Ophiorupta, Martynov, 2010

Martynov, Alexander, 2010, Reassessment of the classification of the Ophiuroidea (Echinodermata), based on morphological characters. I. General character evaluation and delineation of the families Ophiomyxidae and Ophiacanthidae 2697, Zootaxa 2697, pp. 1-154 : 91-93

Ophiorupta gen. nov.

Type species: Ophiomedea discrepans Koehler, 1922

Diagnosis. The disk plates are concealed by numerous small granules. Radial shields are small and concealed under disc scales. Genital slits are long. Each jaw bears several spiniform apical papillae. The latter are clearly distinguished at least from the dorsally placed teeth, which are elongated and more massive. The adjacent 3–4 lateral oral papillae are similar to the ventralmost teeth (apical papillae) and together form a row of similar spiniform papillae. Between the lateral and adoral papillae there is a more or less clear gap, which is however in some interradii almost indistinghuishable. The adoral papillae are 2–3 times larger than the 3–4 lateral papillae. The oral shield is wide, arrow-shaped with small narrow distal lobe. The dorsal arm plates are well developed. Arms have more or less conspicuous nodes and lateral arm plates have a low lateral ridge, on which the large spine articulations are placed. Spines are flattened, pointed apically. The first ventral arm plate is irregularly trapezoid. Suceeding ventral arm plates have a characteristic pole-axe shape throughout the entire arm length. The tentacle pores are larger and more conspicuous proximally than towards the distal end of the arm. Two first proximal segments bear three large elongated, flattened, slightly apically pointed tentacle scales, two on the lateral arm plates and one opposite to the others on the ventral plate. Further segments lack the third tentacle scale and bear only two scales on the lateral plates. The tentacle scales are shorter the arm spines but conspicuous and capable of covering the whole tentacle pore.

Jaws are slightly elongated. Dental plate is without folds, with few small rounded sockets in the ventral part and few elongated ones dorsally. Arm spine articulations are well developed and represent the doubleopening type, but the second opening is not always distinct. The muscle opening is placed within the somewhat transversally compressed and elongated ridge, with a distinct distal lobe. The sigmoidal fold is absent. The nerve opening is placed laterally and distally from the muscle opening, but indistinct. Vertebrae are rather long, not keeled and with well developed zygospondylous articulation. Vertebral dorsal median groove is indistinct, but broad. Podial basins are large.

Etymology. Ophio - (after ophiuroid) and ruptus (Latin), break, in reference to the easily damaged disk.

Remarks. The “Vityaz” specimen of Ophiorupta discrepans comb. nov. was found 40 years later after Koehler’s work (1922a) near the type locality at similar depths and essentially agrees with the first description. In addition, the holotype of Ophiomedea discrepans Koehler, 1922 (USNM 41196) was examined, including SEM investigation of the arm spine articulations, spines and vertebrae ( Fig. 63) and no reliable differences from the “Vityaz” specimen ( Fig. 64) were found.

Koehler (1922a) considered O. discrepans within his genus Ophiomedea based on some similarities of the oral structures (long adoral shield papillae) and the presence of several tentacle scales both on lateral and ventral arm plates in both O. discrepans and the type species, Ophiomedea duplicata Koehler, 1906 . However these features are not restricted only to the genus Ophiomedea ; instead several different ophiomyxids and ophiacanthids possess these characters. The type species of the genus Ophiomedea , O. duplicata has a single massive, wide ventralmost tooth, thus differs significantly from O. discrepans , which possesses a cluster of narrow teeth instead of the apical papilla. In addition, the disk of Ophiomedea is covered by long spinelets, whereas in Ophiorupta gen. nov. there are small numerous granules. The taxonomic position of O. duplicata was unclear and it has never been recorded after the first description, which mentioned only the small, possibly juvenile specimen of 4 mm dd. In the present study the holotype of Ophiomedea duplicata (MNHN EcOs 20426) has been re-investigated ( Fig. 50). The features mentioned above are confirmed. The rounded, regular disk without evident skin layer, covered with numerous long spines and massive teeth (both ventral and dorsal) are clearly different from “ Ophiomedea ” discrepans . In addition, arm spine articulations of the Ophiomedea duplicata holotype reveal the presence of a well-defined ophiacanthid volute, including the sigmoidal fold ( Figs 50F–G). “ Ophiomedea ” discrepans in turn does not possess typical ophiacanthid voluteshaped articulation and the sigmoidal fold is definitely absent ( Figs 63F, G–H; 64H–P). In this respect the current placement of “ Ophiomedea ” discrepans within the family Ophiacanthidae is questioned. Paterson (1985) indicated that juveniles of Ophiotrema alberti are quite similar to Ophiomedea duplicata . This did not exclude the possibility that Ophiomedea duplicata may be only a juvenile stage of a larger ophiacanthid species. The question whether Ophiomedea duplicata is a paedomorphic adult species or just an ontogenetic stage of an ophiacanthid taxon requires further investigation. However, it is possible to conclude that various features of Ophiomedea , including distinctly noded arms ( Figs 50C, F, G) are not similar with the proximal and distal segments of any species of the genus Ophiotoma ( Figs 66–70).

Apparently most closely related to Ophiorupta gen. nov. is the genus Ophiophrura H.L. Clark, 1911 . Koehler (1922a) questioned the synonymy of the genera Ophiomedea and Ophiophrura , and O’Hara and Stöhr (2006) used the binomen Ophiomedea liodisca (H.L. Clark, 1911) . Ophiomedea , Ophiophrura and Ophiorupta gen. nov. share similar oral frames and the presence of several tentacles scales. However, these characters are typical for almost all taxa of the ophiacanthid subfamily Ophiotominae and also for some ophiomyxid taxa. The holotype of Ophiophrura liodisca (USNM 25672) was examined, including SEM study of the arm spine articulations, spines and vertebrae ( Fig. 51). There are several characters that distinguish the genera Ophiophrura and Ophiorupta gen.nov.: reasonable well-defined radial shields, massive dorsalmost teeth, a cluster of the spiniform teeth, arm spine articulations with large muscle opening and very distinct distal lobe ( Fig. 63H) occur in Ophiorupta . Ophiophrura has subparallell arm spine articulations ( Fig. 51H). In addition, dorsal disk is covered with a dense coat of granules in Ophirupta gen.nov. ( Figs 63–64A, B) and a thin but evident skin layer in Ophiophrura ( Figs 51A–C).

The new genus clearly differs from the ophiacanthid genus Ophiotoma , in the general disk and oral frame appearance (compare Figs 63–64 and 66–70), shape of the arm spine articulations and dental plate. The genus Ophiopristis differs considerably from Ophiorupta gen. nov. in the general oral frame appearance, presence of the typical ophiacanthid massive teeth over most of the dental plate length, and typical ophiacanthid sigmoidal fold of the spine articulations ( Figs 57–60). Various species of the genus Ophiopristis , including the type species, also have a peculiar appearance due to the presence of numerous densely placed long disk spinelets, thus clearly differing from Ophiorupta gen. nov. The genus Ophioprium is somewhat similar to Ophiorupta gen. nov. in having long spiniform adoral shield papillae. For comparison, the holotype of Ophioprium cervicornis ( Lyman, 1883) from MCZ and an additional specimen of Ophioprium permixtum ( Koehler, 1914) were studied, including SEM-preparation of lateral arm plates and vertebrae ( Figs 61–62). Ophiorupta gen. nov. clearly differs from the genus Ophioprium in a number of important characters, including the presence of numerous ventralmost teeth covering about half of the dental plate. Although the oral papillae in Ophioprium are similar in shape and size to the apical papillae, they are distinguished from the latter, forming a distinct long regular row along the jaw. Unlike the genus Ophioprium , oral papillae of Ophiorupta gen. nov. are hardly distinct from the apical papillae. In addition, there is an indistinct gap between the apical and oral papillae and the adoral shield papillae of Ophiorupta gen. nov. Adoral shield papillae of the genus Ophioprium and Ophiorupta gen. nov. are markedly different – the former genus has long spiniform papillae, whereas the adoral papillae of Ophiorupta gen. nov. are considerably flattened. Arm spine articulations of both Ophiorupta gen. nov. and Ophioprium , though, belong to the non-ophiacanthid type, lacking a sigmoidal fold ( Figs 62A–B and 63F–G, H, 64H–P).

The dental plate morphology is of special interest, since it combines several small rounded openings (typical for the family Ophiomyxidae ) with a few elongated sockets (characteristic for Ophiacanthidae ) ( Fig. 13A). Murakami (1963) studied the dental plate of Ophiophrura liodisca and found a few small, round sockets, arranged in a way that is considerably different from Ophiorupta discrepans comb. nov. – ventral teeth cluster and correspond to irregularly placed sockets, and dorsally there are a few slightly elongate imperforate sockets. The present study of the holotype of Ophiphrura liodisca confirmed the presence of a short dental plate covered with a few alternately placed spiniform teeth ( Fig. 51E). The number of parallel cases and exceptions from different ophiuroid families (see below for discussion) does not allow using the dental plate shape alone for taxonomic placement.

Finally, an ophiacanthid species reported by Paterson (1985) from the North Atlantic under the name Ophiotrema cf. tertium clearly does not belong to that genus ( O’Hara & Stöhr 2006). The general disk appearance, numerous apical papillae, scarcely distinguished from the oral papillae, and an indistinct gap between adoral and other papillae suggest that it may belong to Ophiorupta gen. nov. Moreover, “ Ophiotrema cf. tertium ” inhabits similar depths (ca. 5000 m) as Ophiorupta gen. nov. Thus, although Ophiorupta discrepans (Koehler, 1922) comb. nov. is known only from two specimens, it represents an important case of possibly transitional morphology between the families Ophiomyxidae and Ophiacanthidae . The relationships of these two large ophiuroid families will be discussed below.