Thelodonti

Thelodonti are an extinct group of jawless fishes, known chiefly from the Silurian and Devonian periods. Their bodies were covered in small scales rather than large bony plates.

Vertebrate: An animal with a backbone or, in early forms, a notochord and related axial structures. Thelodonts belong to the early vertebrate record, before jaws evolved.
Loganellia: An extinct thelodont genus known from small scales and articulated fossil material. Its well-preserved specimens help reconstruct the body form of thelodonts.
Thelodont scale: A small fossilized dermal scale characteristic of thelodont fishes, with a crown and basal region. Scale shape and microstructure often provide the main evidence for identifying thelodonts.
Silurian: A geological period lasting from about 443 to 419 million years ago. Thelodonts are especially well represented in Silurian marine deposits.
Vertebrate paleontology: The study of fossil vertebrates and their evolution through geological time. Thelodont classification depends on interpreting incomplete vertebrate fossils.
Agnatha: A traditional grouping of jawless vertebrates, including living and extinct forms. Thelodonts are commonly discussed among jawless vertebrates, though the grouping's boundaries vary.
Thelodus: An extinct genus of thelodont fishes known from the Silurian fossil record. The genus provides a familiar example of the group named for thelodonts.
Dentine: A mineralized tissue forming much of the hard internal structure of teeth and many dermal denticles. Dentine microstructure is preserved in scales and informs comparisons among thelodonts.
Devonian: A geological period lasting from about 419 to 359 million years ago, marked by major diversification of fishes. Thelodont fossils extend into the Devonian, when jawed fishes also diversified.
Phylogenetics: The study of evolutionary relationships among organisms using shared characteristics and other evidence. Different analyses place thelodonts differently among early jawless vertebrates.

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PteraspidomorphiCompared with: Thelodonts provide a contrasting body-covering pattern among early jawless vertebrates.