The evolution of jaws in early vertebrates was an extremely significant innovation. The fossil record shows that the earliest jawed fish were around at the start of the Devonian Period, just over 400 million years ago. Prior to that, all fish were jawless, much like the hagfish and lampreys of today. Jaws allowed vertebrates to exploit a wide range of food sources, and to become formidable predators.
However, a research team led by the University of Bristol has shown that fish with the earliest jaws were stuck in a compromise between maximizing the strength of their jaws and their bite speed. Big, strong jaws would enable the fish to handle sizeable prey, but the weight of these jaws would make lightning-speed reactions impossible. It seems that, irrespective of the particular way in which early fish species evolved to use their jaws, they were always faced with this same trade-off.
Jaws evolved from the gill arches of jawless fish; these structures supported the gills of early fish and therefore had more of a gaseous exchange function. In fact, the original selective advantage gained by fish with jaws was probably related to increased efficiency in gaseous exchange, rather than to feeding. However, the advantages of jaws for feeding would soon have made this their primary function. All vertebrate jaws, including the human jaw, evolved from the early fish jaw plan.
The new study, which is published in the journal Science Advances, investigates how a breathing structure came to be a successful biting structure and what constraints there were on this evolutionary change. The researchers collected data on the shapes of the earliest fossil jaws and developed mathematical models to characterize them. They then imagined the range of theoretical jaw forms that early fish might have exploited, and ran these through the model in order to characterize them and compare them to the actual fossil jaws.
Previous research had identified that the jaws in early fish fossils are rather similar, showing little of the diversification and radiation that evolutionary biologists might have expected for such a useful innovation. However, when the researchers tested all the different jaw shapes (both actual and theoretical) for their strength (how likely they were to break) and speed (how efficiently they could be opened and closed), they found that the jaws of early fish represented an optimal trade-off between these two factors. They had already reached this state of optimal compromise early on in evolutionary history.
The researchers say their results show that many fish taxa have independently evolved similar jaw morphologies that represent a compromise between strength and snappiness, and that this is evidence for a widespread convergence in jaw shape. Since this optimality was achieved very early in jaw evolution, they feel this supports the idea that early jaws were soon adopted for a predatory function, which would have required both speed and strength.
Some fish groups appear to have evolved away from this optimum balance between strength and speed over time, perhaps as a result of specializing in slower prey or prey that are smaller or weaker. For example, feeding on plankton by filtering the water would not require optimum strength in the jaws.
“Jaws are an extremely important feature to gnathostomes – or jaw-mouths. They are not only extremely widespread, but almost all creatures that have them, use them in the same way – to grab food and process it. That’s more than can be said for an arm or a foot or a tail, which can be used for all sorts of things,” said study lead author William Deakin, who is a PhD student at the University of Bristol.
“This makes jaws extremely useful to anyone studying the evolution of function. Very different jaws from very different animals can be tested in similar ways. Here we have shown that studies on a large variety of jaws, using theoretical morphology and adaptive landscapes to capture their variety in function, can help shed some light on evolutionary questions.”
According to study co-author Professor Philip Donoghue, the earliest jawed vertebrates have jaws in all shapes and sizes, long thought to reflect adaptation to different functions. “Our study shows that most of this variation was equally optimal for strength and speed, making for fearsome predators.”