Diademaspis Janvier, 1985a

Voichyshyn, Victor, 2006, New osteostracans from the Lower Devonian terrigenous deposits of Podolia, Ukraine, Acta Palaeontologica Polonica 51 (1), pp. 131-142 : 135-136

Genus Diademaspis Janvier, 1985a

Type species: Diademaspis poplinae Janvier, 1985a , Lower Devonian , upper part of Wood Bay Formation of Spitsbergen .

Emended diagnosis.— Zenaspididae of small to large size (L = 90–250 mm), with a headshield that is almost as long as wide (L/S = 0.9–1.0). The prepineal part of the shield is considerably shorter than the postpineal one (B/C = 0.6). The abdominal part of the shield narrows backwards and has a median dorsal crest. The hypophysial division of the nasohypophysial opening is much larger than the nasal division. The pineal plate is reduced or absent. The orbital openings are relatively small (S/Os = 27–29) and loosely spaced (Omin/Os is about 4), with a broad circumorbital thickening. The dorsal field is wide (S/Sd index (or ratio) equals about 7). The lateral fields are enlarged posteriorly and reach to the basal (proximal) part of the cornual processes. The distance from the margin of the posterior part of the lateral field to the tip of the pectoral sinus is Ẑ1.5 times larger than that to the lateral margin of the shield. The ornamentation consists of tesserae−like areas displaying groups of tubercles of different sizes. (Modified after Janvier 1985 a and Afanassieva 1989; see also Janvier 1985 b; Afanassieva 1991; Adrain and Wilson 1994).

Species included.— D. jarviki ( Wängsjö, 1952) , Lower Devonian of Spitsbergen; D. stensioei Afanassieva, 1989 , D. sp., both from the Lower Devonian of Podolia. D. sp. 1, D. sp. 2, D. sp. 3, and D. sp. 4 ( Janvier 1985 a) from the Lower Devonian of Spitsbergen.

Remarks.—As to the proportion of the divisions of the nasohypophysial opening of Diademaspis a possible exception is D. jarviki , which has nasal and hypophysial divisions that are almost equal in size ( Wängsjö 1952: text−fig. 59). On the photograph of the holotype this region is unclear, probably unsufficiently preserved ( Wängsjö 1952: pl. 46: 2). However, Janvier (1985a) pointed out that D. jarviki is similar to D. poplinae notably as to the shape of its nasohypophysial opening.

Some shield fragments with the ornamentation typical of Diademaspis have been recorded as Diademaspis sp. from Ustechko and Chervonograd in Podolia ( Janvier 1985 b).

It is doubtful that D.? mackenziensis Adrain and Wilson, 1994 from the Northwest Territories of Canada belongs to the genus Diademaspis because of the considerable morphological difference between this species and the European Diademaspis species. In contrast with the European Diademaspis species, the pineal plate of the Canadian species contacts the orbits, which are considerably larger (S/Os = 15) and more closely−set (Omin/Os = 1.5). Also, the shape and size of the median dorsal field in the compared taxa are different (S/Sd of the Canadian form is 12.5); the headshield is noticeably narrower and its size (L = 30 mm) is strikingly small in comparison with the size of the European Diademaspis headshields. Finally, the exoskeleton ornamentation, which consists of elongated tubercles in D.? mackenziensis, is not characteristic of the genus. The fact, that the hypophysial division of the nasohypophysial opening is larger than the nasal one, may indicate that the Canadian species belongs to the Zenaspidida , but there seems to be no reasons to refer it to Diademaspis and, thus, to make the diagnosis of this genus less precise.

Occurence.—Lower Devonian of Spitsbergen and Podolia.