Monday, November 25, 2013

Valuable lessons from gore-kings and thieves

Oh, yes. This pic again. Lythronax by Andrey Atuchin.
The Cretaceous was undoubtedly one of the most tumultuous times in earth’s history. Life was rapidly changing: over the course of eighty million years, the world saw a wave of never-before-seen groups of life, including flowering plants (and their accompanying pollinators), snakes, and “modern” birds. By the late Cretaceous, the world would have been alien to any animal living in the Jurassic. This was a world of giants, oddballs, tyrants, and tanks. The Cretaceous, despite being the last period of the Mesozoic, was the golden era for dinosaurs worldwide.

As the Cretaceous progressed from its vaguely Jurassic beginnings to its cataclysmic end, faunal groups the world over were surprisingly homogeneous. In North America and Asia, there was a relatively consistent fauna. Generally, the families that were present in such faunal group can be linked to Asian origins. It would make sense, then, that such species, over the course of tens of millions of years, migrated from their Asian motherlands to the brave new world of western North America. With each successive generations moving farther and farther across the land bridge connecting the two continents. However, two newly-described species contest this unidirectional migration. They seem to show that our understanding of late Cretaceous faunal shifts and the evolution of the families composing these faunal groups did not take straightforward paths to reach their eventual burial sites; rather, the migration between the two continents was much more complex.

A "tyrant map" from Wiki. Click to embiggen. The abundance of
tyrannosaurids in North America, and their lack in Asia, were thought
to represent the Asian origin of Tyrannosauridae.
The first recently-described species to raise intriguing questions about dinosaur biogeography and evolution is one that has been making the media rounds lately: Lythronax agrestes, the nowfamous “goreking of the southwest.” Apart from winning Most Badass Scientific Name of the Year, Lythronax reveals interesting aspects of tyrannosaurid evolution. It is the earliest known tyrannosaurid, dating back to about 80 million years, to a time when the North American dinosaur fauna was starting to take the form it would keep until the last day of the Mesozoic. Alongside Lythronax lived some of the first centrosaurine ceratopsians, which themselves would become major ecological players in a few more million years (more on that in an upcoming post), as well as hadrosaurine hadrosaurs. What makes Lythronax special is that it dispels the idea that tyrannosaurids first evolved in Asia. The earliest tyrannosauroids, as well as several species of advanced tyrannosaurids, have been found in China and Mongolia, leading to this logical conclusion. However, it appears that tyrannosaurids may have emerged in North America, evolving their characteristic juggernaut build and binocular vision there before migrating back to Asia. And, although only one Asian ceratopsid has been described thus far, it is likely that the centrosaurines with which Lythronax shared its environment travelled the same way, migrating north in giant herds to the floodplains of Canada and Alaska before returning to Asia.

Acheroraptor by Emily Willoughby. 
Another recently-described species has actually been known for quite a while, but has only recently been given a name. Acheroraptor temertyorum is a dromaeosaurid from Hell Creek, a formation bearing rocks from the end of the Cretaceous. The fauna of this formation is unmistakable, consisting of some of the most wellknown dinosaurs of all time. Tyrannosaurus was the apex predator, stalking lowland plains and forests populated with Triceratops, Ankylosaurus, and Edmontosaurus. For many years, it was labelled as a close relative of the Canadian Dromaeosaurus, a dromaeosaurine dromaeosaurid. Dromaeosaurines were North Americanan through and through, with no other species thus far discovered outside of the continent. It would make sense to assume that Acheroraptor shares a similar story, evolving from endemic early Cretaceous dromaeosaurines. However, Acheroraptor appears to have its roots not in North America, but yet another continent: it was not a dromaeosaurine, but a velociraptorine, an almost entirely Asian group. Even at the very end of the Cretaceous, species were still migrating between the continents of the northern hemisphere.

The relationship between where a species is found and where it comes from are not as straightforward as they may seem. Evolution is an enormous tale of unexpected outcomes and unlikely beginnings, as well as the forces which dictate such results. The wealth of fossil information we have found on ancient biogeography reveals a great deal of surprising new insight into the outward reasons for their long, successful time on this earth. 

Monday, November 18, 2013

Inside the mind of the hadrosaur

Hadrosaurids have been making headlines more often than usual in the past few weeks. Starting with the discovery of hadrosaur tails in both Alberta, Canada and Coahuila, Mexico, new discoveries of the dominant herbivores of the Cretaceous are popping up quite regularly. Since the two tails were uncovered, the youngest-known specimen of Parasaurolophus sp. was discovered and affectionately nicknamed “Joe.” Joe reveals much about lambeosaurine ontogeny in a tribe of lambeosaurines which are pretty poorly understood in terms of how ontogeny and gender affect the development of their signature tube-like crests (Farke et. al, 2013).
Amurosaurus riabinini. Reconstruction by Sergey Krasovskiy. 
Perhaps the most important news relating to hadrosaurs is the recent publication of cranial endocasts of the lambeosaurine Amurosaurus riabinini of Russia. These endocasts allow paleontologists to examine the structure of the brain’s outermost regions, allowing conclusions to be drawn my correlating the size of a given region to its importance to the animal. This, subsequently, allows us to speculate further on the behavior of these animals.

It has been theorized that the volume of the dinosaurian brain took up about half of the available space within the braincase. The recovered Amurosaurus, however, reveals that its brain took up around 60% of the braincase. Being an herbivorous reptile, this does not indicate a significantly higher level of intelligence than any standard reptile; however, this larger brain does indicate that, despite being huge, stolid animals, hadrosaurs were not dull.

Not only were they not dull, but compared to other herbivorous dinosaurs, hadrosaurids had among the highest brain-to-body ratios! The Amurosaurus brain reveals that its Reptilian Encephalization Quotient (REQ), or the ratio of a reptile’s actual brain-to-body mass to its expected mass, was higher than those of ceratopsians and sauropods, but lower than those of even some of the earliest theropods. This latter discovery is not surprising, as the instincts and brainpower needed to hunt are far greater than those needed to graze or browse.

The cranial endocast of A. riabinini. Scale bar represents 2cm for A, B,
and C, and 10cm for D. From Lauters et. al, 2013.
What makes the brain of Amurosaurus particularly important, and what does this show us about hadrosaurs? The detail of the endocast reveals an interesting aspect  of hadrosaur brain physiology: the pituitary gland is enlarged, possibly explaining one part of just how hadrosaurs attained such massive sizes in a relatively short period of time. A hadrosaur could grow from a five-foot-long hatchling to a thirty-foot-long adult in just 12 years, reaching their maximum size in just half the time it took contemporary predators to reach adulthood. The astonishing growth rate of hadrosaurs is part of the reason they prevailed in the late Cretaceous; reaching sexual maturity at just three years old, hadrosaurs could produce many offspring while being too large for most predators to tackle. The larger-than-expected size of the lambeosaurine brain is consistent with the notion that hadrosaurs, in general, were animals with relatively complex social interactions (Lauters et. al, 2013). The need for intraspecies communication between herd members is essential for maintaining herd structure. An enlarged brain processes more auditory and visual signals around it, giving credence to the theory that the large and elaborate crests in lambeosaurines were used for communication as well as courtship displays.

It is a common misconception that dinosaurs were truly dumb animals. Compared to their body size, it is true, their brains are much smaller than a mammal of the same dimensions. However, this does not exclude them from having engaged in complex behaviors which are comparable to those of modern animals. We now know from cranial endocasts of serveral taxa spanning various families that dinosaurs, in general, were much more complex, active, and interesting than previously thought.

References

Sunday, November 17, 2013

What the extinction of the western black rhino should mean to you


Despite the recent increase in social media concerning the extinction of the western black rhinoceros Diceros bicornis longipes, the subspecies was actually declared extinct in 2011, and the last sighting of wild individuals all the way back in 2001. However, the lesson we can learn from the plight of the western black rhino, as well as all other subspecies and species of rhino, is an extremely important one.

The demise of the worldwide rhino population, regardless of range or species, is intrinsically tied to the use of the animal’s horns in traditional Chinese medicine (TCM). Although the TCM trade has largely diminished, with several bans restricting the import of rhinoceros products into Middle Eastern and Eastern Asian countries, poachers still manage toslaughter wild rhinos at an astonishing, not to mention growing, rate.

The situation of the rhino is the same the world over – although some species are more abundant than others, no species is truly common, and two of the six* living species are critically endangered: the Indonesian Javan rhinoceros Rhinoceros sondaicus sondaicus is limited to the last 50 or so members of its subspecies, and the Vietnamese Javan rhino R. s. annamiticus was declared extinct in 2011 as well. The next most common rhino species, the Sumatran Dicerorhinus sumatrensis, has a wild population of about 200 individuals.

*Although most resources cite five rhinoceros species (black, white, Sumatran, Javan, and Indian), the northern and southern subspecies of white rhinoceros Ceratotherium simum have been found to be two distinct species.

The sadness associated with the extinction of a species, especially ones which have such an important ecological role, as well as worldwide recognition and popular appeal, is a sadness largely associated with the regret of not saving a species which the opportunity is present. We, as a species, as doing a fair amount to save the world’s remaining rhinoceros populations, but we truly need to do more if we wish to truly save these animals. Of course, as any biologist will tell you,  there are a myriad of species out there which are much smaller or lesser known than any rhinoceros species and which require much more of our attention. But no species deserves to be loosed to the whims of extinction in a rapidly changing world.

The fate of rhinoceros populations worldwide lies in our hands.
When the first historical anthropogenic extinctions occurred, the very notion of extinction was an alien thought. The fact that an entire species could be wiped out was beyond comprehension in a world in which an almighty power had personally created each and every species. By the time scientific minds had pieced together why, for example, no one had seen a dodo in years, it was far too late to do anything about it. When concern began over the fate of the western black rhinoceros, it was likewise already too late. An aerial survey tallied just 10 remaining individuals in northern Cameroon, and the odds of those individuals finding one another and breeding the population back into health were infinitesimally small. It is frustrating to admit when something natural is beyond any help, but, with our potential and the resources available to us, this does not have to be the only option. We can still work to save the world’s rhino species, and if we are truly concerned for their well-being, there is nothing to stop us from protecting them.

The earth is a cruel place, and all species eventually go extinct. What separates us, now in the 21st century, from those Portuguese and Dutch explorers who killed and ate all that they discovered, is that we have the awareness and the power to change the destiny of another species. Now, more than ever, we need to realize this potential and be responsible for the planet we are dismantling. What happened to the western black rhino is a reason to pity, but more than anything it is a reason to take action.

To find out more about rhinoceros conservation and how we can all help, visit these pages:

John R. Platt wrote a very detailed synopsis of the history of the western black rhinoceros, including its extinction. Check it out here:

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.

Sunday, September 29, 2013

The Time of Great Weirdening

The Miocene (~23-5 Ma) was a tremendously eventful period in Earth's history. With non-avian dinosaurs long gone, the Age of Mammals was coming to its climax, with mammalian diversity reaching unprecedented levels which remain unmatched to this day. It seemed as though evolution had let go of the reigns of normality, and species, regardless of class, reached sizes unheard of or filling impossible niches. Each continent, and every sea, had its share of brand new weirdo denizens.

These bizarre new creatures didn't appear out of nowhere. Global cooling was causing a drastic change in the world's ecosystems. The land was becoming drier, the seas becoming cooler and more fertile. The Miocene marked the end of the era of primeval forests and swamps the size of continents, and slowly the post-dinosaurian world was fading into the past. Mammals were reaching the peak of their diversity. Many modern families of birds came into existence. The tremendous boas, crocodiles, and turtles were becoming more and more rare, their lineage surviving in a few sole species in the wetlands of South America.

Just as the puzzling world of the Paleogene was slipping into the past, the modern world was coming into view. Recognizable horses, rhinoceroses, elephants, antelope, whales, and many other families made their debut. The great apes were coming into fruition, and in a few tens of millions of years their descendants would becoming mankind.  However, the variety of species, even of recognizable ones, was far greater than any period of the Cenozoic, and has never been matched. In this post, I hope to highlight for you some of the most fascinating and, of course, the weirdest of the great Miocene menagerie.

Rise of the Planet of the Artiodactyls
With the disappearance of the rainforests in North America, a whole new environment was created. It may come as a surprise that, up until approximately 20Ma, grasses were rare if not completely absent from major environments. Now that large swathes of land were clear of overgrowth, vast prairies and savannahs began to form. It was on these savannahs that herbivorous mammalian diversity began to skyrocket, and artiodactyls, the even-hoofed mammals, had their first moment of glory.

For tens of millions of years, from the end-Cretaceous extinction until the rise of the grasslands, the majority of herbivorous mammals were perissodactyls, the family of mammals which includes horses, rhinoceroses, and tapirs. Their success was due, in part, to the fact that their simpler stomachs were more suited to digest nutritious plants of the understory. With rainforests so widespread, they found their niche as ground-level browsers. Slowly, artiodactyls were replacing even the largest perissodactyls; in China, the last of the indricotheres, the largest mammals to ever walk the earth, lived alongside some of the first giraffes. While perissodactyls were still commonplace on most plains, they became far outnumbered by strange artiodactyls.

A glimpse at life from the late Miocene of North America, by Jay Matternes. While rhinoceroses and horses were still common, they were outnumbered by camels, pigs, and other artiodactyls.
Grass is low in nutrients and requires a complex multi-chambered stomach to fully reap its benefits. Artiodactyls were already well-equipped - with three to four stomachs, depending on the family, they were quick to radiate onto the prairies and begin grazing. All over the world, brand new families of mammals were springing up - giraffes, true antelope, cattle, and hippopotami were widespread from mainland Europe and Africa all the way to eastern China. Camels, pigs, and pronghorns filled virtually every herbivorous niche in North America.

Some artiodactyls even became some of the most fearsome predators to roam the American plains. The entelodont Daeodon was as tall as a man at the shoulders and twice as long as a man's height, and was the veritable tyrannosaur of the Miocene. It was far larger than any other mammalian carnivore of its environment, and its power and jaws would have scared any other predator from its kill in an instant.

The terrible pig Daeodon approaching the carcass of the rhinoceros Teleoceras on the plains of North America. The artist Chavez describes Daeodon was "the T. rex of the Miocene."

In a few million years, grasslands the world over were populated by a myriad of never-before-seen grazers and browsers. Stephen Jay Gould's The Book of Life has an awesome two-page spread comparing Miocene North American and modern African mammals, and the similarity between the two faunas is incredible. However, the land wasn't the only place which was experiencing a faunal sea change.

The Endless Coast: Life in the oceans
The fertile oceans of the Miocene created an explosion in the abundance of large marine life. Not since the late Cretaceous, when seas were full of serpents, had the oceans seen such diversity and size. Cetaceans, pinnipeds, seabirds, and sharks were all at the peak of their diversity, most inhabiting the tremendous proto-Pacific Ocean. In those days, the Bering Sea had not yet opened up, and an expansive land bridge still connected Siberia to Alaska.

The physeteroid Zygophyseter, a whale not as large as sperm whales
today but just as intimidating.
All along this endless coastline, marine life flourished. The first recognizable dolphins, baleen whales, belugas, and porpoises abounded. Physeteroid whales, represented today by the sperm whale, were the first cetaceans evolved to prey upon other cetaceans. With their huge, pointed teeth, they were the orcas of their day. In recent years, the discovery of the enormous physeteroid Livyatan revealed just how large these predators could be - the size of a modern sperm whale, Livyatan (formerly Leviathan) sported both top and bottom teeth which were so large they have previously been mistaken for the tusks of mastodons. These carnivores likely specialized in feeding on smaller baleen whales, as well as smaller cetaceans and sharks. Strange rhabdosteid whales, like giant river dolphins, also patrolled the waters, feeding on small fish and cephalopods. Whales would never again be so diverse, though their prominent role in marine ecology would remain.

The four-tusked walrus Gomphotaria pugnax (top)
alongside other Miocene pinnipeds and flightless
auks on the ancient Californian coast. Reconstruction
by avancna on deviantArt.
Pinnipeds, including seals, sea lions, and walruses, were also incredibly diverse. A variety of predatory pinnipeds evolved to feed on the abundance of flesh in the water. Many evolved stranger dentition than any pinniped seen today; the walrus-relative Pelagiarctos (literally, "sea bear") sported sharp canines ideal for hunting smaller seals. Another relative, Gomphotaria, sported two pairs of tusks, blunter and shorter than the modern walrus's, ideal for rooting up and smashing shellfish. All along the proto-Pacific coast, pinnipeds of all sizes and relations were common. Some reached sizes comparable to those of modern elephant seals, making them the largest seals to ever swim.


A few coastal mammals during the Miocene were some of the strangest to ever live. From Baja California to Japan, one group of tubby mammals stood out from all the rest: the bizarre desmostylians. These mammals are closely related to both sirenians, including dugongs and manatees, as well as proboscideans such as elephants. However, their affinity with these groups, beyond the fact that there is some affinity, is largely unknown. The desmostylians were a chimera of oddities, sort of like a hippopotamus with a strunken head and giant, paddle-like feet. They were herbivorous, filling niches that sirenians would have in more tropical waters; the coldness of their environment allowed them to exploit such a niche. The desmostylians were a relatively short-lived group, evolving in the late Oligocene, just before the Miocene, and lasting to the end of the next period. They have no living relations beyond questionably related elephants and manatees.

Skeletal reconstruction and life restoration of the bizarre aquatic
mammal Paleoparadoxia. Life restoration by Roman Uchytel.

While marine life was strange throughout the world, the shores of one continent held perhaps the most bizarre and unlikely of all coastal creatures. This island continent was a land seemingly lost in time, still ruled by tremendous reptiles, towering rodents, birds that could run down and kill horses, and vultures the size of airplanes.

The Real Lost World
South America has always been a land of the strange, largely due to the fact that, up until a couple million years ago, it was still an island continent. Since the Mesozoic, it had been separated from all other continents, and thus South American fauna evolved independently of all other life on earth. The huge equatorial continent was a labyrinth of forests and swamps, and had any man set foot on such a land, they would have surely though they had stumbled into a nightmare.

A bit south of the endless coastline from southern California to southeastern Asia, the diversity and weirdness of the proto-Pacific did not stop in South America. The Peruvian coast has provided exquisite fossils representing a Galapagos-like environment chock full of seabirds, including several species of giant penguin, boobies (teehee), gannets, and saw-toothed pelicans. But beyond the myriad of bird species, one mammal ventured where none of its kind had been before: for a brief moment in time, sloths became ocean explorers.

The sea-sloth Thalassocnus. Reconstruction by Guillermo Navalon Fernandez. Alright, I'll admit, this represents the Pliocene Pisco Formation, a few million years later, but it's the best reconstruction I could find.
That's right, there were once marine ground sloths. Thalassocnus was a large sloth which evolved in a very different direction of its giant cousins of the savannahs, for instead of browsing treetops, it was an able swimmer that grazed on sea grasses. The sea-sloths lived alongside the bizarre whales of the coastal proto-Pacific, and if they had ventured far enough out at sea, they would have likely fallen prey to physeteroids and sharks, including the mega-shark Carcharocles (/Carcharodon) megalodon, which swam just beyond the South American coastline.

The giant caiman Purussaurus, which shared its environment
with several other giant crocodilians.
Northern South America at the time was largely covered in wetlands, and these huge steamy areas of water and vegetation were the ideal environment for giant reptiles. Enormous crocodilians, including the giant caiman Purussaurus, the huge gharial Gryposuchus, and the huge filter-feeding croc Mourasuchus all lived alongside one another, each reaching lengths upwards of 10m. The combination of heat, humidity, and area all contributed to the continued reign of giant reptiles. These tremendous creatures shared their environment with a variety of other strange reptiles including the stupendous turtle Stupendemys, as well as the largest rodents to ever live. Some, like Phoberomys, the "fearsome mouse," reached lengths of 3m, and would have made ideal prey for the crocodilians of the area.

Perhaps even more bizarre, and a bit frightening, is the fact that South American crocodilians at the time were not restricted to the water. Like taking a glance back to the Mesozoic, big land-crocodiles were still terrorizing the land, preying upon large mammals both in the wetlands and on dry land. Sebecosuchians, like rauisuchians from the Triassic, searched for prey on long legs which held their bodies high above the ground for easy movement on land. The fact that such dinosaurian wildlife continued to exist in South America, long after the reign of reptiles had ended across the rest of the globe (except for Australia, where enormous monitor lizards were predators par none), is an eye-opening discovery, and holds testament to just how isolated the continent was from the rest of the planet.

"Return to the Triassic," an aptly-named reconstruction of Miocene Venezuela. The sebecid Langstonia attacks the giant tapir-like mammal Granastrapotherium. Reconstruction by Zimices, from deviantArt.
The Miocene was a time of strangeness in every aspect of life. The shift in climate left behind a primordial world of unrecognizable mammals and created the brave new world of modern life on Earth. Many species of the Miocene would have been fairly recognizable to those living today, as many of the families of animals found today had their origins during this time. Yet, in the first period of "modernity," life was still extremely foreign - life had not been this big, this diverse, and this weird since the time of the dinosaurs.

Saturday, September 7, 2013

Moves like Jaguar

When big cats inhabit a certain region for long enough, they learn to specialize in whatever prey is available to them. (Duh.) It's a true testament to their variability and adaptability, and many local populations of cats pick up behaviors unseen anywhere else in their natural ranges. Some lions, for example, build Schwarzenegger-style muscles by charging through water to tackle buffalo in central Africa. Others are the typical plains-dwellers, chasing down wildebeest and zebras.

Jaguars Panthera onca are among the most adaptable of the big cats, and are powerful to boot. Ranging from South America to southern Arizona and Texas, they can be found in habitats ranging from wetlands to rainforests to savannahs and deserts. In each environment, they are the kings of their territory, able to hunt virtually any large prey in their area. In Costa Rica, jaguars have been hunting sea turtles which have come to shore to nest. They have also been known to attack the world's largest snake, the anaconda.

With powerful windpipe-crushing jaws and puncturing canines, jaguars make short work of whatever has the misfortune to wind up in their mouths. Turtle and armadillos shells and snake scales don't stand a chance. They are truly built for general predation, and will at least try to eat whatever they can manage.

Recently, wildlife photographer Justin Black captured footage of a large male jaguar known to local biologists as, awesomely enough, "Mick Jaguar." Looking at photos of Mick, you can see he's been through his fair share of life: blind in one eye and riddled with scars, the bulky cat has clearly been around the block more than a few times. In Black's series of photos, however, Mick reveals his true power as he swam across the River Cuiaba in the Brazilian Pantanal solely to hunt a basking caiman.




 
The photos, needless to say, are sick, in a holy-crap-I-wish-I-had-seen-this-in-person way. Such an encounter is not uncommon in the wild, but capturing such incredible footage is definitely rare. The eight-foot-long Yacare caiman Caiman yacare was carried off by Mick "like it was a doggie bone," according to Black.

For more photos and a full article, follow this link to Daily Mail Online.

Wednesday, August 21, 2013

The Plating Game: Probable sexual dimorphism in stegosaurs

Famed (unfairly so) for their small intellectual potential and vast body size, as well as two rows of bony plates running down the back and a tail tipped with spikes, stegosaurs are among the most easily recognizable dinosaurs out there. They co-dominated the Jurassic Period alongside sauropods as the most abundant herbivorous dinosaurs in the world, stolidly grazing on fern prairies all across Laurasia, the northern landmass.

As famous and popular as they are, we know very little about stegosaur biology. Infant stegosaurs are known from a few extremely fragmentary remains, and eggs are practically, if not totally, unknown. However, a recent discovery (thanks to CT scanning) made by Evan Saitta, a senior at Princeton, has revealed that there is a surefire way to differentiate between male and female stegosaurs: by looking at their plates.

A male-female pair of stegosaurs in repose. Another male lingers in the background. Illustration by Connor Ross.
Many dinosaurs, especially ornithischians, are instantly recognizable by their outlandish anatomical features. It's true that no one pays as much attention to a relatively "bland" little hypsilophodontian than they to do the frills of ceratopsids, the crests of lambeosaurs, or, in this case, the plates of stegosaurs. These features are sometimes identified as "nametags" within species or thermoregulatory structures; however, it is now becoming more and more clear that these bizarre appendages were probably used in sexual display. Such structures are sexually selected, a process of evolution differing from natural selection*. Those individuals with larger, brighter, or generally more impressive ________ (frills, horns, antlers, plates, sails, etc.) will pass on those impressive genes to their offspring, making them irresistible to the opposite sex.

So, how does one tell the difference between male and female stegosaurs by solely examining fossils? It turns out that one gender sported broader, more ovoid plates than the other, which carried more taller, more slender plates. Which plates belong to which gender remains to be seen, but I wouldn't be surprised if the males had the wider, larger plates than the females. When it comes to sexual selection, this is generally the case; males sport larger and flashier features than females.

While other uses for stegosaur plates, including thermoregulation, have not been ruled out, and still may be very likely, but confirming at least one suspicion about the still-very-mysterious biology of stegosaurs is one step in the right direction to uncovering just how these animals lived.

Thursday, July 18, 2013

Giant theropods will NIBBLE YOU TO DEATH

Sorry for the lack of updates for a while, everyone! I've been keeping busy with summer class and work, but now I finally have a bit of free time and will start writing again.

More often than not, the fossils of truly gigantic sauropods are found in the same formations as equally gargantuan theropods. There is no doubt that these predators fed on sauropods once in a while but, being extinct and all, it's extremely difficult to determine just how they fed. Most depictions show allosaurs and carcharodontosaurs leaping onto the backs of unsuspecting saurpods, or clamping their toothy jaws around the herbivore's neck.

When the sizes of contemporary sauropods and theropods are compared, we notice something: sauropods were friggin' HUGE. Even those theropods which are theorized to have been pack-hunters would struggle with taking down an adult super-sauropod. Of course, this doesn't mean that the carnivores were forced to starve; there were plenty of smaller, more easily-subdued critters to eat. Facing an animal which weighed as much as an entire herd of elephants could not only fail, it poses a serious risk of injury and even death to the hunters. How, then, could a predator get a taste of sauropod without risking fatal injuries?

The contemporary giants Tyrannosaurus rex and Alamosaurus sanjuanensis. You can see that, as big as T. rex was, the sauropod is tremendously bigger.

Welcome to the wonderful world of flesh-grazing!
The cookiecutter shark is a species of shark which barely reaches 2ft in length. However, its main prey includes animals which are monumentally larger than it is, including, among other things, whales, seals, dolphins, tuna, and other large sharks. How does such a tiny creature eat such big meals?

It manages to do so because it doesn't prey upon the entire animal. The cookiecutter shark rasps small plugs of skin from its prey, which not only provides it with a meal, but also leaves the rest of the animal alive for repetetive feedings. If you think about it, killing an animal which is much larger than yourself is not only dangerous, it is wasteful: if an allosaur was to take down a giant sauropod by itself, there is no way it could eat the entire animal. Even a pack would leave remains, which would be picked up by other carnivores or simply left to rot.

Even the modestly-sized (for a sauropod) Camarasaurus may have been a bit too much to handle for an adult Allosaurus.

Recent analyses of theropod skull biomechanics show that even large predators had relatively fragile skulls. In a recent post, I summarized a study of Allosaurus feeding mechanics which showed that the animal fed with rapid, jerking motions of its head like a modern falcon. A previous study, concerning Allosaurus' "hatchet" method of feeding, showed that running agape directly at the side of a giant sauropod would not only inflict minimal damage to the sauropod, it would break the jaw and possibly the neck of the Allosaurus.  The hatchet-like hunting method would be much more effective on smaller animals.

Even Tyrannosaurus displays certain features of its skull which indicate rapid, birdlike head motions. Perhaps, as these giants ran alongside titantic sauropods, they could quickly strip morsels of meat from the sides of their prey and run off unnoticed. This would not only leave the prey alive for future feedings, it would greatly reduce the possibility of risk or failed attacked by the predators.

The Case of the Leaping Acrocanthosaurus
Fossilized footprints from the carcharodontosaur Acrocanthosaurus atokensis are particularly well-known throughout Texas, where it once dwelled beside the super-sauropod Sauroposeidon proteles. One preserved trackway shows the footprints of the predator trailing a dozen or so of these sauropods, and in one segment of the track, the Acrocanthosaurus appears to have leaped onto the side of one of them. However, the tracks of the sauropod do not appear to change in speed or direction, suggesting that the predator did not hang onto the herbivore for very long, if at all.


As if the Tyrannosaurus/Alamosaurus comparison wasn't startling enough, there's a snowball's chance in Hell that a lone Acrocanthosaurus could ever take down an adult Sauroposeidon. The possible "attack" footprints are shown below the skeletals.
Perhaps this is evidence of Acrocanthosaurus not viciously attacking Sauroposeidon, but simply snagging a strip of meat from its side before parting from the herd. It is certainly much more plausible than an Acrocanthosaurus actually bringing down one of the giants, especially when the sizes of both animals are compared...

Being an apex predator means that you have the size and tenacity to eat basically every bit of flesh in your environment. None of these giant theropods would have had trouble finding something to eat. The question at hand is whether or not they were bold enough to tackle such monumental prey as the biggest of the sauropods, or whether their main quarry were smaller animals. And, while sauropods could certainly fend for themselves, it is very possible that unnoticed bits of muscle and skin could be torn off once in a while.

Monday, July 15, 2013

What do you get when you cross a turtle, a seahorse, and a beaked whale?

Turns out, you would get the newly-described turtle dubbed Ocepechelon bouyai. Hailing from the latest Cretaceous of Morocco, this turtle was far from out of place: at the time, North Africa was covered in a shallow sea that hosted a variety of strange marine reptiles, including sea snakes and mosasaurs, as well as other species of turtle. But nothing could even come close to the weirdness of Ocepechelon.

Dorsal (left) and ventral views of the holotype skull of Ocepechelon bouyai. From Bardet et. al, 2013.

All we know of the turtle thus far is its skull, but skulls are often the most revealing fossils, especially when they're as outlandish as that of Ocepechelon. Measuring 70cm long, it is clear that this turtle was a giant, definitely one of the largest to ever live. But what makes the skull particularly unique is its shape and, thus, its function: although it is very long, it tapers into a very slender and narrow mouth, the opening of which measures only 6cm in diameter.

Lateral view of the holotype skull of Ocepechelon. From Bardet et. al, 2013.

This pipe-like tube of a mouth is reminiscent of animals which aren't even slightly related to turtles, namely beaked whales*, pipefish and seahorses. All of these animals have tiny mouths when compared to the size of their heads, and all feed on tiny prey using suction as their primary hunting method. It would appear that, without any adaptations to pin down, grasp, or pierce prey items, Ocepechelon was also a suction-feeder, spending its time close to the surface of the open Cretaceous seas, sucking in jellies and cephalopods which lingered nearby.

*Also strangely similar to whales is the position of Ocepechelon's nostrils: they are located far back on the skull, as far as turtles go. This also suggests it spent more time at the surface of the water.

Ocepechelon bouyai, in the flesh. I... Uh, I don't really know... What to say. It's just a weird, weird creature.
We often think of turtles as relicts of a bygone era, unchanged by the ages, deserving of that oft-misunderstood title of "living fossils." However, over the course of their 250-million-year-old history, it is more than clear that turtles have evolved into an incredibly diverse group. Surely Ocepechelon is the tube-mouthed figurehead of turtle diversity.

References
Bardet, Nathalie, Nour-Eddine Jalil, France de Lapparent de Broin, Damien Germain, Olivier Lambert, and Mbarek Amaghzaz. 2013. “A Giant Chelonioid Turtle from the Late Cretaceous of Morocco with a Suction Feeding Apparatus Unique Among Tetrapods.” PLoS ONE 8 (7) (July 11): e63586. doi:10.1371/journal.pone.0063586.

For more on giant, weird turtles, see...

Friday, July 12, 2013

A Brief History of Caenagnathids in Film and Television


"Skinny hands and new jaws," a caenagnathid panoply. Reconstructions by Qilong, from deviantArt.

The family Caenagnathidae derives its name from the Latin "recent jaws," as the great Charles H. Sternberg (the pater familis of a family of historic paleontologists), assigned the fossilized jaw he discovered to an advanced Cretaceous bird. It's not such an uncommon phenomenon to misidentify particularly birdlike non-avian dinosaur fossils as birds; the same has happened with a variety of alvarezsaurs, the tiny short-armed anteater theropods, as well as troodontids and other families of coelurosaurs. Of course, if we were able to observe caegnathids and other coelurosaurs in the flesh, they would even act like big birds, so the confusion is totally understandable.

Anyway, the caenagnathids were a very bizarre bunch, even compared to other oviraptors. To start, they were completely toothless, a feature not found in many non-avian theropods. This indicates a varied diet - the caenagnathids were probably herbivorous, though their toothlessness allowed them to take anything, including small prey. As most species were fairly large to absolutely gigantic in size, the likelihood for herbivory is high. Cliched though it is, it's likely that the caenagnathids had a diet similar to large extant ratites such as ostriches and cassowaries. And, like the cassowary, the caenagnathids display another odd feature - a large, bony crest on the top of their heads. Like so many bizarre skeletal features found on extinct species, we're not quite sure what the crest was used for, but it likely had some function in heat dispersal, call amplification, or courtship.

On to my main point: being as bizarre and obscure as they are, it's always a treat to see caenagnathids portrayed in documentaries and [a] movie. However, out of the variety of caenagnathids which have been discovered, only two genera have been portrayed so far: the gargantuan Gigantoraptor and, in the upcoming Walking With Dinosaurs 3D, Epichirostenotes*. 

*Although the genus has not been confirmed, Epichirostenotes was a contemporary of Pachyrhinosaurus and Edmontonia, which can also be seen in the available trailers.

The Gigantoraptor of Dinosaur Revolution, complete with
wattles and glossy feathers. You go, DR!
It makes sense that Gigantoraptor was the first to be portrayed; like Tyrannosaurus, it's the biggest of its family, which, in pop culture, gives its automatic popularity. The big beast has been reconstructed in two documentaries, Dinosaur Revolution and Planet Dinosaur (both from 2011). In both, oddly, it was depicted performing elaborate courtship displays. I only say "oddly" because, while it totally makes sense that they did engage in such displays, we have no direct evidence of any sort of courtship behavior. It just makes me wonder how both production companies making the documentaries ended up having the same animals doing basically the same thing independently of each other. Anyway, I have to say that I much prefer DR's Gigantoraptor; they even gave the male this awesome wattle clearly* based on that of a Temminck's tragopan. The 'raptor from Planet Dinosaur was a li'l too reptilian for me and looked like a big ol' rubber chicken.


A pair of Gigantoraptor performing a courtship display from
Planet Dinosaur (2011). I wasn't too impressed with these guys. Meh.
*Clearly. Jeez, I sound like such a bird snob sometimes.

A couple of days ago, yet another preview for the upcoming philm phenomenon Walking With Dinosaurs 3D was released. Although a lot of it is reused footage from the first trailer, a few interesting bits popped out (including the fact that it will probably be narrated by a pachyrhinosaur... ugh.). Among these bits was a dinosaur which I, and many others, was surprised to see: a caenagnathid, presumably representing the genus Epichirostenotes of the 72-million-year-old Horseshoe Canyon Formation. I'm especially excited to see this animal on the big screen, even though the it is portrayed in the trailer biting at some young pachyrhinosaurs in the dead of night. I don't support this notion that they were nocturnal predators, but to be fair, this is the first I've ever seen this idea and it is a movie.

Epichirostenotes from the new WWD3D trailer, seen here as a creepy nocturnal baby-killer, which it probably was not.

Tuesday, July 9, 2013

True Blood (from a mammoth)

Frozen woolly mammoths Mammuthus primigenius are exciting and very revealing discoveries when they are found. They usually reveal important clues on their ecology and biology, as well as their growth cycle; we know of frozen mammoths ranging from infants to elderly individuals. Because of the subzero temperatures of their final resting place (the north of Russia, generally), they are remarkably well-preserved, with internal organs, skin, and hair all frozen in time.

A test tube of mammoth blood. Hoooooly crap. Photograph by Semyon Grigoriev.

This week, something even more startling was recovered from one of these frozen titans. Liquid blood. This isn't an internet scam or a too-good-to-be-true hoax: a small test tube of liquid blood from the sample was collected, and will be analyzed to determine just how it managed to stay liquid for 10,000 years. The mammoth, a female, was found with the lower half of her body very well preserved, though the upper half was visibly gnawed on by other animals. This indicates that she probably died in a bog or swamp, becoming stuck in deep water or mud.

What does this liquid blood imply? Could this fuel the increasingly common idea to clone extinct animals if we can? And what would we be able to do with a cloned mammoth? Only time will tell how this startling new discovery fuels the future for extinct animals.

Tuesday, June 4, 2013

Hipster Earth: Uniting nations before the UN was cool

This map, created by Massimo Pietrobon, shows the world circa 300Ma with modern political boundaries. It's amazing to think that our planet, at one point, was lumped together this way, at a time when such borders and names mean  absolutely nothing.

Plus, think of what monsters swam that gigantic ocean...


Friday, May 31, 2013

The "Lion of the Jurassic" fed like a falcon

People seem to think that giant predatory dinosaurs were absolute monsters. Of course, they were monstrous in size, with some species reaching upwards of 50ft in length. However, just because they attained these great sizes does not mean they were reckless, ruthless killers. The more we discover about the giant theropods, the more we are able to compose a lifelike and accurate picture of their biology. In recent years, technological advances have allowed us to take a look not only at the surface of fossils, but test their strength and stress under conditions which they would have faced in life.

Recently, the juggernaut theropod Allosaurus, the lion of the Jurassic, was the subject of a study on feeding mechanisms. Combining engineering and technology with a biological perspective allowed Eric Snivley and his team from Ohio University to examine the living mechanics of the predator and determine how it would have fed. They determined that, although Allosaurus was a large predator, it had a very light skull, and its muscles were not suited for vigorous shaking as much as they were for plucking and tearing at flesh.

Allosaurus and Falco. Image coutresy of WitmerLab, Ohio University.

A major find from this study was the placement of the longissimus capitis superficialis muscles on Allosaurus' neck. These muscles, Snivley explains, are comparable to "a rider pulling on the reins of a horse's bridle." If a muscle on either side contracts, then the head moves in that direction, allowing the head to shake from side to side. However, if both muscles contract, the head is pulled directly down. On Allosaurus, these muscles were located very low on the skull, and indicated that the animal drove its head into its prey, held it there, and then tore straight back and up ( This feeding strategy can be seen in modern raptors, including kestrels.

The Jurassic Paleodiet
This comparatively delicate method of feeding, along with the animal's lightweight skull, raise some interesting questions. Allosaurus is commonly portrayed hunting and feeding on the huge sauropods with which it shared its environment. It is true that Allosaurus likely hunted with hatchet-like movements of its skull, slashing through skin and muscle and bleeding out its prey; however, I believe that the possibility of Allosaurus hunting adult sauropods is becoming slimmer and slimmer with each new discovery we make.

Although it was the most abundant large carnivore of the late Jurassic (at least in North America and Europe), it probaby did not hunt the most abundant herbivores, which were the sauropods. I'm sure that allosaurs attempted to tackle juveniles frequently, and probably with some success, but the bulk of their prey likely consisted of animals which were much easier to tackle. Small ornithopods, juvenile stegosaurs, and other small herbivores, as well as small carnivores and many non-dinosaurian prey items, were likely higher of a priority to Allosaurus than the titanic sauropods.

Sunday, May 19, 2013

Mock-turtles and Reptilian Otters: The pioneering placodonts

Throughout the Mesozoic, marine life was dominated by reptiles of all shapes and sizes. Some of the most bizarre and striking species lived at the very beginning of the era, during the Triassic period. Among ridiculously long-necked protorosaurs and ichthyosaurs as large as sperm whales lived a family of very bizarre, and extremely specialized, animals known as the placodonts.

Placodonts, meaning "flat teeth," were a family of reptiles which lived across the globe and only survived during the Triassic. They were not giants like so many others; the largest species grew to 3m (10ft) long. They were not superpredators, nor were they diminuitive prey items. In fact, at first glance, there doesn't appear to be anything too exciting about them. But to see what makes placodonts so special, we need to look in their mouths.

The placodont Placodus, a bizarre chimera of marine iguana and sea otter. Reconstruction by Dan Varner.
The placodonts weren't named for nothin'. They were armed with almost stone-like, crushing teeth which were perfect for crushing shellfish. These were the first - and last, for a very long time - reptiles to specialize in eating hard-shelled invertebrates. For a long time, the origins of these very specialized reptiles was unknown. Recently, however, fossils of the most basal placodont known were discovered. This specimen, dubbed Palatodonta bleekeri, lacked the crushing teeth of its later relatives - instead, it had small, sharp, peg-like teeth, perfect for gripping much softer prey (Neenan et. al, 2013). These teeth, in comparison, are nothing special in the world of reptiles.

The basal placodontiform Palatodonta bleekeri. Reconstruction by Jaime Chirinos.
The Triassic, as discussed in a previous post, was an age of extremely rapid radiation, with reptiles of all sorts quickly evolving to fill any and all niches left open by the Permo-Triassic Extinction. In oceans filled with species adapted to feed on fish and cephalopods, the placodonts evolved a niche avoided by all other reptiles: scouring the sea floor for bivalves and other hard-shelled goodies.

The "mock-turtle" Henodus. Despite all similarities, it was a placodont, not even closely related to true turtles. Reconstruction by Jim Robbins.
In their day, the largest placodonts were too large to be preyed upon by most animals in their environment - smaller sharks and fish-eating reptiles were the only others to share the shallow seas in which they swam. However, as marine predators began to evolve, the placodonts had to adapt: many species evolved bony armor. This armor may cause some confusion, as some armored species were so well-protected that, at first, they look just like turtles, which had yet to evolve. Some species, such as the 2m (6ft) Psephoderma, took this armor to even greater lengths, with articulated "suits" of armor which allowed for greater mobility in the water.

Another mock-turtle, Psephoderma, and a nothosaurid. Illustration by Kahless28 on deviantArt.
The placodonts were true pioneers in the world of reptiles. They were the first reptiles to evolve a bony shell, and some of the first animals to exploit a niche which would later be filled by animals such as sea otters. Unfortunately, like so many unique species of the Triassic, they disappeared at the end of the period, paving the way for shellfish-eating sharks, turtles, and a myriad of other sea-going oddities.

References
James M. Neenan, Nicole Klein, Torsten M. Scheyer. 2013. European origin of placodont marine

reptiles and the evolution of crushing dentition in Placodontia. Nature Communications. March 27, 2013. doi:10.1038/ncomms2633