Plutonides hicksii ( Salter, 1866b )

Unger, Tanja, Hildenbrand, Anne, Stinnesbeck, Wolfgang & Austermann, Gregor, 2022, Biostratigraphy and taxonomy of polymerid trilobites of the Manuels River Formation (Drumian, middle Cambrian), Newfoundland, Canada, Geodiversitas 44 (33), pp. 1051-1087 : 1071-1074

Plutonides hicksii ( Salter, 1866b)

( Figs 13 ; 14 ; 15 )

Paradoxides hicksii Salter, 1866b: 299 , 300, pl. 4, fig. 12. — Salter & Hicks 1869: 55, pl. 3, figs 1-10. — Linnarsson 1882: 14, 15, pl. 3, figs 1-5. — Grönwall 1902: 117, 118. — Illing 1915: 428, 429, pl. 36, figs 1-7. — Nicholas 1915: 467, 469.

Paradoxides hicksii var. palpebrosus – Linnarsson 1879: 9-11, pl. 1, figs 5-11.

Paradoxides hicksi – Cobbold 1913: 47, 48, pl. 4, figs 1-5. — Lake 1934: 196, pl. 25, figs 4-9 (?);1935: 197-200, pl. 36, figs 1, 2, 2a. — Hutchinson 1952: 76, pl. 2, fig. 5(?); 1962: 113, 114, pl. 18, figs 4-12; pl. 19, figs 1, 2. — Egorova et al. 1982: 77, pl. 1, figs 4-6; pl. 2, figs 10, 11; pl. 3, figs 11, 12.

Hydrocephalus hicksii – Fletcher 1972b: 93, 94, pl. 47, fig. 15; pl. 48, figs 2-6; pl. 49, figs 1-5, 7-10 (?).

Paradoxides (Paradoxides) hicksii – Morris & Fortey 1985: 7, fig. 2.

Paradoxides (Hydrocephalus) hicksii – Martin & Dean 1988: 19, pl. 3, figs 4, 7.

Plutonides hicksii – Whittington et al. 1997: figs 306.1a-c. — Young et al. 2002: pl. 4, figs xi, xii. — Rushton et al. 2007: fig. 8e (non Vaněk et al. 1999: 36, pl. 1, figs 1-6).

Paradoxides (= Plutonides ) hicksii (sic) – Bridge et al. 1998: pl. 8, figs g-h, j.

Paradoxides (subgen. nov.) hicksii (sic) – Fletcher 2006: pl. 34, figs 6-8.

Paradoxides (Mawddachites) hicksii – Fletcher 2007: 48, figs 8A- F. — Rees et al. 2014: figs 1.5g, h, j.

Mawddachites hicksii – Weidner & Nielsen 2014: 80, 81, fig. 48.

Paradoxides aff. hicksi – Bushuev & Makarova 2016: 20, 21, pl. 1, figs 7, 7a.

HOLOTYPE. — Specimen no. BGS(GSM) 10113 , British Geological Survey, London, United Kingdom by original designation, from the Clogau Formation, Menevian Group of the Mawddach valley , Gwynedd, Wales, United Kingdom.

DIAGNOSIS. — S1 shallowing and curving backwards axially, S2 and S3 curving forward, S3 and S4 short, all well-defined; occipital ring broadens axially; pygidium nearly circular in outline with wide axis ending in a blunt point at about half pygidial length (based on Lake 1934, 1935; Fletcher 1972b, with modifications).

MATERIAL EXAMINED. — 101 cranidia and 21 pygidia of Plutonides hicksii (for NFM numbers see Appendix 1). All specimens range between 1.96 and 10.37 m ( Fig. 2 ) of the Manuels River Formation, type locality, Conception Bay South, Newfoundland, Canada .

OCCURRENCE. — Plutonides hicksii is a regional biostratigraphical marker for the middle Cambrian ( Howell 1925; Lake 1935; Hutchinson 1962) and has been documented from southeastern Canada, eastern Newfoundland, in the Paradoxides hicksi and Paradoxides davidis zones ( Hutchinson 1962; Whittington et al. 1997; Fletcher 2006, 2007). Further occurrences are in southeastern Canada in Nova Scotia ( Hutchinson 1952), United Kingdom in Wales ( Ptychagnostus gibbus to Tomagnostus fissus zones; Salter & Hicks 1869; Lake 1934; Thomas et al. 1984; Whittington et al. 1997) and England ( Ptychagnostus gibbus to Tomagnostus fissus zones; Cobbold 1913; Thomas et al. 1984; Bridge et al. 1998), Denmark ( Acidusus atavus Zone ; Grönwall 1902; Weidner & Nielsen 2014) and Sweden ( Tomagnostus fissus and Ptychagnostus atavus zones; Linnarsson 1879, 1882; Westergård 1953).

DESCRIPTION

The cranidia range from 3.0 mm to 51.0 mm width and from 2.5 mm to 41.0 mm length. The width of the pygidia varies from 5.0 to 28.0 mm and from 4.0 mm to 24.0 mm in length. Both cranidia and pygidia are well-preserved as internal casts and moulds. Some cranidial internal cast have a groove running from the frontal margin across the glabella towards the outer margin of the middle of the occipital ring. In at least two thirds of the specimens, the occipital ring bears a small node. On the cranidia, the finely reticulate, ridged surface ornamentation is best preserved on the genae. In smaller specimens, the glabellar furrows are more deeply impressed, with S2 almost transglabellar. The glabella is short with a long preglabellar field towards the outer margin. The glabella in the anterior third is more domed than in bigger specimens. In specimens with an occipital ring, the latter also bears a small node.

REMARKS

Plutonides hicksii has been assigned to different genera (e.g., Fletcher 1972b, 2007; Whittington et al. 1997; Geyer et al. 2022). It was first described as Paradoxides , but this assignment is outdated as discussed above (see remarks Plutonides ). Šnajdr (1958) assigned Hydrocephalus hicksii hicksii as a species within the genus Hydrocephalus without further discussion. Later, Fletcher (1972b) reported the species as Hydrocephalus hicksii , followed by Martin & Dean (1988) who assigned it to Paradoxides (Hydrocephalus) hicksii , but no discussion was presented by the authors. According to the diagnosis for the genus Hydrocephalus , given by Šnajdr (1958) and Whittington et al. (1997), the genus is characterized by a transglabellar S2 furrow, a subcircular glabellar front, short palpebral lobes not extending to close towards the posterior margin, and a posterior margin curving forward to an acute inner spine angle. These characteristics do not match Pl. hicksii . Whittington et al. (1997) mentioned that supposed examples described as the genus Hydrocephalus , from eastern Canada are referred to Plutonides . Morris (1988) marked the generic reference of ‘ Paradoxides ’ hicksii as provisional, pointing out the problem of the generic ranking of the species. Fletcher (2007) rejected the assignment of Pl. hicksii to Plutonides as done by Whittington et al. (1997). He referred to Fletcher et al. (2005), who described the cranidium of Acadoparadoxides as indistinguishable from Plutonides . He based his argument on a reconstructed photograph of the distorted lectotype of the type species of Plutonia sedgwickii Hicks, 1871 . The image shows a small-sized specimen with deep, transglabellar S1 and S2 furrows, a rounded glabellar front and relatively long palpebral lobes. The bigger cranidia of the type species of Plutonides figured by Fletcher et al. (2005) have an S2 furrow that shallows axially to a degree in which they are not transglabellar any more, and palpebral lobes reaching from S1 to L1. These characteristics differ from the characteristics given for Acadoparadoxides , e.g., a transglabellar S2 furrow, long palpebral lobes reaching from L0 to the posterior marginal furrow ( Šnajdr 1958; Whittington et al. 1997). The assignment of Plutonides as a senior synonym of Acadoparadoxides as done by Fletcher et al. (2005) is not followed here. Rather, we follow the assignment of the species Pl. hicksii to the genus Plutonides as done by e.g., Whittington et al. (1997) and Young et al. (2002). Whittington et al. (1997) described the meraspis stage of Plutonides and documented a short frontal lobe, a long preglabellar field, and deep glabellar furrows. All these characteristics are present in small-sized specimens of Pl. hicksii in this work (e.g., NFM F-3177, NFM F-3220, NFM F-3026). It is noteworthy that various authors (e.g., Cobbold 1913; Lake 1934, 1935; Egorova et al. 1982; Bushuev & Makarova 2016) have referred to Pl. hicksii as “ Pl. hicksi ”, with no further discussion.

Paradoxides hicksii var. palpebrosus described by Linnarsson (1879) matches Pl. hicksii and is here taken as a synonym of Pl. hicksii . Lake (1934: fig. 4) figured a hypostome, and Hutchinson (1952: pl. 2, fig. 5) a fragmentary librigena, but these assignments are questionable. This is also the case for two cranidia illustrated in Fletcher (1972b: pl. 47, fig. 15 and pl. 48, fig. 2) which are poorly preserved, both with a deep crack crossing the specimens. Cranidia figured byVaněk et al. (1999: pl. 1, figs 1-3, 5, 6) have transglabellar S1 and S2 furrows and no S3 or S4 furrow. The glabellar furrows do not match the characteristic four furrows of Pl. hicksii . Vaněk et al. ’s plate 1, figure 3 only shows a hypostome and plate 1, figure 4, illustrates a pygidium. Both pictures have a poor resolution and the pygidium appears to be elongate and not as rounded as in Pl. hicksii . Consequently, all illustrated specimens are excluded from Pl. hicksii . Instead, their characteristics match Pa. davidis and specimens illustrated on plate 1, figures 1-6, are assigned to this species.

Figures

FIG. 13. — Plutonides hicksii (Salter, 1866b); A, B, internal cast and mould of the same specimen, occipital node visible in both (NFM F-3717 from 10.38 m); C, internal cast of a thorax attached to the pygidium, pleural spines preserved, NFM F-3299 from 5.22 m; D, internal cast of a cranidium attached to the anterior 11 thoracic segments (NFM F-3352 from 7.56 m). Scale bars: 5 mm.

FIG. 14. — Plutonides hicksii (Salter, 1866b): A, internal cast of a juvenile cranidium, smallest described specimen in this work with long preglabellar field (NFM F-3177 from 10.27 m); B, internal cast of a juvenile cranidium,deep glabellar furrows,ornamentation on fixigenae preserved (NFM F-3220 from 7.59 m); C, internal cast of a juvenile cranidium, deep glabellar furrows (NFM F-3026 from 9.63 m); D, internal cast of a juvenile cranidium, with occipital node and ornamentation on fixigenae preserved (NFM F-3176 from 10.27 m); E, internal cast of a juvenile cranidium, short preglabellar field, occipital node preserved (NFM F-3067 from 3.33 m); F, internal cast of a juvenile cranidium, short preglabellar field, glabellar furrows less deeply impressed than in junger specimens A-D, S2 furrow shallower axial to discontinuous (NFM F-3947 from 4.28 m). Scale bars: 1 mm.

FIG. 15. — Plutonides hicksii (Salter, 1866b): A, internal cast of a cranidium,occipital node preserved, with a vertical groove crossing the cranidium (NFM F-2781 from 4.52 m); B, internal cast of a cranidium with preserved exoskleleton (NFM F-3894 from 2.96 m); C, internal cast of a cranidium, occipital node and ornamentation on fixigenae preserved (NFM F-2861 from 2.53); D, internal cast of a cranidium, with a vertical groove crossing the cranidium (NFM F-3258 from 4.06 m). Scale bar: 3 mm.

FIG. 2. — Lithology of the type locality of the Manuels River Formation, range chart of the here described species. Agnostid biozones from Hildenbrand et al. (2021) and polymerid biozones from Howell (1925).