Ophiosparte, Koehler, 1922

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 : 134-135

The genus Ophiosparte and higher ophiuroid taxonomy

There are more examples showing that situations with "ambiguous" boundaries between apparently distantly related families are quite usual within ophiuroids. In this respect the conservative suggestion of Mortensen (1927), to use only two groups (orders) for all recent Ophiuroidea ― Euryalida and Ophiurida ―, i.e. Müller and Troschel’s (1842) system, agrees better with the results of the present study. However, even if according to most characters euryalids are well distinguished from other Ophiuroidea, the arm spine articulation analysis discovered an essential similarity of the family Ophiuridae with euryalids. Smith et al. (1995) suggested a possible relationship between the Euryalida and the ophiomyxid subfamily Ophiobyrsinae , which, however, was not supported by additional characters, and the latter group was retained as an incertae sedis. Lateral arm plates, including spine articulations, figured by Byrne and Hendler (1988) in Ophiobyrsa and Ophiobyrsella , typically have ophiomyxid double-opening articulation. This is confirmed in the present study for the type species of the genus Ophiobyrsa , O. rudis Lyman, 1878 ( Figs 79F–H). The subfamily Ophiobyrsinae is similar to Euryalida due to a convergence in the streptospondylous vertebral articulation. However, under Mortensen’s and Fell’s (1960) system, most ophiuroid families are still only defined by external characters, which has led to the numerous mistakes in assignment of various genera to a particular family that are repeatedly mentioned in the present paper.

The family Ophiuridae that gave its name to the order Ophiurida has similar arm spine articulations to the order Euryalida . Externally, euryalids and ophiurids are very different. Ophiosparte Koehler, 1922 , an enigmatic taxon early referred either to Ophiacanthidae ( Koehler 1922a; Paterson 1985) or to Ophiomyxidae ( Dearborn et al. 1996) is especially interesting in this regard. However, a number of charcters, i.e. true arm comb (papillae placed on the genital plates ( Figs 65F, G), arm spine articulations, spines, genital plates and radial shield, and also a set of numerous comb-like tentacle scales ( Fig. 65), suggest that Ophiosparte is a member of the family Ophiuridae . The special pattern of the Ophiosparte vertebral articulation surface (“comb”-shaped) ( Figs 19A, D–F), a variant of the zygospondylous articulation, also clearly place it in the Ophiuridae , since that state is not found beyond this family. Somewhat similar small papillae bordering the genital slits in the species of the genus Ophionereis are clearly a convergent feature, because in both Ophiosparte and Ophiuridae , and unlike Ophionereis , the arm comb papillae sit on the genital plates and form a peculiar incision laterally to the insertion of the most proximal arm segments. Ophionereis papillae are always placed only along the borders of the genital slits and never on the genital plates near the insertion of the arm into the disk. The arm spine articulations of Ophiosparte gigas are clearly a euryalid feature with the distantly placed nerve opening on the lateral plate and a spine nerve opening correspondingly shifted to the latero-ventral side of the spine ( Figs 23A–E). Massive well-developed condyles on both adradial genital plate and radial shield articulation surfaces ( Fig. 5B) are also similar to the euryalid condition ( Figs 4A–D). The numerous long, spiniform teeth of O. gigas do not differ from the adjacent oral papillae ( Figs 65L–N) and are very similar to some euryalids, e.g. the genus Gorgonocephalus ( Fig. 15A). However, the genus Ophiosparte also has all the synapomorphies of the family Ophiuridae , including the true arm-comb (compare arm combs in Ophiosparte , Fig. 65F and in Ophiura sarsii , Fig. 65G) and lateral ridge of the adradial genital plate, fused with the articulation surface ( Figs 5B; 65H, I)), present in most ophiurid taxa in various degree of development ( Figs 5A–K, lr). In this respect Ophiosparte may be considered as an earlier offshoot of an ancestral group common with a euryalid, before acquiring the streptospondylous vertebral articulation and other arm features allowing a climbing mode of life. Thus Ophiosparte gigas appears to be an intermediate between the Ophiuridae and Euryalida .

Most of the other ophiuroid families (including Ophiomyxidae , Ophiacanthidae , Ophiodermatidae , Ophiocomidae , Ophionereididae , Ophiochitonidae , Amphilepididae , Amphiuridae , Ophiactidae , Ophiolepididae , Hemieuryalidae , Ophiotrichidae ) form a compact group with numerous intermediate taxa even between apparently very different families as for instance Ophiomyxidae and Ophiacanthidae (genera Ophioplexa gen. nov., Ophioprium , Ophiorupta gen. nov. and others) or Ophiacanthidae and Ophiodermatidae ( Ophiolimna and Ophioconis ). The intermediate status of these genera, as confirmed by complicated microstructural features of the spine articulations compared to peculiarities of the external morphology, taken together makes the independent evolution of these features hardly possible. An appropriate name for this higher ophiuroid group will be suggested after a detailed analysis of other ophiuroid groups, which will be presented in further publications of this series.