Thelodonti Totally Explained

Everything about Thelodonti totally explained

There is much debate over whether the clade of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish.
   Thelodonts (from Greek: "Nipple teeth") are characterised by being small and jawless, and united in possession of "thelodont scales". This defining character isn't necessarily a result of shared ancestry, as it may have been evolved independently by different groups.
   "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. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.
   The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian - Fammenian extinction event of the Late Devonian. They were predominantly deposit-feeding bottom dwellers, although there's evidence to suggest that some species took to the water column to be free-swimming organisms.

Description

Very few complete thelodont specimens are known; fewer still are preserved in three dimensions. This is due in part to the lack of an internal ossified (for example bony) skeleton; it doesn't help that the scales are poorly, if at all, attached to one another.
   Consequently, we're best able to describe the exoskeleton, which was composed of many tooth-like scales, usually around 0.5-1.5mm in size. These scales didn't overlap, were aligned to point backwards along the fish, in the most streamlined direction, but beyond that often appear haphazard in their orientation. The scales themselves approximate the form of a teardrop mounted on a small, bulky base, with the base often containing a small rootlet with which the scale was attached to the fish. The "teardrop" often contains lines, ridges, furrows and spikes running down its length in an array of sometimes complex patterns. Scales found around the gill region were generally smaller than the larger, bulkier scales found on the dorsal/ventral sides of the fish; some genera display rows of longer spikes.
   The scaly covering contrasts them with most other jawless fish (Cephalaspidomorphi), which were armor-plated with large, flat scales.
   Aside from scattered scales, some specimens do appear to display imprints, giving an indication of the structure of the whole animal - which appeared to reach 15-30 cm in length. Tentative studies appear to suggest that the fish possessed a more developed braincase than the lampreys, with an almost shark-like outline. 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.
   The fins of the thelodonts are useful in reconstructing their mode of life. Their paired pectoral fins combined with single, usually well-developed, dorsal and anal fins; Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. with intermediate forms appearing between different areas - and to make matters worse, scale morphology may not even be constant within one area! To confuse things further, scale morphologies are not unique to taxa, and may be indistinguishable on the same area of two different species.

Ecology

Most thelodonts were probably deposit feeders, although nektonic forms were not. They are mainly known from open shelf environments, but are also found nearer the shore and in some freshwater settings.

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).

List of Genera

†Amaltheolepis

†Angaralepis

†Apalolepis

†Archipelepis

†Australolepis B

†Barlowodus

†Boothialepis

†Boreania

†Bystrowia C

†Canonia

†Cephalopterus

†Chattertonodus

†Coelolepides

†Coelolepis

†Cometicercus D

†Drepanolepis E

†Eestilepis

†Erepsilepis F

†Furcacauda G

†Gampsolepis

†Glacialepis

†Goniophorus

†Gonioporus

†Goniporus H

†Helenolepis I

†Illoganellia J

†Jesslepis K

†Katoporodus

†Katoporus L

†Lanarkia

†Larolepis

†Loganella L (cont.)

†Loganellia

†Logania

†Longodus N

†Nethertonodus

†Nikolivia

†Nunavutia O

†Oeselia

†Overia P

†Pachylepis

†Paralogania

†Parathelodus

†Paraturinia

†Pezopallichthys

†Phillipsilepis

†Phlebolepis

†Praetrilogania S

†Sandivia S (cont.)

†Shielia

†Sigurdia

†Skamolepis

†Sophialepis

†Sphenonectris

†Stroinolepis T

†Talimaalepis

†Talivalia

†Thelodus

†Thelolepis

†Thelolepsis

†Thulolepis

†Trimerolepis

†Turinea

†Turinia V

†Valiukia

†Valyalepis Z

†Zuegelepis

Further Information

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