Understanding Bone Growth Marks: The Foundation of Skeletochronology
Vertebrates, much like trees, often exhibit cyclical growth patterns that leave discernible marks in their bones, akin to annual tree rings. This technique, known as skeletochronology (from skeleton + chronology), relies on counting lines of arrested growth (LAGs) in the compact bone cortex to estimate age and reconstruct growth trajectories. First formalized in amphibians and reptiles, it has been widely applied to birds, mammals, and notably, extinct reptiles including non-avian dinosaurs.
In reptiles, these LAGs form during periods of slower growth, typically winter or dry seasons when resources are scarce, contrasting with faster summer growth zones. For Nile crocodiles (Crocodylus niloticus), Africa's largest predatory reptile and a key species in South African ecosystems like Kruger National Park, accurate aging informs population dynamics, conservation strategies, and sustainable harvesting.
However, validation studies have shown inconsistencies. In lizards like Gallotia galloti, LAG numbers vary with climate and altitude, while sea turtles deposit multiple LAGs per year. Alligators and caimans exhibit inter-bone variability, challenging the one-LAG-per-year assumption.
UCT's Pioneering Experiment: Methods Behind the Crocodile Bone Aging Study
At the University of Cape Town (UCT), palaeobiologist Professor Anusuya Chinsamy-Turan and postdoctoral researcher Dr. Maria-Eugenia Pereyra conducted a controlled in vivo labelling study on four captive Nile crocodiles. Hatched in March-April 2011 at Le Bonheur Reptile Park near Cape Town, the juveniles were raised in shared enclosures with summer feeding and winter fasting, mimicking natural South African conditions.
Fluorochrome markers—Engemycin (green fluorescence) in April 2012 and May 2013, Alizarin red (red) in November 2012—were injected at body-weight-adjusted doses to timestamp bone deposition. Sacrificed in June 2013 at approximately two years old, long bones (humerus, radius, ulna, femur, tibia, fibula) were sectioned undecalcified, examined under petrographic (Zeiss E200) and confocal (ZEISS LSM 880) microscopes.
- Expected: Two LAGs (one pre-first injection, one between second/third) plus hatching line (HL).
- Observed: Fluorochromes confirmed summer deposition; LAGs exceeded expectations.
This rigorous setup allowed precise correlation of markers with LAG formation, revealing stochastic (random) patterns.
Key Findings: Stochastic Growth Marks Exceed Age Expectations
All four crocodiles displayed more LAGs than their two-year age warranted. Proximal bones (humerus, femur, tibia) showed fewer (2-3 total), while distal ones (radius, ulna, fibula) had up to five LAGs, including HL and extras. One female specimen's radius/ulna/fibula recorded five LAGs each.
| Specimen | Radius LAGs | Ulna LAGs | Fibula LAGs |
|---|---|---|---|
| Male SR | - | 4 | 3 |
| Male LR | 4 | 3 | - |
| Female SG | 5 | 4 | 5 |
| Female LG | - | 3 | 3 |
Confocal imaging pinpointed extras during favorable summer growth, not strictly annually. Intraskeletal variation highlighted bone-specific histories; no final green stain near periosteum indicated ongoing deposition. Simply counting LAGs would misage these juveniles at 5-6 years.
Implications for Nile Crocodile Conservation in South Africa
South Africa's Nile crocodile populations, protected under CITES Appendix I but farmed commercially, require precise aging for quota management and translocation. Traditional LAG counts from osteoderms or femurs overestimate maturity, risking overhunting of young adults or underestimating population recovery.
Le Bonheur Reptile Park's collaboration exemplifies public-private partnerships aiding UCT research. Wild crocs in iSimangaliso Wetland Park or Okavango Delta face variable climates, amplifying stochastic LAGs from food scarcity or dominance hierarchies. Future studies should integrate genetics and mark-recapture for robust demographics.Explore palaeontology roles at South African universities.
This UCT breakthrough underscores bone histology's role in wildlife management, positioning local expertise globally.
Revolutionizing Dinosaur Age Estimation and Growth Models
As archosaur relatives, crocodiles proxy dinosaur bone dynamics. Prior studies assumed annual LAGs, estimating T. rex adulthood at 20+ years. UCT findings suggest overestimation; stochastic marks during growth spurts (e.g., wet seasons) inflate counts, implying faster maturation.
- Massospondylus (SA prosauropod): Variable LAGs noted.
- Plateosaurus, Mussaurus: Similar plasticity.
- Extinct crocs (Diplocynodon): Multi-LAG evidence.
Back-calculated growth curves falter without distinguishing stochastic from annual marks. Paleontologists must prioritize minimum ages, multi-bone sampling, and environmental context.Read the full UCT study.
Spotlight on UCT Researchers: Chinsamy-Turan and Pereyra
Professor Anusuya Chinsamy-Turan, UCT's bone microstructure authority, has authored books on dinosaurs and chairs the 2026 IPC7 in Cape Town—the first in Africa. Her work spans avian growth to fossil diseases.
Dr. Maria-Eugenia Pereyra, Argentinian postdoc, excels in histological prep, contributing to caiman and plesiosaur papers. Their synergy yields four publications since 2023.Rate UCT professors like Chinsamy-Turan.
UCT's Role in Global Palaeontology and IPC7 2026
UCT leads African palaeontology, hosting IPC7 (Nov 30-Dec 3, 2026) at Baxter Theatre and Iziko Museum. Symposia cover dinosaur reproduction to Cenozoic vertebrates; field trips to Karoo and Cradle of Humankind. Travel grants prioritize African scholars.
This crocodile study exemplifies UCT's impact, blending local reptile research with global questions. Aspiring researchers can engage via university programs.Academic CV tips for palaeontology careers.
Broader Context: Growth Plasticity Across Reptiles
Supporting evidence abounds: Woodward et al. (2014) on alligators; Pereyra et al. (2024) on caimans. Lizards and turtles show multi-LAG years. Kiwi birds deposit stochastic marks. Environmental stressors—temperature, nutrition—drive plasticity, conserved in archosaurs.
For South African endemics like Karoo fossils, UCT's methods refine life histories.
Photo by Joel M Mathey on Unsplash
Future Directions and Actionable Insights
Validate wild crocs; expand to alligators, varanids. Integrate isotopes, genetics for precise aging. For dinosaurs, multi-proxy approaches (ontogenetic markers, fusion). UCT's fluorochrome technique sets a gold standard.
- Conserve SA crocs via refined demographics.
- Revise dinosaur growth models.
- Attend IPC7 for networking.
Prospective students: UCT's Biological Sciences offers palaeontology paths.Find lecturer jobs in South African palaeontology. Faculty positions.
This UCT crocodile bone aging study reshapes vertebrate biology, urging caution in LAG-based inferences while highlighting South African research prowess.
