Editing Echidna (section)

==Evolution==
[[File:Echidna skeleton.jpg|thumb|Short-beaked echidna skeleton]]
The divergence between oviparous (egg-laying) and viviparous (offspring develop internally) mammals is believed to date to the [[Triassic]] period.<ref>{{cite journal |vauthors=Rowe T, Rich TH, Vickers-Rich P, Springer M, Woodburne MO |title=The oldest platypus and its bearing on divergence timing of the platypus and echidna clades |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=4 |pages=1238–42 |year=2008 |pmid=18216270 |pmc=2234122 |doi=10.1073/pnas.0706385105 |bibcode=2008PNAS..105.1238R |doi-access=free }}</ref> Most findings from genetics studies (especially of nuclear genes) are in agreement with the paleontological dating, but some other evidence, like mitochondrial DNA, give slightly different dates.<ref>{{cite journal |author=Musser AM |title=Review of the monotreme fossil record and comparison of palaeontological and molecular data |journal=Comp. Biochem. Physiol. A|volume=136 |issue=4 |pages=927–42 |year=2003 |pmid=14667856 |doi=10.1016/s1095-6433(03)00275-7}}</ref>

[[Molecular clock]] data suggest echidnas split from platypuses between 19 and 48&nbsp;million years ago, so that [[Teinolophos|platypus-like fossils]] dating back to over 112.5&nbsp;million years ago represent [[basal (phylogenetics)|basal]] forms, rather than close relatives of the modern platypus.<ref name=Phillips/> This would imply that echidnas evolved from [[Kollikodon|water-foraging ancestors that returned to land living]], which put them in competition with marsupials.{{explain|date=November 2020}} Although extant monotremes lack adult teeth (platypuses have teeth only as juveniles), many extinct monotreme species have been identified based on the morphology of their teeth.<ref name="Bullock-2005" /> Of the eight genes involved in tooth development, four have been lost in both platypus and echidna, indicating that the loss of teeth occurred before the echidna-platypus split.<ref name="ONeil" /> 

Further evidence of [[Australosphenida|water-foraging ancestors]] can be found in some of the echidna's anatomy, including [[hydrodynamic]] streamlining, dorsally projecting hind limbs acting as rudders, and locomotion founded on hypertrophied humeral long-axis rotation, which provides an efficient swimming stroke.<ref name=Phillips/> 

[[Oviparous]] reproduction in monotremes may give them an advantage over [[marsupials]] in some environments.<ref name="Phillips" /> Their observed adaptive radiation contradicts the assumption that monotremes are frozen in morphological and [[molecular evolution]].

It has been suggested that echidnas originally evolved in [[New Guinea]] when it was isolated from Australia and from marsupials. This would explain their rarity in the fossil record, their abundance in present times in New Guinea, and their original adaptation to terrestrial niches, presumably without competition from marsupials.<ref>{{cite journal | doi=10.1080/03115518.2022.2025900 | title=A review of monotreme (Monotremata) evolution | year=2022 | last1=Flannery | first1=Timothy F. | last2=Rich | first2=Thomas H. | last3=Vickers-Rich | first3=Patricia | last4=Ziegler | first4=Tim | last5=Veatch | first5=E. Grace | last6=Helgen | first6=Kristofer M. | journal=Alcheringa: An Australasian Journal of Palaeontology | volume=46 | issue=1 | pages=3–20 | bibcode=2022Alch...46....3F | s2cid=247542433 | doi-access=free }}</ref>