3 ms·

Nobody has yet explained to me how platypuses can be a "missing link" (I'm aware that that's not a real concept) when the last common ancestor between birds and

Nobody has yet explained to me how platypuses can be a "missing link" (I'm aware that that's not a real concept) when the last common ancestor between birds and mammals is so far back -- I continue to believe that monotremata are the result of a very aberrant mating.

masklinn 4y ago
Why would the platypus be a missing link to birds? Monotremes are rather obviously a very early mammalian offshoot, having retained significant original character (and a lot of weirdnesses) compared to therians. And the platypus’ bill is convergent, if you look at a platypus skull it doesn’t look like a duck’s.
- throwawaymaths 4y ago
  Z/w sex chromosome elements.
  - deleted 4y ago
    [deleted]
  - masklinn 4y ago
    Z/w is not exclusive to birds, and is present in a number of reptiles (as well as fishes and crustaceans). So it could be a recurring sex determination scheme, or it could be a common amniote trait. Either would make a lot more sense than the idea that platypuses would be descendants of birds or whatever.
- thaumasiotes 4y ago
  I assume throwawaymaths is talking about this paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2413164/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2413164/ (or maybe to one of the papers this paper refers to as false?) It finds that platypuses have five sex chromosomes which arrange themselves in a fixed order, that these five chromosomes do not share homologous material with the ancestral therian X chromosome, and that several of them (possibly all?) do share homologous material with the (single) chicken Z chromosome. It says that earlier findings indicated (erroneously) that one end of the platypus sex chromosome sequence shared homology with the therian X, and the other end shared homology with the bird Z, and that this was taken as evidence contradicting the established view that the bird and mammal sex determination systems evolved independently of each other. But it goes on to contradict those earlier findings, which would seem to leave the even earlier view of independent evolution of sex determination systems in place. It is not clear why this would make the platypus a missing link between mammals and birds. The focus seems to be pretty squarely on the evolution of the standard mammalian X chromosome, not on mammals generally.
jjtheblunt 4y ago
aberrant mating would depend on compatible chromosome layouts?
loonster 4y ago
Viruses DNA can be added into animal DNA. Maybe a virus infected a bird, then a mammal that later evolved into the platypus.

masklinn 4y ago

Why would the platypus be a missing link to birds? Monotremes are rather obviously a very early mammalian offshoot, having retained significant original character (and a lot of weirdnesses) compared to therians. And the platypus’ bill is convergent, if you look at a platypus skull it doesn’t look like a duck’s.

throwawaymaths 4y ago
Z/w sex chromosome elements.
- deleted 4y ago
  [deleted]
- masklinn 4y ago
  Z/w is not exclusive to birds, and is present in a number of reptiles (as well as fishes and crustaceans). So it could be a recurring sex determination scheme, or it could be a common amniote trait. Either would make a lot more sense than the idea that platypuses would be descendants of birds or whatever.
thaumasiotes 4y ago
I assume throwawaymaths is talking about this paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2413164/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2413164/ (or maybe to one of the papers this paper refers to as false?) It finds that platypuses have five sex chromosomes which arrange themselves in a fixed order, that these five chromosomes do not share homologous material with the ancestral therian X chromosome, and that several of them (possibly all?) do share homologous material with the (single) chicken Z chromosome. It says that earlier findings indicated (erroneously) that one end of the platypus sex chromosome sequence shared homology with the therian X, and the other end shared homology with the bird Z, and that this was taken as evidence contradicting the established view that the bird and mammal sex determination systems evolved independently of each other. But it goes on to contradict those earlier findings, which would seem to leave the even earlier view of independent evolution of sex determination systems in place. It is not clear why this would make the platypus a missing link between mammals and birds. The focus seems to be pretty squarely on the evolution of the standard mammalian X chromosome, not on mammals generally.

jjtheblunt 4y ago

aberrant mating would depend on compatible chromosome layouts?

loonster 4y ago

Viruses DNA can be added into animal DNA. Maybe a virus infected a bird, then a mammal that later evolved into the platypus.