Wednesday, September 28, 2016

The ugly mammoth

Toward the end of the last ice age, there were three(ish) types of mammoths in the world. Last week, in California, paleontologists excavated a skull that doesn't match any of the known three.

The Dig. Source.

The first of the major mammoth types is Mammuthus columbi, the Columbian mammoth. The Columbian was one of the biggest proboscideans that ever lived (putting it in the top ten largest land mammals). Mammoths are closer related to Asian elephants than they are to African elephants. They split off when all three genera lived in Africa. Asian elephants moved northeast through Arabia into Southern Asia, while mammoths looped through Europe into Central Eurasia and eventually reached Japan and the Siberian Pacific coast about three million years ago.

Who were these mammoths? The first mammoths in Africa were medium sized and probably resembled modern Asian elephants. As the mammoths moved out Africa they became larger and, we assume, better adapted to a cooler climate. The mammoths that arrived on the Pacific were quite large and somewhat hairy, but not woolly. They were adapted to the winters of inner Eurasia, but those winters were not that cold yet. The ice ages were just beginning. As these Eurasian mammoths changed and adapted, we define different stages of their evolution as distinct species even though there are no dramatic breaks along the way. The number and names of these progressive species are debatable.* I'm going to call the one that arrived at the eastern edge of Eurasia, Mammuthus trogontherii, the steppe mammoth.

At the right time, 1.5 or so million years ago, the steppe mammoths were able to walk straight into Alaska. In its coldest phases, the ice age locked up enough water in land-based icecaps to lower the oceans over 400 feet (130m). This dried up the Chukchi Sea and the northern half of the Bering Sea. Beringia, the "land bridge" connecting Alaska and Siberia, wasn't a narrow isthmus; it was wider than Alaska. Most of that lost water sat on Canada in a layer of ice two miles, or more, thick, blocking the way further into the continent. Due to a trick of the weather patterns, western Siberia, Alaska, the Yukon, and Beringia were dry at that time except for some mountain glaciers. When the ice caps melted, the water flooded Beringia, stranding some steppe mammoth herds in North America while opening the whole continent to them. Once enough ice had melted off Canada to make traveling easy, they found tasty prairie grass all the way south to Florida and Mexico. Although the steppe mammoth didn't change much after arriving in America, I'm going to call it the Columbian mammoth from here on.

Beringia. Source.

The ice age was not a singular event. It's a convergence of geologic conditions that make it possible for great continental ice sheets to form. Once those preconditions are in place, other factors, mostly astronomical, push the climate over the edge into a glacial period. We are still living in ice age preconditions and are about six thousand years beyond what should have been the peak warm centuries of this cycle. Things should be very slowly cooling, not rapidly warming, but we kind of screwed that up. We're not sure yet how many glacial advances there have been. Once the idea of an ice age was accepted, European geologists began mapping where the front edge of the ice had been. In places, they found a second, older front edge further out, then a third, then a forth. The idea that there had been four glacial advances, about 100,000 years apart, held for most of a century but, after WWII, we started sailing around the world, drilling holes in things, counting layers, and teasing apart isotope ratios. It turns out there have been eight big 100,000 year cycles. And before that, there were milder 41,000 year cycles. A lot of them.

The steppe mammoths passed through Beringia during one of the small cycles approximately 1.5 million years ago. The long cycles began 740,000 years, ago leaving 15-20 cycles for Columbian and steppe mammoths to periodically connect in Beringia before then.** What happened next? Each cold cycle was colder than the previous one. Columbian mammoths lived on a prairie that extended north-south. When the ice sheets grew in Canada, they could move south to a zone that better suited them. Dealing with the cold phases was harder for the steppe mammoths back in Eurasia. Their prairie extended east-west inland. Moving away from Beringia into Asia, the climate got worse, not better for them. They needed to evolve to survive.

By the time the long, deep ice age cycles began, the steppe mammoths closest to Beringia had accumulated enough useful mutations that we can call them a new species. These mammoths not only had long, thick hair, they had two layers of shedable wool under it. Their bodies had taken on a shorter, more compact form. Their blood hemoglobin found a way to more efficiently bond with oxygen at low temperatures. All of their extremities had modifications to resist cold. We call them Mammuthus primigenius, the woolly mammoth (you probably guessed that). In a short period, woolly mammoths expanded westward and replaced the last steppe mammoths all the way to the Atlantic Ocean. Eastward, they were able to colonize Beringia, after which they ran into ice-covered Canada.

During a later interglacial, when the path through Canada opened and the Bering Strait returned, woolly mammoths expanded southward. Unlike in Europe, they did not replace their cousins oon North America. Woolly mammoths co-evolved with a mosaic of clumping grasses and flowering herbs to form a distinct environment called the mammoth steppe. This environment was distinct from the prairie the Columbian mammoths' preferred and the Arctic tundra that currently covers much of the woolly mammoths' old territory. Although the two species mixed along the boundary of their preferred grazing lands, neither penetrated very deeply into the other's turf.***

The third mammoth was both a type and a species. Let me explain. The action around the Bering Strait/Land Bridge happened all over the world. When the seas went down, new lands were created or made easily accessible. Humans took advantage low water to colonize Australia and North America. In Europe, mammoths and straight-tusked elephants took advantage of narrow straits to colonize big islands in the Mediterranean. In California, a group of Columbian mammoths swam out to, Santarosae Island, an island  that was created when low water joined the northern Channel Islands into one mass. Where the islands were large enough, these intrepid mammoth (and S-T elephant) explorers established permanent populations. Then something interesting happened. They shrank. It's called the island effect. If there are no major carnivores, birds tend to become big, fat, and flightless. Other small animals also become large. Big herbivores, however, become smaller. Huge size, which was once useful for resisting predators, is a liability in a place with a limited food supply. The mammoth that only needs a third or an eighth the fodder of a mainland mammoth is the one who will survive a drought on an island. At the last glacial maximum, there were at least six species of pygmy elephants living around the world who came from at least three different ancestral lines (Tori Herridge has a book on them coming out next year).

Mainland and Island Mammoths. Source.

The Channel Islands pygmy mammoths, Mammuthus exilis, appeared and shrank sometime before the second to last glacial maximum. They survived the last warm period, even though the island shrank, broke into four parts, and then rejoined into one. The colonization of the island was not quite a one time thing. Santarosae Island was only about four miles from the mainland when the sea level was lowest. Elephants are excellent swimmers; twenty miles is not a problem for a planned swim. Over the entire time pygmy mammoths inhabited the Channel Islands, full sized Columbian mammoths continued to appear on the islands. One tenth of the mammoth bones found on the islands are Columbians.

Santarosae Island. Source.

The skull in question is not the most recent mammoth found on the island. This one is about 13,000 years old. The youngest is about 11,000 years old and within the margin of error of the first appearance of humans on the island. There's no indication of human contact wit this fossil and, if there was, it wouldn't be that big of a deal. We have evidence of direct contact (hunting) in New Mexico, Washington, and Siberia at earlier dates than this. What makes this interesting is the skull itself. It doesn't look like any of the other three mammoth species, and it's also not a mastodon.

Dwarfed mammoths/elephants are not perfect miniatures of their ancestors; they make adjustments to their specific environments because that's how evolution works. The proportions of the leg bones of the Channel Islands mammoths changed to better climb the steep hills on the island[s]. In common with other island dwarf elephants, their tusks shrank much more than their bodies. Large tusks are a big energy drain. The whole point of dwarfing is to conserve energy.

The Santarosae Skull. Source.

The new discovery is interesting because it doesn't neatly fit into any of our existing mammoth categories. It's bigger than a typical Island mammoth, but smaller than a mainland Columbian. Since this is near the end of the reign of the mammoths, it's not a partially dwarfed mammoth. Is it a young Columbian that swam out to the island? Probably, but here's what makes it intriguing. It has two well developed tusks, which are very asymmetrical. The left tusk curves down and forward in a gentle arc like a young Columbian or woolly mammoth. The right tusk curves down, out, back up, and inward like a very mature Columbian or woolly mammoth. One tusk is probably pathological, misshapen due to an injury or disease when it was younger. But, which one. So far, they have only the skull. With it out, they'll begin excavating beneath and around the site hunting for other bones. These will help answer exactly what the skull is. Is it merely a deformed individual or is it evidence of something new--not an asymmetrical mammoth, but perhaps an unknown disease.

Like all discovery stories, this one trails off while we wait for more details. Maybe, in a few months, after more of the skeleton has been excavated and the skull cleaned and examined, the Santa Barbara Museum of Natural History will issue a press release telling us about their progress, and the science press will deem it interesting enough to tell us about it. If not, we'll wait a couple years until one of the investigators publishes a scientific paper on it.

Meanwhile, we have only the mammoth. It would be great if this mammoth told us something amazing and universal about mammoths, evolution, extinction, human impact on the environment, or how climate change happens. It probably will not. It probably will only tell us about this mammoth. And that's great. Every living thing is an individual with its own life story and its own death story. Sue the T-rex is covered with scars that ripped into her very bones showing battles and injuries that she survived. Most of the famous frozen mammoths died horrible deaths by drowning or burial alive. My favorite, however, died a natural death on a spring day. He simply wore out and fell to the ground. He had a belly full of willow twigs, which is not typical mammoth fodder. But willow is a natural source of aspirin. The old mammoth had several arthritic vertebrae and was self-medicating. I suspect what's going to be most interesting about this mammoth is going to be its personal life history. What happened to its tusks? Why did it swim out from the mainland? Is there some datable event on the mainland, like a fire, that we can tie to its voyage? As a young mammoth on an island, younger and bigger than most of the others, what killed it? Do humans fit into this story somehow?

We know more about mammoths than any prehistorically extinct animal. By far. We have recovered scores of complete mammoth skeletons illustrating a large stretch of their evolution. We have recovered soft tissues of over forty individuals. We have DNA from over 100. We have stomach and gut contents of over a dozen. We have human illustrations of them in life. The next step is life histories of individual mammoths. This is how one ugly mammoth can be immortalized. It probably did not have an easy life and it died young. Let's not lose it in the back room of a museum and forget it. Let's take a good look at it and remember it. Even if it was an evil mammoth fleeing mammoth justice, its story deserves to be told. I want to hear it.

* Taxonimists are divided into two camps, spliters and lumpers. The former create new species based on any perceived difference in fossils (or living populations). The latter follow them around grumpily sweeping their profuse numbers of species back into a manageable number of piles. By 1940, the authoritative work on elephant taxonomy, Henry Fairfield Osborn's Proboscidea, identified 362 species in 44 genera including sixteen species of mammoths in North America alone. Today, about 175 species of probiscideans are recognized and that includes all the new species discovered since Osborn's time.

** As far as I can tell, no one has calculated a timeline of when the Strait was open and when it was closed. Geology in Alaska is years, even decades, behind the rest of the country. It's a big, empty place; difficult to get to; and plagued by hostile climate extremes, irritable bears, and armed libertarians. For many questions, gathering data is tricky and dangerous. But for others, the oil companies have collected great data, but it remains un-analysed and un-published. My question of when the Bering Strait was open or closed is a perfect example. While hunting for an answer, I came across scientific paper after paper with maps of drilling projects in the Chukchi and Bering Seas. But no one has tried using all that data to create a timeline.

*** Get it, turf?

1 comment:

Gav said...

Er, planning a sequel to "Discovering the Mammoth" already, are we?