Thelodonti (from Greek: "feeble teeth")
[Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996.] is a
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
of extinct
jawless fish
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostomes ...
es with distinctive
scales
Scale or scales may refer to:
Mathematics
* Scale (descriptive set theory), an object defined on a set of points
* Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original
* Scale factor, a number w ...
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
Gnathostomata (; from Greek language, Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposi ...
.
![Shielia tiati](https://upload.wikimedia.org/wikipedia/commons/2/24/Shielia_tiati.svg)
Thelodonts are united in possession of "
thelodont scales
Thelodonti (from Greek: "feeble teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class (biology), class of extinct jawless fishes with distinctive Fish scale, scales instead of large pl ...
". 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
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...
group, 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.
"Thelodonts" were
morphologically very similar, and probably closely related, to fish of the classes
Heterostraci
Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those itha different shell" i is pl. of -us is an extinct subclass of pteraspidomorph jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci ex ...
and
Anaspida
Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived primarily during the Silurian period, and became extinct soon after the start of the Devonian. They were classically regarded as the ancestors of lampre ...
, 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 early and mid Silurian. See ]
The fish lived in both freshwater and marine environments, first appearing during the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
, and perishing during the
Frasnian–Famennian extinction event of the Late
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
. 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
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
, 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.
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,
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
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
, with an almost shark-like outline. Internal scales have also been recovered, some fused into plates resembling
gnathostome
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") 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 ...
tooth-whorls to such a degree that some researchers favour a close link between the families.
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
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") 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 ...
) – 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 fin
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...
s were combined with a single, usually well-developed,
dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal co ...
and
anal fin
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
s;
these and the hypercercal and much larger hypocercal lobes forming a heterocercal
tail
The tail is the section at the rear end of certain kinds of animals’ bodies; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, r ...
resemble features of modern fish that are associated with their deftness at predation and evasion.
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
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
Loganelliiformes, Katoporiida and Shieliiformes into
Thelodontiformes
Thelodontiformes is an extinct order of jawless fish of the Silurian.Kiaer, J., 1932: New coelolepids from the Upper Silurian on Oesel (Esthonia). Eesti Loodusteaduse Arhiiv, Seeria 1, 10: 167-176.
Because the paucity of intact fossils, especial ...
, places
families
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideall ...
Lanarkiidae and Nikoliviidae into
Furcacaudiformes
Furcacaudiformes is an extinct order of jawless fish in the class Thelodonti.
Because the paucity of intact fossils, especially since some families are known entirely from scale fossils, taxonomy of thelodonts is based primarily on scale morpho ...
on the basis of scale morphology, and establishes
Archipelepidiformes
Archipelepidiformes is an order of extinct jawless fishes in the class Thelodonti.
Archipelepidiforms are regarded as the basalmost thelodonts primarily because the histology and morphology of archipelepidiforms have many similarities with the h ...
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
''Oeselia mosaica'' is a fossil jawless fish and is the only species in its genus and in the family Oeseliidae (Oesel being the old name for Saaremaa, Saaremaa Island in Estonia). It is a member of an extinct family of Thelodonti, thelodonts or ...
Märss 2005
*** Order †
Archipelepidiformes
Archipelepidiformes is an order of extinct jawless fishes in the class Thelodonti.
Archipelepidiforms are regarded as the basalmost thelodonts primarily because the histology and morphology of archipelepidiforms have many similarities with the h ...
Wilson & Märss 2009
**** Family †
Boothialepididae Märss 1999
**** Family †
Archipelepididae Märss ex Soehn et al. 2001
*** Order †
Furcacaudiformes
Furcacaudiformes is an extinct order of jawless fish in the class Thelodonti.
Because the paucity of intact fossils, especially since some families are known entirely from scale fossils, taxonomy of thelodonts is based primarily on scale morpho ...
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
Thelodontiformes is an extinct order of jawless fish of the Silurian.Kiaer, J., 1932: New coelolepids from the Upper Silurian on Oesel (Esthonia). Eesti Loodusteaduse Arhiiv, Seeria 1, 10: 167-176.
Because the paucity of intact fossils, especial ...
***** 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
Scales
![Thelodont denticles](https://upload.wikimedia.org/wikipedia/commons/c/c0/Thelodont_denticles.png)
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
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
and growth of the scales to be studied in detail. The scales consist of a non-growing "crown" composed of
dentine
Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...
, with a sometimes-ornamented
enameloid
Enameloid, also known as durodentine or vitrodentine, is an enamel-like tissue found in many fish. It is the primary outer component of shark odontodes (teeth and dermal denticles). Although the origin of enameloid is debated, it is probably hom ...
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.
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.
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.
Scale morphology, alone, has limited value for distinguishing thelodont species. Within each organism, scale shape varies greatly 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
![Furcacauda heintzae](https://upload.wikimedia.org/wikipedia/commons/4/44/Furcacauda_heintzae.jpg)
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
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
, until their extinction in the Late Devonian (Frasnian).
The morphology of some species diversified rapidly enough for the scales to rival the
conodont
Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...
s in utility as
biostratigraphic
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...
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
Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...
n group, which existed during the Cambrian-Ordovician time. However, the thelodonts (as well as the conodonts,
placoderms,
acanthodian
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...
s, and
chondrichthyans
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. C ...
) 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
Morphometrics (from Greek μορϕή ''morphe'', "shape, form", and -μετρία ''metria'', "measurement") or morphometry refers to the quantitative analysis of ''form'', a concept that encompasses size and shape. Morphometric analyses are co ...
Further reading
*
* A range of images of scales are available in
References
External links
* Contains some images.
* A phylogeny.
{{Taxonbar, from=Q138088
Prehistoric fish classes
Paleozoic jawless fish
Middle Ordovician first appearances
Famennian extinctions