Details of the endolymphatic structures are described for the first time in the headshields of the osteostracan genus Tremataspis from the Silurian of Estonia. Tiny platelets, here termed covering platelets, are located within the openings of the endolymphatic duct. The details of their shapes and arrangements differ among the four studied species. In Tremataspis schmidti Rohon and Tremataspis milleri Patten, the covering platelets are usually arranged in an oval or circular shape around the opening of the endolymphatic duct. These species can have smooth, flat covering platelets at the mouth of an irregularly funnel-shaped aperture in the dorsal shield; the funnel often has a posterior extension. In Tremataspis rohoni Robertson and Tremataspis mammillata Patten, a distinct circular arrangement of platelets does not occur; instead, their funnel was capped with a few (2–3) smooth, flat covering platelets. The funnel of T. milleri sometimes has a long postero-lateral extension, while that of T. mammillata can have a short extension; no extensions of the funnel have been observed in T. schmidti or T. rohoni. The diameter of the pores in the covering platelets is larger than that of the pores in the superficial layer of the headshield and much larger than the diameter of the pores in the porous fields (these are thin perforated bony septa subdividing the sensory canals horizontally into lower and upper parts). In T. milleri, the larger pores are located on the side of the covering platelets that is closer to the body midline. The discovered system of covering platelets possibly functioned as a sieve for allowing in suitable grains of material and preventing material that is too fine or too coarse, or not sufficiently dense, from entering the inner ear.
Bölau, E. 1951. Das Sinnesliniensystem der Tremataspiden und dessen Beziehungen zu anderen Gefäss-Systemen der Exoskeletts (The sensory line system of tremataspids and its relations to other vascular systems of the exoskeleton). Acta Zoologica, 32(1–2), 31–40 (in German).
https://doi.org/10.1111/j.1463-6395.1951.tb00358.x
Bond, C. E. 1979. Biology of Fishes. W. B. Saunders, Philadelphia.
Bremer, O., Qu, Q., Sanchez, S., Märss, T., Fernandez, V. and Blom, H. 2021. The emergence of a complex pore-canal system in the dermal skeleton of Tremataspis (Osteostraci). Journal of Morphology, 282(2), 1141–1157.
https://doi.org/10.1002/jmor.21359
Chapuis, L. and Collin, S. P. 2022. The auditory system of cartilaginous fishes. Reviews in Fish Biology and Fisheries, 32(2), 521–554.
https://doi.org/10.1007/s11160-022-09698-8
Coad, B. W. and McAllister, D. E. 2020. Dictionary of Ichthyology.
http://www.briancoad.com/dictionary/completedictionary.htm (accessed 2021-08).
Dearden, R. P. and Giles, S. 2021. Diverse stem-chondrichthyan oral structures and evidence for an independently acquired acanthodid dentition. Royal Society Open Science, 8(11), 210822.
https://doi.org/10.1098/rsos.210822
Denison, R. H. 1947. The exoskeleton of Tremataspis. American Journal of Science, 245(6), 337–365.
https://doi.org/10.2475/ajs.245.6.337
Denison, R. H. 1951. Evolution and classification of the Osteostraci. Fieldiana: Geology, 11(3–4), 155–196.
Denison, R. H. 1966. The origin of the lateral-line sensory system. American Zoologist, 6(3), 368–370.
https://doi.org/10.1093/icb/6.3.368
Eichwald, E. 1854. Die Grauwackenschichten von Liv- und Estland (Greywacke layers of Livonia and Estonia). Bulletin de la Société Impériale des Naturalistes de Moscou, 27(1), 1–111 (in German).
Gross, W. 1968. Beobachtungen mit dem Elektronenraster-Auflichtmikroskop an den Siebplatten und dem Isopedin von Dartmuthia (Osteostraci) (Observations with the electron scanning reflected light microscope on the sieve plates and the isopedine of Dartmuthia (Osteostraci)). Paläontologische Zeitschrift, 42, 73–82 (in German).
https://doi.org/10.1007/BF02987129
Hanson, M., Westerberg, H. and Öblad, M. 1990. The role of magnetic statoconia in dogfish (Squalus acanthias). Journal of Experimental Biology, 151, 205–218.
https://doi.org/10.1242/jeb.151.1.205
Janvier, P. 1985. Les thyestidiens (Osteostraci) du Silurien de Saaremaa (Estonie). Première Partie: Morphologie et Anatomie (Thyestiida (Osteostraci) from the Silurian of Saaremaa (Estonia). Part I: Morphology and anatomy). Annales de Paléontologie, 71(3), 83–147 (in French).
Janvier, P. 1996. Early Vertebrates. Oxford Monographs on Geology and Geophysics, 33. Clarendon Press, Oxford.
Jürgenson, E. 1988. Осадконакопление в силуре Прибалтики (Deposition of the Silurian beds in the Baltic). Valgus, Tallinn (in Russian).
Kasumyan, A. O. 2004. Vestibular system and sense of equilibrium in fish. Journal of Ichthyology, 44 (Supplement 2), S224–S268.
Ladich, F. and Schulz-Mirbach, T. 2016. Diversity in fish auditory systems: One of the riddles of sensory biology. Frontiers in Ecology and Evolution, 31.
https://doi.org/10.3389/fevo.2016.00028
Maisey, J. G. 1985. Cranial morphology of the fossil fish elasmobranch Synechodus dubrisiensis. American Museum Novitates, 2804, 1–28.
Maisey, J. G. 2001. Remarks on the inner ear of elasmobranchs and its interpretation from skeletal labyrinth morphology. Journal of Morphology, 250(3), 236–264.
https://doi.org/10.1002/jmor.1068
Märss, T., Afanassieva, O. and Blom, H. 2014. Biodiversity of the Silurian osteostracans of the East Baltic. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 105, 73–148.
https://doi.org/10.1017/S1755691014000218
Märss, T., Wilson, M. V. H. and Viljus, M. 2022. Species-specific morphology of the endolymphatic openings in the headshields of the osteostracan genus Tremataspis (Agnatha), from the Silurian of Estonia. In Special Publication of the Ichthyolith Issues:16th International Symposium on Early and Lower Vertebrates, Valencia, Spain, 20–26 June 2022 (Paredes Aliaga, M. V., Manzanares, E., Mondéjar Fernández, J., Ros Franch, S., Botella, H. and Martínez Pérez, C., eds). University of Valencia, Valencia, 15, 6.
Mills, M., Rasch, R., Siebeck, U. E. and Collin, S. P. 2011. Exogenous material in the inner ear of the adult Port Jackson shark, Heterodontus portusjacksoni(Elasmbranchii). The Anatomical Record, 294(3), 373–378.
https://doi.org/10.1002/ar.21338
Nolf, D. 1985. Otolithi Piscium. Handbook of Paleoichthyology, 10. Gustav Fischer Verlag, Stuttgart.
O’Shea, J., Keating, J. N. and Donoghue, P. C. J. 2019. The dermal skeleton of the jawless vertebrate Tremataspis mammillata (Osteostraci, stem-Gnathostomata). Journal of Morphology, 280(7), 999–1025.
https://doi.org/10.1002/jmor.20997
Popper, A. N. and Fay, R. R. 1977. Structure and function of the elasmobranch auditory system. American Zoologist, 17, 443–452.
https://doi.org/10.1093/icb/17.2.443
Robertson, G. M. 1938. The Tremataspididae. American Journal of Science, Part I, 35(207), 172–206; Part II, 35(208), 273–296.
https://doi.org/10.2475/ajs.s5-35.208.273
Rojo, A. L. 2017. Dictionary of Evolutionary Fish Osteology. CRC Press, Boca Raton,.
https://doi.org/10.4324/9781315150932
Sahney, S. and Wilson, M. V. H. 2001. Extrinsic labyrinth infillings imply open endolymphatic ducts in Lower Devonian osteostracans, acanthodians, and putative chondrichthyans. Journal of Vertebrate Paleontology, 21, 660–669.
Sansom, R. S. 2009. Phylogeny, classification and character polarity of the Osteostraci (Vertebrata). Journal of Systematic Palaeontology, 7(1), 95–115.
https://doi.org/10.1017/S1477201908002551
Schnetz, L., Pfaff, C., Libowitzky, E., Johanson, Z., Stepanek, R. and Kriwet, J. 2019. Morphology and evolutionary significance of phosphatic otoliths within the inner ears of cartilaginous fishes (Chondrichthyes). BMC Evolutionary Biology, 19, 238.
https://doi.org/10.1186/s12862-019-1568-z
Sinisalu, R. and Kleesment, A. 2002. Purdsetendite granulomeetrilisest klassifikatsioonist (On grain size scale of siliclastic particles). Bulletin of the Geological Survey of Estonia, 10(1), 20–26 (in Estonian).
Stensiö, E. 1927. The Downtonian and Devonian vertebrates of Spitsbergen. Part I. Family Cephalaspidae. Skrifter om Svalbard og Nordishavet, 12, 1–391.
Stensiö, E. 1964. Les Cyclostomes fossiles ou Ostracodermes (The fossils Cyclostomes or Ostracoderms). In Traité de Paléontologie (Piveteau, J., ed.). Masson, Paris, IV(1), 96–382 (in French).
Stewart, C. 1906. On the membranous labyrinth of Echinorhinus, Cestracion and Rhina. Zoological Journal of the Linnean Society of London, 29(194), 439–442.
https://doi.org/10.1111/j.1096-3642.1906.tb00445.x
Watson, D. M. S. 1937. The acanthodian fishes. Philosophical Transactions of the Royal Society of London, Series B, 228(549), 49–146.
https://doi.org/10.1098/rstb.1937.0009
