Mammals (Part 2): Evolution of Mammals

Author: Malcolm Pullan, volunteer guideDate: October 2024

According to a very unscientific survey carried out by examining my gut feeling, many people can answer the question, “When did mammals first appear?”  Further similar unscientific analysis reveals that most of these answers will be something like, “About 60 MYA (million years ago), when dinosaurs became extinct”.  Unfortunately, this common answer is incorrect (in fact, it’s not even true either…).

The purpose of this article is to put the record straight—and to give a little evolutionary history of mammals as whole, concluding with a little look at the evolutionary history of Australian mammals.  The story of New Zealand mammals will follow on from this in final part of this series of articles.

Before beginning the discussion, I should warn you that the evolutionary history of mammals is an ever-changing minefield.  There are controversies over dating and controversies of definitions and names of different groups of mammals, on top of which new information is popping up all the time to throw spanners in the works.  Rather than trying to weigh up the recent academic literature myself (which is never really wise for a layperson), in what follows I will leave it to the experts (i.e. a well-respected and recent university-level textbook (1)).

With that proviso in mind, let’s begin at the beginning.  When did mammals first appear?  There are a range of figures out there.  All of them, though, are way beyond 60 MYA.  They’re actually in the region of about 205 to 225 MYA.  Far from arising when dinosaurs became extinct (which, to be precise, occurred about 66 MYA)(2), it’s actually more accurate to say the opposite—that mammals and dinosaurs both appeared at similar times! (3)

The range of dates for the first appearance of mammals wafting around seem to be just as much a result of different definitions of mammals as anything else.  I’m sure no-one reading this will want to get bogged down in semantics for the sake of a few million years, so I’m just going to take one of the common definitions of mammals—the group of animals known scientifically as Mammalia.  This group first appeared about 205 to 210 MYA and is quite easy to define. It’s the common ancestor of all living mammals and all descendants of that common ancestor. In other words, it’s all living mammals, along with all those animals in the same family tree that have become extinct.

Figure 1 shows a basic evolutionary diagram for mammals (i.e. Mammalia).  Living mammals fall into two main groups: Prototheria and Theria, with Theria being split further into Methatheria and Eutheria.  The living members of each of these groups are:

  • Prototheria:  Monotremes (egg laying mammals), i.e. echidnas and the platypus (see Figure 2).
  • Metatheria:  Marsupials (mammals with pouches for young), e.g. kangaroos and koalas (see Figure 3).
  • Eutheria:  Placentals (4), i.e. all other living mammals, e.g. rats, cows, bears, dolphins, humans… (see Figure 4).

Although it goes without saying there is considerable debate about when each of these separate groups originated, it is generally agreed that they all predate the extinction of dinosaurs by at least several tens of millions of years (see Figure 1).  So where did this common misperception about mammals only evolving after the extinction of dinosaurs come from?  Well, it arose because of what happened after the dinosaurs became extinct.  When dinosaurs were around, they ruled the world.  There were big ones and small ones, occupying all manner of niches.  This would have made it pretty difficult for any other animal to find a place to live—except… Dinosaurs were mostly daytime creatures.  The night-time was generally free of those pesky dinosaurs, especially if you were going to be very small and crawl around quickly.  So that’s what early mammals did.  They were generally small nocturnal, shrew-like creatures (see Figure 5)(5).  Even by the time dinosaurs became extinct, mammals hadn’t become any bigger than the size of a badger (6).  But when the dinosaurs did become extinct… It was party time!  Suddenly there were loads of free niches, especially in the daytime.  Although many mammals also died off with the dinosaurs (7), there were still plenty around to party.  Those that remained quickly evolved to exploit the new niches left by the dinosaurs.  Having said that, some mammals were better party animals than others.  Monotremes stayed in their own quiet corner and carried on much as before.  They’ve never amounted to much.  Metatherians (marsupials) had a bit of a party.  But it was the eutherians (placentals) who were the real party animals.  Placentals really took off after the extinction of the dinosaurs and diversified into many different groups.  And this is where the idea that mammals only evolved after the extinction of dinosaurs comes from.  It was only after the extinction of the dinosaurs that mammals (i.e. placentals) really began to take on the shape, size and forms that we are familiar with today.  It was only once the dinosaurs disappeared that mammals (i.e. placentals) could rise to rule the world as they do today.

So that’s the evolutionary story of mammals living today in (very) brief.  In order to talk about New Zealand’s past though, there’s a bit missing.  Once upon a time there was a successful and wide-spread mammalian lineage sitting between Prototheria and Theria called Allotheria (see Figures 6 and 7).  This lineage suffered badly in the mass extinction event that killed off the dinosaurs.  The remaining allotherians managed to party a bit afterwards, but in the end they were probably outcompeted by placentals, and they became extinct by about 30 to 35 MYA.  But that’s not all.  Allotheria wasn’t the only mammalian lineage sitting between Prototheria and Theria.  There were, in fact, many more mammalian lineages sitting between the two that have long since become extinct (imagine a diagram like Figure 6, but with lots of branches in it).  It’s just that Allotheria is probably the most well-known.  Since I’m not being particularly formal, I’m going to call all the lineages between Prototheria and Theria, “Allotheria (and whānau)”.  (Please don’t tell any evolutionary biologist I’ve just written that.)(8)

In summary, mammals have been around for hundreds of millions of years.  Up until the extinction of the dinosaurs most were very small, and none were big.  Mammals divided into the extant groups prototherians (including monotremes), metatherians (including marsupials) and eutherians (including placentals) long before the extinction of the dinosaurs.  After dinosaurs became extinct one group in particular took off, diversified and took over—and that is the placentals.  Prior to the extinction of the dinosaurs there were also many other mammalian lineages as well (which I’m calling “Allotheria (and whānau)”), some of which were quite successful in their time.  However they have long since become extinct.

Mammals in Australia

With that global picture in place, let’s have a quick look at the story of mammals in Australia.  As noted already, this will give the background for the New Zealand story in the next article.  The Australian story can be summarised under the headings of the main mammalian groups.

  • Prototheria (monotremes):  These originated in the Australian region a very long time ago and have always been there.
  • Allotheria (and whānau):  There are a couple of fossils of these in Australia from about 120 MYA.
  • Metatheria (marsupials):  This one is interesting, because, although Australia is known for its marsupials, Australia is not where they originated.  The oldest marsupial fossils are actually from North America.  However it was in South America where marsupials first took off properly.  From there they made their way to Australia.  The earliest marsupial fossils in Australia are from about 55 MYA.  How did marsupials get to Australia, you may well ask?  As seen in Part One of this series of articles, at 55 MYA Australia was joined to Antarctica.  What wasn’t shown in Part One was that at the time Antarctica was also joined at the tip to South America, or at least almost so.  It seems, then, that sometime around 55 MYA marsupials travelled from South America, to Antarctica and then on to Australia (Antarctica was much warmer then and covered in forests).(9)
  • Eutheria (placentals): There is single fossil in Australia from about 55 MYA that many claim is a placental.  However this is not definitive.  Definitive placental fossils don’t appear in Australia until much later.  There is a fossil of a bat from 26 MYA.  As with the marsupials, these bats probably originated in South America.  When it comes to non-flying mammals in Australia, the first definitive placental fossils are from around only 4.5 MYA.  These (rodents) originated from South East Asia.  The reason it took so long for these mammals to get to Australia is that Australia and South East Asia used to be a long way apart.  The two moved closer together over a period of millions of years, and it would seem it is only in the recent geological past that the two have been close enough for animals to jump from one to the other.

And that’s the Australian story.  If you’ve made it up to this point, you might be putting two and two together and feeling somewhat perplexed.  Why on earth aren’t there any native (non-flying) mammals in New Zealand?  If mammals have been around for 200 or so million years, and they were in Australia when New Zealand was still part of Australia, why weren’t they in New Zealand when it broke away?  Or perhaps they were?  If so, whatever happened to them?

Tune in next time for the final episode of the story of mammals in New Zealand.

Footnotes:

  1. Vertebrate Life (11th edition) by Pough et al.
  2. And to be further precise, we’re talking here about non-avian dinosaurs, i.e. dinosaurs that are not birds. Birds are actually dinosaurs, although most people aren’t aware of that. But that’s a story for another article…
  3.  Dinosaurs first appeared about 230 to 245 MYA.
  4.  As the name suggests, during gestation placentals attach to the mother via a placenta. However, marsupials also have placentas. What is different is that a marsupial placenta is generally less well-developed than a placental’s. A marsupial also has a very short gestation period and its placenta is only present for about 20% of that time. Conversely, a placental has a comparatively long gestation time and the placenta is present for most of it.
  5.  This explains the exceptional sense of smell and hearing most mammals have today (humans, and related primates, have a very poor sense of smell compared with most mammals), not to mention good night vision. Incidentally, even today most mammals are small and nocturnal.
  6.  This was Repenomamus giganticus from about 125 MYA.
  7.  There has been much debate about what caused this general mass extinction. There is no doubt an asteroid hit the earth 66 MYA and caused major devastation. Did this cause the mass extinction? The best answer is probably that it was the final straw in a period in earth’s history when life as a whole was under considerable strain. This strain was possibly caused by prolonged and intense volcanic activity. 
  8.  Who made that label bold?
  9.  Incidentally, monotreme fossils have been found in South America, so there was clearly two-way traffic of mammals between South America and Australia via Antarctica. Also incidentally, before humans arrived in Australia about 40,000 to 50,000 years ago there were many more species of marsupials there than there are now, including marsupial lions and giant kangaroos. 

References:

Gibbs, G. 2016. Ghost of Gondwana: The History of Life in New Zealand (revised edition). Potter & Burton, Nelson. (pp. 72–74)

Lomolino, M.V., Riddle, B.R. & Whittaker, R.J. 2018. Biogeography: Biological Diversity across Space and Time (5th ed.). Sinauer Associates, Sunderland, MA. (pp. 247–249)

Pough, F.H, Bemis, W.E, McGuire, B. & Janis, C.M. 2023. Vertebrate Life (11th ed.). Sinauer Associates, New York. (p. 233–234, Chapters 22 & 23)

Thomsen, T. 2021. The Lonely Islands: The evolutionary phenomenon that is New Zealand. Reed New Holland Publishers. (pp. 94–96, 194–196)

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