What Prevents Gigantic Monotremes From Evolving?

Question

Monotremes are the oddballs of the mammal family, the echidnas and especially the platypus are absolutely absurd creatures with their bizarre anatomical features and mishmash of reptilian and mammalian traits. Of particular note is that they lay eggs and run a much lower core body temperature than other mammals.

These traits at first appeared to be nothing more than a a curiosity and sign of their primitiveness, but I pondered the advantages such a “toolkit” might bring. I came to the conclusion that monotremes actually have features that would be extremely beneficial for growing to massive sizes that placental mammals cannot reach.

Although the idea seems absurd on its face, we can examine what constrains the size of placental mammals and examine traits of what we know to be the largest land animals to ever exist; the sauropod dinosaurs.

Placental mammal size is quite limited compared to the dinosaurs for three primary reasons; metabolism, reproduction, and biomechanics. Placental mammals have to eat a lot of food to keep their fast metabolisms going, while reptiles and monotremes need to eat a lot less. Pregnancy in placental mammals is arduous on the mother and simply put the larger the baby the longer and more costly the pregnancy. Egg laying gets around this as it takes far less energy and maternal commitment. Dinosaurs also had a different bone structure and air sacs that allowed them to grow larger, so both placentals and monotremes lose out on those.

I’m not expecting the monotremes to reach sauropod size, but given that they have the metabolism and reproductive method suited for large sizes of expect them to at least have be reaching larger sizes than the placentals.

But my conjecture isn’t borne out by evolutionary history, as the largest monotreme was about sheep sized. So what prevents the monotremes from getting big besides getting pushed to the very fringe by the other two mammal families? What would it take for gigantic monotremes to evolve? Would being isolated on a continent free of other mammals suffice? Or is there something else?

Answer 1

Nothing is really preventing them from getting big

Something to point out first, nobody really knows what the monotreme family tree looks like. Their fossil record is restricted to two continents (South America and Australia) and Australia is infamous for having a bad fossil record. There are maybe twelve species of monotremes known in the fossil record, along with some loosely related forms in a broader group called Ausktribosphenida, and most of them are some type of semi-aquatic platypus-like form. In fact, echidnas are believed to have become secondarily terrestrial from an aquatic platypus-like ancestor.

We have pretty much no idea what was happening on land with mammals during the Cretaceous in Australia and Antarctica. For all we know there is some species of giant carnivorous monotreme akin to Repenomamus from China in Cretaceous polar Australia and we just haven't found it yet. Even in places where you had monotremes like South America there were often other groups like dryolestoids experimenting with terrestrial niches. What seems to have happened is marsupials and placentals moved into South America and Australia and prevented monotremes from really diversifying. The best way to get monotremes to diversify would be to either stop northern mammals from dispersing to South America around the K-T boundary, or stop them from dispersing to Australia, which would give monotremes a whole continent to play with (if they can outcompete the dryolestoids and gondwanatheres).

On the whole, there are no features to suggest that monotremes are constrained to be the size they are, but there is nothing to suggest they can be gigantic either. We simply have no idea as to this group's evolutionary limits.

What is keeping mammals from reaching sauropod size is probably the presence of teeth. The largest mammals (Paraceratherium) are about the same size as the largest non-sauropod dinosaurs (big hadrosaurs and ceratopsians, large theropods). It's been suggested that because mammals and many ornithischian dinosaurs all have complex chewing batteries this puts a constraint on their size, because their head increases disproportionally relative to size in order to chew efficiently enough to process food. Sauropods (and to a lesser degree stegosaurs) just swallow food whole, so they can get away with having tiny heads relative to their body size.