Thelodonti
Encyclopedia
Thelodonts are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armor.
There is much debate over whether the group 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 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, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.
"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–Famennian extinction event
of the Late Devonian
. They were predominantly deposit-feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free-swimming organisms.
Consequently, we are best able to describe the exoskeleton, which was composed of many tooth-like scales, usually around 0.5-1.5mm in size. These scales did not 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.
Despite the rarity of complete fossils, very rarely specimens do allow us to gain an insight into the innards 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 flat 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.
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; these and the prolonged anterior tube-like handle, followed by a heterocercal tail resemble features of modern fish that associated with their deftness at predation and evasion.
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.
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 comprise a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid
upper surface and an aspidine base. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. 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. Interestingly, 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.
However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area, 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.
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 comprising three groups has been proposed based upon scale morphology and histology.
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.
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.
, until their extinction in the Late Devonian (Frasnian). The morphology of some species diversified rapidly enough for the scales to rival the conodont
s in utility as biostratigraphic markers, allowing precise correlation of widely-spaced sediments.
n 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.
A
B
C
D
E
F
G
H
I
J
K
L
N
O
P
S
T
V
Z
There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
, or disparate stem groups to the major lines of jawless
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
and jawed fish.
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
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.
"Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes Heterostraci
Heterostraci
Heterostraci is an extinct class of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Ordovician, and all, save for the Psammosteids, became extinct during the late Devonian...
and Anaspida
Anaspida
The Anaspida are stem gnathostomes, and are classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked scales and paired fins, but have a striking highly hypocercal tail...
, 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
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
, and perishing during the Frasnian–Famennian extinction event
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...
of the Late Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
. They were predominantly deposit-feeding bottom dwellers, although there is 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 (i.e. bony) skeleton; it does not help that the scales are poorly, if at all, attached to one another.Consequently, we are best able to describe the exoskeleton, which was composed of many tooth-like scales, usually around 0.5-1.5mm in size. These scales did not 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
Cephalaspidomorphi
Cephalaspidomorphs are a group of jawless fishes named for the cephalaspids, a group of osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids...
), 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.
Despite the rarity of complete fossils, very rarely specimens do allow us to gain an insight into the innards 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 flat 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.
The fins of the thelodonts are useful in reconstructing their mode of life. Their paired pectoral fins combined with single, usually well-developed, dorsal
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
and anal fins; these and the prolonged anterior tube-like handle, followed by a heterocercal tail resemble features of modern fish that associated with their deftness at predation and evasion.
Scales
Bone - being one of the most resistant materials to the process of fossilisation - often preserves internal detail, which allows the histology
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
and growth of the scales to be studied in detail. The scales comprise a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
upper surface and an aspidine base. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. 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. Interestingly, 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.
However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area, 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.
The morphology and histology of the thelodonts provides the main tool for quantifying their diversity and distinguishing between species - although ultimately using such convergent
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
traits is prone to errors. Nonetheless, a framework comprising three groups has been proposed based upon scale morphology and histology.
Ecology
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 GondwanaGondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, until their extinction in the Late Devonian (Frasnian). The morphology of some species diversified rapidly enough for the scales to rival the conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
s 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 LaurentiaLaurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
n 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.
Genera
The following genera are included:A
- †Amaltheolepis
- †Angaralepis
- †Apalolepis
- †Archipelepis
- †Australolepis
B
- †Barlowodus
- †Boothialepis
- †Boreania
- †Bystrowia
C
- †Canonia
- †Cephalopterus
- †Chattertonodus
- †Coelolepides
- †Coelolepis
- †CometicercusCometicercusCometicercus is an extinct genus of thelodont which lived in Canada during the Early Devonian period. It is only known from its caudal fin and parts of its dorsal surface, including its dorsal fin....
D
- †DrepanolepisDrepanolepisDrepanolepis is an extinct genus of thelodont which lived in Canada during the Early Devonian period....
E
- †Eestilepis
- †Erepsilepis
F
- †FurcacaudaFurcacaudaAccording to the Systema Naturae 2000 Taxonomic classification, Furcacauda is a genus within the familyof Furcacaudidae, within the order Thelodonti, the Class Agnatha, and the Subphylum Vertebrata....
G
- †Gampsolepis
- †Glacialepis
- †Goniophorus
- †Gonioporus
- †Goniporus
H
- †Helenolepis
I
- †Illoganellia
J
- †Jesslepis
K
- †Katoporodus
- †Katoporus
L
- †LanarkiaLanarkiaLanarkia is an extinct genus of jawless fish which existed in what is now Scotland during the upper Silurian period. It contains the species Lanarkia horrida and Lanarkia spinosa....
- †Larolepis
- †Loganella
- †Loganellia
- †Logania
- †LongodusLongodusLongodus is an extinct genus of thelodont, placed in its own family – Longopdidae – which existed in what is now Estonia during the Ludlow epoch of the upper Silurian period. The type and only species is Longodus acicularis....
N
- †Nethertonodus
- †Nikolivia
- †Nunavutia
O
- †Oeselia
- †Overia
P
- †Pachylepis
- †ParaloganiaParaloganiaParalogania is a genus of thelodont known from the Upper Silurian ; the best deposits are from the Baltic region, but it is widespread in the northern hemisphere....
- †Parathelodus
- †Paraturinia
- †Pezopallichthys
- †Phillipsilepis
- †PhlebolepisPhlebolepisPhlebolepis was an extinct genus of Thelodont fish belonging to the Katoporidae family, whiched lived in the Early Silurian to Late Silurian . Fossils have been found in the Baltic, Europe, North America, Russia and Siberia. Phlebolepis was small, 10 cm long....
- †Praetrilogania
S
- †Sandivia
- †Shielia
- †Sigurdia
- †Skamolepis
- †Sophialepis
- †SphenonectrisSphenonectrisSphenonectris is an extinct genus of thelodont, which lived in Canada during the Early Devonian period.-External links:* http://fossils.valdosta.edu/fossil_pages/fossils_dev/f5.html...
- †StroinolepisStroinolepisStroinolepis is a genus of thelodontid that lived during the Middle Ordovician period in what is now Russia....
T
- †Talimaalepis
- †Talivalia
- †ThelodusThelodusThelodus is an extinct genus of primitive jawless fish that lived in the Silurian period. Fossils have been found worldwide.Thelodus belonged to the Thelodonti, a group of jawless fish lacking the armored plating characteristic of the related Heterostraci. The position of Thelodus`s mouth suggests...
- †Thelolepis
- †Thelolepsis
- †Thulolepis
- †Trimerolepis
- †Turinea
- †Turinia
V
- †Valiukia
- †Valyalepis
Z
- †Zuegelepis
Further reading
- A range of images of scales are available in