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Paleontologists today look at more than just fossil evidence to learn about organisms that lived millions of years ago. For this episode we visited Dr. Katrina Jones at Harvard's Museum of Comparative Zoology to learn how she dissects and examines animals living today in the search for answers about the movements and evolution of early synapsids!

The specimens in this episode were found dead in the wild, and legally obtained with government-issued salvage permits. Do not touch, pick up, or otherwise attempt to obtain parts of or entire animals you may find dead without properly authorized permits.

This is part two 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 (!!!!). Watch Part 1 here: https://youtu.be/opBalCaq5m8

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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This episode is supported in part by:
The Field Museum in Chicago, IL
(http://www.fieldmuseum.org)

And filmed on location at:
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)

 (00:00) to (02:00)


This series of episodes is brought to you by The Field Museum, the Harvard Museum of Comparative Zoology, and the National Science Foundation.

Hey! We're here at Harvard's Museum of Comparative Zoology with Dr. Ken Angielczyk-

Ken: And I am the associate curator of fossil mammals at The Field Museum of Natural History

E: And we're here today to do some experimenting' on some dead animals, to learn more about those fossil mammals, it's gonna be great.  Let's go.

[intro]

K: In the episodes that we've been doing so far we've talked a lot about synapsids, but you might not know what those are.  So a synapsid is a member of a group of animals that includes living mammals as their sort of living, but also a number of fossil forms that, in some cases, look very different.  And all of our different synapsids are characterized by one feature, this opening back here in the back of the skull, just behind the eye socket.  That's an area where jaw muscles attach to the skull, and all synapsids have those.  So if you compare an animal like this, this is eyothyrus, one of the most primitive synapsids that we know about, to a living mammal, you can see the skull looks very different, but we have that same synapsid temporal opening back here in the skull the way we did with eyothyrus.

Um, so this is an animal called Massetognathus which is a more derived or a more advanced kind of synapsid, you can see that it has a skull much more similar to our living mammel.  Synapsids have undergone a massive amount of evolutionary change over the course of their history, so there's actually very few features other than that that you can point to  in all synapsid specimens.

E: And this is kind of why we're here today, right? So we're going to be looking at some of the changes and the other parts of the skeletons of these animals, and primarily what is it that we're going to be looking at?

K: Yeah, so we're interested in looking at the backbone of synapsids.  

 (02:00) to (04:00)


So living mammals have very distinct vertebral columns or backbones that have lots of different regions and they have very specific functions in those regions.  And if we look at more primitive synapsids, they have a much more uniform backbone or vertebral column.  We can only get so much information out of the fossils, we need to get some living mammals with their soft tissues like muscles attached to their backbone or vertebral column, and what we can do is learn about how the backbone functions in those animals and then go back to our fossils and, um, essentially model how those vertebral columns would have been working when they had all of their soft tissues attached.

E: So, we're going to the prep lab.

(2:40)

E: We are here in the prep lab at Harvard's Museum of Comparative Zoology with Dr. Katrina Jones.  Katrina, what is is that you do?

Katrina: Hi Emily, I'm a researcher here at the museum and I study the anatomy and evolution of mammals.

E: And that is what we have in this bag here in front of us.

K: Exactly, yeah.  So, this is a fisher cat and today we're gonna be dissecting out its vertebral column so that we can do experiments on it.

E: But it's not, it's not a fish, kind of.  It's a derived fish.  And it's not a cat.  It's like a big ferret.

K: Yeah, kinda like a giant ferret, otter. So the skin was taken off and its gone to be used in our collections, and all of the organs have been taken out, but now we need to remove the head, the limbs, and get down to the vertebral column, which is what we're interested in.

E: Let's do it!  Should I grab this end?

K: Yep, you can grab it and pull it out.

E: Oooh, oh boy.  You can kind of already, if I'm gonna hold this guy up, you can already sort of see-whoops, now it's bleeding on the table.

K: That's what it does.

E: That the ribs, the thoracic vertebrae and all the muscles that are attached to them are kind of contained in that area, and then there's something here at the lumbar region.

K: Yeah, so mammals actually have a region that's free of ribs, called the lumbar region and that's different than some of the ancient mammal ancestors that you were looking at earlier.  The synapses, which had ribs pretty much all the way along their trunk.  

E: Yeah, then you can kind of see where like, that would have perhaps inhibited a bit of movement or range of motion.

 (04:00) to (06:00)


K: Yeah, definitely.  So that's one of the factors, but also the individual shape of the vertebrae and how they fit together is really important for determining how much mobility can happen at each joint.  And that's what we wanna get a handle on today.  Okay, so, do you wanna take a- we'll start with a forelimb?

E: It pooed on me.

K: Yeah it probably did poo on you.  Sorry about that.  It happens.

E: It's okay.

K: First thing we're gonna do is we're gonna remove a forelimb.  So what we're gonna do is cut down the midline, this is the pectoral muscle, so...

E: Wow, that's cool ,so I can actually see the different muscle that you just removed.

K: Yeah, so that was the pec major that we just, we call it retracted, it's rectracted out.  And so now I'm gonna cut through this connective tissue on the neck so that we can reove the connection with the forelimb there.

E: So what is it, you're kind of cutting the fascia between the muscles?

K: Mhm.. So some muscle attaches into bones, to move bones, but some muscles actually attach to other muscles, and they do that by attaching to these like, thick sheets of connective tissue that we call fascia, and um, that sort of helps them to move.  And it's kinda cool when you think of all the different things that mammals do with their arms like bats fllying or like...

E: Whales swimming.

K: Whales swimming, exactly, like a brachiating primate swinging througgh the treetops.  There's a lot of different functions.

E: A lot of Googling, a lot of Facebooking.

K: Exactly.  Yeah, so this muscle, serratus ventralus, actually has muscle fibers that insert in each rib, so it kind of looks jagged and that's where the name comes from.

E: So it goes all the way from the back to individual ribs.

K: Mhm.  And this is quite cool right here: this is what's called the brachial plexus whcih is a really dense web of nerves.  So this is how you get nerve impulses in to move the muscles and sensation out.

 (06:00) to (08:00)


K: So I'm actually going to cut through the brachial plexus, sorry brachial plexus.  Of  you go.

E: Oh no, now he can't use his arm.  Cuz he's dead.  Also that.

K: Notice how I'm dissecting a lot with my scissors?  Even though it's called dissect you really want to split tissues along their natural planes more than actually cutting through tissues.

E: Hmm.  So you're just seperating things.

K: Seperating things, yeah.

E: Aw  this looks nothing like an arm anymore.

K: Yeah, it's easy to get disorienting once you get it detached from the track.

E: And also it's missing it's little hand.

K: So we're gonna work on this other limb now.  Maybe if you wanted to have a go at doing something you...

E: I'd love to.

K: This is the pec muscle, which is here. It sort of flexes the arm like this.  So what you're gonna do is detach it from the trunk.  Start with the scalpel and then when it starts coming loose you can actually just get in there with the scissors.

E: So like that?

K: Mhm.

E: And then use the scissors?

K: So I'll take that and you use the scissors and you kind of pull it towards me to get some tension and then you sort of 

E: I'm trying to mimick your technique here.

K: Sort of spread apart, do you feel how the tissues just spread along their natural plane, and then where there's a connection you just snip.

E: Like right there.  Right there?

K: Yeah.  So that's where the muscle is actually inserting into ribs, so we have to cut that.

E: This guy has an obscenely thick neck.

K: Yeah, I feel like this is an aggresive, really muscley male.

E: Like he was probably getting some stuff done.

K: Mhm.  Yeah, probably didn't wanna mess with him.  Okay, so we're definitely making progress now. What's holding it on down here still is the serratus muscles. 

 (08:00) to (10:00)


K:  And what we have over here that we saw earlier..

E: The connective

K: The brachial plexus.

E: Yeah! That one.  That's amazing, they are actually-you call them cables they are like cables.

K: Data cables.

E: Data cables.

K: Communicating information from the brain to the..

E: If I spend to much time thinking about that I just have an exsistential crisis.  I'm like "Oh god there are so many things happening in my body right now that I don't even think about."  And then I'm cutting them, oh no.

K:  Oh there we go.  There's actually like a web of muscle that runs all the way over the neck called platysma.  And it's what helps, it's one of the muscles that helps open the jaw.  And it's kinda cool cuz whales have really giant ones.

E: Whales?

K:  Mhm.  It helps them to pucsh out the water when they gulp feed.

E: Like baleen whales?

K: Mhm.  People think dinosaurs are cool but really mammals are much cooler. 

E: Hear that guys?  You're gonna start some stuff in the comments of this video.

K: Okay, so there's only, only the serratus is left, so what I'm gonna have you do, we're gonna put it like that and you're gonna hold it and run your scalpel, and cut that, tand that's gonna bring the limb off.  

E: Just like that.

K: There you go, you have a limb.

E: Another limb.

K: Two of them.

E: Wow!

K: Okay, should we do a head next?

E: Yeah!

K: Lots of very important htings connecting the head and the body going on here.

E: Yeah, I mean, I would hope there are important things connecting the head to the body, otherwise they'd be a lot more likely that it could fall off.

K: Yeah that would be, that would be bad news.   We're working our way through these layers of muscles hoping to get to the joint that conencts the head and the neck.  Its called the atlanto occipital joint.

E: There's just one joint?

K: Mhm.

E: That attaches the head to the neck?

K: That attaches the head to the neck.

E: What happens if you cut it?

K: Then your head falls off.

E: No way!  I mean-

K; But you'll see, it's quite hard to get to, we're still working our way through muscle layers.

 (10:00) to (12:00)


K: Just to try and get to it, so it's not something that's gonna happen overnight.  Don't have to lose sleep over it Emily.  The top two vertebrae in mammals, teh two that are closest to the head are really really really weirdly shaped.  The one that connects the head to the neck, the atlas, only allows movement like this *nods head up and down*,  and the one beneath it, the axis, only allows movement like this *shakes head side to side*.  So it's like the yes joint and the no joint.

E: And they work together.

K; And they work together to produce all other movements.

E: Like a bobblehead.  So you've got a lot of these neck muscles kind of dissected off.

K: Yep and we can actually start to feel the joint between the head and the neck if you just pinch with your thumb and forefinger just there, I'm gonna do a yes action with the head and that's where you're feeling  is the atlanto occipital joint moving.  And so that's what we're gonna try and cut through to separate the head off. 

E: I mean it's prabably for the best that it's difficult for your head to be removed.

K: We're almost through.  Just one more bit.  Okay, let's see.  You ready

E: Yes! Get it.  Here you go.  Oh!

K: Very satisfying.

E: And it lost its head.

K: There we go.  The head and all of the neck muscles.

E: It's somehow more disturbing with just like the neck muscles as a part of it.  *Puppets the head*"Hey, how ya doin'  Katrina?"

K: I'm good, thank you dead fisher head.

E: We'll just leave this here.

K: Okay, I think it's time for you to do the honors, remove the baculum, since you're wearing the baculum earrings.

E: Alright.  I am.

K: Cut along this connective tissue here.

E: Here?  I've never so surgically clipped a penis bone off.

K: Great, and along that side too.

 (12:00) to (14:00)


E: Wow, I feel a bit-

K: We can keep that.  We can keep it as well, you know, someone might be interested in studying it one day.  You wanna cut through the urethra?

E; Here we go.  Keep going?

K: Yeah, just take it all off.

E: Take it all off.

K: There we go, perfect.

E: Alright, well.  Just lay that out there.  The bits.  This is just saline solution, we gotta keep those muscles hydrated.  Hydration's important even post mortem.

K: So this is like the equivalent of hamstrings in humans.

E: Kinda the groin area.

K: Mhm.  So just running the scalpel along the pelvis and cutting all the connections.  Okay, then we'll cut the quads.  Okay now all that's left connecting is the head of the femur into the acetabulum.

E: Which is the ball and socket joint.

K: Ball and socket joint.  I've just cut into the opening of the joint there.

E: I can see it, yeah.

K: So now I'm just cutting the ligaments.  So we've opened up the capsule of the joint, so this is the head of the femur, and that fits in here into the acetabulum and see... I put my probe under here, there's like a connection?

E: Yeah.

K: That's called the ligament of the head of the femur, and that actually keeps the femur in its socket.  So if you wanna do the honors, this is gonna sort of allow the whole thing to just pop open.  You take the scalpel and cut through the ligamet right where my thing is.  And it pops out, once that's cut.

E: That's it.  And then the femur's so tiny compared to all of the muscle that's around it.  I mean the bone is just not even as wide as my pinky finger and then all these other muscles are just coming in and... this is a powerful animal.

K: Okay, you wanna have a try with the other limb?

E: Sure.  

K: Yep.  And then if you wiggle, you can feel, so you already got to the joint there.

 (14:00) to (16:00)


E: Okay.

K: And then we're gonna open up the joint capsule.  Can you, wanna put your finger and feel where it is?

E: Joint, oh it's right there.

K: Yep, feel it?

E: Yep.

K: Great.  So then, cut where you felt the joint moving.

E: And you just keep cutting around?

K: Yep, so you just cut into the ligament.

E: There it is!  I just don't wanna cut your finger right there.

K: Right, right;  that's good, thank you.

E: Ah, I did it.

K: There we go.

E: Aaaand the leg is off.

K: The leg is off, right.

E: We'll just put that over there, make sure we're hydrating those legs.

K: And so now what we have left is the vertabral column, the pelvis, and the ribs.  And so before we do our experiments we'll also remove the pelvis and we'll remove the ribs;  that takes kind of a long time, so we might skip to the next specimen.

E: Yeah.  Cuz we already have it, like a cooking show, already done.  So this is the one that we just dissected.

K: So the next step would be to remove the ribcage and to atart cleaning off some of these muscles.  These are called epaxial muscles which means muscles that run on top of the spine.  So I've started to do that in this smaller fisher that we have form earlier, and one thing that we can see here which is super cool: notice how there are really long strips of muscle running all the way down the back.  And this is sort of a neat mammal feature.  So if you were to look at the epaxial  muscles of, say, reptiles, theire muscles would be a lot shorter, but we have these really long-they call them tracts-of muscles so that when the spine wants to flex like this, they can contract and form the flexion and extention which we know is really important when mammals are using their assymetric gaits like galloping and bounding. So this is sort of before and then after we remove the ribs and all of the muscles we end up with something like this.  

 (16:00) to (18:00)


K: And then notice how flextion-extension is happening a lot here, but if we do twisting there's no action happening down here, and that's because the vertebral joints down here lock together and prevent twisting, whereas those up here can move by each other.

E: Yeah, they help enable the twisting.

K: We can sort of see them twisting.

E: So do you, do you hypothesize that those early synapsids like dimetrodon or adaphosaurus, that they're spind was more like the lumbar here or more like the thorasic, like was it more or less flexible?

K: It was less flexible but when you look at the vertebrae they don't look like either lumbar or thorasic.  From the shape of the joints we expect that they were able to do more lateral flexion, but we don't think that they were able to do any of this *bends spind up and down* bending.

E: So they kind of moves back and forth like a lizard.

K: Mhm, exactly, and we hypothesize that the evolution of specialized joints which allow bending flexion-extension like this might be related to the evolution of assymetric gaits in mammals.  So the division ofthe vertebral column inter regions is controlled by a set of genes called the hox genes.

E: So it was the evolution of the genes that enabled the evolution of the morphology of the spine.

K: Well yeah, I mean it'sthe ggenes which evolve, and it's nice here because we have such a direct link. So we can clearly see that where a particular gene is expressed is where a particular region forms in the adults, so if we can measure the presence of different regions based off of vertebrae in the fossil record, we can infer the expression of genes in animals that lived millions of years ago.

E: Genetic research on animals that are like 200 million years old.

K: Indirectly, indirectly, by mapping out how regionalization has changed through time.

 (18:00) to (18:23)


E: That's pretty exciting.  That's pretty cool stuff.

K: Very exciting.

E: Wecome to the twenty first century.  Awesome.