Among the flat-bodied forms are Lanarkia (top left), provided with long, spine-shaped scales, and Loganellia (top right and middle). Other thelodonts, such as Furcacauda from the Devonian of Canada (bottom) are deep-bodied, with lateral gill openings and a very large, forked tail.[3]
Thelodonts are united in possession of "thelodont scales". This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group,[5] although there is no firm agreement on this point. On the basis that they are monophyletic, they are reconstructed as being ancestrally marine and invading freshwater on multiple occasions.[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. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.[7][8]
The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. Traditionally they were considered predominantly deposit-feeding bottom dwellers, but more recent studies have showed they occupied various ecological roles in various parts of the water column, much like modern bony fishes and sharks. In particular, a large variety of species preferred reef ecosystems, and it has been suggested that this preference was the reason for the development of their unique scales, protecting against abrasion and allowing for the development of more flexible bodies than other jawless fish, which had inflexible armors and were restricted to open habitats.[9]
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 (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,[10] 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.[11] 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.[12]
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 15–30cm in length.[13] 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.[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. Some specimens described in 1993 were the first to be found with a significant degree of three-dimensionality, ending speculations that the Thelodonts were flattened fish. Further, these fossils allowed the gut morphology to be interpreted, which generated much excitement: their guts were unlike those of any other agnathans, and a stomach was clearly visible: this was unexpected, as it was previously thought that stomachs evolved after jaws. Distinctive fork-shaped tails – usually characteristic of the jawed fish (gnathostomes) – were also found, linking the two groups to an unexpected degree.[14]
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;[13] these and the hypercercal and much larger hypocercal lobes forming a heterocercal tail resemble features of modern fish that are associated with their deftness at predation and evasion.[12]
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.[15]
A newer taxonomy based on the work of Nelson, Grande and Wilson 2016[16] and van der Laan 2016.[17]
The bony scales of the thelodont group, as the most abundant form of fossil, are also the best understood – and thus most useful. The scales were formed and shed throughout the organisms' lifetimes, and quickly separated after their death.[18]
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.[19] Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish.[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.[20] 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.[11]
Scale morphology, alone, has limited value for distinguishing thelodont species. Within each organism, scale shape varies greatly according to body area,[21] 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.[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. Nonetheless, a framework of three groups has been proposed, based upon scale morphology and histology.[20]
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.[9]
Ecology
Furcacauda heintzae
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.[9] They are mainly known from open shelf environments, but are also found nearer the shore and in some freshwater settings.[7] 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.[22]
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).[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.
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.[23]
Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.
Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living gnathostomes have true teeth, paired appendages, the elastomeric protein of elastin, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons, and an adaptive immune system that has the discrete lymphoid organs of spleen and thymus, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.
Ostracoderms are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.
Heterostraci is an extinct subclass of pteraspidomorph jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of the Devonian.
Anaspida is an extinct group of evolutionarily underived jawless vertebrates that existed from the early Silurian period to the late Devonian period. They were classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.
The class Osteostraci is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.
Thelodus is an extinct genus of thelodont agnathan that lived during the Silurian period. Fossils have been found in Europe, Asia and North America. Unlike many thelodonts, species of Thelodus are known not only from scales, but from impressions in rocks. Some species, such as the Canadian T. inauditus, are thought to be comparable in size to other thelodonts, i.e., from 5 to 15 centimeters in length. The scales of the type species, T. parvidens of Silurian Great Britain, however, reach the size of coins, and, if proportioned like other thelodonts, such as Loganellia, the living animal would have been about one meter in length.
Andreolepis is an extinct genus of prehistoric fish, which lived around 420 million years ago. It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in Gotland, Sweden. It is placed in the monogeneric family Andreolepididae and is generally regarded as a primitive member of the class Actinopterygii based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans.
Furcacauda is a genus of thelodontid agnathan from the Lower Devonian of Canada, and is the type genus of the order Furcacaudiformes. Furcacaudiform thelodontids were deep water jawless vertebrates with symmetrical fork and lobed-finned tails and scales smaller than typical loganellid and nikoliviid thelodonti scales. Furcacaudiform thelodonts are noted as having a laterally compressed body, large anterior eyes, slightly posterior, lateral, and vertical to a small mouth, and a condensed curved row of branchial openings (gills) directly posterior to the eyes. Many but not all had laterally paired fins. Wilson and Caldwell also note the presence of a caudal peduncle and a long caudal fin made of two large lobes, one dorsal and one ventral separated by 8 to 14 smaller intermediate lobes, giving the appearance of a striated half-moon shaped tail resembling the tail of a heterostracan. A large square cavity within the gut connecting a small intestine to an anal opening lead many to believe that it is this genus that exhibits the first vertebrate stomach. According to Wilson and Caldwell their discovery, based on sediment infillings of fossils of the Furcacauda heintze, gives credence to the evolutionary development of stomach before jaws.
Longodus is an extinct genus of thelodont, placed in its own family – Longodidae – which existed in what is now Estonia during the Ludlow epoch of the upper Silurian period. The type and only species is Longodus acicularis. They are most noted for their long, needle-like scales, which run vertically along their trunk.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Pteraspidiformes is an extinct order of heterostracan agnathan vertebrates known from extensive fossil remains primarily from Early Devonian strata of Europe and North America, and from Upper Silurian Canada.
Furcacaudiformes is an extinct order of jawless fish in the class Thelodonti.
Thelodontiformes is an extinct order of jawless fish of the Silurian.
Furcacaudidae is a family of thelodontid agnathan from the Lower Devonian. It is the type family of the order Furcacaudiformes.
Archipelepidiformes is an order of extinct jawless fishes in the class Thelodonti.
Archipelepis is a genus of extinct thelodont agnathans, and are the most primitive recognized thelodonts of which whole body fossils are known. Fossils of bodies and scales are currently known from Late Telychian to Wenlock-aged marine strata of northern Canada.
Erepsilepis is an extinct thelodont agnathan genus in the family Phlebolepididae.
Cape October is a headland in Severnaya Zemlya, Russia.
↑ Turner, S.; R. S. Dring (1981). "Late Devonian thelodonts (Agnatha) from the Gneudna Formation, Carnarvon Basin, Western Australia". Alcheringa. 5: 39–48. doi:10.1080/03115518108565432.
1 2 Turner, S. (1999). "Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance". In A. J. Boucot; J. Lawson (eds.). Palaeocommunities, International Geological Correlation Programme 53, Project Ecostratigraphy, Final Report. Cambridge University Press. pp.42–78.
↑ The early and mid Silurian. See Kazlev, M.A., White, T (March 6, 2001). "Thelodonti". Palaeos.com. Archived from the original on October 28, 2007. Retrieved October 30, 2007.{{cite web}}: CS1 maint: multiple names: authors list (link)
1 2 Donoghue, P. C. J.; M. P. Smith (2001). "The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti". Transactions of the Royal Society of Edinburgh: Earth Sciences. 92 (1): 15–37. doi:10.1017/S0263593301000025.
1 2 Turner, S. (1991). "Monophyly and interrelationships of the Thelodonti". In M. M. Chang; Y. H. Liu; G. R. Zhang (eds.). Early Vertebrates and Related Problems of Evolutionary Biology. Science Press, Beijing. pp.87–119.
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