Fossil Friday - tusk fragment

One of the prominent features of modern elephants are tusks, which are greatly enlarged upper incisor teeth. The presence of tusks is shared with most of the elephants' extinct proboscidean relatives, including mammoths and mastodons.Besides their size, elephant tusks are a bit different from most mammal teeth in that they lack enamel; tusks are made of ivory, which is dense dentine covered with cementum. Elephants use their tusks for a variety of tasks, including fighting, scraping bark off trees, and helping to loosen sediment when digging water holes, and the ivory tusks will get worn and polished at the tips. If the use is too vigorous the tip of the tusk may break off entirely. This isn't necessarily a problem for the elephant, because the tusks grow continuously. But it does mean that elephants may leave certain areas littered with broken tusk fragments.Mammoths and mastodons had tusks that were structurally very similar to those of modern elephants, and there they appear to have used them in similar ways. Their tusks were also just as prone to breakage, and isolated fragments often turn up as fossils. Shown above is the broken tip of a tusk that found near the western end of Diamond Valley Lake. Unfortunately, there are no morphological features present to tell if this came from a mammoth or a mastodon (at least none that I can identify). The tip (on the left) is polished and rounded from heavy wear, while the broken edge on the right is sharp and jagged. Below is a view of the other side of the same fragment:

This surface has a wrinkled appearance, and a rounded groove running for most of its length. I believe this represents an earlier break, in which the broken surface was smoothed off and polished prior to the entire tip breaking off. If that's correct, then this small fragment actually records two tusk-breakage events with a substantial amount of time between them.It's interesting to note that, according to Haynes (1991), while African elephants use their tusks for a variety of activities, they normally only break them when fighting each other. This seems to be especially prevalent drought years, when elephants will fight each other over access to water holes, although male elephants will fight in dominance displays, and there's evidence that mammoths and mastodons engaged in similar activities.Reference: Haynes, G., 1991. Mammoths, Mastodonts, and Elephants. Cambridge University Press, Cambridge, 413 p.

Fossil Friday - mastodon premolar

I'm starting 2015 at Valley of the Mastodon with an example of the blog's namesake, a tooth from a mastodon.This tooth is much smaller than the one I showed for Fossil Friday back in October, and represents a much younger animal. Like elephants, mastodons grow a total of six teeth in each quarter of their jaws (not including tusks), deciduous premolars 2-4 and molars 1-3. However, these teeth erupt sequentially as if they were on a conveyer belt, so there are never more than three teeth in the mouth at any one time.The tooth is shown above in occlusal view, with the front of the tooth to the right. It seems to have been rolled around in a stream or otherwise eroded, as there is a fair amount of damage including missing roots. There is also a lot of wear on the tooth caused by chewing, so that the enamel ridges characteristic of mastodons have been worn down, leaving three oval enamel ridges on the occlusal surface of the tooth. The fact that there are three of these enamel ovals suggests that this was not the 2nd or 3rd premolar, as these teeth usually only have two enamel ridges. The tooth is also small, only about 7 cm long, and is about twice as long as it is wide, suggesting that it is the 4th premolar. Moreover, the enamel ridges are not perpendicular to the long axis of the tooth, suggesting that this is a lower premolar rather than an upper one.Below is a labial view of the same tooth, showing the heavily worn crown and the damaged roots:

Because of the sequential tooth replacement in mastodons and elephants, the 4th premolar is only present in the mouth for a fairly short period of time. For modern elephants this tooth is only present from about 3 to 10 years of age. Assuming mastodonts grew at the same rate as elephants, this tooth probably represents a mastodon that was roughly 10 years old.

Fossil Friday - mastodon lower molar

For this week's Fossil Friday we have a tooth from this blog's namesake, the mastodon Mammut americanum. This particular tooth was collected about 15 miles southwest of the museum, not far from Temecula in Riverside County.This tooth is the lower left 3rd molar. The image above is the side, or lateral view, often referred to in teeth as the labial view (literally "near the lips"). The front of the tooth is to the left, and a lot of wear is visible on the crown at the anterior edge. Below is the same tooth in medial, or lingual view (literally, "near the tongue"):

In this view the front of the tooth is to the right, and the wear is much less obvious. The discrepancy in wear between the lingual and labial sides of the tooth are even more obvious when looking at the chewing surface of the tooth (occlusal view):

The front of the tooth is to the left. Mastodon teeth have enamel cusps arranged into a series of transverse ridges. There are five of them on this tooth. The first ridge is almost completely worn away, especially on the labial side where the enamel is completely gone. The second ridge is also heavily worn. The third and fourth ridges show moderate wear, with only the top of the enamel ridges worn away, while the small fifth ridge has very little wear. In occlusal view an unworn mastodon tooth is completely covered in enamel, but as the tooth is used the enamel wears away, exposing the softer underlying dentine. In the image below the dentine is colored red, giving an idea of how much enamel has been lost:

There are a few things that we can say about this mastodon based on the variations in wear patterns across this tooth. Because of the way mastodon teeth occlude when the mouth is closed, it's not unusual for the labial side of the lower teeth to wear more rapidly that the lingual side. The upper teeth usually show the reverse, with heavier wear initially on the lingual side. In both the upper and lower jaws the teeth eventually wear down until are essentially flat, but his tooth never reached that level of wear.The difference in wear across the tooth from the front to back is a result of the unusual method of tooth replacement in elephants and many other proboscideans, including mammoths and mastodons. Instead of having a mouthful of teeth all at once, mastodons grow in their teeth gradually and sequentially. As a tooth wears down, it moves forward in the jaw, eventually falling out and being replaced by the next tooth in the sequence. Since an individual tooth moves into position gradually, the front part of the tooth starts wearing down before the back part of the tooth has erupted. That's what we see in this tooth, in which the first enamel ridge is almost completely worn away while the last enamel ridge has almost no wear at all.Because of the sequential tooth replacement, we can also say a little about the age of this mastodon when it died. This tooth is the 3rd molar, which is the last one in the sequence, and it shows only a moderate amount of wear. This suggests that this mastodon was a fully mature adult, but not elderly, when it died. In modern elephants the 3rd molar doesn't erupt and begin to wear until the animal is about 25-30 years old. Assuming mastodon growth and tooth replacement rates were comparable to living elephants, this mastodon would likely have been 30-40 years old when it died.