Loganellia Fredholm 1990

Genus Loganellia Fredholm, 1990

TYPE SPECIES. — Loganellia scotica (Traquair, 1898).

Complemented synonymy list after Märss et al. (2006, 2007).

Thelodus scoticus Traquair, 1898: 72; 1899: 829, partim, pl. 1, figs 1, 2, 5-10; non pl. 1, figs 3, 4; 1905: 880, pl. 1, figs 1-4. — Stetson 1931: 141, fig. 1a, b. — Stensiö 1958: 417, fig. 218; 1964: 371, fig. 124a.

Thelodus planus Traquair, 1899: 831, pl. 2, figs 1-3.

Logania taiti – (Stetson) Gross 1967: 33, pl. 5, figs 26- 42, text-fig. 13N-Q.

Katoporus ? sp. – Gross 1967. — Aldridge &Turner 1975: 419-420, pl. 1, figs 4-6.

Logania scotica – (Traquair) Gross 1967: 32, pl. 5, figs 12- 25, text-fig. 13c-f. — Aldridge & Turner: 419, 420, pl. 1, figs 1-3, 7-9. — Turner 1991: fig. 1f; 1992: 26, textfig. 2. — Vergoosen 1992: 51, figs 2, 3, 5-24. — Van der Brugghen 1993: 88, figs 1-3; 1994: figs 2, 3, 5-8. — Turner & Van der Brugghen 1993: 132, fig. 2. — Märss & Ritchie 1998: 147, figs 7- 21, 33e. — Märss et al. 1998a: 56, 60, figs 2, 3; 1998b: 37, fig. 1; 2006: 17-20, pl. 1, figs 1-5, 7-17, text-fig. 7A-Z; 2007: 49-52, fig. 42a, b. — Blom 1999: 98, fig. 2.

LECTOTYPE. — Traquair (1899: pl. 1, fig. 1) specimen GSE5996 from the British Geological Survey Museum, Keyworth, stored in NMS.

TYPE LOCALITY AND HORIZON. — Patrick Burn Formation, Priesthill Group, upper Llandovery; Logan Water, Lesmahagow Inlier, southern Scotland.

DIAGNOSIS. — (modified after Märss et al. 2007): medium to large thelodonts with fusiform body, with dorsoventrally

flattened anterior part, and strongly rounded rostral part. The scale cover is comparatively uniform. Postpectoral and precaudal scales are elongate, wedge-shaped, elliptical and sometimes carinate; raised median rhombic area of the crown is common; base tending to bulge anteriorly in older scales and an anterior process is common. The pulp cavity is deep and open in young scales, reduced to a slit-like groove in older scales, which leads to a single pulp canal opening being located at the distal end of the scale. Possess specialized branchial denticles.

OCCURRENCE. — Lower Silurian, Upper Llandovery, lower fish beds (articulated), Patrick Burn, Kip Burn & Blaeberry formations, Lesmahagow, Scotland; upper Llandovery, Wych Formation, Pentamerus beds, Purple Shale Formation,Welsh Borderland; Upper Llandovery, Kilbride Formation, Ireland (in part); Upper Llandovery, Lafayette Bugt Formation, Hall Land, northern Greenland; Upper Llandovery, Wulff Land Formation, Thors Fjord Member, Monograptus spiralis Biozone, Greenland; Upper Llandovery, Rumba Formation, Raikkula Stage, Estonia; Upper Llandovery, Lower Visby Formation, Gotland, Sweden; Lower Llandovery,Clemville andWeir formations, Quebec,eastern Canada; Upper Llandovery, Limestone Point Formation, New Brunswick, eastern Canada; Upper Llandovery, Anse Cascon and Anse a la Pierre Loiselle formations, Quebec, eastern Canada; Cape Phillips Formation, Devon Island, CanadianArctic;UpperLlandovery,Baillie-HamiltonIsland, Canadian Arctic, Avalanche Lake, Mackenzie Mountains, northern Canada (Märss et al. 2007).

MATERIAL STUDIED. — MNHN.F. GBP 360, 361, 366, 367, 374, 376, and 381; see Material and methods.

LOCALITY AND HORIZON OF THE STUDIED MATERIAL. — Jamoytius Horizon, Patrick Burn Formation, Priesthill Group, upper Llandovery; Birk Knowes, Lesmahagow Inlier, southern Scotland.

DESCRIPTION

Size of the articulated specimens studied varies from 107 to 450 mm in length. According to Turner (1991: 89; 1992: text-fig.3B) the body of L. scotica usually is 275 mm in length, but may reach 300- 400 mm (Märss & Ritchie 1998). The material studied in this work comprises one small thelodont specimen (MNHN.F.GBP374), three medium sized ones (MNHN.F.GBP360, 361 and 376), and three very large exoskeletons (MNHN.F.GBP366, 367 and 381). Though only postpectoral, precaudal and caudal parts of the large specimens have been preserved. The total body length estimations based on the width of caudal peduncle of the specimen enables us to consider that the maximum total body length of L. scotica may have exceeded 40 cm (Fig. 5B). The measurements of the specimens studied, as well as their derived total body lengths on the basis of the length ratio of caudal peduncle, are given in Table 1. Three of the seven articulated specimens are partly incomplete exoskeletons of large animals; the smaller ones are complete animal fossils, even if the best parts are preserved as natural moulds, lacking the bulk of their squamation (Fig. 4 A-C). Tails of all specimens possess a clearly hypocercal asymmetrical caudal fin with a very wide ventral lobe, best contrasted on the tail of MNHN.F.GBP361 (Fig. 4B), with numerous narrow fin rays branching off from it. The ventral lobe is longer, thicker and much more massive than the dorsal lobe. This wide ventral lobe has already been noticed in its first description by Van der Brugghen (1994).

Scales are small to medium in size. The rostral, cephalo-pectoral, postpectoral, precaudal and pinnal squamation types (Fig. 3) have been observed and studied on the specimens. The pinnal squamation pattern is particularly well preserved on several articulated caudal fins (Fig. 5A 1, A 3). Pinnal squamation of pectoral fins or pectoral flaps (Märss et al. 2007), of dorsal and anal fins are less clearly preserved within the specimens studied. Neither scale cover of bucco-pharyngeal area, with its specific denticles, well described by Märss & Ritchie in 1998, and named lateral squamation, nor orbital squamation patterns have been observed within the material studied (see Märss & Ritchie 1998: figs 9, 10).

Rostral squamation pattern

This, or an anterior cephalo-pectoral squamation, according to Märss & Ritchie (1998), has been observed within the specimens MNHN.F. GBP361 and 374. A clear rostral squamation type is present on the anterior bucco-lateral sides of the head of the complete articulated specimen MNHN.F. GBP376 (Fig. 4C 1). The rostral scales are round to oval in shape, very small, 0.2-0.4 mm in diameter; the crown is of a symmetrical oak-leaf-like shape, with a smooth and slightly bulging surface, and crenulations on the margins (Fig. 4C 2). Natural moulds of rostral squamation have been observed on the specimens MNHN.F. GBP361 and 374.

Cephalo-pectoral squamation pattern

It is particularly well preserved within the exoskeleton of the specimen MNHN.F.GBP361 (Fig. 4B), and it has also been observed as natural moulds within the specimens MNHN.F.GBP374 (Fig. 4A) and 376 (Fig. 4C 1). Cephalo-pectoral scales are rhomboidal in shape, with comparatively dim surface sculpture, and one or two lateral areas which are rather indistinct, expressed as shallow grooves on the anterior part of the crown. In average they are 0.3 to 0.4 mm long. The cephalo-pectoral scales cover the anterior (but rostral) and middle parts of the body, and may also be found on the pectoral flap (pectoral fin), which is the case in the specimen MNHN.F.GBP360 (Fig. 6A).

Postpectoral squamation pattern

It is the main squamation type of thelodonts (see the discussion below), and it comprises the best part of the articulated specimens studied. It is very well preserved within the specimens MNHN.F.GBP360 (Fig. 6 B-D), 361, 366, 367 and 381 (Fig. 7A, B). Natural moulds of postpectoral squamation have also been observed within the specimens MNHN.F.GBP374, and 376, with an exception of few patches of postpectoral scale cover on the latter one. Postpectoral scales are rhomboidal to wedge-shaped, 0.25 to 0.45 mm in length. The crown is elongated, with relatively wide anterior part and sharp posterior apex of the crown. It is clearly divided into elevated median area with oblong shallow furrow, and two lower lateral ridges, separated by sharp longitudinal grooves, which meet at the posterior apex of the crown.

Precaudal squamation pattern

It is characterized by very thin and elongated scales, and covers the zone of the tail up to the caudal fin. Precaudal scales have general morphology quite similar to the postpectoral ones, however they are comparatively smaller (0.15-0.35 mm long), more narrow and elongated. Referring to the specimens observed it is necessary to admit that the change of squamation pattern going from postpectoral to precaudal is very gradual.

Pinnal squamation pattern

It is the most peculiar. Pinnal scales of the caudal fin are tiny (0.1-0.25 mm in diameter) and very narrow (less than 0.01 mm in width). Scales are strongly elongated, less rhomboidal and more bubbin-shaped. Their general crown sclupture is similar to that of postpectoral and precaudal scales, only the median areas are narrower, as well as are the lateral ridges. Being much smaller and more elongate compared to precaudal ones, they cover continuously the flexible ray area of the caudal fin. The caudal fin squamation is exceptionally well preserved on the specimen MNHN.F.GBP367, possessing fully articulated continuous pinnal squamation, with caudal rays as distinct accentuated lines of more compact scale cover (Fig. 5A 1-3). The scale cover is significantly denser in the area of the rays forming arch-shaped ramifications, wider proximally near the base of the caudal lobes, and narrowing distally towards the posterior end of caudal fin (Fig. 5A 2). At least 20 caudal fin rays can be observed on the ventral lobe of the specimen MNHN.F.GBP367. The tail itself is strongly asymmetrical, referring to a rather young age of the thelodont animal. The maximum width of the ventral lobe reaches 2.5 cm, while the dorsal lobe is less than 1.0 cm in width. The ventral lobe still retains rather postpectorallike squamation. These well expressed caudal fin rays have also been observed within the specimens MNHN.F.GBP366and 381, though their scale cover is much less well preserved. Distinct squamation patterns have been observed on the leading edges of caudal fins (specimens MNHN.F.GBP366 and 381), present as denser linear structures of particular pinnal scales, ontop of the precaudal squamation pattern (Fig. 7C, G).

The squamation pattern of the dorsal fin has been observed with the specimens MNHN.F.GBP360 and 361, and that of the anal fin is rather well preserved in the specimen MNHN.F.GBP381 (Fig. 7D). The shape of the dorsal fin of a young L. scotica is well preserved as a natural mould in the specimen MNHN.F.GBP374 (Fig. 4A). The anal fin has been observed only as a natural mould on the specimen MNHN.F.GBP361 (Fig. 4B), and as an incomplete articulated squamation on the specimen MNHN.F.GBP381 (Fig. 7 A-D).