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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 


"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 

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).