The Devonian was a time of wonder and mystique. The Age of Fishes, it capped the rise of vertebrates and heralded the rise of skeletal diversity. Fish in this age began to inch towards the shore; some would have crawled through the muck of it; some others still out of the water.
But before the Carboniferous rolled in, with its skeleton-enhanced fish, giant insects, and great plants, the Devonian paid heed to a relatively diverse group of “fish,” the placoderms, by giving us the “hinge necked” fish, the Arthrodira. Placoderms are relatively simple looking, with a generally scaly body, simple fins, and a large head fashioned into a sort of shield. The mouth has become enhanced with a bony core for the mandible, otherwise a fleshy-flap within which lay a cartilage framework, and its margin was developed into a variety of shapes, including prongs, saws, or a relatively smooth edge. But amongst the greatest of arthrodires was Dunkelosteus which, along with other giants like Titanichthys, Dinichthys, Eastmannosteus, formed a cadre of super fishes, most of whom were predators (Titanichthys being considered a filter feeder; see Janvier, 1998). Their jaws arranged into an elaborate system of sawing edges, their maws must have been utterly terrifying had we, us wee unarmored humans, ever seen them in the flesh.
Those jaws are not apparently for show: Some research by Philip Anderson and Mark Westneat on one species (Dunkleosteus terrelli, the largest) indicated the ability to produce bites of between 4-5k Newtons of force, which is small in relation to, say, white sharks (Carcharias sp.) at 20k, or the Hell Creek tyrant (Tyrannosaurus rex) at over 35k Newtons (with work by Bates and Falkingham). When you consider even the Newton-to-weight ratio, this is pretty high: Dunkleosteus is smaller than White Sharks, but had a comparable bite; and though much smaller bite than in tyrants was only about 1/10th as strong. So chock one up for tyrants: He needs more publicity.
But that’s nothing compared to piranhas. Newtons to weight, piranhas have a much stronger bite: merely inches long, but producing upwards of 300 N in Piaractus brachypomus, the giant Megapiranha (totally not a made up name) paranensis had a bite of 4k N, but was “only” 70cm long (that’s almost two feet for use ‘Merkans; this revises an earlier, 1m estimate). This study from Justin Grubich of the Egyptian Dept. of Biology at American University in Cairo and colleagues took the bite forces of a variety of fishes and scaled them by their size, with clear outliers being the tiny fish who gave Teddy Roosevelt a second look.
But here’s the thing: When a white shark or a piranha closes its mouth, the teeth disappear. They are enveloped in soft-tissue, the formation of which preceded the bony jaws within them. Arthrodires have jaw bones with shearing edges, bone that has been scraped against bone, resembling a butcher’s blade stuck to the end of each jaw and then rasped against another set. Giant butcher’s cleavers scraping in the Devonian seas, making a horrible shrp shrrr shrp shrrr sound. They ground against one another, forming an eversharpening edge. The edges are curved, too, which would have helps focus force to the spaces between the cusps, the “biting teeth” of the jaw’s margin, the teeth in the saw.
Certainly, some part of the jaws wouldn’t be embedded in fleshy stuff, but certainly not all of it, exposing the mandibles to full view. We’re not talking bare bones here; some soft tissue much be present around the jaws. The question is how much? and would the horrifying cleavers from some stygian abyss been visible as the jaw was closed? Or, instead, would we get this:
Probably an unfair comparison, but it certainly softens the image, somewhat. Makes it look more like a beluga, less like a skeletal horror from beyond. This follows comparison to living fish:
The pliability of the oral integument differs from piranha species to piranha species, and this one (Pygocentrus nattereri) is an especially “lippy” one, whilst some others are far thinner in the tissue. But when all piranhas close their mouths, the teeth disappear. The traditional method of restoring arthrodires, amongst other placoderms, has been to leave the head in a sort of thin shroud of tissue, and this may be true of some fish, but around the oral margin things differ, so thinking about this leads me to suspect a stranger look. One that is, perhaps, fishier, and less frightsome.
That is, until it opens its jaws…
Anderson, P. S. L. & Westneat M. W. 2007. Feeding mechanics and bite force modeling of the skull of Dunkleosteus terrelli, an ancient apex predator. Biology Letters 3: 77-80.
Bates, K. T. & Falkingham, P. L. 2012. Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics. Biology Letters 8, 4: 660-664.
Grubich, J. R., Huskey, S., Crofts, S., Orti, G. & Porto, J. 2012. Mega-bites: Extreme jaw forces of living and extinct piranhas (Serrasalmidae). Nature – Scientific Reports 2: e1009.
Janvier, P. 1998. Early Vertebrates. Oxford University Press (Oxford, England & New York).