Ornitorrinco y equidna
edge19214 de Junio de 2015
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e semiaquatic platypus and terrestrial echidnas (spiny anteaters)
are the only living egg-laying mammals (monotremes). The fossil
record has provided few clues as to their origins and the evolution
of their ecological specializations; however, recent reassignment
of the Early Cretaceous Teinolophos and Steropodon to the platypus
lineage implies that platypuses and echidnas diverged >112.5
million years ago, reinforcing the notion of monotremes as living
fossils. This placement is based primarily on characters related to
a single feature, the enlarged mandibular canal, which supplies
blood vessels and dense electrosensory receptors to the platypus
bill. Our reevaluation of the morphological data instead groups
platypus and echidnas to the exclusion of Teinolophos and Steropodon
and suggests that an enlarged mandibular canal is ancestral
for monotremes (partly reversed in echidnas, in association with
general mandibular reduction). A multigene evaluation of the
echidna–platypus divergence using both a relaxed molecular clock
and direct fossil calibrations reveals a recent split of 19 – 48 million
years ago. Platypus-like monotremes (Monotrematum) predate
this divergence, indicating that echidnas had aquatically foraging
ancestors that reinvaded terrestrial ecosystems. This ecological
shift and the associated radiation of echidnas represent a recent
expansion of niche space despite potential competition from marsupials.
Monotremes might have survived the invasion of marsupials
into Australasia by exploiting ecological niches in which
marsupials are restricted by their reproductive mode. Morphology,
ecology, and molecular biology together indicate that Teinolophos
and Steropodon are basal monotremes rather than platypus relatives,
and that living monotremes are a relatively recent radiation.
calibration molecular dating Monotremata niche phylogeny
More than 99% of the 5,400 extant mammal species are
therian (marsupials and placentals) (1). Monotremes, the
only egg-laying mammals, are their living sister group and
comprise just 5 extant species. One of these species is the
semiaquatic, invertebrate feeding platypus (Ornithorhynchus
anatinus) of eastern and southern Australia; the others are the
terrestrial echidnas (Tachyglossidae), the short-beaked echidna
or spiny anteater, Tachyglossus aculeatus of Australia and New
Guinea, and three species of New Guinean long-beaked echidnas
(Zaglossus bruijni, Z. attenboroughi, and Z. bartoni), which feed
on worms and arthropod larvae. Fossil monotremes, such as
Teinolophos trusleri and Steropodon galmani (2), along with their
putative relatives, the insectivore-like ausktribosphenids (3–6),
make up the bulk of the known Australian Cretaceous mammal
fauna. Known monotreme diversity then contracts to only
platypus-like taxa, subsequent to the arrival of marsupials from
South America via Antarctica 71–54.6 million years ago (Ma)
(7, 8). Monotrematum sudamericanum (9, 10) from the Palaeocene
(61 Ma) of South America is known from two platypuslike
distal femora and several molar teeth that closely match
those of the extinct Australian platypus, Obdurodon (11, 12). The
later appearance (25 Ma) of Obdurodon probably reflects the
sparseness of earlier Tertiary mammal-bearing sites in Australia.
Fossil echidnas do not appear until the mid-Miocene (13
Ma) (13), despite excellent late Oligocene–Early Miocene mammal
fossil records in both northern and southern Australia. This
absence has
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