Ordovician jawless fishes

After the Cambrian radiation of basal vertebrates, with and without skeletons, and the conodonts, a diversity of groups of fishes appeared in the Ordovician, but most are represented only by dermal elements. Two 'agnathan' clades, the Astraspida and Arandaspida, are known, however, from more complete specimens in the Upper Ordovician (Sansom et al., 2001).

Astraspids and arandaspids are small fishes, about 200 mm long. They have a mobile tail covered with small protruding pointed plates, and a massive bony head shield made from several large plates that cover the head and most of the body.

Astraspis from North America (Sansom et al., 1997) has an extensive head shield composed of a complex of separate bony plates that surround large eyes on either side, and a series of eight separate gill openings in a row (Figure 3.7(a)). The body is oval in cross-section, and covered with broad overlapping scales, but the tip of the tail is unclear. The bony plates are composed of aspidin (see section 3.2) covered by tubercles composed of dentine capped with enameloid. The tubercles are typically star-shaped over much of the body, hence the name Astraspis ('star shield').

Sacabambaspis from Bolivia (Gagnier, 1993) has a head shield (Figure 3.7(b)) made from a large dorsal (upper) plate that rose to a slight ridge in the midline, and a deep curved ventral (lower) plate. Narrow branchial plates link these two along the sides, and cover the gill area. Long, strap-like scales cover the rest of the body behind the head shield. The eyes are far forward and between them are possibly two small nostrils, and the mouth is armed with very thin plates.

The fossils of Sacabambaspis and Astraspis show clear evidence of a sensory structure that is peculiar to all fishes (except hagfishes) —the lateral line system. This is a line of open pores within each of which are open nerve endings that can detect slight movements in the water, produced for example by predators. The arrangement of these organs in regular lines allows the fish to detect the direction and distance from which the disturbance is coming.

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