EVOLUTION
The oldest platypus and its bearing on divergence timing of the platypus and echidna clades
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*Jackson School of Geosciences, University of Texas, C1100, Austin, TX 78712;
Museum Victoria, PO Box 666, Melbourne, Victoria 3001, Australia;
School of Geosciences, PO Box 28E, Monash University, Victoria 3800, Australia;
¶Department of Biology, University of California, Riverside, CA 92521; and
||Department of Geology, Museum of Northern Arizona, Flagstaff, AZ 86001
Edited by David B. Wake, University of California, Berkeley, CA, and approved October 31, 2007 (received for review July 7, 2007)
Abstract
Monotremes have left a poor fossil record, and paleontology has been virtually mute during two decades of discussion about molecular clock estimates of the timing of divergence between the platypus and echidna clades. We describe evidence from high-resolution x-ray computed tomography indicating that Teinolophos, an Early Cretaceous fossil from Australia's Flat Rocks locality (121–112.5 Ma), lies within the crown clade Monotremata, as a basal platypus. Strict molecular clock estimates of the divergence between platypus and echidnas range from 17 to 80 Ma, but Teinolophos suggests that the two monotreme clades were already distinct in the Early Cretaceous, and that their divergence may predate even the oldest strict molecular estimates by at least 50%. We generated relaxed molecular clock models using three different data sets, but only one yielded a date overlapping with the age of Teinolophos. Morphology suggests that Teinolophos is a platypus in both phylogenetic and ecological aspects, and tends to contradict the popular view of rapid Cenozoic monotreme diversification. Whereas the monotreme fossil record is still sparse and open to interpretation, the new data are consistent with much slower ecological, morphological, and taxonomic diversification rates for monotremes than in their sister taxon, the therian mammals. This alternative view of a deep geological history for monotremes suggests that rate heterogeneities may have affected mammalian evolution in such a way as to defeat strict molecular clock models and to challenge even relaxed molecular clock models when applied to mammalian history at a deep temporal scale.
Mammalia | Monotremata | phylogeny | molecular clock
Author contributions: T.R., T.H.R., M.S., and M.O.W. designed research; T.R., T.H.R., P.V.-R., M.S., and M.O.W. performed research; M.S. contributed new reagents/analytic tools; T.R., M.S., and M.O.W. analyzed data; and T.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
To whom correspondence should be addressed. E-mail: rowe{at}mail.utexas.edu
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