Hacker News Reader

Comment by bj85 | Hacker News Reader

Giant sloths was not really on my list of expected answers but "One species, Glossotherium robustum, a South American sloth that lived between 4 million and 12,500 years ago, reached more than 3 metres in length and weighed up to 1,500 kilograms"

But who was really in charge?

View on HN

> Giant sloths was not really on my list of expected answers

Despite not looking very alike at the first glance, sloths are grouped together with armadillos in the clade Xenarthra[1]. Both lineages descend from a common ancestor from around the time the dinosaurs went extinct. So it's not that surprising there were burrowing sloths. I guess the burrowing behaviour surely came handy to their shared ancestor by the time the dinosaurs were screwed by the asteroid that carved the Chicxulub crater[2]. What is mind-boggling for me is that once there were aquatic sloths[3]. Some sloths followed the steps from the ancestors of whales and dolphins, and rehearsed a return to sea. Unfortunately that evolutionary experiment was cut short when the gap between south and north america closed, isolating atlantic from pacific oceans and dooming the niche where the critters thrived. Had they persisted, I wonder if by now we could have whale-like sloths (whaloths?).

[1] https://en.wikipedia.org/wiki/Xenarthra

[2] https://en.wikipedia.org/wiki/Chicxulub_crater

[3] https://en.wikipedia.org/wiki/Thalassocnus

Thank you for the aquatic sloth link, that's so cool!

Manatees and dugongs feel like "whale-like sloths", though I have no idea if they're taxonomically related

They are more closely related to elephants.

The giant sloth was the first thing that came to my mind. Nearly every time I'm at the grocery I think about the giant ground sloth theory of avocado evolution:

  A number of authors, including Connie Barlow in her 2001 book The Ghosts of 
  Evolution, have speculated that the avocado is an "evolutionary anachronism" 
  with megafaunal dispersal syndrome (a concept originally proposed in the 
  1980s by Paul S. Martin and Daniel H. Janzen), arguing that the avocado 
  likely coevolved dispersal of its large seed by now-extinct megafauna. Barlow 
  proposed that the dispersers included the gomphothere (elephant relative) 
  Cuvieronius, as well as ground sloths, toxodontids, and glyptodonts. The 
  concept of evolutionary anachronisms/megafaunal dispersal syndrome has been 
  criticised by some authors, who note that many large fruit are readily 
  dispersed by non-megafaunal animals, with it being noted that living agoutis 
  disperse avocado seeds, with spectacled bears have also having been observed 
  eating domestic avocados.

https://en.wikipedia.org/wiki/Avocado

https://link.springer.com/article/10.1007/BF02904012

Early wild avocado seeds were smaller at around 2cm/.8in in diameter.

My aunt has a big avocado tree in the north west of Argentina. The avocato are very small, probably an inch long. They are very tasty, but you must collect a lot of them to prepare a dish.

Glossotherium robustum looks quite chunky. There's a skeleton of one in the Natural History Museum London https://en.wikipedia.org/wiki/Glossotherium

and an artists impression https://static.wikia.nocookie.net/fossil/images/9/95/Reconst...

Good question, species do not put that much compulsive effort into preemptive escape without cause.

Maybe humans just used the claws of dead giant sloths to carve the caves for their own purposes.

as is often the case, one species will make a burro only to have other species take over and modify it, with choice locations, used contiously for several millenia, so an odd scratch here and one there adds up, and we have no other behavioral evidence for this last mega fauna, so who knows? might find more of these in different stages, with perhaps fossil evidence

Tunels are 100 millions year old. We split from chimps 5 millions years ago.

Modern humans are .11 million year old, and we arrived to America .015 million years ago.

the sandstone formation is 100 million years old, not the tunnels

No, what is 100 million years old is the rock layer. The tunnels are `recent`.

Just in time to kill the giants sloths?

Thermal regulation is a possibility mentioned outside of predation.

Bears aren’t afraid of being eaten, but they still dig dens because they’re warm in the winter.

Reply on news.ycombinator.com