Telling time is important to scientists who work in the deep past. As humans, we find it difficult to tell time beyond the scale of a lifetime. Last week runs into last month. Those months build up into years and decades. To those of us born in the 1970s, 25 years ago (1990—pre-internet) is half of our remembered life! So it is difficult for us to conceive of geologic time in thousands to billions of years. Geologists, paleontologists, and archaeologists get around these limitations by using natural clocks to measure time. The go-to technique for those of us working in the last 50,000 years is radiocarbon dating. Radiocarbon (aka 14C dating) is a classic illustration of atomic decay. It is a regular feature of college chemistry classes everywhere, demonstrating how an unstable isotope of carbon (14C) decays into the stable isotope of Nitrogen (14N). Since all living organisms incorporate carbon into their body tissues in ratios relative to their living environment, if we can measure the ratio of 14C to the stable isotope (12C) in a fossil, and we know the time it takes for 14C to uniformly decay (it’s half-life), then it is a simple mathematical function (more or less) to calculate how long ago that 14C decay clock started (i.e., the organism died). The physics behind 14C dating is elegant in its simplicity–if only the “real-world” were the same way!
A pragmatic primer in 14C dating
It may seem obvious, but the first question we ask ourselves is…”exactly what is it we are trying to date?” Are we interested in when an animal lived, when a change in climate occurred, or when people occupied a campsite? Since 14C occurs in pretty much anything that was living—we have choices. Archaeologists will use charcoal from a fireplace to date the last time it was used. Paleobotanists like to use seeds and leaves of plants to date when plants were living in a certain area. Since we want to understand when animals went extinct—we probably want to date the bones of these critters themselves.
There are also a few things going on in the environment that can alter a 14C date. Although atomic decay is a uniform process, the concentration of 14C in the atmosphere has changed through time, so not all dead things start out with the same about of 14C—which literally changes the equation. Over the last few decades there has been a global effort to “calibrate” 14C ages to calendar ages by reconstructing the atmospheric concentration of 14C through tree rings and cave speleothems (mostly). These materials archive yearly changes in chemical composition of the atmosphere. Although the decay of 14C to 14N is uniform, the variability in the atmospheric 14C through time is not, and estimating an age in calendar years requires calibrating a 14C date against other records. This can do some pretty funky things to a dataset. For instance, if we are trying to measure how long something lasted the differences between radiocarbon time and real-time can be significant. Consider a chronologically well-defined archaeological horizon—like the Clovis horizon. Although there may be some quibbles about specific dates, this period lasts ~600 years in radiocarbon time (10,900-11,500 14C years before present). However, if we calibrate those dates to work in real time the period is compressed by ~200 years (13,200-12,800 cal BP). Which is significant!
We are also always interested in possible contamination. The amount of 14C in an organism is very small—and only gets smaller as it decays. When things are buried in the ground they are subject to all kinds of biological and geological processes. They are eaten by bacteria and mold. Roots and worms burrow into the outer surface. Water percolating through the soil can leave trace amounts of Carbon-rich minerals on the surface. So all of these things must be removed before we measure the amount of 14C in a sample. When we date a bone, we want the date to reflect the time that the animal was alive—not all of the things that happened to it after it was buried. The “inorganic” or mineral component of bone (the part that gives bone its strength) is highly susceptible to this sort of contamination so we usually remove it and focus on the “organic” component—a mix of proteins, lipids, amino acids, and other goo that would have been present in the animal when it was alive. In the last decade or so we’ve gotten much better at dating bones. We pay close attention to the amounts of carbon and nitrogen in our samples to be sure that they are within the range of bones. If they fall outside of that range, it is possible they have been contaminated by post-depositional processes, or even the chemicals we use to stabilize and prepare specimens! The ability to measure very small samples also means we can isolate specific compounds from the bone itself, whether they be individual amino acids or short lengths of protein chains.
So what happens when you start dating lots of individual events? How do you make sense of broader spatial and chronological patterns? For the M-cubed project, we are dealing with just such a dataset. We’ve amassed a small mountain of data on where mammoths and mastodons were recovered, what they looked like, and importantly, how old they are. In collaboration with Greg Hodgins at the U of Az AMS lab, our dataset has increased to 96 reliable dates on mammoth and mastodon bones and teeth, spanning the last 50,000 years (some are even older). Although not as robust as the samples that modern ecologists amass by observing modern animals, this is a really decent dataset, and a far cry from the 17 well-dated sites that we had before starting the project. Our intent is to tighten up our estimates of when these species went in extinct in the Midwest—so there are some bits of this dataset that help us understand these extinctions in more detail.
When is the youngest not the youngest, and how do we date something that isn’t there?
First, we can look at the youngest dates on mammoths (13,260 cal BP) and mastodons (12,710 cal BP). This is the traditional way of dealing with extinctions and makes intuitive sense. The youngest date on an animal is solid, concrete evidence that those animals were still around when this particular individual was alive. However, is it the only way of dating an extinction? After all, the odds of actually dating the last living individual of a species are pretty slim, a statistical improbability at best. Can we do better? Can we use the rest of our dataset to 1) get a better estimate on the actual age of extinction and 2) understand a bit more about how these animals went extinct?
The real challenge about putting a date (with error bars) on the extinction of Ice Age megafauna is that you can’t date what isn’t there. In other words, with more samples and wider coverage, we might capture younger and younger specimens, but the odds of getting that last mastodon standing are still really small. Unlike a rock stratum, we can’t constrain the date by the next youngest layer, so the error in our estimates will remain fairly large. But larger numbers of dates do help. And with a larger sample size, we can get a better idea of whether we are picking up the last dribbles, or whether we’re missing the last elephant standing in the corner of the room.
Stacey Lengyel, our chronology-specialist on this project, uses sophisticated statistics and modeling to estimate the error in our window of extinction (see Oxcal 4.2; calibration and Bayesian Modeling software). Take mastodons for example. Before we started the project we had 35 dates from 16 localities. Although the terminal age was 10,055+/-40 14C BP (aside: a date we could not replicate), the estimated error for the actual extinction window was fairly large 11,810-9380 cal BP. With the addition of 96 new dates (from 67 sites, with a few samples still pending) to the mix, the estimate of the extinction window narrowed drastically (12,500-12,780 cal BP). Part of the reason for this is that we increased the sample size of dated mastodons in the last few centuries prior to actual extinction. We have more dated animals in the years leading up to that terminal date, which suggests if mastodons are around, then they are present in high enough numbers that we will sample them. It also means that—barring any massive changes in the region-wide preservation of these animals—the absence of dated mastodons AFTER this terminal date is “real” and not simply a function of poor sampling.
It’s a numbers game, and with more dates, we are on more solid footing when we attempt to order events in time. There are a whole slew of ideas about why these animals went extinct, and chronological contemporaneity is an important component of more than a few. Why is all of this important? Doesn’t it seem like we’re splitting hairs? What’s a few hundred years among friends? At the time these animals went extinct, there were lots of things going on. The first widespread evidence of a North American human presence, the Clovis period, dates from 13,200 to 12,800 years ago. There is a global return to glacial conditions called the Younger Dryas that begins ~12,800 years ago. Other species also went extinct at roughly the same time–but were they truly contemporary extinctions (yes, the verdict is still out on this)? We need maximum chronological resolution to establish an order of events. At the moment, it looks like mammoths and mastodons survived the Clovis period in the Midwest, becoming extinct at the beginning of the Younger Dryas. But before you think “that settles it”, it is worth considering that most of our midwestern climate indicators (i.e., a network of lakes skirting the central and southern Great Lakes) suggest this climate shift was very gradual–at least at a regional scale. It was nowhere near drastic enough to unambiguously finger climate-change as the cause of the terminal Pleistocene extinctions. For this, we’ll need other, complementary datasets on animal ecology, as well as region-specific information on human presence and environmental changes. More on that in posts to come.
Note: This is the first post in a series focused on a 4-year, National Science Foundation funded project to look at the extinction of Mammoths and Mastodonts in the Midwest.
For the last few years we’ve been traveling…a lot. We started a project in 2011 to better understand 1) when mammoths and mastodonts went extinct, and 2) the ecological mechanisms that might have played a major role in how they went extinct. The major foundation of this project is a museum-by-museum survey of mammoths and mastodonts in collections from nine states and one province (MN, WI, IA, MO, IL, IN, OH, KY, MI, and ON). Over the last 2.5 years, we’ve documented mammoths and mastodonts from 576 localities.
When we started this project, we knew that the Midwest was a hotbed for Pleistocene proboscideans. A compilation of known/published localities showed a continent-wide distribution, but definitely a concentration in the Great Lakes. Of course, as with most things paleontological, the best represented individuals are the youngest, and both genera overlap with the first humans in to the New World. The last standing mammoths on the continent are widely separated in space, found from the South Dakota Badlands to upstate New York. After the last-glacial maximum, mastodonts seem to be limited to forested areas of the Great Lakes region and Northwestern North America.
Mammoth and mastodont studies lie at the intersection of major research questions in a number of different disciplines. The reason that they are so important is primarily due to the fact that they are so common and widespread in the fossil record. Why, you ask? Probably size and distribution. Their remains are big enough to be seen from the cab of a tractor or backhoe, and were distributed coast to coast during the last half of the Pleistocene. Since they are relatively widespread and common components of the fossil record, we can get an elephant’s eye view of ecological changes, IF we know what questions to ask. Their remains are also much more common in museum collections than other victims of the terminal Pleistocene extinction event, so they might give us a glimpse into HOW the extinction occurred.
Why (Part I): Preposterous Proboscidean Paradigm Shifts
The 2005 discovery of a mammoth tusk in the bed of Sugar Creek (central Illinois) started it all. Dennis Campbell, biology professor at Lincoln College (and ISM research associate), had brought a class out to the creek to census freshwater mussels when Judd McCullum, (then a student in the class), stumbled across a large cylindrical object. Despite good-natured ribbing that it was “just a tree trunk”, Judd was convinced it was a mammoth tusk…and he was right. ISM paleontologist Jeff Saunders identified the tusk as a woolly mammoth. Conventional thinking had woolly mammoths in Illinois at the same time as the glaciers. We thought that they occupied the narrow band of tundra in front of the massive continent-grinding glaciers that covered the Midwest up until ~18,000 years ago.
To be thorough, Jeff submitted a sample for radiocarbon dating anyway. The results were surprising. Rather than dating to the time of the glaciers, the Lincoln College mammoth dated a few thousand years later, when central Illinois was covered by a cold swamp, with black ash and spruce as the dominant vegetation, not a grassland. This was a game-changer. Not only were woolly mammoths found outside of their traditional tundra habitat, but when the glaciers left the area, they stayed and survived in changing Midwestern ecosystems until their extinction, ~12,000 years ago.
Meanwhile, a graduate student at the University of Utah developed an interest in the ancient DNA of North American mammoths. Jake Enk, now finishing his PhD at MacMaster University in Ontario, managed to extract a good chunk of mitochondrial DNA from the Huntington Mammoth in Utah. The Huntington mammoth is the epitome of a Columbian Mammoth. It’s from the heart of the Columbian mammoth range, Utah. It’s cheek teeth, although fairly worn (this animal was 55-60 years old), consist of 7+ enamel ridge-plates spread out into a relatively long tooth (~6 plates per 10 cm). For good measure, Enk also extracted DNA from two additional Columbian mammoth teeth from Wyoming. Surprisingly, when compared to woolly mammoth DNA from Alaska and Siberia, these Columbian mammoths were similar. Actually, they were VERY similar. The three Columbian mammoth mtDNA sequences nested nicely within one of the Alaskan woolly clades. The take home message was that morphological variability in mammoths is much greater than genetic differences. These were not separate species–they probably don’t even merit being a sub-species.
But mastodonts were not immune to paradigm shifts. In 2011, Neal Woodman and Nancy Beaven published a report on the dating of a mastodon in northern Indiana, the Overmyer mastodon. The date they reported was 1500 years younger than expected. The typical pattern was that mastodonts went extinct ~12,900 BP, only a few hundred years after the first major human cultural group (Clovis) appears on the scene. The Overmyer animal, if the dates were to be believed, meant that mastodons survived not only the first wave of human colonization, but lived side-by-side with human groups almost into the Holocene!
Studies like these got us to thinking. What if there are other assumptions about the habitat preferences and behaviors of mammoths and mastodonts that we are wrong about? What would happen if we dated more specimens–or used new techniques for insight into paleodiets and behavior (i.e., stable isotopes) or population dynamics (i.e., ancient DNA)? Was the Lincoln College Mammoth the exception? Or the rule? What do the major morphological differences between different mammoth populations mean if they don’t reflect relatedness–or evolutionary history? Did Mastodonts really hang on so late? Why was there such a large gap between the Overmyer mastodont and other dated animals in the Midwest? All of a sudden, there were a lot of questions that we didn’t know the answer to.
Why (Part II): Elaborate Extinction Scenarios needing Evaluation!
These questions are important not only for understanding past ecological conditions, but for understanding one of those BIG questions…why did 35 genera of North American megafauna (species >100 kg) go extinct at the end of the last Ice Age? This extinction event is considered one of the BIG 5 mass extinctions in the history of life on Earth. Yet it is unique from earlier mass extinctions. In addition to being the most recent, the majority of the victims were the largest of the large fauna. Small fauna were spared, more or less, or managed to migrate to new ranges. Furthermore, the extinction of these species coincided with major climate changes AND the introduction of a novel, supposedly predatory species known to profoundly alter its environment and potentially overhunt its prey, Homo sapiens. The discussion surrounding this extinction event in recent years has become increasingly polarized. There are a number of scenarios that have been proposed to explain this extinction. Perhaps some of the megafauna were killed off by colonizing human populations, with the rest doomed as the result of ecosystem reorganization after the loss of keystone species such as mammoths. Alternatively, abrupt climate changes may have stressed megafaunal populations to the breaking point. Deglaciation was not simply a gradual warming. The glacial spring came in stops and starts, and may have presented megafaunal populations with a moving target. Never quite able to adjust to changing conditions. These are the main working hypotheses, but of course, there are others. Was it the mid-air explosion of a comet over glacial ice in Canada? A hypervirulent disease? A combination of the above? It is hard to say without more hard data on the timing and ecology of key extinct species such as mammoths and mastodonts. Beware of TV documentaries claiming that we now know the answer to what caused these extinctions. Most scientists agree (although there a vocal few who don’t) that we don’t have enough data to tease out the smoking gun…let alone identify who or what pulled the trigger!
How (Part I): New Techniques
But how do you tackle something as big as megafaunal extinctions? This is a global pattern involving many different species and ecosystems. What sort of data do you need to distinguish between different extinction scenarios? Obviously, timing is everything. In the last decade or so, direct dating of megafaunal bones has become more accurate and commonplace. For this project, we’ve been dating a lot fossils from museums, trying to fill in the gaps in space and time. We hope to say something about when these animals ultimately went extinct using new and improved chronological datasets. We also believe that animal ecology is an important aspect of survival, so we are utilizing techniques that capture the details of individual life histories. Specifically, chemical signatures from bones and teeth (in the form of stable isotopes) that can tell us about animal diets and mobility. (more on what we are learning from these techniques in future posts)
How (Part II): The Team
Modern paleontology does not happen without a team of experts, each providing critical data for hypothesis testing. This project is a collaboration between many different experts. Jeff Saunders (ISM) and myself are vertebrate paleontologists/paleoecologists who are tasked with understanding biogeographic variation in space. Stacey Lengyel (ISM) is an expert in dating techniques–she also happens to be creating a great website on Ice Ace mammals that will be launched this spring. Greg Hodgins is a bone chemist and dating expert at the University of Arizona. J. Douglas Walker (University of Kansas) and Alan Walker (Iowa State University) are experts is different types of isotopic analyses. Others have also contributed to our understanding of proboscidean paleoecology. Veterinarian Dennis Lawler (ISM) has been instrumental in exploring the impact of disease on mammoths and mastodonts and Eric Grimm (ISM) has provided environmental context for dated specimens through his work on ancient pollen recovered from the mud of midwestern lakes.
As we scale back the data acquisition phase of this project and focus more on analyzing the datasets that we’ve collected, we’ll have more to say about how mammoths and mastodonts lived and died, at least across the Midwest. A significant component of this project is dedicated to communicating our results to the public, primarily through online resources like this blog and the aforementioned website. So stay tuned for future developments. The data have started rolling in.
Enk, J., Devault, A., Debruyne, R., King, C. E., Treangen, T., O’Rourke, D., … & Poinar, H. (2011). Complete Columbian mammoth mitogenome suggests interbreeding with woolly mammoths. Genome biology, 12(5), R51.
Saunders, J. J., Grimm, E. C., Widga, C. C., Campbell, G. D., Curry, B. B., Grimley, D. A., … & Treworgy, J. D. (2010). Paradigms and proboscideans in the southern Great Lakes region, USA. Quaternary International, 217(1), 175-187.
Woodman, N., & Beavan Athfield, N. (2009). Post-Clovis survival of American mastodon in the southern Great Lakes region of North America. Quaternary Research, 72(3), 359-363.