Palaeobiology and Biodiversity Research Group, Department of Earth Sciences, University of Bristol

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Exploring the mechanics and evolution of a major innovation

A sample of lower jaw diversity from 400 million years ago which include from top to bottom: a giant 8-metre apex predator, a lungfish with a duck-like snout, a reef-dwelling representative of a totally extinct group of vertebrates, a fish-like relative of land animals, and a shark-like cousin of bony fishes. Jaws are not to scale and all are oriented so their front end is to the left.

Using a combination of physics, engineering and palaeobiological methods, researchers at Bristol have been able to shed light on the evolution of the very first jawed animals. These new methods not only allow the early rise of jawed animals to be explored quantitatively, but also can be used to test classic ideas of competition and evolution in early vertebrates - that is all animals with a backbone, including ourselves.

420 million years ago in the Silurian, the world was a remarkably alien place. The warm Silurian seas were teeming with an array of unfamiliar animals including strange shelled 'squids', giant scorpions and perhaps most bizarre, a large diversity of armoured fishes with mouths but lacking any sort of jaw structure.

These jawless fishes were the dominant vertebrate animals of the time (plants and insects had already colonized land, but, vertebrates had not yet taken that step - no mammals, dinosaurs, crocodiles, lizards, birds) and are believed to have used their oral and throat musculature to suck prey into their mouths. Vertebrates with jaws had just evolved. However, they were an oddity, relegated (at least initially) to bottom feeders who potentially utilized their new jaws to help move water through their mouths more efficiently. However 10 million years later, by the Early Devonian, these jawed vertebrates had evolved a wide array of jaw structures allowing them to take on a range of new ecological niches such as crushing and flesh cutting.

By the end of the Devonian period, the jawless fish all but disappeared, while the jawed vertebrates became dominant. Many scenarios have been put forward regarding the initial rise of jawed vertebrates and how they grew to supremacy over their jawless cousins, including direct competition, where the jawed fishes outcompeted their jawless predecessors due to their supposed superior mode of feeding. However, little data has ever been collected to test any of these ideas directly.

Researchers at Bristol, along with colleagues at the University of Oxford and NCB Naturalis in Leiden (The Netherlands), have taken the first steps to unraveling this story by quantifying the variation in feeding function amongst the earliest jawed vertebrates. It is possible to assess the potential feeding function of a fossil organism by using principled derived from physics and engineering. By applying these methods to a range of fossils, the group at Bristol was able to estimate the amount of functional variation in the deep geological past.

Jawed vertebrates achieved high levels of functional (and probably ecological) diversity by the earliest Devonian, when they were still just a minor part of the vertebrate fauna. They then maintained this level of diversity throughout the rest of the period. In fact the variation in jaw types within communities is the same regardless of whether they include a large number of jawless fishes or not. These results argue against scenarios of direct competition between jawed fish and their jawless cousins.

The Devonian period was a tumultuous time marked by rising oxygen levels and punctuated by a series of major extinctions near the end of the period which caused a major collapse in global reef diversity and drove several of the jawed fish groups extinct. However, overall jawed vertebrate functional diversity remained stable. As new species evolved after extinction events, they took on similar functions to their recently extinct ancestors. Essentially new animal "actors" filled in empty ecological "roles."

Unusual jawed ancestors

Although perhaps more recognizable than their jawless contemporaries, the early jawed vertebrates would not have looked that much more familiar. The jawed fish fauna was dominated both in terms of numbers of kinds and functional variety by the relatives of modern lungfish and an extinct group of armoured fish called placoderms. The ancestors of the majority of modern fish (the actinopterygians) show very little functional innovation and few species.

The end of the Devonian also saw the first potential exploration of land by jawed vertebrates. These early tetrapods and their relatives had modified fins with the first evidence of fingers and toes. However, the functional analysis shows they were still very fish-like in terms of their jaw design illustrating that not all parts of an animal evolve at the same time or rate.

See the original press release here. (Opens in a new window)
Read more about "Jaws vs. Jawless" at Pete Wilton's Oxford Science Blog (Opens in a new window)

A high resolution version of the image above is available here.