Helminthoidichnites tenuis Fitch, 1850

Diagnosis: Relatively thin, horizontal, irregularly meandering or winding trails with occasional loops.

Emended diagnosis.—Horizontal, small, thin, unbranched, simple, straight or curved to, more rarely, circular trails or bur­rows that commonly display overlap among specimens, but lack self-overcrossing.

Description: Slightly curved to straight, non meandering, unbranched, horizontal trails. Smooth surface. Crossing between different individuals is common. Diameter relatively constant within each specimen. Diameter ranges between 1.0-3.0 mm. Length varies between 25-95 mm. Preserved as hypichnial ridges.

Remarks: Although placed in synonymy with Gordia by Häntzschel (1975), Helminthoidichnites was more recently considered as a distinct form by Hofmann and Patel (1989). Helminthoidichnites is not meandering as Helminthopsis and does not show overcrossing as Gordia (Hofmann and Patel, 1989; Buatois and Mângano, 1993a). A single ichnospecies, Helminthoidichnnites tenuis Fitch, is known. It is interpreted as grazing trails (pascichnia) probably produced by arthropods.

Simple, unbranched, straight to gently curved trails, oriented parallel to the bedding plane. Fill is identical to the host rock. Overlapping among different individuals is common. Diameter is 0.8–2.1 mm and may slightly vary along the course of individual trails. Maximum observed length is 17.0 mm. Preserved in full relief on sandstone tops.

Helminthoidichnites tenuis is one of the most common components of Ediacaran and lower Cambrian ichnofaunas (e.g., Hofmann et al., 1994; MacNaughton et al.,2000; Jensen et al., 2006). This ichnotaxon is interpreted as a grazing trace (pascichnion) most likely produced by vermiform animals (Buatois et al., 1998).