Asterolepis sp. 1

Olive, Sébastien, 2015, Devonian antiarch placoderms from Belgium revisited, Acta Palaeontologica Polonica 60 (3), pp. 711-731 : 721-722

Asterolepis sp. 1

Figs. 6 , 7 .

1965 Asterolepis ; Gross 1965: 3–5, fig. 1.

Material.— Nu: IRSNB P.1456; Pp: IRSNB P.1457; PMD: IRSNB P.1458; AVL: IRSNB P.1459a, b; CV1: IRSNB P.1460, IRSNB P.1461a, b; MM1: IRSNB P.1463; distal part of the pectoral fin: IRSNB P.1462a, b. From Hingeon, Namur Province, Belgium, Mazy Member, Bois de Bordeaux Formation, Upper Givetian, Middle Devonian.

Description

Headshield.— Nuchal plate, Nu ( Fig. 6B, C ).—Only the middle part of this plate is preserved. The posterior margin is convex with a small posterior median process on the objected nuchal area. A portion of the anterior margin is preserved. It constitutes a part of the postpineal notch. The middle pit-line groove is well marked and connected to the supratemporal pit-line groove, in turn linked to the external openings for the endolymphatic duct.

Postpineal plate, Pp ( Fig. 6A, C ): It is preserved almost entire, on the same block as the nuchal plate, but disconnected. It fits well with the shape of the preserved part of the postpineal notch of the nuchal plate. Thus, both plates certainly belonged to the same individual. The postpineal plate is broader than long. Anteriorly, it shows the limits of a smaller plate corresponding to a younger growth stage.

Trunkshield.— Posterior median dorsal plate, PMD ( Fig. 6D ): The PMD is relatively small compared to the rest of the material and might correspond to a juvenile individual. The dorsal median ridge and median ventral ridge are very sharp, forming an angle of about 90°. This plate tapers towards the anterior end. It displays an extended lateral process and a smooth posterior angle. The area overlapping the AMD (is extended, contrary to the narrow areas overlapping MxLs.

Anterior ventro-lateral plate, AVL ( Fig. 6E, F ): The subcephalic part is missing and the posterior part is badly preserved. The articular area with the pectoral girdle is missing too. Only the axillary foramen is present as a mold in the matrix. The ventral lamina is better preserved than the lateral one. The ventro-lateral ridge forms an angle of about 140°. The area overlapped by the right AVL is slightly observable medio-anterally. Concentric lines are present on the visceral surface of the ventral and lateral laminae.

Pectoral fin.— Ventral central plate 1, CV1 ( Fig. 7A, F ): There are two specimens from Hingeon: a small one and a large one. They are both incomplete in the posterior portion. The ventro-medial margin is prominent and the external ventral articular area quite extended.

Medial marginal plate 1, Mm1 ( Fig. 7E ): This plate is entirely preserved and relatively short. The proximal part is forked and unornamented whereas the distal part is ornamented. It is crossed proximo-distally by the pectoral pit-line groove. The posterior margin is slightly sinusoidal.

Distal part of the pectoral fin ( Fig. 7B–D ): A large part of the pectoral fin distal segment is preserved in ventral view. Neither the anteriormost part, nor the posteriormost part is preserved. The medial marginal plate 4 displays laterally short, spaced spines that are absent on the distal part of the plate. Gross (1965) noticed that a medial marginal plate 5 could be attached to the Mm4 but this is not observable. On the contrary, it is linked to the ventral central plate 4 (Cv4) by a strong edge. The posterior part of this plate is narrow, whereas the anterior part is large and flattened. It certainly corresponds to the insertion area of the pectoral fin proximal segment. Immersed in water or alcohol, the limit between the Cv4 and the lateral marginal 5 ( ML 5) is clearly observed. The ML5 is a long plate with a regular width. Its posterior limit is neither observable nor preserved. The terminal plate is not preserved, but a fragment of its impression might be present on the counterpart of the pectoral fin distal segment. However, its limits are not distinguishable.

Ornament.— The ornament is tuberculate on the nuchal plate. On the postpineal plate, some tubercles fuse to form small ridges. Some of them radiate from the centre of the plate. On the posterior median dorsal plate, Gross (1965: 4) defined the ornament as forming ramparts (“Wallbildung”). It is rather a reticulate ornament with some tubercles present. On the anterior ventro-lateral plate, the ornament is clearly tuberculate, with tubercles arranged in parallel rows. As for the ventral central plate 1, it bears few small, spaced tubercles, and the medial marginal plate bears huge, rough tubercles sometimes associated. The distal segment of the pectoral fin is devoid of ornament.

1

2

Remarks.— Compared to what Gross (1965) published the present work provides further details and the counterparts of some plates are figured here. The drawings made by Gross are very accurate and have been used here for the figures.

Gross (1965) described an indeterminate species of antiarch from the Givetian of Hingeon ( Asterolepis sp. 1 in this paper). In spite of the small number of plates preserved, he assigned this form to Asterolepis , because all the bones matched that genus, and not those of other genera of antiarch. In fact, Pterichthyodes and Byssacanthus display shorter and wider postpineal plates. In Gerdalepis , the same plate bulges forward. The genus Bothriolepis is excluded because its medial marginal plate 1 is longer and narrower. The genus Remigolepis was not considered by Gross (1965), but the species from Hingeon clearly differs from that genus, notably by the organization of the pectoral fin plates. To date, and without supplementary material, the attribution of this material to Asterolepis by Gross (1965) is not reappraised and seems correct. He considered that the Asterolepis material from Hingeon resembles more A. ornata than any other species, but that the material was insufficient to assign it to any particular species of Asterolepis . His cautiousness is here followed.

Figures

Fig. 6. Dermal plates of antiarch placoderm Asterolepis sp. 1, from Hingeon (Namur Province), Upper Givetian, Devonian. A. IRSNB P.1457, Pp, external view. B. IRSNB P.1456, Nu, external view. C. IRSNB P.1456 and 1457, Pp and Nu, external view. D. IRSNB P.1458, PMD, external and internal views; photograph (D 1), explanatory drawing (D 2). E. IRSNB P.1459b, AVL, external and internal views, counter-part of IRSNB P.1459a. F. IRSNB P.1459a, AVL, external and internal views; photograph (F ), explanatory drawing (F ). Drawings from Gross 1965. Scale bars 10 mm.

Fig. 7. Dermal plates of antiarch placoderm Asterolepis sp. 1, from Hingeon (Namur Province), Upper Givetian, Devonian. A. IRSNB P.1460, CV1, external view; photograph (A 1), explanatory drawing (A 2). B. IRSNB 1462a, distal part of the pectoral fin, external view. C. IRSNB 1462b, distal part of the pectoral fin, external view, counterpart of IRSNB 1462a. D. Drawing based on IRSNB 1462a and b, distal part of the pectoral fin, external view. E. IRSNB P.1463, MM1, external view; photograph (E ), explanatory drawing (E ). F. IRSNB P.1461a, CV1, internal view. Drawings from Gross 1965. Scale bars 10 mm.