Belonometra, Obuchi, Masami & Omori, Akihito, 2015

Obuchi, Masami & Omori, Akihito, 2015, A new genus and new species of family Antedonidae (Echinodermata: Crinoidea) from southern Japan, Zootaxa 3972 (3), pp. 441-449 : 442-443

Belonometra n. gen.

Type species. Belonometra kogoi n. sp. by present designation.

Diagnosis. A genus of Antedonidae having numerous cirri, short oral pinnules with short segments, and remarkably long subsequent pinnules. Cirri laterally compressed, closely crowded on low hemispherical centrodorsal, XC–CXXX, composed of 15–22 segments, longest of which is three times longer than wide; penultimate cirral longer than broad, usually longer than antepenultimate, with straight opposing spine. Rays proximally concealed by centrodorsal until base of axillary. Basal rosette with rod-shaped rays. Radial facet concave, taller than broad. Ten arms 175–190 mm long; brachials on distal half of arms short, three to four times broader than long; brachials of small specimens approximately twice as broad as long. Pinnules flagellate; proximal two similar, short, but second with slightly longer and fewer segments; following pinnules very elongated with segments more than three times longer than broad; third usually longest proximal pinnules, but rarely shorter than fourth; proximal pinnules with proximal segments faintly projected aborally: projection more distinct in small specimens; middle to distal pinnules reaching 20% of arm length, composed of more than 40 elongated segments, crowded due to short arm brachials, oriented in three directions along each side of arm, creating a unique configuration.

Etymology. Combination of “ belona ”, needle in Greek, and “ -metra ”, a suffix generally used for comatulids, named for the extremely slender, needle-like pinnules of the new genus.

Remarks. The new genus includes only the type species, Belonometra kogoi n. sp., which has an unusual appearance, characterized by extremely long pinnules crowded in a three-dimensional rather than planar configuration. In addition, it is distinguishable by the combination of ten long arms, and numerous cirri without aboral process.

Distal pinnules of Belonometra n. gen. reach 20% of the arm length. Although distal pinnules tend to be relatively longer in ten-armed comatulids, for example 19% of arm length in Florometra A.H. Clark, 1913 (Antedonidae) and in Colobometra A.H. Clark, 1909 (Colobometridae) , 13% in Tropiometra A.H. Clark, 1907 (Tropiometridae) and Heliometra A.H. Clark, 1907 (Antedonidae) ( A.H. Clark 1947; A.H. Clark & A.M. Clark 1967; Kogo 1998), the new genus has the relatively longest distal pinnules yet observed. The crowding of the pinnules of the new genus is also distinctive. This character derives from the shortness of the distal arm brachials, which occurs commonly in Comasteridae , Himerometridae , but only rarely in Antedonidae . In addition, Belonometra n. gen. is unique in the shapes of cirrus segments. Although the penultimate cirral generally tends to be proportionately elongated in contrast to shorter preceding ones ( A.H. Clark 1915), this is very distinct in the new genus. Such characters make the new genus easily identifiable. However, these features may not reflect its systematic position.

Belonometra n. gen. exhibits skeletal features peculiar to many taxa included in superfamily Antedonoidea: high, thin and flange-like muscular fossae, and a narrow radial cavity with a scarce calcareous filling. Among the three families of Antedonoidea, the new genus is referred to family Antedonidae by having bifurcated rays and the basals transformed into a rosette within the radial cavity. Although the size of the centrodorsal cavity in Belonometra n. gen., 17% of the centrodorsal diameter, is much smaller than those in previously known antedonid species (> 20%) ( A.H. Clark 1915; Meyer 1972), the overhanging rim on the adoral opening of the cavity conforms with other members of the family. A.H. Clark (1915) mentioned a tendency for the larger forms of Antedonidae to have a smaller centrodoral cavity, and Hess & Messing (2011) mentioned the size of the centrodorsal cavity decreases with individual growth, both of which can be applied to Belonometra n. gen. The new genus, with arm length reaching 190 mm, is relatively large for antedonid species. In the smallest specimen, the centrodorsal opening is larger, reaching 28% of the centrodorsal diameter. Skeletal features of the smallest specimen better agree with antedonid species in that the overhanging lip on the centrodorsal cavity is distinct, and in that the radial cavity is totally lacking a calcareous deposit.

Some additional characters support assignment of Belonometra n. gen. to family Antedonidae , although they may be found in other ten-arm comatulids belonging to other families: second syzygy mostly at br9+10 ( A.H. Clark & A.M. Clark 1967; Hess & Messing 2011), short and regular intersyzygial interval of 3 articulations ( A.H. Clark 1921; Kogo & Fujita 2014), and a globular disk with a central mouth ( A.H. Clark 1921).

Of the six subfamilies of Antedonidae , the one in which Belonometra n. gen. should be placed is unclear. The subfamilies are classified on the basis of the arrangement and structure of cirri, and the structure and composition of oral pinnules ( A.H. Clark & A.M. Clark 1967; Hess & Messing 2011). In Belonometra n. gen., cirri are approximately a hundred in number, and in the oral pinnules the proximal two of each side of the arm are shortened and with short segments unlike the very elongated following pinnules. According to the keys in A.H. Clark & A.M. Clark (1967), this species seems to most closely fit Heliometrinae . The two taxa share relatively large body sizes, high radial facets, and rod-shaped basal rays. However, the oral pinnules are longer (often the longest on the arms) in Heliometrinae . Thus, the new genus cannot be placed in this subfamily as currently defined. Members of Thysanometrinae and a few Antedoninae also have oral pinnules with short segments, although Belonometra n. gen. does not share other general morphological characters with these subfamilies. In Thysanometrinae , the second pinnule is similar to the following genital pinnules rather than to the first pinnule, and the cirri are straight without a distinct opposing spine. Belonometra n. gen. has curved cirri with a distinct opposing spine. Annametra occidentalis is unlike other Antedoninae in having the proximal two pinnules composed of short segments (the other congener, Annametra minuta (A.H. Clark, 1907) has oral pinnule segments longer than wide as in other antedonines). Annametra occidentalis resembles Belonometra n. gen. in having proximal to middle pinnules with a saw-like profile produced by aboral projections on basal segments, as in the small specimens of Belonometra kogoi n. sp. However, the two are otherwise different, e.g., A. occidentalis bears fewer than 50 stout cirri, and has distal pinnules shorter than the proximal ones. The general characters of Belonometra n. gen. are also not similar to any other antedonid genus currently treated as subfamily incertae sedis. The systematic position of the new genus should be reconsidered when a complete revision of Antedonidae is conducted.