Zenaspis dzieduszyckii, Voichyshyn, 2006

Voichyshyn, Victor, 2006, New osteostracans from the Lower Devonian terrigenous deposits of Podolia, Ukraine, Acta Palaeontologica Polonica 51 (1), pp. 131-142 : 133-134

Zenaspis dzieduszyckii sp. nov.

Figs. 3 , 4 .

1985 Zenaspis cf. salweyi ( Egerton, 1857) ; Janvier 1985 b: 323–325, fig. 14.

Derivation of the name: In honour of the founder of SMNH, the late Count Włodzimierz Dzieduszycki (1825–1899).

Holotype: SMNH BP.103; natural mould and impression of a headshield, lacking the posterior part and cornual process of the rigth side.

Type locality: Ukraine, Podolia, right bank of Dniester, Horodnytsia ( Fig. 1C ).

Type horizon: Lower part of the Old Red Formation (Dniester Series, most probably its Ustechko Member).

Material.— Besides the holotype, specimens SMNH BP.1193, BP.1215 .

Diagnosis.—Small Zenaspis species with relatively large orbits, wide lateral fields, and short and slender cornual processes.

Description.—The length of the headshield is about 100– 110 mm, its maximum width (120–130 mm) is at the tips of the cornual processes. The lateral margins of the shield are feebly convex. The cornual processes are oriented caudolaterally (or postero−laterally), short and rapidly tapered off. The abdominal part of the headshield was most probably broad (Si = 50–60 mm). The median dorsal crest is elongated along the body axis, extending beyond the posterior margin of the shield. Its total length is up to 35 mm, the length at the base of the median dorsal crest is 28–30 mm, and its height is about 10 mm. The headshield deep, its height reaching the level of the orbits of 20 mm in the holotype. The nasohypophysial opening is relatively remote from the anterior margin of the shield (Q/A = 1.5), with a large hypophysial division (in a depression) and a smaller nasal one (opening in the anterior slope of a slight elevation). The orbits are small (Ol/A = 0.26, Os/A = 0.2), oval to rounded, widely spaced (Omin/A = 0.6), with broad (up to 2 mm) circumorbital thickenings. The pineal plate is very narrow, possibly narrower than the pineal foramen, which exceeds the limits of the pineal plate posteriorly. The pineal foramen is oval in shape, laterally elongated, with a length of 0.9 mm and a width of 1.5 mm. The median dorsal field is elongated and wide, with more or less equal width (S/Sd = 9.8). Its anterior margin is straight, its lateral ones slightly convex, and its posterior one forms an obtuse angle. The lateral fields are long (G = 78–81 mm in the holotype) and wide, with their anterior ends somewhat broadened and thus relatively closer to each other, whereas the posterior ones reach the base of the cornual processes and have postero−medial angles. The distance between the posterolateral margin of the field and the lateral margin of the shield is 2 mm, the same as between posterior margin of the field and tip (anterior margin) of the pectoral sinus. In the preorbital part of the shield the lateral fields are rather narrow, so that their width (in the left lateral field of the holotype) at the anterior end, at the point of narrowing and at the level of the posteromedial angles is 9.3, 7.5 and 10.8 mm, respectively. The distance between the lateral fields and the margin of the shield does not exceed 3 mm. The ornamentation of the exoskeleton of the shield is not preserved. Only traces of small (1–1.5 mm) polygonal fields probably corresponding to tesserae with single rounded tubercles, can be seen.

Comparison.— Zenaspis dzieduszyckii sp. nov. differs from all other representatives of the genus by its smaller overall size and relatively larger orbits. Besides, it differs from Z. salweyi by the details of the outline of lateral fields, shorter cornual processes, and shape of abdominal part of the shield; both species share the shape of the median dorsal field, especially the contour of its posterior margin. In contrast to Z. metopias , the new species has a narrower shield, and relatively wider lateral fields, whereas the size of polygonal fields in the middle layer of the exoskeleton is similar in both species. Z. dzieduszyckii sp. nov. differs from Z. podolica by its smaller and posterolaterally directed cornual processes, the shape of both the abdominal part and median dorsal crest, and its considerably smaller polygonal fields in the middle layer of the exoskeleton.

Remarks.—I believe that two specimens (on a large plate of sandstone, probably from Horodnytsia) kept in the GMLSU collection belong to the new species, but at present they have no numbers to be included in the type series.

Remains of Z. dzieduszyckii sp. nov. occur in the sandstones of Horodnytsia, alongside the Pteraspidiformes Podolaspis sp. and Larnovaspis sp.

Janvier ascribed some specimens from Horodnytsia to Zenaspis cf. salweyi ( Janvier 1985 b: fig. 14). Most probably, this figured specimen belongs to Zenaspis dzieduszyckii sp. nov., judging from the shape of the shield and morphological details. In addition, it comes from the same locality and horizon as the holotype of Z. dzieduszyckii sp. nov.

Figures

Fig. 3. Zenaspis dzieduszyckii sp. nov., the holotype, SMNH BP.103, Horodnytsia, Ukraine, Dniester Series, Lower Devonian, dorsal (A, B, D) and lateral (C) views. The mould (A, C) and the impression (B) of the shield; D, the impression of the proximal part of the cornual process. Scale bars 10 mm.

Fig. 4. Zenaspis dzieduszyckii sp. nov. Reconstruction of the shield, mainly based on the holotype, SMNH BP.103, in dorsal view.

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).