The Heterostracan Fishes

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.

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

“…Obruchev (1941Obruchev ( , 1964 later resurrected Rohon's (1893) hypothesis, interpreting the tissue as cellular and the thread-like cavities as cell lacunae or canaliculi. This interpretation was supported by Halstead (1969Halstead ( , 1973Halstead ( , 1974Halstead Tarlo, 1963, b, 1965 and Novitskaya (1966). The opposing view, that the elongate spaces represent voids of unmineralised collagen fibres (most frequently identified as Sharpey's fibres), was maintained by Gross (1935), Bystrow (1955), Ørvig (1951, 1965, 1967, 1968), and Moss (1968b).…”

“…Inevitably, confusion over the identity of the unmineralised aspidin spaces spilled into the wider debate concerning the primitiveness of cellular versus acellular bone. Within this context, aspidin has variously been interpreted as a primitive acellular type of bone (Denison, 1963;Halstead, 1987), a secondarily acellular bone (Stensi€ o, 1927;Ørvig, 1951, a primitive type of cellular bone (Obruchev, 1941(Obruchev, , 1964Halstead Tarlo, 1963Halstead, 1969), a type of dentine (Urist, 1962(Urist, , 1964aDenison, 1967) and an intermediate grade between dentine and bone (Halstead, 1973). Urist (1962Urist ( , 1964a argued that aspidin could not be a type of bone on the grounds that it did not undergo remodelling, yet tentative evidence of resorption was reported in the psammosteids in the form of scalloping of concentric aspidin lamellae (Gross, 1930;Halstead Tarlo, 1963.…”

“…On the one hand, it appears different taxa exhibit markedly different adult dermal skeletal architectures. For example, in some heterostracans, the superficial layer shows evidence of addition of tubercles (Gross, 1961), while in others it does not (e.g., Denison, 1973;Halstead, 1973). In some taxa, the middle layer can be subdivided into two histologically distinct layers (Lindstr€ om, 1895;Kiaer, 1932), whereas in others it appears homogenous (Gross, 1930).…”

“…On the other hand, differing interpretations regarding the microstructure of discrete tissues have confused, rather than informed, understanding of heterostracan histology. For example, the published literature is equivocal on the presence of enameloid (Ørvig, 1967(Ørvig, vs. Halstead, 1973 and whether the middle layer is a type of bone (Ørvig, 1951;Halstead, 1987), dentine (Urist, 1962;Denison, 1967) or an intermediate tissue grade (Halstead, 1973).…”

“…The histological structure of these tissues is routinely preserved in exquisite detail and has been the subject of study for as long as the fossils have been known (e.g., Agassiz, 1835). However, despite these numerous studies, the nature of the primitive dermal skeleton remains unclear, principally because early attempts to synthesise these data were derived primarily from a strict stratigraphic reading of fossil occurrences (Halstead, 1969(Halstead, , 1973Ørvig, 1967). More recently, cladistics has provided a framework in which to integrate these disparate data, yet current rudimentary knowledge of the diversity and evolution of the dermal skeleton in heterostracans precludes understanding the plesiomorphic vertebrate dermal skeleton.…”