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42 ADVANCES IN PALAEOICHTHYOLOGY
this situation it is surprising that none of the dorsal plates of Obruchevia show any
indication of the presence of the sensory canal system.
Halstead Tarlo (1964) placed Obruchevia together with Traquairosteus in the Family
Obrucheviidae, an arrangement that we agree with here. It is also clear that the new
genus Perscheia should be included within this family. Higher level phylogenetic relationships
are poorly understood, as no attempt has been made to assess relationships
within the Psammosteiformes since the study by Halstead Tarlo (1964). This study was
flawed somewhat by the inclusion of a number of early species that are now not considered
to be psammosteids, such as Tesseraspis, Corvaspis, and Psephaspis. Halstead
Tarlo used a variety of features such as the type of ornamentation, and the shape of the
branchial and ventral plates to show that advanced psammosteids derive from
Guerichosteus, from the Emsian of central Poland, which subsequently gave rise to
two lineages. In one the posterior notch in the ventral plate was deepened and widened,
and the branchial plate became broad and triangular (Pycnolepis, Pycnosteus). In the
other lineage the posterior ventral notch was progressively eliminated, and the branchial
plates became greatly shortened and broadened, forming spine-like plates in some forms
(Psammosteus). Obruchevia and Traquairosteus were very incompletely known at the
time Halstead Tarlo attempted his analysis of phylogeny; however, he placed them in a
tentative relationship with psammolepids based on the fact that the branchial plates
were strongly downturned, a feature that is present to a much lesser degree in the
psammolepids, and despite the fact that the branchial plates of Obruchevia are long
and narrow, not short and broad. The new information presented here shows that both
Obruchevia and Perscheia had a ventral plate in which a well-developed posterior
notch was present, therefore indicating that the obrucheviids are more likely to be connected
to the group including Pycnolepis, Pycnosteus, and Tartuosteus in which this is
a major feature. However, until a modern phylogenetic analysis of the Psammosteiformes
has been carried out, a study that is beyond the scope of this paper, no firm conclusion
can be reached as to the relationship of the obrucheviids.
Acknowledgments. - We would like to thank Hans Arne Nakrem of the Geologisk Museum, Oslo
and Ervīns Lukševičs of the Natural History Museum, Riga for arranging the loan of specimens
in their collections, and Oleg Lebedev, Moscow, for photographs of the branchial plates of
Obruchevia. D.K.E. acknowledges the receipt of a COBASE grant that supported reciprocal
visits with E. M.-K., and support from the Academy of Natural Sciences, Philadelphia for visits
to the collections. E. M.-K. is grateful to the Estonian Science Foundation (Grant No.5275) and
IGCP Project 491 for financial help. This work is also supported by the following grants to
E.B.D.: National Science Foundation Grant No. 0207721 and National Geographic Society
Committee for Research and Exploration Grant No. 7223-02. Photographs of the Latvian
specimens and the Perscheia thin sections were made by G. Baranov, Tallinn. We particularly
thank S. Swift, Flagstaff, who photographed the Canadian material and assembled the photographic
figures, and Drs. A. Blieck (Lille), R. Thorsteinsson (Calgary), and M. Wilson (Edmonton) who
provided constructive criticism of the manuscript.