Biogeography: Why are there so many marsupials in Australia?

Marsupials are a group of mammals generally characterized by having their young develop in a pouch, known as a marsupium. Besides this trait, there isn’t anything obvious about them that screams out that they are more closely related to each other than they are to other mammals. When you place a kangaroo, Tasmanian devil, koala, marsupial mole, antechinus, bilby and cuscus side-by-side, they probably don’t strike you as cousins.

And yet, when we look at their DNA, we find that they are more genetically similar to each other than they are to other mammals. Below is a phylogeny [1] estimated from thousands of letters of DNA, and you can see that the marsupials (at the top in purple) cluster together to the exclusion of all other mammals.

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What’s particularly interesting about their genetic similarity, is that where they live also seems to point to a common ancestry. Some marsupials, namely shrew opossums, opossums, and the monito del monte, live in South America and, in the case of the Virginia opossum, North America as well. The rest of the marsupials all live in Australia and the surrounding islands. Below is a world map showing the distribution of modern day marsupials.

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What adds to this interesting pattern is that when you look at the marsupial phylogeny above, the American marsupial lineages (Caenolestidae, Didelphidae+Caluromyidae, Microbiotheriidae) split off one by one. All of the remaining marsupials, which are clustered together, hail from Australasia. To state this in no uncertain terms: Australasian marsupials are more genetically similar to each other than they are to American marsupials.

So here we have an example where both biogeography and DNA tell the same story: a single marsupial ancestor colonized Australasia and then split into many different species with a multitude of distinct body forms, ranging from kangaroos to marsupial moles and koalas to cuscuses.

Questions for Creationists

Why do marsupials, despite looking so different from each other, have such similar DNA? If God created the DNA ‘blueprint’ for all life, and DNA makes bodies look the way they do, shouldn’t marsupial moles have DNA more like other moles, and Tasmanian devils have DNA more like other carnivorous mammals? Why do most marsupials live in and around Australia? Is it just a coincidence that all Australasian marsupials are more genetically similar to one another than they are to American marsupials? If they didn’t evolve from a common ancestor, did they all walk together from Noah’s ark to Australia? If people brought them to Australia, why did they mostly only bring marsupials?

References

1. Meredith, R. W., Janečka, J. E., Gatesy, J., Ryder, O. A., Fisher, C. A., Teeling, E. C., … & Rabosky, D. L. (2011). Impacts of the Cretaceous Terrestrial Revolution and KPg extinction on mammal diversification. Science334(6055), 521-524.

Photo credit

Kangaroo, Tasmanian devil, koala, antechinus, bilby, cuscus

Biogeography: The biggest birds all live down south

As described in my previous post, DNA and anatomy suggests that large flightless birds, such as ostriches, emus and cassowaries, evolved from a common ancestor shared with tinamous, a group of quail-like birds. These birds, known as palaeognaths, all share a relatively reptilian-like jaw, unlike most other modern birds. Other large-bodied palaeognaths are known from the fossil record, some of which have gone extinct very recently.

One such lineage included the New Zealand moa (Dinornithoformes), a group of nine described species that reached up to 12 ft in height. The moa are the only birds known to have completely lost their wings, presumably as a result of remaining flightless throughout much of their evolutionary history. Though a few reports suggest they may have survived until the 1800’s, they effectively went extinct around 1440. This occurred less than 200 years after the Maori people arrived in New Zealand, suggesting overpredation led to their demise.

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A female (larger) and male (smaller) South Island giant moa (Dinornis robustus), with a  pigeon for comparison.

Another major lineage of recently extinct palaeognaths is the elephant birds (Aepyornithoformes). These were similarly enormous birds, weighing up to half a ton, that lived on Madagascar and became extinct in the 1600’s or 1700’s.

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Aepyornis maximus

Elephant birds maintain the world record for the largest known bird eggs: up to 3.3 ft in circumference, 13 inches long, 22 lbs, and a volume of approximately 160 times that of a chicken egg.

Christie's specialist James Hyslop holds a chicken egg next to a pre-17th century, sub-fossilised Elephant Bird egg in London

In addition to the New Zealand Moa and Malagasy elephant birds, all of today’s living palaeognaths live on southern continents: Africa (ostriches), South America (rheas, tinamous), and Australia, New Guinea and New Zealand (cassowaries, emu, kiwis). This is significant because all of these species are more genetically similar to each other than they are to any other birds.

One evolutionary model posits that a flighted palaeognath ancestor lived in the southern hemisphere, its descendant lineages dispersed to various southern continents, and many of these species became flightless over time. A flighted ancestry would explain why large-bodied palaeognaths possess tiny wings, even though they are incapable of flight. Alternatively, a creationist model suggests that all palaeognaths coincidentally headed south from Noah’s Ark, and flightless species somehow crossed oceans to arrive in Madagascar, New Guinea, Australia, New Zealand and South America.

Questions for Creationists

Why do all of the palaeognath birds live on southern continents? How did the flightless species cross oceans to get where they are today? Is it a coincidence that they are both more genetically similar to each other than to other birds and they all live on southern continents? Why would God create birds with wings that don’t allow them to fly?

Photo credit

Moa, elephant bird, elephant bird egg

 

Biogeography: Did 189 gecko species migrate to Australia together?

Lizards have a widespread distribution, having conquered much of the earth, but certain groups of lizards are localized to a single continent. Here I illustrate an example from geckos.

Carphodactylidae includes 28 species of geckos, all of which inhabit Australia. Species include the long-necked Northern leaf-tailed gecko (Orraya occultus)

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and the smooth knob-tailed gecko (Nephrurus laevissimus).

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Then there is Diplodactylidae, a family of 126 species of geckos that live in Australia, New Zealand and New Caledonia, an island east of Australia. This group includes the crested gecko (Correlophus ciliatus)

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and the Northern spiny-tailed gecko (Strophurus ciliaris).

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Finally, a group known as Pygopodidae, the 35 species of legless geckos, is found in Australia and New Guinea. Below is the hooded scaly-foot (Pygopus nigriceps).

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These anatomically disparate gecko species have something in common besides all living in or very near Australia: they are all more genetically similar to each other than they are to other lizards or even to other geckos. Below is a molecular phylogeny by Pyron et al. [1] containing 4161 species of lizards and using up to 12896 letters of DNA for comparison. What you should notice is that the Australian geckos, Carphodactylidae, Diplodactylidae and Pygopodidae, are all clustered together at the top of the phylogeny to the exclusion of all other geckos (Eublepharidae, Sphaerodactylidae, Phyllodactylidae, Gekkonidae).

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In the context of evolutionary theory, this has a very simple explanation: an ancestral gecko species invaded Australia and diversified into the many Australian geckos that survive today. An alternative creationist hypothesis posits that 189 species of gecko (along with any extinct forms) all walked/slithered across Asia and crossed some portion of the Indian ocean together. This hypothesis suggests that it is simply a coincidence that they are all genetically more similar to each other than they are to other geckos.

Questions for Creationists

If these geckos were on Noah’s ark, how would they know to head to Australia together? Did they swim across the ocean together? Why did they not head elsewhere in the world? Why are they more genetically similar to each other than to other geckos?

References

1. Pyron, R. A., Burbrink, F. T., & Wiens, J. J. (2013). A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC evolutionary biology13(1), 93.

Photo Credit

Orraya occultus, Nephrurus laevissimus, Correlophus ciliatus, Strophurus ciliaris

Biogeography: Xenarthrans have been stuck in the New World for their entire existence

Sloths, armadillos and anteaters (xenarthrans), as different as they look, are genetically more similar to each other than they are to any other mammals. This suggests that these three very different groups of animals descended from a common ancestor. If this common ancestor lived on an isolated island, we would expect that all of its descendants might live on that same island.

Interestingly, all modern and extinct xenarthrans indeed live(d) on a very large island: the Americas. Based on fossil evidence, we think that the earliest xenarthrans lived in South America and some of them later dispersed North, particularly the extinct ground sloths, glyptodonts and pampatheres.

Xenarthra

The only living xenarthran that naturally occurs on the northern continent is the nine-banded armadillo (Dasypus novemcinctus).

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This makes sense as an evolutionary biogeographic scenario: xenarthrans originated in South America, diversified into many species, and dispersed to North America. They probably didn’t travel farther than this simply because the Americas are separated from other continents by oceans.

Questions for Creationists

Is it simply a coincidence that sloths, armadillos and anteaters are genetically similar and they all live in the Americas? If all xenarthrans lived on Noah’s Ark and left from it after the floodwaters receded, how did they get over to North and South America? Why did they go to North and South America at all? How did the slow-moving tree sloths make their way across such a vast distance? Given their poor vision, how did the xenarthrans effectively navigate such a tremendous journey?

Biogeography: Whales originated in Pakistan

As I was composing my previous three posts, I searched the paleobiology database for information regarding the geological layers in which the various primitive whales were found. As I recorded the chronology of these fossils, I happened to notice an interesting pattern regarding their biogeography that is consistent with the theory that whales descended from terrestrial mammals. I will illustrate this by re-listing the fossils in the order described in the previous posts, along with the recorded localities in which they’ve been discovered. In parentheses are the estimated time periods of the geological ranges they were found in millions of years before the present. Try to see if you notice the same pattern that I did.

1. Terrestrial to semi-aquatic species

Indohyus: India and Pakistan (55.8-40.4 Ma)

Pakicetus: India and Pakistan (55.8-40.4 Ma)

Ambulocetus: Pakistan (48.6-40.4 Ma)

Remingtonocetus: India and Pakistan (48.6-40.4 Ma)

2. Species with evidence of strongly aquatic adaptations, e.g., presence of tail flukes, early formation of blowholes

Rhodocetus: Pakistan (48.6-40.4 Ma)

Artiocetus: Pakistan (48.6-40.4 Ma)

Protocetus: Egypt (48.6-40.4 Ma)

Georgiacetus: Southeastern USA (48.6-37.2 Ma)

3. Fully aquatic species, e.g., vestigial hind limbs

Dorudon: Egypt, New Zealand, SE USA, Western Sahara (40.4-33.9 Ma)

Basilosaurus: Antarctica, Egypt, Jordan, Pakistan, UK, Southeastern USA, Western Sahara (40.4-33.9 Ma)

Simocetus: Oregon (33.9-28.4 Ma)

Waipatia: New Zealand (27.3-25.2 Ma)

Janjucetus: Australia (28.4-23.03 Ma)

Mammalodon: New Zealand, Australia (25.2-23.03 Ma)

Aetiocetus: Japan, Mexico, Oregon (33.9-23.03 Ma)

Eomysticetus: South Carolina (28.4-23.03 Ma)

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What should be taken from this is that the earlier whale species, which were still terrestrial or only semi-aquatic, were all found in Pakistan and India. When whales started becoming more aquatic, as demonstrated by the presence of tail flukes and the early formation of blowholes, some of them were still found near Pakistan while others began to disperse to other locations. By the time fully aquatic whales appear, their distribution is circumglobal. From an evolutionary perspective, this makes sense –terrestrial mammals would have a more difficult time dispersing from Pakistan and India to other continents. But once certain key innovations evolved, particularly to facilitate a predominantly aquatic lifestyle, dispersal became much easier for these primitive whales.

Questions for Creationists

Why does it appear that the earliest whale-like mammals all lived in one location (Pakistan and India), whereas those that have more aquatic specializations are found in other parts of the world? If all mammals left Noah’s Ark, why did all of the primitive whale-like mammals head to Pakistan and India and not to other places in the world?

Why biogeography matters

Another theme I will discuss in this blog is biogeography. Biogeography is the study of where organisms live and why they live there.

Why are biologists interested in biogeography?
Evolutionary biologists believe that organisms have been limited in their dispersal to different environments by barriers. For example, shellfish that live in the Pacific off the coast of Central America have difficulties dispersing to the Atlantic due to the Isthmus of Panama. In fact, scientists think that the formation of the Isthmus of Panama some 3.5 million years ago created a barrier to dispersal for marine organisms while simultaneously allowing for increased dispersal for terrestrial organisms, leading to the Great American Interchange of North and South American fauna.

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Biologists and paleontologists study the geographic distribution of living and fossil organisms, plate tectonic movements and the relationships of these organisms in attempts to understand why animals live where they do today. We believe that the evolution of organisms paired with plate tectonic theory, the rising and lowering of sea levels and each species’ ability to disperse best explains the geographic distribution of life.

Creationists need to think about the distributions of living and extinct animals in the context of Noah’s Flood. If the ark contained all terrestrial animals that have ever lived, then Creationists need to ask themselves: 1) how did all of the animals find their way to the ark (i.e., did they all live in the Middle East)?, 2) how did the animals reach the areas of the globe that they did (e.g., how did sloths get to South America)?, and 3) why do some animals that are very similar to each other live in only very specific parts of the earth as opposed to possessing a circumglobal distribution?

Though Creationists typically think of evolutionary theory in the context of things like fossils, biogeography is a very important aspect of the history of life and needs to be properly understood in order to consider evolutionary theory vs. creationism.

Questions for creationists

If life evolved, what would you predict in terms of biogeographical patterns? If life was created within the past 6,000-10,000 years how would it be different? How may that be altered when factoring in Noah’s Ark?