Conodonta

One of the longest-lived groups of early vertebrates, the conodonts, were identified with certainty as fishes only in 1983. Conodont elements, small (0.25-2 mm) toothlike structures made from apatite, have been known since 1856, and they are so abundant in many marine rocks from the Late Cambrian to the end of the Triassic that they are used for stratigraphic dating. Particular conodont species, and groups of species, are characteristic of certain stratigraphic zones, and they form the basis of a worldwide international standard of relative dating. Over the years, these small phosphatic fossils have been assigned to many groups: annelid or nemertean worms, chaetognaths (arrow worms), molluscs, representatives of a separate phylum, or even plants.

The enigma was solved when the first complete conodont, Clydagnathus, was reported from the Lower Carboniferous of Edinburgh (Briggs et al., 1983), and since then nine further specimens have been located in the Edinburgh rocks (Aldridge et al.,1993;Donoghue et al., 1998,2000), as well as different conodont taxa from the Upper Ordovician of South Africa and the Lower Silurian of Wisconsin, USA. The first Edinburgh specimen (Figure 3.5(a)) is a 40.5 mm long eel-like creature that appears to show several chordate synapomorphies: a head with eyes, a notochord and myomeres. Specimen 5 (Figure 3.5(b)) provides additional evidence of the large eyes, including cartilages that supported the eyeballs themselves. Behind the eyes in specimen 1 are possible remnants of the otic capsules, structures

Fig. 3.5 The conodont animal Clydagnathus from the Lower Carboniferous Granton Shrimp Bed of Edinburgh, Scotland: (a) specimen 1; (b) specimen 5. In places, fossil shrimps lie across the conodont bodies. The animal is 40 mm long. (Courtesy of Dick Aldridge.)

Fig. 3.5 The conodont animal Clydagnathus from the Lower Carboniferous Granton Shrimp Bed of Edinburgh, Scotland: (a) specimen 1; (b) specimen 5. In places, fossil shrimps lie across the conodont bodies. The animal is 40 mm long. (Courtesy of Dick Aldridge.)

associated with hearing and balance, and traces of what may be branchial bars. The phosphatic conodont elements lie beneath the head region, in the oral cavity on the pharynx.

Conodont elements occasionally had been found in associations of several types, usually arranged in a particular way. These conodont apparatuses (Figure 3.6(a)) were interpreted as the jaw or filter-feeding structures of some unknown animal, and the 1983 find proved that they were indeed complex feeding baskets. More recent work has revealed microwear patterns on different conodont elements, which demonstrates that they functioned in feeding, in seizing prey and chopping it into pieces (Purnell, 1995). The backwardly-directed teeth helped the conodont stuff its food into its mouth, and perhaps prevent any live prey from escaping.

The body region of the Edinburgh conodont animals shows a clear midline structure (Figure 3.5) that has been interpreted as the notochord. Some specimens show traces of a possible dorsal nerve cord above the notochord. The other obvious feature of the body is its division into V-shaped tissue blocks, or myomeres (Figure 3.5), muscle units that contracted on alternate sides to provide a powerful eel-like swimming motion. Conodonts had narrow tail fins, as shown by tissue

Conodont Diagram Label
Fig. 3.6 The conodont animal: (a) a complete conodont apparatus ofthe type possessed by Clydagnathus, showing different tooth elements (P, S, and M types), and location of the apparatus; (b) restoration of Clydagnathus in life, showing the eyes and the eel-like body. (Courtesy of Mark Purnell.)

traces on either side in some specimens (Figure 3.5(a)). Overall, the conodont animal looked very like a small lamprey (Figure 3.6(b)).

The affinities of conodonts have long been debated, and the new whole-body specimens did not at first resolve the issue. The consensus now is that conodonts are vertebrates, and more derived than the extant lampreys and hagfishes (see Box 3.1). Chordate synapomorphies are the notochord, the dorsal nerve cord, the myomeres, the tail and the midline tail fin. Vertebrate synapomor-phies are the cranium in front of the notochord, the paired sense organs, the extrinsic eye musculature (absent in hagfishes) and the caudal fin with radial supports. The dentine and enamel of conodonts, as well as the eyes, with their sclerotic eye capsule, presumably developed from neural crest tissues of the early embryo, a clear vertebrate feature (see section 1.4.4; Donoghue et al., 2000). Conodonts are more derived than living lampreys and hagfishes, as they share the presence of bone-like calcified tissue with other vertebrates, and there are at least two types: dentine and enamel-like tissue.

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