Ottoia Walcott, 1911

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 : 666-669

Genus Ottoia Walcott, 1911

Type species: Ottoia prolifica Walcott, 1911 ; Burgess Shale Formation , Wulian, Cambrian, British Columbia, Canada .

Ottoia ? sp.

Figs. 2 , 3 .

1986 Ottoia prolifica ( Walcott, 1911) ; Conway Morris and Robison 1986: figs. 1, 2.

1991 Ottoia prolifica ( Walcott, 1911) ; Robison 1991: fig. 6.2.

2015 Scathascolex minor gen. et sp. nov.; Smith 2015: 974.

2016 Ottoia prolifica ; Foster and Gaines 2016: 311, fig. 20A.

2017 Ottoia prolifica ; Broce and Schiffbauer 2017: 602, fig. 1A–C.

Material.— KUMIP 204770 (part and counterpart) from the Sponge Gully locality, Utah, USA ( Fig. 1B 2 ). Middle part ( Ptychagnostus punctuosus Agnostoid Zone ) of the Drumian Marjum Formation .

Description.— KUMIP 204770 measures 4.92 mm in maximum width and 48 mm in length, when uncoiled and represents a complete individual ( Fig. 2 ). It displays anteriorly a partially everted proboscis (eversion stage 3 of Conway Morris 1977; Fig. 2A 1, A 2 , A 6), which is approx. 1.87 mm long and widest distally (approx. 1.28 mm); there is no clear evidence for the preservation of teeth on this organ, however, there are several observable introvert hooks present ( Fig. 2A 2 , A 6). The trunk is almost parallel-sided, except in its posterior fifth where it progressively tapers posteriorly. About 110 well-delimitated, densely distributed (approx. 5 annuli per mm), thin trunk annuli are visible ( Fig. 2A 4 ). The presence of caudal hooks cannot be confidently established, but two rather large structures resembling the “conical projections” of Harvey et al. (2010: fig. 2E–G) are interpreted as the basal parts of a single pair of strong hooks ( Fig. 2A 3 ). A non-everted part of the proboscis, about as long as the everted part, is visible in the anterior trunk region ( Fig. 2A 2 ). It is continued posteriorly by a short portion of a slightly wider and not as well-delimitated intestine, represented by a darker coloured, carbon-rich region. No plates, phosphatic or otherwise, were observed, despite careful examination of the surface of the trunk using SEM/ EDS ( Fig. 3 ). EDS analysis reveals that the fossil is predominantly composed of C, O, Fe, and Mg.

Remarks.—This specimen was originally assigned to the scalidophoran Ottoia prolifica by Conway Morris and Robison (1986), which purportedly extended the geographic and biostratigraphic distributions of this Wuliuan Burgess Shale species to the Drumian strata of Utah. More recently, Smith et al. (2015) questioned this Marjum Formation occurrence of Ottoia prolifica on the grounds that it lacked a detailed description. Later that year, Smith (2015: 974) tentatively reassigned KUMIP 204770 to the new palaeoscolecid taxon, Scathascolex minor Smith, 2015 , but provided no support for this claim.

Palaeoscolecid affinities for KUMIP 204770 might be argued based on the presence of a subparallel-sided trunk,

feature exhibited by all members of the group, but not exclusive to it. The possible remains of a single pair of large caudal hooks would also be more compatible with an assignment to Palaeoscolecida than Ottoia ( Harvey et al. 2010) , the latter taxon possessing eight smaller caudal hooks ( Conway Morris 1977). However, our SEM-EDS investigations demonstrate the complete absence of plates covering the trunk in this specimen, which strongly argues against a close relationship with S. minor or any member of the Palaeoscolecida. Moreover, S. minor has four, not two, caudal hooks and a significantly more elongate body (approx. 16 times longer than wide) than KUMIP 204770 (approx. 10 times longer than wide), despite being a rather short form for a palaeoscolecid. On the other hand, the present description of the Marjum specimen does not provide support for its assignment to Ottoia prolifica either. Emended by Smith et al. (2015), the diagnosis of this species now solely concerns characters pertaining to the teeth borne by the proboscis, whereas KUMIP 204770 preserves no such structures. A tentative assignment to the genus Ottoia may be proposed based on the observations of many of the characters forming the unique combination differentiating this genus: a cylindrical and extensible body with a pronounced external symmetry anteriorly, which is divided into an introvert with an extensible anterior end, and a trunk flexible in a single direction (U-shaped body) and comprising about a hundred annuli ( Conway Morris 1977; Smith et al. 2015; Yang et al. 2016). Most of the diagnostic characters of Ottoia that cannot be observed, such as those pertaining to the proboscis teeth or the internal anatomy (gut, musculature), are likely absent due to the insufficient quality of preservation of KUMIP 204770. Additionally, whether a bursa is present along with the exact number of caudal hooks remain unresolved for KUMIP 204770. The investigated specimen only departs from the diagnosis of Ottoia in the absence of an expanded and bulbous posterior trunk region ( Conway Morris 1977; Smith et al. 2015; Yang et al. 2016). However, if this morphological trait is generally well-expressed in Ottoia prolifica (for an exception, see Conway Morris 1977: pl. 2: 3) and Ottoia tricuspida ( Smith et al. 2015) , it is absent in Ottoia guizhouensis (Yang et al. 2016) and may therefore be inadequate for the definition of the whole genus Ottoia .

In summary, our investigations do not support a palaeoscolecid affinity for KUMIP 204770, but we concur with Smith et al. (2015) that this specimen cannot be confidently assigned to Ottoia prolifica either. Awaiting the study of three other specimens of this scalidophoran ( Robison et al. 2015: fig. 52; Broce and Schiffbauer 2017: fig. 1F–H; RLA personal observation), we propose to tentatively assign this fossil to an undetermined species of Ottoia .

Class Palaeoscolecida Conway Morris and Robison, 1986 (sensu Smith 2015)

Family uncertain

Genus Arrakiscolex nov.

Zoobank LCID: urn:lsid:zoobank.org:act:04D3C924-950D-4E27-97D6-C2F0E3B1D79F

Type species: Arrakiscolex aasei gen. et sp. nov. (by monotypy); see below.

Etymology: The name refers to the fictional planet of Arrakis in the novel “Dune” by Frank Herbert, which is inhabited by a species of armoured worm.

Diagnosis.— As for the monotypic type species.

Remarks.—The assignment of the new genus to the class Palaeoscolecida (sensu Smith 2015) is supported by the presence of a markedly annulated cuticle ( Fig. 4 ) bearing numerous phosphatic plates. Arrakiscolex gen. nov. possesses small plates distributed over the surfaces of the preserved cuticle fragments, and as such can be regarded as a representative of the Palaeoscolecida sensu stricto of Harvey et al. (2010). The plate size, abundance, and distribution easily differentiate members of this new genus from taxa with few, large plates per annulus, such as Cricocosmia Hou and Sun, 1988 and Tabelliscolex Han, Zhang, Zhang, and Shu, 2003a . The small plates of the new genus are discoid, which excludes close relationships with a series of derived, mostly Ordovician genera characterized by elongate plates, whether these plates are oblong (e.g., Milaculum Müller, 1973 , Palaeoscolex Whittard, 1953 , Shergoldiscolex Müller and Hinz-Schallreuter, 1993 ) or rectangular (e.g., Loriciscolex Botting, Muir, Van Roy, Bates, and Upton, 2012 , “ Palaeoscolex ” tenensis Kraft and Mergl, 1989, Plasmuscolex Kraft and Mergl, 1989 , Pluoscolex Botting, Muir, Van Roy, Bates, and Upton, 2012 ). Overall, the plates of Arrakiscolex gen. nov. are somewhat reminiscent to Hadimopanella - type plates, except for the presence of a well-differentiated marginal rim and, more importantly, their entirely smooth surface. The latter character alone allows the discrimination of the new genus from most of the aforementioned genera and the remaining ones composing the class. Indeed, the largest plates of the scleritomes of the overwhelming majority of palaeoscolecids bear at least one, and usually more, nodes, whether they are granules (e.g., Bullascolex Botting, Muir, Van Roy, Bates, and Upton, 2012 , Wudingscolex Hu, Steiner, Zhu, Luo, Forchielli, Keupp, Zhao, and Liu, 2012 ), well-developed tubercles (e.g., Palaeoscolex , some Wronascolex Ivantsov and Zhuralev, 2005 ; García-Bellido et al. 2013; Conway Morris 1997), or even fully formed spines (e.g., some Wronascolex ; Ivantsov and Wrona 2004). Completely smooth discoid plates have been previously reported in only two formally described palaeoscolecid genera, the Wuliuan Scathascolex Smith, 2015 and Utahscolex Whitaker, Jamison, Schiffbauer, and Kimmig, 2020.

The scleritomes of Arrakiscolex gen. nov. and the Burgess Shale Formation Scathascolex (Smith, 2015) are both solely composed of homogeneously sized discoid plates (i.e., no platelets or microplates). The diameters of those plates are comparable in the two taxa (23–28 μm and 24–25 μm in the Marjum Formation taxon, 15–25 μm in the Burgess Shale Formation taxon), as are their gross morphologies, a moderately swollen, essentially smooth central region surrounded by a marginal rim. Yet, the marginal rim is more pronounced in the plates of Arrakiscolex gen. nov., which also show no indication of the five nodes/depressions sometimes observed on those of Scathascolex ; the absence of these faint ornamental features is not taphonomic in origin, as the plates are excellently preserved in some parts of the Marjum Formation specimens ( Fig. 5 ). More importantly, the two taxa can be easily distinguished by the radically different distribution patterns of their plates. In Scathascolex , each annulus bears two adjacent rows of plates near each of its boundaries with neighbouring annuli, and these pairs of rows are separated by a wide plate-free central region (Smith 2015). This strongly contrasts with the condition observed in Arrakiscolex gen. nov., where the plates are homogeneously distributed over most of the annulus, only loosely forming “rows” (18–25 per annulus) and with only rare and small plate-free areas associated with annulus boundaries.

The Spence Shale Utahscolex resembles Scathascolex in possessing homogeneously single-sized plates, which form two sets of two rows separated by a plate-free central area on each annulus. However, those rows of plates may occasionally bifurcate when approaching one body margin (Whitaker et al. 2020), a feature unreported in Scathascolex . As mentioned above, this organization remains fundamentally different from that of Arrakiscolex gen. nov. Additionally, the new Marjum Formation taxon possesses smaller and morphologically more complex plates compared to Utahscolex, in which the plates are approx. 70–80 μm in diameter and devoid of marginal rim or any morphological feature. In summary, the two Cambrian palaeoscolecids from Utah are not only found in stratigraphically different (Wuliuan vs. Drumian) deposits roughly 275 km apart, they also possess highly distinguishable plates and plate organisation.

Figures

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. 3. SEM micrograph and SEM-EDS elemental maps of the posterior region of KUMIP 204770 (Ottoia? sp.), from the Drumian, Cambrian Marjum Formation in the House Range of Utah, USA (Sponge Gully locality).A1, SEM micrograph of the analyzed area (posterior trunk region);A2–A6, SEM-EDS elemental maps of P, Ca, C, Fe, and Mg, respectively.

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.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.