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32 ADVANCES IN PALAEOICHTHYOLOGY
The branchial plates in Obruchevia were briefly described and schematically figured
by Halstead Tarlo (1965, fig. 48). The Moscow collection contains five branchial
plates (PIN # 87/9-13, O. Lebedev pers. comm., 2003), four of them fairly complete. The
plates are of either a wider (Fig. 5C, 5D, PIN 87/11) or narrower type (Fig. 5A, 5B, PIN 87/
10). In either case they are broadly triangular with a somewhat concave medial margin
and convex lateral margin. Where preserved the posterior margin is transverse and
shows a well-developed lobe towards the lateral edge of the plate. The dorsal surface is
convex and generally unornamented, as it was mostly covered by soft tissue in life. In
PIN 87/11 (Fig. 5C, a right branchial plate) some growth lines are visible on the dorsal
side of the vertically oriented lateral portion of the plate, which was presumably exposed.
The growth lines on the concave ventral side of the plate are more strongly
developed. On this side the plate shows radially arranged furrows, similar to those on
the external surface of the ventral plate (Fig. 5D). The branchial plates have strongly
downturned lateral free margins, which would have functioned as runners. One of the
Riga specimens is an incomplete right branchial plate (Fig. 4A, 4B, Pl 10/8). It includes
part of the distal edge and posterior margin, although much of the proximal part of the
plate is missing. It is 133 mm long, 70 mm wide, and 11 mm in maximum thickness and is
‘j’ shaped in cross-section. The concave ventral surface (Fig. 3A) is ornamented with
irregular rounded pits and is shiny due to the addition of pleromin to the outer part of
the spongy aspidin. A strongly developed growth line parallels the outer margin of the
plate. Laterally the plate margin is slightly concave and strongly downturned. The
margin is broken, revealing a cross section of the bone and showing that it has a central
spongy layer (2.5-5 mm thick), covered on both sides by compact pleromin (2 mm thick).
The margin is finely vertically striated, more strongly on the dorsolateral side, giving
evidence of abrasion of the “runners” against the bottom sediment.
The microstructure of the Obruchevia dorsal plate (Obruchev 1941, pl. II) does not
differ very much from that known in other psammosteids (except in the absence of
dentine tubercles). The upper part of the spongy aspidin layer is compact and solidified
by the deposition of pleromin; further down the spongiosa is coarser. The basal layer is
comparatively thin and finely laminated (in Obruchev 1941, pl. II fig. 1 the laminated
structure is not visible: in the photo the basal layer is completely black). Halstead Tarlo
also figured pleromin in Obruchevia heckeri (1964, pl. VII, fig.1, 2, 4; pl. XII, fig. 5). His
fig. 4 (pl. VII) shows pleromin in polarized light.
Discussion. Pleromin in the free margins of Obruchevia branchial plates shows the
same phenomenon known in other psammosteids in which particularly the lateral corners
of the branchial plates but also the central part of the ventral plates and the
posterior ends of ventral ridge scales show additional deposition of this hard tissue.
However, pleromin is not thought to have developed in obrucheviids as a tissue to
counteract wear only. The tissue had several different functions, to counteract wear of
the carapace and squamation, but also to reinforce the fabric of the carapace (Ørvig
1976; Mark-Kurik 1984). Ørvig established that pleromin started to appear between and
below the tubercles before the exoskeletal plates were attacked by wear. This can be
seen in several of the specimens described here in which pleromin can be seen on
surfaces that were not subjected to wear (e.g. the growth lines visible on the lateral
dorsal surface of PIN 87/11, Fig. 5C).