Grossilepis rikiki, Olive, 2015

Olive, Sébastien, 2015, Devonian antiarch placoderms from Belgium revisited, Acta Palaeontologica Polonica 60 (3), pp. 711-731 : 718-721

Grossilepis rikiki sp. nov.

Fig. 5 .

2009 Bothriolepis ; Clément and Prestianni 2009: 107: pl. 3: 2.

Etymology: From French colloquial language word rikiki , small; referring to the small size of the new species. According to article 31.2.3 of the International Code of Zoological Nomenclature, the name “rikiki ” is to be treated as indeclinable because it is not a Latin or Latinized word.

Type material: Holotype: AMD: IRSNB P.9255a, b . Paratypes: Skull roof: IRSNB P.9253a, b; AMD: IRSNB P.9254; PMD: IRSNB P.9256a, b; CV1: IRSNB P.9258a, b; ML2: IRSNB P.9257 .

Type locality: Strud , Namur Province , Belgium ( IRSNB P.9255, IRSNB P.9254, IRSNB P.9258, IRSNB P.9257) and Moresnet, Liège Province, Belgium ( IRSNB P.9253, IRSNB P.9256) .

Type horizon: Evieux Formation , upper Famennian , Upper Devoni- an ( IRSNB P.9255, IRSNB P.9254, IRSNB P.9258, IRSNB P.9257); Montfort / Evieux Formation , upper Famennian, Upper Devonian IRSNB P.9253, IRSNB P.9256 ).

Material.— AMD: IRSNB vert 31594-001a, b; PMD: IRSNB vert 6845-001, PALULG-2011.12.15.18; CV1: IRSNB vert 31594-008a, b, IRSNB vert 32164-002a, b; ML2: IRSNB vert 31594-004a, b, IRSNB vert 31594-005a, b, IRSNB vert 31594-006a, b, IRSNB vert 31594-007a, b, IRSNB vert 31594-009a, b, IRSNB vert 31913-001a, b, IRSNB vert 31913-002a–c, IRSNB vert 32164-001. From Strud, Namur Province, Belgium, Evieux Formation, upper Famennian, Upper Devonian. IRSNB vert 15025-001a, b, from Moresnet, Liège Province, Belgium, Famennian, Upper Devonian.

Diagnosis.— Small bothriolepidoid with estimated length of dorsal wall of trunk-armour reaching about 4–5 cm. Headshield moderately vaulted, with a large orbital fenestra. Lateral plates elongated. Nuchal plate with an obtected nuchal area extending along the entire breadth of the plate, with a medial, complex posterior median process displaying several straight fingerings. Lateral division of the paranuchal plate almost as broad as the median division. Dorsal wall of trunk armour quite arched, with a well developed dorsal median ridge. Tergal angle situated between the anterior and middle thirds of the plate. Levator fossa well marked. Anterior margin of the anterior median dorsal plate sinusoidal and 1.5 times the length of the posterior margin. Area of the AMD overlapped by the posterior median dorsal plate quite extend- ed, with a strong posterior median process. Area of the AMD overlapping the anterior dorso-lateral plate extended, and area overlapping the mixilateral plate elongated. Lateral processes of the posterior median dorsal plate quite extended. Lateral marginal plate 2 very elongate, up to six times longer than broad, with the mesial corner located very anterior. Ornament clearly nodose on the skull roof and anterior median dorsal plate and reticulate to nodose on the pectoral fin bones.

Description

Headshield ( Fig. 5A ).—The anterior portion of the nuchal plate and the postpineal plate are not preserved, but it seems, however, that the orbital fenestra reaches a large size. Two symmetric elements are badly preserved in the fenestra. They could belong to the sclerotic ring and could be anterior sclerotic plates based on their position in the fenestra.

Premedian plate, Prm.—This plate is broader than long. The breadth is maximal at the anterior margin and gently decreases until it reaches half the value at the posterior margin. The principal section of infraorbital sensory line is not easily observed, because of the damaged bone.

Lateral plate, La.—This plate is rather long. It is narrow in its anterior part and broad in its posterior part. The orbital margin is concave, with the highest flexure at the middle length of the plate. The posterior portion of the plate, adjoining the nuchal and paranuchal plates, is quite narrow. The principal section of infraorbital sensory line (runs in the medial part of the plate. The branch of infraorbital sensory line is quite long. There is no trace of central sensory line grooves.

Nuchal plate, Nu.—The anterior part of the plate is not preserved. The posterior portion of the lateral margin is long and concave. The objected nuchal area extends along the entire breadth of the plate. It bears medially a posterior median process with several posterior straight fingerings. There is no observable trace of central sensory line grooves.

Paranuchal plate, Pn.—The principal section of the infraorbital sensory line crosses the plate in its middle. The lateral division of the plate is almost as broad as the median division. The postmarginal plate completes the lateral corner of the skull roof.

Trunkshield.— Anterior median dorsal plate, AMD ( Fig. 5B, D ): This plate is slightly longer than broad. Externally, the plate is arched with a salient and well-developed dorsal median ridge forming an obtuse angle, and running from the tergal angle backwards to the posterior margin. The tergal angle is situated between the anterior and middle thirds of the plate. The top of the dorsal median ridge forms a tiny flat strip on IRSNB P.9254, probably resulting from the post-mortem wear of a sharper dorsal median ridge. This strip is absent on the other AMDs. The anterior margin, 1.5 times the length of the posterior margin, is sinusoidal and the antero-lateral corners are well marked. The breadth at the level of the antero-lateral corners is slightly shorter than at that of the medio-lateral corners. The anterior part of the lateral margin, thus joining both corners, is concave. The posterior part of the same margin is quite straight and slightly shorter than the anterior part. The posterior margin is clearly concave. Underneath, the area overlapped by the posterior median dorsal plate is quite extended and the posterior median process salient. On IRSNB P.9254, the slightly marked left anterior oblique dorsal sensory line groove runs in a smooth depression corresponding internally to the area between the levator fossa and postlevator thickening, from the tergal angle toward the lateral margin. It seems to stop gently before the lateral margin. The right anterior oblique dorsal sensory line groove is not observable. On IRSNB vert 15025-001, both anterior oblique dorsal sensory line grooves are present but very slightly marked, whereas the posterior oblique dorsal sensory line grooves are strongly marked. There is a slight bilateral asymmetry of the posterior oblique dorsal sensory line grooves of IRSNB P.9254. The right dlg2 forms a sharp- er angle with the median dorsal ridge than does the left one. Internally the levator fossa is well marked and the postlevator thickenings quite hilly. The anterior ventral pit (is oval and elongated, the median ventral ridge sharp and the ventral median groove (grm) elongated. The areas overlapping the anterior dorso-lateral plates (cf. ADL) and those overlapping the mixilateral plates are quite extended.

Posterior median dorsal plate, PMD ( Fig. 5E ): The PMD is slightly wider than long. The maximum width is across the lateral corners but the plate is quite uniform in breadth throughout its extent. The anterior margin is convex and the anterior median corner obtuse. The antero-lateral corner is located at the first anterior third of the plate. The lateral process is well marked with prominent lateral and postero-lateral corners. The postero-lateral corners are obtuse. The posterior margin is convex and the posterior median corner protruding. The plate is quite arched with a well-defined dorsal median ridge. The crista transversalis interna posterior is smooth and quite hilly.

Pectoral fin.— Ventral central plate 1, CV1 ( Fig. 5F ): The posterior part is not preserved but this plate is at least twice longer than broad. The external ventral articular area is well preserved. The lateral corner is smooth. The ventro-medial margin is well marked.

Lateral marginal plate 2, ML2 ( Fig. 5C ): This plate is at least six times longer than broad. This is a very elongate bone. The obtuse mesial corner, usually located at the middle length of the plate, occupies here a very anterior position well observed on IRSNB vert 31594-007). The anterior part of the mesial margin is thus short. The slight incision on the posterior part of the mesial margin corresponds certainly to the anterior end of the notch for the dorsal central plate 2. The lateral margin is convex and bears spines that are numerous, slightly spaced out and mostly turned proximally.

Ornament.—The ornament on the anterior median dorsal plate is nodose; composed of a delicate and regular network of interconnecting ridges with tubercles developed at the junctions between ridges. On the ML2 plates, the ornament is reticulate (IRSNB vert 31913-002) or nodose (IRSNB vert 31594-009).

Remarks.— Bothriolepis canadensis is known for all its growth stages ( Stensiö 1948). Referring to Stensiö’s observations, and considering that the genus Bothriolepis is closely related to the genus Grossilepis , the material described here is certainly from half-mature individuals. In fact, the material presents a combination of (i) juvenile characters, i.e., ornament reticulate to nodose, large orbito-nasal fenestra ( Werdelin and Long [1986] and Cloutier [2010] made the same observation), dorsal median ridge well developed, anterior oblique dorsal sensory line grooves present on several AMDs, anterior dorsal depression corresponding ventrally to the position of the postlevator thickening and posterior median process strongly developed; and (ii) mature characters, i.e., absence of central sensory line grooves on the skull roof, slightly-marked anterior oblique dorsal sensory line grooves, absence of the second anterior oblique dorsal sensory line groove on IRSNB P.9254 and presence of some tubercles in the ornament.

The bothriolepidid material from Strud and Moresnet is assigned to the genus Grossilepis based on the AMD overlap relations with the surrounding plates. Indeed, the AMD overlapping both the ADL and MxL is one of the diagnostic characters of the genus Grossilepis . Since no MxL or ADL of this taxon has been found, it is impossible to check the presence of the other diagnostic characters of this genus.

The genus Grossilepis includes three named species. G. tuberculata , from the Frasnian of Latvia, Lithuania and Russia ( Gross 1941; Stensiö 1948; Lukševičs 2001), is the oldest described species of the genus, and the most complete one. It was described on the basis of numerous adult plates and its validity is not discussed here. G. spinosa , from the Middle Frasnian of Latvia ( Gross 1942; Lukševičs 2001), was first described as Bothriolepis spinosa Gross, 1942 . It was reassigned, with reservations, to the genus Grossilepis by Stensiö (1948: 615) in regard to “the shape of certain of its dermal bones, the sutural connection of the AMD and MxL plate, and the ornament”. Lukševičs (2001) confidently assigned this species to the genus Grossilepis . G. brandi from the Frasnian of Scotland ( Miles 1968) was described on the basis of very few remains. The AMD, ADL, and MxL are absent, so the diagnostic characters of the genus are impossible to check. The attribution of G. brandi to Grossilepis is therefore doubtful and it is proposed here to consider this species as a nomen dubium.

Grossilepis rikiki from Strud and Moresnet differs chiefly from G. tuberculata by (i) the shape of the lateral plate, which is larger in G. tuberculata , (ii) the shape of the orbital fenestra, larger in G. rikiki , (iii) the complex obtected nuchal area of the nuchal plate, (iv) the shape of the AMD, which is narrower in G. tuberculata , (v) the absence of central sensory line grooves on the nuchal and lateral plates, (vi) the overlapping areas of the AMD, which are clearly more developed in G. rikiki , (vii) the size of the dorsal wall of the trunk armour, G. tuberculata possesses a longer trunk armour roof, and (viii) by the ornament, tuberculate for G. tuberculata and mainly nodose for the Grossilepis species from Strud and Moresnet. It differs from G. spinosa in characters (iii), (v), (vi), (vii), (viii) and by the size and shape of the lateral spines of the proximal segment of the pectoral fin, which are strongly developed in G. spinosa .

Matukhin et al. (1980) described a species of Grossilepis (remaining in open nomenclature) from the Frasnian of Russia in the Marshrutninskaya locality, Krasnoyarsk region (northwestern Siberian Platform). Moloshnikov (2012) states that this species differs from the other species of Grossilepis by its narrow AMD and the short posterior margin of the AMD. It differs from G. rikiki by those characters too, but also by a more elongated levator fossa, less extended areas overlapping MxLs and ADLs, smoother lateral corners and the absence of antero-lateral corners.

Although the material from Moresnet and Strud seems to be half-mature nature, the differences with the few other species of Grossilepis , from a morphological and a stratigraphical point of views, are enough to justify the erection of a new species.

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

Figures

Fig. 5. Dermal plates of antiarch placoderm Grossilepis rikiki sp. nov., from Moresnet, Liège Province (A, E), Strud, Namur Province (B–D, F), Famennian, Devonian. A. IRSNB P.9253a, skull roof, external view; photograph (A 1), under water immersion; explanatory drawing (A 2), dotted line obtained by mirror image. B. IRSNB P.9254, AMD, external view, whitened with ammonium chloride. C. IRSNB P.9257, ML2, external and internal views, dotted line based on IRSNB vert 31594-007. D. IRSNB P.9255a, AMD, internal view; photograph (D 1), explanatory drawing (D 2). E. IRSNB P.9256a, PMD, external and internal views, dotted line based on the counterpart IRSNB P.9256b. F. IRSNB P.9258a, CV1, external view. Scale bars 10 mm.