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Mysteries of the tar pits

by Donald Prothero, Mar 27 2013
The "Lake Pit" in Hancock Park, with the fiberglass mammoth family on its edge. The Page Museum is in the background.

The “Lake Pit” in Hancock Park, with the fiberglass mammoth family on its edge. The Page Museum is in the background.

Two years ago this week, I began my weekly contributions to SkepticBlog. It seems amazing to realize that it has been that long, or that I’ve written over 100 essays in that time span. I now begin to appreciate how difficult and stressful it can be, and how newspaper columnists must work, always on the lookout for some germ of an idea to expand into 2000-3000 words. But it’s even harder in a science-based blog, where I’m not just flagging recent stories that I’ve encountered, but also try to write a column of substance full of background that is carefully researched, and adding some of my own scientific perspective on its importance. It’s more like the columns Stephen Jay Gould had to write for Natural History magazine, but he only did them once a month!

Given the occasion, I thought I’d indulge myself and actually blog about some of my own recently published research. I was one of those kids who got  hooked on dinosaurs at age 4, and never grew up—except when I was a kid in the 1950s, dinosaurs were not cool with every kid under 12 as they are today. I was the only kid in the school who liked dinosaurs, and I was considered a freak because I knew all about them and could pronounce their names. Now every kid over 7 can do it, apparently. As soon as I knew what a paleontologist was, I knew that’s what I wanted to do. In sixth grade, my teacher Mrs. Helene treated her top boy and girl to a trip out to the Miocene fossil beds at Redrock Canyon (she was a member of the L.A. Natural History Museum, so we got to join her on a member’s tour). By the time I reached tenth grade, I had mapped out where I was going to college and what I was going to study. (I went to U.C. Riverside because it was then cheap for California residents, less than $200 a quarter, not too far from home, yet the large campus had only 4000 students but outstanding geology and biology programs with two paleontologists in the faculty). In the summer after 10th grade (1970), the La Brea tar pits were allowing volunteers to work on their new excavation in Pit 91, and I was eager to join in. As an untrained high school kid, I was relegated to the beginner’s task for all volunteers: sorting out the microfossils (tiny rodent and bird bones, snail and clam shells, insect and plant remains, etc. from the concentrated material left after they wash all the tar out with solvents). They plunked us down on a table beneath a big sycamore tree, and we each had a large lighted magnifier on a stand to see what we were doing hands-free, while we used a tiny wetted paintbrush to pick up these minuscule fossils and place them in the keeper vials. It was dull, tedious work most of the time, but every once in a while we’d find a spectacularly preserved tiny bird bone or rodent jaw, or beetle wing cases which are still iridescent, which made things interesting. I didn’t drive yet, so I had to spend almost 4 hours riding the buses down from Glendale to downtown Skid Row, and then out Wilshire Boulevard and back, just to work for about 4-5 hours a day. But it was lots of fun, and convinced me that no matter how difficult or tedious the work, I was determined to become a paleontologist.

Fast-forward 35 years, as my career has taken me to every state west of the Mississippi River to do research, and I’ve published dozens of papers on rhinos, horses, camels, and many other mammal groups. In all this journey, I’d never found an excuse to return to Rancho La Brea and do a research project there—even though I taught at a school just 30 minutes’ drive away, and knew all the staff there, and took my historical geology students on a tour there every spring. In 2006-2007, I was teaching basic paleontology to an outstanding group of Occidental students, then a few months later to a large group of Caltech students (since paleontology isn’t taught there unless I come to offer a course). From that Oxy class I had 5 students who wanted to do their senior research project with me, and 3 more from the Caltech group also wanted to do something with fossils. So I had an interesting dilemma: I love working with students and starting them on their research careers, but supervising 5-8 different undergraduate projects can be a LOT of work, since they’re not grad students who can be pointed in the general direction and be expected to produce with minimal supervision. There aren’t a lot of paleontology projects that undergrads, with their limited background, can jump right and and solve without me doing the heavy lifting. Then it occurred to me: Rancho la Brea! The collections are enormous, with over 3.5 million specimens of about 660 species of animals and plants, including 59 species of mammals and 135 species of birds. They are housed in a special museum, the Page Museum of La Brea Discoveries, named after its major donor George C. Page, who made his fortune years ago selling Mission Pak fruit baskets at Christmas time. It is located right in Hancock Park, the main site for the tar seeps that trapped these animals between 35,000 years ago until about 9000 years ago. The outside of the museum looks like a huge Mesoamerican pyramid, and inside the building are the exhibits on a loop around the central atrium full of prehistoric-looking plants. But hidden under the grassy slopes of that pyramid and invisible to the public are huge storage rooms to contain all these fossils, plus additional rooms for preparation, classrooms, offices, and labs. If you pass down the east wall of the building, you go down a long dark corridor with thousands of plastic trays of bones, all laid out neatly in rows, organized by species, then by which bone it is, then by pit, so you can quickly find just about anything. For some animals, like the dire wolf and the sabertooth cat, the sample is enormous, with hundreds of specimens of every single bone in their skeleton from every major pit. Other animals (such as pronghorns or peccaries or tapirs or rodents) are relatively rare. The bird room is similarly impressive, hundreds of drawers of delicate bird bones, normally not well preserved in the fossil record. The striking thing about the sample is that predators and scavengers are far more abundant than their prey species, an apparent violation of ecological rules, which can be explained by the fact that it was a predator death trap, with trapped one prey species acting as bait to lure many predators and scavengers.

Kristina Raymond (Oxy'08) measuring trays and trays of sloth bones from the racks. These racks of trays extend back into the darkness over 300 feet, and wrap completely around the outside of the Page Museum.

Kristina Raymond (Oxy’08) measuring trays and trays of sloth bones from the racks. These racks of trays extend back into the darkness over 300 feet, and wrap completely around the outside of the Page Museum.

The biggest challenge of doing research in this collection is figuring out an interesting scientific question that can be solved with these fossils. The bones are completely disarticulated, so you can’t do anything that requires specimens known to belong to a single individual, nor do you have partial or complete skeletons or associated elements. But the enormous sample size is excellent for statistical approaches, and for looking at change through time, since each pit sample has been radiocarbon dated, and gives us a good sample of populations in the area through almost 30,000 years of the last glacial-interglacial cycle. Many people have done functional anatomy with the well-preserved skulls and teeth, or looked at the incredible collection of sick and diseased individuals and juveniles that are common here but rarely fossilized elsewhere.

A typical tray in the Page Museum, showing hundreds of sabertooth cat left middle hand bones (third metatarsals), all from a single pit

A typical tray in the Page Museum, showing hundreds of sabertooth cat left middle foot bones (third metatarsals), all from a single pit

Then it occurred to me that with a large gang of students, we could look at the issue of stasis in fossil species through time. As I discussed in a previous post, one of the important problems in paleontology first mentioned by Stephen Jay Gould and Niles Eldredge in their 1972 “punctuated equilibrium” paper was the fact that most species remain static through time (“equilibrium”) and only change during speciation events (“punctation”). Stasis had been documented hundreds of times in the past 40 years, but only a few such studies examined whether species are static despite the selection pressure of climatic change. I had documented several such instances in Eocene-Oligocene rocks of the Badlands and during other time intervals (Prothero and Heaton, 1996; Prothero, 1999), but here was an outstanding data set on which to test the hypothesis, since we had large well-dated population samples from a single region that spanned the dramatic climatic changes of the last glacial-interglacial cycle. Based on the pollen data and isotopic data, the climate change in the L.A. Basin was quite impressive, with snowy closed-cone coniferous forests covering the low elevations during the peak glacial 20,000 years ago. So I arranged for each student to work with a different species, loaned them a pair of calipers, got them access to the collections, established our measurement protocols, and then let them go crazy with their laptops running Excel. Kristina Raymond (Oxy ’08) worked on the ground sloths, bison, and several other species; Sarah Molina (Oxy ’08) worked on the golden eagles (by far the most common bird at La Brea, believe it or not); Anastasiya Sutyagina (Oxy ’08) worked on the cats; Valerie Syverson (Caltech ’07) worked on the condors; Gina Gage (Caltech ’09) worked on the camels. The next year I had additional recruits: Meena Madan (a UC Irvine student working at La Brea) finished the cats (both sabertooths and Ice Age lions); Ashley Fragomeni (Assistant Curator of Paleontology at the Museum) and I worked on the turkeys, caracaras, and bald eagles (thus covering the 5 most common birds). Then in 2010 we finished the project with Sylvana de Santis (Oxy ’11) tackling the horses; and Edward Linden (Oxy ’11) tackling the largest sample of all, the dire wolves. Each student published their own work individually (see bibliography below), then they got to be co-authors on several joint papers where we looked at the overall big picture of our results (Prothero et al., 2012; Prothero et al., in press).

And the results were no surprise to people who work on Ice Age mammals, since it has long been known that most Pleistocene species are static through millions of years of glacial-interglacial cycles, with no apparent response to climate change. This was first shown by Darwin’s friend Hugh Falconer in 1863 working with mammoths, and corroborated by the major Pleistocene mammal workers, such as Tony Barnosky and his students at UC Berkeley. Still, the La Brea sample was an ideal place to test the hypothesis because it spanned the full range of mammals and birds, and could be directly dated and tied to well-known climate changes in the region. Sure enough, there was complete stasis with no statistically significant size or shape changes through the entire interval, even during the peak glacial when cooler and drier conditions might predict that they would have larger body size (Bergmann’s Rule) or shorter limbs (Allen’s Rule). Why stasis is so prevalent in the region is still not completely understood. It may be, as Bruce Lieberman and Niles Eldredge have suggested, that most species are much more broadly adapted than we realize, and can live in a wider variety of climatic settings without a noticeably difference in size or proportions than we used to think. Either way, it shoots down the oversold idea from Neo-Darwinism that organisms are delicately adapted to small environmental changes and adapt in response to every little blip in climate, as in the famous example of the Galapagos finches. For forty years now, the fossil record has abundantly demonstrated that this is not true, at least not over the long spans of geologic time that really matter for evolution.

Now that the results of this long-term project are all published, it’s great to see the consequences. The paper has been given a lot of publicity, mostly from the press that still doesn’t understand stasis and how it is the universal pattern of the fossil record. Naturally, the Discovery Institute creationists jumped on the bandwagon, and repeated their usual lies and misconceptions about evolution and stasis that they have since the first publication of the 1972 punctuated equilibrium paper. But the best product of this research is how it launched the careers of so many of those students. Valerie Syverson is now earning her Ph.D. in paleontology at the University of Michigan; Kristina Raymond went to East Tennessee State University to get her master’s in paleontology; and Meena Madan is applying to grad programs in vertebrate paleontology right now. Others went in different directions: Sarah Molina and Sylvana de Santis went into high school teaching; Gina Gage is getting her degree in architecture;  Ashley Fragomeni is still working at the Page Museum, Natural History Museum, and Alf Museum; and Edward Linden has been a consulting environmental paleontologist for over a year now, but now is headed back to grad school. That’s some of the best results of scientific research: it not only increases our store of knowledge, but it increases the human capital as well, and positively improves the careers of so many people. Even though we did the entire project on a shoestring budget with no grant support, every dollar and every hour was worth it!



  • DeSantis, S.N., Prothero, D.R., and Gage, G.L. 2011. Size and shape stasis in late Pleistocene horses and camels from Rancho La Brea during the last glacial-interglacial cycle. New Mexico Museum of Natural History Bulletin 53:505-510.
  • Fragomeni, A., and Prothero, D.R., 2011. Stasis in late Quaternary birds from the La Brea tar pits during the last glacial-interglacial cycle. New Mexico Museum of Natural History Bulletin 53:511-516.
  • Madan, M., Prothero, D.R., and Sutyagina, A. 2011. Did felids from Rancho La Brea change size or shape in the last Pleistocene? New Mexico Museum of Natural History Bulletin 53:554-563.
  • Molina, S., and Prothero, D.R. 2011. Evolutionary stasis in late Pleistocene golden eagles. New Mexico Museum of Natural History Bulletin 53:564-569.
  • Prothero, D.R. 1999. Does climatic change drive mammalian evolution? GSA Today  9(9):1-5.
  • Prothero, D.R. 2012. Cenozoic mammals and climate change: the contrast between coarse-scale versus high-resolution studies explained by species sorting. Geosciences 2:24-41.
  • Prothero, D.R., and T.H. Heaton, 1996, Faunal stability during the early Oligocene climatic crash. Palaeogeography, Palaeoclimatology, Palaeoecology 127:239-256.
  • Prothero, D.R., and K. Raymond. 2008. Variability and sexual size dimorphism in Pleistocene ground sloths (Xenarthra), New Mexico Museum of Natural History and Science Bulletin 44:331-334.
  • Prothero, D.R., and K.R. Raymond. 2011. Stasis in late Pleistocene ground sloths (Paramylodon harlani) from Rancho La Brea, California. New Mexico Museum of Natural History Bulletin 53:624-628.
  • Prothero, D.R., V. Syverson, K.R. Raymond, M.A. Madan, A. Fragomeni, S. Molina, A. Sutyagina, S. DeSantis, and G.L. Gage. 2012. Stasis in the face of climatic change in late Pleistocene mammals and birds from Rancho La Brea, California. Quaternary Science Reviews 56: 1-10.
  • Raymond, K.R., and Prothero, D.R. 2010. Comparative variability in intermembranous vs. endochondral bones in Pleistocene mammals. Palaeontologica Electronica
  • Raymond, K.R., and D.R. Prothero. 2011. Did climate change affect size in late Pleistocene bison? New Mexico Museum of Natural History Bulletin 53:636-640.
  • Syverson, V., and D.R. Prothero. 2010. Evolutionary patterns in late Quaternary California condors. PalArch Journal of Vertebrate Paleontology 7(10):1-18.
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8 Responses to “Mysteries of the tar pits”

  1. Christopher Webb says:

    Thank you for a most interesting piece. As someone who in another life might well have aimed for a career in palaeontology, it showed me what might have been.

  2. Trimegistus says:

    It’s always surprising to realize that downtown Los Angeles holds one of the world’s most important paleontology sites. I had no idea the La Brea collection was so vast. Fascinating article!

    • Ed Graham says:

      When I was a kid, Bob Hope and others made jokes about the Tar Pits – - I thought I would check them out on my first LA visit in the 60s. They were not a punchline. I recommend this fascinating spot to everyone I know who is going to the west coast. I’ve been there at least 5 times.

  3. Carl says:

    Wish I’d had any teachers as good as you when I was an undergrad. (Not that all of mine were terrible, mind you.)

  4. markx says:

    A great article and wonderful outcomes for those young students.

    And I will apologize now if I am stating the obvious below:

    It seems surprising ‘stasis’ is at all a controversial topic. No doubt Darwin’s initial summation that evolution was a series of small gradual changes in response to environmental pressures is perhaps now seen as slightly flawed or simplified, but that in no way detracts from the overall concept.

    It seems to me any species living in an ‘ideal’ environment will (basically) all survive. Almost everyone gets a chance to breed, no matter their suitability to deal with extremes. Humans serve as a good example here – all over the world, barring time limited extremes here and there, every couple gets to have children, and their children get to have children. There is no selection pressure in any particular direction, so the population survives as is with a myriad of minor, largely irrelevant changes developing over time.

    Getting away from humans for a moment, amongst any population, genes are changed/damaged by the environment, by mishaps during the fertilization process, and perhaps by accidental incorporation of viral or bacterial genetic matter, and some of these ‘products’ survive. In a benign environment, as long as they still have the basic functions of the original organism there is no pressure against their survival or their breeding. A small (and probably later paleontologically undetectable) sub population forms and survives the benign conditions. When along comes a major environmental change that the species cannot cope with, in almost every case, the various tiny sub populations perish with them, and a species is lost.

    But very, very, very occasionally, a very much ‘damaged’(?) subgroup has arisen which has an adaptation suited to the new situation, and these survive the ‘bottleneck’ of intense selection pressure, and then the subsequent ‘weeding out’ of maladapted individuals in the new environment. I’d imagine that most of the time a species getting through a major environmental change with sufficient changes to thrive would be an extremely dicey process, and an equally high proportion of species losses would occur during the interim adaptation process.

    The oft argued concept that any ‘unnecessary deformity’ must always put a member of a species under negative selection pressure is wrong. The advent of motile bacteria sometimes given as an example: eg: “A subset of components would constitute a collection of superfluous parts which according to Darwinian theory should be eliminated by natural selection”.

    But, as long as conditions were optimal for that species, these individuals would survive, and breed. Not caring they has the beginnings of flagella developments protruding from their bodies. When conditions changed so that motile bacteria were advantaged, there they were, and THAT is when selection pressure got a chance to really refine the new model.

  5. markx says:

    “When conditions changed so that motile bacteria were advantaged, there they were, and THAT is when selection pressure got a chance to really refine the new model.”

    Furthermore, it needs to be said that a loss of the original species is not always necessarily part of the process.

    Sometimes it would have simply been a result of a population subset suddenly finding itself in a new extreme environment.

    For example, consider the situation a bunch of E coli were swallowed with the food of some primitive, simple gutted organism and on one occasion some of the E coli actually managed to survive the new environment and eventually thrived. Over the millenia that followed, their descendants found themselves(as the food chain lengthened) swallowed by more and more complex organisms … and some kept surviving each stage of the process.

  6. madscientist says:

    That’s very good (but tedious) work – but as far as the measurements go I’m a fundamentally lazy person and would be inclined to build machines to do the measurements. Then again there’s probably not enough funding to develop such machines.

    • Donald Prothero says:

      It’s not so simple as that. The researcher has to visually select bones that are complete and adult and unbroken–not a trivial task, and not one that a machine could learn easily. And let’s say it was some sort of optical measuring device, where you placed a bone in a scanner and got the dimensions visually. It would still take more time for the researcher to pick up and insert each bone, make sure that it was oriented properly, and return it to the tray, than it does to pick it up and make five quick measurements with a calipers (especially if you have an assistant typing in the data as you call them out).
      In short, paleontology is not a field for lazy people. This is about as easy as lab research gets–and the grueling days in the desert heat looking for fossils are far more strenuous…