined to the mid-latitudinal zone in the Northern Hemisphere and has a temporal range of about 20 Ma (Santonian to Maastrichtian). SDNHM 25417 is significant because it constitutes the first confirmed record of I. bifurcatus from the North Pacific region (Figure 3). In addition, the present fossil record of I. bifurcatus (Figure 3; Appendix 1) suggests that this taxon first evolved in the region of the present-day eastern and southeastern United States during the late Santonian (i.e., Mississippi, Alabama, and Delaware records) and dispersed eastward as far as present-day northern Kazakhstan via the northern shoreline of the Tethys Sea during the late Maastrichtian, and westward as far as present-day southern California, likely via the southern shoreline of the Late Cretaceous ‘Laramidia,’ during the late Campanian (Appendix 1; Figure 3).
Ischyodus yanshini from the lower Campanian of western Kazakhstan is a species of the ‘bifurcatus-group’ that is characterized by mandibular plates with a narrow median tritor and short anterior outer tritor (see Averianov, 1991). Ischyodus yanshini is considered to be closest phylogenetically to I. bifurcatus on the basis of their morphological resemblance. Based on isolated tooth plates from North Dakota, Colorado and Russia, Hoganson and Erickson (2005) erected a new Maastrichtian species of Ischyodus, I. rayhaasi, here included in the ‘bifurcatus-group’ (see also Hoganson et al., 2015). Mandibular plates of I. rayhaasi differ from those of I. bifurcatus by an unequally bifurcated median tritor with the tip of its outer branch lying directly posterior to the anterior outer tritor and with its inner branch extending beyond the anterior tip of the anterior outer tritor (Hoganson and Erickson, 2005). The presence of an ‘indistinct accessory median tritor’ on the mandibular plate diagnosed for this species in the original description is not unique to I. rayhaasi because a similar additional tritor is also present in the holotype of I. bifurcatus (see above). It should be noted that palatine and vomerine tooth plates among these ‘bifurcatus-group’ species are less taxonomically diagnostic compared to their mandibular tooth plates (Popov, 2004; the vomerine tooth plate is unknown for I. rayhaasi). The present fossil record suggests that I. bifurcatus has the geologically oldest origin among the three ‘bifurcatus-group’ species (i.e., Ischyodus bifurcatus, I. rayhaasi, and I. yanshini; Appendix 1). The geographic distribution of this group strongly indicates that the three species preferred shallow epicontinental seas and shelf environments of temperate latitudes in the Northern Hemisphere (Figure 3).
Other reports of chimaeroid fossils in the North Pacific include, in addition to SDNHM 25417, a dorsal fin spine of Edaphodon sp. from the lower Miocene of Kern County, California (Takeuchi and Huddleston, 2006), as well as numerous tooth plates of Hydrolagus cf. colliei (Lay and Bennett, 1839) and Harriotta cf. raleighana Goode and Bean, 1895, from the lower Miocene—early Pliocene at several localities in southern California (Popov and Takeuchi, 2011). Applegate (1975) reported a tooth plate of Ischyodus zinsmeisteri Applegate, 1975, from the Paleocene of Ventura County, California; however, this specimen is considered to belong to the genus Edaphodon, rather than Ischyodus (EVP, personal observation). An additional eastern North Pacific record comes from the lower Campanian of Vancouver Island, British Columbia, Canada, represented by Edaphodon hesperis Shin, 2010, based on an associated dentition (Shin, 2010). In contrast to the eastern North Pacific, the western North Pacific record of fossil chimaeroids is limited. Examples include an isolated mandibular tooth plate of Edaphodon sp. from the late Oligocene of Japan (Okazaki, 1991), and another such of Chimaera sp. from the middle Miocene of Japan (Nomura, 2000). In addition, Uyeno and Takahashi (1997) briefly noted an occurrence of an associated(?) specimen of Ischyodus(?) from the Upper Cretaceous (Coniacian) of Japan, consisting of two (vomerine and mandibular) tooth plates and a dorsal fin spine; however, the specimen has not yet been described formally and its exact taxonomic identity remains uncertain. The scarcity of chimaeroid fossil records in the region is at odds with the fact that numerous Cretaceous and Cenozoic deposits rich in teeth of fossil elasmobranchs are present in Japan (e.g. Yabumoto and Uyeno, 1994; Goto et al., 1996) and chimaeroid materials commonly co-occur with elasmobranch remains elsewhere (e.g. Popov and Machalski, 2014). Such chimaeroid fossils may exist in museum and private collections in Japan, but the diversity and abundance, as well as stratigraphic and geographic distributions, of fossil chimaeroids in the western North Pacific remain poorly understood. Taking all these occurrence data into account, SDNHM 25417 is important because it constitutes the first known record of I. bifurcatus from the North Pacific region and it represents the only known definite Mesozoic record of the genus in the entire North Pacific region.
Conclusions
SDNHM 25417 is a nearly complete right mandibular tooth plate of Ischyodus bifurcatus that was collected from the upper Campanian portion of the Point Loma Formation in San Diego County, California, USA. It likely came from an individual that measured nearly 1 m TL. A review of occurrence records shows that I. bifurcatus geologically ranges for about 20 Ma from the Santonian to Maastrichtian. Specimens of I. bifurcatus occur in deposits formed in epicontinental seas and shelf paleoenvironments of temperate latitudes in the Northern Hemisphere. Published localities include Delaware, New Jersey, North Carolina, Georgia, Alabama, Arkansas, Montana, and Wyoming of the United States as well as Sweden and European Russia. SDNHM 25417 is significant because (1) it constitutes the first confirmed record of I. bifurcatus from the circum-Pacific, and (2) it represents the only known Mesozoic record of Ischyodus in the entire North Pacific region.
Author Contributions
E. D. J.-R. initiated and carried out this study. E. V. P. is primarily responsible for the taxonomic and technical aspects. T. A. D. supplied relevant geographic and stratigraphic contextual data of the specimen examined. K. S. oversaw the entire project and supplemented additional geological and paleontological information and literature. All authors contributed to the writing of the paper.
Acknowledgments
We thank the Department of Environmental Science and Studies and the Department of Biological Sciences at DePaul University as well as DePaul's McNair Scholars and Arnold L. Mitchem Fellowship Program for providing various support especially to EDJ-R as well as to the present project. The work of EVP in particular was supported by the Russian Foundation for Basic Research (project no. 18-05-01045) and by the subsidy of the Russian Government to support the ‘Program of Competitive Growth of Kazan Federal University among World's Leading Academic Centers.’ Comments and suggestions made by D. J. Cicimurri, Y. Takakuwa, and the editor greatly improved the quality of this paper.
References
Appendices
Appendix 1.
Records of Late Cretaceous Ischyodus bifurcatus and related species of the ‘bifurcatus-group’ with relevant references or referable specimens, including the Californian record described in this study. Numbered occurrences correspond to localities plotted in Figure 3; asterisk (*) indicates unverifiable record.
Ischyodus bifurcatus Case, 1978
1. Point Loma Formation (upper Campanian), Carlsbad, California, USA (this study)
2. Mesaverde Formation (upper Campanian), Worland, Wyoming, USA (Case and Schwimmer, 1992)
3. Marlbrook Marl (upper Campanian), Arkadelphia, Arkansas, USA (Case, 1978)
4. Eutaw Formation (upper Santonian), Columbus, Mississippi, USA (Case and Schwimmer, 1992)
5. Mooreville Chalk (uppermost Santonian–mid-Campanian), Alabama, USA (Cicimurri and Ebersole, 2014, 2015)
6. Cassetta Formation (basal portion; mid-Campanian), Hannahatchee Creek, Georgia, USA (Case and Schwimmer, 1992, fig. 2)
7. Black Creek Formation (upper Campanian), Phoebus Landing, North Carolina, USA (Robb, 1989, fig. 13; referred to as Ischyodus cf. bifurcatus)
8. Merchantville Formation (upper Santonian–lower Campanian), St. Georges, Delaware, USA (Case, 1978)
9. Mount Laurel Sandstone (lower Maastrichtian), Monmouth County, New Jersey, USA (Case, 1978)
10. Navesink Formation (derived; lower—middle Maastrichtian), Monmouth County, New Jersey, USA (Case, 1978)
11. ‘Campanian’, Sweden (Davis, 1890; referred to as Ischyodus brevirostris)
12. Rybushka Formation (lower Campanian), numerous localities, Saratov region, Russia (Averianov and Popov, 1995, 2014; Panteleyev et al., 2004; Zverkov et al., 2017; SSU 154/108, 143, and 144)
13. Rybushka Formation (lower Campanian), several localities, Volgograd region, Russia (Nessov and Averianov, 1996; SSU 154/651 and 652)
14. Pudovkino Formation (upper Campanian), several localities, Penza Region, Russia (Yarkov, 2001; SSU 154/653 and 654)
15. Zhuravlev Formation (base; lower—upper Maastrichtian), Kushmurun, Kostanai Province, northern Kazakhstan (Nessov and Averianov, 1996, p. 16; SSU 154/655 and 656)
*Chico Fauna (lower Campanian), Chico, California, USA (Case, 1978)
*Woodbury Clay (mid-Campanian), Belmawr, New Jersey, USA (Case, 1978)
*Oburg Chalk (base, Cm2a; upper Campanian), Havre, Belgium (Case, 1978)
*Fruitland Formation (upper Campanian–lower Maastrichtian), San Juan Basin, New Mexico, USA (Case and Schwimmer, 1992)
*Zhuravlev Formation (base; upper Maastrichtian), Zhuravlevskyi, Kostanai region, northern Kazakhstan (Nessov and Averianov, 1996, p. 16)
Ischyodus rayhaasi Hoganson and Erickson, 2005
16. Fox Hill Formation and Hell Creek Formation (Breien Member; upper Maastrichtian), numerous localities, North Dakota, USA (Hoganson and Erickson, 2005)
17. Fox Hill Formation (Campanian—Maastrichtian), Poison Springs Locality, Colorado, USA (Hoganson et al., 2015)
18. Berezovaya Beds (base; Danian, Paleocene; reworked from Bereslavka Formation; upper? Maastrichtian), Rasstrygin, Volgograd region, Russia (Popov and Ivanov, 1996; Popov and Yarkov, 2001; referred to as Ischyodus bifurcatus? or I. ‘bifurcatus’)
Ischyodus yanshini Averianov, 1991
19. Zhurun Beds (lower Campanian), Actobe region, western Kazakhstan (Averianov, 1991)
*‘Lower Campanian,’ Tykbytak, Actobe region, western Kazakhstan (Nessov and Averianov, 1996, p. 16; referred to as Ischyodus cf. yanshini)
