Conodonts

Australia: The Land Where Time Began

Conodonts

Conodonts were known for a long time only as isolated structures, ranging in size from microscopic to 3 mm long. They are made of phosphatic material, so when limestone is dissolved in weak acetic acid to release fossils they are often found, as they are not dissolved by the acid. They are often very abundant in rocks from the Cambrian to the Triassic. A worm-like animal, Clydagnathus, has been found that was about 4 cm long and 2 cm wide, that had several types of conodont in its mouth area. They appear to have functioned as either teeth or a base for a filter mechanism.

They are known mainly from microscopic remains of jaw-like structures. They are found in many forms, simple rods, cone-like forms, tooth-like protuberances on blades and platform shapes that are rather complex. Sets of conodonts have been found associated with each other, indicating the animal producing them had a number of differently shaped conodonts together in a single structure. Vertebrate affinities were suggested by microscopic studies of conodonts. A study in the journal Science has suggested that true bone cells were present in some conodonts. This finding was not universally accepted by workers in this field. Though the tooth-like cusps along the ridges have the appearance of jaws, the lack of wear on the 'teeth' suggests they were not used in this way. This has led to the suggestion that they may have been part of support structures for either gills or filter feeding structures

Because of the large variety of conodont forms they enabled stratigraphical correlation of fossil beds with others of known age according to the variety of conodont teeth found in them. It is now known from 11 fossil imprints, as at the time of writing, that they were eel-like fish. The known specimens range in size from about a centimetre to the 40-cm-long Promissum from the South African Ordovician. The animal had muscle blocks that were well-developed chevron-shapes, eye cartilages that were paired, as well as possible muscles (Long, 2011). According to the authors⁴ in 1982 the fossil that showed that conodonts were part of an animal was found (Briggs et al., 1983), that was a chordate possibly related to primitive jawless (agnathan) fish such as amphioxus or hagfish. The animal from the Soom Shale in South Africa was Promissum pulchrum. In 1986 it was first described as an very early land plant that possibly had an important position in land plant evolution (Kovács-Endrõdy, 1987). Promissum was believed to be a plant because when they are large enough to be seen on bedding planes they are often seen to be arranged in apparatuses that are suggested by the authors⁴ to possibly be feeding structures, but following acid digestion they are often found as isolated elements.

Some who studied the fossils were sure they were not plants (e.g. Rayner, 1986), they were unsure what they were, it was suggested they might possibly be giant graptolites. Barry Rickards, a brachiopod specialist, suggested they might be conodonts. A conodont expert Dick Aldridge confirmed their identification as conodonts. Promissum pulchrum was eventually recognised as a large conodont animal, the largest conodont animal known (Theron et al., 1990). A specimen of P. pulchrum, was found in 1993 (Aldridge & Theron, 1993) in which the soft tissue had been preserved.

According to the authors⁴, among later finds from the Sandfontein locality was an animal that was up to 400 mm (16 inches) long that had conodont elements, at the front end it had a pair of eyes, a notochord in the form of a stiff rod along the trunk, and myomeres (muscle blocks), suggesting it could swim in an eel-like manner, a wave passing down the trunk and the tail. The conodont elements are situated behind and below the eyes which indicates they were involved in feeding. A problem that remains to be solved is the mechanism of operation of the conodont apparatus. How did it feed? It has been suggested that the elements were used as teeth or raking devices. Others suggest that as there is no sign of wear and the growth pattern they were more likely to have been internal support for structures such as tentacles.

The first conodont animal was found near Edinburgh in the Carboniferous Granton Shrimp Beds, dating to about 340 Ma. A cluster of small conodont elements were found in the head region of long worm-like animals with supporting fin rays in their tails. According to Long (1995), together with the evidence of the suggested 'bone' in some conodonts indicates they may have been close to the line leading to vertebrates. Features not supporting the argument for them being near the ancestral line of vertebrates include the lack of dentine covering bony plates that are sculptured (Long, 1995). Long has placed them 1 level down from fish.

Because the teeth are a phosphatic structure they change colour permanently according to the temperature they are subjected to as fossils.

The earliest vertebrates known to have had hard tissue are from the Late Cambrian, based on conodonts and isolated pieces of dermal armour, most notably Anatolepis found in Wyoming and Greenland, that have been assumed to have come from jawless fish. The knobby surface ornaments on these very small fish have been suggested to possibly be some kind of scale, when seen with a microscope. They are composed of apatite (calcium phosphate), the constituent of bone that is mineralised. This is found in most vertebrates but not in non-vertebrae chordates, or in vertebrates from the Early and Middle Cambrian. A dentine-like tissue surrounding a pulp cavity, making them similar to teeth, has been found to be a component on the fragments of dermal armour of the exoskeleton of Anatolepis (Smith et al., 1996). As well as showing that apatitic tissue of vertebrates may be complex, it also demonstrates that bone evolved some time following the appearance of vertebrates.

Conodonts all possessed hard parts that were mineralised that are composed of white matter that is similar to cellular dermal bone (Long, 2011). Because of this they are now regarded as being more advanced than the extant lampreys.

A detailed study carried out in the UK by Mark Purnell and Phil Donoghue of conodonts has revealed similarities with the higher vertebrates that were unexpected. They found evidence suggesting links to the advanced jawless fishes.

See Early Chordates

Sources & Further reading

John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011
Patricia Vickers-Rich, Thomas Hewitt Rich, Wildlife of Gondwana, Reed Australia, 1993
Benton, Michael J., 2005, Vertebrate Palaeontology, 3rd Ed. , Blackwell Science.
Selden, Paul & Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing.

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