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.

2 comments:

  1. Hi Zach, I love this blog so far! Is there any way that I can follow it with a wordpress account, or do I have to have a blogspot to do so?

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    1. I am clearly very bad about responding to comments, my apologies Max! Anyways, I am actually planning on moving my blog to Wordpress in the next few weeks, so it'll be easier to follow me then!

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