Belemnites were a highly successful group of Mesozoic cephalopods. Their early evolutionary history is, however, still poorly understood. Previously it has been suggested that the Order Belemnitida originated in the earliest Jurassic (Hettangian) with small forms in Europe, and their occurrence was restricted to Europe until the Pliensbachian. Here we report a large specimen of the Suborder Belemnitina from the upper Pliensbachian of the Kuruma Group in Central Japan. This is the first reliable record of Pliensbachian belemnites from outside Europe and the Mediterranean area. The specimen from Japan differs from those of the coeval European genera by its large rostrum. The rostrum diameter of this specimen rather resembles that of the Middle Jurassic belemnite genus Megateuthis, the largest belemnite ever observed. This new finding provides additional evidence of the existence of belemnites with extremely large rostra in the Early Jurassic of the western paleo-Pacific area, thereby shedding new light on the early evolutionary history of belemnites.
Introduction
Belemnites (Order Belemnitida) were a highly successful group of coleoid cephalopods, which flourished in the world oceans in the Jurassic and Cretaceous (e.g. Mutterlose, 1990; Doyle, 1993). Although the evolutionary history of belemnites through the late Early Jurassic to Cretaceous has been well studied (e.g. Mutterlose, 1990, 1998; Doyle, 1993; Schlegelmilch, 1998; Christensen, 2002; Kostak, 2004; Iba et al., 2011; Weis et al., 2012), their early evolution is not yet fully understood. Previously the Hettangian (earliest Jurassic) genus Schwegleria (Suborder Belemnitina) has been considered as the root of all belemnites (e.g. Schlegelmilch, 1998; Weis and Delsate, 2006). It was assumed that belemnites originated in northern Europe as very small forms with less than 5 mm in rostrum diameter in the earliest Jurassic and were restricted to the European shelf areas until the Pliensbachian (Doyle, 1994). In the Toarcian they spread geographically, obtaining a global distribution (Doyle, 1994; Schlegelmilch, 1998).
In contrast to this hypothesis, Iba et al. (2012) described recently Hettangian belemnites from the Niranohama Formation in northeastern Japan. These authors assigned the specimens to a new species, which was included in the family Sinobelemnitidae. Although sinobelemnitid belemnites were already described from the Carnian (Upper Triassic) of China by Zhu and Bian (1984), this occurrence has been overlooked without detailed investigation of the specimens in previous European studies (e.g. Doyle, 1993, 1994; Doyle et al., 1994; Weis and Delsate, 2006). Iba et al. (2012) reconfirmed the belemnite affinity of Chinese Triassic sinobelemnitids based on their morphological features. These East Asian records of early belemnites extend the origin of the belemnites back by ∼33 Myr into the Late Triassic, and imply that belemnites did not originate in northern Europe. In addition to this new sinobelemnitid belemnite species Iba et al. (2012) also described an extremely large specimen of the Suborder Belemnitina based on two specimens from the Niranohama Formation. Based on this fossil evidence, they concluded that already in the earliest Jurassic the Belemnitida had a higher diversity than previously thought. The stratigraphically and geographically isolated occurrence of a larger form of the Belemnitina from northeastern Japan, however, still leaves an open question the evolutionary relationship between the Hettangian species from Japan and other larger forms from the late Early to Middle Jurassic of Europe.
Here we describe a single incomplete but extremely large belemnite rostrum from the Lower Jurassic (upper Pliensbachian) Teradani Formation, Kuruma Group in Central Japan (Figure 1). This is the first reliable record of a Pliensbachian belemnite from outside Europe and the Mediterranean area (e.g. Turkey). This finding fills the stratigraphic gap between the Hettangian forms from Japan and the large-sized European species. This study therefore sheds new light on the early evolutionary history of larger belemnites in the Early Jurassic.
Figure 1.
Map showing the locality of the examined Pliensbachian belemnite in Japan. Geological map is modified from Goto and Takizawa (1988). Star indicates the belemnite fossil locality.
Geological setting
The belemnite specimen described herein was discovered in a floated block of dark-gray-colored muddy sandstone, which comes from the Teradani Formation of the Kuruma Group exposed along the Teradani Ravine (Figure 1). The Kuruma Group (Lower Jurassic) is mainly composed of siliciclastic deposits of shallow marine to brackish origin, and is subdivided into the Jogodani, Kitamatadani, Negoya, Teradani, Shinatani, and Otakidani formations, in ascending order (e.g. Kobayashi et al., 1957; Goto and Takizawa, 1988; Shiraishi, 1992). The Teradani Formation contains ammonites attributed to Amaltheus sp. and Canavaria sp. ex gr. geyeriana, and are therefore assigned in age to the late Pliensbachian (Sato, 1955, 1962; Sato and Westermann, 1991).
Systematic description
Repository.—The specimen here described is deposited in the Tateyama Caldera Sabo Museum (Tateyama Town, Toyama Prefecture). The collection number is TCSM-J1-0001.
Suborder Belemnitina Zittel, 1895
Remarks.—The present specimen can be clearly distinguished from representatives of the Order Aulacocerida, which diversified mainly in the Paleozoic and Triassic. One of the diagnostic features of the Order Belemnitida: the calcitic rostrum with concentric growth lines, is developed (Figure 2a–f). The aragonitic rostra of the Aulacocerida show irregularly-shaped growth rings, which usually disappeared during diagenesis, causing recrystallisation into calcite. In almost all rostra of the known Aulacocerida growth rings are not preserved (e.g. Mariotti and Pignatti, 1993) with only a few exceptions of well preserved material (e.g. Dictyoconites in Bandel, 1985, figures 22 and 23). Due to the fragmentary nature of the specimen, the apical and alveolar regions are not preserved. The presence or absence of the apical and/or alveolar groove can therefore not be confirmed. These grooves are the diagnostic features of the two different suborders (Belemnitina and Pachybelemnopseina) of the Belemnitida. The large size of the preserved rostrum is, however, only comparable to those of the largest forms of the Belemnitina. The specimen is therefore preliminarily assigned to the Suborder Belemnitina.
Family Uncertain
Genus and species indeterminate
Figure 2
Material.—A single incomplete specimen (TCSM-J1-0001) derived from the Teradani Formation of the Kuruma Group in Teradani, Asahi Town, Toyama Prefecture, Central Japan (Figure 1).
Description.—The rostrum is extremely large, and possibly conical in outline (Figure 2a–e). Outline and profile are symmetrical (Figure 2a–c). Concentrated growth lines can be observed in the transverse and longitudinal sections of the rostrum (Figure 2c–f). The total preserved length is 45 mm. Transverse section of rostrum is circular and there is no flattened side (Figure 2d, e). Diameters at the anterior end and posterior end are 30 mm and 25 mm respectively. The section of the early growth stage of the rostrum (<20 mm in diameter) is elliptical (possibly laterally compressed) (Figure 2f). Neither grooves or lateral lines have been observed. There is no structure indicating the presence of an epirostrum in this specimen (Figure 2c–f).
Comparison.—Since the apical and alveolar regions are not preserved in the Teradani specimen, only the middle (stem) region of the rostrum is available for this study. In the Belemnitina, the diameter of the alveolar region is generally larger than those of the apical and stem regions. Thus maximum rostrum diameter of the Teradani specimen is estimated to reach much more than 30 mm, and differs clearly from those of the coeval European genera like Bairstowius (maximum dorsoventral diameter of rostrum (Dvm): 8 mm), Hastites (Dvm: 8 mm), Passaloteuthis (Dvm: 26 mm), and Gastrobelus (Dvm: 15 mm) (e.g. Doyle, 1994; Schlegelmilch, 1998). The large diameter of the Teradani specimen corresponds to the largest belemnites ever observed. They are the Niranohama specimens (Dvm at alveolar region (Dvma) > 33 mm) from the Hettangian of Japan (Figure 2g–i), Megateuthis of the Megateuthididae (Dvm at stem region (Dvms) = 30 mm; Dvma = 50 mm) from the Middle Jurassic of Europe, Acroteuthis (Dvms = 39 mm; Dvma = 42 mm), and Pachyteuthis (Dvms = 39 mm; Dvma = 40 mm) from the latest Jurassic to Early Cretaceous of Europe (Cylindroteuthididae) (Saks and Nal'nyaeva, 1966; Weis and Mariotti, 2007; Iba et al., 2012). For the Hettangian Niranohama belemnites the dorsoventral and lateral diameters of the rostrum near the protoconch (Figure 2g, h) are estimated at 26.4 and 20.9 mm, respectively (Iba et al., 2012). The rostrum diameters of the Teradani specimen (30 mm at the anterior end of the preserved stem region of the rostrum) are probably larger than those of the Niranohama specimens in the diameter of the stem region (Figure 2). It is therefore suggested that the Teradani belemnite has a larger rostrum than the Niranohama specimens (Figure 2g–j), and can sizewise be rather compared to Megateuthis (Figure 2k), Acroteuthis, and Pachyteuthis.
Both the Niranohama specimens and Megateuthis have a laterally compressed rostrum, a feature that is missing in the Teradani specimen except for its early growth stage (<20 mm in diameter). Transverse sections of the rostrum of Megateuthis frequently show the shape of an 8 (Figure 2k), due to the very incised dorsolaterally placed grooves (Weis and Mariotti, 2007), a feature that is missing in the Teradani specimen. Rostra of Acroteuthis and Pachyteuthis are dorsoventrally compressed and their apical line is strongly shifted to the ventral side. These features cannot be observed in the present specimen. The Teradani belemnite is therefore not assigned to any of the Megateuthididae, Cylindroteuthididae or other well-known taxa. Further specimens are needed to reveal its systematic assignment.
Figure 2.
Extremely large belemnite rostra from the Jurassic of Japan and Europe. a–f, Belemnitina gen. et sp. indet. (TCSM-J1-0001) from the Teradani Formation (upper Pliensbachian) from the northern part of Central Japan; a, possible dorsal or ventral view; b, possible lateral view; c, longitudinal section; d, transverse section of the anterior end of the preserved rostrum; e, f, transverse section of the posterior end of the preserved rostrum; g–j, Belemnitina gen et sp. indet. from the Niranohama Formation (Hettangian) from northeastern Japan (Iba et al. 2012, figure 3K, L); g, lateral view (UMUT MM 07078: University Museum, University of Tokyo); h, longitudinal section (UMUT MM 07078); i, transverse section of the alveolar part (UMUT MM 07078); j, transverse section of the alveolar part (UMUT MM 31003). Ventral side to the bottom; k, Megateuthis suevica from the Bajocian of Luxembourg (Weis and Mariotti, 2007, pl. 6, fig. 3) (MnhnL BM466: Musée National d'Histoire Naturelle du Luxembourg). Transverse section of the rostrum just behind the alveolar part. Ventral side to the bottom.
Discussion and implications
Previously it has been suggested that the Order Belemnitida originated in the Hettangian (earliest Jurassic) with small forms in Europe, and its occurrences was restricted to Europe and the Mediterranean area (e.g. Turkey) until the Pliensbachian (e.g. Doyle, 1994). A large taxon of the Suborder Belemnitina reported here is the first reliable record of Pliensbachian belemnites from outside Europe and the Mediterranean area. The Japanese specimen clearly differs from those of the coeval European genera by its large-sized rostrum. Previously such large-sized belemnite rostra have been seen as typical for the Bajocian (Middle Jurassic) (Schlegelmilch, 1998). For example, Megateuthis (Figure 2k), the largest belemnite ever observed, reached about 50 mm and 700 mm in maximum diameter and length of rostrum, respectively (e.g. Schlegelmilch, 1998; Weis and Mariotti, 2007). The recent discovery of a large belemnite rostrum from the Hettangian Niranohama Formation in northeastern Japan revealed that belemnites with extremely large rostra (Dvma > 33 mm) already existed in the earliest Jurassic (Figure 2g–j). The evolutionary relationship between the large forms of the Hettangian and the Middle Jurassic megateuthidids is still enigmatic. The Teradani form potentially fills this stratigraphic gap, though its evolutionary relationship with either of these forms remains problematic.
This new finding from Japan provides additional evidence for the presence of belemnites with extremely large rostra in the Early Jurassic. It also suggests that such large belemnites were already common in the Early Jurassic, at least from the Hettangian to the late Pliensbachian in the western paleo-Pacific. The early evolutionary history of the belemnites, especially of their larger forms, should therefore be reconsidered focusing on belemnite records from the paleo-Pacific area.
Acknowledgements
Kazumi Nabeshima, Kazuhito Nabeshima (Toyama), Tadashi Sato (Fukada Geological Institute), and the Tateyama Caldera Sabo Museum provided the belemnite specimen for this study. The specimen was collected by Kazumi Nabeshima and Kazuhito Nabeshima from a protected area in 1987. Toyama Prefectural Board of Education has approved Kazuhito Nabeshima's application to collect this specimen ex post facto in 2013. This research was financially supported by Japan Society for Promotion of Science grant no. 25800285 (to YI).
