Ophiotreta Verrill, 1899

Thuy, Ben, 2013, Temporary expansion to shelf depths rather than an onshore-offshore trend: the shallow-water rise and demise of the modern deep-sea brittle star family Ophiacanthidae (Echinodermata: Ophiuroidea), European Journal of Taxonomy 48, pp. 1-242 : 83-84

Genus Ophiotreta Verrill, 1899

Type species

Ophiacantha lineolata Lyman, 1883 , by original designation.

Diagnosis

LAPs with nearly equal-sized spine articulations freestanding in continuous vertical row on elevated distal portion; outer surface commonly with vertical striation composed of thin overlapping lamellae, distalmost of which largest and undulose, sharply bordering spine articulations; commonly thin and poorly to moderately and well-defined connecting ridge between ventral of spine articulations and distalmost lamella; inner side of LAPs with very large conspicuous ridge composed of short, oblique ventral portion and very long, near-vertical dorsal portion paralleling proximal edge of LAP; kink between dorsal and ventral portions of ridge commonly with ventro-proximally pointing angle.

Remarks

The LAPs of extant species of Ophiotreta ( Fig. 16 : 1-4) almost invariably combine numerous near-equally sized spine articulations on an elevated distal portion, proximally bordered by the distalmost lamella of the outer surface striation, and a ridge on the inner side which displays a distinctively long, narrow, near-vertical dorsal portion very close to the proximal edge separated from the shorter, oblique ventral portion by a kink with a ventro-proximally pointing angle. While this highly distinctive combination of characters clearly sets the LAPs of Ophiotreta apart from those of almost all other ophiacanthid lineages, the LAPs of extant Ophiopristis Verrill, 1899 ( Fig. 16 : 5-6) are almost identical to those of Ophiotreta . At first sight, this appears puzzling considering that the two genera were formerly assigned into separate ophiacanthid subfamilies (e.g. Paterson 1985; Stöhr & O’Hara 2007). Ophiotreta has been generally considered to be closely related to Ophiacantha Müller & Troschel, 1842 , while Ophiopristis has been interpreted as a member of the more basal, large-pored lineages (e.g. Martynov 2010).

The strong similarities in general skeletal morphology between both genera, especially considering the long jaws, the number, shape and arrangement of the oral papillae and the size of the tentacle pores, however, were mentioned, although briefly, by Paterson (1985). These imply that the two genera in question are either much more closely related than previously thought, or that they independently evolved similarly shaped jaws and tentacle pores. The above-mentioned, striking similarities in LAP morphology favour the former interpretation, in line with the observations by Thuy & Stöhr (2011) concerning similarities in LAP morphology and close phylogenetic relationships.

The size of the tentacle pore needs more detailed consideration here since it was identified as one of the major characters separating ophiacanthid genera into the basal, large-pored lineages and the large clade of the small-pored genera in the phylogenetic analysis of Thuy et al. (2012). In the revised phylogenetic analysis presented here, Ophiotreta is basalmost among the large-pored genera and thus close to Ophiopristis . As already pointed out by Paterson (1985), tentacle pores in Ophiopristis are rather small in comparison with other former ophiotominids, and in Ophiotreta they are rather large for a former ophiacanthinid. The definition of the tentacle pore size as proposed by Thuy et al. (2012) might not be suited to reflect the close relationship between Ophiotreta and Ophiopristis adequately, which obviously seem to hold an intermediate position between the basal large-pored lineages and the more derived small-pored ones. Whether Ophiotreta and Ophiopristis represent an unusually small-pored group within the large-pored lineages or vice versa requires a careful reassessment of the definition of the tentacle pore size with respect to its application in cladistic phylogenetic analyses.

The fossil Ophiotreta - and Ophiopristis -like LAPs described herein mostly display connecting ridges between the distalmost lamella of the outer surface ornament and the ventral lobe of the spine articulation, a feature shared by both Ophiopristis and Ophiotreta but more commonly observed in the latter, especially when in proximal LAPs. Thus, the fossil LAPs in question are here assigned to Ophiotreta , stressing, however, that Ophiotreta and Ophiopristis share barely distinguishable LAP morphologies, and that at least some of the fossil LAPs most probably belong to undescribed genera close to the two extant taxa and best erected on the basis of articulated finds. Ophiotreta and Ophiopristis most probably represent a single lineage, of which the fossil LAPs described herein, although assigned to Ophiotreta , constitute the fossil record.

Figures

Fig. 16. Skeletal plates of fossil and Recent ophiacanthid brittle stars; lateral arm plates (LAPs) in external (a) and internal (b) views. 1-3. Ophiotreta durbanensis (Mortensen, 1933), Recent. 1. Proximal LAP. 2. Distal LAP. 3. Arm spine. 4. Ophiotreta valenciennesi (Lyman, 1879), Recent; proximal to median LAP. 5-6. Ophiopristis procera (Koehler, 1904), Recent. 5. Proximal LAP. 6. Distal LAP. 7-10. Ophiotreta stefaniae sp. nov. from the late Oxfordian (Late Jurassic) of the Plettenberg, Germany. 7. GZG.INV.78583 (holotype), proximal LAP. 8. GZG.INV.78584 (paratype), proximal LAP. 9. GZG. INV.78585 (paratype), median LAP. 10. GZG.INV.78586 (paratype), distal LAP. One common scale bar per species is given, except for 3.