Lapidaster mastodon, 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 : 42-44

Lapidaster mastodon sp. nov.

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Fig. 9 : 6-8

Diagnosis

Species of Lapidaster gen. nov. with moderately large LAPs displaying a round, sickle-shaped outline; up to three spine articulations in proximal LAPs.

Etymology

Species named in honour of Brent Hinds, Brann Dailor, Bill Kelliher and Troy Sanders of the rockgroup Mastodon, whose music was indispensable during lengthy nightly sessions at the scanning electron microscope.

Type material

Holotype GZG.INV.78517 .

Paratypes

GZG.INV.78518 and GZG.INV.78519.

Type locality and horizon

Plettenberg near Balingen, southern Germany; clay pockets between sponge associations in the lowest bed exposed at the quarry, Bimammatum Zone, Late Oxfordian, Late Jurassic.

Additional material

GZG.INV.78520 (194 dissociated LAPs) from the Late Oxfordian of Plettenberg near Balingen; GZG. INV.78521 (2 dissociated LAPs) from the Early Kimmeridgian of Geisingen, southern Germany.

Description

Holotype

GZG.INV.78517 is an isolated, small proximal LAP, of equal height and width, with sickle-shaped outline; ventral third of LAP protruding; ventro-distal tip of LAP only slightly so; proximal edge strongly concave, distal and dorsal edges convex; dorso-proximal and ventro-proximal tips of LAP pointed; no constriction; poorly defined but slightly prominent and protruding spur in ventral third of proximal edge, composed of slightly denser stereom than remaining edge. Outer surface with finely reticulate stereom; trabeculae showing tendency to merge into irregular vertical stripes, especially in ventral half of outer surface; reticulate stereom grading into finely meshed stereom towards proximal edge; dorsalmost spine articulation directly surrounded by reticulate stereom; two ventral spine articulations surrounded by slightly sunken areole of more finely meshed stereom, encompassed by reticulate stereom of outer surface. Three large, ear-shaped spine articulations freestanding in continuous row near distal edge, of equal size, dorsal gap larger than ventral one; dorsal and ventral lobes separated in all three spine articulations by irregularly crenulate central proximal knob. Ventral edge of LAP with large, gently concave tentacle notch.

Inner side of LAP with moderately large, well-defined and prominent ridge; dorsal half of ridge oblique and nearly straight, not reaching dorsal or proximal edges of LAP, with slightly widened dorsal tip; ventral half of ridge confluent with thickened ventral edge of LAP, less prominent than dorsal one, separated from the latter by a rounded kink; no perforations discernible on inner side; inner side of ventro-distal tip of LAP with large, well-defined, round, prominent spur. Inner side of tentacle notch with horizontally stretched stereom.

Paratype supplements and variation

GZG.INV.78518 is a dissociated median LAP, slightly wider than high, differing from holotype in having a slightly better-defined spur on the ventral third of the outer proximal edge, and in addition a very poorly defined, almost indiscernible spur in the middle of the outer proximal edge. Inner side well in agreement with holotype morphology.

GZG.INV.78519 is a dissociated distal LAP, approximately twice wider than high; dorsal edge straight to very slightly concave; only very small ventral portion of LAP protruding ventro-proximalwards. Proximal edge with well-defined, prominent and slightly protruding spur near ventro-proximal tip of LAP; second, smaller, less well-defined but clearly protruding spur in the middle of the proximal edge. Three spine articulations; dorsal gap between spine articulations only slightly larger than ventral one. Tentacle notch large but shallow.

Inner side with sharply defined, prominent, oblique and nearly straight ridge, widest ventrally, with short ventro-proximalwards pointing protrusion. Inner side of ventro-distal tip of LAP with large, welldefined and prominent spur. Tentacle notch wide, with slightly horizontally stretched stereom, dorsally bordered by single irregular perforation.

Remarks

The round, sickle-shaped outline, the unusually weakly protruding ventro-distal tip of the LAPs, and the low number of spine articulations (three in proximal segments) are a combination of characters which is unique among the LAP types assignable to Lapidaster gen. nov. It is therefore here described as a new species within this genus. At least in terms of LAP morphology, however, Lapidaster mastodon sp. nov., L. fasciatus comb. nov. and L. varuna sp. nov. appear to be very closely related.

From a palaeoecological perspective, both known occurrences of Lapidaster mastodon sp. nov. are from clayey crevasse infills within sponge reefs and from clay levels interbedded with sponge reefs. It thus seems to be the sole species known of the genus which is exclusively found associated with sponge build-ups rather than on muddy bottoms.

Occurrence

Late Oxfordian to early Kimmeridgian of Germany.

Figures

Fig. 9. Fossil lateral arm plates (LAPs) of ophiacanthid brittle stars in external (a) and internal (b) views and articulated arm fragments. 1-5. Lapidaster etteri gen. et sp. nov. from the early Oxfordian (Late Jurassic) of Longecombe, France. 1. NHMB M11209 (holotype), proximal LAP. 2. NHMB 11210 (paratype), median LAP. 3. NHMB M11211 (paratype), distal LAP. 4. NHMB M11212 (paratype), distal arm fragment in lateral view. 5. NHMB M11213 (paratype), distal arm fragment in dorsal (a) and ventral (b) views. 6-8. Lapidaster mastodon gen. et sp. nov. from the late Oxfordian (Late Jurassic) of the Plettenberg, Germany. 6. GZG.INV.78517 (holotype), proximal LAP. 7. GZG.INV.78518 (paratype), median LAP. 8. GZG.INV.78519 (paratype), distal LAP. One common scale bar per species is given, except for 4 and 5.