Following is the monotreme file, as mentioned by Kathleen Hunt. Subject: Monotremes and th

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Organization: Swiss Federal Institute of Technology (ETH), Zurich, CH From: (Peter Lamb) Message-ID: prl.697368250@iis Newsgroups: Following is the monotreme file, as mentioned by Kathleen Hunt. ========================== Subject: Monotremes and the reptile-mammal transition. In <>, (Kathleen Hunt) writes: >Those wondering how egg-laying reptiles could make the transition to >placental mammals may wish to study the reproductive biology of the >monotremes (egg-laying mammals) and the marsupials. The monotremes >in particular could almost be considered "living transitional fossils". This has prompted me to prepare a post about the monotremes. Introduction -- What are monotremes Reptile-like and mammal-like features of monotremes Where do they fit in the evolution of mammals Introduction -- What are monotremes Monotremes are egg-laying mammals found in Australia and on the island of New Guinea (politically Papua-New Guinea in the east and the Indonesian province of Irian Jaya in the west). There are only three extant species, Common name(s) Ornithorhynchus anatinus Platypus Tachyglossus aculeatus Echidna, spiny anteater Zaglossus bruijni Long beaked echidna The platypus is found in rivers on the eastern and south-eastern coasts of the Australian mainland, and on the island of Tasmania, the echidna in most of the eastern half of Australia and in New Guinea, and the long beaked echidna in the highlands of New Guinea[2:ch 2]. While the laying of eggs by the monotremes is the best-known and most obvious similarity that they have to the reptiles, there are a number of other similarities, which will be examined in the next section. It is inaccurate, however, to consider the monotremes and marsupials as "living fossils" -- they have had an independent evolutionary history of some 180Myr, and perform better than placental mammals at some "typically mammalian" tasks, like thermoregulation [3:ch 4,9]. Neither are they a "missing link"; they are neither missing nor are modern monotremes a link between any other modern groups. However, they have retained some features which we would expect to have seen in the more mammal-like of the transitional forms between reptiles and mammals. Reptile-like and mammal-like features of monotremes Mammal-like features Griffiths [2:app] lists 17 important common features of marsupials and placental mammals, of these, Dawson lists 13 as also common to monotremes. These are, in abbreviated form: 1) Typically mammalian jaw joints 2) 3 bones in the middle ear 3) Young are small, naked and raised on milk 4) Mammary glands; growth and differentiation influenced by ovarian hormones 5) Temperature regulation, assisted by internal heat production and hair 6) Separate left and right sides in the heart 7) Red blood cells have no nucleus 8) Typically mammalian kidney structure & blood supply 9) Nitrogenous waste mostly excreted as urea 10) Respiration using alveolar lungs and diaphragm 11) 7 cervical vertebrae 12) Typically mammalian pelvis 13) Large forebrain, left and right halves connected Reptilian and mixed features 1) Egg laying -- the eggs have a leathery, rather than hard, shell. The eggs develop in the uterus for a relatively long time (28 days) and a large part of the embryonic development occurs there. The eggs are incubated about 10 days. In reptiles more of the embryonic development occurs after the egg is laid. "[monotreme] hatchlings have a sharp egg tooth on the upper jaw (as do many reptiles) which enables them to tear open their rubbery shells"[1]. 2) Excretion and reproduction in the female is all carried out through one orifice, as in reptiles -- the name monotreme (one hole) denotes this. (Marsupial females have a common orifice for reproduction and urine, but a separate anus). 3) Internal testes; this is a feature of reptiles, but some placental mammals (eg. beavers) also have internal testes. 4) Some features of the skeletons are intermediate between those of reptiles and placental mammals. Griffiths[1] notes that the pectoral girdle and the epipubic bones of the pelvic girdle in the platypus have a structure similar to the fossil therapsids and to reptiles. Therapsids are mammal-like fossile reptiles, see also, for example, [4] or Kathleen Hunt's transitional forms FAQ. 5) "Monotreme chromosomes also reflect a mixture of reptilian and mammalian traits. Although these animals are unique in having two categories of chromosomes, large and small, the large ones (macrochromosomes) are typical of those found in mammals, whereas the small ones (microchromosomes) are similar to those found in many species of reptiles, and do not occur in mammals"[1]. 6) "Platypus sperm is long and slender, with a filiform, or threadlike head, much like the sperm of reptiles, the arrangement of the subcellular elements called microtubules, however, are typical of mammalian sperm"[1]. One feature of monotremes which I don't know where it fits is the mechanism for sex determination: "The male is heterogametic, that is, he produces two kinds of sperm, as humans do, one with a Y chromosome and the other with an X chromosome ... platypus and echidna males differ from all other mammals in that sex is determined by the presence of a multivalent XY/XX complex: at meiosis in the male the X and Y chromosomes are associated with small autosomal chromosomes, four unpaired, and four paired"[1]. Where do they fit in the evolution of mammals Few fossil monotremes have been found. Dawson[3] lists: in the Tachyglossidae, Zaglossus ramsayi, Z. hacketti and Z. robusta; in the Ornithorhynchidae, Obdurodon insignis. Unfortunately, these are all relatively recent fossils, the oldest being that of Ob. insignis, about 15Myr, of which only a few teeth were preserved. Griffith[1] mentions two more recent finds. Ob. dicksoni, a complete skull of an adult platypus, again about 15Myr old, and a piece of lower jaw containing three molars, roughly 100Myr old. This leaves a substantial gap to the fossils to which the monotremes are considered to be most closely related, the Morganucodontids, and little structural information about the early monotremes. Morganucodontids have a pelvic structure features very similar to that of modern monotremes. Griffiths [2:app] and Colbert&Morales[4:p241] review current theories about the affinities of the monotremes. To conclude: "[the monotremes] represent a branch of mammals that is quite ancient. They are more closely related to marsupial and placental mammals than to any group of reptiles, yet they have retained a surprising number of ancestral reptilian traits over the course of evolution and posses an interesting mosaic of mammalian and reptilian characteristics". Mervyn Griffiths[1] Peter Lamb ( Thu Feb 6 10:34:55 MET 1992 [1] Mervyn Griffiths, "The Platypus", Scientific American, May 1988, pp 60-67. [2] Mervyn Griffiths, "The Biology of the Monotremes", Academic Press, New York a.o., 1978 [3] Terence J. Dawson, "Monotremes and Marsupials: the other Mammals", Arnold, London, 1983 [4] E. H. Colbert & M. Morales, "Evolution of the Vertebrates", 4ed, Wiley-Liss, New York, 1991. [5] S. Gould, "To be a Platypus", in "Bully for Brontosaurus", W. W. Norton, New York & London. [6] S. Gould, "Bligh's Bounty", ibid. Notes: [3] is an excellent introduction to monotremes and marsupials for the interested non-biologist (like me). [1] and [4] (an earlier edition is mentioned in Kathleen Hunt's transitional form FAQ sheet) are also directed to the general reader. [4] also contains information about marsupial evolution, which is far better documented in the fossil record than monotreme evolution. I found [2] heavy going. [5] and [6] are not referenced, and contain little information beyond what is in the referenced texts, but is likely to be easier for readers with only access to general libraries to find. [5] is a short history of how monotremes came to be described scientifically and the recognition by biologists of their peculiar mode of reproduction. 82 years elapsed between the first scientific description of a monotreme and the recognition that they indeed lay eggs. [6] demonstrates that although the monotremes retain many primitive features, they are not, in general, less well-adapted than placental mammals (thanks to Herbert A. Huston, for the reference). Any corrections, or suggestions for additions or improvements, are most welcome.


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