THE HETEROSTRACAN FISHES

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Abstract
Summary: 1. The heterostracan fishes were jawless, microphagous, devoid of paired fins and encased in a bony armour. The classification is based on the arrangement of the plates of the carapace — the primitive forms possessed a tessellated armour made up of numerous small polygonal plates. Several genera had a tessellated carapace ventrally but large discrete plates dorsally. All further groups are characterized by distinct patterns of plates which remain constant within each order. The proportions in some species suggest sexual dimorphism.2. A study of the superficial ornamentation reveals patterns of growth. From the primitive tessellated condition different evolutionary lines can be followed leading to the fusion of these small elements into large discrete plates. Among the latest group of heterostracans there was a secondary redevelopment of tesserae.3. Impressions on the inner surface of the plates of the carapace enable certain aspects of the internal anatomy to be reconstructed. The nasal sacs were double, the acousticolateralis system was primitive. The brain was little more than the nerve cord swollen in three places, there was no cranial flexure. Impressions of two pre‐otic somites were present, indicating that they had not migrated to form the extrinsic eye muscles. The branchial arches appear to have been of gnathostome type and in some genera spiracles were formed as an adaptation to a benthonic mode of life.4. Microscopic sections of the armour demonstrate the existence of four tissues: aspidin, dentine, enameloid and calcified cartilage. Aspidin was originally acellular but later became cellular; the organic matrix was first organized like dentine, but subsequently like bone. Furthermore aspidin was capable of remodelling. Dentine appears to have acted as a skin‐like tissue and was capable of regeneration. The significance of enameloid and calcified cartilage in the dermal armour is not well understood.5. Until the end of the Silurian the heterostracans inhabited marine waters but from the beginning of the Devonian they colonized the fresh‐water lakes and rivers of the Old Red Continent. One major group flourished in a large embayment on the edge of the Tungussian land mass. When the stratigraphical range and geographical distribution of the heterostracans is listed, evolutionary centres can be recognized and also migration routes: during the Upper Silurian the cyathaspids in the Canadian Arctic, the Lower Devonian pteraspids in Eastern Europe, the later Lower Devonian amphiaspids in north‐west Siberia (Tungussian Realm), and the Middle and Upper Devonian psammosteids in the Baltic province. Periods of migration from the Baltic to Scotland, the Timan, Ellesmereland, the Urals and Donbas have been documented.For the illustrations the author is indebted to Miss Jennifer Middleton and Mr John Smith.

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