Mike Keesey  

Mammals of the Mesozoic

September 26th, 2006 by Mike Keesey :: see related comic

(You knew it had to happen, Debbie.)

Even though the Mesozoic is called the “Age of Reptiles” and the ensuing Cenozoic the “Age of Mammals”, there were a fair number of mammals around in the Mesozoic. Many people don’t realize it, but mammals and dinosaurs originated right around the same time as each other, in the early Late Triassic (or shortly before).

By the end of the Mesozoic (around Carney’s time), mammals had diversified into many of the groups we know today: monotremes, marsupials, xenarthrans, afrotheres, archontans, glireans, carnivorans, pholidotans, artiodactyls, perissodactlys, chiropterans, and lipotyphlans. Of course, they hadn’t diverged in appearance much—they all looked more or less like Ferd and Dan, and didn’t get any larger than small dogs.

One type of mammal relative did grow large enough to prey on dinosaurs, though. A specimen of Repenomamus robustus was found with a juvenile Psittacosaurus in its belly. Doubtless a tasty last meal.

No such luck for Ferd and Dan.


  • Hu, Y., J. Meng, Y. Wang, and C. Li. 2005. Large Mesozoic mammals fed on young dinosaurs. Nature 433:159–152
  • Springer, M.S., W. J. Murphy, E. Eizirik, and S. J. O’Brien. 2003. Placental mammal diversification and the Cretaceous–Tertiary boundary. Proc. Nat’l Acad. Sci. 100(3):1056–1061
16 Responses to “Mammals of the Mesozoic”
Albert (Netherlands) wrote:

I guess we have to live with it, Debbie.
Mike, there also was another “big” mammal, the Didelphodon from the late cretaceous if I remember it right..

Mike Keesey wrote:

(Repenomamus was probably not a mammal, but close.)

Yes, and also Didelphodon’s fellow metatherian, Alphadon, which is known only from teeth, but seems to have gotten rather large (relatively speaking, of course).

Albert (netherlands) wrote:

I wonder what your opinion is on the reason why mammals remained relatively small all through the mesozoic period. Is the fact that there were dinosaurs around really a good reason for that?

Debbie wrote:

Oh Albert, I suppose you’re right…it’s time to bow to the inevitable. Alas poor Ferd and Dan, I liked thee well!

Debbie wrote:

re: “Large Mesozoic mammals fed on young dinosaurs” — What a horribly morbid title for a paper!

“Placental mammal diversification and the Cretaceous–Tertiary boundary” is somewhat more uplifting, but just barely.

Mike Keesey wrote:

Paleontology is a morbid science! Basically we study remains of past tragedies. ;)

Mike Keesey wrote:

As to Albert’s question, I think that’s a perfectly good explanation. After the K/T K/Pg* event, the only dinosaurs left were some of the smallest ones: modern birds. Both birds and therian mammals made bids for the newly-vacated “large terrestrial animal” niches, and therian mammals generally did much better.

* “Tertiary” is now an antiquated term. Better to use “Paleogene” here.

Derksen wrote:

Any good resources for hypotheses as to why mammals might have outcompeted birds for “large terrestrial organism”? I mean, the birds definitely had a good start there with the terror-cranes in a world full of eohippae. I’d hate to fall back on the familiar relative cost of producing and defending an egg or eggs vs. marsupial offspring… it just doesn’t feel justifiable given that this model appears to have been competitively successful for dinosaurs… so are there better theories or papers you could reference that would be relevant?


Mike Keesey wrote:

I’m not aware of any papers on this. My purely off-the-cuff theory is that crown-avians have a lot of adaptations for flight that might compromise their ability to compete as land-dwellers (toothlessness, highly modified forelimbs, rigid skeleton, etc.), while mammals were always (up to that point) land-dwellers.

Derksen wrote:

I dunno… could one describe the birds that made it over the K/T as being relatively adaptive inflexible when compared to the early mammals of the period? Were birds as a group really ‘too derived’ to compete with the ‘generalist’ mammals? I’m not sure how one could really go about demonstrating this experimentally, either…

Thanks - you’ve given me an interesting question to ponder over tomorrow…

Mike Keesey wrote:

This question requires an environmental qualifier: the birds of the Danian Age were perfectly adaptable insofar as flying niches are involved. There are about twice as many bird species alive today as mammal species.

Mammals won the land, but birds already ruled the air.

Neutrino Cannon wrote:

There seems to be a maximum size that birds can attain. The largest dinornithids, dromornithids, and aepyornithids were all around the same size. Their eggs were all around the same size too, so that may have been the limiting biological factor.

Granted, one could argue that both New Zealand and Madagasgar were not capable of supporting larger animals than the moa and elephant bird respectively, but that clearly doesn’t hold up for Australia. Australia did support larger animals than the mihirung, namely the diprotodonts. Thus it seems that dromornis stritoni was not only the bird ever to have existed, it may well have been the largest bird biomechanically possible.

That constraint on size precluded the birds from ever being serious competators to the mammals in the large-bodied land niches.

Raymond wrote:

“By the end of the Mesozoic (around Carney’s time), mammals had diversified into many of the groups we know today: monotremes, marsupials, xenarthrans, afrotheres, archontans, glireans, carnivorans, pholidotans, artiodactyls, perissodactlys, chiropterans, and lipotyphlans. Of course, they hadn’t diverged in appearance much—they all looked more or less like Ferd and Dan, and didn’t get any larger than small dogs.”

Belated I know, but what was the ratio of sprawling mammals such as multis,monotremes and company compared to the parasagittal therians?

BTW, aside from the crown Monotremes,crown Metatheres, and a few descriptions of Eutherian Placentals like Xenarthrans,Afrotheres and possibly Lipotyphlans, you are treading on grounds eligible for rebuttals describing all these clades as Mesozoic in origin.Placentals may well have even been a minority,(significent nonetheless) among the Eutherians until the Eocene.

Mike Keesey wrote:

According to Kielan-Jaworowska and Hurum (2006), the parasagittal stance is a synapomorphy of Boreosphenida (including, but not limited to, Theria). As for the ratio of boreosphenidan to non-boreosphenidan mammals at a given time, I’m not exactly sure. All I know is that the earliest known boreosphenidans are Barremian (unless the Berriasian Tribactonodon is boreosphenidan), and nowadays we outnumber the non-boreosphenidan mammal species by about 4500 to 3. What the ratio was at the K/T K/Pg boundary, I’m not sure.

It’s possible for those placentalian clades to have originated in the Mesozoic and for placentals to have been a minority. See Springer et al. (mentioned above) for more on the diversification of placentals.


* Kielan−Jaworowska, Z. and Hurum, J. H. 2006. Limb posture in early mammals: Sprawling or parasagittal. Acta Palaeontologica Polonica 51(3): 393–406.

Raymond wrote:

Ah, thanks for the reminder regarding Boreosphenida.

Thanks for the Springer link!:)

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