Heliolitine corals of the topmost Douro and overlying formations (Upper Silurian), Canadian Artic Islands

Journal of Paleontology, Nov 1998 by Dixon, Owen A

Jones and Hill (1940) interpreted H. daintreei Nicholson and Etheridge as a highly morphologically varied and long-ranging species in the Siluro-Devonian of Australia. Bondarenko (1966 considered one of their illustrated specimens (Jones and Hill 1940, pl. 8, fig. 2) from the Silurian of New South Wales to be synonymous with H. arcuatus; the synonymy remains a possibility considering the similarities in corallite size, spacing, and septal structure.

Among the six species of Heliolites from the Boothia Uplift region that have similarly small corallite diameters (mean CL less than 1.2 mm), H. arcuatus has many characters in common with H. subdecipiens, including the distinctively wide corallite spacing (Table 1; Figs. 5 and 6). However, the strongly developed septa and smaller average corallite size in H. arcuatus clearly distinguish it from the sparsely septate H. subdecipiens. Its wide corallite spacing clearly separates H. arcuatus from H. furyi and H. cf. H. rariformis in this study, and from H. diligens and H. aff. H. luxarboreus previously reported from the lower Douro Formation (Dixon, 1989).

Collections and occurrence.-In Fauna 2, two specimens (not previously reported in Dixon, 1996), from Douro Formation it Pressure Point area (Locality 1), Somerset Island. Twenty- seven specimens in Fauna 3 on Devon Island: hypotypes GSC 117396, 117397, 117398, and 20 others from topmost Douro Formation in Dragleybeck Inlet area (Locality 5); hypotype GSC 117399 and one other specimen from topmost Douro Formation, and one from basal 7 m of Barlow Inlet Formation in Gascoyne Inlet area (Locality 3.1); one from basal Barlow Inlet Formation in Griffin Inlet area (Locality 4). Upper Silurian (upper Ludlow) of Canadian Arctic; upper Ludlow, possibly Pridoli, of Central Kazakhstan; possibly Upper Silurian of New South Wales, Australia.

HELIOLITES FURYI Dixon, 1996

Figure 3.5, 3.6

Heliolites sp. MCMILLAN in FORTIER ET AL., 1963, p. 133.

Heliolites sp. DixoN, 1993, p. 20 (H1).

Heliolites furyi DIxoN, 1996, p. 721-725, figs. 2.1-2.4, 3.1, 3.2.

Diagnosis.-Corallites 0.8-1.2 mm (mean 1.0 mm) in diameter, with smoothly rounded, locally faceted walls. Septal spines usually sparse, short, less commonly longer and up-curved adaxially to less than one-half radius; rarely aseptate. Corallites widely spaced in coenenchyme: 23-72 (mean 43) corallites per cm2 of cross section; corallites occupy 20-52 percent (mean 31 percent) of cross-sectional area; corallite centers 1.2-2.1 mm (mean 1.6 mm) apart; 1.54.5 (mean 3) rows of coenenchymal tubules between corallites. Coenenchyme of regular tubules, polygonal in cross section, 0.20.37 mm (mean 0.27 mm) in diameter. Diaphragms distinctly more crowded than adjacent tabulae. (Original diagnosis; minor modifications using larger morphometric database)

Description.-Representatives of H. furyi in Fauna 3 from Devon Island conform well to the species as originally described in Fauna 2 (Dixon, 1996). Corallum growth medial (see Morphology, above). Morphometric data (Table 1) essentially as before, but with slightly higher maximum values of CA and NTC. Fauna 3 contains fewer specimens with relatively thick corallite walls (i.e., thicknesses usually 0.02-0.04 mm, compared to 0.04-0.07 mm in Fauna 2), and lesser maximum wall thickness (0.08 mm) in dense skeletal zones. Scatter plots (e.g., Figs. 5, 6) based on a large data set portray considerable intraspecific morphological variation. Although "typical" species character can be inferred from the central regions of scatter plots, assignment of some peripheral variants remains problematic, notably for a few forms morphologically intermediate between H. furyi and H. greineri (see also Dixon, 1996, p. 729).