Sunday, October 20, 2013

Game of Bones: Erratum

Upon sharing yesterday's post to Facebook, Dr. Thomas Holtz mentioned something important which I completely didn't realize. Triceratops and Torosaurus were not just "floating heads." This is, of course, obviously true, but represents a side of the debate unexamined in Longrich & Field (2013). In order to truly determine whether the two warrant their own respective genera can only be determined once the postcranial elements of both are examined.

Tyrannosaurus asks Torosaurus if he is, after all, a valid genus. By the great Luis V. Rey.

This is tricky because, well, we don't really have too many postcranial Torosaurus fossils. That's why the study focused entirely on the skull. If more Torosaurus bodies were known, the debate would probably never have arisen; indeed, the fact that such few bodies are recovered is part of the mystery of the animal in the first place. But, as any paleontologist can tell you, fossilization relies on very precise conditions, and even though we know a huge amount about the world of the past, there is much that we will never know.

Let's hope one that one day the debate can really come to an end. As of right now, although it seems likely that Torosaurus is not Triceratops, and while the evidence is present, the case is not closed just yet.

Wednesday, October 16, 2013

In Montana, it rains sheep and cougars

Scenes of intense predatory action are commonplace in nature documentaries. The poise, grace, and stealth of apex predators is unparalleled, and we are all captivated by such incredible feats of athleticism in the natural world.

Well, even top predators manage to botch things once in a while.

In Glacier National Park, a photographer (anyone know who?) spotted two bodies lying on a closed road. Not human bodies, thankfully, but the bodies of a predator and its prey that both met their demise in the heat of pursuit. The bodies were of a Dall sheep Ovis dalli and a cougar (mountain lion, puma, panther, what-have-you) Puma concolor. 

Lying at the bottom of a sheer cliff, the bodies not only tell of an incredible and clearly lethal fall, but of just how hard the two animals fell. The sheath of the sheep's horn came clean off upon impact (seen above, left), and its hind right leg had a severe compound fracture.

Interestingly enough, it doesn't seem like the cat was too far off from getting a nice mutton meal before the two met their untimely departure: the cougar died with a tuft of the sheep's fur in its mouth!

In the wild, slight miscalculations of distance or steepness, bad weather, and plain old bad timing can lead to terrible consequences. No animal is perfect at what it does. There is no species which kills 100% of the prey it intends to, and many risk death in the pursuit of one meal. And this doesn't just apply to today's animals; we have many instances of accidental death in the fossil record as well. Perhaps, if the conditions were right, the sheep and the cougar would form a fossil just like this...

For more photos of this scene, click here. Some of the photos are pretty gruesome, so proceed at your own discretion.

Tuesday, October 15, 2013

Game of Bones: The true identities of America's last ceratopsians

While we can learn a lot from fossils, there is still much we cannot determine just by examining petrified bones, feathers, and other structures. For one, it is difficult to determine whether or not a specimen represents an already-known species or is an entirely new one. This phenomenon occurs more often than one would think, and many extinct genera have a score of junior synonyms which were once considered to be different species. These often turn out just to be juveniles of already-known species, or display some pathology which had rendered them to appear different from the rest of their kind.

Of course, all adult animals known from fossils had to start somewhere – obviously, we have fossils of various dinosaurs ranging in age from embryonic to adult-most forms. Our knowledge of the ontogeny of dinosaurs ranges from species to species – in the case of some, such as Allosaurus and Maiasaura, we have records of complete life histories. In other species, juveniles can often be reassigned as other species or genera simply due to a lack of information on the species as a whole.

The two three-horned titans in question: Triceratops (left) and Torosaurus (right). Illustration by Nicholas Longrich.

One such ontogenetic debate originated last year concerning one of America’s fossilized sweethearts – none other than the three-horned darling that is Triceratops. Fossilized remains of Triceratops are abundant in the American west and are known from young individuals to adults. However, Triceratops wasn’t the only three-horned beast roaming North America at the end of the Cretaceous. Torosaurus,  a closely-related chasmosaurine, has been found alongside Triceratops from Colorado to Montana. However, unlike the remarkably complete life history we have of Triceratops, Torosaurusfossils are largely adult specimens.

Triceratops growth series by Gregory S. Paul. The bottom-most skull, and those
at right, represent Torosaurus. This series represents the logic of Horner et. al (2010).

The absence of young Torosaurus in the fossil record led Jack Horner et. al (2010) to believe that Torosaurus is not its own species, rather it represents the most mature specimens of Triceratops. It makes sense at first: we know animals get larger and, in some cases, more impressive as they age, and Torosaurus’ massive head and elongated frill are much more spectacular than the shorter, square frill of Triceratops. However, size and exaggeration of features do not necessarily contribute to the maturity of any given specimen, and this determination is even harder to make when all we have to work with are fossils.

When news broke that the world may lose Triceratops to cladistic lumping, a panic spread: what would we do without everyone’s favorite three-horn? How would museums cope with the innumerable info-graphics which would need to be reprinted? And the children… How would our children grow up in a world in which Triceratops was no longer a valid genus?

Thankfully, there is no need to worry. First of all, Triceratops was named in 1889, whereas Torosaurus was named in 1891, giving our herbivorous hero precedent over the long-frilled foe. Second of all, a paper has finally been published which puts this whole debate to rest. Well-known specimens of both genera were compared and analyzed, put through a gauntlet of 24 visibly-testable features which diagnose the chasmosaurines as juveniles or more mature specimens.

Testing such features as the curvature of the postorbital horns, the degree of scalloping of the parietals (bumps around the edge of the frill) and squamosals (pointed cheek bones), and the fusion of major bones of the skull allowed Nicholas Longrich and Daniel Field (2013) to put an end to this long-winded argument.  A total of 36 specimens from both genera were analyzed for the test.

Comparative ages and diagnostic features of the ontogenetic stages of Triceratops and Torosaurus. From
Longrich & Field, 2013.

When Horner cited Torosaurus as being just a mature Triceratops, he cited the fact that no juvenile specimens of the former have been discovered, whereas many young Triceratops have been discovered. Longrich and Field discovered that, while the overwhelming majority of Torosaurus specimens are, in fact, adults, at least one specimen represents a slightly younger animal, which instantly puts a hole in Horner’s logic. If young Torosaurus did exist, displaying ontogenetic characteristics diagnostic of a young animal far different from adult Triceratops, then it is impossible for the latter to be a “stepping stone” to the former. So, in your FACE, Horner! Torosaurus juveniles DO exist! Which means…

"And you read this in my voice!"

But, in all seriousness, both genera are valid. While they are both extremely similar in body-shape and lifestyle, they represent two different animals. And I’m sure Tyrannosaurus thought they tasted the same anyway.

A similar story of mistaken identity occurred between these three sympatric pachycephalosaurs. From left
to right, the specimens range from young to old.

Horner has proposed this lumping before with other species; coincidentally (or not), they all hail from about the same time and place. Besides Triceratops/Torosaurus, he is a strong proponent of Nanotyrannus representing a juvenile Tyrannosaurus, which has also recently been tested and supported. More closely related to the horned creatures in question, he has also suggested that the pachycephalosaurids Dracorex, Stygimoloch, and Pachycephalosaurus also represent three ontogenetic stages in the life of the lattermost species. I’m a bit hesitant to accept that Stygimoloch represents an intermediate between the other two, partially because I grew up with ol’ Styg as one of my favorite dinosaurs. Plus, the name is just awesome: Stygimoloch translates to “devil of the River Styx.”

I think we can all rest a bit easier tonight knowing that Longrich and Field have put an end to the attempted assassination of Triceratops as a genus. I know I can, anyway. 

Tuesday, October 1, 2013

Turn To Stone (on Lake Natron)

Lake Natron, in northern Tanzania, is an alkaline lake whose water has the pH of household ammonia. The water temperature can reach well above 100 degrees Fahrenheit, and supports only those species which are evolved enough to handle such a deadly environment. If there was ever a little slice of Hell on Earth, Lake Natron would be a contender.

A calcified African fish eagle Haliaeetus vocifer perches
above deadly Lake Natron. Photograph by Nick Brandt.

The water is so deadly to those not used to it that anything that merely touches it is calcified. Photographer Nick Brandt captured several spectacular shots of several calcified creatures who fell victim to the illusion of a crystalline, placid lake, only to emerge from the water and become preserved in stone.

A whydah Vidula sp.

When islands form on the caustic lake, they attract scores of both lesser and greater flamingos. The flamingos take advantage of the disappearing islands, constructing their mud-tower nests and breeding, all the while feeding on invertebrates in the more saline areas of the lake. However, like bathers on a shark-infested beach, even these seasonal visitors sometimes fall victim to the deadly water.

Even seasonal visitors, such as this lesser flamingo Phoenicopterus minor, still fall victim to the caustic water. Photograph by Nick Brandt.