Reply to Camens: How Recently Did Modern Monotremes Diversify?

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LETTER consistent with (rather than compelling evidence for) the hypothesis. Camens interprets the ﬁrst two characteristics as fossorial adaptations and the third as absent in platypuses. Reply to Camens: How recently did However, similarly fossorial mammals such as the giant ant- modern monotremes diversify? eater and aardvark possess none of these characteristics. In contrast, hydrodynamic dorso-ventral compression and hyper- trophied humeral long-axis rotation are present in platypuses. Camens (1) responds to our analysis of morphological data (2) Finally, although the platypus hindfoot is rotated posteriorly in which platypuses (Ornithorhynchidae) and echidnas (Ta- only when swimming, it remains partly rotated (i.e., directed chyglossidae) were inferred to be each other's closest relatives, laterally) when walking—as in certain other aquatic groups to the exclusion of Early Cretaceous forms, Teinolophos and such as sea lions (Otariidae). This condition, associated with Steropodon. Our phylogeny is consistent with the late appear- aquatic habits, is a plausible precursor to the posteriorly di- ance of undisputed fossil echidnas and platypuses. Molecular rected hindfoot in echidnas. dating provided important independent corroboration, reveal- Finally, there is no evidence for the hypothesis that echidna- – ing that platypuses and echidnas diverged only 19 48 Ma, im- like monotremes have occupied myrmecophagic (ant/termite- – plying that Teinolophos and Steropodon (105 121 Ma) must lie eating) niches since the Cretaceous, to the exclusion of marsu- – outside the platypus echidna dichotomy. pials (1). Among echidnas, only Tachyglossus is myrmecophagic; Camens (1) presents different fossil evidence for an ancient both its sister taxon Zaglossus (from which it diverged <10.6 Ma: ≈ divergence, suggesting that Kryoryctes ( 105 Ma), known from Table 1 in ref. 2) and the paleontologically older ﬁ only a fossil humerus, has af nities with echidnas. However, Megalibgwilia retained more generalized monotreme diets of such a relationship is rejected in the only published description worms and large insect larvae (5). Instead, the presence of (3), which states that “primitive features of the bone exclude the semifossorial invertebrate-feeding bandicoots and rat-kangaroos, animal concerned from the extant families Tachyglossidae and contemporaneous with the platypus–echidna divergence and Ornithorhynchidae and suggest that, if it is a monotreme, it is a pre-dating Tachyglossus (2), supports our suggestion that stem-group monotreme.” Hence, Kryoryctes does not falsify the echidnas expanded into new ecospace despite potential 19–48 Ma estimate for the platypus–echidna divergence. Sim- competition from marsupials. ilarly, “Considerable capacity for rotation digging” in Kryoryctes Matthew J. Phillipsa,1, T. H. Bennetta and Michael S. Y. Leeb,c is shared by both echidnas and platypuses and so is irrelevant for aCentre for Macroevolution and Macroecology, Research School of distinguishing terrestrial from semiaquatic origins. Indeed, as Biology, Australian National University, Canberra, ACT 0200, noted already (3), the Kryoryctes humerus could belong to the Australia; bSchool of Earth and Environmental Sciences, University contemporaneous and similarly sized Steropodon, which is a of Adelaide, SA 5005, Australia; and cEarth Sciences Section, stem monotreme (2) and probably platypus-like (4). Camens (1) South Australian Museum, Adelaide, SA 5000, Australia

also suggests taphonomic biases might explain the late occur- 1. Camens AB (2009) Were early Tertiary monotremes really all aquatic? Inferring pa- rence of stem-echidnas in the fossil record. However, we con- laeobiology and phylogeny from a depauperate fossil record. Proc Natl Acad Sci USA 107:E12. sidered the absence of echidnas among abundant Oligocene/ 2. Phillips MJ, Bennett TH, Lee MSY (2009) Molecules, morphology, and ecology indicate early Miocene terrestrial mammal faunas to be consistent with a recent, amphibious ancestry for echidnas. Proc Natl Acad Sci USA 106:17089–17094. 3. Pridmore PA, Rich TH, Vickers-Rich P, Gambaryan PP (2005) A tachyglossid-like humerus (rather than compelling evidence for) recent origins and indeed from the Early Cretaceous of South-Eastern Australia. J Mammal Evol 12:359–378. offered a more compelling reason for nonpreservation: Echid- 4. Musser AM (2003) Review of the monotreme fossil record and comparison of palae- ontological and molecular data. Comp Biochem Physiol A Mol Integr Physiol 136:927–942. nas lack teeth, the most common mammal fossil remains. 5. Grifﬁths M, Wells RT, Barrie DJ (1991) Observations on the skulls of fossil and extant We proposed an aquatic ancestry for echidnas based on echidnas (Monotremata: Tachyglossidae). Australian Mammalogy 14:87–101. platypus-like morphology among stem-monotreme fossils pre-

dating the divergence of echidnas from the platypus lineage Author contributions: T.H.B., M.J.P, and M.S.Y.L. performed research; and M.J.P. and M.S. (2, 4). Dorso-ventral compression of the body, “front-wheel Y.L. wrote the paper. drive locomotion” based on humeral long-axis rotation, and The authors declare no conﬂict of interest. 1 reversed hind-foot posture in echidnas again were considered To whom correspondence should be addressed. E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.0913152107 PNAS | January 26, 2010 | vol. 107 | no. 4 | E13 Downloaded by guest on September 28, 2021