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Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος ( kōnos), meaning "cone", and ὀδούς ( odoús), meaning "tooth"). They are primarily known from their hard, mineralised tooth-like structures called "conodont elements" that in life were present in the oral cavity and used to process food. Rare soft tissue remains suggest that they had elongate eel-like bodies with large eyes. Conodonts were a long-lasting group with over 300 million years of existence from the Cambrian (over 500 million years ago) to the beginning of the Jurassic (around 200 million years ago). Conodont elements are highly distinctive to particular species and are widely used in biostratigraphy as indicative of particular periods of geological time.
It was only in the early 1980s that the first fossil evidence of the rest of the animal was found (see below). In the 1990s exquisite fossils were found in South Africa in which the soft tissue had been converted to clay, preserving even muscle fibres. The presence of muscles for rotating the eyes showed definitively that the animals were primitive vertebrates.
Conodonta and Conodontophorida are by far the most common scientific names used to refer to conodonts, though inconsistencies regarding their taxonomic rank still persist. Bengtson (1976)'s research on conodont evolution identified three morphological tiers of early conodont-like fossils: , Paraconodontida, and "true conodonts" (euconodonts). Further investigations revealed that protoconodonts were probably more closely related to Chaetognatha (arrow worms) rather than true conodonts. On the other hand, paraconodonts are still considered a likely ancestral stock or sister group to euconodonts.
The 1981 Treatise on Invertebrate Paleontology volume on the conodonts (Part W revised, supplement 2) lists Conodonta as the name of both a phylum and a class, with Conodontophorida as a subordinate order for "true conodonts". All three ranks were attributed to Eichenberg, and Paraconodontida was also included as an order under Conodonta.David L. Clark (1981). 9780813730288, Geological Society of America; University of Kansas. . ISBN 9780813730288 This approach was criticized by Fåhraeus (1983), who argued that it overlooked Pander's historical relevance as a founder and primary figure in conodontology. Fåhraeus proposed to retain Conodonta as a phylum (attributed to Pander), with the single class Conodontata (Pander) and the single order Conodontophorida (Eichenberg). Subsequent authors continued to regard Conodonta as a phylum with an ever-increasing number of subgroups.
With increasingly strong evidence that conodonts lie within the phylum Chordata, more recent studies generally refer to "true conodonts" as the class Conodonta, containing multiple smaller orders. Paraconodonts are typically excluded from the group, though still regarded as close relatives. In practice, Conodonta, Conodontophorida, and Euconodonta are equivalent terms and are used interchangeably.
The element array constituted a feeding apparatus that is radically different from the jaws of modern animals. They are now termed "conodont elements" to avoid confusion. The three forms of teeth, i.e., coniform cones, ramiform bars, and pectiniform platforms, probably performed different functions.
For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometers to 5 millimeters in length), which occur commonly, but not always, in isolation and were not associated with any other fossil. Until the early 1980s, conodont teeth had not been found in association with fossils of the host organism, in a konservat lagerstätte. This is because the conodont animal was soft-bodied, thus everything but the teeth was unsuited for preservation under normal circumstances.
These microfossils are made of hydroxylapatite (a phosphatic mineral). The conodont elements can be extracted from rock using adequate solvents. (2025). 9789048140138 ISBN 9789048140138
They are widely used in biostratigraphy. Conodont elements are also used as , a proxy for thermal alteration in the host rock, because under higher temperatures, the phosphate undergoes predictable and permanent color changes, measured with the conodont alteration index. This has made them useful for petroleum exploration where they are known, in rocks dating from the Cambrian to the Late Triassic.
In the 1930s, the concept of conodont assemblages was described by Hermann Schmidt and by Harold W. Scott in 1934.Harold W. Scott, "The Zoological Relationships of the Conodonts. Journal of Paleontology, Vol. 8, No. 4 (Dec., 1934), pages 448-455 ( Stable URL)
The S and M elements are ramiform, elongate, and comb-like structures. An individual element has a single row of many cusps running down the midline along its top side. These conodont elements are arranged towards the animal's anterior oral surface, forming an interlocking basket of cusps within the mouth. Cusp may point out towards the head of the animal, or back towards the tail. The number of S and M elements present as well as the direction they point may vary by taxonomic group. M (makellate) elements have a higher position in the mouth and commonly form a symmetrical shape akin to a horseshoe or pick. S elements are further divided into three subtypes:
In P elements, a pectiniform (comb-shaped) row of cusps transitions into a broad flat or ridged platform moving towards the base of the element. Platforms and cusps are only found along one side of the structure. Individual elements oriented vertically and arranged in pairs, with platforms and cusps pointing towards the animal's midline. They occur deeper in the throat than the S and M elements. P elements are further divided into two subtypes:
== The conodont animal ==
According to these fossils, conodonts had large eyes, fins with fin rays, Myomere and axial line, which were interpreted as notochord or the dorsal nerve cord. While Clydagnathus and Panderodus had lengths only reaching , Promissum is estimated to reach in length, if it had the same proportions as Clydagnathus.
Another hypothesis contests that the conodont elements were used to actively catch and process prey. S and M elements could have been independently movable, allowing prey to be captured in the oral region of the animal. Modern hagfish and lampreys scrape at flesh using blades supported by a simple but effective pulley-like system, involving a string of muscles around a Cartilage core. An equivalent system might have been present in conodonts. S and M elements would be able to open and close at will to firmly grasp or pinch at prey, before rotating back to consume the prey element. The blade-like P elements deeper in the throat would process the food by slicing against their counterparts like a pair of scissors, or grinding against each other like molar teeth.
Current consensus supports the latter hypothesis in which elements are used for predation, not suspension feeding. One line of evidence for this includes the isometric growth pattern exhibited by S, M, and P elements. If the conodont animal relied upon a filter feeding strategy then this growth pattern would not provide the necessary surface area needed to support ciliated tissue as the animal grew. There is some evidence for cartilaginous structures similar to those present in modern jawless fish, which are both and . Wear on some conodont elements suggests that they functioned like teeth, with both wear marks likely created by food as well as by occlusion with other elements.
It is possible that multiple feeding strategies may have arisen in different groups of conodonts, as they are a diverse clade. A 2009 paper suggested that the genus Panderodus may have utilized venom in the acquisition of prey. Evidence of longitudinal grooves are present on some conodont elements associated with the feeding apparatus of this particular animal. These sorts of grooves are analogous to those present in some extant groups of venomous vertebrates.
A study on the population dynamics of Alternognathus has been published. Among other things, it demonstrates that at least this taxon had short lifespans lasting around a month. A study Strontium/Calcium and Barium/Ca ratios of a population of conodonts from a carbonate platform from the Silurian of Sweden found that the different conodont species and genera likely occupied different Trophic level.
Milsom and Sue Rigby envision them as vertebrates similar in appearance to modern hagfish and lampreys, and phylogenetic analysis suggests they are more Synapomorphy than either of these groups. However, this analysis comes with one caveat: the earliest conodont-like fossils, the , appear to form a distinct clade from the later Paraconodontida and euconodonts. Protoconodonts are probably not relatives of true conodonts, but likely represent a stem group to Chaetognatha, an unrelated phylum that includes arrow worms.
Moreover, some analyses do not regard conodonts as either or Craniata, because they lack the main characteristics of these groups. More recently it has been proposed that conodonts may be stem-Cyclostomi, more closely related to hagfish and than to Gnathostomata, based on similarities in the shape of their fins, and the idea that the conodont elements may be homologous with lamprey and hagfish tooth plates.
Only a few studies approach the question of conodont ingroup relationships from a Cladistics perspective, as informed by phylogenetic analyses. One of the broadest studies of this nature was the analysis of Donoghue et al. (2008), which focused on "complex" conodonts (Prioniodontida and other descendant groups):
Diversity continued to decline during the Middle and Late Triassic, culminating in their extinction soon after the Triassic-Jurassic boundary. Much of their diversity during the Paleozoic was likely controlled by sea levels and temperature, with the major declines during the Late Ordovician and Late Carboniferous due to cooler temperatures, especially and associated marine regressions which reduced continental shelf area. However, their final demise is more likely related to biotic interactions, perhaps competition with new Mesozoic taxa.
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