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Paleontologists today look at more than just fossil evidence to learn about organisms that lived millions of years ago. In this case, we're seeking to answer the question: how, and when, did mammals evolve their specialized movements? Turns out, the next step in this process involves dissecting a giant weasel...

This is part one in a three-part series supported in part by The Field Museum, the Museum of Comparative Zoology at Harvard University, and The National Science Foundation (!!!!).

Big thanks to Drs. Ken Angielczyk, Stephanie Pierce, and Katrina Jones for their immense help and accommodation during the creation of this series.

Want to learn more about this research? Here's the gist:

Mammals are known for their great range of locomotor behaviors, including unique gaits such as galloping and bounding. These gaits are made possible by the subdivision of the backbone into two distinct regions: the thoracic region, which bears ribs and aids in breathing; and the lumbar region, which is ribless, highly mobile and functions in locomotion. Combined, these two sections of the backbone allow mammals to breathe and move simultaneously, permitting the use of high speed gaits for prolonged periods of time. But, how did this key mammalian trait evolve? Using cutting-edge 3D technology, along with the rich fossil record of mammals and their ancestors, this research will trace the origin and evolution of the mammalian backbone and its link with the development of mammal-specific locomotor behaviors. The work will deepen our understanding of the history of a key characteristic of mammals and part of the skeleton that is of great medical importance.
Retrieved from: http://1.usa.gov/1MHCXQm

"Dimetrodon is Not a Dinosaur"
https://youtu.be/-tdVPiyVDsQ

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Producer, Writer, Creator, Host:
Emily Graslie

Producer, Editor, Camera, Graphics:
Brandon Brungard
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Filmed on Location and Supported by:
The Field Museum in Chicago, IL
(http://www.fieldmuseum.org)

The Museum of Comparative Zoology at Harvard University:
(http://www.mcz.harvard.edu/)

The National Science Foundation:
Grants NSF EAR-1524938 and EAR-1524523
(http://www.nsf.gov)
Emilie: Hey, we're going to be doing something really cool! The next 3 episodes will be brought to you in part by the Field Museum, the Museum of Comparative Zoology at Harvard University, and the National Science Foundation.
 
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Do you ever wonder how paleontologists figure out how prehistoric animals move? Based off of only skeletal evidence as well as trace fossils, like fossil footprints, we can start to get an understanding. For instance, take Dimetrodon. An early proto-mammal, and not a dinosaur. A paleontologist was able to take measurements of the shoulders, and hipbones of a Dimetrodon fossil and then compare the range of motion of those joints to fossil trackways. The results give us a good ballpark estimate for how this creature, which lived nearly 300 hundred million years ago, moved. We know from this research that Dimetrodon probably walked like an alligator. Since they had sprawling short limbs and very rigid shoulders, they had to swing their bodies from side to side in order to move forward.

That swinging motion, is called "lateral undulation" and is employed by living animals like lizards, and snakes today. However, this movement makes it tricky for reptiles to breathe because as the animal moves, they end up compressing one lung, and expanding the other. Reptiles lack the muscular diaphragm that mammals have. As a result, they need to move their chest muscles in and out in order to breathe... which isn't possible when moving in a side to side pattern. Because of this, lizards can move very fast but only in short bursts before needing to stop and catch their breath. Which means, it's likely that Dimetrodon was also limited to running in bursts and, probably wasn't winning any marathon races.

Mammals on the other hand can endure running, and traveling long distances. We've got some of the greatest stamina of any animal on the planet. That's because the thoracic part of a mammal's body is highly specialized into a stiff cage with a muscular diaphragm that acts as a piston for breathing. This frees our lower back, the lumbar region to help with movement. Instead of our movements going side to side, our spines move in an up and down and wave like pattern, which actually helps to push the air in and out of both lungs when running. Together this handy setup allows us to move and breathe at the same time.

Even though Dimetrodon and its relatives were proto-mammals, they didn't have the same kind of distinct trunk division that mammals today have. Instead, their ribs extended all the way to their pelvic bones. Paleontologists today want to know. To understand how a regionalized trunk could've evolved, we need to understand how the spine develops.

Vertebrae form as a long series of repeating segments, and which type, or shape, they become is determined by where, between the head and the tail, they are. This patterning is controlled by a special set of genes called "homeotic genes" which spell out the overall body plan of an animal during development. These homeotic genes ensure the parts don't go in places they're not supposed to. When these genes cluster together they're called, "Hox genes" and they create a map of every animal to determine which parts develop where. Similar Hox genes are found in animals as diverse as insects, fish, humans, and even starfish, and they're always linked to body blueprints. This means it may be possible to trace their patterns back through the fossil record.

By using the spine as a kind of map for Hox genes, the researchers studying this hope that mammals living today, and their early relatives can be used together to create models for Hox gene evolution. This is one way that paleontologists can begin to understand how two distinctive trunk regions in mammals evolved from a single region in their ancestors.

In a way we can still get clues to the genetic patterning of extinct animals even if we're physically unable to sequence their genes. Paleontologists suspect the shift in mammal body plan began to happen around 260 million years ago in a group of mammal relatives called cynodonts. But, we need to test this hypothesis.

Understanding genetic evolution is only one way to understand how the evolution of mammal movement came to be. The next part of this research project is going to take us to Harvard's Museum of Comparative Zoology, where Field Museum curator, Dr. Ken Angielczyk, is collaborating with other paleontologists in search of an answer to this question: "When and how mammals evolved their specialized spines?" First we're going to dissect out the spinal columns of a variety of modern day mammals. Then we'll CT scan those spines to create 3D models in order to compare those to scanned fossil models. And, finally from comparative stress and mobility tests we hope to better understand the movements of mammals that went extinct millions of years ago.

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Emilie: It still has brains on it.