In biology and palaeontology (the study of extinct organisms) there are a few ways to estimate the age of an animal’s skeleton. One is the extent of fusion of sutures in the skeleton – how much the plates of bone have joined together as the animal matured. Another is the texture of the bone surfaces. Then there are growth marks recorded in the microscopic structure of bone.
Many modern animals grow in periodic spurts (fast at times, slowly at other times). It’s generally thought that they grow fast in the good seasons when the environment is better for them in terms of food, temperature and water. They are thought to grow more slowly during unfavourable seasons, when the growth marks form in their bones, rather like the rings formed in trees. By counting the number of growth marks inside the bone tissues, scientists estimate the age of the animal. This method is called skeletochronology.
Over the years there have been a few studies that have determined when the different growth cycles formed, and have proposed the utility of skeletochronology for age determination.
The application of skeletochronology has been particularly important in working out the age of extinct reptiles like dinosaurs. It’s also been used as the basis for constructing graphs showing how the animal grew over time and comparing the rate of growth of different dinosaurs. This is very useful when trying to assess how extinct animals (like dinosaurs) grew up, and in some cases reached gigantic proportions.
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Our work in our palaeobiology laboratory at the University of Cape Town has shown that juvenile (wild and captive) caimans, American reptiles related to crocodiles and alligators, under one year of age showed growth marks in their bones. This was unexpected because the animals were too young to show annual periods of quick and slow growth.
This study by our team suggested there was a need for a more cautious approach to estimating the age of skeletons. This caution was reinforced by similar findings in our later work on Nile crocodiles.
More growth marks than expected
Our work on the Nile crocodiles began as an investigation into their growth dynamics. On three occasions we administered antibiotics to two-year-old crocodiles at the Le Bonheur Reptiles and Adventures farm, about 60km from Cape Town in South Africa. These antibiotics became incorporated into the bones of the growing crocodiles.
Later, when the crocodiles died, we skeletonised the carcasses and prepared thin sections of their bones which we examined under the microscope. The antibiotic markers allowed us to deduce how much bone growth had occurred in specific time periods.
Much to our surprise, we found that aside from a slowdown in growth during the unfavourable (winter) season, extra growth marks formed during the favourable (summer) season when fast growth was expected. These extra growth marks tell us that the crocodile responded to some environmental factors (perhaps temperature, rainfall, or competition) by slowing down their growth and forming a growth ring.
We found that the two-year-old crocodiles had as many as five or six growth cycles in their bones. We would have expected only one per year. This meant that if we applied skeletochronology, we would have overestimated the age of the crocodiles. Until now, most of the time when skeletochronology was applied, the concern has been about under-estimating the age of the animal (because growth marks are sometimes removed during normal growth processes).
Questions about method of establishing bone age
Our study of these living relatives of dinosaurs raises questions regarding the accuracy of using skeletochronology for estimating the age of dinosaurs. We know the four crocodiles were raised on a crocodile farm, which perhaps does not ideally reflect their a natural environment. But we are also aware that on the farm, they would have had optimal conditions for growth – and yet, under these ideal circumstances, they formed extra marks.
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Currently investigations into dinosaur skeletochronology are plagued by several issues such as the presence of multiple closely spaced growth marks that are difficult to separate out, as well as some growth marks that cannot be followed around the whole circumference of the cross section of the bone. Added to this, we suggest that since living relatives of dinosaurs (birds and crocodiles) can form extra growth marks, some of the growth marks in dinosaur bones could well be “extra” and therefore unrelated to their age.
More research is clearly needed to investigate this matter. An obvious first step is to undertake a similar study of crocodiles and alligators in the wild – a feat easier said than done.
Anusuya Chinsamy-Turan, Professor, Biological Sciences Department, University of Cape Town
Maria-Eugenia Pereyra, Postdoctoral fellow, University of Cape Town