Diademaspis sp.

Voichyshyn, Victor, 2006, New osteostracans from the Lower Devonian terrigenous deposits of Podolia, Ukraine, Acta Palaeontologica Polonica 51 (1), pp. 131-142 : 136

Diademaspis sp.

Figs. 5 , 6 .

Locality and horizon: Ukraine, Podolia ( Fig. 1B ), precise locality unknown; Lower Devonian, probably lower part of the Old Red Formation (Dniester Series), the first Old Red faunal zone after Brotzen (1933).

Material.—Mould of the central part of a headshield (from the nasohypophysial opening to the median dorsal crest), GMLSU 478, 4 ( Fig. 5A, B ).

Description.—The length of the headshield is 90–100 mm (the distance between the anterior limit of the nasohypophysial opening and the posterior limit of the median dorsal crest is about 70 mm). The orbits are oval in shape and relatively small (their length is 7 mm and the width is about 5 mm). The circumorbital thickening is 1.2–1.3 mm wide (broad). The distance between the orbits is relatively small for Diademaspis , 13 mm. The nasohypophysial opening has an enlarged hypophysial division that is sunken into a depression and a small nasal division. The pineal plate seems to be reduced, without any contact with the orbits; its width does not exceed the extent of the anterior margin of the median dorsal field. The recess of the pineal plate has a chink−like shape with a maximum length (along the body axis) of 1.5 mm and width of 6.5 mm. The median dorsal field is 22 mm long and has concave and narrowed anterior and posterior margins, and convex lateral ones. The posterior margin of the dorsal field displays two lateral processes separated by a 2 mm deep cavity between them. The maximum width of the median dorsal field (13 mm) is at the level of its second third. The median dorsal crest is well developed (its length is about 25 mm, the height up from the level of the median dorsal field is 10 mm), with a slope that begins immediately behind the posterior margin of the median dorsal field. The ornamentation of the exoskeleton is poorly preserved. It seems to consist mainly of tesserae−like areas (about 1 mm in size), which include relatively large rounded tubercles surrounded by considerably smaller ones.

Comparison.—This form differs from D. poplinae by its chink−like shape of the pineal plate, the shape of the median dorsal field (in particular, its more convex lateral margins) ( Fig. 6A ), the more developed median dorsal crest ( Fig. 6B ), and the smaller overall size.

It differs from D. jarviki by its greater size of the hypophysial division of the nasohypophysial opening, the presence of a pineal plate and the shape of the median dorsal field ( Fig. 6A ).

This specimen differs from D. stensioei by its type of ornamentation ( Fig. 5 ). Other material for comparison with this species is unavailable. The central part of the headshield, which is preserved in the specimen described above, is unknown in the material referred to D. stensioei . For the same reason it is not possible to compare the form described here to the shield fragment from Ustechko, assigned to Diademaspis sp. ( Janvier 1985 b: fig. 16A). Due to the state of preservation of the ornamentation in the form described here it is not clear, whether the tubercles formed groups, like in the specimen from Ustechko. In any case, they were more densely spaced than in the fragment from Ustechko.

This specimen differs from D. sp. 2 ( Janvier 1985 a) by the shape and the greater size of the orbits, the longer (along the body axis) pineal plate, and the concave anterior margin of the median dorsal field, which becomes conspicuously broader in its middle part. This form is, however, similar to D. sp. 2 by the lateral extent of its pineal plate. Like in the following cases, other features cannot be compared because of the poor preservation of the specimens.

It differs from D. sp. 3 by the less steep rise of the median dorsal crest ( Figs. 5B , 6B ).

It differs from D. sp. 4 by shorter orbits, the smaller distance between the anterior margin of the nasohypophysial opening and the line connecting the anterior margins of the orbits, and, probably, by its different type of ornamentation.

Besides, this form differs from all Diademaspis species from Spitsbergen, including D. sp. 1, by its smaller overall size, and, except for D. sp. 4, by its more densely spaced orbits.

Remarks.—I should avoid to name the Diademaspis sp. described here which is completely unknown in outline of its shield, although it should be a really new form.

Figures

Fig. 5. Diademaspis sp., GMLSU 478,4. Podolia, Ukraine, Dniester Series, Lower Devonian, dorsal (A) and lateral (B) views. Scale bars 10 mm.

Fig. 6. Diademaspis sp., reconstruction of the central part of the shield, based on specimen GMLSU 478,4, dorsal (A) and lateral (B) views.

Fig. 1. Contours of Ukraine and the neighbouring countries (A), simplified map of Podolia with main cities and major tributaries of Dniester (B), locality map, showing position of the points of finds of the studied material (indicated by an asterisk) (C).