Overview of What crocodile bones teach us about dinosaurs
This Shortwave (NPR) episode reports on new research from the University of Cape Town showing that growth rings in crocodile bones — long used as a model for estimating dinosaur ages — can form more often than once per year. The findings suggest that counting growth rings in fossil bones may sometimes overestimate dinosaur ages. The story follows researcher Anousia Chinsami-Taran’s work at a crocodile facility near Cape Town, describes the methods used to “time-stamp” bones, and presents expert reactions and implications for paleontology.
Key points and main takeaways
- Traditional method: Paleontologists often estimate dinosaur age by counting growth rings in fossil bones, assuming they form annually (like tree rings).
- New observation: In marked Nile crocodiles, multiple growth marks appeared within a single year; a two‑year‑old croc sometimes showed up to five growth marks.
- Implication: If dinosaurs formed rings like these crocodiles did, many dinosaurs might have been younger at death than previously estimated — ring counts may reflect growth cycles, not strictly years.
- Support: Similar non-annual ring patterns have been observed in other reptiles and in kiwi birds, indicating this phenomenon is not unique to crocodiles.
- Caution: The result doesn’t overturn ring-counting as a tool but underscores that it’s an estimate with taxon- and context-dependent reliability.
Methods used (concise)
- Study subjects: Four Nile crocodiles raised at Le Bon Air Reptiles and Adventures (near Cape Town).
- Experimental marking: Crocodiles were injected with an antibiotic over months; the antibiotic becomes incorporated into growing bone and serves as a time marker.
- Bone analysis: After the animals were later culled (in 2013) and bones recovered, thin cross-sections of limb bones were prepared and examined microscopically for growth bands and the antibiotic marker.
- Finding: Presence of more growth bands than chronological years indicated multiple bands formed within individual years.
Expert reactions and perspectives
- Anousia Chinsami-Taran (University of Cape Town): Urges caution — growth marks should be seen as estimates and may represent cycles, not strictly annual increments. “It’s all in the bones.”
- Holly Woodward (Oklahoma State University, paleohistologist): Study is important ground-truthing; growth rings are a useful starting point, but patterns vary across species and more work is needed to understand causes.
- Christy Curry (Macalester College, dinosaur paleobiologist): Treats this as a cautionary example against over-interpreting bone microstructure; we still lack a complete understanding of how living vertebrate bones record environmental and physiological signals.
Implications for dinosaur science
- Age and growth-rate estimates: Some dinosaurs previously thought to be decades old might have been younger; growth-rate curves derived from ring counts could need revision in certain cases.
- Life-history reconstructions: Estimates of maturation age, longevity, and population dynamics based on ring counts may be biased if rings are non-annual.
- Methodology: Paleontologists should combine histology with other lines of evidence (e.g., isotopes, cross-species comparisons, phylogenetic context) and avoid assuming universal annual periodicity.
Limitations and open questions
- Sample size and context: The study used a small number (four) of captive crocodiles; captive conditions, diet, stress, or husbandry could affect growth marks.
- Taxonomic variation: Some modern taxa do show clearly annual rings — why patterns differ across species is still unclear.
- Mechanism unknown: Possible drivers include hormone cycles, circadian or seasonal rhythms, environmental stressors — more experimental work is needed.
- Fossil preservation/remodeling: In older individuals (including humans), bone remodeling can erase earlier growth marks, complicating counts.
Notable quotes
- “We always thought that these rings are formed annually.” — summary of traditional assumption.
- “This is a two-year-old crocodile, and in many cases we found up to five growth marks in the bones.” — observation that challenges the annual assumption.
- “It’s all in the bones.” — Anousia on the potential for bone studies to reveal life history, with the caveat that interpretation requires care.
Recommended next steps (for researchers)
- Expand ground-truthing to more species (wild and captive), ages, and environments to map when rings are annual vs. non-annual.
- Use controlled experiments (time markers, hormonal monitoring, environmental manipulation) to identify physiological or environmental causes of ring formation.
- Integrate histology with other proxies (isotopes, skeletochronology cross-checks) when reconstructing fossil growth and life history.
- Reassess key dinosaur age/growth datasets where ring counts were taken at face value, especially if based on small samples or taxa closely related to reptiles showing non-annual patterns.
Quick take for listeners
Growth rings in bones remain a useful tool, but this crocodile study shows they aren't a simple “one ring = one year” clock for every species. Paleontologists should treat ring counts as informed estimates and continue cross-validating methods with modern animals.