The heterostracans were a group of armoured jawless fish, or ‘ostracoderm’, which lived in both saltwater and freshwater environments from the Ordovician to the Devonian period. They were notable for their characteristic armour, with which they evolved a wide range of forms during their existence: some encased in box-like plates, others in smaller tesserae, some with flattened bodies, others with bizarre pointy nasal structures. I’ll probably be subjecting you to a post on heterostracan morphological diversity later on, so don’t worry too much about taking notes.
Heterostracans are important to palaeobiology because, despite being jawless themselves, they lie somewhere on the ‘stem’ of the phylogenetic tree of the gnathostomes, or jawed vertebrates. In the phylogeny below, the only two living clades are the gnathostomes (jawed fish, including us), which have jaws, paired fins and a bony skeleton, and the cyclostomes (hagfish and lampreys), which have neither jaws, paired fins nor bone. This means that fossils are the only way we have of learning about how these characters evolved. Heterostracans themselves are particularly interesting due to their bone, one of the first occurrences of this tissue amongst vertebrates.
Enter our hero, Pteraspis. This 20cm long animal is pretty famous as heterostracans go, and can usually be found swimming his way through any marine Devonian diorama (there’s a particularly good one in the National Museum of Scotland, Edinburgh). The most striking thing about Pteraspis initially is its large bony head shield. This is characteristic of all heterostracans, although the types of plate forming it and the shape vary dependent on the species. Characters peculiar to Pteraspis and related genera are the long rostral (ie. nose) section and the large dorsal spine. Also worth noting are the characters it lacked. It had no paired fins, although it’s been suggested that the two wing-like protrusions coming out of its head shield served a similar function. It also had no jaws or teeth.
As the vast majority of living vertebrates have jaws and teeth, it might be quite hard to imagine how Pteraspis would have fed. The two groups of living jawless fish- hagfish and lampreys- have highly specialised mouthparts: the former for deep sea scavenging and the other as a ‘sucker’. Heterostracan mouthparts aren’t similar to either of these however, and they also have a more generalised swimming body plan; one thing they do have though is a series of oral plates. These were parts of the head shield (not teeth!) that lay over the oral cavity. Many functions have been suggested: scooping sediment, grabbing onto prey and suspension feeding. Patterns of wear seem to exclude scooping and tiny little outwards pointing denticles on the plates would have prevented big prey from entering the mouth. As such it seems that heterostracans like Pteraspis would have suspension fed on small animals, like many groups of animal today.
Pteraspis and the heterostracans have a lot to tell us about how the jawed vertebrates evolved. They possess bone, but no jaws or paired fins, immediately telling us that these characters of the gnathostome body plan didn’t evolve together. Although we’ve not really discussed it here, the structure of their bone itself can also teach us about the evolution of bone, and how it came to prominence as a tissue. These animals, completely different to anything living in the oceans today, also present their own mysteries, such as how they fed without jaws. We’ve only briefly talked about heterostracans here, but fear not! There’s plenty more to talk about in the future.
- Janvier, P. (1996) The Early Vertebrates, OUP Oxford
- Purnell, M. (2002) Feeding in extinct jawless heterostracan fishes and testing scenarios of early vertebrate evolution. Proc. Roy. Soc. B.