South America and colossal animals

[LeopJag]

I wonder what is it about South America (in prehistoric times at least) that has begot such huge land animals? the largest ever found dinosaurs, bison-sized cavie rodent, biggest ever bear, ground sloths and some others that i can't think of,, where from that continent... what are the most widely accepted scientific theories regarding this?

thanx

[Derdadort]

I don't think it has anything to do with South America. If you look at other continents you can also find a lot of big animals. Take for example North America: Sauroposeidon, T.rex, "Amphiceolias", Columbian Mammoth,...
or take Africa: Carcharodontosaurus, Spinosaurus, Sarcosuchus, Paralititan, Deinotherium,...

During the middle Cretaceous was especially the African megafauna very similar to the South American one. On both continents you found very large Sauropods and big Theropods like Carcharodontosaurids or Spinosaurids. So in this case it has nothing or less to do with the geography of South America.

In my opinion, those South American mammals aren't also anything special. You can find colossoal mammals nearly everywhere on the planet.

[Grey]

That feeling about South America is because the discoveries, especially of big giants, have been all close each other.

[blaze]

The biggest known specimens of Arctodus simus are actually larger than the biggest of Arctotherium angustidens, anyway both animals are very close in size and their genera are sister taxa. Is interesting that Arctodus grew in size since it first appeared, with the biggest species, A. simus appearing during the late Pleistocene while Arcthotherium was larger during the early Pleistocene with subsequent species shrinking in size.

With respect to ground sloths, the tittle of biggest one is shared between Megatherium americanum and Eremotherium laurillardi, which was present in north America as well as south/central America, but they are not bigger than the biggest species of mammoth so I don't see how it is special that they're big, though I certainly find prehistoric south American fauna fascinating, those that evolved in isolation from the world for millions of years until the panama land bridge was formed, that is the reason, I think, why caviomorph rodents grew so large there, there were no ungulates and after the extinction of the astrapotheres, the niche of large grazing animal, was completely at their disposal.

[Fragillimus335]

Yes, I think it's a media bias that makes South America seem like the land of giants.

[theropod]

From some of the animals we know it may appear like SA is a land of giants. But in fact, there isn't much basis of this. Some truly huge animals (Arctotherium, Josephoartigasias, Glybtodon, Giganotosaurus, Mapusaurus, Megatherium, Argentinosaurus, Puertasaurus etc.) have lived there, no question.

But most of those are not unmatched in their respective "titles", some also common on other continents in a similar form, some undoubtedly exceeded, and not all are necessarily huge in absolute terms, just for their respective subtaxa.

Eg. a 1t rodent or armadillo is huge for their respective groups. But not for a herbivorous mammal. This kind of gigantism in otherwise mostly small-sized taxa is often found on isolated continents. There simply is no other animal to effectively fill the niche, and consequently no better-suited competition.

When talking about the Argentine titanosaurs many people seem to forget that almost certainly sauropods known from NA, Europe, Africa and Australia (A. fragillimus, "B. nougaredi", Plagne trackmaker, Parabrontopodus distercii, Broome trackmaker...) easily reach or exceed their size.

The same applies to carcharodontosaurs like Giganotosaurus or Mapusaurus, similar-sized as well as undoubtedly bigger theropods are known from other continents, especially Africa.
These are merely the most well-popularized giants, but if one has a critical look at them they are not that gigantic at all.

The reason is the somewhat peculiar fauna that evolved on the southern continents, which were isolated for more than 100 miy. This means macrofauna often consists of more "exotic" or unusual taxa there, that seem bigger because they are so big compared to close relatives (giant rodents and xenarthrans), or simply because some of the major discoveries were recent and well popularized (titanosaurs, carcharodontosaurs, spinosaurs).

[gigadino96]

I've read that the dinosaurs in Cenomanian stage of the late Cretaceous were so big due to much oxigen. I don't know if it's true.

[theropod]

Undoubtedly the athmospheric conditions for animals were better than in the highly vulcanic latest cretaceous. On the other hand, less oxygen but more carbon-dioxide is beneficial for plants. And we also have some similar-sized animals from the Maastrichtian (like Puertasaurus, Alamosaurus, Tyrannosaurus). Probably not the record-holders for their higher-level clades, but definitely gigantic too.

Anyway, the largest dinosaurs (and perhaps animals) ever seem to have lived during the upper Jurassic, not the mid-Cretaceous, and we also have evidence for giant theropods from that period. The cenomanian is not uniquen in this regard, there were almost periodic periods that brought forth particularly large animals.

[gigadino96]

Maybe the largest Sauropods (if Amphicoelias is real) but not the largest Theropods. The largest Jurassic Theropod, Saurophaganax, was ~ 6,5-7 t, and we know Cretaceous Theropods larger (Giganotosaurus, Carcharodontosaurus, Spinosaurus ecc).

[theropod]

I was referring to ichnotaxa. It would be foolish to ignore them, and they are evidence for gigantig theropods.

[Derdadort]

There are Ichnotaxa, which indicate Theropods larger than Spinosaurus?

[gigadino96]

Jul 29, 2013 18:40:36 GMT 5 Derdadort said:

There are Ichnotaxa, which indicate Theropods larger than Spinosaurus?

This ichnotaxon is likely about 15 m and 9-11 t. Yeah, big, but smaller than Spino.

[Grey]

Ichnotaxa can be hints of giants animals, they nowhere and never indicate proper measurements.

[theropod]

Grey: I don't see why that matters. This is just about a rough idea of its size, and that is enough.

Jul 29, 2013 18:40:36 GMT 5 Derdadort said:

There are Ichnotaxa, which indicate Theropods larger than Spinosaurus?

Not Spinosaurus, but on par with or larger than the rest (large Carcharodontosaurs, Tyrannosaurs), proving my point that gigantism in theropods is more widely and evenly distributed than many might think.

Spinosaurus is the exception among theropods due to its specialized piscivorous and semiaquatic lifestyle.

[Grey]

Yes but a rough idea is what it is : a rough idea.