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Lanarkia sp.
Lanarkia sp.
Lanarkia sp. was a genera of flat-bodied, jawless fish (agnathans) covered with protective spines, belonging to the Thelodont ("nipple-tooth") class, dating from the Late Silurian period at about 443-428 mya. The genus Lanarkia is named for the site at Lanarkshire, Scotland where it was first discovered by Traquair in 1898. It belongs to the phylum Chordata, the subphylum vertebrata, the class Thelodonti, and the subclass Katoporida.
Thelodonts first appeared during the Late Ordovician period (450 mya), and lived until the Frasnian–Famennian extinction at the end of the Devonian period (about 359 mya). Instead of being covered with large plates of protective dermal armor like most agnathans, Thelodonts developed more flexible body armor composed of many tooth-like or spiny scales called denticles. These offered some protection against their main predators, giant sea scorpions (eurypterids) up to two meters long such as Pterygotus. Evasive swimming was aided by a dorsal fin, paired pectoral fins,and an anal fin, as shown in the figure.
The Lanarkia were about 30-45 cm. in length. The
type species, Lanarkia horrida, was found in Lower Wenlock (Late Silurian, 433-428 mya) levels at
Lesmahagow in Lanarkshire, in the Dippal Burn Formation of the Waterhead
Group. More recent finds have occurred in 2006 by the Hunterian
Museum. A second Scottish species defined as Lanarkia spinosa instead represents (according to Marss and Ritchie 1998) two different growth stages of the same species, L. horrida. Besides the Scottish finds, examples of L. horrida have also been found in Silurian deposits at Avalanche Lake in Northwest Territories, Canada (Marss et al. 2002).
References:
Lanarkia horrida at fossilworks.org
Marss, T. and A. Ritchie 1998. Articulated thelodonts (Agnatha) of Scotland, Trans. Roy. Soc. Edinburgh, Earth Sci. 88: 143-195.
Marss, T.M., M. V. Wilson, and R. Thorsteinsson. 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(2):88-120
Turner, S. 1991. Monophyly and interrelationships of the Thelodonti. In Early vertebrates and related problems of evolutionary biology (ed. M. M. Chang, Y. H. Liu, and G. R. Zhang), pp. 87-111.
Vertebrates, Thelodonti (Palaeos.org)
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