Now, y’see… If I had said ‘Echidnas have four-headed penises’ you’d have asked, ‘Even the females?’ 
Well, yeah, there would have to have been. They’ve been evolving independently for 150M years. Wikipedia has a list of monotreme fossils.
I don’t know if the pelvis area would indicate that or not, but I suspect that certain skeletal features unrelated to reproduction would identify the fossil as being a monotreme-- either in the skull or joints or something like that.
Don’t care. It’s still screwed up. Seriously, are there any other poisonous/venomous ( I know there’s a difference) mammals?
(I just looked it up - there’s some shrews and a primate)
Some other early ancestors of mammals more advanced than therapsids may have laid eggs, but it depends on their (controversial) position relative to other groups. Docodonts (Docodonta - Wikipedia), for example, are probably more primitive than monotremes and therefore probably laid eggs. Another primitive mammal line of not-quote-certain relationship to modern mammals are the multituberculates (Multituberculata - Wikipedia), which apparently gave birth to underdeveloped live young like marsupials (based on similar pelvic structure). Several other primitive orders of mammals or near-mammals listed here: Mammaliaformes - Wikipedia
Good answer. As far as I know, fossil mammal eggs have never been discovered – heck, within living memory there was a famous, and at the time true, saying that all the Mesozoic mammal fossils known would fit together in a shoebox. (Because both the fossils themselves were small and there were relatively few of them.)
The monotremes clearly have a long separate history, but very little is known of their evolution. While no living monotreme has teeth, and the diagnostic features of nearly all Mesozoic mammals are their teeth (characteristically differentiated and the best-preserved parts), embryonic platypuses grow tiny tooth-buds which are then resorbed as the baby in the egg develops. Those toothbuds have strong similarities to the teeth of the Docodonts. (Which is, admittedly, hanging a lot of implications on a single observed fact – but you work with what you have in paleontology.)
I recently heard an argument (a congenial one) that monotremes aren’t really mammals and should be classified as surviving “mammal-like reptiles” Offered just as food for thought
Except “mammal-like reptiles” generally isn’t used these days, given that those critters weren’t reptiles at all (and likely bore only a passing resemblance to mammals, as well, depending on which group you were examining). The amniote lineages that gave rise to mammals and reptiles (Synapsida and Sauropsida, respectively) parted company some 300 million years ago (with true reptiles showing up ~260 mya). The critters that many folks consider “mammal-like reptiles” are early synapsids, some quite derived from the basal “undifferentiated amniote” stock. Therapsids, dicynodonts, cynodonts, and the aforementioned multituberculates and docodonts were all increasingly-derived pre-mammallian synapsids.
As **Darwin’s Finch ** indicates, monotremes are much more closely related to other mammals than they are to any traditional “reptiles” (turtles, crocodilians, lizards, snakes, and tuatara).
The characteristics that monotremes share with “reptiles” - egg-laying, certain elements of the skeleton - are all what are known in cladistics as plesiomorphies, or shared ancestral (“primitive”) traits. Since they were present in the last common ancestor, cladistically they are useless for determining closeness of relationships between descendant groups.
The characteristics that monotremes share with other mammals - milk glands, hair, other skeletal elements - are what are known as synapomorphies, or shared derived (“advanced”) traits. It is these kinds of traits that cladists use to determine relationships.
As DF also points out, “mammal-like reptile” is an obsolete term these days. However, if anyone was arguing that monotremes are closer to “reptiles” than to mammals they are not doing so from a cladistic point of view.
And magic, too! My laptop speakers died a while ago, and nothing we could do could fix them. The headphone jack was still working, so I went and bought desktop speakers (40$ set on sale at 10$!) and have been using them since… that is, until the Gatling-penis video.
Suddenly my speakers work, and seem to work on all the programs and webpages that they didn’t work on before!
Magic egg-laying-mammal-Gatling penises!
Like female humans, the answer is: Any kind they want.
In addition to what’s been mentioned above, a “mammal” is only what we define as such. You could define “mammal” to mean “the blue whale and all animals more closely related to it than dolphins”. This would make most everything else ‘surviving mammal-like reptiles’.
I think currently, the most widely accepted definition of mammal is what’s called the crown clade–all living groups and all fossil groups more closely related to the living ones than to the extinct ones. Under this and most other uses of the term, monotremes are mammals by definition, not non-mammalian synapsids, simply because the definition itself hinges partially on their inclusion.
Whether some of the other extinct groups mentioned above are technically mammals or not depends, again, on their position relative to monotremes and eutherians (marsupials and placentals).
A question for those who know more than I do about these things:
If we had no living examples of platypuses or echidnas, if ALL we had were the skeletons - would they be classified as mammals or a near-but-not-quite relative? Because, really, mammary glands don’t fossilize, do they… and the fact monotremes secrete milk (I assume) is a significant factor in including them in “mammals”.
The bones of the inner ear and the jaw (joint) can be used to distinguish mammals from non-mammals.
True.
However, I have a vague recollection that those characteristics in monotremes are actually closer to reptile than mammal. But I might be mis-remembering, I do that sometimes, which is why I asked about skeletal remains.
As the Wikipedia article I linked discusses, the skeletons of monotremes are closer in some ways to reptilian skeletons. They have jaws that are non-mammalian, they have side-set legs, like reptiles, and they have ears at the base of the jaw. Their inner ear bones are mammal-like, but there is an assertion that has been made that this is a parallel development. This assertion is disputed.
Your article just said they are different from other mammals, not that they are non-mammalian.
Seeing as the jawbone and inner ear arrangement of mammals is strictly limited to mammals, any other configuration is, arguably, non-mammalian,.
The ability to read and write is strictly limited to mammals as well, but lacking that ability is not in itself non-mammalian.
But monotremes are mammals. By saying “non-mammalian”, that implies “reptilian” in the context of answering the question. So even if monotremes have a feature that is atypical for mammals, that doesn’t make that feature reptilian. Does that make sense?
Yes. That is why I included the qualifier “arguably”. Go back and read the post, it’s really there.
Of course mammals can share characteristics with non-mammals. Having four legs is not restricted to mammals, it’s not a mammalian trait, it’s a trait of a LOT of critters.
However, it is my understanding that one of the reasons monotremes are classed with mammals is that they nurse their young, which we know ONLY because we have living specimens to study. For darn sure no one from Europe or Asia or Africa who first looked at a platypus and said “Damn - that’s OBVIOUSLY a mammal”. They didn’t. Accounts of the time made it very clear that it was a very confusing animal and in fact for a time the existing dead specimens returned to Europe were held to be fakes because the animal was just so weird.
Hence, my question - are their sufficient skeletal differences between monotremes and other mammal types that if all we had were skeletons they might be classed as something other than mammals, albeit closely related? It’s preceisely because of factoids like their jaws being non-standard-mammal that I ask that question. I’m not disputing their current classification, I’m asking that based on skeletons alone would they be definitively in or out of the group “mammals” or just a confusing mystery?