Geromura touertensis, Thuy, 2013

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 : 74-75

Geromura touertensis sp. nov.

urn:lsid:zoobank.org:act:83E97048-1F4F-447A-927A-EA58AE3B4973

Fig. 14 : 8-10

Diagnosis

Species of Geromura gen. nov. with moderately large LAPs, at least twice wider than high; displaying two conspicuous, well-defined and strongly protruding spurs on the outer proximal edge paralleled by two poorly defined areas of densely meshed stereom on the inner distal edge; up to three spine articulations; single large knob in proximal LAPs, divided into two widely separate, smaller knobs in median and distal LAPs.

Etymology

Species named after its type locality Le Touert near Chaudon-Norante, Alpes-de-Haute-Provence, France.

Type material

Holotype GZG.INV.78571.

Paratypes

GZG.INV.78572 and GZG.INV.78573.

Type locality and horizon

Le Touert near Chaudon, France; marl bed at the Bajocian-Bathonian boundary, Middle Jurassic.

Description

Holotype

GZG.INV.78571 is a dissociated, moderately large median to proximal LAP; slightly more than twice wider than high; dorsal edge strongly concave as a result of a well-developed constriction; distal edge nearly straight; ventro-distal tip of LAP strongly protruding; proximal edge wavy, with two moderately large, well-defined, prominent, horizontally elongate and conspicuously protruding spurs composed of densely meshed stereom; ventral spur larger and more strongly protruding than dorsal one; outer surface with very coarsely meshed stereom, replaced by finely meshed stereom in a narrow band along the proximal edge of the LAP; intersections of trabeculae thickened into granules. Two relatively small, rugged, ear-shaped spine articulations, freestanding on slightly elevated distal edge, not in notches or depressions; ventral and dorsal lobes forming nearly circular volute; nerve opening large, ventrodistally bordering sigmoidal fold; dorsal spine articulation slightly larger than ventral one; very large gap separating spine articulations and distal edge of LAP. Ventral edge of LAP concave, with very large, concave tentacle notch.

Inner side of LAP with two widely separate, sharply defined and prominent knobs; the proximal knob three times larger than distal one, nearly triangular; distal knob rounded; inner side of distal edge of LAP with poorly defined, slightly prominent knobs composed of densely meshed stereom, one knob in the dorso-distal corner, the other in the ventro-distal corner. Inner side of tentacle notch with very coarsely meshed and horizontally stretched stereom. No perforations discernible on inner side.

Paratype supplements and variation

GZG.INV.78572 is a dissociated proximal LAP; twice wider than high; generally well in agreement with holotype; dorso-proximal tip fragmentary. Three spine articulations, with very slight dorsalward increase in size; ventral gap larger than dorsal gap.

Inner side with single, large, sharply defined, prominent, nearly wedge-shaped knob.

GZG.INV.78573 is a dissociated distal LAP; rod shaped; almost three times as wide as high; spurs on proximal edge strongly protruding, nearly equal sized. Two spine articulations, ventral one slightly smaller than dorsal one. Ventral edge of LAP evenly concave, tentacle opening developed as perforation ventrally bordering the ventral spine articulation.

Inner side with two sharply defined, rounded, prominent and widely separated knobs; proximal knob approximately three times larger than distal one. Very large tentacle notch in the middle of the distal third of the inner side.

Remarks

These LAPs can be unmistakably distinguished from those of its congener Geromura teckliformis sp. nov. (see below) on account of the two well-developed and strongly protruding spurs on the outer proximal edge paralleled by less well-defined, slightly prominent spurs on the inner distal edge, as well as the lower number of spine articulations. The transformation of two separate knobs into a single larger one from median to proximal LAPs is remarkable. In fact, it suggests that the development of the knobs on the inner side is primarily controlled by biomechanical constraints imposed by the height/ width ration of the LAPs. This is corroborated by G. teckliformis sp. nov. in which all LAPs, including the proximal ones, have a significantly lower height/width ratio than those of G. touertensis sp. nov., and at the same time invariably display two widely separated knobs. Supportive evidence furthermore comes from the distalmost LAPs of Lapidaster etteri sp. nov. (see above). With a height/width ratio of 1:3 and thus comparable to that of most LAPs of species of Geromura gen. nov., the distalmost LAPs of Lapidaster etteri sp. nov. display two widely separate knobs rather than a single knob or ridge.

Because elongate arm segments are a typically juvenile character ( Stöhr 2005), it appears probable that Geromura gen. nov. developed from an ancestor of the Lapidaster - Ophiologimus lineage via paedogenesis. The strong elongation of the arm segments entailed the transformation of a single knob or ridge on the inner side of the LAPs into two widely separate knobs. It cannot be ruled out that this process occurred more than once, which would imply that Geromura gen. nov. is an artificial grouping of superficially similar but independently evolved LAP morphologies. The strong similarities in spine articulation morphology, however, endorse the concept of Geromura gen. nov. as a single lineage, since there is no evidence that a decrease in height/width ratio entails any changes in the structure of the spine articulations.

Occurrence

Bajocian-Bathonian of France.

Figures

Fig. 14. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views. 1-3. Ophiolimna lisae sp. nov. from the early Kimmeridgian of the Pointe du Chay (1), France, and the late Kimmeridgian of Trancoso, Portugal (2-3). 1. GZG.INV.78561 (holotype), proximal LAP. 2. GZG. INV.78562 (paratype), proximal LAP. 3. GZG.INV.78563 (paratype), median LAP. 4-7. Ophiolimna kucerai sp. nov. from the latest Aptian to earliest Albian (Early Cretaceous) of Blake Nose, NE Atlantic. 4. GZG.INV.78566 (holotype), proximal LAP. 5. GZG.INV.78567 (paratype), median LAP. 6. GZG. INV.78568 (paratype), distal LAP. 7. GZG.INV.78569 (paratype), proximal LAP. 8-10. Geromura touertensis gen. et sp. nov. from the Bajocian-Bathonian boundary of Touert, France. 8. GZG.INV.78571 (holotype) median to proximal LAP. 9. GZG.INV.78572 (paratype), proximal LAP. 10. GZG.INV.78573 (paratype), distal LAP. One common scale bar per species is given.