Fossil Vertebrate of the Month: Shonisaurus popularis

Earlier this month, the Society of Vertebrate Paleontology held its annual meeting in Las Vegas.  The conference logo - one of the best I've ever seen for an SVP meeting - featured the ichthyosaur Shonisaurus, Nevada's state fossil (shown in the picture at left in the new Nevada State Museum, Las Vegas).  Shonisaurus was a remarkable animal.  Dating from the Late Triassic, it was not only one of the earliest ichthyosaurs, but at 15 meters in length was also among the biggest (the largest known ichthyosaur was the Triassic Shastasaurus, which has been reported from Oregon, British Columbia, and Northern California, among other places).  Shonisaurus skeletons have been found in large numbers - and in remarkably good condition - in Berlin-Ichthyosaur State Park southeast of Reno, making it among the best-known early ichthyosaurs.  The concentration of skeletons at the locality has been interpreted several different ways through the years.  It was originally thought to represent a stranding site, but the lithology and paleontology of the site indicate a deep water environment.  It has also been interpreted as evidence of an ichthyosaur breeding ground, though the lack of juvenile specimens contradicts this hypothesis.  The generally accepted explanation for the bone bed is that it represents an area of upwelling that would have brought nutrients up from the deep sea, supporting a diverse ecosystem in which Shonisaurus would have been the top predator.  Unfortunately, Shonisaurus has also been the subject of an exceptionally high-profile and exceptionally shoddy study this year that used the arrangement of skeletons as "evidence" of an exceptionally intelligent cephalopod; the readiness with which a talk on the subject was accepted by the Geological Society of America and the eagerness with which it was reported by the media are black eyes for paleontology and for scientific journalism.

The Manis Mastodon

While growing up in Seattle, I often lamented the lack of dinosaurs from the Pacific Northwest, but I always took some solace in the fact that we had some pretty cool mammal fossils.  I was a regular visitor to the Burke Museum to see the Blue Lake Rhino and the Sea-Tac Sloth, and my family indulged me enough to take me on trips to Ginkgo State Park, Republic, and the John Day Fossil Beds.  One of my particular favorite Northwest fossils was the Manis Mastodon, found near the town of Sequim, on the Olympic Peninsula, because, as a proboscidean, it was big, and therefore akin to the dinosaurs I so desperately wanted to study.  Now, of course, things have come full circle, and I'm living in the Northwest again and studying mammal paleontology.  I have a new appreciation for all the fossils I visited as a child (the John Day fauna has, in fact, become a huge part of my research), and it turns out that one of my old favorites was even more important than I realized.  The Manis Mastodon wasn't just big: it turns out that it's one of the only specimens in North America that preserves evidence of humans butchering a mastodon.  It had long been suspected (at least by some) that a bone point embedded in one of the mastodon's ribs was a broken-off projectile point, which would imply that humans not only scavenged mastodon carcasses, but might have actively hunted them as well.  This hypothesis was recently put to the test by a group of researchers that includes WSU's Carl Gustafson, the scientist that conducted the initial study of the site.  Scans of the rib confirm this hypothesis, but perhaps the most exciting finding of the study was that the Manis site was far older than had been expected: about 13,800 years old.  This revelation has two major implications.  First, it supports the evidence of the so-called "Kelp Highway" hypothesis (the main research focus of Oregon's own Jon Erlandson) that humans populated the Americas by travelling south along the West Coast.  Second, it suggests that humans were hunting large animals prior to the development of stone Clovis points, which may itself have implications for the extinction of the North American megafauna.  The moral of this story?  Never let anyone (even a younger version of me) tell you that there aren't any interesting fossils in the Northwest; as long as our region continues to yield finds like the Manis Mastodon, there will be plenty to keep paleontologists here busy for a long time.

Addendum: Adding to the Manis Mastodon's Northwest cred, Knute Berger, my favorite Seattle journalist has supplied a brief article on the subject.

Fossil Vertebrate of the Month - Terror Bird

This is the largest group of organisms I've ever featured as a FVOTM, but given that we're coming up on Halloween, it seemed appropriate to spotlight a family whose common name is based on how terrifying they were.  Terror birds (or, more correctly, phorusrhacids), represented by the LA County Museum's mount of Paraphysornis in the picture at left, were a group of giant, flightless birds related to living seriemas, most of which have been uncovered in South America.  Flightless birds are not unusual, as anyone who's seen an ostrich, emu, or rhea (or fossils of elephant birds, moas, or mihirungs) can attest.  However, phorusrhacids were different in one key respect: they were carnivorous.  Carnivory has been suggested for some other land birds - chief among them the Eocene Gastornis, itself a possible terror bird ancestor - but the huge size, robust build, and raptor-like beaks of phorusrhacids leave no doubt.  In fact, the near absence of large mammalian carnivores in South America for most of the Cenozoic indicates that the top predator niche on that continent was occupied by terror birds (they would have preyed upon one of the strangest herbivore faunas in the world, composed of, among other things, meridiungulates, xenarthrans, and - somewhat inexplicably - platyrrhine primates and hystricomorph rodents).  Phorusrhacids were key players in the American Biotic Interchange; once thought to have gone extinct when mammalian carnivores (including the iconic Smilodon) moved in from the north, it is now known that terror birds actually expanded onto the Gulf Coastal Plain in North America, where they were represented by Titanis, one of the largest birds ever to have lived (though it was not the largest phorusrhacid - that honor is currently bestowed on the recently-described Kelenken from Argentina).

Fossil Vertebrate of the Month: Megalonyx jeffersoni

This month's (somewhat belated) fossil vertebrate is a long-time favorite of mine: Megalonyx jeffersoni, a Pleistocene ground sloth.  The reasons for it being one of my favorites are prosaic enough: there was a skeleton of one in Seattle's Burke Museum while I was growing up (a specimen that was discovered during the construction of Sea-Tac Airport, which I always felt would make it a good candidate for Washington State Fossil, an honor that's since been bestowed on the Columbian mammoth).  Ground sloths are one of the great evolutionary success stories to come out of South America, having been among the first animals from the formerly island continent to expand into North America after the formation of the Isthmus of Panama about 3 million years ago (ground sloths actually seem to have made the jump to North America well before the isthmus was fully formed, suggesting that they, like modern sloths, were very capable swimmers).  Ground sloths thrived in North America until the Pleistocene megafaunal extinction, around 13,000 years ago, that also sounded the death knell for mammoths, horses, camels, and many other types of mammal on the continent.  Megalonyx has the distinction of being the only fossil vertebrate to have been described by a President of the United States: Thomas Jefferson, who described a specimen from a cave in West Virginia as a kind of lion.  The great anatomist Caspar Wistar subsequently reidentified it as a sloth, named the species after the then ex-president, and is thought to have suggested to Meriwether Lewis that he keep a weather eye open for living megafauna, such as Megalonyx, during his expedition west with William Clark.

Orcutt & Hopkins, 2011

It's been a long time coming, but as of today, my first paper is officially published.  It's in this month's Journal of Vertebrate Paleontology and has the thrilling title 'The canid fauna of the Juntura Formation (Late Clarendonian), Oregon.'  It's far from groundbreaking work, as most of it is a redescription of misidentified specimens, but it does have its noteworthy elements.  It includes the description of a jaw of the giant dog Epicyon saevus found during our lab's field work in 2008.  It provides information on the postcrania of the even more giant E. haydeni and the much smaller (but previously unknown from the Northwest) Carpocyon.  Perhaps most importantly, it's the first publication to come out of the Hopkins Lab's Juntura Project.  The Juntura Basin east of Burns in southeast Oregon was the research focus of the pioneering paleoecologist J. Arnold Shotwell (also of the U of O) until the 1970s, but has been largely neglected since Shotwell left the field.  Our lab's field work in the area has been the first concerted research project there in nearly forty years, and if nothing else my paper stands as the first fruits of what will hopefully (and presumably) be a very fruitful paleontological endeavor.

Fossil Vertebrate of the Month: Archaeotherium

It's springtime, which means its time for class field trips, which to a paleontologist in Oregon can only mean it's time to head to the John Day Basin.  The fossil beds of the John Day country are some of the best continuous exposures of Oligo-Miocene sediments in the world, and have yielded everything from tiny "worm-lizards" to gomphotheres.  One of my favorite animals from the area, though, is the entelodont Archaeotherium from the Late Oligocene Turtle Cove Member of the John Day Formation (in the picture at left, Archaeotherium can be seen in the foreground, while the background is Sheep Rock, the most spectacular of the Turtle Cove outcrops).  Entelodonts have been popularly termed 'terminator pigs' or 'hell pigs,' and with good reason.  Opinions are split on whether or not entelodonts were particularly closely related to pigs (they may have been closer relatives of hippos), but they certainly would have been hellish things to encounter.  Their large, flat teeth are similar to those of bears, pigs, and humans (though on a much larger scale than the latter two), and like all these animals they were almost certainly omnivorous, making them some of the only artiodactyls to include meat as a major part of their diet.  The skulls of entelodonts, including Archaeotherium, are generally long and characterized by strange protuberances at the back of the jaw that may have served as anchors for muscles or, perhaps more likely, may have played a role in display or competition for mates.  Archaeotherium was a mid-sized entelodont, but members of the family could grow to huge sizes: the giant Daeodon (once known by the fantastic name Dinohyus, or 'Terrible Pig') grew to the size of a rhinoceros.

Fun With Body Mass Estimates

The latest issue of the Journal of Vertebrate Paleontology has been a minor media sensation because it includes a description of the largest known rabbit, Nuralagus rex from the Pliocene of Menorca (as neat an illustration of the Island Rule as ever there was).  For those of us interested in carnivores, though, the issue also included the description of, as the authors put it, 'a gigantic otter.'  The otter is from the Pliocene Awash region of Ethiopia - site of many a famous hominid discovery - and is a new species in the genus Enhydriodon: E. dikikae.  Giant animals are always fun to read about, and they're even more so to someone writing a dissertation on mammal body size evolution.  The paper includes dental measurements for the new species, including the dimensions of the first lower molar (the carnassial, for those that know your carnivore teeth), which is tightly correlated with body size.  I couldn't resist plugging the E. dikikae measurements into the body mass regression for mustelids developed by Blaire Van Valkenburgh to get some kind of idea of the size of the animal.  There are two specimens included in the paper: the smaller, more complete tooth suggests a 77 kg animal, while the larger, incomplete tooth may represent an animal of 126 kg.  The former mass would put the individual in the same size range as modern giant otter (Pteroneura brasiliensis), but the larger individual would be in a class by itself; a 126 kg otter would be roughly the same size as a large jaguar or a small lion.  Of course, as with any body mass estimate for exceptionally large (or small) animals, these numbers should be taken with a grain of salt: the very fact that E. dikikae is so big means that it falls well outside the range of masses in living mustelids (with the not insignificant exception of the giant otter) and that any estimate of its weight is therefore extrapolating beyond the available data (the same issue has been raised for mass estimates of giant South American rodents).  Still, it's fossils like this that remind me why I consider myself lucky to be a paleobiologist; after all, what other field would publish journals describing giant rabbits and otters in the same issue?  Life, and in particular prehistoric life, is just so cool.

Fossil Vertebrate of the Month: Baryonyx walkeri

Last month's fossil vertebrate was an Irish icon, so in honor of April's St. George's Day, it only seems fair to put the spotlight on an English animal.  The earliest dinosaurs to ever be described were English, but for the most part the end of the Victorian Era was also the end of new dinosaur discoveries in Great Britain.  A few new taxa have come to light in the previous few decades, though, perhaps the most impressive of which is the bizarre Baryonyx walkeri.  Uncovered in Surrey in 1983 and named in 1986, the genus name translates as 'heavy claw,' a reference to the large, recurved claws on the animal's hands.  The other noteworthy feature of Baryonyx is its elongated skull with a kinked jaw, remarkably similar to that of a crocodile.  This skull, in conjunction with fish scales found within the specimen's body cavity, led to the conclusion (since challenged by some, but still largely accepted) that Baryonyx was piscivorous.  It's unusual morphology meant that the relationship of Baryonyx to other dinosaurs remained a mystery for some years, though the discovery of similar dinosaurs have shown that it was a member of the Spinosauridae, a group of large (in at least one case very large), sometimes sail-backed, likely fish-eating group of theropods that lived mainly on the southern continents during the Cretaceous.  Baryonyx was very closely related to the much-publicized African spinosaur Suchomimus tenerensis; in fact, the two were likely members of the same genus.

José María Velasco

Serendipity can be a wonderful thing.  While doing research in Mexico City last September, I spent a day in the historic center of the city, and one of the places I visited was the Antiguo Colegio de San Ildefonso; tourists like me flock there because it was the birthplace of the Mexican muralist movement, but while I was there it was also hosting an exhibit celebrating the centennial of the Universidad Nacional Autónoma de México.  You can imagine how pleasantly surprised I was to round a corner in the geology section of the exhibit to see these:

The paintings above, as well as a third of cave bears that I couldn't find an imagine for online, are by the artist José María Velasco, who I have to admit I'd never heard of before my trip.  He lived and worked in the late 19th and early 20th Centuries and is best remembered for his landscapes of the Valley of Mexico, which have served as a touchstone of Mexican national identity.  He was also a scientist, with a particular interest in natural history (a running theme in his profession, it so transpires, as Mexico's greatest landscape artist, Dr. Atl, was also an amateur volcanologist and advocate for science); he even described a species of salamander, that has since been renamed in his honor.  This may explain why he was commissioned to decorate UNAM's Instituto de Geologia.  Velasco's paintings have adorned the palatial building (itself as glorious an example of early 20th Century museum architecture and design as you'll find anywhere in the world) near central Mexico City since the 1910s, and had been brought over to the UNAM exhibit during some renovations (you can get a sense of how they look in situ in this picture).  Information on the paintings is scarce, but it appears that Velasco painted two series: one tracing the history of marine life and one depicting terrestrial animals and landscapes through time.  These would have been painted at roughly the same time as some of the greatest works of Charles R. Knight and his European counterpart, Heinrich Harder, and I would argue that not only are Velasco's reconstructions in the same league as those of his more famous contemporaries (though it must be said that no one before or since can compete with the vibrancy of Knight's animals), but he in fact surpasses them in many ways; his paleo-landscapes are especially impressive (though sadly underrepresented online).  This should come as no surprise, as Velasco was, after all, a classically trained painter and one of his country's greatest artists of the pre-modern era.  It seems a shame that his contributions to scientific illustration and paleoart should have lapsed into obscurity, and I thought I'd do my humble best to try to share some of those contributions with the world.

Brontomerus, Hell Creek, and Mesozoic Ecology

Two dinosaur-related stories have been getting a lot of press this month.  The first is the naming of the new sauropod Brontomerus mcintoshi (Taylor et al. 2011), which is remarkable for its name (literally 'McIntosh's Thunder Thighs'), its oddly large legs, and its implications for Early Cretaceous sauropod diversity.  The story that is more intriguing to me, though, is the publication of the results of the Hell Creek Project dinosaur census (Horner et al. 2011).  For those who aren't familiar with it, the Hell Creek Formation of eastern Montana and adjacent states has produced one of the richest assemblages of Late Cretaceous vertebrates in the world.  The fauna has been extensively sampled and studied, thanks in large part to the efforts of Jack Horner at MSU's Museum of the Rockies.  The completeness of the Hell Creek fossil record makes it an appealing subject for paleoecological analysis, which is the focus of Horner's new paper.  The authors draw two major conclusions: that the bulk of the large-bodied dinosaurs from Hell Creek represent individuals of intermediate age, while juveniles and old adults are rare, and that Tyrannosaurus was so common that it must have been more ecologically analogous to scavenging, opportunistic hyenas rather than predatory big cats (which require huge amounts of food and are therefore almost always much less common than their prey).  The first point should perhaps not be surprising; as is observed in the paper, there are compelling ecological and taphonomic reasons why very young individuals should not be found at Hell Creek, and it is likewise to be expected that most dinosaurs probably did not survive to extreme old age.  The argument that Tyrannosaurus could not have been an active hunter - the part of the research, incidentally, that has attracted the most media attention - is somewhat more problematic.  Certainly, a modern mammalian predator would not be as abundant as Tyrannosaurus was in the Hell Creek fauna, but using mammals as analogs for dinosaurs has its drawbacks.  Dinosaurs were biologically distinct from mammals (no mammal, for example, could grow to the sizes of sauropods without outstripping their food supply) and the Mesozoic world was fundamentally different from that of today, and as such dinosaurs played by a different set of rules than does anything currently living (including the dinosaurs' descendants, the birds).  Because of this, patterns such as predator/prey ratios that can be very informative when discussing community structure in Cenozoic ecosystems may mean something very different in the Mesozoic, and the preponderance of Tyrannosaurus may be due to biological factors such as metabolism or social structure or to taphonomic biases.  This post may sound like a criticism of Horner et al., but that is not its intent.  In fact, I think the finding that Tyrannosaurus was aberrantly common in the Hell Creek fauna is extremely interesting and certainly the authors' interpretation may be correct.  Further, Hell Creek is one of the only Mesozoic ecosystems that lends itself to fairly robust ecological analysis, and it's excellent that work along those lines is being conducted.  However, at the end of the day, there's a reason dinosaurs are so popular: they are utterly foreign to modern eyes.  This is something of a double-edged sword, because it does make dinosaurs fascinating animals, but it also means they have no good modern analog and that any reconstruction of their ecology will always be cursed with a lower degree of confidence than studies of animals such as reptiles, birds, and, of course, mammals.

Fossil Vertebrate of the Month: Megaloceros giganteus

As my readers are no doubt aware, St. Patrick's Day is this month, and in honor of that March's fossil vertebrate is the extinct animal most strongly associated with Ireland: Megaloceros, the Irish elk.  It's common name, as famously observed by Stephen J. Gould, is a double misnomer, as Megaloceros was not exclusively Irish (it's remains have been found across Eurasia) and while it is a cervid (the largest ever known, in fact), it is not particularly closely related to elk.  However, the earliest specimens to be described were uncovered from Irish bogs, which still yield some of the most impressive Megaloceros fossils.  Because of this, the Irish elk remains something of a national symbol of Ireland, with its remains adorning museums, universities (such as the pair at left from Dublin's Trinity College), and castles alike.  The outsized antlers of Megaloceros males have, unsurprisingly, been the focus of a great deal of research.  Whether they were the product of sexual selection, allometric growth, or some combination of the two has been an area of debate, as has been their role in the animals' extinction.  A long-standing (but somewhat fanciful) hypothesis held that Irish elk went extinct when forests overtook the more open habitats to which they were adapted and that their large bodies and antlers made life in a closed environment impossible.  A more likely culprit is changing climate that ushered in flora that were nutritionally insufficient to support healthy populations of large, antlered animals such as Megaloceros.

Oregon Trail Word Cloud

What's this blog all about?  This word cloud from Wordle pretty succinctly sums up my major themes from the last several months.  Apparently I like ecology a lot, but not as much as the number one.

Humboldt, Bergmann, and Haeckel: The German Roots of Ecology

One of the axioms of science is that any report on your research should include a thorough overview of the topic it addresses. This often means that introductions to scientific papers include some very old citations (for some authors, trying to find the oldest publication you can legitimately cite has become a game, and a pretty fun one at that). I'm in the process of writing the first chapter of my dissertation on the influence - or lack thereof - of climate on body size evolution in mammals, which turns out to be a very long-standing area of study. In working back to the roots of the debate, I've found myself returning to three papers from the early to middle 19th Century, one of which has a direct bearing on my research, another that is a little more tenuously connected, and a third that is only indirectly related but has a profound impact on everything I study. These papers were written by three very different scientists who were studying very different groups of organisms, but all three papers share one major commonality and, as I hope to convince you, several smaller ones as well.

The most recent of these publications is Haeckel (1866): Generelle Morphologie der Organismen by Ernst Heinrich Philipp August Haeckel. Haeckel is remembered today for many things; some of these are positive (he was continental Europe's most eloquent and effective supporter of Darwin and one of the most accomplished scientific illustrators of all time) and some of them very, very negative (he used evolution and his studies on development to justify scientific racism), but he makes an appearance here because it was in his 1866 book that he introduced the word 'ecology' to the world. Haeckel defined his newly-minted word (which roughly translates as 'house study' in Greek) as the study of the environments of organisms. In modern popular culture, environment is often taken to mean the group of abiotic factors - variables such as climate, geography, and geology - that influence an organism, but to ecologists, this is only half the story; organisms also interact with a biotic environment shaped by factors such as predation, competition, and productivity. Ecology, then, is the study of how biotic and abiotic variables influence organisms or, more simply put, the study of why organisms evolve (as opposed to how life has evolved and is evolving, the province of evolutionary biology, though of course there is a huge overlap between the two fields). Because he coined the term, Haeckel is often thought of as the father of ecology, but in fact he would have had no field to provide a name for had it not been for the work of earlier scientists studying the influence of environment on evolution.

One of the most influential of these proto-ecologists was Karl Georg Lucas Christian Bergmann, whose work forms the backbone of my dissertation and of countless other research projects over the course of the last century and a half. Very little biographical information is available for Bergmann: the salient points are that he was born in 1814, attended the University of Göttingen, taught at both his alma mater and at Rostock, and died in 1865, one year before Haeckel wrote his landmark book. While at Göttingen in 1847, Bergmann published the paper for which he is best remembered today: Über die Verhältnisse der Wärmeökomie der Thiere zu ihrer Grösse. The title is a bit of a tongue twister for non-German speakers, but the concept is straightforward enough. Bergmann observed that species of mammals (not individuals within species, as is commonly thought) that lived near the poles tended to be larger than those living towards the equator. Bergmann's explanation for this was that large mammals have small surface area to volume ratios and can therefore retain heat more easily while, conversely, small mammals can shed heat more effectively. Naturally, the poles are colder than temperate regions which are in turn colder than the tropics, and therefore as you head from the former to the latter, you should expect to see a decrease in body size. Bergmann's rule, as this hypothesis has come to be known, has been put to the test several times; his relatively simple explanation has been both supported and attacked by ecologists through the years, but regardless of what you think of his rule, it can't be denied that Bergmann was a pioneering ecologist. By assigning a physical cause to a biotic pattern - and eleven years before Darwin and Wallace introduced the world to natural selection, no less - Bergmann set the tone for generation of ecologists to follow, and as such he deserves to be remembered as a father of the field.

If Bergmann is one of the fathers of ecology, then surely its grandfather was Friedrich Wilhelm Heinrich Alexander Freiherr von Humboldt (1769-1859). As opposed to the relatively obscure Bergmann and the politically distasteful Haeckel, Humboldt is one of the best-known and most beloved figures in the history of science; his Latin American travels and research garnered praise from such luminaries as Edgar Allen Poe, Simon Bolivar, and Thomas Jefferson (himself a scientist of no little reputation) and would inspire Darwin's voyage on the Beagle. While it was his travel narrative that would establish his fame, Humboldt also laid the cornerstone of ecology when, along with his traveling companion Aimee Bonpland, he published Essai sur la géographie des plantes in 1805.  During his sojourn in South America, Humboldt had climbed the volcanic peak of Chimborazo in the Ecuadorian Andes (though he failed to reach the summit) and was struck by the distinct zones of vegetation he encountered during his ascent.  His notes, coupled with observations of similar patterns on European mountains, gave Humboldt the data necessary for his 1805 paper as well as the large-scale figure that accompanied it (itself a milestone of scientific illustration).  By tying vegetation to factors such as temperature, air pressure, and soil type Humboldt became the first scientist to seriously study the influence of the environment on organisms, earning his reputation as a pioneering ecologist as well as countless citations in manuscripts (including mine) over the course of the subsequent two centuries.
The authors of these three papers no doubt had many things in common, but perhaps the most striking is that they shared a country of origin.  At first glance, it seems illogical that ecology should have been born in Germany (which, after all, was only a collection of smaller kingdoms and principalities until 1871).  Germany has always produced great scientists, from Leibniz to Einstein, but in Humboldt's time Paris was the center of the scientific world, and many of the most celebrated accomplishments of 19th Century science took place not on the Continent but across the English Channel.  Even within biology, France and Britain played a dominant role, producing some of the greatest anatomists and, later, evolutionary biologists that have ever lived.  Nonetheless, ecology was, at its root, a uniquely German phenomenon.  This begs a rather obvious question: why?  While I'm a far better paleontologist than historian, I think think I have the glimmer of an answer, but in the interest of keeping unassailable historical facts apart from more baseless arm-waving, I'll save my thoughts for a later post.  Stay tuned...

Fossil Vertebrate of the Month: Megatherium americanum

FVOTM is back from its extended holiday vacation, and because February 12th is Darwin Day, this month's vertebrate is an animal that played a crucial role in the development of evolutionary theory: the giant ground sloth Megatherium americanum.  The species would have been familiar to Darwin before he ever departed on the Beagle: it had been named in 1796 by no less a figure than Baron Georges Cuvier and its size (comparable to that of a modern elephant) and bizarre combination of traits (such as teeth without enamel and claws that the animal evidently walked on) had made it immensely popular.  Darwin himself uncovered fossils of Megatherium - as well as the hippo-like ungulate Toxodon - at Bahia Blanca, south of Buenos Aires.  While it is impossible to pinpoint exactly where or when Darwin first began to understand the patterns that he would later use to support natural selection, his recognition that Megatherium shared many features - including its apparently aberrant teeth and claws - with modern tree sloths certainly represents a milestone, as the great scientist would himself acknowledge in the opening lines of his epochal On the Origin of Species: "When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent.  These facts seemed to me to throw some light on the origin of species - that mystery of mysteries as it has been called by one of our greatest philosophers."