according to body area, with intermediate scale forms appearing between different areas; furthermore, scale morphology may not even be constant within a given body area. To confuse things further, scale morphologies are not unique to specific taxa, and may be indistinguishable on the same area of two different species. 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. Nonetheless, a framework of three groups has been proposed, based upon scale morphology and histology. Thelodonts displayed similar squamation patterns as modern sharks, so functionally they served a similar role. This allows for a clearer observation of their ecological niches. In particular, protection against abrasion seems to have been the original role for these scales. Ecology Most thelodonts were considered deposit feeders, but more recent studies have shown that several species were active swimmers and thus more pelagic. A large variety of species in particular preferred reef ecosystems. They are mainly known from open shelf environments, but are also found nearer the shore and in some freshwater settings. The appearance of the same species in fresh- and salt-water settings has led to suggestions that some thelodonts migrated into fresh water, perhaps to spawn. However, the transition from fresh- to salt- water should be observable, as the scales' composition would change to reflect the different environment. This compositional change has not yet been found. Utility as biostratigraphic markers Thelodont scales are globally widespread during the Silurian and Early Devonian times, becoming restricted in range to Gondwana, until their extinction in the Late Devonian (Frasnian). 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. Evolutionary patterns The first major pattern or group of jawless fish with exoskeletons or plated armour, was the Laurentian group, which existed during the Cambrian-Ordovician time. 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. Due to their similar characteristics and chronological time frame of existence, many believe the thelodonts have Laurentian origins. See also List of thelodont genera Morphometrics Further reading A range of images of scales are available in References External links Contains some