#echidnas

Yang: What'cha doing there, buddy?

Jaune: Trying to erase a picture from my mind that Ren sent me.

Yang: From Ren? Ren's got you wishing for a memory wipe?

Jaune: *Sighing deeply* He... educated me about echidnas.

Yang: The little spikey dudes? What could possibly be so bad about them?

Jaune: *Passes over his Scroll* ...

Yang: *Takes it, opens chat log with Ren* ...

Yang: Well, this doesn't seem so ba- *ECHIDNA GENITALS HAVE 9 HEADS.PNG*

~~~~~~~~~~

Jaune&Yang: *Leaning back on a bench, pressing the palms of their hands hard against their eyelids* ...

Yang: What did you do to Ren for him to retaliate like this?!

A short beaked echidna (Tachyglossus aculeatus) in Freycinet, Australia

Our existing monotremes, the four species of echnidna and the platypus, don't seem like they have all that much in common besides their basic monotreme traits like egg-laying and milk production. Echidnas are spiny and relatively slow-moving as they trundle across the land, while platypodes (platypuses? platypi?) are semi-aquatic and quite adept at swimming.

Scientists have long assumed that the last common ancestor of these species was a terrestrial mammal, since semi-aquatic and aquatic mammals tend to stay in the water once they've evolved the proper adaptations. So when this little fossil monotreme humerus discovered in southeastern Australia was examined in further detail, it suggested a much more unusual evolutionary path.

The bone was much more robust than expected, more resembling the heavy bones of the platypus than the comparatively gracile bones of the echidnas. Denser bones help the semi-aquatic platypodes to stay submerged in water with less effort, but they would be a detriment to a relatively small land animal.

What this suggests is that the last common ancestor of today's monotremes may have been semi-aquatic. The platypus's origins wouldn't be surprising, but it is exceedingly rare for a mammal lineage that headed into the water to then return to a fully terrestrial existence later on.

Of course, there are a lot of ifs involved. We don't know for sure that the newly found stem-monotreme humerus represents the direct ancestors of modern monotremes. Even as much as paleontologists have learned about the living beings of the past and the ecosystems they inhabited, it is likely that less than one tenth of one percent of all species that have ever lived ended up fossilized. We can only guess at the biodiversity we don't have direct evidence for, and it may be that another species living at the same time as the stem-monotreme was the actual last common ancestor of today's monotremes.

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