Osteostraci

Reconstruction of Tremataspis mammillata, copyright Nobu Tamura.

Belongs within: Craniata.

Boneshells

In the modern day, jawless fishes represent a very small part of the surviving fauna, represented only by the eel- or worm-like hagfishes and lampreys. In the earlier part of the Palaeozoic, however, they were a much more significant component of piscine diversity. Among the most diverse of the jawless vertebrates were the Osteostraci, a lineage whose name translates to English as “bony shells”.

Reconstruction of Ateleaspis tessellata, copyright Nobu Tamura.

Members of the Osteostraci are known from the Silurian and Devonian periods, with their greater diversity during the latter stretch of time. Throughout their existence, they appear to have been restricted to the continent of Euramerica (comprising modern Europe and North America), mostly inhabiting quiet marine or brackish environments such as lagoons and river deltas. Members of the Osteostraci varied greatly in size, from the fingernail-sized Kiaeraspidida to the gigantic Parameteoraspis with a head shield roughly 40 cm in width (Janvier 1996).

Model reconstruction of Cephalaspis walweyi in the Manchester Museum, copyright Akhenatenator.

The body of osteostracans had a largely triangular cross-section, broadening towards the front where the head was covered by a more or less horseshoe-shaped bony shield. This head shield might be made up of individual tesserae or fused into a single mass. The rear part of the body, including the fins, was covered with small, tessellated scales. A pair of pectoral fins was often present immediately behind the head shield. One or two dorsal fins and a large caudal fin were also present (at least in those species for which the shape of the posterior body is known), with a fin web on the posterior dorsal and caudal fins but not on the anterior dorsal or pectoral fins. The mouth and gill openings were on the underside of the head. The most distinctive feature of the Osteostraci was the presence of median and lateral depressions on the head shield, referred to as the ‘cephalic fields’. These fields were covered by numerous small, free platelets and are thought to have been sensory structures and/or electrical organs.

Assorted osteostracans, copyright Philippe Janvier. Clockwise from top left: Tauraspis, Hoelaspis, Tremataspis, Zenaspis.

In basal osteostracans such as Ateleaspis and Hemicyclaspis, the scales of the head and trunk grade gradually into those of the pectoral fins. Most osteostracans, however, may be placed in a clade Cornuata in which a more distinct division occurs between head shield and pectoral fins (Sansom 2009a). In many cornuates, a prominent spine extends from the head shield in front of each pectoral fin. A prominent rostrum extends from the front of the head shield in members of the family Boreaspididae. One family of cornuates, the Tremataspididae, is marked by a more oval-shaped head shield together with the apparent loss of the pectoral fins. Tremataspids also exhibit restriction of the cephalic fields, in some species being reduced to small ovals instead of the elongate grooves of most osteostracans.

Reconstruction of Superciliaspis gabrielsei, illustrating growth of individual osteoderms, from Hawthorn et al. (2008).

The osteostracans were presumably microphages, feeding on suspended particles or minute invertebrates. Many species exhibit little size variation among known specimens and this, together with the solid, unified structure of the head shield in many species, has lead to the suggestion that juveniles lacked an ossified exoskeleton until near adult size. Apparent ossified juveniles have been described for one species, Superciliaspis gabrielsei, by Hawthorn et al. (2008), but this is one of those taxa in which the head shield is not fused into a solid mass. Most osteostracan taxa were highly endemic in their distribution, each restricted to a single geographical province, and it has been suggested that this lack of apparent dispersal may have been a factor in their eventual displacement by their jawed relatives (Sansom 2009b).

Systematics of Osteostraci
<==Osteostraci [Cephalaspides]
    |--Sclerodus [Sclerodontidae, Sclerodontiformes]H93
    |    `--S. pustulliferus Agassiz 1839H93
    |--Endeiolepis [Endeiolepididae, Endeiolepidiformes]H93
    |    `--E. aneri Stensiö 1939H93
    |--Thyestiidae [Thyestiiformes]H93
    |    |--Fieldingaspis egertoni Lankester 1870H93
    |    `--ThyestesH93
    |         |--T. egertoni (Lankester 1870)BMM02
    |         `--T. verrucosus Eichwald 1854H93 [=Cephalaspis verrucosusP56]
    |--Ateleaspididae [Ateleaspidiformes]H93
    |    |--Witaaspis schrenkii (Pander 1856)H93
    |    |--Hemicyclaspis murchisoni (Egerton 1857)H93
    |    `--AteleaspisH93
    |         |--A. robustus (Kiaer 1911) [incl. Hirella gracilis (Kiaer 1911)]H93
    |         `--A. tesselata Traquair 1899H93
    |--Scolenaspididae [Scolenaspidiformes]H93
    |    |--Scolenaspis signata (Wängsjö 1952)H93
    |    |--Escuminaspis laticeps (Traquair 1899)H93
    |    `--AlaspisH93
    |         |--A. macrotuberculata Ørvig 1957H93
    |         `--A. rosamondae (Robertson 1937)H93
    |--KiaeraspidiformesH93
    |    |--Axinaspis [Axinaspididae]H93
    |    |    `--A. whitei Wängsjö 1952H93
    |    |--KiaeraspididaeH93
    |    |    |--Kiaeraspis auchenaspidoides Stensiö 1927H93
    |    |    `--Norselaspis glacialis Janvier 1981H93
    |    `--NectaspididaeH93
    |         |--Acrotomaspis instabilis Wängsjö 1953H93
    |         `--Gustavaspis trinodis (Wängsjö 1952)H93
    |--CephalaspidiformesH93
    |    |--ProcephalaspididaeH93
    |    |    |--Procephalaspis oeselensis (Robertson 1939)H93
    |    |    `--Auchenaspis salteri Egerton 1857H93
    |    `--CephalaspididaeH93
    |         |--Cephalaspis Agassiz 1835BMM02
    |         |    |--C. lyelli Agassiz 1835H93
    |         |    `--C. schrenkii Pander 1856P56
    |         `--MeteoraspisH93
    |              |--M. moythomasi (Wangsjö 1953)H93
    |              `--M. semicircularis (Wangsjö 1952)H93
    |--BenneviaspidiformesH93
    |    |--Benneviaspis [Benneviaspididae]H93
    |    |    |--B. anglica Stensiö 1932H93
    |    |    |--B. holtedahli Stensiö 1927H93
    |    |    `--B lankesteri Stensiö 1932H93
    |    `--BoreaspididaeH93
    |         |--Dicranaspis curtirostris (Wängsjö 1952)H93
    |         |--Spatulaspis costata (Wängsjö 1952)H93
    |         `--BoreaspisH93
    |              |--B. batoides Wängsjö 1952H93
    |              |--B. ceratops Wängsjö 1953H93
    |              `--B. ginsburgi Janvier 1977H93
    `--TremataspidiformesH93
         |--Oeselaspis [Oeselaspididae]H93
         |    `--O. pustulata (Patten 1931)H93
         |--Didymaspis Lankester 1867BMM02 [DidymaspididaeH93]
         |    `--D. grindrodi Lankester 1867H93
         |--Timanaspis [Timanaspididae]H93
         |    `--T. kossovoii Obruchev 1962H93
         |--Tyriaspis [Tyriaspididae]H93
         |    `--T. whitei Heintz 1967H93
         |--DatmuthiidaeH93
         |    |--Dartmuthia gemmifera Patten 1931H93
         |    `--Saaremaaspis mickwitzi (Rohon 1892)H93
         |--TannuaspididaeH93
         |    |--Tuvaspis margaritae Obruchev 1956H93
         |    `--Tannuaspis levenkoi Obruchev 1956H93
         `--Tremataspis [Tremataspididae]H93
              |--T. mammillata Patten 1931H93
              |--T. milleri Patten 1931H93
              |--T. rohoni Robertson 1938H93
              `--T. schmidtii Rohon 1892H93

*Type species of generic name indicated

References

[BMM02] Blom, H., T. Märss & C. G. Miller. 2002. Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere. Transactions of the Royal Society of Edinburgh: Earth Sciences 92: 263–323.

[H93] Halstead, L. B. 1993. Agnatha. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 573–581. Chapman & Hall: London.

Hawthorn, J. R., M. V. H. Wilson & A. B. Falkenberg. 2008. Development of the dermoskeleton in Superciliaspis gabrielsei (Agnatha: Osteostraci). Journal of Vertebrate Paleontology 28 (4): 951–960.

Janvier, P. 1996. Early Vertebrates. Clarendon Press: Oxford.

[P56] Pander, C. H. 1856. Monographie der Fossilen Fische des Silurischen Systems der Russisch-Baltischen Gouvernements. Buchdruckerei der Kaiserlichen Akademie der Wissenschaften: St Petersburg.

Sansom, R. S. 2009a. Phylogeny, classification and character polarity of the Osteostraci (Vertebrata). Journal of Systematic Palaeontology 7 (1): 95–115.

Sansom, R. S. 2009b. Endemicity and palaeobiogeography of the Osteostraci and Galeaspida: a test of scenarios of gnathostome evolution. Palaeontology 52 (6): 1257–1273.

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