Thelodonti

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 (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. The exoskeleton is composed of many tooth-like scales, usually around 0.5–1.5mm in size. These scales did not overlap, and 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 fishes, 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 were combined with a single, usually well-developed, dorsal and anal fins; Bone – being one of the most resistant materials to the process of fossilisation – often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. 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. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish. 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. == Taxonomy ==

Due to the small number of intact fossils, the taxonomy of thelodonts is based primarily on scale morphology. In fact, some thelodont families are only recognised based on their scale fossils. A recent assessment of thelodont taxonomy by Wilson and Märss in 2009 merges the orders Loganelliiformes, Katoporiida and Shieliiformes into Thelodontiformes, places families Lanarkiidae and Nikoliviidae into Furcacaudiformes on the basis of scale morphology, and establishes Archipelepidiformes as the basal-most order. A newer taxonomy based on the work of Nelson, Grande and Wilson 2016 and van der Laan 2016. • Superclass †Thelodontomorphi Jaekel 1911 • Class †Thelodonti Kiaer 1932 • Family †Oeseliidae Märss 2005 • Order †Archipelepidiformes Wilson & Märss 2009 • Family †Boothialepididae Märss 1999 • Family †Archipelepididae Märss ex Soehn et al. 2001 • Order †Furcacaudiformes Wilson & Caldwell 1998 (Fork-tailed thelodonts) • Family †Nikoliviidae Karatajūtė-Talimaa 1978 • Family †Lanarkiidae Obručhev 1949 • Family †Pezopallichthyidae Wilson & Caldwell 1998 • Family †Drepanolepididae Wilson & Marss 2009 • Family †Barlowodidae Märss, Wilson & Thorsteinsson 2002 • Family †Apalolepididae Turner 1976 • Family †Furcacaudidae Wilson & Caldwell 1998 • Clade Thelodontida Stensiö 1958 non Kiaer 1932 s.l. • Family †Talivaliidae Marss, Wilson & Thorsteinsson 2002 • Family †Longodidae Märss 2006b • Family †Helenolepididae Wilson & Märss 2009 • Order †Sandiviiformes Karatajūtė-Talimaa & Märss 2004 • Family †Angaralepididae Karatajūtė-Talimaa & Märss 2004 • Family †Stroinolepididae Karatajūtė-Talimaa & Märss 2002 • Family †Sandiviidae Karatajūtė-Talimaa & Märss 2004 • Order †Turiniida Stensiö 1958 • Family †Turiniidae Obručhev 1964 • Order †Thelodontiformes • Family †Thelodontidae Jordan 1905 • Order †Loganelliiformes Turner 1991 • Family †Nunavutiidae Marss, Wilson & Thorsteinsson 2002 • Family †Loganelliidae Märss, Wilson & Thorsteinsson 2002 • Order †Phlebolepidiformes Berg 1937 s.s. • Family †Phlebolepididae Berg 1937 corrig. • Family †Shieliidae Märss, Wilson & Thorsteinsson 2002 • Family †Katoporodidae Soehn et al. 2001 ex Märss, Wilson & Thorsteinsson 2002 The following cladogram, simplified from Ferrón et al. (2018), reflects the analysis of Wilson & Märss (2009): }} == Ecology ==

fredholmae'' 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. == 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. The oldest named thelodonts are from the Sandbian stage of the Late Ordovician: Larolepis (from St. Joseph Island, Canada) and Stroinolepis (from Severnaya Zemlya, Russia). Both belong to the family Loganelliidae. This family is not the earliest-diverging group of thelodonts, suggesting that their origin goes back further. The two earliest-diverging thelodont groups, the archipelepidiforms and furcacaudiforms, are mainly distributed in Laurentia (the ancient tropical continent roughly equivalent to North America), with only the latter dispersing to nearby continents a few times. Thelodontiforms, on the other hand, may have originated from Baltica (equivalent to much of Eastern Europe). Thelodontiforms attained a much wider distribution than the other two orders, despite not appearing to be adapted for active swimming as adults. Their ability to reach new regions relate to new dispersal adaptations in their larvae. == See also ==