I googled and googled, with no luck. Did the LCE of marsupials and placentals reproduce more like one than the other? A combination? Some other method? Or do we not know?
Probably like the echinda or spiny anteater, which are mammals that lay eggs.
Googling did tell me that marsupials and placental mammals are much more closely related to each other than either are to monotremes (egg-laying mammals), so it seems more likely to me that their common ancestor evolved something aside from egg-laying prior to splitting into the two non-egg methods of reproduction.
“Something aside from egg-laying” would presumably have been some form of ovoviviparity. Mammals are not alone in discovering this method; it has evolved independently in some species of reptiles and amphibians too.
The LCE would likely have been a bit of both. There’s a tendency to think of marsupials as “more primitive” than placentals, and hence that placental evolution must have passed through a marsupial-like stage, but that’s a misconception.
Agreed. I think that pouches with mammary glands followed live birth and not directly following any variety of oviparous birth. It may be the development of the pouch that kept marsupial births at their early stage of development instead of further development pre-birth like placental mammals. Mammary glands must have been well evolved before pouches formed.
Actually echidnas (but not platypuses) have pouches in which the egg is deposited and where the undeveloped young develops for a time, like marsupials. Like platypuses, echidnas lack teats and secrete milk directly on the skin, where the young laps it up.
And not all marsupials have pouches. In some, like the short-tailed opossum, the young, attached to the nipples, are carried completely exposed.
And marsupials have placentas, they just don’t last very long, only a few days before the young are born. The innovation of placentals was being able to suppress the mother’s immune response to carrying genetically different offspring so they could develop internally longer.
While I haven’t looked into it, I would suspect the pouches of echidnas and marsupials are independently evolved, rather than having been present in the common ancestor (and then secondarily lost in platypuses, placentals, and some marsupials). Pouches are present in the most basal lineage of marsupials, the New World Opossums, and so were probably present in the common ancestor of marsupials and lost in some lineages.
No idea how monotremes developed, IIRC they developed a separate line prior to the division of placentals and marsupials. Is ‘placental’ the proper designation for non-marsupial non-monotreme mammals? Maybe I’m confused with that terminology.
Still, I see mammary glands, teats or not, developing before any pouch that could support the early developmental stage of marsupial newborns, and the existence of a pouch reducing the evolutionary pressure for longer development in the womb. Of course a lot of intermediary species may have developed that we’ll never find evidence of, there might have been some pouched mammals who bore fully developed offspring.
Monotremes, or Prototheria, split very early from the Theria, the lineage that includes both marsupials and placentals. One estimate is 220 million years ago. Like other mammals, they have free ear ossicles (rather than having these bones attached to the lower jaw as in reptiles). However, it has been proposed that they acquired this modification independently from other mammals. Their fossil record is very poor. However, a fossil platypus has been found in Argentina, indicating that like marsupials they once occurred in South America as well as Australia.
The technical terms for placentals and marsupials are Eutheria and Metatheria respectively.
Thanks, I’ll try to stick to those terms for clarity.
The platypus is one of the coolest mammals, not surprising early examples were considered hoaxes. Looking at an article I was just reminded how platypi have no corpus callosum. I didn’t know that was the case for all the monotremes and metatheria. To me that’s far more interesting than how they give birth and something new to learn about.
The common ancestor of placentals and marsupials probably looked a lot like Juramaia, from 160 million years ago. It’s actually considered to be the first known placental, but is very close to the time the two lineages diverged.
These early placentals and marsupials are identified as such based on differences in the teeth of the two lineages. Unfortunately reproductive characteristics don’t fossilize, so we can only guess what those of the common ancestor would have been.
A common characteristic of marsupials is epipublic bones, which help support the pouch. However, they are also found in most other non-placental groups, including monotremes, some fossil groups, and even some primitive eutherians. Their presence probably isn’t an indication that ancestral mammals had pouches.
So how does that work? I assume it goes like this - Egg-laying animals develop secretion glands (modifications of sweat glands) that provide more and more nutrients to the newly-hatched. As this become more beneficial to the offspring, the volume of nutrient excretions becomes bigger, meaning more development can happen after hatching. The sooner the animal hatches, to sooner it can use additional resources directly from mother rather than the mother having to assemble to complete kit for the egg earlier and more quickly. I assume the embryonic sac is the development/remains of what used to be an egg?
Thanks for the great info to every who contributed. Sounds like this is a question that can only be guessed at right now, since it would require soft tissue data which is very unlikely to fossilize from that long ago.
There’s some evidence in the bones used for suckling. I’ll pull a reference when I’m not on my phone.
Yes. The amniotic sac which contains the developing fetus in mammals is the same as the membrane that contains the embryo in birds and reptiles. The amnion is the defining feature of the amniotes, including reptiles, birds, and mammals (but not fish or amphibians). The other membranes that surround the fetus in mammals are also homologous with those in shelled eggs.
Then I’m going to extend my wild-ass guess one step further…
As the process of developing an egg lasts longer and longer, developing into a form of placenta - and as the availability of nutrients outside the womb means the newborn can survive from an earlier and earlier age - then the eggshell becomes wasted resources, and going directly from womb to teat is more efficient.
The next step is that some animals develop longer and longer in the womb - which as pointed out, necessitates the development of an efficient placental nutrient exchange and avoiding rejection; but being more developed when emerging is certainly advantageous provided it does not put a big load on the mother. We note that most animals have evolved to an optimal level - nesting animals like mice, cats, etc. can be quite immature, nomadic animals like horses or elephants pop out ready to walk. Humans pop out while the head is still smaller and somewhat flexible because that’s another limitation.
Mother nature seems to push every possibility to its limits.
https://science.sciencemag.org/content/365/6450/276
It’s very paywalled but you should be able to read the abstract. I think these pictures are not paywalled:
https://science.sciencemag.org/content/sci/365/6450/276/F3.large.jpg
https://science.sciencemag.org/content/sci/365/6450/222/F1.large.jpg
The actual article doesn’t mention suckling, but the fluff piece (the one most of us read when reading Science) mentions:
So the skeletal configuration that today is important in suckling is seen back in extinct stuff that existed before anything we label as “mammal”. Although the setup is important for eating food too. So while it supports suckling evolving early, I wouldn’t call it a slam dunk.