Fossil Friday – partial horse skull

Horses are among the most common large animals in the Diamond Valley Lake fauna, and there are several skulls in various states of preservation in the Western Science Center collection. One of these skulls stands out, however, because of the strange way in which it was preserved.

The skull, which was collected at the East Dam of Diamond Valley Lake, is shown above in ventral view; essentially, you’re looking at the roof of the mouth. While it has been mostly cleaned, it’s still in the original field jacket, so this how the specimen was preserved in the ground. This view shows the complete set of posterior teeth (premolars and molars) from both sides of the upper jaw; the left ones are labeled below: 

You may have noticed that, while there are a lot of teeth in this jacket, there is no bone. Now, teeth tend to preserve better than bones, since tooth enamel is tougher than bone. But in this case we have the complete, well-preserved upper posterior dentition – 12 tooth positions – all in their correct positions relative to each other, with no bone to hold them in place! This suggests that the bone was removed after the skull was buried; otherwise the teeth would have shifted out of position. The only explanation I’ve been able to come up with is that, after burial, the skull was exposed to sediment or ground water chemistry that caused the bone to dissolve while leaving the teeth untouched. Differential dissolution rates are not unusual in fossils, but I don’t recall previously seeing an example in which the bone was completely dissolved while the teeth were apparently untouched.

This strange preservational history does provide us with certain advantages. With the bones removed, we can see features of the dentition that would normally require X-rays or CT-scans. Notice that M3 has a different appearance than the other teeth. That’s because it had not yet erupted when this horse died, and was likely still imbedded in the upper jaw. This also enables us to estimate the horse’s age. In modern horses M3 typically erupts when the horse is 3 to 4 years old, and this M3 was not even close to erupting, so this horse was almost certainly between 2 and 3 years old, likely closer to 2. That also means that its deciduous (milk) premolars should still be in the mouth, with the permanent premolars still developing. So let’s look at a side view of the left premolars: 

Here’s an annotated version:

As expected, the permanent premolars were still developing when this horse died, and are present beneath the deciduous premolars (actually above them, since this is the upper jaw sitting upside down). Their presence also helps refine our age estimate, since P2 should erupt around age 2 to 3 years. Since dP2 is still in place but almost worn away, this horse was around 2 years old when it died.

While in general I prefer to use X-rays or CT to look inside a bone, this skull gives us a great view of what was happening in this horse’s dental development, and provides some interesting hints about the geochemistry of the sediments from this site.


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