Conodonta: Conodonta

Characters: bilaterally symmetrical vermiform (worm-like) animals; eyes present; conodont elements appear to be mouth parts; elements grew through successive addition of apatite layers around a basal cavity & usually capped by a tooth-like cusp; shapes depend n unequal addition of apatite; processes created by expansions may be capped with denticles, ridges, etc.; V-shaped myomeres; .

Notes: [1] the existence of a basal cavity is thought to mean that the elements grew within an epithelial pocket of the animal. [2] simple coniform types dominant in Cambrian first half of Ordovician. Compound elements (ramiform, pectiniform, etc.) first occur in Furongian and dominate after the Middle Ordovician. During Early Silurian, coniforms prevalent in off-shore faunas, while compound types were more numerous in nearshore environments. Coniform types disappeared after Early Devonian. [2] conodont elements known from ~50My prior to first bony scales from fish (Sacabambaspis).

Characters: Relatively simple forms with simple cones [SD01]; single tissue layer probably homologous with basal body of euconodonts [D+00] [SD01]; elements relatively poorly mineralized [D+00]; growth proceeds by adding to margins & base, but not tip [D+00].

Note: no soft tissue remains of paraconodonts are known. However, the group may be paraphyletic and include euconodonts. The material of paraconodont elements, as well as of euconodont basal bodies, is believed to be a dentine homologue.

Large, anterior-most cups, believed to be eyes with extrinsic eye muscles; notochord not reaching most anterior portion of cranium; existence of brain inferred from encephalization of sensory structures; complex, poorly-understood tooth-like "conodont elements" formed in calcium phosphate (bone?) in head region; elements include grasping ramiform ('S' and 'M') elements and transverse (biting?) comb-shaped 'P' elements; conodont elements may have rasping function like hagfish and lampreys; 'P' elements may be velum homologue; conodont elements with precise occlusion suggested by microwear; elements formed poorly mineralized, finely crystalline base (probably dentine) and highly mineralized, coarsely crystalline crown probably "homologous to enamel"); possible gill pouches; long, eel-like body with simple, V-shaped myomeres; fast-twitch fibers may not be present; possible longitudinal septum; 2-lobed (hypocercal?) caudal fin with radials, but no evidence of fin musculature; indeterminate growth

Note: Estimated 65% of all jawless vertebrate species are conodonts, but the taxon was known only from conodont elements until 1983. A few body fossils are now known primarily from lagerstätten in Scotland and South Africa. The euconodonts are probably descended from the paraphyletic Lower Cambrian Paraconodontida; but this taxon is only known from crownless basal mineralizations and no soft tissue is known. The still more primitive Pre-Cambrian Protoconodonta are believed to be unrelated. APW040316.

Note: not to be confused with Protoconodontida, which are probably non-chordate (chaetognath) remains. APW040316.

Characters: apparatus having a pair of incisor-like (M?) elements in front, a set of four pairs of relatively gracile (S?) elements connected into a single unit by one posteriorly located symmetrical (S₀?) element, and two pairs of robust (P?) elements hidden within the throat.

Conodonta

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