Fossil Friday – possible mastodon bones

A common theme on this blog is that we can often get a lot of information from very incomplete material. Even so, as a general rule, the more remains we have from a given fossil organism, the more we can say about it. But sometimes we can have multiple bones, and even something as basic as a species identification can be elusive.

The small field jacket shown above was collected from the West Dam of Diamond Valley Lake. There are several bones present, all consistent with a single individual. An annotated version is shown below:

The bone highlighted in blue is the distal end of the left femur. Those in red are ribs, the yellow are thoracic vertebrae, and the purple are unidentified. The size and general shape indicate that these are proboscidean bones, but there are two species from Diamond Valley Lake: the mammoth Mammuthus columbi and the mastodon Mammut americanum. While mastodons are much more common at DVL, numerous mammoth remains were also found. Which species is represented by these bones?

Obviously our unidentified bones are not going to be of much help here. Ribs are also notoriously difficult to identify, especially if they’re incomplete.

The femur is potentially more useful. Femora have lots of distinctive features, and they differ between mammoths and mastodons. The most obvious difference is that mastodon femora are much more robust, while mammoth femora are rather long and slender. Unfortunately, this is difficult to evaluate unless a substantial portion of the femur (roughly half) is preserved. We only have about the distal fourth of the femur, and that is poorly preserved and partially hidden under the other bones. The medial condyle (the large curved knob at the end of the femur) does seem quite large, and this may suggest that a mastodon is more likely, but that’s not much to go on.

How about the vertebrae? In most vertebral positions mammoth and mastodon vertebrae are quite different. For example, in anterior view the lumbar vertebrae of mastodons have nearly rectangular centra that are wider than tall, while in mammoths they are more heart-shaped and are taller than wide. These vertebrae appear to be posterior thoracic vertebrae. Unfortunately, mammoth and mastodon posterior thoracics are very similar to each other. Mammoths tend to have much longer neural spines on these vertebrae, but the spines are incomplete in this specimen. Mastodons have a slightly taller and differently shaped neural canal, that seems a little more similar to these bones, but it seems this trait is quite variable.

The only other hint may be the size. By proboscidean standards, these bones are not particularly large. Even though they’re fairly small, they come from a more-or-less adult animal; the condyles are fused to the femur, and the vertebral epiphyses are fused to their centra, both of which indicate an animal that was mostly finished growing. Mammoths were larger than mastodons on average, so that suggests that a mastodon might be more likely (and, of course, mastodons are three times more common at DVL than mammoths).

So, which is it? I don’t know. I lean toward mastodon, and most of the observations seem to point that way, but that’s with a lot of “mays”, “seems”,  “suggests”, and other tentative qualifiers. Sometimes, it’s hard to say for sure.


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