conodont

[koh-nuh-dont, kon-uh-]

Minute toothlike fossil composed of the mineral apatite (calcium phosphate); conodonts are among the most frequently encountered fossils in marine sedimentary rocks of Paleozoic age. They are the remains of animals that lived 543–248 million years ago that are believed to have been small marine invertebrates living in the open oceans and coastal waters throughout the tropical and temperate zones.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Conodont

Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. The animal is also called conodontophora (conodont bearers) to avoid ambiguity.

Description

The eleven known fossil imprints of conodont animals depict an eel-like creature with 15 or, more rarely, 19 elements forming a bilaterally symmetrical array in the head. This array comprised a feeding apparatus radically different from the jaws of modern animals. There are three forms of teeth, coniform cones, ramiform bars, and pectiniform platforms, which may have performed different roles.

The organisms range from a centimeter or so to the giant Promissum, 40cm in length. It is now widely agreed that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.

Ecology

The "teeth" of some conodonts have been interpreted as filter-feeding apparatuses, filtering out plankton from the water and passing it down the throat. Others have been interpreted as a "grasping and crushing array".

The lateral position of the eyes makes a predatory role unlikely.

The preserved musculature hints that some conodonts (Promissum at least) were efficient cruisers but incapable of bursts of speed.

Classification

The conodonts are currently classified in the phylum Chordata because their fins with fin rays, chevron-shaped muscles and notochord are characteristic of Chordata.

They are considered by Milsom and Rigby to be vertebrates similar in appearance to modern hagfish and lampreys,Milsom, C. & Rigby, S (2004). Fossils at a Glance. Victoria, Australia: Blackwell Publishing, 155 pp. and phylogenetic analysis suggests that they are more derived than either of these groups.Donoghue, P.C.J.; Forey, P.L.; Aldridge, R.J. (2000). "Conodont affinity and chordate phylogeny". Biological Reviews 75 (02): 191–251. Retrieved on 2008-04-07. This analysis, however, comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paraconodonts and euconodonts. It appears likely that the protoconodonts represent a stem group to the phylum containing chaetognath worms, indicating that they are not close relatives of true conodonts.

Conodont teeth fossils

For many years, conodonts were known only from enigmatic tooth-like microfossils, which occur commonly but not always in isolation, and were not associated with any other fossil. These phosphatic microfossils are now termed "conodont elements" to avoid confusion. This confusion is most apparent for the non-specialist in the book "Your Inner Fish", by Neil Shubin, who describes the origin of teeth in chapter 4. In this chapter, the author attaches the name "conodont" to both the "conodont bearer" (the animal) and the "conodont elements" (the teeth), and the reader may have a hard time to make sense of the concept of "animals living in the mouths of animals".

They are widely used in biostratigraphy.

Conodont elements are also used as paleothermometers, a proxy for thermal alteration in the host rock. This is 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.

It was not until early 1980s that the conodont teeth were found in association with fossils of the host organism, in a konservat lagerstätte. This is because most of the conodont animal was soft-bodied, thus everything but the teeth were not suited for preservation under normal circumstances.