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D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia
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two most important in reconstructing the plate are the holotype CMN-NUFV101 (Figs.
6A, 6B, 7A), which is probably from the posterior margin, and CMN-NUFV104 (Figs. 6E-
G, 7C), which is from the posterolateral margin. CMN-NUFV101 can be placed in the
midline based on the presence of symmetrically placed longitudinal sensory canals that
are probably the medial dorsal canals. They run parallel to the radial ornament and are
joined by a transverse commissure that runs parallel to the growth lines. CMN-NUFV104
is a fragment of the lateral part of the plate and contains a shallow embayment that is
probably equivalent to the lateral notch present in Obruchevia. It is suggested here
that this fragment comes from the left side of the plate based on the fact that: (1), the
sensory canal thought to be the lateral dorsal canal continues in one direction (anterior)
while petering out in the other (posterior); (2), the growth lines suggest a very convex
margin to the plate beyond the lateral notch and if this was the anterior part of the
fragment then it would suggest a very anterior position for the notch which is not the
case in Obruchevia; and (3), the diagonal ridges across the embayment, if related in
some way to the branchial duct, are more likely to be oriented posteriorly.
Specimens CMN-NUFV102 (Figs. 6C, 7B) and CMN-NUFV103 (Figs. 6D, 7D)
have no natural edges and can only be positioned on the dorsal plate based on their
ornament and the sensory canals present. CMN-NUFV103 probably extends from the
midline towards the lateral margin and contains a section of the lateral dorsal canal.
CMN-NUFV102 may represent a more posterior part of the plate close to the posterolateral
notch and contains parts of the medial and lateral dorsal canals. Based on these
interpretations the dorsal plate might be as much as 600 mm long and 550 mm wide,
which is close to the size of the dorsal plate of Obruchevia.
The ventral plate is represented only by CMN-NUFV105 (Fig. 8), which clearly
represents the left posterior part of the plate including the margin of the posterior median
notch. It is difficult to estimate the overall size of the plate from this one fragment,
but based on other species that have notched ventral median plates, it could have been
as much as 450 mm long and 350 mm wide (Fig. 10B).
Remarks on the phylogenetic relationships of obrucheviids
The presence of a well-developed and apparently connected canal system on the dorsal
plate of Perscheia is an unusual feature, as the psammosteid canal system is generally
poorly known (Obruchev and Mark-Kurik 1968). Where it is known it normally consists
of a pair of medial dorsal canals and one to three pairs of transverse commissures,
all of which are present below the surface layer of dentine tubercles. The presence of
this system as open grooves on the surface in Perscheia is also unusual, although not
unexpected in a species in which the surface covering of dentine tubercles is missing.
An intermediate stage can be seen in Traquairosteus pustulatus, a Frasnian species
from Scotland in which the outer surface of aspidin is thrown up into conical mounds,
each surmounted by a small crenulated dentine tubercle. The dentine tubercles are thus
very sparse. In the holotype (BM P.8297; Halstead Tarlo 1965, pl. XVII, fig. 1) two
canals can be seen as open grooves on the surface, indicating that as the dentine tubercles
were reduced, the canals became exposed and were only covered by dermis. Given