Finding Tiktaalik, the Fossil Link Between Fish and Land Animals

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00:00:07.22 My name is Neil Shubin, I'm from the University of Chicago.
00:00:10.01 And I'm going to talk today about finding Tiktaalik.
00:00:12.06 My life's quest began in graduate school, in the second year
00:00:16.26 when I saw this exact slide. This is the slide my professor
00:00:19.29 showed back in 1986, and it showed what we knew at the time as
00:00:25.17 the fish to tetrapod transition, which is the transition from life in water
00:00:29.19 to life on land, as seen by these cartoons of these fossil creatures.
00:00:33.12 The one on top is a fish from 390 million years ago, the one on the bottom
00:00:38.18 is a tetrapod from about 365 million years ago. I remember as a student
00:00:43.10 seeing this diagram for the first time and thinking, "Holy cow!
00:00:46.20 What a first class scientific problem!" How did fish evolve to walk on land? It
00:00:52.00 seemed so utterly impossible go to from the thing on top to the thing
00:00:54.28 on the bottom. And that's what became my quest. So like any
00:00:58.00 good paleontologist, I applied the paleontological toolkit to try to find
00:01:01.09 an intermediate, say some sort of creature between these two.
00:01:04.14 And so what do we do when we try to find a new place to discover fossils?
00:01:08.18 What we do is we look for places in the world that have three things.
00:01:11.27 The first is rocks of the right age, you need rocks of the right age
00:01:16.08 to answer whatever question interests you. So in this case, clearly
00:01:19.19 we want something between 390 and 365 million years old.
00:01:24.05 That's of a time period known as the Late Devonian. The other thing
00:01:28.08 that was really important, of course, is to have rocks of the right type.
00:01:31.00 Not every kind of rock holds fossils. Some are super heated, some are
00:01:35.06 highly pressurized. So we needed a particular kind of rock, sedimentary
00:01:39.15 rocks, and not only that, sedimentary rocks in the right environment
00:01:43.20 to hold the kind of intermediates we're looking for. So what's the
00:01:47.14 other variable? Rocks of the right age. Rocks of the right type.
00:01:50.10 Well, it does me no good if my wonderful rocks of the right age
00:01:53.27 and the right type are buried five miles underground, right? These things
00:01:57.08 have to be exposed at the surface. So what we do is we look for places
00:02:00.28 in the world where we have naked bedrock, okay? So that's it. If you
00:02:04.10 ever want to run your own expedition someday, look for places in the world
00:02:07.18 that have rocks of the right age, rocks of the right type, and rocks that
00:02:10.09 are exposed at the surface. There's another criterion, and in this case
00:02:14.06 was accessibility. And this is where I began my first scientific expeditions.
00:02:19.06 These were accessible, because I had just begun my first academic
00:02:24.12 job at the University of Pennsylvania, here at the southeastern corner
00:02:28.00 of the state of Pennsylvania. And I dug out a geological map of
00:02:31.15 the state. And I'm showing you here what is sort of relevant for our
00:02:35.21 question. If you look at this map of Pennsylvania, what you see is
00:02:39.00 everything in purple are Devonian age rocks. So I have lots of
00:02:43.22 rocks of the right age throughout the state of Pennsylvania. But what about
00:02:47.02 that second variable, rocks of the right type? Well if you want to
00:02:50.29 think about what Pennsylvania looked like in the Devonian, get Pittsburg,
00:02:54.29 Harrisburg, Philadelphia, out of your brain. And think amazon delta.
00:03:00.14 This is a cartoon of what Pennsylvania looked like 365 million years ago.
00:03:04.24 You had highlands to the east, an inland sea to the west, and a series
00:03:09.12 of rivers draining from east to west. Now if you're a paleontologist
00:03:14.01 wanting to find fossils at the cusp of the transition from life in water
00:03:18.01 to life on land, this is perfect. Because if you have the right exposures,
00:03:22.19 you can sample ancient seaways, ancient rivers and streams, even ancient
00:03:27.15 upland deposits. Now the problem is Pennsylvania doesn't have a whole lot
00:03:31.21 of exposures. So that third variable is very tough. So my initial
00:03:36.06 expeditions, if you will, became following the Pennsylvania Department of
00:03:40.24 Transportation around as they build new roads. And if we were really lucky,
00:03:45.02 what they would do is they would be digging a new road in a place of Devonian
00:03:48.23 rock. And that's what you see here. This is a set of rocks about an hour north
00:03:53.02 of State College Pennsylvania. And this expanse extends about half a mile
00:03:57.21 long. It's a big red hill of rock -- it's called Red Hill, no surprise.
00:04:01.12 But what you see in these layers is the strata, the ancient layers
00:04:05.21 of ancient streams. And this is really perfect. Almost as soon as we
00:04:10.02 hit this site in the early '90s, we started to find fossils. The first thing
00:04:15.07 we started to find were teeth the size of railroad spikes. So big monsters, almost the size of
00:04:19.15 your thumb. Then we started to find the jaws of these creatures. Here's the
00:04:22.27 front end of one of these jaws. These jaws would be the length of your arm.
00:04:25.27 Alright, so they're really big. These big animals are probably about
00:04:29.13 16 feet long. Then we started to find all kinds of other fish, like
00:04:33.01 this little mashed thing here. This is actually a mashed fossil
00:04:36.25 of a lobed fin fish, which is related to lungfish today. So that's a head
00:04:41.14 and a body. But then my colleague, Ted Daeschler, who's been my partner
00:04:45.07 in all these expeditions for the last number of decades. Ted found
00:04:49.11 early tetrapods. One day I had left, one week I had left to do
00:04:53.28 expeditions elsewhere, and he had just discovered a series of
00:04:56.04 bones. Arm bones, leg bones, from creatures that represented
00:04:59.16 early limbed animals. And working with the National Geographic Society,
00:05:03.18 Ted and I were able to reconstruct what this ancient road in Pennsylvania
00:05:07.16 looked like 365 million years ago. I mean we had some of the first
00:05:12.10 forests, you can see this, and ancient streams, see that in the middle
00:05:16.04 here is that big monster fish, about 16 foot long fish. And then you see
00:05:20.28 a series of other kinds of fish, in particular, you see the little creatures
00:05:23.21 with limbs crawling around both on land and in the water.
00:05:27.16 Now this was great, but Ted and I realized we had a problem.
00:05:31.23 Why? We're in rocks about 365 million years old and we're already finding
00:05:38.00 pretty advanced limbed creatures. Remember what motivated this
00:05:41.02 whole quest was this whole slide I saw in graduate school.
00:05:44.22 We're not finding intermediates, we're already finding pretty advanced
00:05:47.20 limbed animals, I want something in the middle here. Well, to give you
00:05:50.18 a sense of what we're after, let's just go through some of the
00:05:52.26 traits that we look for. When you look at the creature on top, it has a
00:05:56.06 conical head with eyes on either side. The creature on the bottom
00:06:00.01 has a flat head with eyes on top. So really one of the kind of features
00:06:03.28 that changes in this transition is the shape of the head.
00:06:08.08 The other thing that changes is if you look at the creature on top,
00:06:10.27 the fish on top, you'll see the head is connected to the shoulder girdle by a series
00:06:14.21 of bones. It doesn't have a neck. Fish don't have necks where the head
00:06:18.20 can bend independently of the body. Early limbed animals, early
00:06:22.23 tetrapods, have a neck where the head can swivel independently
00:06:26.16 and it can look around as its legs are on the ground. Finally, another big feature
00:06:31.23 which obviously gives it its name, tetrapods, four-legged animal,
00:06:35.02 is fish have fins and the earliest tetrapods have limbs with fingers
00:06:39.03 and toes and wrists and ankles. We're finding a lot of fingers, toes, and wrists,
00:06:43.08 and ankles, right? We're not finding any intermediate. What Ted and I
00:06:46.14 are looking for is a flat headed fish with say fins or something in that
00:06:50.18 intermediate zone. To get that, what we had to do was move back in time,
00:06:55.02 back in time about 15 million years using analyses that other colleagues had done.
00:07:00.06 So Ted and I pulled out the paleontological rule book and began looking for
00:07:04.21 places that have rocks that are the right age, rocks that are the right type,
00:07:07.11 and rocks that are exposed. Wed had an idea to work in Brazil.
00:07:11.16 We had an idea to work in western North America. Then everything
00:07:15.28 changed one day in my office, back in the late '90s, when we had a
00:07:20.28 geological debate. And to settle the debate, I pulled out a college
00:07:24.15 geology textbook, this one right here. It's now in its eleventh edition.
00:07:28.19 So that gives you a sense of how old I am. But anyway, so we
00:07:31.01 finished the debate and I was paging through the book, and we ran
00:07:35.08 into this figure. This is the figure that changed the traction of research
00:07:39.25 for me, about 15 years ago. What it is is what we look for in a nutshell
00:07:45.01 as paleontologist. You'll see the caption, it says upper Devonian sedimentary
00:07:49.11 facies. Rocks more or less of the right age and the right type.
00:07:52.27 But here you see a map of North America, and superimposed on that
00:07:56.18 map is an interpretation of the geology of the Devonian age
00:08:00.27 rocks. Now you'll notice in the western part of North America,
00:08:03.20 there are Devonian rocks mapped as ancient oceans, but then the authors
00:08:08.05 identify three different areas, shown in red here, that had rocks
00:08:12.03 that had formed in ancient rivers and streams. Ancient rivers and stream
00:08:15.06 environments, delta systems. The first one, you've seen before.
00:08:20.01 That's the one I just showed you from Pennsylvania, that's the ongoing Catskill
00:08:23.08 project that we're working on along the roadsides of Pennsylvania.
00:08:26.29 There's another one on top here in East Greenland, that was well studied.
00:08:31.01 That was discovered in the '20s by Danes and Swedes. And
00:08:34.29 had produced a number of early tetrapods or limbed animals, in fact
00:08:38.03 the cartoon I've been showing you is the cartoon derived from
00:08:40.29 one of the fossils from this area. You see where I'm going?
00:08:43.25 Extending 1500 kilometers east to west across the Canadian arctic,
00:08:48.04 was mapped Devonian aged rock, completely unexplored. Not only that,
00:08:53.05 that rock was perfect. It was mapped as being from ancient rivers
00:08:56.03 and streams, ancient delta systems. So Ted and I were really excited,
00:08:59.13 and just a little anecdote here is, we were so excited. It's all happening
00:09:02.18 one morning, so we ran to a Chinese food restaurant across the street
00:09:05.23 and had a fortune cookie. And that fortune cookie said, "Soon you'll
00:09:09.24 be at the top of the world." So yes, here we go! We had a fortune cookie,
00:09:13.02 off we go! So anyway, this is a colorized version of one of the scientific papers
00:09:17.15 and it shows the stratigraphic section right here, and as well as the
00:09:21.07 map. And you can see, super-imposed over the map of the Canadian
00:09:25.15 arctic is a huge area, shown in red here, of Devonian age rock
00:09:30.18 So you can see it's all over. This is 1500 kilometers of section.
00:09:35.23 So now we had a new problem, instead of driving in our station
00:09:39.23 wagons to central Pennsylvania, we're now working several hundred
00:09:42.28 miles from the North Pole, where the arrow is in this slide.
00:09:46.02 To give you a sense, the nearest town is about 300 miles away.
00:09:50.06 I'm going to show you a picture of that town, with about 170 permanent residents.
00:09:54.07 This is a picture of that town in spring, Beresford, Canada. This is a big city!
00:09:58.22 And so this is resupply. Anyway, as you can imagine, much of our
00:10:02.16 supply is based on planes and helicopters. And so we're highly
00:10:06.20 dependent on aircraft. Now the way this affects science, is that the supply chain
00:10:12.23 is very long. And fossils are rocks, they're very heavy. So that means we
00:10:16.29 can't take home everything we find. So much of what we do in the field
00:10:20.26 is trying to determine whether we have really good material that is
00:10:23.20 worth spending the money to freight it home. So we started in 1998,
00:10:30.27 we had that fortune cookie in 1998. Our first expedition was in 1999.
00:10:35.02 It took us about a year to get the permits, the money, all that good stuff.
00:10:37.22 And we started with great hope in the western part of the Canadian arctic.
00:10:41.06 And you can see what that area looks like. It looks kind of flat, doesn't it?
00:10:45.15 And you can see our camp just below me here. This is Devonian rock.
00:10:49.02 So we'd wake up in the morning and essentially walk all over these
00:10:51.25 Devonian rocks, looking for the bones that are weathering on the surface.
00:10:56.02 We realized we had a big problem, this is the wrong part of the Arctic.
00:10:58.28 We were in all these rocks, which were marine rocks formed in ancient
00:11:02.22 oceans. It was pretty clear based on other people's discoveries
00:11:06.12 that we needed to hit rocks in rivers and streams. So what that meant
00:11:11.07 is using the geological map, this is a version of what I showed you
00:11:14.29 before for Pennsylvania, it applied also to the arctic. We were in the
00:11:18.24 ancient seaways, shown in blue. We had to move upstream. To do
00:11:22.12 that geologically, it meant moving east. So we retooled, the next year we went back
00:11:27.23 and we moved east, here to Southern Ellesmere Island.
00:11:30.25 And as soon as we did that, you can see these beautiful landscapes
00:11:33.29 here, of mountains and areas. And these are all red beds that contain
00:11:38.09 Devonian rocks formed in ancient rivers and streams.
00:11:41.15 Took another few years, we eventually honed in on an individual
00:11:45.17 valley that had good fossil preservation. And this is that valley here.
00:11:49.04 What you see, in front of us, is a big hole in the middle of this
00:11:52.21 slide, that shows a layer that was preserving hundreds of fossil
00:11:56.13 fish, preserved one on top of the other. So, the arctic's a big place. Fossils
00:12:01.16 -- you know they're not very big, so you're trying to find the little
00:12:03.19 tiny patches of rock that will have those fossils inside. And what we use
00:12:08.01 are the tools of geology and sedimentology. Well, we worked this layer,
00:12:12.02 and everything changed one day in 2004, when my colleague,
00:12:16.09 Steve Gates, he's shown in blue here to the left, removed a rock
00:12:19.18 from this layer. And he saw a little notch, a little v, you can see that v
00:12:24.01 in the middle of this slide. If you look very carefully you can see it.
00:12:27.11 He says, hey guys, what's this? We run over to it and we saw what
00:12:31.04 we spent multiple years looking for and several decades of a quest. What we had
00:12:35.09 looking at us here is the snout of a fish, and not just any fish, the snout
00:12:39.22 of a flat headed fish. Remember I told you the conical head and the flathead
00:12:43.05 was the big piece of this transition? What I had here, in the middle of this
00:12:46.15 slide, was a flat headed fish sticking right out at me. With any luck at all,
00:12:50.11 the rest of the fish would be preserved in the rock. So we removed
00:12:53.29 this thing and they come home at the base of the helicopter, so this is a
00:12:57.08 first year graduate student for scale. And it comes back several hundred
00:13:00.17 miles to a supply base. And as we remove this first specimen, we
00:13:07.01 found several more. So at the end of the 2004 season, we had multiple
00:13:10.08 specimens of this flat headed fish. We come back to the lab, we're really
00:13:15.27 excited. We come back to the lab and the preparers start removing
00:13:19.05 the rock, grain by grain, to expose the fossils within. This is what
00:13:23.25 Steve's specimen looked like several months after preparation.
00:13:27.14 You can see what's emerging is sort of a flat head with eyes on top.
00:13:30.15 You can see the tool that's used here to remove the specimen, after
00:13:34.29 several more months, what's exposed. Look here we sort of have a flatheaded
00:13:38.14 animal with eyes on top, even a shoulder that looks separated from
00:13:42.00 the rest of the head. Remember what began this quest? In graduate
00:13:45.23 school I saw this slide, fish on top, tetrapod on bottom.
00:13:48.25 We wanted a flat headed fish. After several decades, using
00:13:52.05 tools of evolutionary biology, using tools of centrography, using the tools
00:13:55.22 of geology, this is the flat headed fish. This is Steve's specimen
00:13:59.19 as removed from that hole. Flathead, eyes on top, has a neck,
00:14:03.14 you can see the head is separated from the shoulder, it has fins,
00:14:06.22 it has fin rays, and it has scales on its back. Like a fish, it has
00:14:10.21 scales and fins, like an early limbed animal, it has eyes on top,
00:14:15.18 it has a neck. And indeed, when we crack open the fin, what
00:14:18.06 do we find? Bones that correspond with the upper arm, forearm, and even parts
00:14:22.13 of a wrist. This is a real mix of characteristics. Found at the right
00:14:25.28 part of the fossil record. So this creature we call Tiktaalik, Tiktaalik
00:14:30.05 roseae. And you can see here, what's sort of salient about it. It has a
00:14:33.16 mix of tetrapod and fish characteristics, like a lobed fin fish it has fins,
00:14:37.26 scales, and primitive jaws. Like an early tetrapod, it has a neck, a wrist,
00:14:43.16 a flathead, and so forth. So here's a creature with lungs and gills, here's a
00:14:49.26 creature with limbs that have limb bones in them but also fin bones,
00:14:52.25 as well. Fined rays. It's really a mix between fish and tetrapod.
00:14:56.19 And when we put it in the phylogenetic analysis, the evolutionary
00:14:59.29 tree, you can see Tiktaalik drawn right in the middle here. It really
00:15:04.03 shows a fine sequence of the emergence of tetrapod characteristics
00:15:08.11 throughout the Devonian. So what's the take home message here?
00:15:12.15 The take home message here is that we can predict likely and unlikely
00:15:16.04 places to find fossils. We can, in doing that, learn about the great
00:15:20.27 transitions in the history of life. In particular, this one, the transition
00:15:23.28 from life in water to life on land. But the story doesn't stop there, it begins there.
00:15:29.10 Why? Because we can trace the bones in a creature like Tiktaalik.
00:15:33.09 LIke the arm bone, the upper arm bone, the humerus of Tiktaalik,
00:15:36.17 we can trace it all the way to other mammals and to people.
00:15:39.23 This story of the origin of the neck, the origin of a wrist, is not
00:15:44.27 just some esoteric event in the history of life. It's an event
00:15:48.19 embedded in our own bodies. The origin of the neck, the first time we see
00:15:52.09 in Tiktaalik and its evolutionary cousins, is something that is to become
00:15:55.09 our own neck. Likewise with the arm bones. So every time you
00:15:58.11 shake your head, every time you bend your wrists, you can say
00:16:01.07 "Thank you, Tiktaalik and its evolutionary cousins that lived in the
00:16:04.22 Devonian streams 375 million years ago."

This Talk

Speaker: Neil Shubin

Audience:

General Public
Educators of H. School / Intro Undergrad
Student
Educators of Adv. Undergrad / Grad
Researcher
Educators

Recorded: June 2015

Educator Resources for this Talk

Talk Overview

Ever since he was a graduate student, Neil Shubin has been obsessed with finding fossils of the creature that marked the transition from fish to land-dwelling animals. He explains how he scoured maps to find rocks of the right age and type that were accessible at the earth’s surface. This led him to the Canadian arctic where, in 2004, Shubin and his colleagues found Tiktaalik, a fossil of a creature with traits found in both fish and tetrapods. Tiktaalik was the link between fish and land animals.

Speaker Bio

Neil Shubin

Dr. Neil Shubin is a Professor in the Department of Organismal Biology and Anatomy and the Committee on Evolutionary Biology at the University of Chicago. Shubin’s research focuses on understanding the evolutionary origins of new anatomical features such as limbs. Shubin is well known for his discovery of Tiktaalik roseae,the 375 million year old fossil… Continue Reading