INTRODUCTION

The teeth of the Neogene elasmobranchs on the Sakhalin Island (Russia) are extremely rare, unlike in the neighboring Hokkaido (North Japan), where several finds of the fossil sharks are known (Kuga, 1985; Kuga and Nakata, 1980). In the 20th Century, the single tooth of the extinct shark Isurus hastalis (Agassiz, 1838) from the Upper Miocene beds of Sertunai Formation at northwest coast of the island was reported (Kuzina and Ratnovsky, 1970a, b; Zhidkova et al., 1974), but this specimen is absent in the appropriate collections and is probably lost. To date, only two teeth have been reliably known. Both teeth came from two different outcrops of the Middle-Upper Miocene Kurasi Formation (Savitskyi, 1982; Zhidkova, 1986; Gladenkov et al., 2002) in the south-western coast of the Sakhalin in the Tomari City district. The tooth belonging to a thresher shark closely related to the modern species Alopias superciliosus Lowe, 1841, was found in the sediments that were previously assigned to the Late Oligocene Kholmsk Formation (Nazarkin and Malyshkina, 2012), but presently treated as belonging to the Kurasi Formation. Another tooth, attributed to the hook-toothed mako Isurus planus Agassiz, 1856, was discovered from the same beds of Kurasi Formation, but cropped out in 13 km north (Nazarkin, 2013) (Figs. 1a, 1b).

Fig. 1.
figure 1

Novosiolka-1 locality, Chekhov Formation, Lower Miocene, Tomari district, Sakhalin Island: (a) geographic position of the Novosiolka-1 locality, (b) scheme of the geological structure of the Upper Oligocene–Lower Pliocene in the Tomari District of the Sakhalin (adapted from Gladenkov et al., 2002), (c) sandstone outcrop of the Chekhov Formation in the northern part of the Novosiolka-1 locality, (d) nodules on the beach under the outcrop, (e) fragments of a nodule containing a carbonate-siliceous core with a C. planus tooth described in the paper. (★) Novosiolka-1 on the map, () place of the first find of the C.planus tooth in the Kurasi Formation, Middle-Upper Miocene (Nazarkin, 2013).

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Due to the rarity of the Neogene selachians remains in the Sakhalin, in this article we report a one more finding of the hook-toothed mako recently discovered in the deposits of the Chekhov Formation of the south-western Sakhalin by the third author. This formation deposits are dated back to the Early Miocene (Gladenkov et al., 2002). The new finding became the third reliable record of the Neogene shark on the Sakhalin Island.

In the World, the remains of the hook-toothed mako are not common. They are known only from the Pacific basin, and recorded in Japan, Sakhalin, South Korea, California, and Australia, mainly from the Middle and Late Miocene and Pliocene beds (Agassiz, 1856; Jordan, 2007, 1922; Kemp, 1991; Yabumoto, Uyeno, 1994; Nakano, 1999; Takakuwa et al., 2009; Nazarkin, 2013; Yun, 2022; Malyshkina et al., 2023). Finds of this species are extremely rare in the Lower Miocene deposits, and the tooth described herein is probably the most ancient record of the species. The taxonomic position of the hook-toothed mako is currently a debatable issue. According to various researchers, it assigns to the modern mako shark genus Isurus Rafinesque, 1810, or to the modern great white shark genus Carcharodon Müller et Henle, 1838, or to the extinct genus Cosmopolitodus Glickman, 1964. Based on the morphological features of the dental system of these three genera, we support the third version; the placement of hook-toothed mako in Cosmopolitodus is discussed below.

MATERIALS AND METHODS

Geology

The Chekhov Formation (N1čh) is distributed locally in the southern part of the Sakhalin and is exposed in the coastal cliffs on the western and eastern sides of the island. The formation is represented by unevenly alternating tuffs, volcanic breccias, tuffogenic sandstones, which are dominated by tuffaceous siltstones and mudstones, interlayers and lenses of coals, carbonaceous-argillaceous shales, secant and bedded bodies of basaltic andesites and basalts. Higher up the section, the rocks of the Chekhov Formation become finer clastic (Kuzina and Ratnovsky, 1970b; Zhidkova et al., 1974; Savitskii, 1982; Gladenkov et al., 2002). In the vicinity of the city of Tomari, the Chekhov Formation is represented by tuffaceous sandstones exposed in the cliffs of the sea terrace, containing marl nodules in the lower part of the outcrop and single thin interlayer of brown coals in its upper part (Vereshchagin, 1972). In the vicinity of Chekhov city, the deposits of the Chekhov Formation overlap with erosion by the Lower Miocene Verkhnedui Formation. The beds of the Chekhov Formation were formed in the Early Miocene, under conditions of marine sedimentation and intense volcanism (Gladenkov et al., 2002). Two assemblages of thermophilic bivalve mollusks originate from the deposits of the Chekhov Formation. The lower part of the deposits contains numerous Chlamys branneri, Mizuhopecten cf. subyessoensis, Mytilus ochotensis, Thyasira cf. bisecta, Macoma cf. simizuensis. A characteristic thermophilic assemblage of fauna with Glycimeris, Chlamys, Dosinia, Macrocallista, and others was discovered at the top of the Chekhov Formation (Gladenkov et al., 2002). The composition of these assemblages evident the Early Miocene age of the deposits (Kuzina and Ratnovsky, 1970b; Vereshchagin, 1972; Zhidkova et al., 1974; Gladenkov et al., 2002; Golozubov et al., 2012). In the middle part of the formation, a peculiar floristic complex with the dominance of ferns was found (Krasilov et al., 1984).

Locality and Material

The vertebrate fossils from the Chekhov Formation were not previously known. The isolate shark tooth was collected in the coastal cliff of the Tartar Strait at Novosiolka-1 locality, 1.3 km north from the Novosiolka River mouth (47.6734° N; 142.0037° E). This locality is placed 30 km north from the Chekhov City, and 14 km south of the Tomari City (Fig. 1). The deposits are exposed in a coastal cliff terrace 460 m long and about 30 m high (Fig. 1c). The section reveals sediments of the Chekhov Formation with light gray sandstones of tuffogenic origin with rare thin interlayers of brown coals and rare spherical nodules about 15 cm in average diameter. The nodules consist of gray fine-grained sandstone with siliceous-carbonate cement, and contain small inclusions of plant remains and shells of bivalves Macoma sp. and Nuculana sp. The shark tooth described herein was found in a nodule lying on a sandy beach at the foot of the outcrop (Figs. 1d, 1e). The specimen is kept in the private collection of A.V. Solovyow.

RESULTS AND DISCUSSION

Systematic Part

Subclass: Elasmobranchii

Cohort: Euselachii

Subcohort: Neoselachii

Superorder: Galeomorphii

Order LAMNIFORMES Berg, 1958

Family LAMNIDAE Bonaparte, 1838

Genus Cosmopolitodus Gliсkman, 1964

Type species: Oxyrhina hastalis Agassiz, 1843.

Cosmopolitodus planus (Agassiz, 1856)

(Fig. 2)

Fig. 2.
figure 2

Tooth of Cosmopolitodus planus, Novosiolka-1 locality, Sakhalin Island; lower part of the Chekhov Formation, Lower Miocene, (a) photograph, (b) drawing. Scale: 1 cm.

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Oxyrhina plana: Agassiz, 1856. P. 274.

Material. One tooth from the Novosiolka-1 locality, Sakhalin Island; lower part of the Chekhov Formation, Lower Miocene.

Description. The right upper lateral tooth, 27 mm in total height, is embedded in the matrix with its lingual side. The height of the crown is 15 mm, the height of the root is about 12 mm. The crown is strongly inclined and curved distally, the angle of inclination is 57°. The cutting edges are smooth. The distal cutting edge is straight in the upper part, steeply curved at the level of 1/4 of the crown height. The mesial crown edge is smoothly curved in the upper half of the crown height. The lower part of the mesial crown edge is destroyed. The cusplets are absent. In the middle of the crown base there is a small vertical fold of enameloid. The line of contact between the crown and the root in the center is smoothly curved upward. The labial face of the root is eroded. The root branches are wide, rounded, and, apparently, symmetrical.

DISCUSSION

Comparison

The tooth under discussion belongs to a shark of the family Lamnidae, as indicated by its relatively large size, the triangle cusp, and the short and wide root branches. The combination of these features with the distally bent crown and reduced cusplets leaves no doubt that this is the upper lateral tooth of the hook-toothed mako C. planus. The described tooth differs from the upper lateral teeth of the closest species C. hastalis (Agassiz, 1843) by a strong inclination and curvature of the crown: in C. hastalis, the teeth of this jaw position have a more vertical crown, which is characterized by a slight inclination (about 80°), rather than a curve (compare Malyshkina et al., 2023. Figs. 4, 5A–5N). This tooth differs from the upper lateral teeth of sharks of the genus Isurus in the rounded branches of the root, the absence of accentuated contact between the crown and root on the labial side, as well as the large distal inclination and curvature of the crown, and the absence of cusplets and shoulders (compare Malyshkina et al., 2023. Figs. 4, 5O−5Z).

Taxonomy of the Fossil Makos

The assignment of this fossil species to the modern genera Carcharodon Müller et Henle, 1838 (with one living species), or Isurus Rafinesque, 1810 (with two living species), or to the extinct genus Cosmopolitodus Glickman, 1964, is currently a debatable issue. We note right away that the evolution and taxonomy of these three genera require further careful study and revision, but we will express only some considerations.

Glickman (1964) erected the genus Cosmopolitodus for the Miocene lamnids with a smooth cutting tooth edges (i.e. Cosmopolitodus hastalis and C. plicatilis), to separate the species with smooth tooth crowns from those with serrated teeth, grouped into Carcharodon. Cappetta (2012) also attributed hastalis, the ancestral form of the great white shark, to Cosmopolitodus. Recently (Ehret et al., 2012), the trend to unite and enlarge taxa has led to a proposal to combine Cosmopolitodus and Carcharodon under Carcharodon. Kent (2018), justifying the merging of Cosmopolitodus and Carcharodon, indicates that the main difference between Cosmopolitodus and Carcharodon consists in the smooth or serrated cutting edge, respectively. In our opinion, this feature does not exhaust the entire spectrum of differences. The dentition of the Middle Miocene Cosmopolitodus hastalis exhibits a significantly greater degree of both mono- and dignathic heterodonty than that of the Carcharodon.

Monognathic heterodonty is expressed in the fact, that the posterior teeth of the upper jaw have significantly greater distal crown inclination than the anterior ones, as well as a more pronounced asymmetry of the root branches and a large mesio-distal thickness. These features in Carcharodon are faintly expressed or absent. In the lower jaw, the differences between the anterior and posterior teeth in C. hastalis are much more pronounced than in Carcharodon: the former has rather slender lower anterior teeth, whereas in the latter the lower anterior teeth are wide, with shortened branches.

Dignathic heterodonty in C. hastalis is manifested in a significant difference in the inclination of the crowns, in the length of the root branches, and in the general design of the teeth of the same position in the opposite jaws. In Carcharodon dignathic heterodonty is almost not observable. Obviously, differences in dentition cause the different mechanics of capturing, holding and fragmenting the prey, what influences on feeding behavior (ability to whale-eating), ecology, and other morphological and physiological features of these sharks.

Actually, the diet of the C. carcharias is very various including different cetaceans whereas the smooth-toothed large lamnid sharks feed mainly fish. For example, Isurus oxyrinchus feeds mainly on fishes and squids and only very large adults may take small cetaceans (Compagno et al., 2005). Collareta et al. (2017) indicated hunting of semi-serrated Carcharodon hubbelli Ehret, 2012 on cetaceans on the base of analysis of the damaged mammal bones.

Ehret et al., (2012) described semi-serrated form, Carcharodon hubbelli, and examined Carcharodon teeth from Late Miocene (Messinian) to Early Pliocene (Zanclean) marine fossils in California, the San Mateo, Capistrano, and Purisima Formations, where they postulated the Carcharodon hastalis-hubbelli-carcharias transition. According to these authors, geochronological and biostratigraphic data show that the entire evolutionary transition from C. hastalis to C. carcharias occurred between 6.9 and 5.3 Ma (Messinian), with this process coinciding in California, Peru and Chile, and also with the first appearance of C. carcharias elsewhere in the world. The evolutionary sequence observed in the dentition of the Carcharodon lineage is a gradual morphological transition from ancestral non-serrated C. hastalis through semi-serrated C. hubbelli to fully serrated C. carcharias. The specialization seems to have been associated with the transition from picsivorous C. hastalis to cetivorous C. hubbelli. Ehret et al. (2012) cite whale bones with teeth of C. hubbelli embedded in them as evidence that C. hubbelli feeds on marine mammals. In the Pisco Formation in the Late Miocene, teeth of C. hastalis of a typical non-serrated form were found (Takakuwa, 2014), that is, simultaneously with the serrated form of C. hubbelli. In addition, in the Miocene to Pliocene there is the large-toothed Atlantic and Pacific form with a smooth cutting edge, attributed to Carcharodon plicatilis (Carrillo-Briceño et al., 2015; Staig et al., 2015) or Cosmopolitodus plicatilis (Landini et al., 2019). This species seems to be a sister extinct taxon to C. hastalis.

In our opinion, above arguments are the sufficient basis for differentiation of smooth-toothed Cosmopolitodus and serrae-toothed Carcharodon and attribution of C. hastalis with closest species hook-toothed mako C. planus and C. plicatilis to Cosmopolitodus. The combination Cosmopolitodus planus was first used by Yun (2022).

Another popular affiliation of hooked tooth mako into genus Isurus seems inappropriate because of less degree of similarity of C. planus teeth with any representatives of Isurus, than with C. hastalis. Cappetta (2012. P. 218) remarked: that “the peculiar tooth morphology of “Isurus” planus (Agassiz, 1856) … indicates that the species could represent a different genus”. The validity of this view was confirmed by evolutionary data that suppose the origin of C. carcharias from a common ancestor with other lamnids (Nyberg et al., 2006). According to molecular data (Martin, 1996; Martin et al., 2002), C. carcharias divergented from the common ancestor of Recent Isurus species, I. oxyrhinchus and I. paucus, about 40 Ma.

Geographic and Stratigraphic Distribution

(Fig. 3)

Fig. 3.
figure 3

Map of the world finds of Cosmopolitodus planus, compiled according to the literature data cited in the text: (◼) Pliocene, () Upper Miocene, (⚫) Middle Miocene, (★) Lower Miocene.

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Cosmopolitodus planus is a rather rare case of a pelagic species that lived in only one ocean. Its findings are confined exclusively to the Neogene deposits of the Pacific. Although the oldest finds were recorded by Kemp in the Upper Oligocene Jan Juc Formation of Australia (Kemp, 1991), the teeth apparently of the lower jaw, cited by him as an illustration of C. planus (Kemp, 1991. Figs. 23A, 23B), should be referred to a different taxon of Lamnidae due to its convex labial side of the crown and too prominent central protuberance of the root. Since the author indicates I. planus occurring together in the same formation with Striatolamia macrota Agassiz, 1843 (Fig. 13M), which is the strictly Eocene species, one can assume either an error in the interpretation of the material or its heterosynchrony. Fitzgerald (2004) confirmed the Late Oligocene age of the Jun Juc Formation and also reported (without illustration) I. planus teeth, associated with the bones of the Neogene cetaceans together with the teeth of sharks Carcharias taurus (which first appeared in the Pliocene), and Carcharodon (Carcharocles) angustidens (the typical Oligocene species). Such mixture may indicate heterosynchrony of the material or wrong species identification. Accordingly, it is impossible to confirm the existence of C. planus in the Oligocene.

In the Early Miocene, findings of C. planus are exceptionally rare. Kemp (1991) recorded teeth of C. planus from the both Lower Miocene Batesford Limestone and Muddy Creek Marl of Australia. However, the data obtaining from the foraminifera assemblages studying shows that the former were deposited in the latest Early Miocene to early Middle Miocene, while the latter were deposited in the middle Middle Miocene to lower Late Miocene (Fitzgerald, 2004). The teeth of C. planus from the Lower Miocene Puebla Formation of Australia (Kemp, 1991. Pls. 23A−23C) obviously belong to another species similar with C. hastalis. Record of Welton (1972) from the Lower Miocene Astoria Formation of Oregon is listed under question mark without illustration, and cannot be confirmed. Thus, the tooth described here is most likely the earliest find of the species in the world.

Most findings of C. planus are known in the Middle and Late Miocene: it recorded in Sakhalin (Nazarkin, 2013), Japan (Uyeno and Tuematsu, 1984; Kuga, 1985), South Korea (Malyshkina et al., 2021, 2023; Yun, 2022), Australia (Kemp, 1991; Fitzgerald, 2004), Mexico (González-Rodríguez et al., 2013), and several localities of California (Jordan, 1907; Domning, 1978; Deméré et al., 1984; Perry, 1993; Bossenecker, 2011) and Oregon (Arnold and Hannibal, 1913). Pliocene records are scarce and restricted to California (Ashby and Minch, 1894; Jordan, 1922).

Thus, the hook-toothed mako remains are quite rare in the fossil record, and each finding is still of scientific interest.