Remigolepis durnalensis, Olive, 2015

Olive, Sébastien, 2015, Devonian antiarch placoderms from Belgium revisited, Acta Palaeontologica Polonica 60 (3), pp. 711-731 : 723-728

Remigolepis durnalensis sp. nov.

Figs. 9–12 .

2009 Bothriolepis ; Clément and Prestianni 2009: 107.

Etymology: In reference to the locality, where the material has been found.

Type material: Holotype: AMD: IRSNB P.9266a, b . Paratypes: Prm: IRSNB P.9262; La: IRSNB P.9263a, b; Pp : IRSNB P.9264a, b; AMD: IRSNB P.9265a, b; PMD: IRSNB P.9267a, b; IRSNB P.9268a, b; ADL: IRSNB P.9269a, b; PDL: IRSNB P.9270a, b; IRSNB P.9271a, b; PVL: IRSNB P.9272; PVL and PL: IRSNB P.9273a–d; CV1: IRSNB P.9261a, b; pectoral fin and AVL: IRSNB P.9274a, b .

Type locality: Tienne-des-Marteaux quarry also called Durnal 2, Bocq valley , Spontin, Namur Province, Belgium .

Type horizon: Montfort / Evieux Formation , Famennian, Upper Devonian .

Material.— PMD: IRSNB vert 32049-001a, b; ADL: IRSNB vert 32049-002 from the type locality .

Diagnosis.— Species of moderate size. Anterior edge of the Prm displaying a strong notch. Short postpineal plate. Dorsal wall of trunk-armour quite flat and reaching an estimated length of 15-16 cm. Tergal angle situated in the middle of the anterior median dorsal plate. Anterior margin of the anterior median dorsal plate narrow and representing one quarter of the posterior margin. Internal surface of the AMD displaying a median ventral ridge. Anterior and posterior oblique dorsal sensory line grooves well defined. Posterior median dorsal plate as long as broad.Anterior median angle of the posterior median dorsal plate prominent. Crista interna transversalis posterior slightly developed. Processus obstans well developed and articular fossa of the anterior dorso-lateral plate extended. Crista transversalis interna anterior oblique and high. Pectoral fin massive, almost three times as long as broad and stretching slightly less far than the posterior margin of the anterior ventro-lateral plate. Vermiculate ornament forming on several specimens a radiating and concentric network. Tuberculate ornament on the largest plates.

Description

Headshield.— Premedian plate, Prm ( Fig. 9A ): It is almost as long as broad and crossed medially by the principal section of infraorbital canal. The anterior edge is marked in its middle by a not extended but well marked notch.

Lateral plate, La ( Fig. 9B ): According to its size, the only preserved lateral plate (IRSNB P.9263a, b) is most probably that of a juvenile. This plate is rather narrow with the posterior part broader than the anterior one. The principal and secondary sections of infraorbital sensory line are deeply incised. The orbit seems small.

Postpineal plate, Pp ( Fig. 9C ): The postpineal plate is wid- er than long. The anterior margin is slightly convex whereas the posterior margin is strongly convex. The antero-lateral margin forms a 160° angle with the postero-lateral margin. The postpineal plate shows anteriorly the limits of a smaller plate corresponding probably to a younger growth stage.

Trunkshield.— Anterior median dorsal plate, AMD ( Fig. 9D–G ): This plate is elongate. Externally, it is quite flat, with a slight arch constituted by the dorsal median ridge. The latter forms an obtuse angle and runs from the tergal angle, situated in the middle of the plate, backwards to the posterior margin. The dorsal median ridge is sharper on the smallest AMD (IRSNB P.9266) than on the largest one (IRSNB P.9265) where it is almost absent. The length of the anterior division of the plate is roughly the same as the posterior one. The anterior straight margin is narrow and represents one quarter of the straight posterior margin length. The lateral corners are more or less marked depending on the specimen considered. They are quite rounded on IRSNB P.9266 and more angular on IRSNB P.9265. The anterior part of the lateral margin is about twice longer than the posterior one. There is a dissymmetry of the lateral corners of IRSNB P.9266. The right lateral corner is situated more anteriorly than the left one and is therefore at half the length of the lateral margin. The posterior portion of the lateral margin has the typical sigmoid shape of the genus Remigolepis . It overlaps the posterior dorso-lateral plate anteriorly and is overlapped by the posterior dorso-lateral plate posteriorly. The posterior oblique dorsal sensory line grooves are well defined. They form an angle of about 45° with the dorsal median ridge. The posterior oblique dorsal sensory line grooves run from the tergal angle to the middle of the posterior part of the lateral margin. The anterior oblique dorsal sensory line grooves are less well defined but distinguishable. They form a right angle with the posterior oblique dorsal sensory line grooves, and run from the tergal angle to the middle of the anterior part of the lateral margin. A small anterior part of the visceral surface is observable. It shows a medi- an ventral ridge, unusual for the genus Remigolepis , and a part of the area overlapping the anterior dorso-lateral plate on all its length. Otherwise, the rest of the visceral surface is quite smooth. On the internal surface of IRSNB P.9265, there are three areas corresponding to the compression of the right and left areas overlapped by the posterior dorso-lateral plates and to the compression of the area overlapped by the posterior median dorsal plate.

Posterior median dorsal plate, PMD ( Fig. 10A, B ): The plate is as broad as long. It is quite flat, with a smooth dorsal median ridge surmounting the plate and stretching from the posterior to the anterior angles. The anterior margin is strongly convex, with a strong median angle. The lateral corner is well pronounced whereas the antero-lateral corner is slightly marked. The posterior margin is also convex and the posterior angle rounded. The three overlapping areas are well indicated by prominent areas on the external surface, revealed by the fossilization. They are connected and well extended. IRSNB vert 32049-001 is an abnormal plate with the right side narrower than the left one. Ventrally (IRSNB P.9268), the crista transversalis interna posterior and the median ventral ridge are slightly developed but present.

Anterior dorso-lateral plate, ADL ( Fig. 10C ): The lateral and dorsal laminae are equivalent in width, even if the anterior part of the lateral lamina is much reduced, compared to the anterior part of the dorsal lamina. The processus obstans is well developed and the articular fossa is extended. The anterior part of the lateral lamina is very small, compared to the observable expanded posterior area. The dorso-lateral ridge is sharp, but the angle formed between the lateral and dorsal laminae is difficult to estimate because of the strong compression of the plate during fossilization (about 170° on IRSNB P.9269). The main lateral line groove is not clearly visible. It probably runs from the posterior margin of the lateral lamina to the anterior third of the same lamina, along the dorso-lateral ridge. The area overlapping the posterior dorso-lateral plate is indirectly observable in the form of a prominent area on the external surface due to the compression during fossilization. The area overlapped by the anterior median dorsal plate is clearly visible.

Posterior dorso-lateral plate, PDL ( Fig. 10D, E ): The dorsal lamina bulges slightly in its middle. The angle formed by the two laminae is approximately 150°, with a quite strong dorsal median ridge. The dorsal lamina is about twice as long as broad. The dorso-medial corner is well pronounced. The anterior margin is straight, contrary to the concave posterior margin. The dorso-medial margin displays a sigmoid anterior part due to an elongated process. The posterior part of the dorso-medial margin is straight and about as long as the anterior part. A little part of the area overlapped by the PMD is observed on IRSNB P.9271. The lateral lamina is four times longer than wide. It is less developed than the dorsal lamina, with a width about four times less than that of the latter. This lamina extends over the anterior two-thirds of the plate. The main lateral line groove is not clearly visible. It may run alongside the convex ventral margin of the lateral lamina. The posterior oblique dorsal sensory line groove runs from the posterior area of the dorsal median ridge to the anterior end of the dorsal process. On IRSNB P.9270, the visceral surface shows several growth lines that have their ossification centre on the posterior part of the dorsal median ridge.

Posterior ventro-lateral plate, PVL ( Figs. 10F , 11A ): The ventro-lateral ridge is strongly pronounced. The top of the ridge is widened in the middle on IRSNB P.9272. IRSNB P.9273 displays a less marked ventral lateral ridge because of the compression. The anterior margin of the ventral lamina is straight, and the margin, corresponding to the border with the median ventral plate, is straight and quite long. The lateral lamina is elongated and wider through its dorsal corner. The margin running from the dorsal corner to the anterior dorso-lateral corner is concave. The margin, joining the dorsal corner to the posterior dorso-lateral corner, is also concave and quite extended. The lateral process itself is well developed. The posterior margin of the lateral lamina is concave. The subanal division is relatively large because it occupies about one fourth of the PVL total length. The area overlapped by the left PVL is elongated and narrow. The area overlapped by the AVL is quite narrow too.

Posterior lateral plate, PL ( Fig. 11A ): The only preserved posterior lateral plate (IRSNB P.9273) is in connection with a posterior ventro-lateral plate. It is very low relatively to its length and higher anteriorly than posteriorly. The anterior margin forms a rather acute angle with the anterior portion of the dorsal margin. The posterior dorsal angle is rounded and the height at this level reaches one third of the anterior height. The ventral margin displays posteriorly a re-entrant angle, into which the PVL inserts. The area overlapped by the AVL is thin.

Anterior ventro-lateral plate, AVL ( Fig. 11C ): Only one specimen of AVL is available (IRSNB P.9274). The anterior portion, and thus the anterodorsal process rising to overlap the ADL, is not preserved. The joint with the pectoral fin is masked by the pectoral fin, which is connected to the AVL. The ventro-lateral ridge is strongly compressed, and the lateral lamina rather damaged but extended. The margin shared with the median ventral plate is long and concave. The median ventral plate should occupy a large place in the ventral shield. Internally, the crista transversalis interna anterior is oblique. It seems high but is partly broken.

Pectoral fin ( Fig. 11B, C ).—The ventral side of the pectoral fin is entirely visible. This fin extends posteriorly slightly less than the posterior margin of the anterior ventro-lateral plate. It is quite broad, giving it a stout shape. It is about three times as long as broad. It is divided into two rows of plates with unique proximal and distal plates. The ventral central plate 1 (CV1) is as long as broad. The ventro-medial marginal plate series (VM1–VM3) displays three polygonal plates with the VM1 badly preserved. The lateral marginal plate series (ML1– ML4) displays four polygonal plates. The ML1 is preserved as an impression, and its shape is not clearly distinguishable. The terminal plate (T) is posteriorly sharp.

The dorsal central plate 1 (CD1) is at least twice longer than broad (IRSNB P.9261), with a well-defined lateral corner.

Ornament.—The ornament of the trunk elements is made of vermiculating, coarse, short pieces of ridges. It can form a radiating network, on the smallest AMD (IRSNB P.9266), and a radiating and concentric network, according to the area of the plate, on the largest AMD (IRSNB P.9265). On the largest plates, it is made of individual or anastomosed tubercles (IRSNB P.9273). On IRSNB P.9267, some tubercles are organized in sensory groove-like structures and form a cross-like structure extending over all the posterior part of the PMD. Except for the lateral plate, which seems unornamented, the other cephalic plates display the same ornament than the trunk plates, i.e., vermiculating, coarse, short pieces of ridges.

Remarks.— Even on the largest AMD (IRSNB P.9265), the anterior oblique dorsal sensory line grooves are present. This character is typical of juvenile individuals in Bothriolepis ( Stensiö 1948) but seems to be retained in adults of R. durnalensis .

It seems that R. durnalensis , at least its trunk armour, becomes more dorso-ventrally compressed during its growth. In fact, the lateral lamina of the PVL is deeper in smaller individuals (IRSNB P.9272) than in larger ones (IRSNB P.9273). However, this lower plate is perhaps compensated by a high- er posterior lateral plate and/or a higher lateral lamina of the posterior dorso-lateral plate. There is currently not enough material to test such a hypothesis.

The antiarch from Spontin belongs to Remigolepis because of the absence of the joint in the pectoral appendages, the short and broad premedian plate notched anteriorly, the AMD overlapping PDLs anteriorly but overlapped by them posteriorly, and the PDLs and PLs not fused.

Stensiö (1931) erected the genus Remigolepis and named several species from the Upper Devonian of East Greenland. The material referable to the type species, R. incisa ( Woodward, 1900) , consists only, and with certainty, of two AMDs Stensiö 1931). They resemble the AMD of R. durnalensis by the shape and the ornament consisting of coarse, rounded or slightly elongated tubercles arranged in rather distinct rows that radiate from the dorsal angle. However, the anterior oblique dorsal sensory line grooves, observable in R. durnalensis , are absent in R. incisa , and further comparisons are impossible because of the small amount of material.

R. cristata Stensiö, 1931 differs from R. durnalensis by the longer anterior margin of the AMD and by the spine-like process on the dorsal median ridge. R. kochi Stensiö, 1931 differs from R. durnalensis by its ornament, which forms low striae radiating from the tergal angle on the AMD. The ornament of R. durnalensis is more vermiculating. R. kochi also differs from R. durnalensis by a more elongated PMD, by a strong dorsal process on the PVL ( Stensiö 1931: pl. 26: 5) and by a smaller overall length of the trunk shield. R. kullingi Stensiö, 1931 differs from R. durnalensis notably by a more elongated PMD and by a more delicate ornament (concentric arrangement on the AMD). R. acuta Stensiö, 1931 differs from R. durnalensis by its larger size (about twice longer than R. durnalensis ) and by a narrower PMD.

Of the two Australian species, R. redcliffensis Johanson, 1997b differs from R. durnalensis by a stronger dorsal median ridge, by the absence of oblique dorsal sensory line grooves and by a reduced processus obstans. R. walkeri Johanson, 1997a differs from R. durnalensis by narrower Prm and Pp plates, by the shape of the Pp plate, by a stronger dorsal median ridge and by a lower crista transversalis interna anterior.

The Kirgizian R. suusamyrensis Panteleyev, 1992 and R. karakoliensis Panteleyev, 1992 are small species, very different from R. durnalensis . The total length of their trunk armour equals the length of the sole AMD of R. durnalensis . Moreover, the shape of their AMDs does not match with that one of R. durnalensis ; i.e. more rounded in R. suusamyrensis and more elongated in R. karakoliensis .

Among the Chinese species, R. major Pan, 1987 (in Pan et al. 1987) and R. xixiaensis Pan, 1987 (in Pan et al. 1987) are large species that differ radically from R. durnalensis . The first one could reach a head and trunkshield length of 45 cm, and the latter of 30 cm, whereas in R. durnalensis it is nearly 20 cm. They also differ from R. durnalensis by a smooth ventral surface of the AMD. Moreover, the AMD of R. major is anteriorly pointed, whereas the AMD of R. durnalensis presents a short anterior margin. By contrast, R. xiangshanensis Pan, 1987 (in Pan et al. 1987) differs from R. durnalensis by its small size. Moreover, the angle between the posterior oblique dorsal sensory line grooves on the AMD is more obtuse in R. xiangshanensis and the ventral surface of the AMD smooth. R. zhongweiensis Pan, 1987 (in Pan et al. 1987) and R. zhongningensis Pan, Wang, Liu, Gu, and Hang, 1980 are in the size range of R. durnalensis . However the first one differs from the Belgian species by narrower Pp and PMD and by a more elongated AMD without oblique dorsal sensory line grooves. The latter differs from R. durnalensis by the shape of the AMD, which displays almost parallel lateral margins, by a tergal angle located anteriorly, by a slender PMD with well-developed cristae transversalis interna posterior and by a slender pectoral fin. Finally, R. microcephala Pan, 1987 (in Pan et al. 1987) differs from R. durnalensis by a Prm anterior notch slightly defined, by a massive Pp and by wider AMD and PMD.

The material defining the Scottish species? Remigolepis sp. only consists of an isolated AMD ( Andrews 1978). It comes from a large species because the AMD alone is 15 cm in length. It differs from R. durnalensis by the size and by coarse tubercles arranged in rows parallel to the margins. R. armata Lukševičs, 1991 from Russia differs notably from R. durnalensis by the spine on the dorsal surface of the PMD plate and by the AMD, which does not taper anteriorly.

The material of Remigolepis from Belgium is sufficient and diagnostic enough to erect a new species. This discovery enlarges the spatial distribution of the genus to Western Europe and confirms its cosmopolitan distribution during the Late Devonian.

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

Figures

Fig. 9. Dermal plates of antiarch placoderm Remigolepis durnalensis sp. nov., from Spontin (Namur Province), Famennian, Devonian. A. IRSNB P.9262, Prm, external view, photographed under water immersion. B. IRSNB P.9263a, La, external view, photographed under water immersion. C. IRSNB P.9264a, Pp, external view, dotted line based on the counterpart IRSNB P.9264b, photographed under water immersion. D. IRSNB P.9265a, AMD, external and internal views, dotted line based on IRSNB P.9265b. E. IRSNB P.9265b, AMD, external and internal views. F. IRSNB P.9266b, AMD, external and internal views. G. IRSNB P.9266a, AMD, external and internal views. Photographs (A1–D1, E–G); explanatory drawings (A2–D2). Scale bars 10 mm.

Fig. 10. Dermal plates of antiarch placoderm Remigolepis durnalensis sp. nov., from Spontin (Namur Province), Famennian, Devonian. A. IRSNB P.9267a, PMD, external view, dotted line based on IRSNB P.9268. B. IRSNB P.9268a, PMD, internal view, photographed under water immersion. C. IRSNB P.9269a,ADL, external view. D. IRSNB P.9271a, PDL, external and internal views. E. IRSNB P.9270a, PDL, external and internal views. F. IRSNB P.9272, PVL, external view. Photographs (A1, B, C1, D, E1, F1); explanatory drawings (A2, C2, E2,F2). Scale bars 10 mm.

Fig. 11. Dermal plates of antiarch placoderm Remigolepis durnalensis sp. nov., from Spontin (Namur Province), Famennian, Devonian. A. IRSNB P.9273a, PVL + PL, external view; photograph of natural cast (A1), explanatory drawing (A2), dotted line based on PVL and PL of other Remigolepis species. B. IRSNB P.9261a, CV1, internal and external views.C. IRSNB P.9274a, pectoral fin + AVL, ventral view; photograph (C1); explanatory drawing (C2), dotted line based on IRSNB P.9274b.Abbreviations: CV, ventral central plate; ML, lateral marginal plate; VM, ventro-medial marginal plate. Scale bars 10 mm.

Fig. 12. Reconstructions of the dermal shield of antiarch placoderm Remigolepis durnalensis sp. nov., from Spontin (Namur Province), Famennian, Devonian. A. Reconstruction of the incomplete head shield (the left lateral plate is the mirror image of the right one), dorsal view. B. Trunk reconstruction, dorsal view. Scale bars 10 mm.