Dinosaur Aging Reconsidered: Cape Town Crocodiles Offer New Clues to Growth Rings

In this NPR Short Wave episode, Regina Barber and Ari Daniel explore how researchers study dinosaur ages through bone growth markers. At a crocodile center outside Cape Town, scientists mark year-old crocs with antibiotics to create time markers in their bones, then examine growth rings under a microscope. Unexpectedly, they find multiple growth marks in short-lived individuals, suggesting rings may reflect cycles of growth rather than strictly annual rings. If dinosaur bones behave similarly, some dinosaurs may be younger at death than previously estimated. The discussion features perspectives from researchers including Anusia Chinsami-Taran, Maria Eugenia Pereira, Holly Woodward, and Kristi Curry Rogers, emphasizing that the science is evolving and that growth rings are a starting point, not a definitive measure.

Overview and premise

NPR’s Short Wave hosts Regina Barber and Ari Daniel investigate how paleobiologists estimate dinosaur ages and what Crocodilian biology can teach us about this long-standing question. The focal point is a team from the University of Cape Town, led by Anusia Chinsami-Taran, who studied crocodiles as modern analogues for dinosaur growth. Traditional methods counted growth rings in fossil bones, assuming one ring per year, to determine growth rates and time to maturity. Yet birds are living dinosaurs and offer a related line of evidence, and the team uses a novel approach to test ring reliability in a living system. The episode also situates the story near Cape Town, at Le Bonheur Reptiles and Adventures, and in Anusia’s lab at the University of Cape Town, setting the stage for a reassessment of how rings should be interpreted in ancient bones.

As Ari and Regina describe, the main question is whether growth rings in dinosaur bones were truly annual or whether they reflect broader growth cycles. The answer has implications for how we infer the ages of extinct species at death, and whether dinosaurs grew more quickly or more slowly than previously thought. The episode blends field observations, lab work, and expert commentary to illuminate the nuance involved in reading the fossil record, and it emphasizes that the methods we rely on are still being tested against modern vertebrates.

Methods: time markers in bones

The Cape Town team took a bold step beyond counting rings: they injected four year-old crocodiles with an antibiotic regimen over several months. The antibiotic becomes incorporated into developing bone and leaves a tracer in bone tissue, effectively marking time in a way that can be read later under a microscope. This approach, described by Anusia, aims to anchor growth records to real time and to examine how bone formation correlates with growth in living animals. Maria Eugenia Pereira, another biologist on the project, notes that the four crocodiles did not grow identically; they hatch together and grow together for a while, but each follows a distinct growth trajectory. The largest individual surpassed 80 pounds, illustrating the diversity of growth patterns within a single cohort and highlighting the value of multiple specimens for understanding growth biology.

Findings: more rings than expected and a cautionary tale

When the croc bones were sliced and polished for microscopic analysis, the researchers were surprised to find extra growth marks in bones that had formed over just a couple of years. In some cases, a two-year-old crocodile displayed up to five growth marks, implying that growth rings are not strictly annual for these animals. This finding raises the possibility that dinosaur bones might also accumulate time markers in a non-annual fashion, which would mean that counting rings could overestimate the animal’s true age at death. The episode connects this observation to other reptiles and kiwi birds, which show similar patterns in growth mark formation, suggesting a broader biological principle that rings may reflect growth cycles rather than fixed yearly intervals.

Implications for dinosaur aging

The implications are profound. If some dinosaur bones record cycles instead of annual rings, then many previous estimates of age at death could be biased upward. The idea that dinosaurs might have perished younger than assumed would alter our understanding of their life histories, metabolism, and population dynamics. Ari explains that although this finding does not overturn established estimates, it urges caution in interpreting fossil bone tissue and underscores the need for “ground truthing” with modern animals to calibrate our expectations about fossil growth patterns. The episode emphasizes that the science is not settled, and researchers such as Holly Woodward and Kristi Curry Rogers stress that growth rings are a useful starting point but require more evidence to become a definitive tool for aging dinosaurs.

Expert perspectives: grounding the debate

Holly Woodward, a paleohistologist at Oklahoma State University, argues that studies like this one are essential for expanding knowledge about how growth rings form in different species. She notes that variations across living vertebrates mean there is not a universal rule about ring formation, and hormones or circadian cycles could influence ring deposition. Kristi Curry Rogers, a dinosaur paleobiologist, agrees that this serves as a cautionary tale: not all growth rings are equivalent across species, and the environment and biology can complicate interpretation. Anusia, reflecting on the Cape Town work, acknowledges that there is still much to learn, and the team’s results invite further research rather than definitive conclusions. The discussion also touches on the human side of science, including the fact that humans also have growth rings, though remodeling tends to erase older rings as we age.

Humans and growth rings: a parallel reminder

To close, the reporters address a frequently asked question: do humans have growth rings? Ari shares that humans do have growth rings in bone, but they are less often reported because most reads come from older skeletons where remodeling hides or erases earlier rings. The key takeaway is that while growth rings exist in humans, interpreting them for age is more complex due to remodeling processes. The episode ends with a reminder that dinosaur age estimation remains an estimation, albeit a better-informed one when combined with modern biology and careful interpretation of bone tissue.

Quotes from the episode

"These marks may be better thought of as cycles of growth." - Anusia Chinsami-Taran, paleobiologist

"We've always estimated the age of a dinosaur, and what this means is that we can still get a rough estimate, but people have to realize that it's an estimation." - Anusia Chinsami-Taran, paleobiologist

"Humans do have growth rings. They are just not reported that often because we usually stop growing in our 20s." - Ari Daniel

"Growth rings remain at least a useful starting point for understanding dinosaur growth." - Holly Woodward, paleohistologist

Concluding note

This episode demonstrates how a living model such as crocodiles can illuminate the limits of a long-standing method used to study the deep past. It shows the iterative nature of science, where a surprising finding leads to new questions, wider collaborations, and a more nuanced view of how to read the signals preserved in bone tissue. The podcast reinforces that while growth rings are a valuable line of evidence, they are not the final word on dinosaur ages and life histories.