Acontheus inarmatus Hutchinson, 1962

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 : 1076

Acontheus inarmatus Hutchinson, 1962

( Fig. 17 )

Acontheus inarmatus Hutchinson, 1962: 109 , pl. 16, figs 8a, b, 9.

Acontheus inarmatus minutus Sdzuy, 2000: 307 , pl. 3, figs 1-5 (partim). — Geyer 2010: unnamed plate p. 84, fig. 7. — Heuse et al. 2010: fig. 4.16.

HOLOTYPE. — Specimen no. GSC No. 12053 , Geological Survey of Canada, Ottawa, Canada, by original designation, from the Manuels River Formation on the north shore of Highland Cove , Trinity Bay, Newfoundland, Canada.

DIAGNOSIS. — Glabella expands anteriorly up to three times wider than posterior part; faint glabellar furrows; rounded, inflated fixigenae; rounded genal angle; punctuate exoskeleton.

MATERIAL EXAMINED. — 14 cranidia of Acontheus inarmatus (for NFM numbers seeAppendix 1). All specimens range between 15.63 and 16.67 m ( Fig. 2 ) of the Manuels River Formation, type locality, Conception Bay South, Newfoundland, Canada.

OCCURRENCE. — Acontheus inarmatus is a rare middle Cambrian taxon, reported from southeastern Canada, eastern Newfoundland, in the Paradoxides davidis Zone ( Hutchinson 1962) . It has further been documented from Germany ( Sdzuy 2000).

DESCRIPTION

The cranidia range from 1.4 mm to 4.1 mm in width and from 1.3 mm to 3.9 mm in length. The cranidia are well-preserved as internal casts and moulds. The glabella is more domed than the cheeks and glabellar furrows are faintly preserved in two specimens (NFM F-3143 and NFM F-3659). The specimens have a smooth to slightly punctuate surface and none has the exoskeleton preserved.Two specimens bear an occipital node (NFM F-3211 and NFM F-3701) and three are pyritized.

REMARKS

Sdzuy (2000) introduced Acontheus inarmatus minutus based on a smoother exoskeleton than in Hutchinson’s (1962) specimens and a slightly differing occipital ring. Sdzuy (2000) acknowledged that these differences in eight cranidia and one pygidium, may have resulted from poor preservation of his specimens. We also refer to these differences as preservational and the result of intraspecific variation and assign Ac. inarmatus minutus as a synonym. The pygidium figured by Sdzuy (2000: pl. 3, fig. 6) does not match the characteristics of Acontheus and is here excluded from the genus. Geyer (2010) and Heuse et al. (2010) followed Sdzuy (2000) in applying the subspecies and illustrated both one of Sdzuy’s (2000) cranidia. The cranidia show the characteristics of Ac. inarmatus , so the specimens are here assigned to the species.

Figures

FIG. 17. — Acontheus inarmatus Hutchinson, 1962: A, internal cast of a cranidium, faint glabellar furrows preserved, slightly punctuated ornamentation (NFM F-3143 from 16.67 m); B, internal cast of a cranidium, occipital node preserved (NFM F-3211 from 15.63 m); C, internal cast of a cranidium, occipital node preserved (NFM F-3701 from 15.68 m); D, internal cast of a cranidium,slightly punctuated ornamentation (NFM F-3150 from 16.67 m); E, internal cast of a cranidium (NFM F-3167 from 16.67 m); F, internal cast of a cranidium,glabellar furrows on left side of glabella slightly visible (NFM F-3659 from 16.67 m). Scale bars:1 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).