Cordilleria mutabile ( Kelly, 1942 )

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 : 691-695

Cordilleria mutabile ( Kelly, 1942)

1942 Diphyphyllum mutabile Kelly, 1942: 358 , pl. 51: 7, 8.

1966 Lithostrotion mutabile Kelly, 1942 ; Bamber 1966: 4, pl. 1: 1a–e.

2007 “ Diphyphyllum ” mutabile Kelly, 1942 ; Fedorowski and Bamber 2007: pl. 2: 1–6, 10–12; pl. 7: 1.

Holotype: GSC 9642 .

Type locality: “Southwest slope of the mountain northwest of the junction of the McLeod River with Whitehorse Creek” ( Kelly 1942: 359).

Type horizon: Lower Rundle Group, upper Tournaisian.

Emended diagnosis.—Fasciculate colonial coral with n:d value of 20:3.9 mm to 25:6.0 mm ( Fig. 10 ). Major septa weakly amplexoid, reach up to 1/2 corallite radius on tabulae surfaces, reduced to 1/3–1/4 corallite radius when sectioned between tabulae, i.e., terminated on steep, peripheral parts of tabulae. Protosepta indistinguishable. Cardinal fossula absent. Minor septa almost as long as major septa when latter not elongated. Pseudocolumella short, simple, slightly thickened or thin, rarely underdeveloped. Tabulae complete, slope peripherally under 10–20° when pseudocolumella present, elevated slightly upward next to pseudocolumella. Tabulae horizontal, slightly convex or concave when pseudocolumella absent; both kinds strongly deflected at periphery, rare inner parts of tabulae rest upon underlying tabulae, most meet dissepimentarium or external wall under a variable angle. In immature specimens, tabulae span entire corallite lumen, slightly convex or horizontal with weakly marked peripheral deflection, or straight. Dissepimentarium in one incomplete row of regular, interseptal dissepiments. Microstructure of septa finely trabecular.

Remarks.—The synonymy is restricted to the papers discussing with the holotype. The emended diagnosis, comprising all main characteristics of skeleton of the holotype, is based on description and illustrations by Kelly (1942: pl. 51: 7, 8), restudy of the holotype by Bamber (1966: pl. 1: 1a– e) and on the further restudy of the holotype by Fedorowski and Bamber (2007: pl. 1: 1–11, pl. 2: 1–6, 10–12).

Stratigraphic and geographic range.— Type horizon and locality only.

Cordilleria aff. mutabile ( Kelly, 1942) Figs. 9–11 .

Material.—Three fragments of colonies ( Fig. 6 ): C-47930 = GSC 142469, C-52546 = GSC 142470, and C-52124 = GSC 142471 from the Flett Formation, Rundle Group, Viséan, Liard Basin, Northwest Territories, Canada and colony C-7349 = GSC 142468 from Rundle Group, Viséan, Rocky Mountains, Bone Mountain, east-central British Columbia, Canada, all comparable in age ( Table 1), included in this description as bearing morphological features important for the discussion on this species and the new genus Cordilleria . All colonies recrystallized.

Description.—N:d values of all colonies vary, some comparable to those of holotype ( Fig. 10 ). However, increasing number of septa does not always follow increase of corallite diameter in all colonies, holotype included. Corallites in GSC 142469 display smallest number of septa; largest of them equal in septal number to smallest corallites of holotype. In contrast, corallites in GSC 142470 display number of septa comparable to holotype, but diameters larger. Thus, n:d values of those two colonies differ from that in holotype and remaining specimens ( Fig. 10 ). Also, they differ from one another in morphology ( Figs. 9A 3 , B 4 and 11A 1 , B 2 , respectively) suggesting limits of intraspecific variability provisionally accepted here for C. mutabile are too broad.

Amplexoid character of major septa differentiated. Most advanced in GSC 142469 ( Fig. 9B 4 ), where some major septa reach more than 2/3 corallite radius. In GSC 142471 and GSC 142470 ( Figs. 9A 3 and 11B 2 , respectively), amplexoid character of major septa hardly recognizable. Longest major septa in corallites of those colonies reach only slightly more than 1/3 corallite radius. In all specimens studied, major septa thin and protosepta indistinguishable irrespective of presence or absence of pseudocolumella. Minor septa longest in GSC 142469 ( Fig. 9B 4 ), closely comparable to those in holotype. In remaining specimens, minor septa vary in length both between corallites and during growth of individual corallites; longest reach 2/3 length of major septa or slightly more. Those reaching 1/2–2/3 length of major septa prevail, but slightly shorter ones occur. Occurrence of pseudocolumellae strongly differentiated; present as a slightly thickened body in all corallites observed in transverse section of colony GSC 142469, but not in fragments of longitudinal sections of several corallites within that colony ( Fig. 9B 4 and B 1 –B 3 , respectively). Pseudocolumella absent from all corallites in two other colonies studied from the Flett Formation ( Figs. 9A , 11B 2 ). Differences in longitudinal section depend mainly on development of pseudocolumella. Closely resembling each other, including holotype ( Fedorowski and Bamber 2007: pl. 2: 5, 6, 10), when pseudocolumella not developed ( Figs. 9A 1 , A 2 , B 1 (white arrow), B 2, B 3, 11B 1). In columellate part of longitudinal section of GSC 142469 ( Fig. 9B 1 , black arrow), inner tabulae elevated steeply towards thickened pseudocolumella. Many of them rest upon underlying tabulae to form short lasting columnotheca. Columnotheca rarely occurs in holotype. Also, inner parts of its tabulae elevated much less steeply than tabulae in the illustrated GSC 142469. Colony GSC 142468 additionally included in this paper, ( Fig. 11A ) displays morphology intermediate between GSC 142469 and holotype. This especially concerns the development vs. absence of pseudocolumella. Pseudocolumella in colony GSC 142468 strongly interrupted, making acolumellate colonies from the Liard Basin an extreme variant of a sequence starting with colonies with pseudocolumella almost invariably continuous in all corallites to colonies composed of acolumellate corallites. Also, it documents secretion of pseudocolumella by epidermis of polyp’s floor ( Fig. 11A 2 , A 4 ), discussed in more detail in general remarks. Dissepimentarium mostly in one incomplete row of rectangular, interseptal dissepiments, i.e., like in holotype, but in colony GSC 142471 may occasionally increase to 2–3 rows in small parts of corallite and during short period of the corallite growth ( Fig. 9A 1 ). Offsetting ( Fig. 11C, D ) documented by offsets in various growth stages appearing by chance in transverse thin sections. Their morphology follows particular growth stages illustrated by Fedorowski and Bamber (2007: pl. 1: 1–11) in holotype.

Remarks.—The morphological variability in almost all characters of specimens described from the Flett Formation extends beyond the parameters established above for the holotype. The small n:d value of GSC 142469 and its rather strongly amplexoid major septa resemble those in C. flexuosum ( Warren, 1927) , whereas its very long minor septa and its morphology in longitudinal section distinguishes it clearly from that species. The morphology in transverse sections of the other colonies and the slightly reduced length of some of their minor septa resemble C. oculinum ( Sando, 1963) , whereas the great length of most of the minor septa, the weakly marked amplexoid character of the major septa and the morphology in longitudinal sections form a set of characters distinguishing them from the latter species. All those characters, the long minor septa and the morphology in the longitudinal sections allow all the colonies discussed to be distinguished from C. warreni ( Nelson, 1960) .

Figures

Fig. 10. Variation in septal number and corallite diameters of rugose coral Cordilleria aff. mutabile (Kelly, 1942) and the holotype colony of Cordilleria mutabile (Kelly, 1942) added for comparison. Large differences in that character between colonies comparable to that noted by Sando (1963: fig. 1) in “Lithostrotion (Siphonodendron) oculinum” (Sando, 1963) = Cordilleria oculina (Sando, 1963) (see Fig. 12).

Fig. 9. Rugose corals Cordilleria aff. mutabile (Kelly, 1942), from Flett Formation (Mississipian), Rundle Group, Liard Basin, Northwest Territories, Canada. A. C-52124 = GSC142471, locality 3 (Fig. 2); longitudinal thin sections (A1, A2), transverse thin section (A3). B. C-47930 = GSC-142469, locality 14 (Fig. 2), Flett Formation undivided; longitudinal sections, peels (B1–B3), transverse thin section (B4). Note in B1 steeply arranged tabulae when pseudocolumella present (black arrow), flat tabulae when pseudocolumella absent (white arrow).

Fig. 11. Rugose corals Cordilleria aff. mutabile (Kelly, 1942), from Rundle Group undivided, east-central British Columbia, Canada (A) and Meilleur Member, Flett Formation (Mississipian), Rundle Group, Liard Basin, Northwest Territories, Canada (B–D). A. C-07349 = GSC 142468, 8.05 km NW of Bone Mountain, east-central British Columbia; transverse thin section (A1), longitudinal thin sections (A2,A3), pseudocolumella (A4, black arrows) created on tabula surface (white arrow) and tabulae/pseudocolumella relationships in lower part of picture. B. C-52546 = GSC 142470, locality 3 (Fig. 2); longitudinal thin section (B1), transverse thin section (B2). C. C-47930 = GSC 142469, locality 2 (Fig. 2); transverse thin section; very early growth stage of offset. D. C-52124 = GSC 142471, locality 3 (Fig. 2); thin section of three offsets in different growth stage; one cut longitudinally, two transversally.

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. 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. 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).

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. 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).