7.7. Mammal-Marsupial Exchange: There Goes the Neighborhood

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En provenance du cours de University of Illinois at Urbana-Champaign

Emergence of Life

58 notes

University of Illinois at Urbana-Champaign

Emergence of Life

58 notes

How did life emerge on Earth? How have life and Earth co-evolved through geological time? Is life elsewhere in the universe? Take a look through the 4-billion-year history of life on Earth through the lens of the modern Tree of Life! This course will evaluate the entire history of life on Earth within the context of our cutting-edge understanding of the Tree of Life. This includes the pioneering work of Professor Carl Woese on the University of Illinois Urbana-Champaign campus which revolutionized our understanding with a new "Tree of Life." Other themes include: -Reconnaissance of ancient primordial life before the first cell evolved -The entire ~4-billion-year development of single- and multi-celled life through the lens of the Tree of Life -The influence of Earth system processes (meteor impacts, volcanoes, ice sheets) on shaping and structuring the Tree of Life This synthesis emphasizes the universality of the emergence of life as a prelude for the search for extraterrestrial life.

À partir de la leçon

Week 7 - Cenozoic Mammals and Global Environmental Change

With the demise of the dinosaurs, mammals rapidly radiated and diversified during the Cenozoic. The combination of abundant food sources and significant fluctuations in global climate fostered extreme variations in morphology, body size, and interaction with the environment. You'll learn why mammals had a distinct advantage over previous groups, and you'll also have the opportunity to explore multiple theories on primate and hominoid evolution in the context of global conditions during this period.

7.1. House of the Rising Mammals4:54

7.2. Mammals Conquer the Earth7:31

7.3. Big is Beautiful6:05

7.4. Sophistication of Mammal Skulls8:16

7.5. Mammalian Dentition: My, What Big Teeth You Have!11:25

7.6. Mammals: Taxonomy is in the Eye of the Beholder12:19

7.7. Mammal-Marsupial Exchange: There Goes the Neighborhood7:56

7.8. The Primates11:38

7.9. The Rise of Humans: Getting a Leg Up on the Competition9:37

7.10. Early Hominids: Trees versus Savannahs8:17

Rencontrer les enseignants

Bruce W. Fouke, Ph.D.
Director of the Roy J. Carver Biotechnology Center
Department of Geology, Department of Microbiology, and Institute for Genomic Biology

[MUSIC]

Remarkably, in history, we have see the continents move apart and come together,

and move apart and come together in different places around the world.

And during the Cenozoic time period when these large scale crusto processes and

mechanisms were in motion, they had a dramatic effect on what was going on in

terms of global evolutionary biology.

And so one of the mechanisms that was laid out by Darwin as a way to

generate new types of species was something called allopatric speciation.

And that is the idea that once you have some kind of a physical process,

like a mountain built, a new glacier forming,

volcanics that shut off land areas where you could have migration.

You have flooding and suddenly have a new ocean pathway that was formed.

Any kind of a physical barrier that could prevent the migration of organisms back

and forth would be a good way to have the gene pools,

via sexual reproduction isolated from each other.

And then generate a new group of organisms.

And so, this idea of allopatric speciation, geographic separation,

enhancing the ability to radiate new species, has been a really important

tenet, in looking at the, evolution of macro eukaryotic organisms.

So we've seen that played out extremely well in

terms of North and South America as one example.

So let me just go through the details of this South America's contextual stage.

And we'll see how the drama played out over the organisms between the two

different continents.

So with respect to South America,

now South American rifted from Africa in about the late Cretaceous, so

again this is a late Cretaceous going into Cenozoic ongoing process that took place.

And once that we had the separation from Africa and

South America physically, then we started seeing that marsupials and

placental herbivores migrated to South America.

And again, this all took place at the end of the Cretaceous.

And once that isolation continued and the ocean became larger,

the southern part of the Atlantic, then that isolation locked in and

the allopatric speciation kicked in.

And so, we saw this remarkably play out for South America.

So, the mammals were dominant then and remained on Africa.

And then the marsupials took over and

capitalized on the South American continent.

Remarkably, we saw all kinds of evolution of marsupials which ended

up looking a lot like the placental mammals that had remained back on Africa.

But they filled the same kind of ecosystem space, but

they were definitely not placental mammals, they were marsupials.

And some of the largest carnivores that we've had on the planet in the Cenozoic

time period were actually sabertoothed marsupials, and

we'll see that in just one moment.

So if we look at kind of an overlay of what some of these organisms look like

we'll see that we had a one for one counterpart between Africa and

South America.

And in terms of the wolf like organisms,

the placental wolf, there was a Tasmanian wolf.

Again, it was a marsupial, not a placental mammal.

A flying phalanger versus a flying squirrel.

We had an anteater develop that were both marsupial and placental.

And then we had, as an example, moles that were developed, that evolved that were

both either placental or marsupial, so we had this one for one exchange.

And we've seen this before when the same kind of ecosystem allows the radiation and

dominance of a different organism that looks somewhat

similar to what was there before.

We've also seen this play out on the stage of Australia, of course, and

even until recently, Australia has been dominated by the marsupials, kangaroos,

wallabies, wombats, phalangers, koala, all kinds of organisms.

So, but what's happening in the modern day, is that human beings introduced

invasive species, and these species have out competed many of the marsupials.

So it's causing some dramatic shifts in the ecosystems of Australia.

So, going back to North and South America,

there was this great drama that was played out.

When we had the two continents go from being isolated,

the placental mammals were dominating on the North American continent,

similarly to what happened in Africa.

And, again, the South American continent,

when it separated the marsupials, dominated there.

And we call this the great American interchange of the mammals.

So before the isthmus of Panama in Central America rose up in the Cenozoic,

those two continents were isolated.

Once that rose, then it was a pathway for organisms to intersperse and

come together.

So that land bridge allowed the invasion marsupials to the north, and

the placental mammals to the south.

Now unfortunately for the marsupials,

whenever marsupials compete with placental mammals, we find that they don't do well,

and they get out competed, and are often times driven to extinction.

And we saw this played out very quickly in the Paleocene time

period when the Central American land mass was brought up.

Now, some of the things that migrated back and

forth from North America to South America were the bears, the camels,

the cats, the deer, the dogs, raccoons, skunks, tapirs.

Some of the organisms that we're familiar with still from the modern day

North American template of what lives here.

The ones moving northward, anteaters, armadillos, opossums, porcupines, sloths.

We had this great exchange that took place across this interface.

Unfortunately again, the marsupials consistently

lost out in most of these areas where this change took place.

But one of the ones that are most remarkable that I want to point out

are the marsupial saber tooth tigers.

Now when you say the word saber toothed tiger it brings up

an immediate image, right?

The large fangs and the incisors as we see in this reconstructed diagram.

And fearsome predators that are dominating the landscape.

But even more fearsome than the North American placental sabre-tooth cats were

the South American marsupial sabre-tooth cats.

And more fearsome because they were even larger but

also they had even bigger fangs that came down.

Their canines were even bigger and they were so

large that they evolved a sheath, so as we see in these fossils here,

there is a bone sheath that developed down from the jaw.

And so when the incisors were closed, when the mouth was closed,

they were so long and strong and sharp

that they developed these bone masses that keep themselves from stabbing themselves.

So, here's a reconstruction of one of the South American marsupial sabre-tooth cats.

And these were actually larger than the North American sabre-tooth placental cats.

But in the end they lost out, they were unable to compete on a consistent basis.

So, again the concept of geology, the moving of the continents,

the development of ice sheets and volcanics, and all kinds of

physical impediments to having organisms move back and forth and be able to

interbreed between themselves has been very important in geological time.

But then, equally important, is having these physical barriers dropped.

And in the case of North and South America,

this land mass developing that allowed the exchange of the mammals and

seeing the mammals from the placental lineages out competing the marsupials.

[MUSIC]

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