A partly preserved squamation of Thelodus laevis (Pander) from the Himmiste outcrop (Saaremaa Island, Estonia) is described. Most of the trunk with a pectoral fin fragment and three fragments of posterior fins are present, while part of the head and the caudal fin are absent. Ontogenetic development of the scale cover of T. laevis is explained by comparing it with the squamation of Lanarkia horrida Traquair, Loganellia scotica (Traquair) and a modern fish. It is supposed that newly forming trunk scales of early juvenile Thelodus were smooth and rhomboidal. When the individual grew, ridged scales were added between smooth ones in irregular rows in the posterior part of the trunk. Adult individuals were covered with species-specific scales with particular morphology and sculpture of the crown. Co-occurrence of smooth and ridged scales on the trunk in the specimen studied corresponds to a young stage of ontogeny.
Aaloe, A. 1963. On the stratigraphic position and conditions of sedimentation containing Tremataspis mammillata. ENSV Teaduste Akadeemia Geoloogia Instituudi Uurimused, 13, 83–90 [in Russian, with Estonian and English summaries].
Blom, H. 1999. Vertebrate remains from Upper Silurian–Lower Devonian beds of Hall Land, North Greenland. Geology of Greenland Survey Bulletin, 182, 1–80.
Burdak, V. D. 1979. Funktsional'naya morfologiya cheshujnogo pokrova ryb [Functional Morphology of the Squamation of Fishes]. Naukova Dumka, Kiev, 163 pp. [in Russian, with English summary].
Fredholm, D. 1988. Vertebrates in the Ludlovian Hemse Beds of Gotland, Sweden. Geologiska Föreningens i Stockholm Förhandlingar, 110, 157–179.
Gross, W. 1947. Die Agnathen und Acanthodier der Obersilurischen Beyrichienkalks. Palaeontographica, Abt. A, 96, 91–161.
Gross, W. 1967. Über Thelodontier‑Schuppen. Palaeontographica, Abt. A, 127, 1–67.
Hill, L. G. 1971. Scale development and patterns of squamation on the spring cavefish, Chologaster agassizi (Amblyopsidae). Proceedings of the Oklahoma Academy of Sciences, 51, 13–14.
Hoppe, K.-H. 1931. Die Coelolepiden und Acanthodier des Obersilurs der Insel Ösel. Ihre Paläobiologie und Paläontologie. Palaeontographica, 76, 35–94.
Karatajūtė-Talimaa, V. 1978. Telodonty silura i devona SSSR i Spitsbergena [Silurian and Devonian Thelodonts of the SSSR and Spitsbergen]. Mokslas, Vilnius, 334 pp. [in Russian].
Märss, T. 1986a. Pozvonochnye silura Éstonii i Zapadnoj Latvii [Silurian Vertebrates of Estonia and West Latvia]. Fossilia Baltica, 1, Valgus, Tallinn, 104 pp. [in Russian, with English summary].
Märss, T. 1986b. Squamation of the thelodont agnathan Phlebolepis. Journal of Vertebrate Paleontology, 6, 1–11.
Märss, T. 2005. Oeselia mosaica gen. et sp. nov. from the Lower Silurian of Estonia. Proceedings of the Estonian Academy of Sciences, Geology, 54, 181–190.
Märss, T. & Karatajūtė-Talimaa, V. 2002. Ordovician and Lower Silurian thelodonts from Severnaya Zemlya Archipelago (Russia). Geodiversitas, 24, 381–404.
Märss, T. & Ritchie, A. 1998. Articulated thelodonts (Agnatha) of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 88, 143–195.
Märss, T., Wilson, M. V. H. & Thorsteinsson, R. 2002. New thelodont (Agnatha) and possible chondrichthyan (Gnathostomata) taxa established in the Silurian and Lower Devonian of the Canadian Arctic Archipelago. Proceedings of the Estonian Academy of Sciences, Geology, 51, 88–120.
Märss, T., Perens, H. & Klaos, T. 2003. Sedimentation of the Himmiste-Kuigu fish bed (Ludlow of Estonia), and taphonomy of the Phlebolepis elegans Pander (Thelodonti) shoal. Proceedings of the Estonian Academy of Sciences, Geology, 52, 239–264.
Märss, T., Wilson, M. V. H. & Thorsteinsson, R. 2006. Silurian and Lower Devonian thelodonts and putative chondrichthyans from the Canadian Arctic Archipelago. Special Papers in Palaeontology, 75, 1–140.
Märss, T., Turner, S. & Karatajūtė-Talimaa, V. 2007. Handbook of Paleoichthyology, Vol. 1B: “Agnatha” II. Thelodonti (Schultze, H.-P., ed.). Verlag Dr. Friedrich Pfeil, München, 143 pp.
Pander, C. H. 1856. Monographie der fossilen Fische des silurischen Systems der russisch-baltischen Gouvernements. Obersilurische Fische. Buchdruckerei der Kaiserlichen Akademie des Wissenschaften, St. Petersburg, 91 pp.
Reif, W.-E. 1985. Squamation and ecology of sharks. Courier Forschungsinstitut Senckenberg, 78, 1–255.
Rohon, J. V. 1893. Die obersilurischen Fische von Oesel. II Theil. Selachii, Dipnoi, Ganoidei, Pteraspidae und Cephalaspidae. Mémoires de L’Académie Impériale des Sciences de St.-Petersbourg, 41, 1–124.
Sire, J.-Y. & Akimenko, M.-A. 2004. Scale development in fish: a review, with description of sonic hedgehog (shh) expression in the zebrafish (Danio rerio). The International Journal of Developmental Biology, 48, 233–247.
http://dx.doi.org/10.1387/ijdb.15272389
Stetson, H. C. 1928. A new American Thelodus. American Journal of Science, 16, 221–231.
http://dx.doi.org/10.2475/ajs.s5-16.93.221
Turner, S. 1984. Studies of Palaeozoic Thelodonti (Craniata: Agnatha). Unpublished Ph.D. thesis, 2 volumes. University of Newcastle-upon-Tyne, 272 pp.
Turner, S. 1986. Thelodus macintoshi Stetson 1928, the largest known thelodont (Agnatha: Thelodonti). Breviora, 486, 1–18.
Turner, S. 1991. Monophyly and interrelationships of the Thelodonti. In Early Vertebrates and Related Problems of Evolutionary Biology (Chang Mee-mann, Liu Yu-hai & Zhang Guo-rui, eds), pp. 87–119. Science Press, Beijing.
Wilson, M. V. H. & Caldwell, M. W. 1998. The Furcacaudiformes: a new order of jawless vertebrates with thelodont scales, based on articulated Silurian and Devonian fossils from Northern Canada. Journal of Vertebrate Paleontology, 18, 10–29.http://dx.doi.org/10.1080/02724634.1998.10011031
