Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia
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Obruchev (1940) published the description of the Luka section in his paper on the
Devonian river delta on the Lovat´ River. In 1958 about 10 m of the section was documented
by Elga Mark. The Luka locality (Fig. 2, upper left) is an extensive outcrop,
consisting of rather soft terrigenous rocks of the Frasnian Prilovat´ Formation (Fig. 2,
lower left). It consists of purple, brown, red or greenish clay, and white, yellow or
brownish siltstone and sandstone (Fig. 2, right). Fossil fishes were discovered in a clay
bed (IV fish horizon) and three siltstone and/or sandstone beds in the lower part of the
section, the II fish horizon being the richest. In both these horizons the same fishes
occurred: Obruchevia heckeri, Bothriolepis maxima and the sarcopterygian
Platycephalichthys bishoffi. The II horizon yielded an additional psammosteid,
Psammosteus falcatus. According to Esin et al. (2000) the above mostly large or very
large forms characterize the Snezha-Prilovat´ (or Katleši-Pamušis) interval of the Upper
Frasnian of the NW of the East European Platform (Fig. 2, lower left). Psammosteus
falcatus and Bothriolepis maxima are Late Frasnian index fossils (Lukševics 2001). The
marine basin during the Pamušis (Ogre)-Prilovat´ time was shallow and dominated by
clastic sedimentation. The main source area of rich detrital material was situated to the
north and northwest in the area occupied by the modern Baltic Shield (Sorokin 1978, p.
236, fig. 50).
Obruchevia heckeri (Obruchev, 1936)
(Figs. 3-5)
Diagnosis. (Modified after Halstead Tarlo, 1965) Dorsal plate thick with wide, shallow
re-entrant angle at anterior margin, and marked notch in posterior part of each lateral
margin. Ornamentation of smooth, radial grooves and shallow, circular pits. Histological
structure of spongy aspidin reinforced by pleromic dentine towards external surface.
Branchial plates thin, long and narrow, and bent at right angles so that the lateral
part is oriented vertically. Ventral plate thick with a long, posterior median notch.
Material. Six branchial plates (PIN 87/9-13; Pl 10/8); three dorsal plate fragments (Pl 10/
9, 10/11, 10/12); ventral plate fragment (Pl 10/10).
Locality. Lovat´ River, near Peryesy and Luka, Novgorod, Region, Russia. Prilovat´
Formation, Upper Devonian, Frasnian.
Description. The description of Obruchevia (Aspidosteus) was based on its dorsal plates
(Obruchev 1941, pl. I), the other plates only becoming known later. The dorsal plates are
very large, notably thick, and cardiform. The length of the holotype (housed in the
CNIGR Museum, St. Petersburg, coll. # 1/4680) is 510 mm and the maximum width is 480
mm. The ornament of the external surface of the plates varies: it consists either of radial
furrows (Obruchev 1941; pl. I, fig. 1) or of pits or both (Obruchev 1941; pl. I, fig. 2; pl. II,
fig. 3). Two fragments from the Riga collection show variations of the ornament (Fig. 3).
One specimen (Pl 10/12, Fig. 3B) has strongly developed furrows, but in the other (Pl 10/
11, Fig. 3A) the furrows are wider, wavy, and more delicate. The dorsal plates appear to
have grown by the addition of lateral flanges that developed from the lower surface of
the margins. In Pl 10/12 four such flanges can be counted (Fig. 3B). The visceral surface
of the fragments has marginal zones, lacking the basal layer, that are 30-50 mm wide.