Why Are Those Teeth Like Steak Knives? Cutting Flesh Needs Edges
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The teeth of meat-eating dinosaurs carry fine serrations along their edges that work exactly as a serrated blade does, and their size and spacing differ enough between species to identify the animal.
Why serrations cut better
A smooth blade cuts by pressing through material, which works on something that parts readily and works poorly on tough fibrous tissue such as muscle and skin. A serrated edge cuts differently, concentrating force at the points of the serrations so that each one initiates a tear, and the spaces between them grip the material and prevent it sliding away from the blade. Drawing such an edge through flesh therefore starts many small cuts that join into one, which is why a serrated knife cuts a tomato and a smooth one slips off. Teeth built this way slice rather than crush, which suits an animal that swallows pieces of a carcass whole.
What the teeth are like
The pattern is consistent enough to be diagnostic:
- •Blade-like rather than conical, flattened from side to side
- •Curved backwards, so prey pulling away drives the tooth in further
- •Serrations along both the front and back edges in most species
- •Serration size measured as the number per millimetre, which varies by species
- •Small cracks at the base of each serration that stop it splitting further
- •Replaced continuously throughout life, so shed teeth are common fossils
What the differences tell palaeontologists
Because the pattern differs between groups, an isolated tooth found on its own can frequently be identified without anything else. Counting serrations per unit length, measuring their shape, and examining the cross section of the tooth distinguishes major groups reliably and sometimes individual species. That matters enormously, since teeth are the commonest vertebrate fossil by a wide margin, being hard, numerous and shed continuously, so a site yielding no bones may yield teeth that establish which animals were present. Teeth also record wear and breakage, which indicates how the animal fed, and chemical analysis of the enamel supplies information about diet and about the water the animal drank.
How they are studied
Working out what a tooth actually did requires more than looking at it, and several approaches are combined. Microscopic wear patterns on the surface record what the tooth contacted, with scratches indicating slicing and pits indicating contact with bone. Bite marks on fossil bones are matched against teeth to identify who made them, and a small number of specimens preserve a broken tooth embedded in the bone of the prey, which is the most direct evidence available. Engineering models estimate the forces a skull could generate and the stresses a tooth could survive. And replicas are drawn through meat and bone experimentally, which is a standard and slightly gruesome part of the field.
What else has them
Serrated cutting teeth evolved repeatedly in unrelated animals, which indicates they solve the problem well. Sharks have them, in a range of forms that similarly distinguishes species and that appear in the fossil record in enormous numbers. Komodo dragons have them and use the same slicing action. Several extinct marine reptiles had them. Among mammals the arrangement is different, since mammals chew and dinosaurs did not, so carnivorous mammals evolved paired blade-like teeth that shear against each other rather than serrated single teeth. And the engineering has been borrowed back, with the geometry of fossil teeth studied directly by people designing cutting tools.
The takeaway
Serrations concentrate force at points that each start a tear while the gaps grip the material, so a drawn edge slices tough tissue that a smooth blade slips off. The teeth are flattened, backward curved and serrated on both edges, with small cracks at the base of each serration preventing splitting. Serration count and shape identify species from an isolated tooth, which matters since teeth are the commonest vertebrate fossil.