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In lecture 1, we discussed the characteristics of the protochordates. One particularly important characteristic was their filter feeding mechanism using cilia, employed because they lack a distinct head, mouth and jaws.

In lecture 2, we have looked at the development of the head enabled by the acquirement of HOX genes. This allowed the advancement from cilia to the evolution of tooth plates in some agnathans, although filter feeding is still employed.

The next group to consider is the ostracoderms, which are jawless but well armoured vertebrates.

Ostracoderms share the following characteristics:

1. Mouth plates arranged around a small circular mouth; located farther forward in the head than jawed vertebrates.

2. Share a covering of dermal bone in the form of carapace or scales. They have definite bone and mineral tissue.

3. Lack jaws but have moveable mouth plates.

4. Possess a midline dorsal fin and only derived forms had lateral paired appendages.

5. Separate pharyngeal gill pouches along the side of the head.

6. The notochord is the main axial support throughout adult life

7. A muscular (not ciliary) pump creates suction to drag in small and slow moving prey

8. Gills are used for respiration not feeding

9. They co-existed with gnathostomes, which are therefore not thought to be responsible for ostracoderm extinctions

There are 4 main orders within the ostracoderm group.

Order Pteraspida. “Wing shields”.

	Flattened, heavily armoured, fish, but with four or more plates fitting around the head/trunk and covering up to 1/3 of the body. These are pierced by the mouth. Oral plates later developed enlarged tooth-like projections for scraping.
	A lateral eye and paired olfactory pouches are present but no nostrils
	No internal skeleton is known, but impressions on inside of head shield suggest a very primitive, ammocoete-like, anatomy, with eye-muscles of other vertebrates here still somites.
	External gill openings present but lacked paired appendages.

Order Cephalaspida. “Head-shields”.

	Flattened, heavily armoured, fish from the Ordovician - Devonian.
	Solid dorsal head-shield enclosed brain, nerves and blood vessels.
	Had single dorsal nostril, not running through to pharynx, like lampreys.
	Flexible throat allowed some sort of pumping/breathing/filter-feeding.
	10 pairs of gill openings around the edge and paired appendages

Order Anaspida. “No shields”.

	Fish-shaped Silurian-Devonian fish, with thin scales or plates covering the body in most genera.
	Bethnic detritus feeders
	Single dorsal nostril.
	Oblique row of gill openings and stabilising dorsal, anal and lateral projections. This is a unique feature thought to have advanced agility and locomotory ability.
	A hypocercal tail.
	Jamoytius, from the Silurian of Scotland, has a trace of a cartilaginous branchial skeleton but it was naked anteriorly.

Order Thelodonta

	Covered in tiny denticles instead of plates, the armour is lighter and is thought to have contributed to the improved swimming by the members of order.
	Pharyngeal denticles are thought to have been used to crush prey and trap fragments with pharyngeal teeth and gill rakers.
	Well developed stomach

In the next lecture we will discuss how 'proper jaws' for feeding evolved from the toothplates mentioned in this lecture.

Further Reading:

Pough, F.H., Janis, C.M. & Heiser J.B. (2002) Vertebrate Life. Chapter 3: pages 64 - 80

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