Friday, September 18, 2015

Dino Tootsies and the Reverend Hitchcock

Dinosaurs and birds share many things in common, and one of the most important characteristics that links the two is their feet.  Looking at the foot of a chicken and a tyrannosaur, you can see a number of fascinating similarities.  In both animals, you can see that the toe bones, the metatarsal bones, form a surface with which the tibia, astragalus, and calcaneum. This is one of the most important defining characteristics of the dinosaurs, and a fascinating link with modern birds! 
Above is a leg of a Gorgosaurus, a type of tyrannosaur that lived about ten million years prior to the famed Tyrant Lizard King. You can see the flat section at the top (or proximal) end of the fused metatarsals, and you can just see the bottom of the astragalus and calcaneum at the top of the photo. This leg is currently on display at the Denver Gem and Mineral Show, and can be rented or purchased from The Dinosaur Brokers. 

Gorgosaurus, ballet dancers, cats, penguins, and hobbits all share one thing in common: they spend a lot of time on their toes. When you walk on the balls of your feet, you are walking like a dinosaur, cat, dog, or deer. For all of these animals, the metatarsals rarely, if ever, touch the ground, and instead help to make the leg just a bit longer. Ever wonder why a flamingo's knee bends backwards? It actually doesn't, they just have an extremely long ankle!
Here's how I would walk if I was a dinosaur, dog, cat, bird, or deer....
....and here's how I would walk if I was a human, bear, or raccoon. 

During the 1800s, a man named Alfred Hitchcock was able to glean fantastic insight into dinosaur anatomy simply by studying their fossil footprints in the Connecticut River Valley. Reverend Hitchcock determined that these animals looked like modern birds, walking on their toes, and their tails held well off the ground. Some critics of Hitchcock point out that the good Reverend never actually said that his bird-like track makers were dinosaurs. At the time, however, the best scientific minds in Europe believed dinosaurs were colossal, lumbering quadrupedal behemoths, with sprawling legs, a crocodile-like gait, and mammal-like paws. No one in their right mind would have put the two together, and Hitchcock was clearly blessed with a brilliant, highly analytical mind. 
The foot of the primitive, Late Triassic dinosaur Herrerasaurus, also from The Dinosaur Brokers. Notice that, unlike the Gorgosaurus (who would live nearly 160 million years later), the metatarsal bones have undergone fusion to a much lesser degree. 

Friday, June 26, 2015

Prominent Cheekbones: Abe Lincoln, Peter Cushing, and Archaeotherium

Today at the Morrison Natural History Museum, I ended up talking with Dr. Robert Bakker about the development of pronounced cheekbones in various groups of animals.  The flared cheekbones of the animals presented in this post would have almost certainly been used primarily for display, both to attract a mate and to appear larger against rivals and predators (i.e. a bull elephant flapping its ears, or your cat arching her back and raising her hair to appear larger and more intimidating).  What's interesting is that, unlike some display structures (think the plates on the back of stegosaurs and the fin on the back of Spinosaurus and Dimetrodon), most of these cheekbone structures are derived from the same bone, the jugal (or as we refer to it in mammals, the zygomatic).  Dr. Bob suggested that the cheeks might consistently evolve for display due to the fact that they are so close to the eye, and eyes can be pretty dang important for behavioral interactions.  Given the frequency with which the flared cheekbones has evolved, and the enormous disparity between the animals who have evolved it, there has to be some explanation!  Here's a look at some animals with those prominent cheekbones, arranged in order from oldest (geologically speaking) to youngest.  And a special shoutout to Dr. Bob for helping me out with this post, and for letting me use his images of Hypsognathus (apparently pronounced with a silent "g"), and Archaeotherium!
Bradysaurus, a large pareiasaur reptile from the Middle Permian Period of South Africa.  You can see both the flaring cheekbones, as well as two smaller bumps pointing downwards from the jaw, on this specimen.  I got to see this guy six days ago at the Field Museum of Natural History (FMNH) in Chicago, Illinois!  As requested by the Field Museum, this photo is Attribution Non-Commercial Share-Alike (CC BY-NC-SA 2.5).
Scutosaurus karpinskii, another pareiasaur, but from the Late Permian Period of Russia.  Although the skull looks slightly crushed, you can still easily see the massive, sharply pointed cheekbones of the animal.  Specimen on display at the American Museum of Natural History (AMNH) in New York
An illustration of Hypsognathus, a procolophonid reptile from New Jersey.  This animal lived during the Late Triassic Period, at the same time as the earliest dinosaurs.  Check out those crazy massive cheekbones!  Thanks again, Dr. Bob, for letting me use this photo.  Photo Credit and Copyright: Dr. Robert T. Bakker
A skull of the small, primitive ceratopsian dinosaur Psittacosaurus, from the Early Cretaceous of Asia.  I originally took this picture to compare the braincase of different ceratopsian dinosaurs (note the interesting ball-and-socket joint where the head articulates to the neck), which is why it's from behind.  But it does a nice job of demonstrating the flaring of the cheekbones.  The ceratopsian dinosaurs often have very nicely flared jugals, and in later ceratopsians, they would actually evolve the epijugal, a separate bone, like the nasal horn of the same group.  This specimen cast is in the collections of the Morrison Natural History Museum
Head-on view of a Protoceratops andrewsi skull cast at the Wyoming Dinosaur Center in Thermopolis, Wyoming.  Although not as pronounced as the cheeks of Psittacosaurus, Protoceratops, from the Late Cretaceous of Mongolia, does still have those flared cheekbones.
Skull of Ceratops montanus, on display at the Best Western Denver Southwest "Dino Hotel" in Lakewood, Colorado.  Noticeable jugal flare, but not as pronounced as it is in other ceratopsian dinosaurs like Psittacosaurus or Pentaceratops.
The most famous ceratopsian dinosaur of all, Triceratops.  Triceratops has decent-sized jugal flares, but nowhere near as pronounced as those of Pentaceratops, whose name actually derives from those two extra horns on the cheeks, bringing their grand total of horns up to five (versus the two brow horns and the nasal horn of Triceratops, the "three-horned face").  Triceratops, from the Late Cretaceous Period of North America, was first found in Denver, Colorado, and this skull cast can be seen on display at the Morrison Natural History Museum.
A skull of the ankylosaur dinosaur Ankylosaurus magniventris from the Late Cretaceous of Alberta, Canada.  Although flared cheeks are visible, they aren't as pronounced as they are in many of the animals we have looked at in this post.
Side-view of the skull of Archaeotherium, an entelodont mammal from the Late Paleogene.  Two morphs of Archaeotherium have been discovered, one with more pronounced cheekbones, the other with less pronounced cheekbones.  Most paleontologists suspect that the male Archaeotherium had the pronounced cheekbones, and the females had the less pronounced cheekbones, a classic case of sexual dimorphism.  Skeleton on display at the Denver Museum of Nature and Science (DMNS).
Female Archaeotherium, also on display at the DMNS.  Compare the cheekbones with the male in the picture above, and you can definitely see a difference.
A dramatic, frontal view of the male Archaeotherium, and you can immediately see how pronounced those cheekbones are.  Peter Cushing wishes. 
An illustration of the skeleton of Archaeotherium.  You can see how pronounced the cheekbones of the animal are, even in profile.  Much thanks to Dr. Bob for letting me use this illustration as well!  Photo Credit and Copyright: Dr. Robert T. Bakker
Another entelodont, Dinohyus, also on display at the DMNS.  Dinohyus, from the Early Neogene, is much larger and more robust than Archaeotherium, and still has some nicely pronounced cheekbones.
A skull of the Neogene Macrogenis crassigenis, an extinct peccary from Nebraska.  I really wish that I'd gotten a picture of this skull straight-on, but even in profile you can see how strange the enormous cheeks of this animal are.  On display at the AMNH.
The skull of the massive, armadillo-like glyptodont Panochthus frenzelianus, from the Pleistocene of Argentina.  Some glyptodonts, as well as their cousins the giant ground sloths, have large cheekbones.  Also on display at the AMNH.
Finally, even some modern animals have the enlarged cheekbones!  Here, we have a common warthog (Phacochoerus africanus) at The Snake Farm in New Braunfels, Texas.  It's difficult to see the left cheek of the animal, but the right cheek is nicely delineated against the background of the picture.
As you can see, the prominent cheekbones have evolved many times over the last several hundred million years.  Display seems like the most obvious and the most likely answer to the evolution of these structures, and it would have been quite exciting to see a pair of pareiasaurs squaring off during the breeding season.  Instead, to get your daily dose of prominent cheekbones, you'll just have to find something starring Peter Cushing.

Monday, June 15, 2015

Jurassic World: Facultative Bipedalism

In the new Jurassic World movie, the main movie monster is a critter that the movie characters called Indominus rex.  The dinosaur is a big guy, and its forearms are especially large, especially when compared to the tiny arms of Tyrannosaurus rex.  There were a few funky things going on with the Jurassic World dinosaur hands, but we can talk about that in a later post.  Today, we are going to talk about an interesting type of behavior exhibited by Indominus: facultative bipedalism.
Two baby Stegosaurus models on display at the Morrison Natural History Museum.  Stegosaurus individuals of all sizes would have been able to switch between walking on two and four legs, facultative bipedalism, which is the topic of this post.
Facultative bipedalism is an animal that can walk on both two or four legs, at least for a little bit.  For example, the gerenuk (Litocranius walleri), a type of African antelope, can rear up on their hind legs to nab plants off of some higher branches.  But they can't really walk on their hind legs, so they wouldn't really be considered facultative bipeds.  My little Djungarian hamster (Phodopus sungorus) will lean back on his hind limbs to manipulate food with his forepaws, as do many other types of rodents.  Meerkats (Suricata suricatta) often sit on their hind legs to observe their surroundings.  The extinct giant ground sloths would have reclined on their haunches to browse from the higher branches of trees.  But none of them would have moved around on their hind limbs, and therefore would not be classified as bipeds, facultative or otherwise.
Several gerenuk at Walt Disney World in Florida using their hind legs to eat some food off of the higher branches.  Photo Credit: Julie Neher
A little golden-mantled ground squirrel (Callospermophilus lateralis) using its forepaws to manipulate its meal at the Cheyenne Mountain Zoo in Colorado Springs.  Forepaw manipulation is something that many rodents can do. 
Another rodent manipulating an object with its forepaws. This North American porcupine (Erethizon dorsatum), also at the Cheyenne Mountain Zoo, is holding a primitive slingshot weapon.  Zookeepers have had a very difficult time controlling these animals.
A meerkat at the Cheyenne Mountain Zoo, using part of its termite mound as a lookout area.  It'll stand on its hind legs, but runs around on all fours. 
A Jefferson's ground sloth (Megalonyx jeffersoni) exaggerates the size of the fish it caught last summer at the Rocky Mountain Dinosaur Resource Center (RMDRC) in Woodland Park, Colorado.  Just like the gerenuk, the ground sloths would have reared up on their hind legs to consume vegetation off of a higher level.
A Glossotherium skeleton on display at the American Museum of Natural History in New York, walking quadrupedally.
The duck-billed hadrosaur dinosaurs and the iguanodonts are good examples of facultative bipeds.  They would have been capable of moving around on both two or four legs, depending on whether they were grazing lazily (quadrupedal) or moving more quickly (bipedal).
An Anatotitan skeleton on display at the American Museum of Natural History in New York.  You can see this skeleton is moving around on all fours....
....but the animal could also walk bipedally.
Stegosaurus was initially thought to be bipedal, and although most modern reconstructions show the armored dinosaur as a quadruped, fossil trackways at the Morrison Natural History Museum (MNHM) in Colorado demonstrate quite clearly that the baby Stegosaurus, tiny little six pound hatchlings, were entirely capable of moving around on their back legs!
Baby Stegosaurus model on display at the Morrison Natural History Museum, right next to the very first baby Stegosaurus track ever discovered by museum director Matthew Mossbrucker in 2007.
For stegosaur and hadrosaur dinosaurs, most of their weight was centered over their hips, and the same is true of some of the long-necked dinosaurs, such as Apatosaurus and Brontosaurus.  Fossil trackways of baby Apatosaurus at the MNHM show that the babies could run around on their hind legs, to keep up with their parents, and probably keep out of their way as well!  The little baby Apatosaurus tracks at the MNHM show the animal scooting along on its back legs, sort of like the modern basilisk lizard from South America.
Two trackways made by infant apatosaur dinosaurs, on display at the Morrison Natural History Museum.  The lower trackway has tracks from both the front and back feet, while the upper trackway has only hind foot tracks, and are spaced two to three times further apart than the ones in the lower trackway.  This shows that these baby dinosaurs would have been capable of running around on their hind legs!
Very few mammals are facultative bipeds, or even bipeds at all, with exceptions such as pangolins, jerboas, and kangaroo rats, as well as the regular old kangaroos.  Apparently, even cockroaches in the genus Periplaneta can run on their hind legs as well, if they get going fast enough!
A Parma wallaby at the Cheyenne Mountain Zoo, moving around on its hind limbs.
Here, you can see the same Parma wallaby, moving on all four legs.
A mounted skeleton of the Pleistocene kangaroo Simosthenurus at the American Museum of Natural History in New York.  As you can see, even though it looks like its twerking, it is on its hind legs, grabbing some vegetation.
Although Indominus was never shown really running on all four legs, and most of the time seemed like a biped, there were a few times where the beast would drop down to all four legs.  At those times, Indominus looked a bit like some of the earliest dinosaurs might have, as well as their close cousins.  Poposaurus and Postosuchus are both dinosaur cousins for whom the possibility of facultative bipedalism, or just regular bipedalism, has been suggested in the past.  Even if those specific critters weren't facultative bipeds, there were definitely cousins of theirs that were.

Works Cited:

Alexander, R. (n.d.). Bipedal animals, and their differences from humans. J Anatomy Journal of Anatomy, 321-330.

Weinbaum, J. (2013). Postcranial skeleton of Postosuchus kirkpatricki (Archosauria: Paracrocodylomorpha), from the Upper Triassic of the United States. Geological Society, London, Special Publications, 525-553.

Sunday, June 14, 2015

Jurassic World: Shed Teeth

At one point in the new Jurassic World movie, the main character Owen Grady (played by popular actor Andy Dwyer) pulls a broken tooth from the main monster Indominus rex from the shell of one of the Gyrospheres.  Believe it or not, broken teeth just like this are incredibly important for paleontologists when it comes to studying many different aspects of dinosaur habits and behavior!  First, let's take a quick look at the anatomy of a dinosaur tooth.
A pair of Tyranosaurus rex tooth casts with my camera lens for scale.
In the picture above, both teeth belong to the famed tyrant lizard king Robert California Tyrannosaurus rex itself!  The tooth on the top is larger, but it wouldn't have appeared that much larger in the mouth of the animal.  The reason behind that is the darker brown part on the left of the tooth is actually the root of the tooth, and would have been inside the animals skull.  The tooth underneath, the darker brown one, is a shed tooth crown.  Dinosaurs, unlike mammals, have an infinite supply of teeth, and if they lost a tooth it simply didn't matter!  They would grow another one in its place in a few weeks.
Stan, the Tyrannosaurus rex skull on display at the Morrison Natural History Museum.  Look on the upper jaw.  See the largest tooth, just about in the middle of the tooth row?  Let's zoom in and take an inside look! 
This is a view from the inside of the Tyrannosaurus skull from above, a view that the lawyer from Jurassic Park probably didn't find quite as fascinating.  See the largest tooth in the middle of the picture?  Notice how there's another little bump at the top of the tooth row, where the teeth are emerging from the maxilla bone.  That's actually another tooth growing in underneath!  If we CT scanned the original fossil, you'd be able to see all sorts of teeth growing in underneath!
Here we have a dental battery of the famous Triceratops, on display at the Rocky Mountain Dinosaur Resource Center (RMDRC) in Woodland Park, Colorado.  Now take a look at the picture below. 
Here, we have an individual tooth of Triceratops, out of the dental battery that you can see in the picture above.  This specimen is from the Hell Creek Formation of South Dakota, and also on display at the RMDRC.
The reconstructed jaws of the enormous, sixty foot long shark Megalodon, on display at the Mace Brown Museum of Natural History at the College of Charleston in South Carolina.  Like other sharks, as well as dinosaurs, you can see the several rows of teeth in the jaws of this guy, as well as the enormous biceps on the arms of the sexy Chris Pratt look-alike on the right.  Biceps for scale are approximately 36 inches in diameter.
Here and below, we have pictures of part of the dentary of the large carnivorous theropod dinosaur Torvosaurus from the Late Jurassic Morrison Formation.  You can see on the end in the picture above, the tooth is growing in, while the rest of the teeth are pretty well established.  This fossil is on display at the University of Colorado Museum of Natural History in Boulder
Another shot of that Torvosaurus dentary seen above, you can see another tooth growing in as well, second from the left.
Shed teeth can be quite important for paleontologists when it comes to determining behavior of these extinct creatures.  When paleontologists discover shed teeth of an animal, it can be a good indicator that the dinosaur was feeding on something nearby.  Unfortunately, shed teeth are easily recognized as fossils by most laymen, and are therefore often picked up by the public or fossil collectors looking to make a quick buck, thereby destroying any information we could potentially gain from such knowledge.  With good samples of shed teeth, like those employed by paleontologist Dr. Robert Bakker at the Late Jurassic Morrison Formation site of Como Bluff in Wyoming, scientists can learn about dinosaur diets, habits, habitats, and behavior, such as group movement, pack hunting, and even whether dinosaurs cared for their young!
A shed tooth crown of a Tyrannosaurus on display at the RMDRC.
Several shed phytosaur teeth on display at the Denver Museum of Nature and Science.  Phytosaurs were distant cousins of dinosaurs, and looked a lot like crocodilians.
A shed tooth of Nanotyrannus, a small cousin of Tyrannosaurus rex, from South Dakota.
Leidyosuchus, a type of Cretaceous crocodilian, with several shed teeth.
Brachychampsa, another Cretaceous crocodilian.
In 1877, local geologist Arthur Lakes discovered the very first bones of the dinosaurs Stegosaurus armatus and Apatosaurus ajax in Morrison, Colorado, and if you check out the Morrison Natural History Museum, you can actually see them there today!  Surrounding the genoholotype of Apatosaurus ajax, the very first specimen called YPM 1860, was reported by Lakes to have seven shed teeth belonging to an allosaur surrounding the specimen.  This indicates some that the predatory allosaurs were actually feeding on the Apatosaurus, which is very interesting information for paleontologists to have!  Below are two pictures of part of that YPM 1860 specimen, with the Director and Chief Curator of the Morrison Natural History Museum Matthew Mossbrucker pointing to the shed allosaur tooth crown.  These pictures are from the collections of the Yale Peabody Museum in Connecticut.
Shed allosaur tooth crown in the matrix of YPM 1860.  Photo Credit: Matthew Mossbrucker 
Shed allosaur tooth crown in the matrix of YPM 1860.  Photo Credit: Matthew Mossbrucker
Non mammals rarely have more than one type of tooth in their mouth, and when they do, it can often be the cause of celebration.  For example, in our previous post about the Latin and Greek root of two, we discussed two animals called Dimetrodon and Dimorphodon.  Dimetrodon is an early ancestor of modern mammals, and its name means "two measures of teeth," as it has two different types of teeth in its mouth.  Dimorphodon is a type of pterosaur (sometimes referred to as pterodactyls), a distant cousin of Pteranodon, whose name means "two morphs of teeth," again in reference to the fact that two types of teeth are in the animals mouth.  The animal below is a dinosaur called Heterodontosaurus, who belongs to the eponymously named family of dinosaurs, the Heterodontosauridae.  As you can see in the picture below, Heterodontosaurus has several larger teeth in the front of their mouth, and smaller teeth in the back.
Heterodontosaurus, a small little Early Jurassic dinosaur from South Africa.  As you can see, there are two different types of teeth in their mouth, larger ones in the front and smaller ones in the back.
One of the things that make mammals special is our teeth.  One of the most classic features of us mammals is our varied teeth.  In us humans, we have our incisors and canines in front, and our chewing molars in the back.  Since most mammals were only about the size of a shrew back during the Mesozoic Era, the time of the dinosaurs, in many places such as Como Bluff in Wyoming, paleontologists rely almost exclusively on the teeth of the tiny little mammals, since the teeth are much harder and more durable than the rest of the skeleton.  Below, we have a trio of elephantid molars on display at the Mace Brown Museum of Natural History at the College of Charleston in South Carolina.  Look at how varied the teeth are!  The first two belong to animals whose teeth were better adapted for crushing and grinding tougher vegetation, while the last molar would have been better for mashing up grasses.
Cuvieronius tropicus, a Pliocene-aged elephantid from South Carolina.  Large, high-cusped molars for crushing and grinding tougher vegetation.
Stegomastodon mirificus, a Pliocene and Pleistocene-aged elephantid, discovered in the Ashepoo River of South Carolina.  Like Cuvieronius, Stegomastodon also has those large, high-cusped molars that are great for demolishing tough plant matter.
The Imperial mammoth (Mammuthus imperator) from the Pleistocene of Florida.  These guys have a similar design of tooth to the dental battery of the ceratopsian dinosaurs mentioned above.  The duck-billed dinosaurs, or hadrosaurs, also had a similar design.  These teeth are broad and flat and good for mashing up grasses.
Shed teeth can be pretty important when paleontologists study fossils and extinct animals.  They are good at establishing behavior, and can be pretty important for long-term studies of paleoenvironments.  So when Owen uses the shed Indominus tooth in Jurassic World, believe it or not, that's actually something that paleontologists do from time to time!
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