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Adult and juvenile desmostylians. Art by Justin Santos.

Rare Fossil Offers a View to a Beast’s Old Age

Picture a prehistoric animal. Don’t worry, I won’t judge you if it’s Tyrannosaurus or another super-popular species. Any creature will do. Now, what age is the animal you imagined? I’d be willing to bet that it’s an adult, representing a healthy individual in the prime of life. (If not, and you pictured a hatching or an aged or injured animal, take your moment of pride in being ahead of the curve and hang on a second.)

In art and film and even within the science of paleontology itself, we often envision extinct species as healthy adults. There are plenty of intersecting reasons for this. We’re bound by a typology that needs to find a standard, a baseline of normalcy, that represents a species, not to mention that even in our own lives we remember the influential and famous as adults rather than as their childhood selves. Adulthood is when things get interesting. And with fossils, we often lack individuals from the very young and very old parts of their species’ lifespan. Babies are snacks for carnivores and most individuals in a population never make it to old age, meaning that we end up with a fossil collection of subadults and adults who evaded the jaws of predators but expired before they hit old age. We don’t envision creaky old Triceratops or saberkittens very often because we just don’t find them in the same numbers of the midrange individuals.

But every now and then someone’s fortunate enough to stumble across the ontogenetic bookends of a species’ life. One such stroke of luck befell construction workers in 1996 as they were working on southern California’s State Route 241. As it chewed through the 11 million year old rock of the Puente Formation that rings the Los Angeles Basin the excavation turned up the lower jaw of an extremely unusual marine mammal, and it turned out to be a real old timer biologically as well as geologically speaking.

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The jaw of an old Desmostylus, showing a downturned canine but a lack of cheek teeth. From Santos et al., 2016.

That jaw, described by paleontologist and friend of the blog Gabriel-Philip Santos and colleagues, belonged to Desmostylus. Paleontologists are still working out what these fossil mammals were. For a long time they seemed to be cousins of elephants and manatees, although some recent research has suggested that they were early offshoots of the same line that spun off horses and rhinos. Regardless, though, they can immediately be recognized by their teeth. Their premolars and molars are “bonded pillars” – the literal translation of Desmostylus – and their bodies looked like some kind of mashup between hippo and sea lion.

Yet the jaw dug up from the tollway construction project lacked the characteristic molars. The sockets, Santos and coauthors report, are not so much empty as filled with remodeled bone. Throughout the animal’s life a limited set of chewing teeth erupted and moved forward to provide fresh chewing surfaces to cope with all the sand and grit attached to the seagrasses and other vegetation it ate, and apparently the mammal was so adept at this that it wore down its last set of teeth beyond the nubs. The beast even managed to feed itself as its tooth roots eroded away and bone reclaimed the open holes. And so the fossil has filled in just a little bit more about what paleontologists know about this species, but the relic is more than that. The gray, near-toothless Desmostylus reminds us that there was far more to prehistoric life than the vibrant days of ancient youth.