Thelodus

Name: Thelodus.
Phonetic: Fel-o-dus.
Named By: Agassiz‭ ‬-‭ ‬1839.
Classification: Chordata,‭ ‬Agnatha,‭ ‬Thelodonti,‭ ‬Thelodontidae.
Species: T.‭ ‬parvidens‭ (‬type‭?)‬,‭ ‬T.‭ ‬calvus,‭ ‬T.‭ ‬hoskinsi.
Diet: Carnivore‭?
Size: Between‭ ‬15‭ ‬and‭ ‬20‭ ‬centimetres long.
Known locations: Estonia,‭ ‬Norway,‭ ‬Russia,‭ ‬Sweden,‭ ‬Ukraine‭ & ‬United Kingdom.
Time period: Wenlock of the Silurian through to Pragian of the Devonian‭ (‬possibly‭ ‬later in the Devonian‭)‬.
Fossil representation: Multiple individuals.

       Although in fish terms Thelodus were very primitive,‭ ‬this genus shows a development towards stronger open water swimming.‭ ‬Thelodus possessed a better developed tail,‭ ‬a dorsal fin‭ (‬on the back‭)‬,‭ ‬and anal fin‭ (‬underside,‭ ‬near the tail‭) ‬as well as rudimentary pectoral fins in the shape of flaps that could be angled‭ ‬to‭ ‬control pitch when swimming.‭ ‬Add to this a mouth that was faced forwards,‭ ‬not down as seen in bottom feeders of the time,‭ ‬and it‭’‬s easy to appreciate that Thelodus were more likely open water fish that either filtered plankton or sucked in small organisms as they swam forward.‭ ‬Additionally,‭ ‬the lack of heavier body commonly seen in other fish genera of the time,‭ ‬further‭ ‬supports an open water,‭ ‬and reliance upon speed instead of armour for defence.

Further reading
-‭ ‬Ordovician and Lower Silurian thelodonts from Severnaya Zemlya Archipelago‭ (‬Russia‭)‬,‭ ‬T.‭ ‬Marss and V.‭ ‬Karatajute-Talimaa‭ ‬-‭ ‬2002.



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