Aetomylaeus Garman , 1908

White, William T., 2014, A revised generic arrangement for the eagle ray family Myliobatidae, with definitions for the valid genera, Zootaxa 3860 (2), pp. 149-166 : 153-157

Genus Aetomylaeus Garman, 1908

Myliobatis Geoffroy St. Hilaire, 1817 : pl. 26 (Type species Myliobatis bovina Geoffroy St. Hilaire, 1817 , by subsequent designation)

Aetomylaeus Garman, 1908: 252 (Type species Myliobatus maculatus Gray, 1834 , by original designation) Pteromylaeus Garman, 1913: 437 (Type species Myliobatis asperrimus Gilbert in Jordan & Evermann, 1898, by subsequent designation by Jordan, 1920)

Species. Aetomylaeus includes 6 valid nominal species (see Table 1). In addition, there is one undescribed species from northern Australia (W. White & P. Last, in progress).

Definition. Aetomylaeus is distinguished from the other two myliobatid genera in the following combination of characters: Anteriormost part of pectoral fins joins head slightly below eye, above level of snout (vs. at level of eye in Aetobatus and well below eye in Myliobatis , Fig. 1 ). Rostral part of pectoral fins (snout) without subocular ridges connecting them to pectoral disc (vs. connected to pectoral disc in Myliobatis ); pectoral radials interrupted from separate cephalic lobe (vs. continuous below eyes onto rostral lobe in Myliobatis ); rostral radial cartilages much less developed than pectoral radials (vs. no less or only slightly less developed in Myliobatis ). Free rear tip of pectoral-fin with an angular, somewhat pointed apex (vs. broadly rounded apex in Aetobatus , Fig. 2 ). Mesopterygium absent or fused with the scapulocoracoid (vs. mesopterygium consisting of several components that all articulate with scapulocoracoid in Myliobatis ). Head relatively narrow (vs. relatively broad in Myliobatis ). Postorbital process of chondrocranium with the anterior and posterior sections secondarily distally fused, with a large foramen evidence of this fusion (vs. with two separate parts, a small triangular anterior section and an expanded plate-like posterior section in Myliobatis ); lateral margin of process not prolonged and ventrally protruding (vs. prolonged and ventrally protruded, forming a bar-like projection in Aetobatus ). Spiracles lateral on head, openings not visible in dorsal view (vs. dorsolateral on head and openings almost completely visible in dorsal view in Aetobatus , Fig. 3 ). Nasal curtain straight or slightly undulated (vs. deeply notched in Aetobatus , Fig. 4 ). Teeth usually in seven rows in both the upper and lower jaw, lateral rows may be lost in adults of some species but always present in juveniles (vs. in a single row in Aetobatus ); median tooth row widest, transverse (vs. chevronshaped in Aetobatus ). Hyomandibular Accessory Cartilage 1 (HAC-1) small, bar-like and loosely attached to distal end of the hyomandibular cartilage (see Fig. 22A in Nishida, 1990; vs. absent in Aetobatus ). Hypo- and basi-hyoid cartilages absent (vs. present as a pair of small, bar-like cartilages in Myliobatis ; see figs 28B vs. 28A in Nishida, 1990). Dorsal fin without a free rear tip, posterior margin joining to tail without a free portion (vs. with a short free rear tip in Aetobatus and Myliobatis , Fig. 5 ); origin well in front of pelvic-fin free rear tips. Barbed stinging spines either absent ( A. maculatus , A. milvus , A. nichofii and A. vespertilio ) or present ( A. asperrimus and A. bovinus ) on dorsal surface of tail behind dorsal fin; when present, usually not strong (vs. spines strong in Aetobatus and Myliobatis ). Puboischiadic bar of pelvic girdle weakly to moderately arched and robust (vs. strongly arched and only moderately robust in Aetobatus ; see figs 36J vs. 36L in Nishida, 1990). Total vertebrae (excluding synarcual) 80–86 (vs. 80–97 in Aetobatus and 108–114 in Myliobatis ); predorsal diplospondylous vertebrae 5–20 (vs. 13–31 in Aetobatus and 37–48 in Myliobatis ). Pectoral-fin radials (excluding rostral cartilages) 79–92 (vs. 89–116 in Aetobatus and 85–92 in Myliobatis ).

Remarks. The genus Aetomylaeus was proposed by Garman (1908) for M. maculatus , M. milvus , M. nichofii and M. vespertilio . The key characters used to distinguish from Myliobatis and Aetobatus were: absence of a serrated dorsal spine on tail (from both); mesopterygia fused with the shoulder girdle (from Myliobatis ); pectoral rays not connecting the lateral portions of the pectoral fins with the cephalic lobe (from Myliobatis ); teeth with a broad median series with three rows of narrower teeth on either side (from Aetobatus ). The type species for Aetomylaeus was designated by Garman (1908) as A. maculatus ( Gray, 1834) . In 1913, Garman proposed the genus Pteromylaeus for M. bovina , M. asperrimus and M. punctatus (= A. ocellatus ). The type species for this genus is Myliobatis asperrimus , subsequently designated by Jordan (1920). The key characters used to distinguish from the genus Myliobatis were: head more elongate and narrower; pectoral fins more falcate and not continuous with the rostral fin; pelvic fins more elongate and narrow; and spiracles open more upward and less to the side. Additional characters used in the generic key in Garman (1913) are: teeth in more than 3 rows on each jaw (from Aetobatus ); caudal spine present (from Aetomylaeus ). Thus, the only character used to differentiate Aetomylaeus and Pteromylaeus was the presence of a stinging spine on the tail.

The allocation of M. punctatus to Pteromylaeus by Garman (1913) is somewhat confusing given that the illustration and description of this species by Miklouho-Maclay & Macleay (1886) clearly depicts a whitespotted eagle ray, Aetobatus ocellatus . It is likely that this allocation was based on misinterpretation of the dentition from the original description, i.e. [regarding teeth plates] “These plates consist of many longitudinal rows of teeth, of which the middle ones are largest”. Garman (1913) likely misinterpreted it as meaning they have a wide central tooth row and narrower rows on either side, which then given the presence of a spine would make sense to place into Pteromylaeus . However, Miklouho-Maclay & Macleay’s description was referring to the longitudinal series of teeth (in only one row) and that the middle ones are larger than those more anterior or posterior.

The presence of a stinging spine is the only feature used to differentiate Pteromylaeus from Aetomylaeus . However, the spine present in P. bovina and P. asperrimus is far less developed than in Aetobatus and Myliobatis species and it is questionable as to whether this single character is sufficient to maintain separation of Pteromylaeus from Aetomylaeus . Other than colour differences and the presence of the spine, these two species are morphologically very similar to the four Aetomylaeus species and share the characters which separate them from the genera Aetobatus and Myliobatis . The findings of Naylor et al. (2012), where P. bovina nests deeply within the four Aetomylaeus species (see Fig. 2.11 in that publication), casts further doubt on the separation of these two genera. Thus, Pteromylaeus Garman, 1913 is herein considered a junior synonym of Aetomylaeus Garman, 1908 .

Figures

FIGURE 1. Lateral view of head of: A) Aetobatus narutobiei, B) Aetomylaeus milvus, and C) Myliobatis tobijei.

FIGURE 2. Pectoral-fin free rear tip shape in: A) Aetobatus narutobiei, B) Aetomylaeus milvus, and C) Myliobatis tobijei.

FIGURE 3. Dorsal view of head highlighting spiracular opening and rostral lobe in: A) Aetobatus ocellatus, B) Aetomylaeus nichofii, and C) Myliobatis tobijei.

FIGURE 4. Nasal curtain shape in: A) Aetobatus ocellatus, B) Aetomylaeus nichofii, and C) Myliobatis tobijei.

FIGURE 5. Dorsal fin and stinging spine (if present) in: A) Aetobatus narutobiei, B) Aetomylaeus bovinus, C) Aetomylaeus nichofii and D) Myliobatis tobijei.