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Without a doubt, the most iconic mammals of Australia are the pouch-bearing marsupials. You can find less than 250 species in that one island-continent.

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Marsupials have been around for 65 million years, and they are all that's left of a much larger clade called Metatheria, the entire bag of pouch-bearing mammals.

However, Australia is also home to a smaller, more ancient order of mammals--the monotremes, the only extant mammals to retain their reptilian ancestral trait of laying eggs.

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In this alternate history scenario, the metatheres either never existed or have been extinct for tens of millions of years, leaving the monotremes as the next best thing. Once there, considering that monotremes were never ecologically popular (only 16 species so far identified in the group's 210-million-year existence), will the egg-layers radiate into a vast diversity of shapes, sizes and characteristics? Or will they just be incomprehensibly alien variations on the platypus and echidna themes?

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  • $\begingroup$ "will the egg-layers radiate into a vast diversity of shapes, sizes and characteristics? Or will they just be incomprehensibly alien variations on the platypus and echidna themes?" I can't think of any good reason why the former rather than the latter would be true. Monotremes were suppressed by competition from more fit placental and marsupial mammals, not because they had an inherently unworkable reproduction method (which worked fine for reptiles which were very diverse). $\endgroup$ – ohwilleke Nov 12 '16 at 8:15
  • $\begingroup$ You need some reason why egg laying would retain some advantage in context. $\endgroup$ – JDługosz Nov 12 '16 at 10:50
  • $\begingroup$ @ohwilleke That may be the case here, but in this scenario, modern Australia has no marsupials, and the only placentals are bats, seals and whales. $\endgroup$ – JohnWDailey Nov 12 '16 at 14:48
  • $\begingroup$ @JohnWDailey. Agreed, that is why I think that monotremes could thrive and diversify in your world. $\endgroup$ – ohwilleke Nov 12 '16 at 16:27
  • $\begingroup$ Will someone please explain before voting to close? $\endgroup$ – JohnWDailey Nov 13 '16 at 15:31
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In any ecosystem, the existing animals will evolve to occupy all the existing ecological slots. The marsupials did this in Australia, so you can see, for example, a marsupial "bat" (the Flying Fox), a marsupial "big cat" (the Tasmanian tiger), a marsupial scavenger/omnivore (the Tassie Devil), and so on.

If the marsupials didn't exist, monotremes would be able to occupy these niches.

There was also marsupial megafauna when human beings and dogs arrived in Australia about 50,000 years ago, so there would likely be monotreme megafauna unless a large predator species arrived in the ecosystem.

If you don't have a large predator species, you will also have other species occupying the "lazy" ecological niches that are not viable in the presence of animals like cats and dogs, as they did in New Zealand - lots of slow-moving, flightless birds and big lazy lizards.

Of course, you could evolve a monotreme large predator. A megafauna Tasmanian Tiger with the poisonous foot-spears that the platypus has would be fearsome indeed! Or one based on the echidna ancestor that can shoot its spines like some porcupines can ...

You could potentially also evolve fully aquatic monotremes - they would just need to evolve some arrangement for carrying their eggs with them in the water. Since they already have a pouch, all it would take is an extra skin fold to direct the newly-laid eggs into the pouch.

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It looks like you're wondering about the course of the monotreme's evolution. For a short answer? Yeah, if the monotremes were the dominant species in Australia, their evolution would explode with variety.

The first monotremes appeared in Australia about 123 Ma, and it was called the Teinolophos trusleri.

The first metatheres appeared in China some time before this, however, the first Australidelphia, which are the marsupials of Australia, would not appear until 55 Ma (the Djarthia).

In this beginning, the monotreme's evolution was formidable. In 105 Ma, the Steropodon galmani were around, a species of monotreme that was as big as a cat. This may not seem like much, but for the time period this was massive!

However, in 105 Ma, dinosaurs still roamed the earth. In fact, the T-Rex hadn't even appeared yet! It wouldn't be until the Cretaceous-Paleogene extinction event in 66 Ma that mammals would inherit the Earth.

If you had the great extinction wipe out the potential for the Australidelphia along with the dinosaurs, then you'd be set for humongous monotreme growth.

When you asked whether there would be a massive diversity of monotremes, without competitors to stunt their growth, the answer is a definitive yes. The marsupials came in and essentially stole the resources from the monotremes, stunting their evolution and causing most of them to die out. If this never happened, then monotremes have the entirety of Australia essentially to themselves.

With this in mind, the flourish of evolution that created everything from sugar-gliders to kangaroos, could have occurred to the monotremes instead. Sixty-something million years is a lot of time to create a vast array of shapes, sizes, and characteristics! (Just look at what the other types of mammals have accomplished!)

Hope this helped!

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yes the egg layers would radiate into a wide range of niches and forms. The big thing is no whales or dolphins since they will still have to come out on to land to lay eggs.

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