Thelodonti (from Greek: "nipple teeth")[4] is a class of extinct Palaeozoic jawless fishes with distinctive scales instead of large plates of armor.

[6] "Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales.

Traditionally they were considered predominantly deposit-feeding bottom dwellers, but more recent studies have shown they occupied various ecological roles in various parts of the water column, much like modern bony fishes and sharks.

This is due in part to the lack of an internal ossified (i.e. bony) skeleton; it does not help that the scales are poorly, if at all, attached to one another, and that they readily detach from their owners upon death.

Internal scales have also been recovered, some fused into plates resembling gnathostome tooth-whorls to such a degree that some researchers favour a close link between the families.

[11] Despite the rarity of complete fossils, these very rare intact specimens do allow us to gain an insight into the internal organ arrangement of the Thelodonts.

The scales consist of a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine (acellular bony tissue) base.

[11] Beyond that, there appear to be five types of bone-growth, which may represent five natural groupings within the thelodonts – or a spectrum ranging between the end members, meta- (or ortho-) dentine and mesodentine tissues.

[13] The morphology and histology of the thelodonts provides the main tool for quantifying their diversity and distinguishing between species – although ultimately using such convergent traits is prone to errors.

[11] The morphology of some species diversified rapidly enough for the scales to rival the conodonts in utility as biostratigraphic markers, allowing precise correlation of widely spaced sediments.

However, the thelodonts (as well as the conodonts, placoderms, acanthodians, and chondrichthyans) are the second major group which are believed to have emerged in the middle Ordovician and lasted near the Late Devonian period.