Ornitorrinco y equidna

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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