Aluterus shigensis, Miyajima & Ohe & Koike & Matsuoka, 2014

Miyajima, Yusuke, Ohe, Fumio, Koike, Hakuichi & Matsuoka, Hiroshige, 2014, First fossil occurrence of a filefish (Tetraodontiformes; Monacanthidae) in Asia, from the Middle Miocene in Nagano Prefecture, central Japan, Zootaxa 3786 (3), pp. 382-400 : 385-389

publication ID	https://doi.org/ 10.11646/zootaxa.3786.3.7
publication LSID	lsid:zoobank.org:pub:E1F6AF3C-3FBA-494A-BFB8-995F16C76282
DOI	https://doi.org/10.5281/zenodo.4913462
persistent identifier	https://treatment.plazi.org/id/972E984A-FF99-FFCD-FF6D-E62EFB1EFF15
treatment provided by	Felipe
scientific name	Aluterus shigensis
status	sp. nov.

Treatment

Aluterus shigensis sp. nov.

Figures 3–5B View FIGURE 3 View FIGURE 4 View FIGURE 5 , 6–7B View FIGURE 6 View FIGURE 7 , 9 View FIGURE 9

Holotype. MSFM 00606 , part and counterpart, collected by Yusuke Miyajima (Kyoto University , Japan).

Locality and horizon. The riverbed (36°19'37"N, 137°59'40"E) of the Hofukuji River at Sorimachi, Matsumoto City, Nagano Prefecture, central Japan ( Figures 1 View FIGURE 1 and 2 View FIGURE 2 ). The Tazawa black mudstone Member ( Tanaka and Seki, 1966) of the Middle Miocene Bessho Formation ( Honma, 1927), 13.6-13.1 Ma ( Kato et al., 2011).

Etymology. The specific name shigensis is derived from the old name of the region where the fossil was found, Shiga-Mura village.

Diagnosis. Total vertebrae, 21 (7AV + 14CV). Very slender and long first dorsal spine with tiny anterior barbs. Thin, lancet-shaped single basal pterygiophore supporting two dorsal spines, with its ventral margin separated from the skull. Moderately slender first basal pterygiophore of the soft dorsal fin having its proximal tip located above the middle length of the neural spine of the fifth abdominal vertebra. Soft dorsal-fin base longer than the anal-fin base. Caudal peduncle having nearly equal depth and length. Tiny, fine scales with slender, straight spinules.

Description of holotype. The fossil specimen lacks the skull bones and pelvic girdle, being represented mostly by an articulated axial skeleton ( Figures 3 View FIGURE 3 and 4 View FIGURE 4 ). All of the vertebrae and their neural and haemal spines are preserved. Two dorsal spines supported by a single basal pterygiophore, which is a diagnostic feature of the Monacanthidae ( Tyler, 1980) , are preserved. The basal pterygiophores of the soft dorsal fin and the anal fin (on the counterpart) are partially missing. Most of the soft dorsal-fin rays are missing, but several anal-fin rays are preserved. Considering the numbers and preservation states of the pterygiophores and fin rays, the soft dorsal-fin base is longer than the anal-fin base. The ventral part of the compound terminal centrum ( Schultze and Arratia, 2013) and the caudal fin are preserved. Tiny, fine scales with spinules ( Tyler, 1980; Sorbini and Tyler, 2004) are preserved.

The length of the skeleton from the anterior end of the second abdominal vertebra to the base of the caudal fin (shown as l in Figure 3B View FIGURE 3 ) is 124.1 mm. Body depth (BD) is more than 65.0 mm. Measurements of the fossil and of specimens of the extant Aluterus scriptus are shown in Table 1 View TABLE 1 .

Total vertebrae including compound terminal centrum (C-H in Figure 3 View FIGURE 3 ) are 21 (7 AV + 14 CV), and this also distinguishes the fossil from balistids which have almost invariably 18 vertebrae ( Matsuura, 1979; Tyler, 1980). Although the first abdominal vertebra is disarticulated and displaced anteriorly, the other 20 vertebrae are well articulated in a straight vertebral column. The neural spines of the first to sixth abdominal vertebrae are thin and expanded, but their outlines are obscure and can scarcely be seen using a microscope. The neural spine of only the first abdominal vertebra is oriented anteriorly. The distal end of the neural spine of the second abdominal vertebra is immediately posteroventral to the basal pterygiophore of the spiny dorsal fin. The distal end of the neural spine of the fourth abdominal vertebra is situated close to that of the fifth abdominal vertebra. The neural spine of the fifth abdominal vertebra is immediately anterior to the first pterygiophore of the soft dorsal fin (2DP 1 in Figures 3 View FIGURE 3 , 4 View FIGURE 4 and 6A View FIGURE 6 ) and it is relatively more slender than the other expanded neural spines. The neural spine of the sixth abdominal vertebra is more expanded and higher than that of the fifth abdominal vertebra. Parapophysis of the seventh abdominal vertebra is wide, short and oriented posteriorly. Ribs are absent. Haemal spines are present from the first to thirteenth caudal vertebra. The haemal spine of the first caudal vertebra contacts the first pterygiophore of the anal fin, although the boundary of these two bones is indistinct ( Figure 4 View FIGURE 4 ). The thirteenth caudal vertebra has a posteriorly expanded haemal spine.

The first dorsal spine is very slender and long; it is about 1 mm in width and its preserved length is 32 mm. Tiny anterior barbs are present; that is, the anterior margin of this spine is finely serrated in an orderly manner and the posterior margin is smooth ( Figure 5B View FIGURE 5 ). There is an inner cavity running along the central axis of this spine in its cross section ( Figure 5A, B View FIGURE 5 ). The surface of this spine is characterized by grooves parallel to the long axis of the spine. The second dorsal spine, which is placed just behind the base of the first dorsal spine, is tiny and curved posteriorly with a blunt distal end. This is a diagnostic feature of the Monacanthidae , whereas there are three stronger dorsal spines supported by two basal pterygiophores in the Balistidae ( Matsuura, 1979; Tyler, 1980).

The first and second dorsal spines are supported by a single basal pterygiophore (1DBP in Figures 3 View FIGURE 3 , 4 View FIGURE 4 and 5A View FIGURE 5 ). This bone has a thin, lancet-like shape and its ventral margin is separated from the skull. The posterior part of this bone is partly missing, but its posterior end extends to above the neural spine of the second abdominal vertebra. The depth of this basal pterygiophore (BPD) is 4.9 mm ( Table 1 View TABLE 1 ).

Most parts of the basal pterygiophores of the soft dorsal fin are preserved. There are 34 such basal pterygiophores, and the posterior part of the soft dorsal fin is partly missing. The first and thickest pterygiophore of the soft dorsal fin (2DP1) is moderately slender and its proximal tip, which is located above the middle length of the neural spine of the fifth abdominal vertebra, is blunt ( Figure 6A View FIGURE 6 ). The second pterygiophore is longer than the other soft dorsal-fin pterygiophores, and its proximal tip is located lower than those of the other pterygiophores and below the middle length of the neural spine of the sixth abdominal vertebra. The pterygiophores are indistinguishably articulated to one another at their distal ends where radial grooves are present on the surfaces, but they are partly broken and displaced slightly dorsally above the second to fifth caudal vertebrae. In monacanthid fishes, most parts of each slender pterygiophore of the soft dorsal and anal fins are connected with one another anteroposteriorly by very thin flanges but such a feature is indistinct in this fossil specimen. The number of pterygiophores between two adjacent neural spines ranges from two to four except for their being a single pterygiophore between the neural spines of the fifth and sixth abdominal vertebrae ( Table 2 View TABLE 2 ). Only six fin rays of the soft dorsal fin are preserved at the posterior end of the counterpart, but two of them are disarticulated and displaced ( Figures 4 View FIGURE 4 and 6B View FIGURE 6 ). The soft dorsal-fin base, from the second to the last pterygiophore of the fin, is longer than the anal-fin base ( Table 1 View TABLE 1 ). The length of the soft dorsal-fin base of the fossil shown in Table 1 View TABLE 1 corresponds to the length from the second pterygiophore (2DP 2 in Figure 4 View FIGURE 4 ) to the pterygiophore preserved between the neural spines of the ninth and tenth caudal vertebrae (2DPX in Figure 4 View FIGURE 4 ), but it seems to be longer because the soft dorsalfin rays are preserved posteriorly and a slender bone, probably a pterygiophore (2DP? in Figures 3 View FIGURE 3 and 4 View FIGURE 4 ), is present between the neural spines of the tenth and eleventh caudal vertebrae.

The pterygiophores of the anal fin are partially preserved on the counterpart ( Figure 4 View FIGURE 4 ); 16 pterygiophores are recognizable at least in part, and the first pterygiophore (AP 1 in Figure 4 View FIGURE 4 ) is thicker than the others. The pterygiophores of the anal fin are absent at least between the haemal spines of the tenth and the last caudal vertebrae. Although there are no pterygiophores preserved between the haemal spines of the ninth and tenth caudal vertebrae, those bones must have been present there because anal-fin rays are present ventrally. There are 21 analfin rays preserved, but most of them are disarticulated and displaced.

Although the caudal peduncle is incomplete, the depth and length of it are nearly equal ( Table 1 View TABLE 1 ), with depth measured vertically from the posterior end of the anal-fin base (the base of the last anal-fin ray, LAF in Figures 3 View FIGURE 3 and 4 View FIGURE 4 ) to the dorsal margin of the skin of the caudal peduncle, and length measured from the posterior end of the anal-fin base to the ventral edge of the caudal-fin base, following Berry and Vogele (1961). Although the dorsal part of the caudal skeleton is missing, the triangular compound terminal centrum including an unknown number of vertebral centra and hypurals ( Schultze and Arratia, 2013) is preserved. In monacanthid fishes, a horizontal crest for muscle attachment is present at the anterior part of this bone ( Matsuura, 1979), but such a feature is poorly preserved and indistinct in this fossil specimen. The parhypural is preserved between the compound terminal centrum and the expanded haemal spine of the thirteenth caudal vertebra, but its shape is indistinct. There are 12 caudal-fin rays.

The scales are very small and fine, and have slender, straight spinules ( Figure 7A, B View FIGURE 7 ), which are preserved over almost the entire body. Especially along the dorsal margins of the pterygiophores of the spiny and soft dorsal fins, the tiny spinules (one of which is about 0.26 mm in length) of the scales are well preserved.