a girrafe on a unicycle one of the most exciting areas in evolutionary biology at the moment – tracing the origins of gnathostomes, or jawed vertebrates. Before I start, I owe a debt of gratitude to our Latin-America Correspondent, Dr A. C. of Santiago de Chile, who reminded me that I’ve posted on this before. This means that I don’t have to go into as much as a preamble as I’d feared. So, before reading any further, I recommend you look at my earlier post on this issue, and come back later.
The evolution of jawed vertebrates from their jawless ancestors has been a perplexing evolutionary problem for a while. The reason is easily stated – an immense morphological gulf separates the two states. It’s not just a question of bolting a jaw onto a jawless substructure, in the manner of Jacob Marley’s ghost. The transition to the jawed state encompasses the appearance, seemingly wholesale, of many other features, such as paired limbs, notwithstanding inasmuch as which many changes to the head and face. One of these changes includes an entire reorganisation of the face to produce two nostrils rather than just one.
The only extant jawless vertebrates – lampreys and hagfishes – don’t say much (being jawless, you see) and in any case have been evolving along their separate tracks for hundreds of millions of years. This means that their anatomies needn’t reveal much about their common ancestry with gnathostomes.
Luckily we have the fossil record to help. Back in the Silurian and Devonian Periods (between about 443 and 359 million years ago) there existed a wide range of jawless fishes, collectively the ostracoderms. Although now entirely extinct, they sported thick, bony plates on their heads and trunks, which, in some cases, preserved the internal details of their skulls in, on occasion, very fine detail, such that you can trace the courses once taken by nerves and blood vessels. Some of the ostracoderms appear to have had paired pectoral fins, and it is generally thought that paired pectoral fins evolved before jaws. This implies that the ostracoderms were not a natural group, but a varied assemblage whose various forms were more or less related to gnathostomes. Which realization is, you’ll note, a place to start to bridge the morphological gap between the jawed and jawless states. The features we see in jawed vertebrates today needn’t have evolved all at once, but sequentially, and fossils will help us set this sequence in order.
For example, some recent work has shown how some ostracoderms had gone some way towards evolving two nostrils, creating, for the first time a morphological field in the mid-face that made space for jaws. These ostracoderms didn’t have paired fins. Therefore, it seems a fair bet that paired nostrils evolved before paired fins, though the reality was almost certainly more complex – it’s possible to imagine fishes with single nostrils and paired fins, and fishes with dual nostrils and no paired fins. Convergence, as we know, is rife.
Another extinct group of fishes was the placoderms. These had large, bony plates on their heads and trunks – indeed, some of them look at first glance like some ostracoderms – but they also had jaws.
Relating jawed, bony fishes (the osteichthyes, which includes ourselves) to placoderms has always been tough, because the skeletal elements of placoderms do not easily correspond with those of bony fishes. The jaws, in particular, look quite different. Indeed, until recently it wasn’t known whether even the teeth of placoderms were evolutionary homologues of teeth in other jawed vertebrates (they are.) The placoderms comprise a large and diverse group, and it’s now generally thought that they, like the ostracoderms, do not form a natural group with a single common ancestry, but an assemblage whose components are more or less related to gnathostomes.
Amid all this brouhaha come the acanthodians (another extinct group) and the chondrichthyans (sharks and rays, very much still with us.) Unlike placoderms, ostracoderms – and, it has to be said, bony fishes – chondrichthyans and acanthodians don’t have heavily armoured heads and trunks. Rather, they tend to be covered with lots of very small, bony scales, which – in sharks at least – look very much like teeth.
This condition is often assumed to have been primitive for gnathostomes. That is, the common ancestor of gnathostomes is assumed to have been clothed in sharkskin rather than heavy armour: and sharks are cast into the role of living fossils, swimming around much as they always have done for more than 400 million years.
The implications of this view are profound. It means that ostracoderm armour needn’t have anything to do with the armour of anything else, and that placoderm armour had nothing in particular to do with osteichthyan armour, because they’d have to have evolved convergently. This means that there hasn’t been much impulse to forge any close links between placoderms and osteichthyans.
All of this changes following the revelation of Entelognathus, a fossil fish from the Silurian of China which looks in all respects like a placoderm, but with jaws and other features of head anatomy that correspond in detail with osteichthyans. You can find the paper here, with a commentary here, and a news story here. Entelognathus is the earliest fish we know of that has something close to the modern gnathostome face.
Here’s an evolutionary tree summarising the current state of play.
Importantly, the place of Entelognathus in this scheme of things isn’t entirely secure. That’s what happens when one discovers a fossil that contains a combination of characters otherwise seen in quite separate groups – in this case, a fish with placoderm armour and osteichthyan jaws. The effect of this discovery is not to insert a ‘missing link’ into a scheme that’s already well established, but to re-draw that scheme entirely. That’s why the idea of the missing link is entirely erroneous and, in fact, antithetical to scientific progress.
One effect of the discovery of Entelognathus is to show that the condition of sharks, with their clothing of small, tooth-like scales, is not primitive, but advanced (or ‘derived’, to use the correct terminology.) The acanthodians, rather than being a motley assemblage as suspected, are now a group allied with sharks. The common ancestor of both groups would have evolved from a creature, presumably something similar to Entelognathus, with a heavy, placoderm-like armour.
Another effect has been to show that the armoured state of osteichthyans and placoderms might, in fact, stand detailed comparison, both with each other and the ostracoderms.
More broadly, the discovery overturns another old idea about the origin of teeth. If, as had been traditionally assumed, the shark-like state was primitive, it follows that teeth started off as scales found all over the body, and became confined to the mouth only later on. The new research suggests that the reverse is more likely – sharks co-opted their tooth-making skills to spread them all over the body.
As I explained, untangling the early history of the jawed vertebrates is a field currently in much ferment and excitement.
Watch this space.