rh@smds.UUCP (Richard Harter) (04/27/91)
In article <1991Apr26.183747.21006@hollie.rdg.dec.com>, winalski@psw.enet.dec.com (Paul S. Winalski) writes: I wrote: > |>The cases are quite different. The dividing line between the threrapsid > |>reptiles and the mammals is the jaw bone. I.e. fossils in that lineage > |>are classified as one or the other based on whether certain bones are > |>fused or not. And Paul S. Winalski replied PW: The actual dividing line between extant reptiles and mammals is lactation. PW: On the evidence of the extant species, the monotremes deserve classification PW: as mammals. Of course, we cannot determine from the fossil record whether PW: theraspids suckled their young. We have to fall back on bone structure PW: analysis. It seems to me that we have three classifications here: separate PW: bones in the jaw (theraspids), fused bones in the jaw PW: (marsupial and placental PW: mammals), PW: and bones separate at hatching but fused in the adult (monotremes). PW: On the basis of this evidence of the extant species, I would be inclined to PW: group the monotremes with the mammals and to treat the lack of fusion of PW: the jaw in the juvenile as a neontological artifact in the same category PW: as the gill slits in human embryos. Therapsid, not theraspid. Gould's new book has a relevant chapter on naming of species. Tho it is not strictly relevant since we are dealing with classification rather than naming, the issue is prior practice. Platypus was initially classified as a mammal on the basis of hair. [Perhaps some of our informed readers can tell us if there is any extant species of mammal that is completely hairless or any species of reptile that has hair.] Lactation in Platypus wasn't discovered until much later; indeed lactation in Platypus is what you might expect in an intermediate form between reptiles and mammals. The working out of the evolutionary sequence from therapsids to mammals was worked out much later. By prior practice Platypus is classified as a mammal. Since taxonomists are a conservative bunch we may expect that the classification will remain in effect; there is no compelling reason for reclassification. If one considers the historical situation there was, at one time, animals which I will call proto-mammals. That is, they had some mammalian characteristics, but not all. Platypus seems to be in a line of descent that retains more characteristics of proto-mammals. A real question that I don't know the answer to is whether the monotreme classification is legitimate, i.e. whether the descent lines are like therapsid -> monotreme -> marsupial -> placental or whether, instead, there are separate descent lines for the extant monotremes. I rather expect no one knows. -- Richard Harter, Software Maintenance and Development Systems, Inc. Net address: jjmhome!smds!rh Phone: 508-369-7398 US Mail: SMDS Inc., PO Box 555, Concord MA 01742 This sentence no verb. This sentence short. This signature done.
winalski@psw.enet.dec.com (Paul S. Winalski) (04/28/91)
In article <421@smds.UUCP>, rh@smds.UUCP (Richard Harter) writes: |>A real |>question that I don't know the answer to is whether the monotreme |>classification is legitimate, i.e. whether the descent lines are like |> |> therapsid -> monotreme |> -> marsupial |> -> placental |> |>or whether, instead, there are separate descent lines for the extant |>monotremes. I rather expect no one knows. Indeed. This kind of open and unresolvable (by scientific means of hypothesis and testing) question is why I don't care much for Paleontology. Monotreme remains a useful classification for the extant species, regardless of its paleontological legitimacy. --PSW
andrewt@cluster.cs.su.oz.au (Andrew Taylor) (04/28/91)
In article <421@smds.UUCP>, rh@smds.UUCP (Richard Harter) writes: > Lactation in Platypus wasn't discovered until much later; indeed lactation > in Platypus is what you might expect in an intermediate form between > reptiles and mammals. I'm suspicious of the assumption that the platypus lactation is a primitive charcteristic because the platypus is so highly specialised. Perhaps the nipple was lost in the platypus because the bill made it difficult for the young to suckle. I'm probably wrong but ... you've got to be careful. Andrew Taylor
sarima@tdatirv.UUCP (Stanley Friesen) (04/30/91)
In article <2350@cluster.cs.su.oz.au> andrewt@cluster.cs.su.oz.au (Andrew Taylor) writes: >I'm suspicious of the assumption that the platypus lactation is >a primitive charcteristic because the platypus is so highly specialised. >Perhaps the nipple was lost in the platypus because the bill >made it difficult for the young to suckle. I'm probably wrong but ... >you've got to be careful. Well, perhaps it is a specialization, but not for this reason. All monotremes lack nipples, including the three species of echidna, which are essentially 'anteaters', with long, tubular snouts. The echidnas would *very* good at suckling at nipples. Also, I think I remember photos of newborn platypuses, and I seem to remember that the bill is not yet well developed. (I *could* be wrong here, the memory is rather vague). Quite honestly, I think that the lack of nipples is likely to be one of the most certain primitive characteristics of monotremes. The others could easily be specializations, but this one is hard to explain any other way. (Nipples are so much more efficient - they allow much better control over milk production, and thus less waste). -- --------------- uunet!tdatirv!sarima (Stanley Friesen)
andrewt@cluster.cs.su.oz.au (Andrew Taylor) (05/06/91)
In article <227@tdatirv.UUCP>, sarima@tdatirv.UUCP (Stanley Friesen) writes: > All monotremes lack nipples, including the three species of echidna, which > are essentially 'anteaters', with long, tubular snouts. The echidnas > would *very* good at suckling at nipples. Actually I'm not convinced echidnas (there are only 2 species) would be good at suckling at nipples. The bill is well-developed in both echidnas and platypus before the young finish suckling. The next time I get a chance I'll take a close look at an echidna's bill. A big problem for the lost monotreme nipple theory is that it doesn't seem likely the common ancestor of platypus and echidna had a highly-specialised mouth. Nipple loss would have to occurred twice. Andrew Taylor