Following is the monotreme file, as mentioned by Kathleen Hunt. Subject: Monotremes and th
Organization: Swiss Federal Institute of Technology (ETH), Zurich, CH
From: prl@iis.ethz.ch (Peter Lamb)
Message-ID: prl.697368250@iis
Newsgroups: talk.origins
Following is the monotreme file, as mentioned by Kathleen Hunt.
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Subject: Monotremes and the reptile-mammal transition.
In <1991Nov18.023943.12499@milton.u.washington.edu>,
jespah@milton.u.washington.edu (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 (prl@iis.ethz.ch)
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,
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