Michelinia de Koninck, 1841

Fedorowski, Jerzy, Bamber, E. Wayne & Richards, Barry C., 2021, Mississippian colonial tabulate and rugose corals from the Flett Formation, Liard Basin, northwest Canada, Acta Palaeontologica Polonica 66 (3), pp. 679-704 : 687-689

Genus Michelinia de Koninck, 1841

Diagnosis.—See Hill (1981: F561).

Remarks.—Colonies included here in the Michelinia de Konnick, 1841 , belong to a morphologically complex group of species developing both tabulae and abundant dissepiments. That morphological complexity led Stumm (1948) to introduce the new genus Tabellaephyllum Stumm, 1948 , based on the new species T. peculiare Stumm, 1948 , and to include it in the Rugosa. Oliver and Sando (1977) rejected both the Devonian age of T. peculiare and its position within the Rugosa, identifying that species as belonging to the tabulate genus Michelinia . Also, they suggested the synonymy of T. peculiare with Michelinia expansa White, 1883 , “a common and widespread element of the Early Mississippian … coral fauna…” as mentioned by Oliver and Sando (1977: 422). However, Lafuste and Plusquellec (1990) cast doubt on the identification of “ M. expansa ” as Michelinia by placing that generic name in inverted comas as far as the topotypes of that species are concerned. They identified Michelinia expansa of Sando (1969a) as Turnecipora sp. and in the case of Tabellaephyllum peculiare concluded: “The beautiful wording of Oliver and Sando suits us perfectly ‘ Tabellaephyllum Stumm is a Michelinia ’ however, silicification of the wall precludes precise attribution to a genus.” (“La belle formule de Oliver et Sando (1977) nous agrée parfaitement ‘ Tabellaephyllum Stumm is a Michelinia ’ mais la silicification de la muraille empêche toute attribution générique precise”) ( Lafuste and Plusquellec 1990: 23). The morphology of the intercorallite walls, observed in fragments of two colonies ( Figs. 7A 2 , 8C 3 ), suggest they are related to Turnecipora Lafuste and Plusquellec, 1985, whereas the microstructure of the walls in the remaining parts of those colonies and in the other two colonies were destroyed by recrystallization with only the darker middle line recognizable. Thus, our suggestion is not supported firmly, precluding a further discussion of that matter and our genus and species identifications rely on the general morphology of the coral skeletons.

Michelinia expansa White, 1883

Figs. 7 , 8 .

Material.— Four almost complete colonies ( Fig. 6 ) from the Flett Formation , Rundle Group , Liard Basin , Northwest Territories, Canada: IG-17 = UAM-Tc.Can./1, Tlogotsho Member (70 m above base), lower Viséan; IG-91-1 = UAM-Tc. Can/2, IG-91-2 = UAM-Tc.Can/3, IG-93 = UAM-Tc.Can/4, Meilleur Member (270 m above base and 50 m above base of member), middle Viséan. Peripheral parts of colonies, i.e., holotheca and fragments of external corallites corroded. Corallite skeletons recrystallized, partly dolomitized and/or replaced by silica. Microstructure of wall destroyed in most. Septal spines extremely rare and may have been dissolved. Corallite calices preserved in fragments of two colonies .

Description.—Colonies hemispherical, cerioid as documented by tripartite composition of inter-corallite walls ( Fig. 8C ), i.e., partition of Fedorowski and Jull (1976). Corallites within colonies differentiated in size into a few large corallites and many smaller corallites, surrounding large ones ( Figs. 7A 1 , 8B 2 ). Diameters of largest corallites: 6.5 mm in specimens UAM-Tc.Can./1, 3 and 7.5 mm in colonies UAM-Tc.Can./2, 4. Pattern described suggests either differentiated preferences in offsetting with only some polyps within colony being able to offset, or ability to offset appearing late in polyps growth. Offsetting lateral ( Fig. 8A 2 , B 1, C 1 , C 2 , C 4 ). Offsets separated by partitions early in their growth, but a communication duct or pore connecting parental polyp and its offset recognizable at very beginning of offsetting when longitudinal section centric (e.g., Fig. 8A 2 , arrows; B 1, grey arrows). Such communication apparently absent when section eccentric ( Fig. 8B 1 , black arrow) and can be missing from transverse section ( Fig. 8C 1 ). Pores of early growth stage of one offset open towards two mature corallites ( Fig. 8C 4 ) recognized as well. Also, pores connecting offset with adjacent corallites common ( Fig. 8A 2 , arrows; B 1, grey arrows). Septal spines extremely rare, very small when present ( Fig. 7A 3 ). Pores connecting mature corallites rare, but both types (P 1 and P 2) documented; sometimes both types occurring close to one another ( Fig. 7A 4 ). Dissepiments absent from young corallites ( Figs. 7A 1 , 8C 1 , C 2 , C 4 ), growing in number from one row in small corallites to 2–4 rows in large corallites. Long, steeply down sloping dissepiments may occupy up to 1/2 corallite lumen. Only complete tabulae, expanding through entire corallite lumen, present during several millimeters of earliest corallite growth. In mature corallites tabulae vary from horizontal or slightly sagging to convex; all observed in same colony or even same corallite ( Figs. 7A 1 , upper, 8A 2, B 1).

Remarks.—Doubts expressed above in the remarks on the genus resulted in both, removal from the synonymy and identification of species based on the macro-morphology. Also, a wide intraspecific variability is here accepted rather than attempting to identify the colonies as different species. Such characters as “thinner walls and fewer and generally more convex tabulae” listed by Sando (1969a: 311) as the only characters distinguishing M. leptosphragma Armstrong, 1962 , from M. expansa are here considered inadequate for distinction between species. Thickness of walls may vary ( Fig. 8C 2 , C 5). Thus, the synonymy of those two seems very probable. All colonies described here resemble both “ Tabellaephyllum peculiaris Stumm, 1948 ”, ( Stumm 1948: pl. 12: 1, 2, 9, 11; Lafuste and Plusquellec 1990:fig.6)and the type and topotype specimens of Michelinia expansa White, 1883 ( White 1883: pl. 39: 2a, b; Sando 1969a: pl. 39: 1–3; Lafuste and Plusquellec 1990: fig. 7a, b). They differ clearly from Michelinia meekiana Girty, 1910 ( Girty 1910: 189 only; Sando 1969b: pl. 2: 6–9; Plusquellec and Sando 1987) in possessing short tabulae but dissepiments common, long and steeply sloping down.

Stratigraphic and geographic range.—Viséan (Mississipian, Carboniferous), Flett Formation, Rundle Group, Liard Basin, Northwest Territories, Canada; known also from Shunda Formation, Rundle Group, Liard Basin, Northwest Territories, Canada; Chouteau Limestone, Missouri; Redwall Limestone, Arizona; Lodgepole Formation, Utah; all upper Tournaisian (Mississipian, Carboniferous).

Figures

Fig. 7. Tabulate coral Michelinia expansa White, 1883. IG-91-1 = UAM-Tc.Can./2, from locality 3 (Fig. 2), Meilleur Member, Fleet Formation (middle Viséan, Mississipian), Rundle Group, Liard Basin, Northwest Territories, Canada.A1, thin section of colony, transverse in most part and longitudinal at colony edge; A2, immature corallite, note remnants of wall microstructure, arrow points to P1 pore; A3, septal spines attached to tabula; A4, pores P1 and P 2 situated close to each other (arrows).

Fig. 2. Simplified geological map of study region showing distribution of outcrops of Carboniferous Prophet, Flett, and Mattson formations in southwest corner of Northwest Territories and southeastYukon Territory and lines of cross-sectionA–A’ and B–B’ (see Figs.4, 5 for details). Surface and subsurface sections (corallocalities are in orange):1,Twisted Mountain;2, north end Mattson anticline;3, Jackfish Gap (main coral locality); 4, Tlogotsho Plateau;5, Etanda Lakes; 6, Tika Creek; 7, Pan American Home Signal C.S.P. Celibeta Number 7; 8, Texaco N.F.A. Bovie Lake J-72; 9, Pan American A-1 Pointed Mountain P-53; 10, Canada Southern et al. North Beaver River YT 1-27; 11, Pan American Beaver YT G-01; 12, West Flett anticline; 13, Southern Liard Range; 14, Sheaf Cree.Abbreviations:Fm., Formation (formal unit); fm., formation (informal unit); fms.,formations; S., south; W., west;YT,Yukon Territory;C.S.P., Canada Southern Petroleum. Modified from Harker (1963) and an unpublished map by Andrew Okulitch from 2005 (reproduced with permission).

Fig.8. Tabulate corals Michelinia expansa White, 1883, from locality 3 (Fig.2), Meilleur Member, Flett Formation (Viséan, Mississipian), Rundle Group, Liard Basin, Northwest Territories, Canada. Transverse thin sections unless stated otherwise. A. IG-91-2 = UAM-Tc.Can./3; A1, fragment of colony; A2, longitudinal section (peel) of offsetting corallite; showing pores (arrows). B. IG-17 = UAM-Tc.Can./1; B1, drawing on peel image of longitudinal section of corallites in different growth stage; showing pores (grey arrows) and apparent absence of pore resulting from eccentric section (black arrow); B2, fragment of a colony. C. IG-91-1 = UAM-Tc.Can./2, different growth stages; C1, very early growth stage of offset surrounded by thick, solid wall; C2, very early growth stage of offset; showing P2 pores (black arrows) and overgrown pores? (white arrows); note differentiated thickness of partitions; C3, two offsets adjacent to one another with first dissepiments developed; C4, very early growth stage of offset with two P1 pores open to two adjacent mature corallites; C5, partitions differentiated in thickness, showing P2 pores (arrow).

Fig. 3. A. Partly schematic, palinspastic stratigraphic cross-sectionA–A’ showing stratigraphic relationships of Flett Formation in eastern Cordillera of northern part of project area in southwest corner of Northwest Territories and southeast Yukon. B. Environments of deposition. See Fig. 2 for line of cross-section A–A’.Abbreviations:Fm., Formation (formal unit); fm., formation (informal unit); Mbr., Member (formal unit). Modified from Richards et al. (1989).

Fig. 6. Stratigraphic column of type section of Viséan Flett Formation at Jackfish Gap on Yohin Ridge, eastern Cordillera, southwest corner of Northwest Territories. Shows lithostratigraphic positions of coral collections discussed in this paper.Abbreviations: Fm., Formation. Stratigraphic column modified from Richards et al. (1989: fig. 24).

Fig. 1. Map showing location of study area, Carboniferous lithofacies assemblages subcropping beneath Permian and Mesozoic deposits in the Western Canada Sedimentary Basin, principal sub-basins, uplifts, and shelves. In the Liard Basin western occurrences of the Rundle and Matson lithofacies assemblages overlie and pass westwards into the Besa River lithofacies assemblage. See Fig. 3 for the formational composition of the Banff, Rundle, and Mattson lithofacies assemblages. Modified from Richards et al. (1994).

Fig. 4. A. Partly schematic, palinspastic stratigraphic cross-section showing stratigraphic relationships of Flett Formation in southern part of project area. B. Environments of deposition. See Fig. 2 for line of cross-section B–B’. Abbreviations: Fm., Formation (formal unit); fm., formation (informal unit); Mbr., Member (formal unit); mbr., member (informal unit); YT,Yukon Territory; C.S.P., Canada Southern Petroleum. Modified from Richards et al. (1994).