Did Dinosaurs Travel With the Seasons? Evidence Written in Teeth

Movement leaves almost no direct trace in the fossil record. A skeleton records where an animal died rather than where it lived, and finding the same species in two places says nothing about whether individuals travelled between them or whether two populations lived separately. Trackways record a direction for a few dozen metres. Arguments from the modern analogy of large herbivores following seasonal grazing are suggestive and prove nothing, since the geography, climate and vegetation were entirely different. For decades the question was therefore addressed by plausibility arguments alone, and confident statements in popular accounts substantially outran the evidence.

Teeth record the chemistry of the water an animal drank while they grew:

• •Strontium in rocks varies by age and type, and differs between regions
• •That signature passes through plants and water into the animal
• •Tooth enamel incorporates it and does not remodel afterwards
• •A tooth growing over months records a sequence rather than one value
• •Sampling along a tooth therefore gives a chemical travel log
• •Comparing that against a map of regional signatures indicates where the animal was

The technique was applied to a Camarasaurus, a large long-necked sauropod from the Jurassic of North America, in work published in 2011. Sampling along the teeth found signatures that did not match the area where the animals died, and that cycled in a pattern consistent with regular movement between the basin where the fossils were found and highland areas over a hundred kilometres away, apparently timed to the dry season. That established seasonal long-distance movement in at least one species using physical evidence rather than inference. Comparable work on other groups has produced mixed results, with some species showing signatures consistent with staying in one area throughout life.

The chemical approach has real limits and stating them keeps the conclusions honest. It requires a landscape whose regional signatures differ measurably, which is not true everywhere, since areas of geologically uniform rock produce the same value over enormous distances and record nothing. Those regional signatures must be reconstructed for the period in question rather than measured today, since erosion and deposition have altered the surface. Diagenesis, the chemical alteration of a fossil after burial, can overwrite the original signal, so demonstrating that enamel is unaltered is a necessary and non-trivial step. The result gives a sequence of regions rather than a route, so the path taken between them is unknown. And it describes individuals rather than populations.

Several independent lines of evidence bear on the question and none is conclusive alone. Bonebeds containing large numbers of one species of one age class suggest herds moving together, and the interpretation of such deposits is contested since a flood can concentrate animals that never associated. Trackways with many individuals travelling in the same direction at the same speed are strong evidence for group movement over the distance recorded. Growth rings in bone record seasonal variation in growth rate, which indicates the environment changed rather than that the animal moved. Polar dinosaur fossils from within the Cretaceous Arctic Circle prompted arguments about whether those animals migrated away for winter or endured it, and the current evidence favours enduring it.

A skeleton records where an animal died rather than where it lived, so movement was argued from plausibility for decades. Strontium signatures vary by region, pass into growing tooth enamel and are not remodelled, so sampling along a tooth gives a chemical travel log. Work published in 2011 established seasonal movement of over a hundred kilometres in one sauropod, and other species show signatures consistent with staying put.