Ocadia tanegashimensis, Takahashi, Akio, Ōki, Kimihiko, Ishido, Takahiro & Hirayama, Ren, 2013

Takahashi, Akio, Ōki, Kimihiko, Ishido, Takahiro & Hirayama, Ren, 2013, A new species of the genus Ocadia (Testudines: Geoemydidae) from the middle Miocene of Tanegashima Island, southwestern Japan and its paleogeographic implications, Zootaxa 3647 (4), pp. 527-540 : 530-534

Ocadia tanegashimensis sp. nov.

(Suggested Japanese name: Tanegashima hanagame) ( Figs. 2–4 ; Table 1)

Holotype. A shell, deposited in the collection of the Minamitane–cho Board of Education (MTE1).

Type locality. South slope located approximately 1.5 km southwest from the Kaminaka crossroad on Prefectural Road 588, Kawachi, Minamitane–cho, Kagoshima Prefecture, Japan.

Horizon. Middle part of Kawachi Formation (early middle Miocene), Kukinaga Group.

Etymology. The specific epithet refers to the name of the island (Tanegashima Island) from which the fossil was discovered.

Diagnosis. A medium–sized geoemydid species larger than O. sinensis , characterized by a combination of the following character states: the first vertebral expanded anterolaterally, reaching the second marginal; the second and third vertebral scutes much narrower than the first vertebral anteroposteriorly; a medial and two lateral obtuse discontinuous keels on carapace; length of the entoplastron as long as the interhyoplastral suture; the costals dovetailed with one another in outline; the third pleural overlapping only the sixth and seventh peripherals.

Description. Shell. The shell is incompletely preserved and strongly depressed dorsoventrally. It lacks the eighth neural; the first and second suprapygals; the right fifth, eighth and eleventh peripherals; the left sixth and eleventh peripherals; and the pygal. The shell margin is not emarginated anterolaterally and posterolaterally. The shell surface is smooth, lacks growth annuli, and has an obtuse medial and two lateral discontinuous knob–like longitudinal keels. The carapace and plastron are moderately thick (carapace: 10.3 mm thick in the middle of the thickened part of the nuchal, 9.4 mm in the posterior part of the fifth neural, and 8.5 mm in the second peripheral; plastron: 11.5 mm thick in the left lateral part of the gular lip, 8.0 mm in the posterior corner of the entoplastron, 4.9 mm in the posterolateral corner of the right hyoplastron, and 12.8 mm in the posteriormost part of the left femoral lip).

The nuchal is hexagonal and slightly short–sided posteriorly. The first neural is barrel–shaped and the second to seventh neurals are hexagonal and short–sided anteriorly. Widths of the costals are variable; the second, fourth, sixth, and eighth costals are distally expanded, whereas the third and fifth costals are distally contracted. The first costal contacts the first to third peripherals and the second costal connects the third and fourth peripherals laterally. The remaining costals articulate with the adjacent two peripherals distally.

The cervical scute is rectangular and longer than wide in dorsal view and trapezoidal and much wider than long in ventral view. The first vertebral is slightly longer than wide (83.7 mm long and 81.2 mm wide) and laterally notched, reaching the posteromedial corner of the second marginal. The second and third vertebrals are nearly rectangular in shape and narrower than the first vertebral (the second, 67.4 mm long and 67.6 mm wide; the third, 70.3 mm long and 73.2 mm wide). The fourth is obviously wider than the second and third vertebrals. The first pleural contacts the posterolateral corner of the first marginal anteromedially and the second to fifth marginals anterolaterally. The second pleural appears to contact the fifth and sixth marginals laterally, and the third pleural contacts the sixth to eighth marginals. The fourth pleural contacts the eighth to tenth marginals. Of the four pleural scutes, the first covers four and the second and fourth cover three peripherals, whereas the third covers only two peripherals (the sixth and seventh).

Plastron. The plastron preserves all components: the epiplastra, the entoplastron, the hyoplastra, the hypoplastra and the xiphiplastra. The maximum plastron length is estimated to be approximately 325 mm. The presence of a shallow but distinct ventral concavity on the plastron in ventral view and the absence of the laterally rounded posterior lobe suggest that this individual is male (Ernst and Barbour 1989; Zhang et al. 1998).

The epiplastra are mostly preserved, lacking only the lateral margin of the left epiplastron. Of these, the right is 56.7 mm long and 57.2 mm wide. They have a relatively wide epiplastral lip represented by dorsal overlapping of the gular scute (15.7 mm in the right) and lack the epiplastral excavation. The interepiplastral suture is short (24.3 mm) and approximately two–fifths of the median length of the entoplastron (57.4 mm) in ventral view. It is also short (25.3 mm) dorsally and approximately half of the median length of the entoplastron (43.7 mm). The hyoplastra lack most of the axillary buttresses, the anterolateral fringes corresponding to a portion of the bridge to the carapace, and the posterolateral part of the left forelobe. The left hyoplastron (109.6 mm long and 108.7 mm wide) preserves the upper part of the axillary buttress, suturing with the middle parts of the left first costal in ventral view. The interhyoplastral suture is 62.6 mm, slightly longer than the median length of the entoplastron in ventral view. The hypoplastra are also preserved but their anterolateral and anteromedial corners and the ascending processes of the inguinal buttresses are missing. However, the left fifth costal preserves the sutural facet in ventral view, indicating that the inguinal buttresses extend to the middle part of the fifth costals. The xiphiplastra are nearly completely preserved, missing just small portions in their anteromedial parts and the posteromedial part of the left xiphiplastron. The left xiphiplastron is 77.9 mm long and 68.63 mm wide. Length of the interhypoplstral suture is 58.1 mm.

The gular scutes reach the anterior tip of the entoplastron. Length of the intergular sulcus (35.6 mm) is slightly shorter than that of the interhumeral sulcus (36.2 mm). The humero–pectoral sulcus intersects the posterior part of the entoplastron. It slightly bends backward on the hyoplastra. The pectoro–abdominal sulcus arches over the lateral part of the hyoplastra. The axillary and right inguinal scutes are invisible, although the left inguinal is present and is isolated from the femoral scute as in O. sinensis .

Comparisons. The musk duct foramen in the right third peripheral of the present material (MTE1) indicates that this taxon belongs to the family Geoemydidae (Hirayama 1985; Joyce and Bell 2004). MTE1 is similar to Chinemys, Cuvierichelys , Mauremys , Ocadia , Palaeochelys , Sacalia , and Siebenrockiella in lacking the plastral hinge and in having the entoplastron intersected by the humero–pectoral sulcus and the neurals short–sided anteriorly. In addition, MTE1 has four informative character states for genus level taxonomy ( Table 1): (1) second and third vertebrals nearly rectangular, as long as wide (modified from Hirayama et al. 2007), (2) absence of serration on the posterior margin of the carapace (e.g., Ernst and Barbour 1989; Yasukawa et al. 2001), (3) medial length of the gular shorter than the interhumeral sulcus (Hirayama et al. 2007), and (4) the plastral buttresses moderately developed, extending to half way of the costals (Hirayama et al. 2007). Of these, the character (3) and (4) are known as diagnostic characters for the genus Ocadia (Hirayama et al. 2007) , but variably seen in several comparative taxa ( Table 1). Character (1) is shared exclusively with O. nipponica , some O. sinensis , and some Palaeochelys ( P. crocheti and P. laurenti ). A similar state is seen in M. gaudryi , some M. japonica , M. massiliensis , and M. sarmatica , in which the second and third vertebrals are much wider than long (de Broin 1977; Hervet 2004; Takahashi per. obs. on M. japonica ). Character state (2) is informative for discriminating Ocadia from several species of Mauremys as well as a few Palaeochelys ( P. laurenti ), and Siebenrockiella (Sacco 1889; Ernst and Barbour 1989; Hervet 2004; Claude et al. 2007; Chesi et al. 2009). Among the comparative genera, all of these four character states seen in MTE1 are exclusively shared with the genus Ocadia ( Table 1). The other character states (5–10) are not informative for genus level identification. Thus, in the following, we compare MTE1 within Ocadia .

Genus Species Character state

1 2 3 4 5 6 7 8 9 10

MTE1 + + + + + + + + + +

Chinemys Ch. fenhoense – +?? – – – – – – Ch. nigricans – + – – – – – – – – Ch. reevesii – + +/– – + +/– +/– – – –

Cuvierichelys Cu. parisiensis ? + +? –?? – –?

Mauremys M. annamensis – + +/– + – +/– + – – – M. campanii – – –?? + + + – – M. gaudryi –? –? – + + – – – M. japonica – – +/– – – +/– + – – – M. leprosa – + – – + + – – – – M. massiliensis – + –? + + – –? – M. mutica – – +/– – – +/– + + – – M. portisi – – –? – + – – – – M. pygolopha – – – – – – – + – – M. rivulata – + – – + – – – – – M. sarmatica – + – – – + + + – – M. thanhinensis – +/– +/– – – +/– + + – – M. yabei –? – – – + + + – –

Ocadia O. nipponica + + + + – + + – – – O. sinensis +/– + +/– + + – – – – –

Palaeochelys P . crocheti + + + – – – + + – – P. laurenti + – + – – +/– + – – – P. vallisnerii – + + – – + + – – –

Sacalia Sacalia spp. – + +/– – – – +/– – – –

Siebenrockiella S. crassicollis – – – + – – +/– – – – S. leytensis – – – + – – – – – – Within the genus Ocadia , MTE1 exclusively shares the first vertebral expanded anterolaterally and reaching to the second marginal (character 6) with O. nipponica (Hirayama et al. 2007) . On the other hand, it shares the tricarinate discontinuous knob–like keels (character state 5) only with O. sinensis ( Table 1, Smith 1931; Ernst and Barbour 1989; Hirayama et al. 2007). Ernst and Barbour (1989) also described that the dorsal keels on the carapace disappear in aged individuals of O. sinensis . This suggests that absence of the keels in O. nipponica could be attributed to loss resulting from aging. Moreover, the second and third vertebrals in MTE1 are much narrower than the first (character state 7), which is one of the diagnostic characters for O. nipponica (Hirayama et al. 2007) .

Furthermore, MTE1 has three peculiar character states: length of the entoplastron nearly as long as the interhyoplastral suture (character 8), the second to sixth costal bones showing dovetailed shapes (character 9), and the third pleural overlapping only the sixth and seventh peripherals (character 10). Character 8 is also shared with several species of Mauremys ( M. campanii , M. mutica , M. pygolopha , M. sarmatica , M. thanhinensi s, M. yabei , and P. crocheti ; Shikama 1949; de Broin 1977; Hervet 2004; Claude et al. 2007; Takahashi pers. obs. on M. mutica ). In MTE1, the even–numbered costal plates are expanded whereas the odd–numbered plates exclusive of the first one are narrower distally (character 9). In contrast, the costals in the other comparative taxa are nearly rectangular. The third pleural covers only two peripherals, the seventh and eighth (character 10) in MTE1, but overlaps three peripherals in the two congeneric species as well as the other comparative species (the sixth to eighth, generally, but very rarely the fifth to seventh in O. sinensis ). Based on these features, MTE1 is clearly distinguished from O. sinensis and O. nipponica .

Figures

FIGURE 2. Picture showing occurrence of Ocadia tanegashimensis (MTE 1) from the Kawachi Formation in Kawachi, Minamitane – cho, southern part of Tanegashima Island.

FIGURE 3. Ocadia tanegashimensis (MTE 1) from Tanegashima Island. A – C, the carapace in dorsal (A), ventral (B) and anterior (C) views. D, left fifth to seventh costals in ventral view. E – F, the plastron in dorsal (E) and ventral (F) views. Arrows indicate locations of upper ends of the plastral buttresses. Scale bar: 5 cm.

FIGURE 4. Illustration of Ocadia tanegashimensis (MTE 1) from Tanegashima Island. A, the carapace in dorsal and B, the plastron in ventral views. Abbreviations: abd, abdominal; an, anal; ce, cervical; co, costal; ent, entoplastron; epi, epiplastron; fe, femoral; gu, gular; hu, humeral; hyo, hyoplastron; hyp, hypoplastron; LE, length of the epiplastron; LHS, length of the hyoplastral suture; m, marginal; ne, neural; nu, nuchal; pec, pectoral; pl, pleural; v, vertebral; xip, xiphiplastron. Symbol +: locations of upper ends of the plastral buttresses in ventral view. Scale bar: 5 cm.