04 December 2013

Fossil Vertebrate of the Month: Nimravus brachyops

As a paleoecologist, interactions between animals and their environments are my stock-in-trade.  For the most part, paleoecologists study these interactions by observing trends through time in the variables of interest in search of macroscopic patterns.  Every now and then, though, a fossil turns up that captures (or at least appears to capture) an interaction between an individual and some aspect of its environment (usually another animal).  The most famous examples of this are, unsurprisingly, saurian: I had the opportunity to view the amazing Mongolian "Fighting Dinosaurs" when they were on display in New York a few years back, and another pair has made headlines for all the wrong reasons in recent weeks.  However, no specimen provides as good an example of both the power and the pitfalls of "fighting fossils" as a skull in the University of Nebraska State Museum.  It is a specimen of Nimravus brachyops, a member of the eponymous Nimravidae.  Nimravids are a fascinating group of carnivores in their own right: they are mid-sized saber-toothed predators that were particularly abundant in the Oligocene and are nearly indistinguishable from cats but are probably closer relatives of civets.  This specimen, however, is of interest from more than just a scientific point of view.  It was found with a canine embedded in the humerus of another N. brachyops, showing that the two individuals had died fighting.  That, at least, was the conclusion of the field crew that first uncovered the specimen.  This crew included a young Loren Eiseley, who would go on to become one of the most prominent naturalists of the 20th Century.  Eiseley was so impressed by the specimen that it inspired him to write one of his most famous poems, 'The Innocent Assassins.'  The picture painted by the fossils and by the poem is certainly dramatic, but is it accurate?  Paleontology has long been plagued by studies that put good stories in front of the evidence of the fossil record and, unfortunately, this may be one of them.  First of all, there is no irrefutable evidence that more than one individual was present (in paleontological parlance, the Minimum Number of Individuals is 1, meaning that it is impossible to disprove that both bones came from the same animal).  Of course, no one would suggest that the nimravid bit through its own arm.  However, it is possible that the humerus and the canine were driven together after death, either during transport or, probably more likely, during burial.  As others have observed, this latter scenario is supported by the fact that the canine, while broken towards its tip, is largely intact.  One of the major paradoxes of saber-toothed predators is that elongated canines are, for the most part, exceptionally brittle and would have broken remarkably easily if too much stress were applied to them. It beggars belief that this Nimravus had canines robust enough to not only puncture bone but to remain mostly intact during the struggle that would have followed.  Perhaps the Nebraska specimen really does represent a death struggle, but the balance of probability is that taphonomy, not paleoecology, provides the explanation for the association between the two bones.  For those of you who find this depressingly banal, I hasten to add that this does not mean that nimravids never fought.  A well-known specimen from South Dakota seems to represent a Nimravus skull that has been punctured by the saber of the smaller nimravid Eusmilus, and a talk at this year's Geological Society of America Meeting suggested that such injuries might be more common than previously thought.  Nimravids may very well have been preternaturally pugnacious, but for all of Eiseley's eloquence, the true drama of the "Innocent Assassins" specimen lies not in the moment of death but in the evolutionary and ecological story into which the fossil fits.

13 November 2013

Fossil Vertebrate of the Month: Stegomastodon

Holyoke Stegomastodon Tusk
Denver Museum of Nature & Science
My dad grew up in Colorado.  Not in the Colorado of alpine valleys and ski resorts that most of you probably just envisioned, but on the plains of the eastern third of the state, in the town of Holyoke, to be exact.  Colorado is, of course, famous for its fossils, as the state tourism board, the architects of Denver International Airport, and at least one hotelier are always happy to remind you, but its most famous fossils are from the Morrison Formation (named for a suburb of Denver) of the mountainous western side of the state.  Despite sharing a border with Nebraska and its mother lode of Cenozoic fossils, eastern Colorado is a relatively blank spot on the paleontological map.  Or so I thought.  Passing through Holyoke on my way out to Iowa, I was surprised to learn that the Denver Museum of Nature & Science had spent the summer of 2011 excavating Pleistocene fossils from a gravel pit just outside of town (as reported by the local paper here, here, and here and by the DMNS here, here, here, and here).  The site yielded several specimens (including a possible dire wolf), but the unquestioned star of the show was a specimen of the Proboscidean Stegomastodon.  The specimen is currently being prepared at the DMNS, where I was pleasantly surprised to see it while attending last month's Geological Society of America meeting in Denver (the convention center where the meeting took place, incidentally, also housed an excellent series of murals depicting prehistoric landscapes from across Colorado, including 'Dunes,' a Pleistocene scene from Wray, just down the road from Holyoke).
Relevance to my family's history is not the only reason I'm featuring Stegomastodon this month.  It was among the last of the gomphotheres, one of the most prolific (though probably paraphyletic) groups of proboscideans (despite what the name might suggest, it was neither a close relative of the North American mastodon nor of the Asian Stegodon).  Elephants and their relatives are one of the great triumphs of mammal evolution, due in large part to their ability to disperse widely, and Stegomastodon represents an especially important milestone in this history: it was one of only two proboscidean genera to colonize South America during the American Biotic Interchange (the other being Cuvieronius, also a gomphothere).  Instead of being just an isolated specimen from eastern Colorado, then, the Holyoke Stegomastodon was part of the last great success story of a once diverse group of proboscideans, a story that unfolded not just on the Great Plains, but across Panama and into the Pampas of South America.

07 November 2013


In the past few months, I've moved to Iowa, started my new job at Cornell College, and leaped headlong into the deep end of the block-system-teaching pool here.  Having been caught up in these fairly major life changes, I allowed this blog to go fallow (though I did take the time to update the title and the appearance) and seriously considered shuttering it altogether.  However, I spent the last week and a half in Denver and Los Angeles attending the annual meetings of the Geological Society of America and the Society of Vertebrate Paleontology, which came at exactly the right time for me.  They reminded me that the world of paleontology is an exciting one, and as I'll be arguing in a forthcoming post, it is a world in a greater state of flux than ever before.  I can hardly claim to be the most articulate voice for paleontology out there, nor am I the type to write new posts daily, but I do think science blogs have real value (especially in an age where even scientific societies seem to prefer the vapid blurbs of Twitter).  If I can be even a moderately effective medium between paleontology and the general public, then I'll feel I've done a good job.  In the true spirit of my alma mater, then, it's time for the Mammoth Prairie to rise from the ashes of the Oregon Trail; whether or not I fulfill the Chicago motto by growing knowledge and enriching life will be for you all to judge.

04 July 2013

Fossil Vertebrate of the Month: Diceratherium

John Day Fossil Beds National Monument
This is the last FVOTM I'll be publishing in Oregon, so I thought I'd focus it on the most impressive fossil vertebrate I've found during my time in that state (I would have put the spotlight on Metasequoia, the state fossil, but as a plant it falls outside the scope of a feature on fossil vertebrates).  In the summer of 2010, I was TA-ing the UO Geology Department's field camp in eastern Oregon.  We were prospecting for fossils in the gullies of the Turtle Cove Member of the John Day Formation when I almost literally stumbled across what turned out to be a tibia of the rhinoceros Diceratherium.  One of the things I've discovered during my time here is that I have a terrible eye for fossils in the field, which made finding a rhino leg fairly exciting for me.  However, this excitement was tempered by the fact that Diceratherium was an extremely common member of the John Day ecosystem.  This may come as something of a surprise to many people, as rhinos are, of course, not members of the North American megafauna today.  However, the oldest fossil rhinos are from the Eocene of North America.  The two-horned Diceratherium first appeared in the Oligocene, a period of time during which rhinos had begun to spread across the world.  Diceratherium was an especially successful disperser, having spread into Asia and Europe by the Miocene.  By the Pliocene, rhinos had disappeared from North America, and they survive today only in the Old World.  There are a number of unusual features of rhinos, including their horns and broad teeth, but perhaps one of the strangest things about them is a product of their evolutionary history.  The earliest rhinos were small, running animals; enormous size did not evolve until later in the group's history, but even the very large rhinos of today still run frequently (charging being one of their main forms of defense and intimidation).  The stress this puts on their legs is enormous, and as a result most adult rhinos have arthritis.  Research by former UO undergrad Kelsey Stilson has shown that this trend extends far back into rhino evolution and would have occurred even in mid-sized members of the group such as Diceratherium.

01 June 2013

Fossil Vertebrate of the Month: Tylosaurus ivoensis

Tylosaurus. ivoensis and other marine vertebrates from the Karlstad Basin (Sørensen et al 2013)
Many of you may be familiar with my long-standing love of all things Swedish as well as my more recent interest in Denmark (born out of a couple of trips to Copenhagen and the discovery that I am distantly related to the Danish Vikings that invaded Ireland).  Both countries celebrate their national holidays in June (Denmark on the 5th, Sweden on the 6th) and Midsummer, a major holiday throughout the Nordic countries, occurs late in the month, so I thought I'd revel in my Scandinavophilia (Scandinaviophilia?  Scandinaviaphilia?) by highlighting one of the more spectacular fossil vertebrates to have been discovered in Skåne, the historically Danish region of southern Sweden.  During the Late Cretaceous, sea levels worldwide were extremely high, flooding low-lying areas of land, including most of Europe and what are now the Great Plains of North America.  These shallow seas were home to organisms ranging from the plankton whose shells would eventually form the chalk beds that gave the Cretaceous its name (and the White Cliffs of Dover their characteristic color) to the first marine birds to fish of all shapes and sizes to large marine reptiles.  Remains of such reptiles are especially common in the Kristianstad Basin of Skåne, and in particular along the shores of Ivösjön, one of the large lakes dotting the landscape of southern Sweden.  Reptiles recovered from the area include crocodiles, turtles, plesiosaurs, and mosasaurs, relatives of snakes and monitor lizards and the group to which Tylosaurus ivoensis belongs.  Mosasaurs are by no means unusual in the fossil record of northern Europe, but they have an especially rich fossil record in the Kristianstad Basin.  The seas of Skåne played host to a complex food web (as demonstrated by a study published just last month) and while smaller mosasaurs likely preyed upon fish and ammonites, there is direct evidence that large species such as T. ivoensis fed on other marine reptiles.  Mosasaurs, of course, are absent from the world's oceans today, and marine rocks from Scandinavia give some indication as to why.  Not all extraterrestrial impacts are associated with widespread extinctions, as the fantastically-named Mjølnir Crater of Norway shows, but an impact at the end of the Cretaceous seems to have played a major role in sealing the fate of many animals, including mosasaurs.  Early evidence for this impact also came from Scandinavia: the cliffs of Stevns Klint, a Danish site south of Copenhagen, were among the localities studied by Walter Alvarez when he first proposed his now-famous impact hypothesis.

31 May 2013

The Mammoth Prairie

Mammoths & Sabertooth Cats (Zdenek Burian)
For several years now, I have been based in Oregon, first as a grad student and then as an instructor.  This blog, originally intended as a way of staying in touch with friends in family in a pre-Facebook age and subsequently evolving into a sounding board for my thoughts on paleontology, has always been written from an Oregonian perspective (hence its name).  However, I was fortunate enough to get a postdoc at Cornell College in Iowa, where I will be moving later this year.  I have only just started getting back into blogging and it's something I want to not only continue, but to do more of.  I intend to keep this site going, and at this same URL, but as Iowa is a long way from Eugene or Roseburg and since the Oregon Trail began one state to the south of where I'll be, the current title of this blog will clearly need to be changed.  I'm currently leaning towards 'The Mammoth Prairie" as a tip of the cap to the mammoth fossils uncovered near where I'll be living (and in recognition of the fact that mammoths and mastodons are likely to become a stronger focus of my research over the next few years) as well as to the tallgrass prairie ecosystem that historically covered most of Iowa.  I also like the name because it has echoes of the 'mammoth steppe,' the name coined to describe the dry tundra environment of Arctic North America and Eurasia during the Pleistocene and a phrase that I've always found highly atmospheric.  Before I make the change, though, I thought I'd field thoughts on the name and alternative suggestions from whatever is left of my reading audience, so let me know what you think.

24 May 2013

Bellingham's Big Bird

DiatrymaUniversity of Wyoming Geological Museum
Note: I wrote this post some time ago when this was actually breaking news. It no longer is, but at Edward Davis' urging, I'm trying to get back into active blogging, and I thought I'd begin by finally posting this.  Bona fide new posts to follow.  Eventually.

The western half of the Northwest is, for the most part, a geologically young landscape, shaped by the still-growing Cascades and by sediments deposited during the Pleistocene.  The fossils found here are, for the most part, correspondingly young.  In the Puget Sound Lowlands and the Willamette Valley in particular the vertebrate fossil record is dominated by Ice Age mammals (including the Manis Mastodon).  However, there are pockets of older rocks in the region, including the Oligo-Miocene formations of the outer coast that have yielded some of the world's most important specimens of marine mammals and the marine reptile-bearing Cretaceous rocks along the Strait of Georgia.  Among the most unusual vertebrate fossils in the region are those from the Chuckanut Formation near Bellingham.  In the Eocene, the area was a low-lying floodplain in a warm climate (as indicated by the palm fronds that have been found there).  Bones of fossil vertebrates are rare in the formation, but many animals left their footprints in the then-soft sediments of the floodplain, several of which have been preserved as fossil trackways.  Trackways and other trace fossils are invaluable paleoecological tools, as they preserve direct evidence of interactions between organisms and their environment.  A recent publication out of Western Washington University describing a pair of giant bird tracks from the Chuckanut Formation is a nice case study of the use of fossil footprints in making inferences about the behavior of extinct animals.  One of the Eocene's most charismatic animals was the giant flightless bird Diatryma (possibly the same animal as the European Gastornis of BBC fame).  Diatryma bones are well-known from the Eocene beds of Wyoming, and when it was first discovered by Edward Drinker Cope in the late 19th Century, it was thought to be a carnivore and was frequently depicted as preying upon the small horses that were common in the area.  However, it has subsequently been hypothesized that Diatryma was herbivorous, possibly using its large beak to crack nuts or fruit rinds.  The Washington tracks have been tentatively assigned to Diatryma or a close relative; while assigning trace fossils to a taxon previously known from body fossils always entails some risk, but since no other large birds are known from the Eocene of North America, in this case the authors are not going out on too much of a limb.  If the tracks were indeed made by Diatryma, they provide some hints as to the animal's behavior, as they don't seem to show any evidence of the sharp talons that characterize modern predatory birds (including, significantly, the terrestrial secretary bird).  This is not, of course, the final word in the debate; it's entirely possible that if Diatryma were a predator, it relied more on its beak than its feet for hunting, and it's not outside of the realm of possibility that evidence of talons simply wasn't preserved in these tracks.  However, the footprints do provide a novel viewpoint, and it is certainly to be hoped that the Chuckanut Formation will continue to produce fossils that will help elucidate the ecology of Eocene ecosystems.