How Old Was That Dinosaur? Cut the Bone Open and Count

Bone is laid down in layers as an animal grows, and those layers preserve their microscopic structure through fossilisation, so a section cut thin enough to transmit light shows the original tissue in detail. The arrangement of blood vessel channels indicates how fast bone was being deposited, since rapidly growing bone is laid down with a disorganised structure full of channels while slowly growing bone is dense and well organised. Lines where growth paused appear as narrow bands running around the section, comparable to tree rings. The outermost layers show whether growth had stopped, which distinguishes a fully grown adult from a large juvenile.

The technique answers several questions that no other evidence reaches:

• •Age at death, by counting the growth interruption lines
• •Growth rate, from the tissue type and the spacing of those lines
• •Whether the animal had reached full size or was still growing
• •Whether it had reached sexual maturity, which changes growth rate
• •Illness or injury, which leave recognisable disruptions
• •In some cases the sex, from a tissue females deposit while producing eggs

The most consequential result concerns how fast dinosaurs grew, which bears directly on their physiology. The tissue found in most dinosaur bone is the fast-growing disorganised type characteristic of birds and mammals rather than the slow dense type characteristic of modern reptiles, which indicates growth rates far higher than a crocodile of comparable size achieves. Reconstructing growth curves for several species has shown large animals reaching full size within one to two decades rather than over a century, with the largest growing at extraordinary rates during an adolescent spurt. That evidence was central to the shift away from regarding dinosaurs as slow cold-blooded animals.

Several long-running disputes about particular species have been decided by cutting bones rather than by comparing shapes. Whether a number of named small species were separate animals or juveniles of larger known ones has been tested directly, since a juvenile shows actively growing tissue while a small adult shows growth having stopped, and in several cases the result collapsed two names into one. Whether the very largest sauropods grew steadily for a century or rapidly for a few decades was answered by the spacing of growth lines. Whether a famous horned dinosaur and a differently shaped one were the same species at different ages was addressed the same way. These results are frequently unpopular, since collapsing species removes names from the literature.

The method requires cutting a fossil, which museums resisted for decades and which remains a genuine constraint. A specimen must be sawn, a slab removed, mounted on glass and ground to a fraction of a millimetre, and that material is gone permanently. Curators therefore refused access to important specimens, and early work depended on isolated bones and damaged material. The situation changed as the results proved valuable, and practice now uses the smallest possible cores taken from standard positions in a limb bone, since the growth record is most complete there, with the hole filled afterwards. Even so, sampling a famous mounted skeleton requires a case that satisfies a committee, and several key questions remain unanswered because the relevant specimens cannot be cut.

A section cut thin enough to transmit light shows blood vessel arrangement, which indicates deposition rate, and growth interruption lines that can be counted like tree rings. The tissue found in most dinosaur bone is the fast-growing type characteristic of birds and mammals rather than of modern reptiles, which showed large species reaching full size in one to two decades. The method destroys material, which museums resisted for decades.