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Flexible tails can help paleontologists infer how extinct marine predators moved through water, but they cannot tell us a swimming speed by themselves. Tail shape, spinal flexibility, body stiffness, size and propulsion all matter—and any speed quoted for a fossil animal is a model-based estimate, not a direct measurement.
Can a fossil tail tell us how fast an extinct predator swam?
Not on its own. A tail is evidence about swimming mechanics, not a speedometer. Fossils can preserve the bones that supported a tail fin and, more rarely, soft tissue that changes how its shape is interpreted. Those clues constrain plausible ways an animal could have moved; they do not record a top speed.
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To estimate performance, researchers combine anatomy with measurements from living animals, fluid-mechanics principles and assumptions about how the extinct animal moved. The result depends on what the model is designed to estimate: a burst of acceleration, an efficient cruising pace or sustained swimming. Those are different measures and should not be treated as interchangeable.
How flexibility, stiffness and tail shape affect swimming
In tail-driven swimming, a bending wave travels along some part of the body and tail, pushing water backward through the caudal fin. The water’s reaction propels the animal forward. Where the body bends—and how much—helps determine how that force is produced.
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- More extensive flexibility: Bending across a larger part of the trunk may allow a broad propulsive stroke and quick acceleration in some body plans.
- Concentrated flexibility: A relatively stiff torso with movement focused toward the rear produces a different stroke from one that bends along much of its length.
- Tail and fin geometry: The outline, proportions and skeletal support of the tail fin help constrain hypotheses about propulsion, but do not establish speed by themselves.
These are conditional mechanical inferences, not a universal ranking in which more flexible always means faster. Body shape, size and other sources of propulsion also affect performance. A form suited to a rapid burst may not be best suited to economical cruising, and a fossil’s anatomy alone does not settle which performance measure mattered most.
What the ichthyosaur estimate actually says
In a 2002 model, Ryosuke Motani estimated the optimal cruising speed of the Jurassic ichthyosaur Stenopterygius and compared its basal metabolic rate with that of living tunas. The reported similarity is a model result: it is not a measured swim speed, and it does not establish the ichthyosaur’s maximum sprint speed.
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Sharks offer a useful functional comparison for thinking about ichthyosaur tail propulsion because both groups use tail-driven swimming. But an analogy is not proof that they swam identically. Differences in skeletal support, buoyancy and tail geometry can change how a fin works; a shark’s performance cannot simply be assigned to an ichthyosaur.
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A 2011 study of the mosasaur Ectenosaurus interpreted collagen-fiber arrangements as evidence that the anterior torso was relatively stiff. Under that interpretation, movement used for propulsion would have been concentrated farther back, toward the posterior body and tail. This illustrates why a tail cannot be interpreted in isolation: the stiffness of the body in front of it affects how a bending wave travels and where the animal can generate motion.
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Preserved soft tissue can refine or revise ideas about body shape and mechanics, but such preservation is rare. For many extinct swimmers, researchers must work from skeletal anatomy and comparisons rather than a complete record of the animal’s outline.
Why plesiosaurs do not fit a tail-flexibility comparison
Not every large marine reptile relied on its tail for propulsion. Plesiosaurs used their fore and hind flippers. In a 2017 controlled water-tank experiment with reconstructed flippers, thrust and efficiency varied with flipper spacing and the timing of the strokes. The experiment supports four-flipper propulsion in the reconstruction; it is not a direct measurement of every plesiosaur species.
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That distinction matters when comparing extinct predators. A tail-focused explanation of speed does not apply equally to animals whose main propulsive surfaces were their flippers.
What the reported mosasaur lunge speeds mean
A National Geographic article published October 7, 2026, reported model-based estimates for mosasaurs, including a potential lunge of around 15 miles per hour for a digitally scaled, 45-foot Tylosaurus. That figure is a secondary report of a modeled estimate, not an observed speed. The underlying primary article and its methods, inputs and uncertainty ranges are not established here, so the number should be treated as provisional rather than a settled maximum.
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National Geographic quoted vertebrate paleontologist Kiersten Formoso saying, “They were probably much more efficient and dynamic swimmers than previously thought.” That is an interpretation reported in the article, not a substitute for the model’s assumptions or a direct speed measurement. The same coverage quoted mosasaur expert Mike Polcyn of Southern Methodist University proposing that sustained swimming as a pursuit predator, rather than an ambush predator, might have contributed to Tylosaurus giving way to the larger Mosasaurus. This is an ecological interpretation, not a conclusion that a lunge-speed estimate alone can demonstrate.
How to read a speed claim about an extinct swimmer
Before comparing two animals—or repeating a precise number—identify what evidence and performance measure sit behind the claim:
- Propulsion: Was the animal modeled as a tail-driven swimmer or as a flipper-powered swimmer?
- Flexibility: Does the interpretation specify how much of the trunk and tail could bend, and where stiffness was concentrated?
- Fin support and shape: What tail-lobe proportions, fin span and supporting body plan were assumed?
- Performance: Is the number an estimate of burst acceleration, optimal cruising speed or sustained swimming?
- Evidence: Does the claim rest on preserved anatomy, rare soft tissue, comparisons with living animals, a physical experiment or a mathematical model?
A model can be useful without yielding a directly observed speed. Its number is meaningful only alongside the question it answers, the evidence it uses and the assumptions that connect fossil anatomy to movement.
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Further reading
Cambridge University Press lists “Swimming capabilities of Mesozoic marine reptiles: a review” in The Mechanics and Physiology of Animal Swimming, a specialist reference for readers interested in the biomechanics of extinct marine reptiles.
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