Conodonts (Greekkōnos, "cone", +odont, "tooth") are anextinct group ofjawless vertebrates, classified in theclassConodonta. 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 theCambrian (over 500 million years ago) to the beginning of theJurassic (around 200 million years ago). Conodont elements are highly distinctive to particular species and are widely used inbiostratigraphy 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.[3]
Through their history of study, "conodont" is a term which has been applied to both the individual fossils and to the animals to which they belonged. The original German term used by Pander was "conodonten", which was subsequentlyanglicized as "conodonts", though no formallatinized name was provided for several decades. MacFarlane (1923) described them as anorder, Conodontes (a Greek translation), which Huddle (1934) altered to the Latin spelling Conodonta.[4] A few years earlier, Eichenberg (1930) established another name for the animals responsible for conodont fossils: Conodontophorida ("conodont bearers").[1] A few other scientific names were rarely and inconsistently applied to conodonts and their proposed close relatives during 20th century, such as Conodontophoridia, Conodontophora, Conodontochordata, Conodontiformes,[5] and Conodontomorpha.
Conodonta and Conodontophorida are by far the most common scientific names used to refer to conodonts, though inconsistencies regarding theirtaxonomic rank still persist. Bengtson (1976)'s research on conodont evolution identified three morphological tiers of early conodont-like fossils:protoconodonts,paraconodonts, and "true conodonts" (euconodonts).[5] Further investigations revealed that protoconodonts were probably more closely related tochaetognaths (arrow worms) rather than true conodonts. On the other hand, paraconodonts are still considered a likely ancestral stock orsister group to euconodonts.
The 1981Treatise on Invertebrate Paleontology volume on the conodonts (Part W revised, supplement 2) lists Conodonta as the name of both aphylum and aclass, 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.[6] 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).[4][7] Subsequent authors continued to regard Conodonta as a phylum with an ever-increasing number of subgroups.[8]
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.[9][10][11] Paraconodonts are typically excluded from the group, though still regarded as close relatives.[9][10][11] In practice, Conodonta, Conodontophorida, and Euconodonta are equivalent terms and are used interchangeably.
For a long time, the function and arrangement of conodont elements was enigmatic, since the whole animal was soft-bodied, with the sole exception of the mineralized elements. Upon the conodont animal's demise, thesoft tissues would decompose and the individual conodont elements would separate. However, in instances of exceptional preservation the conodont elements may be recovered in articulation.[12] By closely observing these rare specimens, Briggset al. (1983)[13] were able to for the first time study theanatomy of the complexes formed by the conodont elements arranged as they were in life. Other researchers have continued to revise and reinterpret this initial description.[14][15][16]
Conodont elements consist of mineralised teeth-like structures of varying morphology and complexity. The evolution ofmineralized tissues has been puzzling for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodonts or the dermal skeleton of earlyagnathans.
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[17]), 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 akonservat lagerstätte.[13] 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 ofhydroxylapatite (a phosphatic mineral).[18] The conodont elements can be extracted from rock using adequate solvents.[19][20][21]
They are widelyused in biostratigraphy. Conodont elements are also used aspaleothermometers, a proxy for thermal alteration in the host rock, because under higher temperatures, the phosphate undergoes predictable and permanent color changes, measured with theconodont alteration index. This has made them useful forpetroleum exploration where they are known, in rocks dating from theCambrian to the LateTriassic.
Preserved articulated association of conodont elements belonging to the speciesArcheognathus primus (Ordovician, North America)
The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.[22]
In the 1930s, the concept of conodont assemblages was described by Hermann Schmidt[23] and by Harold W. Scott in 1934.[24][25][26][27]
Model of elements ofManticolepis subrecta – a conodont from the Upper Frasnian of Poland – photography taken in the Geological Museum of the Polish Geological Institute in Warsaw
The arrangement of elements inozarkodinids and other complex conodonts was first reconstructed from extremely well-preservedtaxa by Briggset al. (1983),[13] although loosely articulated conodont elements are reported as early as 1971.[28] Conodont elements are organized into three different groups based upon shape. These groups of shapes are termedS,M, andP elements.[15]
TheS andM elements are ramiform, elongate, and comb-like structures.[14] 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.[16] The number ofS andM elements present as well as the direction they point may vary bytaxonomic group.M (makellate) elements have a higher position in the mouth and commonly form a symmetrical shape akin to a horseshoe or pick.[13]S elements are further divided into three subtypes:
Sa element - an unpairedsymmetrical ramiform structure at the front of the mouth. Sometimes known as an S0 element.
Sb element - paired asymmetrical structures
Sc element - paired highly asymmetrical, bipennate structures
InP elements, a pectiniform (comb-shaped) row of cusps transitions into a broad flat or ridged platform moving towards the base of the element.[13] 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:
According to these fossils, conodonts had large eyes, fins with fin rays,chevron-shaped muscles and axial line, which were interpreted asnotochord or thedorsal nerve cord.[31][35] WhileClydagnathus andPanderodus had lengths only reaching 4–5 cm (1.6–2.0 in),Promissum is estimated to reach 40 cm (16 in) in length, if it had the same proportions asClydagnathus.[31][32]
Because they are associated with the oral region of the conodont animal, it is accepted that conodont elements are used in the acquisition of food. Two primaryhypotheses have arisen as to how this is accomplished. One hypothesis proposed that elements acted as support structures for filamentous soft-tissues.[36][28] These small filaments (cilia) would be used to filter smallplanktonic organisms out of the water column, analogous to thecnidoblast cells of acoral or thelophophore of abrachiopod.
Another hypothesis contests that the conodont elements were used to actively catch and process prey.[28][16] 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 usingkeratinous blades supported by a simple but effective pulley-like system, involving a string of muscles around acartilaginous core. An equivalent system might have been present in conodonts.[16] 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,[16] or grinding against each other likemolar teeth.
Current consensus supports the latter hypothesis in which elements are used for predation, notsuspension feeding.[32] One line of evidence for this includes the isometric growth pattern exhibited by S, M, and P elements.[28] 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 bothpredators andscavengers.[16] Wear on some conodont elements suggests that they functioned like teeth, with both wear marks likely created by food as well as byocclusion with other elements.[32][37]
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 genusPanderodus may have utilizedvenom in the acquisition of prey.[38] 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.
Studies have concluded that conodonts taxa occupied bothpelagic (open ocean) andnektobenthic (swimming above the sediment surface) niches.[37] The preserved musculature suggests that some conodonts (Promissum at least) were efficient cruisers, but incapable of bursts of speed.[32] Based on isotopic evidence, some Devonian conodonts have been proposed to have been low-level consumers that fed onzooplankton.[37]
A study on the population dynamics ofAlternognathus has been published. Among other things, it demonstrates that at least this taxon had short lifespans lasting around a month.[39] A studySr/Ca andBa/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 differenttrophic niches.[37]
Milsom andRigby envision them as vertebrates similar in appearance to modern hagfish and lampreys,[41] andphylogenetic analysis suggests they are morederived than either of these groups.[9] However, this analysis comes with one caveat: the earliest conodont-like fossils, theprotoconodonts, appear to form a distinct clade from the laterparaconodonts andeuconodonts. Protoconodonts are probably not relatives of true conodonts, but likely represent a stem group toChaetognatha, an unrelated phylum that includes arrow worms.[42]
Moreover, some analyses do not regard conodonts as eithervertebrates orcraniates, because they lack the main characteristics of these groups.[43] More recently it has been proposed that conodonts may be stem-cyclostomes, more closely related tohagfish andlampreys than tojawed vertebrates.[44]
Individual conodont elements are difficult to classify in a consistent manner, but an increasing number of conodont species are now known from multi-element assemblages, which offer more data to infer how different conodont lineages are related to each other. The following is a simplified cladogram based on Sweet and Donoghue (2001),[10] which summarized previous work by Sweet (1988)[8] and Donoghue et al. (2000):[9]
Only a few studies approach the question of conodont ingroup relationships from acladistic perspective, as informed byphylogenetic 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):[11]
Conodont elements from the Deer Valley Member of theMauch Chunk Formation in Pennsylvania, Maryland, and West Virginia, US
detail
Figures 1, 2. Conodonts from the Deer Valley Member of the Mauch Chunk Formation, Keystone quarry, Pa. This collection (93RS–79c) is from the lower 10 cm of the Deer Valley Member. Note the nonabraded, although slightly broken, conodont elements of the high-energy oolitic marine facies of the Deer Valley Member. 1.Kladognathus sp., Sa element, posterior view, X140 2.Cavusgnathus unicornis, gamma morphotype, Pa element, lateral view, X140 3–9. Conodonts from the uppermost Loyalhanna Limestone Member of the Mauch Chunk Formation, Keystone quarry, Pa. This collection (93RS–79b) is from the upper 10 cm of the Loyalhanna Member. Note the highly abraded and reworked aeolian forms. 3, 4.Kladognathus sp., Sa element, lateral views, X140 5.Cavusgnathus unicornis, alpha morphotype, Pa element, lateral view, X140 6, 7.Cavusgnathus sp., Pa element, lateral view, X140 8.Polygnathus sp., Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, X140 9.Gnathodus texanus?, Pa element, upper view, X140 10–14. Conodonts from the basal 20 cm of the Loyalhanna Limestone Member of the Mauch Chunk Formation, Keystone quarry, Pa. (93RS–79a), and Westernport, Md. (93RS–67), note the highly abraded and reworked aeolian forms 10.Polygnathus sp., Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, 93RS–79a, X140 11.Polygnathus sp., Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, 93RS–67, X140 12.Gnathodus sp., Pa element, upper view, reworked Late Devonian(?) through Mississippian morphotype, 93RS–67, X140 13.Kladognathus sp., M element, lateral views, 93RS–67, X140 14.Cavusgnathus sp., Pa element, lateral view, 93RS–67, X140
The earliest fossils of conodonts are known from the Cambrian period. Conodonts extensively diversified during the early Ordovician, reaching their apex of diversity during the middle part of the period, and experienced a sharp decline during the late Ordovician and Silurian, before reaching another peak of diversity during the mid-late Devonian. Conodont diversity declined during theCarboniferous, with an extinction event at the end of the middleTournaisian[45] and a prolonged period of significant loss of diversity during thePennsylvanian.[46][47] Only a handful of conodont genera were present during the Permian, though diversity increased after the P-T extinction during the Early Triassic.
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, especiallyglacial events and associatedmarine regressions which reducedcontinental shelf area. However, their final demise is more likely related tobiotic interactions, perhaps competition with new Mesozoic taxa.[48]
^abAldridge, R.J.; Briggs, D.E.G.; Smith, M.P.; Clarkson, E.N.K.; Clark, N.D.L. (1993). "The anatomy of conodonts".Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences.340 (1294):405–421.doi:10.1098/rstb.1993.0082.
^Barnes, Christopher R. (1967). "A Questionable Natural Conodont Assemblage from Middle Ordovician Limestone, Ottawa, Canada".Journal of Paleontology.41 (6):1557–1560.JSTOR1302203.
^Nicoli, R.S. (1985). "Multielement composition of the conodont speciesPolygnathus xylus xylus(Stauffer, 1940) andOzarkodina brevis(Bischoff and Ziegler, 1957) from the Upper Devonian of the Canning basin, Western Australia".Journal of Australian Geology and Geophysics.9:133–147.
^Świś, Przemysław (2019). "Population dynamics of the Late Devonian conodont Alternognathus calibrated in days".Historical Biology: An International Journal of Paleobiology:1–9.doi:10.1080/08912963.2018.1427088.S2CID89835464.
Aldridge, R. J.; Briggs, D. E. G.;Smith, M. Paul; Clarkson, E. N. K.; Clark, N. D. L. (1993). "The anatomy of conodonts".Philosophical Transactions of the Royal Society of London, Series B.340 (1294):405–421.doi:10.1098/rstb.1993.0082.
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