Neoancistrocrania

NEOANCISTROCRANIA: RELATIONSHIPS, ADAPTATION, AND TAXONOMY

As previously reported (Cohen et al., 2008) Neoancistrocrania is sister to the Novocrania clade that occurs in the North-East Atlantic and Western Mediterranean, an unexpected relationship according to Laurin (1992) that present results confirm, and the time-tree indicates that their mean age of divergence was ∼90 Ma. A speculative hypothesis to account for the distinctive ventral shell morphology of Neoancistrocrania is suggested by two observations. First, that mineralization of the cemented (‘ventral’) valve of Novocrania specimens varies from ‘normal’ to almost nothing, the latter being seen in several specimens from the Chesterfield Ridge (e.g. D1651– 1653) whose dorsal valves were ∼ 1 cm across but which showed no sign of ventral valve mineralization. Unmineralized ventral shell has also been reported in Novocrania lecointei by J. H. Robinson (2012, personal communication) and described in N. anomala (Cusack & Williams, 2001). Second, a few cemented valves and complete individuals of Neoancistrocrania were recovered on substrate blocks (dredged by Dr B. Richer de Forges). Some of these created an impression that, in life, the animals had been surrounded by a trench a few millimetres wide, which was somehow kept clear of encrusting organisms and limestone deposits. This impression remains incompletely documented because of the fragmentary nature of the material. Nevertheless, coupled with variability in ventral valve mineralization it suggests that, historically, Neoancistrocrania may have originated from common ancestors with Novocrania that exhibited a wide range of valve mineralization, the thick-valved extreme of the range being favoured by differential survival on rapidly growing reefs, resulting in speciation due to competitive exclusion of craniids with thin ventral shells. The closest affinity of Neoancistrocrania with Atlantic–Mediterranean Novocrania suggests that any such divergence may have occurred in the Tethys, in a region and at a time when conditions permitted rapid mineral growth, presumably during a period of elevated water temperatures (∼100 Ma or more, Veevers, 2004).

Neoancistrocrania presents a systematic problem. Morphologically it is readily distinguished from Novocrania and relaxed-clock analyses of rDNAs place the extant craniid root between these genera. These facts endorse separate generic status. Against this may be set the topology of the rDNA gene trees and splits analyses, all of which suggest that Neoancistrocrania is no more distantly related to the Northern clade of Novocrania than are other Novocrania clades. These data therefore favour generic synonymy. No resolution is available for this conflict between molecules and morphology as classification tools, but the present taxonomy is clearly useful, and no change seems necessary unless new evidence is forthcoming.