Thelodus (from , 'nipple' and , 'tooth')[ Thelodus at Fossilworks.org]
Description
Unlike many thelodonts, species of
Thelodus are known not only from scales, but from impressions in rocks. Some species, such as the Canadian
T. inauditus, are thought to be comparable in size to other thelodonts, i.e., from 5 to 15 centimeters in length.
[Märss, Tiiu, Mark VH Wilson, and Raymond Thorsteinsson. "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. Vol. 51. No. 2. Estonian Academy Publishers, 2002.] The scales of the type species,
T. parvidens (
syn. T. macintoshi) of Silurian Great Britain, however, reach the size of coins, and, if proportioned like other thelodonts, such as
Loganellia, the living animal would have been about one meter in length.
[Turner, Susan. Thelodus Macintoshi Stetson 1928: The Largest Known Thelodont (Agnatha: Thelodonti). Museum of Comparative Zoology, 1986.]
Scale-based ecology
Many
Thelodus species have been named, and they may have had different habitat preferences based on their scale structures. The scales of
T. parvidens,
T. sculptilis,
T. traquairi, and
T. calvus are robust and abrasion-resistant, similar to modern sharks which live among rough substrates such as rocky caves or reefs.
T. visvaldi has streamlined scales to minimize drag, similar to modern sharks capable of strong swimming in open waters. A few species (
T. macintoshi,
T. inauditus) are too fragmentary to draw any firm conclusions.
Some species ( T. carinatus, T. marginatus, T. matukhini) are known from two types of isolated scales: abrasion-resistant scales and generalized scales which offer a compromise between streamlining, armor, and anti-parasite protection. Modern sharks which forage on a sandy or muddy seabed tend to have abrasion-resistant scales on the belly and generalized scales on the back. T. laevis has a unique mosaic of generalized and abrasion-resistant scales across the entire body.
