What’s My Age Again?

July 12, 2013 PLOS Blogs Zoology

Earlier this month, an almost 40 pound rockfish was caught in Alaska that was allegedly 200 years old. The angler that caught the enormous fish based this age on body size estimates. Then, earlier this week when the fish was officially aged, it was found to only be 64 years old. Many of you at home may be wondering: how was this fish officially aged? How do we know how old any vertebrate is when all we have a carcass (or in the case of paleontology, just some dry bones)?

The term ‘sclerochronology’ was coined in the 70s to describe the study of accreted hard parts in invertebrates, but this study can be applied to any hard parts that have specific growth patterns. In corals, mollusks, and even vertebrate teeth and bones, we see growth lines that can reflect annual, weekly, or daily increments of time. Studying these lines and how often they form can tell us how old an animal is, since body size estimates are clearly unreliable once an animal reaches adulthood.

The aforementioned rockfish was aged by analyzing its otoliths– small bony structures in the ears used for sensing movement. These structures grow incrementally, and these increments form at known times. When they are counted, the fish can be assigned an age- a method that works in both modern and fossil organisms. The growth lines take on different characteristics in summer and in winter due to nutrient cycling and diet, so a pair of summer and winter bands can be counted as one year of life. Trace elements and isotopes can be measured in each band, which gives us an idea about migration and seasonal dietary change.

Otoliths from a Pacific cod. Uploaded by Edgewise, distributed under GDFL Wikimedia Commons.

Luckily there are also similar growth lines in vertebrate teeth, and sometimes bones, although these lines can be slightly more difficult to interpret. Sometimes their formation is not regular, so they cannot simply be counted the same as tree rings to determine age. In vertebrate teeth, there are a variety of types of growth lines known as perikymata and striae of Retzius, that can be a valuable chronometer if the time between these increments is known. In humans, certain growth lines in teeth are essentially weekly, but since we are unable to perform experiments to study the growth of extinct organisms, the length of time between growth increments of fossils can be difficult to discern.

Last week, Kevin Uno and co-authors published a study in PNAS relating to growth lines in elephant tusks. This paper makes me so excited for a variety of reasons, but mainly because they use carbon-14 dating and stable isotope analysis to make vital points about animal growth, ecology, and wildlife forensics.

Basically, Uno et al. were interested in utilizing the carbon-14 “bomb curve” to date ivory and thereby show if it was legal or not, as ivory harvested before a certain date can be legal for sale in the United States. Nuclear weapons testing from 1952-1962 doubled the atmospheric concentration of carbon-14. Recognizing this spike in radiocarbon in a biological sample can give us an idea of when it was formed.

Figure 3 from Uno et al. 2013 illustrating the C-14 age of growth bands in an elephant molar.

The work of Uno et al. illustrates that the dating can help discern whether the ivory is legal or not, but also, their detailed research into the dating of growth lines in elephant tusks and other mammalian molars allow us to be able to make more detailed paleoecological estimates with stable isotopes. Serial sampling fossilized materials for stable isotopes, such as molars, can tell us about seasonal changes in diets and water use. Understanding how often growth lines are deposited in hard parts of vertebrates by dating them is necessary if we want to have an accurate chronometer for assessing the age at death of a fossil organism. The work in this paper illustrated that elephants also have approximately week long increments of growth, and this knowledge now allows us to have a detailed chronometer for other extinct elephant relatives. I am of the opinion that serial sampling for stable isotopes and dating should be done on growth lines whenever possible so we have quantitative data on just how old these fossils are.

And next time the media throws us a fish story, we can all be a bit more skeptical.

Reference:

Kevin T. Uno, Jay Quade, Daniel C. Fisher, George Wittemyer, Iain Douglas-Hamilton, Samuel Andanje, Patrick Omondi, Moses Litoroh, and Thure E. Cerling. Bomb-curve radiocarbon measurement of recent biologic tissues and applications to wildlife forensics and stable isotope (paleo)ecology. PNAS 2013 : 1302226110v1-201302226.