Introduction

The genus Adocus of moderate to large-sized semiaquatic turtles has widely been recorded from the Cretaceous to the Paleogene of Asia and North America (e.g. Hutchison, 2000; Hirayama et al., 2000; Danilov et al., 2011, 2013). This genus is easily distinguishable from other turtles even in isolated shell materials by its characteristic wide overlapping of the marginal scales onto the costal plates. At least seven valid species have been described (Hirayama, 2002; Danilov et al., 2011). Occurrence of Adocus from the Lower Cretaceous Kitadani Formation of Japan (Barremian to Aptian, Adocus sp., Hirayama, 2002) suggests that the genus Adocus appeared first in Asia. Nonetheless, these materials are rather poorly documented, making it difficult to discuss systematic positions within this genus at the species level.

In this study, we report new materials of Adocus from the Lower Cretaceous Sengoku Formation of the Kanmon Group in Miyawaka City, Fukuoka, Kyushu, Western Japan (Figure 1). We describe these materials as a new species and discuss its systematic implications for understanding the early history of this genus.

Geological setting

The Lower Cretaceous Kanmon Group distributed in northern Kyushu to western Honshu of Southwest Japan is stratigraphically divided into the lower Wakino and the upper Shimonoseki subgroups (Figure 1; Matsumoto, 1951). The Wakino Subgroup is mainly composed of black shale, conglomerate, sandstone, purple siltstone, and acid tuff, and contains abundant fresh-water mollusks (Ota, 1960). It consists of the Sengoku, Nyoraida and Wakamiya formations in ascending order (Ota, 1953). The Sengoku Formation yields a number of vertebrate fossils such as dinosaurs, crocodiles, fishes, and turtles (Okazaki, 1992, 1994). Turtle remains occur from the upper part of the Sengoku Formation that is interpreted as marginal lacustrine deposits by lithofacies, fossil assemblages and sedimentary structures (Seo et al., 1994). The Sengoku Formation has been correlated with the upper Barremian by freshwater molluscan fossils including Trigonioides and Plicatounio (Hase, 1960; Ota, 1960; Matsumoto et al., 1982; Kozai et al., 2005). The radiometric age of this formation has not been dated yet.

The upper Shimonoseki Subgroup, composed of volcaniclastic and tenestrial sediments, consists of the Shiohama, Kitahikoshima and Sujigahama formations in ascending order (Matsumoto, 1951). Although there are few index fossils, the Shimonoseki Subgroup is assigned to the Albian by K-Ar ages and magnetostratigraphic data from andesite and dacite in the Kitahikoshima Formation (Shibata et al., 1978; Imaoka et al., 1993; Matsuura, 1998).

Figure 1.

Fossil locality of the present materials and the distribution of the Kanmon Group in the northern part of Kyushu, Southwest Japan (modified from Hase, 1960).

Materials and methods

The assemblage of specimens described (KMNH VP 000,024, housed in the Kitakyushu Museum of Natural History and Human History, Kitakyushu City, Fukuoka Prefecture, Japan) was collected by Mr. Masahiro Sato in 1994, from a riverside outcrop in the Sengoku gully of Miyawaka City, Fukuoka Prefecture, Japan. These materials comprise seven isolated shell elements (Figure 2). They can be considered to be derived from a single individual because: 1) all elements are different from each other in position of the shell, 2) the seven elements are not very different from one another in size, 3) these materials occurred together within a small block of about 20 cm span, 4) the matrix consists of lacustrine mud where physical transport of coarse clastic particles and shell materials are less likely, and 5) the ornamentation on the shell surface is very well preserved.

Terminology for the shell morphology follows Zangerl (1969). The carapace and plastron lengths are estimated mainly on the basis of Adocus punctatus (Hay, 1908). Our studies on the shell reconstruction and the determination of the systematic position refer to published data on the following taxa of trionychoids: Adocus aksary Nessov in Nessov and Krasovskaya, 1984 (Syromystnikova and Danilov, 2009); A. amtgai Narmandakh, 1985; (Sukhanov and Narmandakh, 2006); A. beatus Leidy, 1865 (Hay, 1908); A. bossi Gilmore, 1919 (Gilmore, 1919); A. inexpectatus Danilov et al., 2013 (Danilov et al., 2013); A. kirtlandius Gilmore, 1919 (Gilmore, 1919); A. punctatus Marsh, 1890 (Hay, 1908); A. dzhurtsensis Syromyatnikova and Danilov, 2009 (Syromyatnikova and Danilov, 2009); Adocus sp. from Katsuyama in Fukui, Japan (Hirayama, 2002).

In addition to the genus Adocus, we used an unnamed trionychoid from the Kuwajima Formation of “Kasekikabe”, Shiramine, Ishikawa Prefecture and Ferganemys verzilini as important outgroup taxa (Hirayama, 2000; Hirayama et al., 2000).

Systematic paleontology

Figure 2.

Adocus sengokuensis sp. nov., holotype, KMNH VP 000,024. A–E, dorsal views of carapace; A, left first peripheral; B, nuchal; C, right first peripheral; D, left fourth peripheral; E, left second costal; F, G, plastron; F₁, ventral view of right hypoplastron; F₂, medial view of right hypoplastron; G, ventral view of left hyoplastron. Scale sulci are filled with grey.

Figure 3.

Line drawings of Adocus sengokuensis sp. nov., holotype, KMNH VP 000,024. A–E, dorsal views of carapace; A, left first peripheral; B, nuchal; C, right first peripheral; D, left fourth peripheral; E, left second costal; F, G, plastron; F₁, ventral view of right hypoplastron; F₂, medial view of right hypoplastron; G, ventral view of left hyoplastron. Scale sulci are drawn with bold lines.

Figure 4.

Shell reconstruction of Adocus sengokuensis sp. nov. A, dorsal view; B, ventral view. Scale sulci are drawn with black and gray thick lines. Black lines (thick and thin) are based on preserved elements. Gray lines (thick and thin) are based on A. punctatus (after Hay, 1908).

Table 1.

Measurements in mm of shell elements of KMNH VP 000,024.

Table 2.

Comparisons of shell characters in Adocus. A. beatus is treated as representative of North American taxa.

Discussion

KMNH VP 000,024 is assigned to the Adocidae based on the presence of the following synapomorphies: shell sculpturing with regular and fine pits, shallow and narrow scale sulci, weakly developed rib heads, and very flat ventral surface of costals (Syromyatnikova and Danilov, 2009; Danilov et al., 2011). Furthermore, the left second costal in the present material (Figures 2D and 3D) is clearly overlapped by the fifth marginal, which has been regarded as an autoapomorphic character of the genus Adocus (Hirayama, 2002; Syromyatnikova and Danilov, 2009; Danilov et al., 2011). Thus, KMNH VP 000, 024 is identified as Adocus.

A. sengokuensis differs from the other species of this genus in having the smallest shell size, a cervical scale wider than it is long, and a narrow distal projection of the first pleural scale on the fourth peripheral (Table 2).

The shell length of A. sengokuensis is estimated as smaller than 30 cm long, whereas those in the Late Cretaceous and Paleogene species, including A. aksary, A. amtgai, A. beatus, A. bossi, A. inexpectatus, and A. kyzirkmensis, have been more than 35 cm in length (Gilmore, 1919; Danilov et al., 2013). In addition, the Early Cretaceous Adocus sp. from the Kitadani Formation of the Tetori Group in Katsuyama, Fukui, Japan, appears to be larger than A. sengokuensis (ca. 37 cm long; Hirayama, 2002). On the other hand, estimated shell length of an unnamed trionychoid taxon from the Lower Cretaceous Kuwajima Formation of the Tetori Group and Ferganemys, a primitive adocid from the Early Cretaceous of Central Asia, are less than 30 cm long (Hirayama, 2000; Hirayama et al., 2000; Syromyatnikova, 2011). This character distribution suggests that the small shell size of A. sengokuensis is a plesiomorphic state among the genus Adocus as well as the trionychoids. The shell size may have gradually become larger in Adocus during the Cretaceous.

A. sengokuensis is unique in the possession of a wide cervical scale within the genus Adocus (Figures 2B and 3B). The cervical is narrow and trapezoidal or rectangular in most species of this genus such as A. beatus, A. bossi, A. aksary, A. amtgai and A. sp. from the Lower Cretaceous Kitadani Formation of Fukui, Japan, or absent in A. sp. from the Upper Cretaceous Tamagawa Formation of Kuji, Iwate, Japan (Table 2; Hirayama, 2002; Syromyatnikova and Danilov, 2009; Hirayama et al., 2010; Danilov et al., 2013). As cervical shape is relatively stable through ontogenetic growth in living turtles (Hirayama, personal observation), this feature has been used as one of the informative character states in turtle taxonomy since the later 19th century (e.g. Boulenger, 1889). In other trionychoids such as the nanhsiungchelyids and Ferganemys, as well as an unnamed trionychoid from the Lower Cretaceous Kuwajima Formation of Shiramine, Ishikawa, Japan (Hirayama, 2000; Sukhanov et al., 2008; Syromyatnikova, 2011). the cervical scale is wider than it is long. Thus, the wide cervical scale in A. sengokuensis is considered as a plesiomorphic condition for Adocus. A narrow distal projection of the first pleural scale bordered by the fourth and fifth marginals of the fourth peripheral plate in A. sengokuensis seems quite unique in Adocus as well as the other turtles (Figures 2D and 3D). Other species of this genus have no similar lateral projection of the first pleural. Thus, this character is considered as an autoapomorphic character of this taxon. Based on the above characters, KMNH VP 000,024 is distinguished from its congeners.

In conclusion, new adocid materials from the Lower Cretaceous Sengoku Formation of Miyawaka, Fukuoka, Japan, are described as the most basal species of the genus Adocus, A. sengokuensis. This species is distinguished from its congeners in the possession of two plesiomorphic characters such as small shell size and wider cervical scale, and one autoapomorphic character, a narrow lateral projection of the first pleural of the fourth peripheral. With reference to the recent results of cladistic analysis for Adocus. A. sengokuensis appears to be the most plesiomorphic taxon within the genus in lacking two synapomorphies: marginal scale overlapping costals beginning with the third or fourth (Figure 5, node 1; e.g. Syromyatnikova and Danilov, 2009; Danilov et al., 2013). and a cervical scale longer than it is wide (node 2; see above). Further research on turtles in the Late Jurassic and Early Cretaceous turtles from Asia will shed light on the origin and early evolution of the superfamily Trionychoidea.

Figure 5.

Phylogenetic tree showing relationships of Adocus and Ferganemys (Adocidae), modified from Danilov et al. (2013). Synapomorphic characters at each node are as follows: 1) marginal scale overlapping costals begins from third or fourth marginal, 2) cervical scale longer than it is wide, 3) fifth and posterior marginals overlapping onto costals, 4) shell sculpturing with fine pits or dots, reduced proximal head of thoracic rib, and large semicircular facet for ischium on xiphiplastron.