How Hard Could It Bite? Reconstructing Muscles That Left No Trace
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Bite force figures for extinct animals are quoted constantly and are calculated rather than measured. The calculation rests on reconstructing muscles from the spaces and scars they left on the skull, and the steps are worth knowing before trusting a number.
What the skull preserves
Muscles do not fossilise and they leave evidence of several kinds. Openings in the skull behind the eye are the spaces the jaw-closing muscles passed through and occupied, so their size sets an upper limit on muscle cross-section, which is the quantity that determines force. Roughened areas and ridges mark where muscle attached, and their extent and texture indicate how much attached and how strongly. The shape of the lower jaw records where the muscle inserted, and the distance from that insertion to the joint is the lever arm, which determines how much of the muscle force reaches the teeth. Comparison with living animals converts these observations into estimates, since the relationship between attachment area, muscle size and measured bite force can be established in crocodilians and birds and applied to the fossil.
The steps in a calculation
A published figure is the end of a sequence, and each step adds uncertainty:
- •Reconstructing which muscles were present, by bracketing between birds and crocodilians
- •Estimating each muscle's cross-sectional area from the space and attachment available
- •Applying a value for how much force muscle produces per unit area, taken from living animals and assumed to be similar
- •Measuring the lever arms from the jaw geometry, which requires a correctly reconstructed skull
- •Computing the force at a chosen tooth position, since force differs along the jaw and is highest at the back
- •Comparing the result against what the skull and teeth could withstand, which is a check rather than an input
The other way to do it
An independent approach uses computational modelling rather than lever arithmetic. A digital model of the skull is built from scans, loads are applied where the muscles attached, and the software calculates how stress distributes through the structure, which is the same technique used to test engineering components. That reveals whether a proposed bite force would break the skull, identifies where stress concentrates and shows how features such as fused bones and reinforced regions contribute, and it has been used to compare feeding styles between species directly. The method is only as good as the model, since bone material properties have to be assumed, since sutures between bones behave differently from solid material and are frequently simplified, and since the loads applied come from the same muscle reconstruction the simpler method uses.
What the teeth say
Teeth supply an independent check on what a jaw was doing, and the checks are frequently more informative than the force estimate. Tooth shape indicates the loading the tooth was built for, with thick conical teeth suiting crushing, blade-like serrated teeth suiting slicing through flesh without contacting bone, and flattened grinding surfaces suiting plant material. Wear patterns record what actually happened, since bone contact leaves distinctive damage and abrasive plant material leaves scratches whose orientation indicates how the jaw moved. Breakage and replacement rates indicate how hard the teeth were being used, and several large predators show high rates of tooth breakage consistent with regular bone contact. Puncture marks matching a particular tooth arrangement on prey bones close the loop by showing the force being delivered rather than calculating what was available.
Reading the published figures
Numbers for the largest predators circulate widely and deserve scepticism of a specific kind. Estimates for the same animal have varied by a factor of several between studies, reflecting different muscle reconstructions and different assumptions rather than new specimens. Figures are quoted without stating the tooth position, which changes the answer substantially. Comparisons across species frequently mix methods, which makes the ranking unreliable. And the comparison with living animals that gives the figures meaning depends on measurements from a small number of species that were willing to bite an instrument, which is a modest reference set. None of this means the estimates are useless, since they support genuine conclusions about relative capability and about what a skull was built to withstand, and it means the headline number is the least reliable part of the work.
The takeaway
Openings behind the eye limit muscle cross-section, attachment scars indicate how much attached, and jaw geometry gives the lever arms, with living crocodilians and birds supplying the conversion. Digital models test whether a proposed force would break the skull. Estimates for the same animal have varied several-fold between studies, and figures are quoted without the tooth position that determines them.