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Armoured vacuums 4 - Thelodonts

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A selection of the extinct agnathans from the class Thelodonti, not to scale.
Shark scales, also known as denticles, are structurely similar to vertebrate teeth and display a wide range of form varying with ecological conditions. For example scales that are more resistant to abrasion can be found on shark species that frequent sandy and muddy substrate on the seafloor. Convergently similar scales evolved in a group of now extinct jawless fish.
All extinct Palaeozoic agnathans possess a combination of compact dermal armor plates and large flat scales (see Armoured vacuums 1 - Osteostracans, Armoured vacuums 2 - Galeaspids, Armoured vacuums 3 - Pteraspidomorphs), except for the thelodonts, which were covered in thousands of tiny placoid scales arranged randomly on the body. So abundant and distinct are the scales in the fossil record, that the palaeogeography and geological strata of a formation can be defined by the presence of specific thelodonts. The teardrop-shaped scales have a complex pattern of linear ridges and spikes along the edge, but vary greatly in form. Scale morphology has been historically used to differentiate groupings, however the shape is not consistent over the body, nor unique to taxa. Like sharks, scale morphology is possibly linked lifestyle habit. Most fossils have been found in marine deposits including shallow water lagoons (Phlebolepis), near shore, or even deep water basins (furcacaudids), while a few species have been found in brackish or freshwater environments (Turinia). Fossils of Loganellia are notable for the presence of denticles lining the inside of the branchial bars, while the dorsoventrally-flattened Lanarkia had addition taller spine-shaped scales that ran in rows along the back.
Fully articulated specimens are known for some species (Turinia, Loganellia, Phlebolepis) which gives a reasonable understanding of body shape. Morphologically thelodonts fall into three main categories: dorso-ventrally-flattened bodies with a large asymmetrical tail fin, fusiforms with similar tails (Shielia), and strong swimmers with a tall laterally-flattened body and multilobed forked tail (Sphenonectris, Drepanolepis). Compared to other agnathans, thelodonts show many features in common with jawed fish. They usually had lateral paired fins, a single dorsal and anal fin. They were the first agnathans to have a stomach, suggesting that amongst vertebrates the gut developed prior to jaws. There is considerable debate as to whether thelodonts represent a distinct group of jawless fish or various stem groups between the major lineages of jawless and jawed fish.
Thelodonts were a widespread group with a global range across the Middle Ordovician to the Early Devonian, however by the Middle and Late Devonian were restricted to Gondwana (Australolepis).
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Fishy-san87's avatar

Aqua roomba confirmed ✔😁  I simply love odd looking fishes :heart: