Conodont

Conodonts are extinct chordates that form the class Conodonta. For many years, conodonts were known only from enigmatic tooth-like microfossils, which despite their common occurrence were always found in isolation, and were not associated with any other fossil. These phosphatic microfossils, now termed conodont elements to avoid confusion (e.g. Zhuravlev 2007), are widely used in biostratigraphy; they are also used as paleothermometers, because phosphate undergoes a predictable series of color changes as it experiences higher temperatures. It was not until early 1980s that the conodont teeth were found in association with fossils of the host organism, in a konservat-lagerstätten. This is because most of the conodont animal was soft-bodied, thus everything but the teeth were not suited for preservation under normal circumstances. A few exceptionally preserved fossils have revealed the full conodont animal; new discoveries have expanded the known body fossil record, and a more consistent impression of the animals has emerged.

Once thought to only exist on the millimetre scale, a well-preserved and unusually large genus, Promissum, was found in 1994. It's now widely agreed that conodonts bore large eyes, fins with fin rays, chevron-shaped muscles and a notochord. The latter three are characteristic features of the phylum Chordata (ie., the vertebrates), leading to their current classification. However, such a classification is far from secure: whilst Milsom and Rigby (2004) consider them to be vertebrates similar in appearance to modern hagfish and lampreys, an opinion supported by cladistic analysis carried out by Donoghue et al. (2000). This analysis, however, comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paracononts and euconodonts: most paleontologists (following Szaniawski) deem the protoconodonts to represent a stem group to the phylum containing chaetognath worms, indicating that they are not in fact close relatives of the true conodonts.

The eleven known fossil imprints of conodont animals, sometimes referred to as conodontophora, depict an eel-like creature with 15 or, more rarely, 19 elements forming a bilaterally symmetrical array in the head, comprising a feeding apparatus radically different from the jaws of modern animals. The conodont animal is considered to have been a herbivore: despite their ferocious appearance, the "teeth" were probably used to filter out plankton and pass it down the throat. However, it's not impossible that some were used at biting teeth; the three forms (in the jargon, coniform cones, ramiform bars, and pectiniform platforms) of teeth may have performed different roles. The lateral position of the eyes makes a carnivorous role unlikely.

Because conodonts are phosphatic, they undergo a series of permanent and predictable color-changes when heated to different temperatures. They are therefore used as a proxy for thermal alteration in the host rock. This feature has made them a useful tool for petroleum exploration where they are known, in rocks dating from the Cambrian to the Late Triassic. The Conodont Alteration Index (CAI) is a scale which correlates conodont color to the maximum rock temperature attained at depth.