Scolecodonts which are acid-resistant cuticulinous jaws of fossil marine free-moving worms referred to as annelid polychete (Polychaeta), have a very long geological record. They are known from rocks varying in age from Ordovician to Recent. However, scolecodonts are reported most commonly from Ordovician, Silurian and Devonian deposits.
The jaws of polychetes were formed of a locally transformed pharyngeal cuticle as a result of its hardening, denticulation and thickening. The most primitive stage of the morphological evolution of the jaws is represented by compound jaws, that is, those having at least two rows of teeth, which, in the course of the phylogenetic development, divided to form simple jaws.

Archaeoprion quadricristatus
Mochtyella angelini
Mochtyella grazynae
Rakvereprion balticus
Vistulella kozlowskii
The earliest finds of the scolecodonts known so far, probably mark in the phylogeny of Polychaeta not the moment of appearances of toothed form, but that at which their jaws acquired resistance to diagenetic factors. This hypothesis is confirmed by the fact that the bristles of the polychetes as a rule do not occur in fossil state, although their distinct impressions were more than once described in literature.
The study of fossil polychete jaws was introduced in the middle nineteenth century by N.P.Angelin (1850,unpublished results), E.Eichwald (1854) and C.H.Pander (1856).
     The term
scolecodont was introduced by C.Croneis and H.W. Scott (1933).
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Pharate scolecodont - each assemblage of morphologically identical jaws composed according to the "jaw-in-jaw" principle.
Transmission electron microscopy
study on pharate scolecodonts.