Friday, July 11, 2014

Not a good way to go

Here's a little story on CT scans of the two baby mammoths Khroma and Lyuba. The two are recent discoveries--found within the last decade--and among the most complete and best preserved ever seen. With such exceptional specimens, it's only natural that researchers would constantly be searching for ways to squeeze a few more facts out of them. Getting an opportunity to run one through an industrial-sized CT scanner is something both teams jumped at. The article mentions some interesting lines of research suggested by the results about how they grew and possible subspecies, but one thing stood out for me: these babies died horrible deaths.

CT scans of Lyuba and Khroma showing the sediment they in haled in their final moments. Source.

Most things that die go straight into the food chain. There are billions of bugs, germs, fungi, roots, wolves, sharks, and birds waiting for their share of anything that dies. If the biosphere has its way, no part will go unused. But, the biosphere does not always get its way and that's the only thing that makes paleontology possible. Some bodies or parts thereof escape the food chain and linger long enough for us look at them long after their parent species have been taken off the menu. There will always be gaps in the fossil record because the processes that lead to fossilization are the exception rather than the rule. Every fossil has a unique story to tell.

Over the last 320 years, fewer than eighty mammoths have been discovered with soft parts preserved (seventy-five by my count). Many of those were no more than a patch of skin with some hair and ligaments attached. Until recently, only about half of those reported were recovered. Only seventeen of the seventy-five were more than half complete when discovered. We know details of the last moments of the lives of only a small number of preserved mammoths. To my knowledge, all but one died a horrible, terrifying death.

The CT scans of Khroma and Lyuba show they drowned, buried in mud, and still gasping hard enough as they went under that they sucked sediment into their lungs. Little Dima, though the story of his death is still disputed, apparently stayed afloat for days in a bog before losing his strength and sinking into the mud. The Berezovka mammoth, an old male, tumbled down a riverbank, breaking his hip and thigh as he fell, and suffocated while struggling to stand up as wet soil slid down the bank and buried him alive.

The taxidermied skin of the Berezovka mammoth in the posture died in, attempting to rise as it was buried. Vladimir Gorodnjanski, 2007.

All of these horrible deaths were preserved because they happened in the Fall. Once the mammoths died, they were quickly frozen, probably that same night, and, for some reason, never thawed. In Lyuba's case, her preservation was aided by settling into an anoxic layer of sediment in the pond where she drowned. Other mammoth carcasses discovered at Yuribei and Fishhook also show a pattern of having died in the late Summer or Fall.

Dutch paleontologist Dick Mol with the head of the Yukagir mammoth. Source.

I'm only aware of one frozen mammoth that died in the Spring and, coincidentally, he's also the only one I'm aware of who died a peaceful death. The Yukagir mammoth was discovered in 2002 on the banks of an oxbow lake east of the Lena River delta. The front part of the body and most of the gut with its contents were recovered and sent to Yakutsk to be studied. The Yukagir mammoth was an old male who died in the early Spring after a tough Winter. He had several deformed vertebrae in his upper back from an infection indirectly caused by inflammatory bowel disease. It was the hungry season just before the plants would begin to bud and bloom. He had been eating a lot of willow twigs, which do not have a very high nutritional value, but they would have filled his stomach and the natural aspirin in them would have soothed his back. It was probably a warm day when he lay down on the shady side of a hill and died. Later, the sun melted some mud higher on the hill which covered the body and froze. Being on the shady side of the hill, it stayed frozen for the next twenty-two thousand years.

That most of the frozen mammoths died in the late Summer or Fall, is not an observation that can be extended as a rule to other fossils. This season was the time of year when large animals on the mammoth steppe had the best odds of being preserved, that is covered in mud and frozen. Other environments had their own best seasons for preservation. I suspect the best time to get preserved in the anoxic depths of a peat bog would be the wettest season. The best time to get deeply buried in sediments of a lake or shallow sea would be the flood season. The least likely time of year for preservation, in any environment, would be any time that left a body exposed on the surface where scavengers and the elements could have their way with your remains. 

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