We vertebrates all have jaws, with the exception of our friends the lamprey and hagfish. Jaws began to evolve in the Silurian, over 400 million years ago, from the front gill arch in an early fish and have been extremely useful to us.
This origin of the jaw in the first gill arch is by now widely accepted. One open question is when and how this arch lost its breathing purpose and assumed a feeding function, and what sort of feeding might have ensued. Did the first jawed fish use their new accoutrement to suction-feed? Or could they bite, snapping their jaws shut on some passing life form?
Now a new analysis published in Science Advances on Friday, based on actual fossil fish jaws and simulated fish jaws, has concluded: the earliest jawed fish could bite, and could do it hard, strong and fast.
The study is based on the earliest fossil fish jaws and mathematical models, taken from engineering, to characterize these jaws. These models enabled the team to extrapolate a wide range of theoretical jaw shapes that early fish could have had. Then they "tested" the theoretical jaws for their strength – how likely they were to break when biting, and speed – how efficiently they could snap shut.
The jaws were found to tread a fine line of optimality, balancing the need for both speed and strength, William Deakin of the University of Bristol and colleagues report. Moreover, in the course of the evolution of jawed fish who begat today's vertebrates, the lower jaw underwent various evolutionary innovations from the Silurian period to the Devonian period. Yet according to the model, over the eons, fish clustered along the optimal line, with the odd deviation.
This is a good time to explain that the entire paper relates solely to the mandible, the lower jaw. It discusses what apparently happened when a gill arch turned into the lower jaw, Deakin explains. It does not relate to the upper jaw, which is related to the skull, not to the lower jaw, appearances notwithstanding.
In other words, "jaws" is a colloquialism. Now we go back to the early mandible, or lower jaw.
There are two main theories about the evolution of said jaw, Deakin explains: that the gill arch became more moveable and was used to bite right away. Or, that the arch first evolved into a better way to pump water across the gills to aid in ventilation, which was an intermediary step towards the evolution of the jaw, for suction or biting.
The paper does not go there – it isn't testing these two hypotheses, but its conclusions support the first hypothesis: that the earliest jaws served for biting. This pattern of optimal balance between biting and speed persisted in all ancient jawed fish groups: Placodermi, Chondrichthyes (sharks and rays), Actinopterygii (ray-finned fishes), and Sarcopterygii (early bony fish), the authors write.
"A thicker jaw will be stronger and more resistant to biting forces but a slimmer jaw will be faster," Deakin says – both being things a fish wants. What is optimal in this respect?
He helpfully compares its evolutionary predicament with car-shopping. You want the cheapest, most fuel-efficient car. Type 1 is cheap but guzzles. Type 2 is expensive but cheap to run. And if there's a Type 3 that is both cheap, fuel efficient it will win, and that's what all those ancient fish were like.
The optimality of the early jaw's balance between strength versus speed was tested with the help of the engineering model, applied here to evolution.
Which brings us to wonder about the jaws – excuse the expression – of fish vertebrate that suck.
Who are suction feeders? Your goldfish is one of them. In fact, it turns out that most fish are suction feeders. While seemingly not as dramatic as a shark attack, the suction function is a predatory event involving a complex system of bones and muscles that expand the head, to suck in water with goldfish pellets, marine life and so on. On the face of it they sound like a natural evolution of a breathing mechanism.
One wonders about insect jaws – convergent evolution? The entomological path of personal development, from egg etc., is categorically different from that of vertebrates, and their "jaws" consist of a lot of fiddly bits, not a lower mandible closing in on a part of the skull known as the upper jaw. But they do serve a similar function, and must, like in our primordially jawed fish and in subsequent jawed vertebrates, withstand stress and work reasonably fast.
Finally, if the primordial mandibles hadn't served for early biting, then their forms would plausibly have been more diverse early on, but they're not, Deakin points out. True, the data we have on the very earliest fish is spotty. But with all that evolutionary space to explore, if jaws stuck to the straight and narrow optimal line, this is quite remarkable, he sums up. It suggests these very early jaws were being used to bite down on stuff, and hard.
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