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Echidnas evolved from amphibious ancestors
This is a very interesting study that was presented at the CAVEPS meeting in
Sydney earlier this year. It finds compelling support for a platypus-echidna
clade to the exclusion of _Steropodon_ and _Teinolophos_, which are recovered
as stem monotremes.
Phillips, MJ, Bennett, TH, and Lee, TSY. Molecules, morphology, and ecology
indicate a recent, amphibious ancestry for echidnas. PNAS Published online
before print September 23, 2009, doi: 10.1073/pnas.0904649106
Abstract
"The 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 echidnaâplatypus divergence using both a relaxed
molecular clock and direct fossil calibrations reveals a recent split of 19â48
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."
Thus, according to this study, the crown clade (ornithorhynchids+tachyglossids)
first appeared in the Cenozoic. The previous view was that the platypus group
(ornithorhynchids) must have split from the echidna group (tachyglossids)
during the Mesozoic, based on the referral of the aquatic Cretaceous monotremes
_Steropodon_ and _Teinolophos_ to the platypus group.
The study is also interesting in light of the recent study of the eutherian
_Maelestes_ by Wible et al. (2009; Bull AMNH 327:1 -123). The phylogeny
included in that study found that no extant placental mammal "order" had
representatives in the Cretaceous. This also appears to hold true for extant
marsupial "orders" (Meredith et al. 2008; J. Mammal Evol. 15:1â36). Now the
crown monotreme clade has been shifted forward to the Cenozoic. So, the
cumulative evidence suggests that no extant mammal "order" appeared in the
Mesozoic.
Also, as noted by Phillip &c, the hypothesized evolution of echidnas from
amphibious ancestors is the second example of this transition in mammals, the
other example being the Proboscidea (elephants and friends).
Cheers
Tim