What Are Growth Rings in Bone? Reading an Animal's Age From a Slice

Bone grows by adding tissue at its outer surface, and the rate of that addition is not constant through the year. When growth slows or stops, typically in a dry or cold season, the tissue deposited is denser and differently organised, and it appears in a thin section as a dark line. Resuming growth lays down a broad zone of faster tissue between the lines. The result is an alternating pattern of zones and lines, with each pair generally representing one year, which is the same logic that makes tree rings countable. The tissue type within a zone is itself informative, since rapidly deposited bone has a disorganised fibrous structure with abundant blood vessel channels while slowly deposited bone is layered and organised, so a section records not just how long the animal lived but how fast it was growing at each stage of that life.

Reading a thin section gives several distinct pieces of information:

• •Age at death, from the number of lines, subject to a correction for the earliest rings
• •Growth rate, from the thickness of each zone, which peaks in adolescence and declines afterwards
• •Maturity, since an external layer of densely packed lines at the outer surface indicates growth had essentially stopped
• •Sexual maturity in some cases, since a specific tissue laid down in female birds before laying has been identified in dinosaur bone
• •Stress events, where an unusually marked line can indicate injury, illness or a severe season
• •Metabolic type, since the overall tissue organisation distinguishes sustained fast growth from the slow intermittent pattern of a typical reptile

The method has a built-in complication that every study has to handle. Bone is continually remodelled during life, with the interior being resorbed and replaced, and that process destroys the innermost rings first, so the earliest years of an animal's life are frequently missing from the section entirely. The standard correction estimates how many rings were lost by comparing the diameter of the remaining innermost ring with the sizes of younger individuals of the same species, which introduces uncertainty proportional to how much remodelling occurred. Large, long-lived animals suffer worst, since they remodel more, which means that exactly the specimens whose ages are most interesting are the ones with the least reliable counts. Sampling also destroys material, since a section requires cutting the bone, which museums resist for important specimens, and weight-bearing limb bones give the best record while being the elements collections are least willing to cut.

Lines are not deposited reliably by every animal, and the exceptions matter for how far the method can be pushed. Animals living in environments without a marked annual cycle have weaker or absent lines, which is a recognised problem in tropical and deep marine settings, and the assumption of one line per year has to be argued rather than assumed for each group. Some living animals deposit lines in response to events other than the season, including drought, illness and reproduction, which introduces extra lines that would inflate an age estimate. Very fast-growing animals may pass through a year without a detectable slowdown. Modern validation uses animals of known age, including marked wild individuals and captive specimens, to check the correspondence before the method is applied to fossils, and that calibration work in living reptiles, birds and mammals is what gives the fossil application its credibility.

The results reshaped several long-standing assumptions. Large dinosaurs turned out to have grown extremely fast, reaching adult size in years to a few decades rather than the century-scale spans once suggested, with growth curves resembling those of birds and mammals rather than reptiles, and that supported a general reappraisal of their physiology. Named species were shown to be growth stages of others, since sectioning revealed that supposedly adult specimens were juveniles still growing rapidly, which has collapsed a number of names and generated continuing argument about several more. The identification of the tissue associated with egg laying allowed individual specimens to be identified as female and sexually mature, which also dated the onset of reproduction relative to full size and showed that several species bred before finishing growth. And the method supplied ages for the famous individual specimens, which turns out to matter for interpreting almost everything else about them.

Seasonal slowdowns leave dense lines in bone, one pair of line and zone per year, and the tissue between them records how fast the animal was growing at the time. Remodelling destroys the innermost rings, so ages need a correction that is least reliable for the largest animals. Sectioning showed large dinosaurs grew fast on bird-like curves and that several named species were juveniles of others.