Microvertebrates

Conodonts

Conodonts are small, phosphatic, tooth-like fossils that formed part of a complex feeding apparatus in an extinct eel-like fish. Ranging from the Cambrian to Triassic, they are important Palaeozoic biostratigraphic indicators. As their color changes with temperature of burial, they are used as indicators of thermal alteration in rocks.

Morphology

Conodonts can be divided into three basic forms (Fig. 13.14):

1 Coniform elements (cones): cusp-shaped conodonts that curve and taper to a narrow tip.

2 Ramiform elements (bars): blade-like, elongated, multicusped forms.

3 Pectiniform elements (platforms): broad-based, multicusped conodonts.

Rare fossil specimens show that conodont associations were arranged symmetrically. These associations, or apparatuses, were usually formed of seven or eight pairs of conodonts.

Cusp

Cusp

Basal cavity

Basal cavity

Main cusp —Denticle

Basal cavity

(a) Coniform

(b) Ramiform Plane of symmetry

Main cusp —Denticle

Basal cavity

Blade-

Platform

Blade-

Pectiniform

Fig. 13.14 The main forms of conodont elements: (a) Hertzina (6 mm) (b) Ozarkodina (c. 1 mm); (c) Polygnathus (c. 1 mm); and (d) natural assemblage of conodont elements in Scottognathus typicus.

The conodont animal

True conodont animals, preserved with the conodonts in situ, have been found from several localities including Scotland. The animal is bilaterally symmetric with a long eel-like body, up to 55 cm in length, a small head, segmented muscles, and fins. Large eyes dominate the head and the conodont apparatus is positioned immediately behind them. The conodont animal was a vertebrate that resembled a hagfish or lamprey and the apparatus may have been used to grasp and crush prey (Fig. 13.15).

Paleoecology

Conodonts are found in a range of marine environments but are most common in tropical nearshore environments. Some Palaeozoic communities show a relationship with depth, and deeper water assemblages are generally less diverse.

Evolutionary history

Simple coniform elements are known from the early Cambrian. Conodont diversity reached its maximum during the mid-Ordovician with 60 genera. Diversity declined sharply through the Silurian, but conodonts radiated again during

Conodonts Scotland

5 mm

Fig. 13.15 Impression of the body of the conodont animal from the Lower Carboniferous Granton Shrimp Bed, near Edinburgh, UK.

5 mm

Fig. 13.15 Impression of the body of the conodont animal from the Lower Carboniferous Granton Shrimp Bed, near Edinburgh, UK.

late Devonian times. After the Devonian extinction, conodont associations were less variable. Only a few forms survived the Permian mass extinction and the entire group disappeared at the end of the Triassic.

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