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

20 ADVANCES IN PALAEOICHTHYOLOGY
surface demonstrates a complicated pattern, composed of pits of different sizes (1-5
microns) and diversely directed grooves. In the present case the enameloid and (partly)
mesodentine tissue are missing as a result of acid-treatment (or/and abrasion), and the
odontocyte cavities and narrow grooves of canaliculi branching off from them become
visible at the horizontal section of the superficial layer (Afanassieva 2000). M.M. Smith
has also described a similar type of tissue in Griphognathus whitei Miles (Smith 1977:
pl. 2, fig. 28-31; pl. 3, fig. 32; the surface of the tooth ridge after acid-treatment). Thus in
osteostracans the characters of the exoskeleton, including fine sculpture, can be used
for taxonomic purposes, and are necessary for the identification of fragmentary material;
however we must take into consideration that an abrasion or acid-etching can cause the
appearance of unusual microrelief on the exoskeletal surface.
The information presented above leads to the conclusion that the sculpture on the
osteostracan exoskeleton, both macrosculpture and microsculpture, reflects processes
of the probable mode of ossification of the osteostracan hard cover. On the other hand,
various types of microsculpture (microtubercles, fine ribs or stripes, microapertures) in
general are related to the functional peculiarities responsible for the animals’ adaptation
to the ambient environment, and were necessary for the implementation of metabolic
processes in different layers (and between the layers) of covering tissues of early
vertebrates.
Acknowledgements. - I would like to thank Dr. Valentina N. Karatajute-Talimaa, who kindly
presented the part of Severnaya Zemlya material for study, for her help. I should also like to
thank Mr. Lev T. Protasevich for his constant assistance with the SEM, and Mrs. Marianna K.
Emelianova for assistance with the computer graphic programs. The work was partly supported
by UNESCO IGCP Project 491.
References
Afanassieva O.B. 1985. External skeletal features of the Thyestinae (Agnatha). Paleontological
Journal, 4 (6): 81-87.
Afanassieva O.B. 1991. Tsefalaspidy Sovetskogo Soyuza (Agnatha) (The Cephalaspids (Agnatha)
of the Soviet Union). Trudy Paleontologicheskogo Instituta Akademii Nauk, 248, Moscow: 1-
144 p. [In Russian with English summary]
Afanassieva O.B. 1999. The exoskeleton of Ungulaspis and Ateleaspis (Osteostraci, Agnatha)
from the Lower Devonian of Severnaya Zemlya, Russia. Acta Geologica Polonica, 49 (2):
119-123.
Afanassieva O.B. 2000. New osteostracans from the Silurian of Severnaya Zemlya Archipelago
(Russia) and some problems relating to the parataxonomy of armored agnathans.
Paleontological Journal, 34, Suppl. 2, S138-S146.
Afanassieva O.B. 2002. The exoskeleton of Thyestes verrucosus (Osteostraci, Agnatha) from
the Silurian of Saaremaa Island: a mode of ossification. – In: Satkunas J., Lazauskiene J.
(eds.). The Fifth Baltic Stratigraphical Conference: Basin stratigraphy – modern methods and
problems: Vilnius, 9-10.
Afanassieva O.B., Karatajute-Talimaa V.N. 1998. New osteostracans (Agnatha) from the Silurian
and Lower Devonian of the Severnaya Zemlya Archipelago (Russia). Paleontological Journal,
32 (6): 605-610.
Afanassieva O.B., Märss T. 1999. New data on osteostracan microremains from the Silurian of
Severnaya Zemlya, Russia. – In: Lukševics E., Stinkulis G., Wilson M.V.H. (eds.). Lower-
Middle Palaeozoic Events Across the Circum-Arctic. Ichthyolith Issues Special Publication, 5:
4-5.