The platypus’s closest living relatives are four species of echidna: three species of long-beaked echidna found in Papua New Guinea (Flannery and Groves 1998), and the short-beaked echidna found in both Australia and Papua New Guinea (as shown above). At first sight, echidnas seem to be very different from the platypus – stocky, rounded animals that feed on land (mainly on insects or worms) and are more or less spiny.
However, the platypus and echidnas share a number of features that collectively distinguish them from all other mammals:
- Both groups have a single opening in their body that is used to excrete solid and liquid waste as well as for reproduction (causing them to be classified as monotremes, meaning “one opening”).
- Both groups lay eggs (though, unlike the platypus, echidnas incubate their eggs in a pouch).
- Both groups lack teeth as adults and have an unfurred bill or beak that contains receptors capable of detecting electrical fields produced by their prey (Manger et al. 1997; Gregory et al. 1989; Proske et al. 1998). Four of the eight genes needed for mammalian tooth development are absent in both the platypus and short-beaked echidna, implying that they probably evolved from a single shared toothless ancestor (Zhou et al. 2021).
- Both groups have a lower active body temperature than most other mammals, averaging about 32oC. (Grigg et al. 1992 a, b).
- In both groups, pointed spurs are present on the ankles of adult males. These are used by the platypus to deliver venom but have little or no functional value in echidnas (Temple-Smith 1973; Griffiths 1978).
- Platypus and echidna skeletons have many reptilian features that are not present in other mammals, including differences in the skull, ear region, ribs, backbone, and limbs and limb girdles (Murray 1984).
Reflecting the fact that the earliest known echidna fossils are no more than 15 million years old (Musser 2003), the evolutionary relationship between echidnas and the platypus remains poorly understood. However, it’s currently believed that echidnas most likely developed from a platypus-like ancestor rather than vice-versa between 19 and 78 million years ago (Westerman and Edwards 1991; Belov and Hellman 2003; Phillips et al. 2009).
Photo: APC
LITERATURE CITED
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