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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?

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    $\begingroup$ So, what makes you think your hypothesis ("features ... extremely beneficial for growing to massive sizes") is right in spite of being contradicted by reality? :grin: $\endgroup$ Feb 29 '20 at 9:39
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    $\begingroup$ @Adrian Colomitchi I know I’m wrong, I just want to know why I’m wrong $\endgroup$ Feb 29 '20 at 9:45
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    $\begingroup$ Maybe their genetics evolved into a dead-end ("5 pairs of sex chromosomes and that one of the X chromosomes resembles the Z chromosome of birds") and the evolved monotremes that you seek are the placentals and marsupials ("suggesting that the two sex chromosomes of marsupial and placental mammals evolved after the split from the monotreme lineage"). Otherwise, yes, the low competition from the others may explain why they survived at all in spite of not being "the fittest". $\endgroup$ Feb 29 '20 at 10:01
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    $\begingroup$ You state that egg-laying is less costly than pregnancy...how is this true? You have to consider that the egg must hold enough nutrients to fuel the growth of whatever is inside until it's ready to hatch, nutrients that must come from the mother. That egg must have fully as much energy and material put into it as a live birth, plus creating the eggshell for protection (in pregnancy, the mother's body does this instead). Egg-laying would seem to take more energy than pregnancy, not less; there are good reasons why evolution went from egg-laying to live birth, and never the reverse. $\endgroup$
    – Palarran
    Feb 29 '20 at 13:23
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    $\begingroup$ @Palarran Egg-laying re-evolved from live birth at least twice in lizards. What stops it from being more common is many of these species lose the complex adaptations that let them lay eggs (like shell production). Similar constraints are why there are no live-birthing birds (can't have viviparity with hard shells). Nothing to do with one being better than the other. $\endgroup$ Feb 29 '20 at 18:04
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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.

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  • $\begingroup$ “ What is keeping mammals from reaching sauropod size is probably the presence of teeth.” Well that’s convenient given the monotremes have no teeth as adults $\endgroup$ Mar 1 '20 at 6:23
  • $\begingroup$ "Australia is infamous for having a bad fossil record" what the heck this even means, mate? $\endgroup$ Mar 1 '20 at 7:27
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    $\begingroup$ @AdrianColomitchi Australia is infamous among paleontologists for having a bad fossil record. It has only a couple of good terrestrial Cretaceous sites (all of the same age), no good Triassic-Jurassic sites, and only a single site between the late Cretaceous and late Oligocene. It has good Devonian fossils and Miocene-Pleistocene record but that's it. That's terrible compared to every other continent except Antarctica. For mammals it's missing two-thirds of the continent's fossil history. $\endgroup$ Mar 1 '20 at 17:25
  • $\begingroup$ @user2352714 thanks $\endgroup$ Mar 1 '20 at 22:41
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    $\begingroup$ @NixonCranium The only monotremes that don't have teeth as adults are the myrmecophagous echidnas and the modern platypus. All extinct platypodes as well as stem-monotremes like Steropodon, Kollikodon, and Teinolophos have teeth. Modern monotremes are all specialized feeders on invertebrates so it's not clear how easy they could become herbivores (and the grinding plates of platypodes would pose the same problem with getting large as the complex teeth of other mammals/ornithischians). $\endgroup$ Mar 2 '20 at 3:33

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