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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.
Olga Afanassieva. Microrelief on the exoskeleton of early osteostracans 21 Besnosov P. 2003. The crown ultrasculpture of Acanthodes type scales in some Devonian- Carboniferous acanthodians. – In: Schultze H.-P., Lukševics E., Unwin D. (eds.). The Gross Symposium 2: Advances in Palaeoichthyology. Ichthyolith Issues Special Publication, 7: 10. Denison R.H. 1951a. Evolution and classification of the Osteostraci. Fieldiana: Geology, 11 (3): 155–196. Denison R.H. 1951b. The exoskeleton of early Osteostraci. Fieldiana: Geology, 11 (4): 197- 218. Deryck C., Chancogne-Weber C. 1995. Histological discovery on acanthodians scales from the Famennian of Belgium. Geobios, M.S., 19: 31-34. Märss T. 2002. Ultrasculpture on the exoskeleton of early agnathans and fishes. – In: Satkunas J., Lazauskiene J. (eds.) The Fifth Baltic Stratigraphical Conference: Basin stratigraphy – modern methods and problems: Vilnius, 119-120. Robertson G.M. 1938. The Tremataspidae. American Journal of Science, 5 (35): 172-206, 273- 295. Schultze H.-P. 1977. Ausgangsform und Entwicklung der rhombischen Schuppen der Osteichthyes (Pisces). Paläontologische Zeitschrift, 51: 152-168. Smith M.M. 1977. The microstructure of the dentition and dermal ornament of three dipnoans from the Devonian of Western Australia: a contribution towards dipnoan interrelations, and morphogenesis, growth and adaptation of the skeletal tissues. Philosophical Transactions of the Royal Society of London, Series B, 281: 29-72. Stensiö E. 1927. The Downtonian and Devonian vertebrates of Spitsbergen. 1. Family Cephalaspidae. Skrifter om Svalbard og Ishavet, 12: 1-391. Stensiö E. 1932. The Cephalaspids of Great Britain. The British Museum (Natural History), London. 220 p. Dažu agrīno kaulvairodžu ārējā skeleta mikroreljefs: tā nozīmīguma sākotnējā analīze OLGA B. AFANASJEVA Kaulvairodžu (Osteostraci) ārējais skelets pētīts skanējošā elektronmikroskopā, izmantojot atseviškas mikroskopiskas atliekas un nelielus fragmentus no veseliem galvkrūšu vairogiem. Izmantots materiāls no Severnaja Zemļas arhipelāga (Krievija) un Sāmsalas (Igaunija) silūra un apakšējā devona nogulumiem. Pirmo reizi ir aprakstīti epidermālo šūnu nospiedumi kaulvairodžu ārējā skeleta virsmā. Secināts, ka kaulvairodžu ārējā skeleta ornamentējums, gan makro-, gan mikroskulptūra, iespējams, ataino kaulvairodžu cietā apvalka pārkaulošanās procesu īpatnējo veidu. No otras puses, dažādu mikroskulptūras paveidu daudzveidība (mikropauguri, smalkas ribas vai švīkas, mikroskopiskas atveres un poras) kopumā ir saistīta ar tām organismu funkcionālām īpatnībām, kas nosaka dzīvnieku pielāgošanos mainīgiem vides apstākļiem, un tā ir nepieciešama vielmainas procesu nodrošināšanai agrīno mugurkaulnieku ārējā apvalka dažādos audos.
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