How Is a Dinosaur Aged? Counting Rings Inside the Bone
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Cutting a thin slice from a fossil bone and examining it under a microscope reveals growth lines comparable to tree rings, and counting them gives an age in years. That method turned a subject that relied on guesswork into one that produces numbers, and it required destroying part of the specimen.
What the bone records
Bone grows by adding layers at its outer surface and the rate varies with conditions, so periods of slow or arrested growth leave visible marks. In many animals those marks are annual, produced by a seasonal slowdown, and they appear in a thin section as lines of arrested growth separating zones of faster deposition. Counting them gives a minimum age, minimum because the innermost lines are destroyed as the marrow cavity expands with growth, which must be corrected for by comparing with younger individuals of the same species. The texture of the bone between the lines is equally informative, since rapidly growing bone has a disorganised fibrous structure with abundant blood vessel channels while slow-growing bone is denser and more organised, which means a section records not only how many years passed but how fast the animal was growing in each. An external fundamental system, meaning tightly packed lines at the outer surface, indicates that growth had essentially stopped and the animal was fully grown.
What the technique established
Applying it across many specimens produced results that changed how these animals are understood:
- •Growth rates far above those of living reptiles and comparable to birds and large mammals, which bears directly on metabolism
- •Adult size reached in a small number of years even for very large species, with the largest reaching most of their mass in under two decades
- •A growth curve shaped like a stretched S, with slow early growth, a rapid adolescent phase and a plateau, which is the mammalian and avian pattern rather than the reptilian one
- •Ages at death for individual famous specimens, which allowed populations to be described rather than individuals
- •Sexual maturity before full size, inferred from medullary bone, a tissue birds deposit while laying eggs and which has been identified in several dinosaur specimens
- •The resolution of several species disputes, since a specimen showing an external fundamental system is an adult and one without it is not, which tests whether a small form is a juvenile of a large one
The cost of the method
Histology is destructive, requiring a section to be cut from a bone and ground thin enough to transmit light, and that has been an obstacle to applying it to important specimens. Institutions weigh the information against the damage, and the usual compromise is to sample a limb bone rather than a skull, to take a core rather than a full section where possible, and to document everything. Non-destructive alternatives have been pursued, with computed tomography able to image internal structure and generally lacking the resolution to resolve growth lines reliably, and with synchrotron imaging achieving much better results on small specimens. The destructive requirement also means that the sample of animals aged this way is biased towards those of which multiple specimens exist, which affects what can be said about rarer species. Nonetheless the accumulation of sections has been substantial enough to support statistical work rather than anecdote, which is the change that mattered.
The wider use
The technique is not confined to dinosaurs and its application elsewhere is what validated it. Growth lines are counted in living reptiles, amphibians, fish and mammals of known age to establish that the marks are annual, which is the calibration the fossil work depends on and which has found exceptions where the marks are not annual in particular species or conditions. Fish otoliths, small structures in the ear, are read the same way and are the standard method for ageing fish in commercial assessment, where knowing the age structure of a stock determines quota decisions. Teeth are sectioned similarly in mammals. Archaeology uses the technique on animal remains to establish season of death and therefore when a site was occupied. Forensic and conservation applications exist for ageing individual animals. Across all of them the principle is the same, which is that a tissue laid down continuously and varying with the seasons is a record of time.
The takeaway
Growth lines in a thin section of bone are counted for a minimum age, minimum because the earliest ones are destroyed as the marrow cavity expands. The texture between them records how fast the animal was growing, and tightly packed lines at the surface mark an adult. The results show mammal-like growth curves, adult size reached in under two decades even in the largest, and the method destroys part of the specimen.