You have to be careful about drawing conclusions about evolution solely from extant species. Just because two groups appear to have common traits does not mean that the “more advanced” group evolved from the “more primitive” group. It is quite possible that both had a common ancestor that shared some of the traits of both but was, in fact even more “primitive” than either. The common ancestor of monotremes and placetals might have been neither a monotreme nor a placental, but something else altogether. I would agree, however, that the common ancestor was an egg layer.
More to the point, it is not at all clear that marsupials represent some intermediate (living missing link) between monotremes and placentals. They might easily be a seperat side branch. It’s like saying chimps represent an intermediate between humans and monkeys, which is absolutely false. Yes, our common ancestor might have looked more like a chimp than a human, but it wasn’t either a chimp or a human. It was something else altogether.
The last sentence in the first paragraph should have read: “I would agree, however, that the common ancestor was most likely an egg layer.” My mistake.
Cladogram
SYNAPSIDA
|
Therapsida
|–Biarmosuchia --+--Dinocephalia | |--Anteosauria | –Tapinocephalia --+--Anomodontia | |--Venyukovioidea | –Dicynodontia --Theriodontia |--Gorgonopsia –±-Therocephalia
`–CYNODONTIA
but the Cretaceous was a melting pot of mammalian sub-classes most of which do not survive to today
However, therapsids are known as the first mammals, and they did lay eggs.
The Triassic Period of the Mesozoic Era
250 to 205 million years ago -when therapsids appeared
Triassic Period–Therapsids, cynodonts, and the first mammals
So they had a 100 million years before the first placental appeared.
Phanerozoic
Mesozoic
Triassic
Late Triassic
Middle Triassic
Early Triassic (Scythian)
Jurassic
Early Jurassic (Lias)
Middle Jurassic (Dogger)
Late Jurassic (Malm)
Cretaceous
Early Cretaceous (Neocomian)
Mid Cretaceous (Gallic)
Late Cretaceous (Senonian)
IF I understand these two comments, then I’m afraid you are wrong. The fact that monotremes may have preceded marsupials and placentals chronologically does not make them ancestors–ancestor implies that placentals descended through monotremes and marsupials. There is no evidence for that. There is no “evolution of traits” that proceeds “from egg to puch to uterus” even if it is true that pouch was earlier than uterus (since uterine mammals did not descend from marsupials) or that eggs were earlier than pouches (since marsupials did not descend from monotremes). This paper on monotreme evolution viewed through DNA (Adobe required) discusses the cladistics on pages 11 and 12, noting that the traditional division separates monotremes from the other therian-descended groups, then shows marsupials and placentals dividing as sister groups (meaning neither gave rise to the other). It then notes that a counter hypothesis suggests that placentals divergd first, leaving monotremes and marsupials as sister (not parental) groups.
So, the use of the word ancestor (which implies descent) when discussing the relationship of monotremes, marsupials, and placentals is clearly wrong.
Marsupials and placentals also have a common ancestor…and marsupials evolved first and claimed the planet for a while before placentals took over. That much is observable, so no conclusions really need to be drawn.
However, you appear to be confusing “egg-laying” with “monotreme.” While monotremes lay eggs, they are not in line of descent with marsupials or placentals. There is more to the identification of montreme than simply their laying of eggs.
Okay, the textbook says the monotremes and placentals branched from a common ancestor 80-100 million years ago, and the ancestry is obscure.
Did the common ancestor have more marsupial traits than placental though? That’s my question…or was it equal in traits of both? Or did it have more monotreme traits than anything, in fact, was it just a monotreme that is considered the common ancestor?
If it was completely monotreme it would mean there is no transition from marsupial to placental…only from monotreme to both marsupial and placental. (you have to look at all bone structures, and other parts of anatomy, not just uterus vs no uterus).
If the ancestor was more marsupial then there is a link from monotreme to marsupial to placental. If it was more placental then it went monotreme-placental-marsupial.
Of course, if there is no connection among the three of them, then neither of these scenarios is valid.
I have never seen any evidence that either marsupials or placentals evolved from monotremes and the literature is quite consistent that neither placentals nor marsupials evolved from the other.
You appear to continue to equate “egg-laying” with “monotreme,” but I have seen no evidence that the therapsids, in general, are legitimately called monotremes.
True. So where is your evidence? In addition to bones to examine, we have genetic analysis (to which I have already provided a link) that denies that any of the three groups under discussion gave rise to either of the others.
Just like this dilemma of finding which ‘transition’ animal is actually reptile or mammal:
I’m sure there are marsupials or monotremes ‘on the edge’ between each other…and then the same between the common ancestor between marsupials and placentals…
Or is that the monotreme at the end of the monotreme line is more of a placental/monotreme? Then there would be the animals that are placental/marsupial.
no, I equate egg laying as a montoreme trait. Just like it is also a reptilian trait…and then as shown as above…the evolution of montremes from reptiles.
I never said therapsids are montremes…rather a link from therapsid from reptile to monotreme, or maybe therapsids are monotremes.
Or you’re saying that the three mammal groups each have their own reptilian ancestor? That’s the only way they would not have common ancestors and no connection. That’s not so, because the marsupials and placentals have a common ancestor.
The literature clearly states a common ancestor for marsupials and placentals…so that common ancestor would have to be connected.
tomndebb, are you a creationist? If so, I don’t care to discuss this anymore because the discussion will never end. It will always be an argument about ‘truth’ and transitions, and the sides of creation vs evolution are divided in such a way that discussion is pointless
My theory will continue to be that marsupials and monotremes have a common ancestor, and it has already been stated in literatue that marsupials and placentals have a common ancestor…whether that ancestor be more placental or marsupial…I have to find out.
So I can see where marsupials and placentals could branch from therapsid instead of monotremes…but I don’t think that is as likely as a branch from monotremes to marsupials and placentals.
Your theory does not agree with any current phylogeny of the three groups. Marsupials and monotremes do not share any common ancestor that does not also include eutherians. Marsupials and eutherians do share a most recent common ancestor (and are collectively known as Theria), and the most recent common ancestor of Theria with monotremes would be the primeval mammal.
The questions you should be asking are not, “Is the common ancestor more marsupial-like or more placental-like?” - that’s putting the cart before the horse. Both groups retain primitive characters which were present in their common ancestor. Both groups have derived characters not present in that ancestral group. Marsupials may or may not retain their reproductive arrangement from that ancestor, but doing so simply means that that character is “primitive”, with respect to that ancestor, not that the ancestor is “more marsupial-like”.
Not that any of this has anything to do with Behe, of course…
My position is that the following is a valid statement:
but that when you continue on with:
you are echoing your still earlier statement, which is not valid:
Mammals arose from cynodont therapsids, but there is no reason to assume that Protherians (Triconodonts, Docodonts, Monotremes, and Multituberculates) gave rise to Therians (Pantotherians, Marsupials, and Eutherians) or that within Theria, Marsupials or Eutherians gave rise to each other (or to Pantotherians).
You may not like the conclusion, but all the current evidence argues against your preference to line them up in descent.
This hijack has gone on far enough. Drabble refernces have been provided showing your hypithesis to be wrong on numerous grounds. Can you please start another thread concerning mammalin evolution, one which I would love to join.
But contimuing the hijack of this thread is really bad manners.