Crocodile Bone Rings Do Not Reliably Tell Age, and That Matters for Dinosaur Science

Count the rings in a tree trunk and you get its age. Count the growth marks in a vertebrate's bone and you should get something similar -- or so the field of skeletochronology has long assumed. A new study from the University of Cape Town suggests the reality is considerably messier, at least for crocodilians.

Five-year-old bones in two-year-old crocodiles

Professor Anusuya Chinsamy-Turan, an expert in bone microstructure of extinct and living vertebrates, and Dr. Marcos Pereyra, an Argentinian postdoctoral researcher at UCT, examined bone slices from four two-year-old crocodiles raised together under similar conditions. Under the microscope, the dense, compact parts of the bones showed very clear growth marks -- but far more than expected for animals of known age.

Had the researchers simply counted the visible marks without knowing the animals' actual age, they would have estimated the crocodiles were five to six years old. The "extra" marks did not correspond to annual growth cycles. Instead, they appear to have been deposited during favorable growing periods related to environmental factors such as competition for food, dominance hierarchies, or temperature fluctuations.

The study was published in Scientific Reports.

Growth rates are flexible, not fixed

The finding that growth mark counts varied substantially even among animals of the same age, raised in the same environment, points to a fundamental flexibility in how crocodilians grow. Their growth rates shift in response to environmental conditions rather than ticking along like a clock.

This is not entirely surprising. Similar flexibility has been documented in alligators, newts, and other vertebrates. But the degree to which it can distort age estimates -- tripling the apparent age in this case -- is striking.

What this means for reading dinosaur bones

The implications extend well beyond living crocodiles. Paleontologists routinely study the microstructure of dinosaur bones to estimate how long extinct animals took to reach full size, how fast they grew, and what their life histories looked like. Crocodiles are among the closest living relatives of dinosaurs, and the growth flexibility documented in this study was likely present in a wide range of extinct vertebrates as well.

Chinsamy-Turan and Pereyra caution against using growth mark counts and derived growth curves to age either modern animals or their extinct relatives. They recommend thinking of growth marks in the dense outer bone as signs of intermittent growth periods rather than markers that formed reliably once per year.

This does not invalidate skeletochronology entirely, but it introduces a significant source of uncertainty that studies relying on the technique will need to address.

A productive collaboration

The Scientific Reports paper is the fourth publication from the Chinsamy-Turan and Pereyra collaboration in three years. Previous studies examined an extinct long-necked marine reptile from Argentina (published in the Journal of Anatomy) and broad-snouted caimans (published in the Journal of Morphology).

Both researchers are also organizing the 7th International Palaeontological Congress (IPC7), which the University of Cape Town will host from November 30 to December 3, 2026 -- the first time the quadrennial event has been held in Africa.