Arthrodires range from a few centimeters to several meters in length. These fishes were very important inhabitants of the lake system and the rivers. They are all “armoured” with big overlapping bony plates, often with many tiny tubercles on the outside.

The head and body were connected with a joint like structure. This probably made movement of the head more independent from the body. A similar structure can be found today in recent armoured (actinopterigii) catfish. This is a case of a structure evolving independently in completely different fish groups.

Only the front third of an individual arthrodire was heavily armored. Sometimes the rest of the body was covered with very tiny scales, which were probably not bony since they fossilize leaving only a faint stain. Only some of the antiarchi species found on Orkney have scales that preserve well.

It is not possible to generalize about placoderm feeding mechanisms because they differ from species to species.  However, most of the placoderms found on Orkney have well developed jaws often with tiny teeth like structures. They may have fed on tiny fish or invertebrates. It is also suggested that they were ambush predators swallowing their prey whole.

The dermal plates are normally constructed in three layers, the basal layer is made up of laminar bone, the middle layer of trabecular bone with vascular spaces and the superficial layer that has the ornamentation. In thin sections you often see tubercles growing on top of each other. Tubercles can form as part of growth process of a dermal plate, or part of a repair process. It does not reflect well the yearly growth is generally not well preserved. Some of the yearly (seasonal) growth can be seen at the edges of the plate, illustrated here with an intro lateral plate of Homostius.

Median ventral plate Homostius with close up of tubercles

Photomicrographs of thin sections of tubercles of Homosteus. Note the growth of tubercles on top of each other.

Photomicrograph of Intro lateral plate of Homostius, transverse section, count the growth zones!

Nuchal and paranuchal plates, close ups and photomicrographs of tubercles.

Photomicrograhs of dermal plates and close up.

Photomicrograph of para nuchal plate and close up.

Para nuchal plate and close up of tubercles.

Photomicrographs of tubercles.

Posterior ventro lateral plate and close up of tubercles.

Photomicrographs of thin sections of dermal plates of Millerosteus

Head and close up of head.

Photomicrograph of thin sections of a pile of dermal plates of Watsonosteus

• Desmond, A. J. 1974. On the coccosteid arthrodire Millerosteus minor. Zoological
• Denison, R. 1978. Placodermi. Handbook of Paleoichthyology. Volume 2. Gustav Fischer Verlag, Stuttgart. 128 pp.
• Gross, W. 1931. Asterolepis ornata Und das Antiarchi-Problem. Palaeontographica, 75:1-61.
• Gross, W. 1940. Acanthodier und Placodermen aus den Heterostius-Schichten Estlands und Lettlands. Publications of the Geological Institution of the University of Tartu, 60:1-88.
• Hemmings, S. K. 1978. The Old Red Sandstone antiarchs of Scotland: Pterichthyodes and Microbrachius. Palaeontographical Society [Monographs]. 64 pp.
• Heintz, A. 1934. Revision of the Estonian Arthrodira, Part 1, Family Homostiidae Jaekel. Publications of the Geological Institution of the University of Tartu, 38, 1-114.
• Heintz, A. 1938. Notes on arthrodires. Norsk Geologisk Tidsskrift, 18, 1-27.
• Hemmings, S. K. and Rostron, J. 1972. A multivariate analysis of the measurements of the Scottish Old Red Sandstone antiarchs fish genus Pterichthyodes Biological Journal of the Linnean Society, 4, 15-28.
• Hemmings, S. K. 1978. The Old Red Sandstone antiarchs of Scotland: Pterichthyodes and Microbrachius. Soc. [Monogr.], 1-64, figs. 1-36, pls. 1-9.
•    Heintz, A. 1968. The spinel plate in Homosteus and Dunkleosteus, 145-151, figs. 1-2. Nobel Symp. 4, ed. Orvig, T. Interscience, London. Pp. 539.
• Long, J.A., Mark-Kurik, E., Johanson, Z., Lee, M.S.Y., Young, G.C., Zhu, M., Ahlberg, P.E., Newman, M., Jones, R., den Blaauwen, J., Choo, B. and Trinajstic, K. 2015. Copulation in antiarch placoderms and the origin of gnathostome internal fertilization. Nature, 517, 196–199.
• Miles, R.S., and Westoll, T. S. 1963. Two new genera of Coccosteid Arthrodira from the Middle Old Red Sandstone of Scotland and their stratigraphical distribution. Transactions of the Royal Society of Edinburgh, 65:179–210.
• Miles, R. S. and Westoll, T. S. 1968. The placoderms fish Coccosteus cuspidatus Miller ex Agassiz from the Middle Old Red Sandstone of Scotland. Part 1. Descriptive morphology. Transaction of the Royal Society of Scotland, 67, 373-476.
• Newman, M.J., Blaauwen, J.L. den and Tuuling, T. 2017. Middle Devonian coccosteid (Arthrodira, Placodermi) biostratigraphy of Scotland and Estonia. Scottish Journal of Geology, 53(2):63–
• Newman, M.J. & den Blaauwen, J.L. 2018. A Redescription of the endemic antiarch placoderm Asterolepis thule from the Middle Devonian (Givetian) of Shetland and its biostratigraphical horizon. Scottish Journal of Geology, 54:69-75.
• Newman, M. J., den Blaauwen, J.L. & Leather, D. 2019 The antiarch fish Asterolepis orcadensis from the Scottish Middle Devonian. Palaeontologica Electronica, 2.37, 1-24.
• Mark-Kurik, E. 1992. The inferognathal in the Middle Devonian arthrodire Homosteus. Lethaia, 25, 173-178, figs. 1-6.
• Mark-Kurik, E. 1993. Remarks on the trunk-shield structure in Homosteus (Placodermi). Estonian Acad. Science. Geol., 42, 176-180, 1 fig., pls. I, II.
• Mark-Kurik, E., Newman, M. J., Toom, U. & den Blaauwen, J.L. 2018 A new species of the antiarch Microbrachius from the Middle Devonian (Givetian) of Belarus. Estonian Journal of Earth Science, 67, 3-13. [Research article].
• Miles, R. S. 1968. Features of placoderm diversification and the evolution of the Arthrodire feeding mechanism. Roy. Soc. Edinburgh, 68, 124-165, figs. 1-10.
• Miles, R. S. and Westoll, T. S. 1963. Two new genera of coccosteid Arthrodira from the Middle Old Red Sandstone of Scotland, and their stratigraphical distribution. Roy. Soc. Edinburgh, LXV, 179-208, figs. 1-11, pls. I-III.
• Miles, R. S. and Westoll, T. S. 1968. The placoderm fish Coccosteus cuspidatus Miller ex Agassiz from the Middle Old Red Sandstone of Scotland part I. Descriptive morphology. Roy. Soc. Edinburgh, 67, 373-476, figs. 1-51, pls. I-XII.
• Pander, C.H. 1857. Über die Placodermen des Devonischen Systems. Buchdruckerei der Kaiserlichen Akademie der Wissenschaften, St Petersburg.
• Stensio, E. A. 1963. Anatomical studies on the arthrodiran head. Part I. Kungliga Svenska Vetenskapsakademiens Handlingar (4) 9(2):419 pp
• Stensio, E. A. 1959. On the pectoral fin and shoulder girdle of the arthrodires. Kungliga Svenska Vetenskapsakademiens Handlinger, 8, 1-229.
• Traquair, R. H. 1894-1914. A monograph of the fossil fishes in the Old Red Sandstone of Britain, Part 2, The Asterolepidae. Palaeontographical Society [Monographs]. 63-130.
• Traquair, R. H. 1889. Homosteus Asmuss, compared with Coccosteus, Agassiz. Mag., (3) VI, 1-8, pl.
• Trinajstic, , Boisvert, C., Long, J., Maksimenko, A., Johanson, Z. Pelvic and reproductive structures in placoderms(stem gnathostomes). Biol. Rev.(2015),90, pp. 467–50
• Traquair, R. 1888a. Notes on the nomenclature of the fishes of the Old Red Sandstone of Great Britain. Geological Magazine, Decade 3, 11, 507-517.
• Traquair, R. H. 1890. On the structure of Coccosteus decipiens Proc. Roy. Phys. Soc. Edin., X, pl. X.
• Traquair, R. H. 1890. On the supposed pectoral limb in Coccosteus decipiens. Mag. (3) 7, 235.
• Traquair, R. 1890. On the fossils found at Achanarras Quarry, Caithness. Annuls and Magazine of Natural History, Series 6, 6, 479-486.
• Traquair, R. H. 1908. On fossil fish remains collected by J. S. Flett, M. A., D. sc., from the Old Red Sandstone of Shetland. Roy. Soc. Edinburgh, XLVI, 321-329, 3 figs., pls. I, II.
• Traquair, R. H. 1894-1904 Part II- The Asterolepidae, in : A monograph of the fishes of the Old Red Sandstone of Britain. Printed for the Palaeontographical society.
• Watson, D. M. 1934. The interpretation of arthrodires. Proceedings of the Zoological Society of London, 3, 437-464.
• Watson, D. M. S. 1908. Coccosteus minor, Hugh Miller, in the Old Red Sandstone of Dalcross, Inverness-shire. Mag., 5 (9), 431.
• Watson, D. M. S. 1961. Some additions to our knowledge of antiarchs. Palaeontology, 4, 210-220, figs. 1-3.
• Woodward, A. S. 1891. Note on some dermal plates of Homosteus from the Old Red Sandstone of Caithness. Zool. Soc. London, 198-201, figs. 1-4.