Euconodonts have long been an enigmatic group of fossil marine animals, represented by minute, comb-shaped or claw-shaped denticles - the 'conodonts' - which were widely used by stratigraphers for dating and correlating geological formations. They are known from the Middle Cambrian (540 million years) to the Late Triassic (230 million years). These denticles, which are made of calcium phosphate, like the vertebrate bones and teeth, have been variously referred to annelids, arthropods, molluscs, chaetognaths, and even plants, although it has been sometimes suggested that they were fish teeth. The clue came in 1983 when the first articulated "conodont animal" was discovered in the Carboniferous of Scotland. Later, yet another "conodont animal" was found in the Ordovician of South Africa. Both forms show an elongated body, with imprints of chevron-shaped muscles, a trace of the notochord, large paired eyes, and a caudal fin strengthened by radials. The conodont organs (i.e. the denticles) are situated in the head, presumably at the entrance of the pharynx. Recent histological studies on the euconodonts have brought to light a variety of hard tissues which recall the enamel, dentine and bone of the vertebrates, but their homology with vertebrate tissues remains controversial.
Euconodonts are characterized by:
- a feeding apparatus which generally consists of three or four types of mineralized denticles. The denticles have a crown of superficial, lamellar, enamel-like tissue which covers a core of dentine- or bone-like tissue (the 'white matter'). These two tissues overlay a third type of tissue, the 'basal body', which has been interpreted as globular calcified cartilage.
- very large, anteriorly-placed eyes
The undoubted presence of well-developed eyes and caudal fin radials in euconodonts are enough to place them among the Craniata, although there position within the craniates is the subject of controversies. Some consider them as the sister-group of all other craniates, others regard them as more closely related to either hagfishes or lampreys, and others place them as the sister-group of all "ostracoderms" + gnathostomes, sharing with them the ability to produce a phosphatic exoskeleton (see Vertebrata page).
The Euconodonta are classically gathered with two other taxa, the Protoconodonta and Paraconodonta, in the phylum Conodonta, despite quite different histological structures. To date, only articulated specimens of the Euconodonta are known, and it is not ruled out that the two other groups have nothing to do with the latter. There is some evidence, for example, that the Protoconodonta are related to modern chaetognaths. There are several classifications of the Euconodonta, but most of them are largely phenetic. They usually include seven orders, the Proconodonta, Belodellida, Protopanderodontida, Panderodontida, Prionodontida, Prioniodinida, and Ozarkodinida.
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