The Empty Pampas: epitome of a biogeographical mystery, part 2

...continued from https://www.inaturalist.org/journal/milewski/55189-the-empty-pampas-epitome-of-a-biogeographical-mystery-part-1#

Everyone knows that

• the megafauna which disappeared from the Americas about ten thousand years ago included the relatives of elephants, camels and zebras;
• a particular feature was an array of extremely large relatives of sloths and armadillos; and
• the most outlandish of the carnivores were sabre-tooth felids and short-faced bears resembling gigantic hyenas.

However, what is less well-known is that, in most respects, Buenos Aires Province is even more deserving of the 'mega-' than the La Brea tar pits of California. On a prey-base of diverse large mammals of both modern and archaic (e.g. see https://www.researchgate.net/profile/Juan-D-Carrillo/publication/324364169_Systematics_of_the_South_American_Native_Ungulates_and_the_Neogene_Evolution_of_Mammals_from_Northern_South_America/links/5b76d884299bf119f6532b78/Systematics-of-the-South-American-Native-Ungulates-and-the-Neogene-Evolution-of-Mammals-from-Northern-South-America.pdf and https://en.wikipedia.org/wiki/Litopterna) lineages, South America supported the largest felid ever recorded (Smilodon populator, https://prehistoric-fauna.com/Smilodon-populator) in addition to a true lion (see https://www.sciencedirect.com/science/article/pii/S1631068317301094).

The capacity of the pampas to support a megafauna outdoing, in some sense, even that of Pleistocene Africa is possibly owing to the extreme nutrient-richness of a coastal plain which at the time reached even farther out on the coastal shelf than it does today (see https://culturalandscape.wordpress.com/2012/11/19/ice-sheets-world-map/). This fertility is owing to the unusual genesis of the soils: repeated dustings of volcanic ash brought on the wind from the southernmost Andes, thousands of kilometers distant, for millions of years (see https://www.researchgate.net/publication/325475166_Soils_of_the_Pampean_Region).

Despite the megafaunal extinction, there remained in South America several genera of ungulates (Ozotoceros, Blastocerus, Hippocamelus, Mazama, Llama, Vicugna, Pecari), plus several genera of rodents (Dolichotis https://www.researchgate.net/figure/Photo-of-a-mara-Dolichotis-patagonum-with-some-adaptations-for-running-being-highlighted_fig3_333504431, Hydrochoerus) with a potential for functional replacement of ungulates.

Body size is one of the most adaptable features of mammals, making it easy to imagine compensation in the pampas within a few thousand years in response to the new opportunities for niche-expansion.

• Ozotoceros, Mazama (https://www.flickr.com/photos/amazon_eco_lodges/4328183289) and the collared peccary could have increased in size.
• Hippocamelus (https://www.alamy.com/south-andean-huemul-female-eating-torres-del-paine-chile-image276135999.html) and Vicugna could have moved in from the Andes.
• Blastocerus (https://www.mammalwatching.com/2013/08/30/brazil-2013/2013-marsh-deer/) and the capybara (https://www.youtube.com/watch?v=T87T_OhqJN0) could have adapted to dry land.
• And indeed that exceptionally adaptable species, the white-tailed deer (Odocoileus virginianus, see https://www.naturallynorthidaho.com/2012/08/white-tailed-deer-far-ranging-species.htm), could have completed its immigration from the north.

Even if just the guanaco (Llama guanicoe, https://www.sciencephoto.com/media/1126732/view/guanaco-grazing-on-grassland-plain-patagonia-chile), plus a long-legged derivative of the capybara, plus one species of deer exceeding 100 kg, had all obeyed the axiom 'nature abhors a vacuum' with enhanced gregariousness, the Empty Pampas would have been restored to a plain full of movement.

But this did not happen, and even the pampas deer - a weakly gregarious 'token' of plains game - remained less than half the body mass of the closely related mule deer (Odocoileus hemionus, https://pxhere.com/en/photo/1385001) of North American grassland.

To be continued in https://www.inaturalist.org/posts/55195-the-empty-pampas-epitome-of-a-biogeographical-mystery-part-3#...

Posted on 12 August, 2021 05:38 by milewski