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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The inner face of the specimen is smooth and shiny, and at least three growth lines,
delineated by either shallow grooves or thickening of the bone, run parallel to the
natural edge (Fig. 6B). The surface is formed of a dense laminar basal layer of aspidin
that is up to 2 mm thick and tends to flake away from the spongy overlying layers. The
surface shows small foramina related to narrow, shallow, radial grooves and large smooth
blisters that are elongated radially. This specimen is assumed to be part of the median
dorsal plate, probably the median posterior part based on the presence of the medial
canals, the orientation of the ornament, and the shape of the natural margin.
CMN–NUFV102 (Figs. 6C, 7B) is a roughly pentagonal, concavo-convex fragment
of plate in which all the edges are broken. Its maximum dimensions are 134 mm x 112 mm
and the thickness varies from 15 mm to 10 mm. The outer surface of the plate is
ornamented with large elongated blisters, apparently radially oriented. Several of these
are ring shaped with a depressed center. Four sections of the sensory canal system can
be seen, forming two parallel pairs almost at right angles to each other (Fig. 7B). One
pair consists of a short section within the plate and a section along the right-hand edge.
They run at an angle to the trend of the ridges, the canal on the plate edge does so
almost at right angles to the ridges. The second pair consists of a canal that runs along
the longest edge, almost at right angles to the first pair and a short section on the
opposite edge. The longest section of canal is almost parallel to the trend of the ornament
suggesting that it is part of the medial dorsal canal; the short section parallel to it
may be part of the lateral dorsal canal, while the other two sections of canals are
probably transverse commissures. The internal surface is concave, smooth, and shiny
with a thick, dense, laminar, basal layer and no obvious growth lines.
CMN–NUFV103 (Figs. 6D, 7D) is an irregular fragment with maximum dimensions of
96 mm and 161 mm. It varies in thickness from 12 mm to 7 mm. The external surface is
covered by large blisters and short ridges arranged roughly parallel to the long axis of
the plate fragment. At about the midpoint of the specimen a sensory canal runs almost
at right angles to the trend of the ridges (Fig. 7D). Six short lateral branches separate
from the main canal and project between the surface ridges. Towards one end a short
section of canal can be see running along one edge and parallel to the radial ornament.
This canal is probably one of the transverse commissures while the longer section is
part of one of the lateral or medial dorsal canals. The internal surface is weathered and
the laminar basal layer is missing, exposing the spongy aspidin above it.
CMN-NUFV104 (Figs. 6E-G, 7C) is an irregular and thick piece of plate, 85 mm x 155
mm and up to 28 mm thick. It has broken margins on all edges except for one small
section 55 mm long on one margin where there is a natural rounded edge to a shallow
embayment. The curved margin adjacent to the embayment is probably close to the
plate margin as it thins rapidly. The outer surface is concavo-convex with a suggestion
of growth lines parallel to the curved margin (Fig. 6F). A single sensory canal is present
running across the middle of the fragment for two-thirds of its width and then branching
at right angles (Fig. 7C). One branch runs parallel to the embayment to the broken
margin in one direction and for about 50 mm parallel to the plate margin in the other
direction before petering out. Presumably this part of the sensory canal is the lateral
dorsal canal. Although damaged the surface shows the same pattern of coarse blisters
present on the other plates.