A colleague recently pointed out a change in my appearance that has become more obvious in recent years. It’s noticeable in the images above, from 2005 (left) and 2010. Here are some closeups emphasizing the change:
Broadly speaking, this is a minor example of ontogenetic change, which is simply the changes that take place in an organism during the course of its life. This can be an interesting issue for paleontologists to deal with, in fossils as well as in themselves. The increasing amount of grey in my beard may not hinder my identification, but many examples of ontogenetic change are much more significant. Still using me as an example, compare these pictures of me:
Notice how, as I got older, the shape of my head in particular changed, as well as the size of my head relative to the rest of my body. Humans exhibit allometric growth (occasionally referred to as anisometric growth). Basically, different parts of our bodies grow at different rates, so our body proportions and shapes change as we grow. The vast majority of organisms exhibit allometric growth. This can cause difficulties for paleontologists trying to identify fossils. If you have two similar but different individuals, how can you tell if the differences are due to the individuals representing different species, or different ages within the same species?
This is often a difficult issue to resolve. The history of paleontology is filled with examples of named species that later lost their identities when it was decided that they were probably juvenile examples of known species. Fortunately skeletons do sometimes leave us clues as to the age of the animal, which helps in determining if the variation we see is ontogenetic.
In whales, one of the features we can look for is the degree of epiphyseal fusion in the vertebrae. Vertebral epiphyses are bony disks that attach to each end of the vertebral centrum. In juveniles the epiphyses are attached with cartilage, but in older animals they fuse to the centrum. The unattached epiphyses are marked below, in this side view of a thoracic vertebra from Squalodon whitmorei:
Whales are also convenient in their style of allometry. As a general rule, the epiphyses at each end of the vertebral column (the neck and the tip of the tail) fuse fairly early in life, while the thoracic vertebral epiphyses fuse much later. So a whale with loose neck epiphyses was probably a very young animal. In contrast, a whale like “Picasso”, which only has loose epiphyses on the thoracic vertebrae, was probably an adolescent or a young adult:
This is a general rule that usually works well, but it’s not ironclad; there are individual exceptions. The type specimen of Squalodon whitmorei, like “Picasso”, has unfused thoracic epiphyses but fused lumbar epiphyses, suggesting that it’s an adolescent or young adult. However, one epiphysis in the neck (the posterior one on the 4th cervical) is also unfused, as an apparent individual variation.
Like the grey in my beard, there are also signs of aging in fully mature animals. It seems that whales often develop osteoarthritis in their vertebrae late in life. This results in the development of osteophytes (bone spurs) in the centrum, like the ones below:
Again, this works as a general rule, but there are exceptions. Osteophytes can develop at a younger age, due to injury or an abnormally early onset of osteoarthritis. Even so, by looking for clues such as epiphyseal fusion and osteophytes, it’s possible to gain insight into the age of an individual at the time of its death. This can help to determine if the variation seen in a fossil might be due to ontogenetic variation.