Abstract
Ophiuroid systematics is currently in a state of upheaval, with recent molecular estimates fundamentally clashing with traditional, morphology-based classifications. Here, we attempt a long overdue recast of a morphological phylogeny estimate of the Ophiuroidea taking into account latest insights on microstructural features of the arm skeleton. Our final estimate is based on a total of 45 ingroup taxa, including 41 recent species covering the full range of extant ophiuroid higher taxon diversity and 4 fossil species known from exceptionally preserved material, and the Lower Carboniferous Aganaster gregarius as the outgroup. A total of 130 characters were scored directly on specimens. The tree resulting from the Bayesian inference analysis of the full data matrix is reasonably well resolved and well supported, and refutes all previous classifications, with most traditional families discredited as poly- or paraphyletic. In contrast, our tree agrees remarkably well with the latest molecular estimate, thus paving the way towards an integrated new classification of the Ophiuroidea. Among the characters which were qualitatively found to accord best with our tree topology, we selected a list of potential synapomorphies for future formal clade definitions. Furthermore, an analysis with 13 of the ingroup taxa reduced to the lateral arm plate characters produced a tree which was essentially similar to the full dataset tree. This suggests that dissociated lateral arm plates can be analysed in combination with fully known taxa and thus effectively unlocks the extensive record of fossil lateral arm plates for phylogenetic estimates. Finally, the age and position within our tree implies that the ophiuroid crown-group had started to diversify by the Early Triassic.
Highlights
Brittle stars or ophiuroids are a major component of modern marine benthic communities, occurring in all oceans from the tropics to the poles and from the intertidal to the hadal trenches
Apart from occasional attempts at family-level classification, the monographs of the mid- and late nineteenth century which laid the foundation for ophiuroid systematics provided little more than anatomical observations and a plethora of new genera and species
The tree resulting from Bayesian inference of the full character matrix (Fig 1) shows a high amount of structure with reasonable support, several relationships remain unresolved
Summary
Brittle stars or ophiuroids are a major component of modern marine benthic communities, occurring in all oceans from the tropics to the poles and from the intertidal to the hadal trenches.PLOS ONE | DOI:10.1371/journal.pone.0156140 May 26, 2016Morphological Phylogeny of the OphiuroideaWith over 2000 living species, they are the largest among the five extant echinoderm classes [1]. On the basis of a few key skeletal characters, in particular vertebral morphology, the presence of dorsal arm plates and the type of articulation between the genital plate and the radial shield, Matsumoto [2,3] subdivided the Ophiuroidea into two subclasses, the Oegophiuroida for the Paleozoic forms with divided vertebrae, and the Myophiuroida with fused vertebrae, comprising all extant forms The latter were further subdivided into four orders accommodating all the families and subfamilies known at that time: the Phrynophiurida (including the euryalids and Ophiomyxidae), Laemophiurida (Ophiacanthidae and Hemieuryalidae), Gnathophiurida (Amphiuridae, Amphilepididae and Ophiotrichidae) and the Chilophiurida (Ophiodermatidae, Ophiochitonidae, Ophiocomidae, Ophioleucidae and Ophiolepididae)
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