Callorhinchus orientalis sp. nov., a new callorhinchid from the Upper Cretaceous Hakobuchi Formation, Yezo Group, Hokkaido, Japan

Abstract

Herein, we describe Callorhinchus orientalis sp. nov., an extinct callorhinchid chondrichthyan species found in the lower Maastrichtian deposits of the Hakobuchi Formation in Hobetsu, Hokkaido, northern Japan. This species is the first record of the genus Callorhinchus from the northern Pacific region, filling the biogeographical gap and implying survival beyond the Cretaceous/Paleogene extinction event because of the broad distribution of the genus. A comparison of Maastrichtian Callorhinchus species, including C. orientalis sp. nov., and Danian species indicates a potential period of temporal dwarfing within the genus across this extinction event.

ZooBank registration: urn:lsid:zoobank.org:pub:7ED050DB-7A04-4999-9CA9-8E04E0B5AA56

Introduction

Chimaeroidei is the only suborder within the subclass Holocephali that includes extant taxa (Stahl, 1999). Modern chimaeroid fishes, represented by three families, six genera, and approximately fifty species, are all benthic and durophagous (Didier et al., 2012). Holocephalian dentition is characterized by ever-growing tooth plates without replacement, with chimaeroid dentition comprising a pair of mandibular tooth plates and two pairs of vomerine and palatine tooth plates (Figure 1A). On the tooth plates’ occlusal surface, hypermineralized grinding surfaces, termed tritors, forms rods or pads. Tooth plates are crucial for identifying extinct and extant species within this group, as tooth morphology varies among taxa.

Figure 1. Schematic images of Callorhinchus dentition. A, dentition of C. milii, from an open-mouthed view, modified from Herman et al. (2001); B, terminology associated with the palatine plate surface in C. milii (left palatine plate), modified from Patterson (1992); C, growth stages of the C. milii palatine plate, modified from Didier et al. (1994). Gray areas indicate tritors (grinding surfaces). Arrows mark stages observed by Didier et al. (1994). Abbreviations: v. t. p, vomerine tooth plate; p. t. p, palatine tooth plate; m. t. p, mandibular tooth plate.

The family Callorhinchidae contains the most primitive living chimaeroids, characterized by the presence of a well-developed descending lamina on the tooth plates’ aboral surface (Dider, 1995; Kriwet and Gaździcki, 2003). All three extant Callorhinchus species, along with most Callorhinchus fossil records, have been documented in the Southern Hemisphere (Cicimurri and Ebersole, 2015; Rizzari and Finucci, 2019). In the Northern Hemisphere, Callorhinchus fossil records exhibit a temporal range from the Middle Jurassic to the Eocene (Brown, 1946; Gurr, 1962; Ward, 1973).

In this study, we present a new species of the genus Callorhinchus from the lowest Maastrichtian (Upper Cretaceous) Hakobuchi Formation at Tomiuchi District in the Hobetsu area of Mukawa Town, Hokkaido, northern Japan. This species holds particular importance as it represents the first documented instance of the genus within the northern Pacific region, including both extant and extinct species. Moreover, this discovery implies a transient reduction in the body size of species of the genus across the Cretaceous/Paleogene (K/Pg) mass extinction event.

Geological setting

The specimen was collected as a float containing bivalves by K. Moriki from Pankerusano-sawa (Panke-tosano) Creek at Tomiuch District in the Hobetsu area of Mukawa Town, Hokkaido, northern Japan (Figure 2A, B). After its donation to the Hobetsu Museum, further fieldwork was conducted around the creek, leading to the discovery of a fine sandstone unit, containing numerous bivalve fossils and some chondrichthyan teeth. The specimen was presumed to have originated from this sandstone unit, located in the upper part of the IVb rock unit in the Hakobuchi Formation of the Yezo Group (Figure 2C) (see Matsumoto, 1942; Takashima et al., 2004), with loc. 0008 (Figure 2B) pinpointed owing to similarities in rock faces and bivalve assemblages. The IVb rock unit is stratigraphically correlated with the earliest Maastrichtian based on comparisons of ammonoids and inoceramids (Shigeta and Nishimura, 2014) and has yielded a plethora of both invertebrate and vertebrate fossils, including ammonoids, mosasaurs, sea turtles, and a hadrosaurid dinosaur (e.g. Suzuki, 1985; Hirayama and Chitoku, 1996; Shigeta and Nishimura, 2014; Konishi et al., 2016; Kobayashi et al., 2019). Bivalves found in the sandstone unit include Inoceramus shikotanensis Nagao and Matsumoto (1940), recorded from upper Campanian–lower Maastrichtian deposits (Morozumi, 1985; Toshimitsu et al., 1995; Matsunaga et al., 2008). The sandstone unit is presumed to be correlated with the lowest Maastrichtian Nostoceras hetonaiense Zone.

Figure 2. Specimen locality and horizon. A, map of the main island of Hokkaido, Japan; B, route map of the study area (Pankerusano-sawa Creek), showing exposure locations and the presumed location of the holotype, HMG-2013, Callorhinchus orientalis sp. nov.; C, stratigraphic column of the study area, showing the horizon of HMG-2013, holotype of C. orientalis. sp. nov. with black star. In the area we conduct the fieldwork, we could not confirm the border of Unit IVb and IVc, so that we express “IVb-c” in Figure 2. Abbreviations: Cam., Campanian; M.S., Mudstone; S.S., Sandstone; Congl., Conglomerate.

Material and methods

Specimens registered under catalog number “ZPAL P.9” are kept at the Institute of Paleobiology of the Polish Academy of Sciences, Warzawa, Poland (Kriwet and Gaździcki, 2003).

The specimen was prepared using the following procedure. First, the exposed tooth plate was coated with acrylic resin. Subsequently, the entire rock was immersed in 10% acetic acid for 42 h, followed by submergence in water overnight to remove residual acid. The dissolved rock material was rinsed and scraped away using hand tools, and the rock was dried. A fresh layer of acrylic resin was then applied to both the newly exposed and previously treated areas of the tooth plate. The above procedure was repeated until the specimen was removed from the rock.

To compare specimen size, we indirectly measured the symphyseal margin length of palatine tooth plates in Callorhinchus spp., as some species possess a “wing” on the distal–labial margin, whereas others do not. Measurements for all specimens, except HMG-2013, were obtained from figures in the cited literature (Nessov and Averianov, 1996; Kriwet and Gaździcki, 2003; Otero et al., 2013; Cicimurri and Ebersole, 2015). Incomplete or partial specimens were reconstructed to obtain approximate outlines with reference to well-preserved specimens (Figure 3).

Figure 3. Schematic illustration of Callorhinchus spp. palatine tooth plates. Gray areas indicate the tritor. Black areas show the reconstructed parts of specimens. Asterisk (*) denotes horizontally flipped images. A, ZPAL P.9/3; B, BMNH P.44039; C, BMNH P.55313; D1, CMM-5354; D2, CMM-V-5787; E, MMNS 6210; F1, SGO.PV.22012d; F2, SGO.PV.22012c; G, HMG-2013; H, BMNH P.2301; I, “TsNIGR Museum No 1/12963”. A–C and H are modified from Kriwet and Gaździcki (2003); D1, D2, and E are modified from Cicimurri and Ebersole (2015); F1 and F2 are modified from Otero et al. (2013); I is modified from Nessov and Averianov (1996). Letters correspond to those in Table 2. Abbreviations: Ypr., Ypresian; Maa., Maastrichtian.

Higher-level taxonomy follows Nelson et al. (2016), and terminology primarily follows Patterson (1992) and Stahl (1999).

Institutional abbreviations.—BMNH, Natural History Museum, London, England; CMM, Calvert Marine Museum, Solomons, Maryland, USA; HMG, Hobetsu Museum, Mukawa, Hokkaido, Japan; IRSNB, Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium; MMNS, Mississippi Museum of Natural Science, Jackson, Mississippi, USA; SGO.PV, Museo Nacional de Historia Natural, Santiago, Chile.

Systematic paleontology

Class Chondrichthyes Huxley, 1880

Subclass Holocephali Bonaparte, 1832

Superorder Holocephalimorpha Nelson, 2006

Order Chimaeriformes Obruchev, 1953

Suborder Chimaeroidei Patterson, 1965

Superfamily Callorhynchoidea Garman, 1901

Family Callorhynchidae Garman, 1901

Genus Callorhinchus Lacépède, 1798

Type species.—Callorhinchus callorhynchus Linnaeus, 1758.

Revised diagnosis.—Considering tooth plates only (Herman et al., 2001; Kriwet and Gaździcki, 2003; Otero et al., 2013): single central tritor on each tooth plate; quadrilateral or lozenge-shaped mandibular tooth plate with a single central tritor pad restricted distally on the oral surface, flanked by narrow tritors on the symphyseal and/or labial edges; elongate triangular or trapezoid-shaped palatine tooth plate; single tritor with one or two projections toward the labial margin; small, quadrilateral or lozenge-shaped vomerine tooth plate with a single central tritor.

Remarks.—A single bifid tritor toward the anterior is considered a diagnostic character for Callorhinchus palatine tooth plates. Although some Callorhinchus alfordi Cicimurri and Ebersole (2015) palatine tooth plates do not display a bifid tritor, Cicimurri and Ebersole (2015) identified this morphological variation within a single species based on size and specific adult fish characteristics.

Callorhinchus orientalis sp. nov.

ZooBank lsid: urn:lsid:zoobank.org:act:50F60CEF-DFDB-4D71-8E05-4F710A41BA6C

Diagnosis.—Palatine tooth plate bearing a single tritor and a distinct anterior projection. This plate differs from that of other species in the genus by the single tritor’s posterior margin extending toward the lingual–labial margin of the plate.

Type specimen.—Holotype, HMG-2013 (Figure 4), a nearly complete left palatine tooth plate.

Figure 4. Callorhinchus orientalis sp. nov. left palatine tooth plate, HMG-2013. A, line drawing of the oral surface of the holotype; B–E, holotype; B, oral view; C, aboral view; D, symphyseal view; E, labial view. In A, gray areas indicate the tritor, broken lines show the margins of missing parts and hatching indicates damaged surfaces of the tritor. Abbreviation: dl, descending lamina.

Locality and horizon.—Pankerusano-sawa Creek, Tomiuchi District, the Hobetsu area of Mukawa Town, Hokkaido, northern Japan. This specimen was collected from the Upper Cretaceous Hakobuchi Formation Unit IVb (Matsumoto, 1942) in the Yezo Group.

Etymology.—Species name refers to “eastern,” from the Latin word orientalis.

Description.—The holotype, HMG-2013, is a nearly complete left palatine tooth plate, missing only the lingual and distal part of the symphyseal margin. The specimen is 37.34 mm in length with a trapezoid shape. The anterior and posterior ends of the plate have widths of 9.09 and 18.36 mm, respectively. The straight, flat symphyseal margin measures 6.18 mm in height. The plate lacks a wing-like lateral expansion distally. A single tritor is present on the oral surface. The mesial end of this tritor is not bifurcated but bears an anteriorly directed inner prong (6.67 mm long). The tritor extends distally along the labial margin. In the aboral view, the plate has a well-developed descending lamina that is more distinct on the labial side than on the symphyseal side (Figure 4C).

Comparison.—Fossil holocephalan classification relies on tooth plate and tritor characteristics (Obruchev, 1964; Kriwet and Gaździcki, 2003). Given that tooth plates grow continuously and are not replaced, even in the adult stage, their morphology can vary due to growth after reaching adulthood and abrasion (Didier et al., 1994; Otero et al., 2013). These variations are crucial for taxonomic identification. Based on its elongated outline and single tritor, the specimen, HMG-2013 is considered a palatine tooth plate of the genus Callorhinchus (see Stahl, 1999; Cicimurri and Ebersole, 2015). The typical palatine tooth plate tritor of Callorhinchus is semicircular. This new species is distinguished from other species by the posterior margin of the tritor extending toward the lingual–labial margin of the palatine tooth plate (Figure 4A). Because the posterior portion of the tooth plate’s oral surface is unworn (Didier, 1995), it likely preserves its original morphology. A palatine tooth plate possessing a tritor with a posterior margin extending to the plate’s lingual margin is also observed in Callorhinchus phillipsi Cicimurri and Ebersole (2015), although the extension is more on the symphyseal side (Figure 3E).

Unlike most Callorhinchus palatine tooth plate tritors, which have a mesial margin that bifurcates and projects anteriorly, the tritor of the new species is not bifid but possesses only an inner prong, resembling some C. alfordi specimens from the Thanetian (CMM-V-5354 and CMM-V-5787) (Figure 3). As mentioned earlier, prong number and shape can vary within species due to growth stage and wear. These specimens of C. alfordi are considered adults, displaying a single large tritor with a clear projection, suggesting a growth pattern similar to that observed in extant species. In the palatine tooth plates of Callorhinchus milii Bory de Saint-Vincent (1823), an extant species, only “fully grown fish” display the species-specific single large tritor (Figure 1C; Didier et al., 1994). Notably, the holotype of new species, HMG-2013 appears larger than all C. alfordi palatine tooth plates described by Cicimurri and Ebersole (2015).

Discussion

Callorhinchus fossil records

Three extant Callorhinchus species are exclusive to the Southern Hemisphere (Figure 5A) (Cicimurri and Ebersole, 2015; Rizzari and Finucci, 2019). Conversely, Callorhinchus fossils have been recorded in both hemispheres (Table 1; Figure 5). Early Late Cretaceous Callorhinchus fossils include specimens from Antarctica (Southern Hemisphere) as well as England and Germany (Tethys region, Northern Hemisphere), whereas the latest Cretaceous (Campanian and Maastrichtian) fossils are restricted to the Southern Hemisphere (Figure 5D, E). During the Paleogene, the geographic distribution of Callorhinchus was concentrated in the northern Atlantic region of the Northern Hemisphere, whereas in the Neogene, it became restricted to the Southern Hemisphere (Figure 5B, C). Previous studies suggested a Campanian–Maastrichtian extinction of Callorhinchus in the Northern Hemisphere. However, the discovery of the new species, C. orientalis sp. nov., from the Maastrichtian of Japan contradicts this extinction hypothesis and supports the continuous presence of Callorhinchus in the Northern Hemisphere since the known origin of the genus until the Eocene. Notably, in the Northern Hemisphere, all pre-Neogene specimens were thus far discovered in the Tethys and Atlantic regions, with no Pacific records. Callorhinchus orientalis sp. nov. from the Hakobuchi Formation in Japan thus represents the first and oldest record of the genus in the northern Pacific region. This finding, along with Callorhinchus sp. discovered in Campanian–Maastrichtian deposits in Chile, indicates that the genus dispersed into the Pacific Ocean during the latest Late Cretaceous. For extant chondrichthyans, geographic range size is negatively correlated with World Conservation Union (IUCN) threat risk (Field et al., 2009). A broader distribution, potentially spanning both hemispheres, may have contributed to the survival of Callorhinchus through the K/Pg mass extinction.

Figure 5. Paleobiogeographic distribution of species of the genus Callorhinchus. A, distribution of extant species (dark gray area) is based on Rizzari and Finucci (2019); B, Neogene; C, Paleogene; D, late Late Cretaceous; E, Early to Late Cretaceous. Paleogeographical map modified from Scotese (2014a–d). Letters correspond to those in Table 1.

Table 1. Stratigraphic and geographic distributions of the genus Callorhinchus. Abbreviations, see Figure 6.

System	Series	Stage	Locality	Taxon	References	Caption in Figure 5
Neogene	Pliocene	—	Sacaco, Peru	C. cf. callorhynchus	Muizon and DeVries, 1985	j
	Miocene	—	Atacama, Chile	Callorhinchus sp.	Suárez et al., 2004	k
		—	Santa Cruz, Argentina	C. crassus	Woodward and White, 1930	l
	Early Miocene	—	La Boca, Chile	Callorhinchus sp.	Suárez et al., 2006	m
Paleogene	Eocene	Upper Ypr.	Seymour Island, Antarctica	C. stahli	Kriwet and Gaździcki, 2003	a
		Ypresian	Kent, U.K.	C. regulbiensis	Gurr, 1962; Ward, 1973	b
	Paleocene	Thanetian	Kent, U.K.	C. newtoni	Ward, 1973	c
		Thanetian	Virginia, U.S.A.	C. alfordi	Cicimurri and Eversole, 2015	d
		Danian	Mississippi, U.S.A.	C. phillipsi	Cicimurri and Eversole, 2015	e
Cretaceous	Late Cretaceous	Upper Maa.	Seymour Island, Antarctica	Callorhinchus sp.	Martin and Crame, 2006	n
		Upper Maa.	Seymour Island, Antarctica	C. torresi	Otero et al., 2013	f
		Lower Maa.	Hokkaido, Japan	C. orientalis sp. nov.	This study	g
		Cam. - Maa.	Quiriquina Island, Chile	Callorhinchus sp.	Suárez et al., 2003	o
		Upper Cam.	James Ross Island, Antarctica	Callorhinchus sp.	Otero et al., 2014	p
		Santonian	Penza Region, Russia	Callorhinchus sp.	Averianov, 1997	q
		Cenomanian	Amuri Bluff, New zealand	C. hectori	Newton, 1876	h
	Early Cretaceeous	Albian	Gubkin, Belgorod region, Russia	C. borealis	Nessov and Averianov, 1996	i
Jurassic	Late Jurassic	Tithonian	Moskow region, Russia	Callorhinchus sp.	Popov and Shapovalov, 2007	—
	Middle Jurassic	—	Württemberg, Germany	C. germaticus	Brown, 1946; Stahl, 1999	—

Size reduction of Callorhinchus across the K/Pg mass extinction

The size of the palatine tooth plates of the genus Callorhinchus increased from the early Late Cretaceous to the later Late Cretaceous. Subsequently, the palatine tooth plate of the Danian species became approximately 70%–80% smaller than those of Maastrichtian species. The size of the tooth plate then apparently increased through the Eocene (Figure 6; Table 2).

Figure 6. Sizes of the palatine tooth plate of the genus Callorhinchus from Late Cretaceous Albian–Eocene Ypresian. Black square indicates a measurement based on an actual specimen; black triangles indicate measurements based on figures in the literature; white triangles indicate measurements based on reconstructed figures. Letters correspond to those in Table 2. Abbreviations: Ypr., Ypresian; Tha., Thanetian; Sel., Selandian; Dan., Danian; Maa., Maastrichtian; Cam., Campanian; San., Santonian; Con., Coniacian; Tur. Turonian; Cen., Cenomanian; Alb., Albian.

Table 2. Measurements of indirect symphyseal margin length in Callorhinchus spp. palatine tooth plates. Abbreviations, see Figure 6.

Caption in Figures 3 and 6	Stages	Taxon	Specimen Number	Indirect Symphyseal Margin Length (mm)	Measured by	References
A	Upper Ypr.	C. stahli	ZPAL P.9/3	24.11	Reconstructed figure	Kriwet and Gaździcki (2003)
B	Ypresian	C. regulbiensis	BMNH P.44039	28.81	Reconstructed figure
C	Thanetian	C. newtoni	BMNH P.55313	25.45	Reconstructed figure
D1	Thanetian	C. alfordi	CMM-5354	21.89	Reconstructed figure	Cicimurri and Ebersole (2015)
D2	Thanetian	C. alfordi	CMM-V-5787	24.36	Reconstructed figure
E	Danian	C. phillipsi	MMNS 6210	10.44	Figure on literature
F1	Upper Maa.	C. torresi	SGO.PV.22012d	51.52	Reconstructed figure	Otero et al. (2013)
F2	Upper Maa.	C. torresi	SGO.PV.22012c	51.02	Reconstructed figure
G	Lower Maa.	C. orientalis sp. nov.	HMG-2013	32.91	Actual specimen	This study
H	Cenomanian	C. hectori	BMNH P.2301	26.47	Reconstructed figure	Kriwet and Gaździcki (2003)
I	Albian	C. borealis	“TsNIGR Museum No 1/12963”	21.48	Figure on literature	Nessov and Averianov (1996)

Regarding the palatine tooth plates of extant C. milii, hatchlings exhibit a single large, ovoid tritor, whereas juveniles display a pair of narrow, elongated tritors (Figure 1C; Didier et al., 1994). As mentioned earlier, only adults possess the characteristic tritor, represented by a large, exposed pad with anteriorly directed projections (Cicimurri and Ebersole, 2015). Assuming similar tooth plate growth patterns in fossil and extant species, most specimens compared in Figure 6 are likely adults based on their single large tritoral pad with distinct projections. “TsNIGR museum No 1/12963” and SGO.PV.22012c possess two parallel tritoral features, resembling juvenile specimens. Otero et al. (2013) suggested that SGO.PV.22012c is likely an adult or subadult specimen owing to its similarity in size and shape to other specimens. “TsNIGR museum No 1/12963” is also considered an adult specimen based on its size similarity with another palatine tooth plate, as per fig. 1, 2 of Nessov and Averianov (1996), showing a species-specific tritor. Assuming a positive correlation between the palatine tooth plates and the body sizes, the body size increased from the early Late Cretaceous to the later Late Cretaceous and deceased across the K/Pg boundary.

Similar body size reductions in Danian species after the K/Pg extinction are documented in various marine organisms, including lamniform sharks and teleost fish (Friedman, 2009; Belben et al., 2017). This temporal dwarfing could be attributed to a combination of several factors, including changes in oxygen levels and food availability (Twitchett, 2007). Mollusks (bivalves), found in same sandstone unit presumed to have yielded the holotype specimen of C. orientalis sp. nov., is a potential food source for Callorhinchus based on extant species’ food habit (e.g. Di Giácomo and Perier, 1996). Mollusks also exhibit a size reduction following the K/Pg extinction (e.g. Aberhan et al., 2007). Evolutionary reduction of prey size is one likely contributor to the observed body size reduction in Callorhinchus.

Acknowledgements

This paper is based on part of master’s thesis by A.O.; we are grateful to Y. Iba at the Hokkaido University for advising on the thesis. We express gratitude to Y. Takakuwa of Gunma Museum of Natural History for advising about the description of tooth fossils and reference data of fossil holocephalans.

Author contributions

A.O. initiated this study, prepared the specimen, wrote the original draft, and revised the draft. T.N. contributed to conducting fieldwork, discussions, and draft revisions. Y.K. supervised the study, contributed to discussions, and revisd the draft. K.M. collected and preliminarily identified the specimen.

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