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Reading trackways: dinosaur speed, evidence and uncertainty
How dinosaurs are defined by ancestry, how speed is estimated from fossil trackways with Alexander's equation, how well that equation really works, and how scientists weighed the evidence for the end-Cretaceous extinction.
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Dinosauria can be defined by ancestry: the group descended from the most recent common ancestor of Diplodocus, Triceratops and the house sparrow. That makes birds dinosaurs; palaeontologists say 'non-avian dinosaurs' for the rest.[1]
Anatomy: all dinosaurs held their limbs directly under the body, with hinge-like ankles. They are split into ornithischians and saurischians by hip structure. Pterosaurs, marine reptiles and crocodiles are not dinosaurs.[2][3][1]
Trackways record movement. The stride length λ is the distance between two prints of the same foot, and the hip height h is often estimated as about 4 × the footprint length.[9][10]
Alexander's equation estimates speed: u = 0.25 g0.5 λ1.67 h−1.17, with u in m/s, g = 9.81 m/s², and λ and h in metres. It assumes the leg is kept straight.[10][9]
How good is it? When researchers tested the equations on living guineafowl walking in mud, none of them predicted speed accurately, so trackway speeds may carry major uncertainty.[10]
The end of the non-avian dinosaurs, 66 million years ago: a panel of 41 experts reviewed 20 years of research and concluded that a 15 km asteroid hitting Chicxulub, Mexico, was the cause. They judged the Deccan Traps eruptions in India (about 1.5 million years of eruptions, about 1,100,000 km³ of lava) not to be the primary cause. More than half of all species died out.[4][2]
Interpretations change with evidence: the Bernissart Iguanodons were mounted in 1882 as upright 'kangaroos', but in 1980 David Norman concluded that the back was held roughly horizontal and that Iguanodon walked on all fours.[7]
Try it: Testing Alexander's equation on yourselves
You need
- A straight, flat 20 m track (playground or hall)
- Tape measure (at least 10 m) and metre rule
- Stopwatch
- Chalk or bean bags to mark footfalls
- Calculator or spreadsheet
Safety
Use a dry, level surface with no obstacles. Jog, do not sprint; only one pair on the track at a time. Wear suitable shoes.
What to do
- Measure your hip height h (floor to the top of the thigh bone at the side of the hip) in metres. Also measure your foot length and compare h with 4 × foot length.
- Mark a 10 m timing zone in the middle of the track, with a run-up before it.
- Walk through the zone at a steady pace. A partner times you over the 10 m; another marks where the heel of the same foot lands twice in a row. Measure the stride length λ.
- Measured speed = 10 m ÷ time. Predicted speed u = 0.25 × √9.81 × λ^1.67 × h^−1.17.
- Repeat for a fast walk and a gentle jog. Do three trials of each and take means.
- Calculate the percentage difference between predicted and measured speed, and estimate the uncertainty in each measurement.
What happens, and why?
Alexander's equation links relative stride length to speed, so it usually predicts higher speeds for longer strides. It will not match exactly: the equation was fitted to data from living animals, assumes straight legs and estimates hip height, and a study with guineafowl in mud found large errors. Your percentage differences show how far a trackway speed can be trusted.[10][9]
Play: Observation or inference?
Is each one measured directly, or worked out using assumptions?
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Quiz
Other ages: Ages 6–7 · Ages 10–11