Mephisto Tyler 1966

Mephisto Tyler 1966a: 1–5 (original description; type species Mephisto fraserbrunneri Tyler 1966; etymology: genus named for the devil Mephisto, second only to Satan in the Faustian legend of Mephistopheles, in allusion to the reddish exterior, blackish interior (peritoneum), and the retrose-barbed dorsal-fin spines being the equivalent of horns in the type species).

Species. The genus Mephisto contains two species: Mephisto fraserbrunneri Tyler 1966 and Mephisto albomaculosus Matsuura, Psomadakis, and Mya Than Tun 2018.

Subfamilial placement. The two subfamilies of Triacanthodidae are diagnosed primarily by features in two different regions of the skeleton: the posterior process of the pelvis and the posterodorsal region of the skull. The width, shape, and structure of the posterior process of the pelvis (Fig. 3A, B) is visible externally; the shape of the supraoccipital and its relationship to the epioccipitals is only visible internally (Fig. 3C, D; see Tyler, 1968: 62; 1980: 56).

The Hollardiinae (Hollardia, Parahollardia, western Atlantic, except one species in Pacific Oceania) have a shaft-like posterior process of the pelvis that is relatively rounded in ventral view and is not much wider between the bases of the pelvic spines than at the blunt posterior end, and they have a dome-like supraoccipital that separates the contralateral epioccipitals posteriorly on the dorsal surface of the skull. The Triacanthodinae (all other extant genera, Indo-Pacific except one species in western Atlantic) have a basin-like posterior process of the pelvis that is flat ventrally, with slightly dorsally upturned lateral edges, and is wider anteriorly between the pelvic spines than posteriorly where it tapers to an end (see Tyler, 1968 for illustrations of the posterior process of the pelvis in all genera of triacanthodids), and they have a flattened supraoccipital with a median crest that does not separate the epioccipitals posteriorly on the dorsal surface of the skull. The only fossil triacanthodids are two taxa from the Oligocene of the Polish Carpathian Mountains, the hollardiin Prohollardia and the triacanthodin Carpathospinus. The two triacanthodid subfamilies diverged no less than 29 to 24 MYA (see Tyler et al., 1993).

* 1 tooth offset from main series of teeth.

Mephisto fraserbrunneri has the diagnostic characters of the Triacanthodinae (Fig. 3). Mephisto albomaculosus has the pelvic characters of triacanthodins, but the shape of its supraoccipital and its articulation with the epioccipitals is not known; presumably it is typical of triacanthodins.

Diagnosis. The genus Mephisto is distinguished from all other triacanthodids by a long gill opening (13.5– 17.6% SL; Table 3), with its lower edge reaching slightly below the lower edge of the lobe of the pectoral-fin base (Fig. 4A; see Tyler, 1968: figs. 137, 150, 164, 173 for comparisons of length of gill opening across ontogeny for all genera).

Description. (1) Pelvis thin and basin-like, its ventral surface flat but with slightly upturned edges; width between the pelvic-fin spines moderate to somewhat narrowed (diagnostically different between the two species, with M. fraserbrunneri 10.6–12.1% SL and M. albomaculosus 7.8% SL); pelvic width into the pelvic length 2.6–4.0 times (likewise diagnostically different between the two species, with M. fraserbrunneri 2.6–3.2 times and M. albomaculosus 4.0 times). (2) Deep bodied (45.8–54.2% SL). (3) Short snouted (12.4–14.5% SL). (4) Long postorbital length (11.6–13.2% SL). (5) Six dorsal-fin spines decreasing gradually in length from the first to the short last spine, all visible externally (Table 2). (6) Origin of the anal fin distinctly posterior to the origin of the soft dorsal fin. (7) Mouth terminal, with a moderate number of conical teeth (17–25 in upper jaw and 19–27 in lower jaw; Table 2; Matsuura et al., 2018) in a single series with no teeth internal to them (one specimen of M. fraserbrunneri has one lower jaw tooth slightly offset posteriorly from the main row, but we do not interpret this as an internal tooth sensu Tyler, 1968:58). (8) Few olfactory lamellae (9–11; Table 2; Matsuura et al., 2018). (9) Moderate number of gill rakers (15–19; Table 2; Matsuura et al., 2018). (10) Pseudobranch with a moderate number of lamellae (16–20; Table 2; Matsuura et al., 2018), the lower edge of the base of the pseudobranch level with the upper edge of the lobe of the pectoral-fin base (Table 4; Matsuura et al., 2018). (11) Few spinules per scale, consisting of one large central spinule and smaller spinules dorsal and ventral to it that increase in number and branching during ontogeny; only a single spinule present in scales of smallest known specimens (DABFUK/FI/304, 48.6 mm SL and ANSP 103314, 52.2 mm SL; Table 4); some spinules are branched in large specimens (e.g., Fig. 5; USNM 350153, 105.8 mm SL, DABFUK/FI/ 302, 102.7 mm SL, and NSMT-P 132271, 94.4 mm SL; Table 4; Matsuura et al., 2018). (12) Retrose barbs on dorsal- and pelvic-fin spines (Fig. 3). (13) Pelvic fin with only one soft ray. (14) Small patch of isolated spinulose scales on middle of upper surface of dorsal lip (Fig. 6).

Geographic and depth distribution and physical environment. Specimens and photographs of Mephisto are known from the Indian Ocean from off Somalia to Myanmar (Fig. 2), from 74 m to 446 m (Table 1). Preliminary analysis, based on World Ocean Atlas 2018, suggests that Mephisto occurs in waters of 10.2–25.3°C and salinity of 34.00–35.43 psu (Table 5); however, more specimens are needed to confirm this because both salinity and temperature ranges were expanded by photographic records of unretained specimens of M. albomaculosus and M. fraserbrunneri.

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