Ophiolofsson obituary, Thuy & Eriksson & Kutscher & Numberger-Thuy, 2024

Thuy, Ben, Eriksson, Mats E., Kutscher, Manfred & Numberger-Thuy, Lea D., 2024, The beginning of a success story: basalmost members of the extant ophiuroid clade from the Silurian of Gotland, Sweden, European Journal of Taxonomy 947 (1), pp. 216-247 : 227-228

Ophiolofsson obituary gen. et sp. nov.

urn:lsid:zoobank.org:act:78904D7E-CE55-4F89-8E26-16E70DB74B82

Figs 3E–L , 7

Diagnosis

Species of Ophiolofsson gen. nov. with rounded trapezoidal lateral arm plates with a slightly concave dorsal edge; outer surface with a coarse, dense tuberculation; up to three very large, vertically elongate spine articulations in ventral portion of distal edge; up to five large, strongly protruding ambulacral groove spine articulations.

Etymology

Species named after American death metal pioneers, Obituary, who were fundamental in developing the death metal genre, and appropriately, O. obituary gen. et sp. nov. represents a basal species in the Ophiolofsson lineage.

Type material

Holotype

SWEDEN • Gotland, Brissund ; NRM PAL Ec38324 .

Paratypes

SWEDEN • 3 specs; same data as for holotype; NRM PAL Ec38325 to NRM PAL Ec38327 .

Type locality and stratum

Wenlock Series, Sheinwoodian Stage, Upper Kockellela ranuliformis conodont Zone, Högklint Formation “undifferentiated”; Brissund, Gotland, Sweden.

Description

Holotype ( Fig. 3E–F )

NRM PAL Ec38324 is a dissociated median to proximal lateral arm plate, approximately 1.3 times as long as high, with nearly straight dorsal edge and strongly convex distal edge; proximal edge weakly convex, without central incision and with shallow, poorly defined, semi-circular depression ventrally bordered by a poorly defined, weakly prominent oval knob; outer surface with coarse tuberculation composed of enlarged trabecular intersections covering entire outer surface except for depressed area at proximal edge; three very large, vertically elongate, equidistant lateral arm spine articulations in vertical to slightly oblique row at some distance from distal edge, freestanding and prominent; dorsalward increase in size of spine articulations; dorsalmost spine articulation separated from dorsal edge of lateral arm plate by one quarter to one fifth of lateral arm plate height; spine articulations consisting of single opening encompassed by coarse, elevated ring; ventral edge of lateral arm plate slightly convex, with strong protrusions of ambulacral groove spine articulations. Inner side of lateral arm plate with very large, sharply defined and prominent vertebral articulation knob at dorso-proximal corner of lateral arm plate; centre of knob slightly depressed. Inner distal edge of lateral arm plate ( Fig. 3F ) slightly elevated and separated from remaining outer surface by low, moderately well-defined vertical ridge; large, well-defined tentacle notch distally sharply bordered by ridge of distal edge; ventral edge of lateral arm plate devoid of incision for tentacle notch but with five large, conspicuous, nearly equidistant ambulacral groove spine articulations.

Paratype (supplements)

NRM PAL Ec38325 ( Fig. 3G–H ) is a dissociated proximal lateral arm plate approximately 1.2 times as long as high; well in agreement with holotype but with slightly concave dorsal edge; lateral arm spine articulations as in holotype; ventral edge with only very weak protrusions of ambulacral groove spine articulations. Inner side as in holotype but with four slightly smaller, less strongly protruding ambulacral groove spine articulations.

NRM PAL Ec38326 ( Fig. 3I–J ) is a dissociated median lateral arm plate similar to holotype but slightly longer and with spine articulations restricted to the ventral half of the plate. Inner side as in holotype.

NRM PAL Ec38327 ( Fig. 3K–L ) is a dissociated distal lateral arm plate approximately two times as long as high; well in agreement with holotype except for more strongly concave proximal edge; outer surface ornamentation and lateral arm spine articulations as in holotype; ventral edge with five conspicuous protrusions of ambulacral groove spine articulations. Inner side as in holotype.

Remarks

Ophiolofsson obituary gen. et sp. nov. is distinguished from other species by its large, vertically elongated spine articulations, arranged in an oblique row along the ventral section of the distal part of the lateral arm plate. It furthermore differs from the morphologically similar and stratigraphically close Ophiolofsson joelmciveri gen. et sp. nov. in having a slightly coarser outer surface tuberculation and larger and more strongly protruding ambulacral groove spines. Remarkably, the two occurrences of O. obituary described in the present paper differ in average plate size while maintaining an otherwise identical morphology: the lateral arm plates of O. obituary from Lickershamn are significantly larger than their Brissund counterparts. This observation agrees with the size decrease described by Thuy et al. (2022) coinciding with the Ireviken Event.

Occurrence

Brissund and Lickershamn.

Figures

Fig. 3. Dissociated lateral arm plates of ophiuroids from the Silurian of Gotland. A–B. Ophiolofsson joelmciveri gen. et sp. nov., holotype, NRM PAL Ec38322, proximal lateral arm plate in external (A) and internal (B) views. C–D. O. joelmciveri, paratype, NRM PAL Ec38323, median to distal lateral arm plate in external (C) and internal (D) views. E–F. Ophiolofsson obituary gen. et sp. nov., holotype, NRM PAL Ec38324, median to proximal lateral arm plate in external (E) and internal (F) views. G–H. O. obituary, paratype, NRM PAL Ec38325, proximal lateral arm plate in external (G) and internal (H) views. I–J. O. obituary, paratype, NRM PAL Ec38326, proximal median lateral arm plate in external (I) and internal (J) views. K–L. O. obituary, paratype, NRM PAL Ec38327, distal lateral arm plate in external (K) and internal (L) views. Abbreviations: AGSA = ambulacral groove spine articulations; di = distal; do = dorsal; LSA = lateral spine articulations; TN = tentacle notch; VAK = vertebral articular knob.

Fig. 7. Stratigraphic position of the taxa described in the present paper with their assumed evolutionary relationships and the nesting of the clades Ankhurida clade nov., Ophiovalida clade nov. and Metophiurida Matsumoto, 1913. Note that some lateral arm plates are mirrored to allow comparison in similar orientation.