winalski@psw.enet.dec.com (Paul S. Winalski) (04/26/91)
In article <416@smds.UUCP>, rh@smds.UUCP (Richard Harter) writes: |> |>Bones. They have mammalian jaws. The inimitable Dr. Asimov argues in one |>of his columns that the fusion of the relevant bones in Platypus is not |>completed until after birth and the correct classification of Platypus |>is a Therapsid and not a mammal at all. I assume that you and the good Dr. Asimov mean hatching, not birth. Reclassifying Platypus on this basis seems on the surface as silly as arguing that human beings should be reclassified because the ductus arteriosus and the fetal hole in the ventricular septum don't close until after birth. What is the common name for the Theraspids? --PSW
rh@smds.UUCP (Richard Harter) (04/26/91)
In article <1991Apr25.182824.18628@hollie.rdg.dec.com>, winalski@psw.enet.dec.com (Paul S. Winalski) writes: --- re classification of platypus --- > I assume that you and the good Dr. Asimov mean hatching, not birth. Yes. > Reclassifying Platypus on this basis seems on the surface as silly as > arguing that human beings should be reclassified because the ductus > arteriosus and the fetal hole in the ventricular septum don't close > until after birth. 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. > What is the common name for the Theraspids? None that I know of. They've been gone for a long time. Therapsids were the dominant branch of reptiles before they displaced by the dinosaurs (circa 200,000,000 BC?). One branch evolved into mammals. There is an interesting issue here. Asimov's proposal is really a bit of sentimentality, sort of saying "See, the therapsids aren't really extinct." However one can take the view that this sentiment is rooted in the notion that the monotremes (and Platypus in particular) are more "primitive" than other mammals. However they are just as modern as any other animal living today -- they are just in a different lineage. -- 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) (04/27/91)
In article <420@smds.UUCP>, rh@smds.UUCP (Richard Harter) writes: |>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. The actual dividing line between extant reptiles and mammals is lactation. On the evidence of the extant species, the monotremes deserve classification as mammals. Of course, we cannot determine from the fossil record whether theraspids suckled their young. We have to fall back on bone structure analysis. It seems to me that we have three classifications here: separate bones in the jaw (theraspids), fused bones in the jaw (marsupial and placental mammals), and bones separate at hatching but fused in the adult (monotremes). On the basis of this evidence of the extant species, I would be inclined to group the monotremes with the mammals and to treat the lack of fusion of the jaw in the juvenile as a neontological artifact in the same category as the gill slits in human embryos. --PSW
sarima@tdatirv.UUCP (Stanley Friesen) (04/28/91)
In article <420@smds.UUCP> rh@smds.UUCP (Richard Harter) writes: >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. Hmm, as I think on this, I seem to remember that it is not a matter of fusion so much as reorganization. The reptilian jaw has several bones that the mammalian jaw does not. In mammals these bones are known to have moved into the middle ear and become ear ossicles. In conjunction with this a new jaw joint developed between two bones that do not even touch in reptiles. In fossils, where lactation is unobservable, it is the existance of this secondary jaw joint that is used as the working definition of mammal. -- --------------- uunet!tdatirv!sarima (Stanley Friesen)