Arrakiscolex aasei, Leibach & Lerosey-Aubril & Whitaker & Schiffbauer & Kimmig, 2021

Leibach, Wade W., Lerosey-Aubril, Rudy, Whitaker, Anna F., Schiffbauer, James D. & Kimmig, Julien, 2021, First palaeoscolecid from the Cambrian (Drumian, Miaolingian) Marjum Formation of western Utah, USA, Acta Palaeontologica Polonica 66 (3), pp. 663-678 : 671-673

Arrakiscolex aasei sp. nov.

Figs. 4 –6.

Zoobank LSID: urn:lsid:zoobank.org:act:6B251ADA-C57D-4E34-BAE3-F8325F504C91

?2010 Priapulida indet.; Bonino and Kier 2010: pl. 74d.?2015 Wronascolex sp. ; Robison et al. 2015: 88, fig. 54 (right).

?2018 Palaeoscolecida gen. et sp. nov. A; Lerosey-Aubril et al. 2018: 708, 711, figs. 3d–g, 5e, f, supplemental tables 1, 3.

Etymology: In honour of Arvid Aase, the collector of the type material. Type material: Holotype, KUMIP 314100, a fragment of cuticle. Paratype, KUMIP 314101, a fragment of cuticle, from the type locality and horizon.

Type locality: The Grey Marjum locality, House Range, Utah, USA .

Type horizon: Middle part ( Ptychagnostus punctuosus Zone ) of the Drumian Marjum Formation.

Material.—The type material and possibly up to fifty, mostly incomplete specimens from the Guzhangian ( Proagnostus bulbus Agnostoid Zone , Cedaria Trilobite Zone ) upper

Weeks Formation of the House Range of Utah, USA (see Remarks below).

Diagnosis.— Each annulus homogeneously covered by hundreds of plates (>500 on average). Plates equally sized, approx. 25 μm in diameter, discoid, and smooth, each composed of a slightly depressed to slightly swollen inner region surrounded by a pronounced marginal rim of varying width. Platelets and microplates are absent. Interannular boundary is represented by plate-free areas that alternate with plate-bearing areas (“columns”) connecting one annulus to the other.

Description.—The fossils are fragments preserved as twodimensional compressions, either straight or slightly curved ( Fig. 4 ). The holotype (KUMIP 314100) has a width of 3.95 mm at its widest and a length (uncoiled) of 38.14 mm ( Fig. 4A ). The paratype (KUMIP 314101) has a width of 1.98 mm at its widest and a length (uncoiled) of 25.12 mm ( Fig. 4B ). In both KUMIP 314100 and KUMIP 314101, there are approx. 3 annuli per mm ( Fig. 4A 2 , B). The plates are similar in discoid morphology and size, ranging from 23–28 μm in diameter in KUMIP 314100 and 24–25 μm in diameter in KUMIP 314101 ( Fig. 5A, B ). Their external surface exhibits a smooth inner region, which varies from slightly depressed to slightly swollen compared to the thin raised rim surrounding it ( Fig. 5B 2 ). The width of this marginal rim is not constant both between and within individual plates, varying from 1.5–3 μm between different plates in KUMIP 314100 and 3–6 μm in KUMIP 314101, and 1–3 μm in thickness within plates in both specimens. With regard to their arrangement, the plates are evenly distributed, with one plate being typically surrounded by six others located, on average, at a spacing distance of approx. 13 μm ( Fig. 5B 1). Alignments of plates in any given direction have proven particularly challenging to follow, and therefore we use the term “rows” in a loose sense to depict gross alignments of plates along the transverse axis of the body. In KUMIP 314100, each annulus comprises 8–14 of such “rows”, which are composed of 60–90 plates as measured in the widest preserved parts of the scleritome ( Fig. 5A 1). Counting of all plates present on at least three annuli per specimen results in a varying plate number per annulus, from 300–860 in KUMIP 314100 and 370–630 in KUMIP 314101. Annuli boundaries are marked by areas lacking plates. These areas never form continuous transverse bands but are interrupted by plate-covered zones or “columns” connecting two consecutives annuli ( Fig. 5A 2 ). It is unclear whether the presence of bands of discontinuous plate-free areas is taphonomic. Both the holotype and paratype of Arrakiscolex aasei gen. et sp. nov. are relatively poorly preserved, missing both their anterior and posterior ends as well as gut tract. Accordingly, the morphology of the proboscis and the presence of caudal spines are unknown in the new taxon. It is unlikely, however, that these organisms were transported or disturbed much either prior to or after burial, given their relatively well-ordered delicate plates.

Plates of the holotype (KUMIP 314100) are preserved as Fe, Mg, and P (Fig. 6A), and plates on the paratype (KUMIP 314101) are preserved as Fe, Mg, P, and Ca (Fig. 6B), which is not atypical of palaeoscolecid plates (e.g., Harvey et al. 2010; Botting et al. 2012; Martin et al. 2016; Whitaker et al. 2020). While both holotype and paratype have similar Fe, Mg, and P elemental maps, KUMIP 314100 has a visibly weaker Ca signature (Fig. 6A 2) likely owing to the presence of two mineral bands with comparatively high Ca signal overwhelming the signature of the plates in relative abundance EDS elemental maps. Despite the presence of well-preserved plates, little soft tissue is preserved within either KUMIP 314100 or 314101, and what remains is primarily replaced by calcium carbonate, later altered to dolomite or high-Mg calcite (Fig. 6A 2, B 2).

Remarks.—We tentatively assigned to Arrakiscolex aasei gen. et sp. nov. about fifty specimens recovered from the overlying Guzhangian Weeks Formation, and reposited in the Department of Geology of the University of Utah and the Natural History Museum of Utah (Salt Lake City, USA), and the Back to the Past Museum (Cancun, Mexico). When studied macroscopically, these specimens all exhibit similar scleritomes (e.g., Lerosey-Aubril et al. 2018: figs. 3d–g, 5e, f), which are characterized by the presence of numerous, densely distributed plates over the whole surface of each annulus. Preliminary microscopic investigations on two specimens also revealed plates comparable in size and morphology to those described in the type material of the new Marjum Formation taxon ( Lerosey-Aubril et al. 2018: fig. 5f). However, these microscopic observations are too scarce and spatially limited to allow their generalization to the whole scleritomes of those two individuals, and a fortiori to all the palaeoscolecid fossils found in the Weeks Formation, hence their tentative assignment to Arrakiscolex aasei gen. et sp. nov. Confirming this assignment will be a major objective of our investigations on this abundant, stratigraphically younger palaeoscolecid material.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig.4.Palaeoscolecid Arrakiscolex aasei gen et sp. nov. from the Drumian,Cambrian Marjum Formation in the House Range of Utah, USA (Grey Marjum locality). A. Holotype (KUMIP 314100). A1, general view of counterpart with location of close-up; A2, detailed view of trunk annulation; A3, general view of part. B. Paratype (KUMIP 314101), general view, showing trunk annulation and location of SEM micrograph in Fig. 5A1 (dashed line rectangle, arrowheads point to the limits of poorly preserved part of the fossil).

Fig. 2. Ottoid scalidophoran Ottoia? sp. (KUMIP 204770) from the Drumian, Cambrian Marjum Formation in the House Range of Utah, USA (Sponge Gully locality). A1, general view showing body orientation and location of close-ups; A2, detailed view of anterior region, showing the partially everted introvert and digestive tract;A3, detailed view of posterior region, showing location of possible bases of caudal hooks;A4, SEM micrograph showing trunk annulation, arrowheads point to fossil margins; A5, SEM micrograph of anterior region, showing introvert and a few introvert hooks, arrowheads point to fossil margins;A6, SEM micrograph of introvert hooks.

Fig.5. Scleritome of the palaeoscolecid Arrakiscolex aasei gen et sp.nov.from the Drumian, Cambrian Marjum Formation in the House Range of Utah, USA (Grey Marjum locality).A. SEM micrographs of paratype (KUMIP 314101).A1, dense and homogeneous distribution of the plates on the surfaces of annuli; A2, interannular region showing alternation between plate-free areas and plate-covered areas (“columns”; arrowheads). B. SEM micrographs of holotype (KUMIP 314100). B1, homogeneous distribution of the plates; B2, single plate, showing the swollen inner region surrounded by a raised marginal rim.