Southern Cross(ings)

A continuing theme in my blog posts is how different South America was millions of years ago compared to today. Although that is certainly true, the difference isn’t nearly so dramatic as that of Antarctica.

Today Antarctica is almost completely covered by ice, and the only mammals (other than humans) that call it home are marine mammals such as seals and sea lions. That has been true for millions of years, but if you were to go back to the Eocene period, more than 34 million years ago, you would have found Antarctica to be a very different place. The continent was forested with southern beech (Nothofagus spp.) and a variety of other plants, and a diversity of mammals were living there alongside birds and other animals. Unfortunately, fossils of these Eocene mammals are difficult to come by, so we only have hints about most of the species, but they do give us some important information about ancient Antarctic ecosystems.

At this point, you may be wondering how anyone finds fossils of any type in Antarctica, given all the ice. A mere dusting of snow usually prevents looking for fossils anywhere else in the world, so how is it possible to find them in Antarctica? The key is to look for fossils in places that aren’t completely covered by ice, at least not during the austral summer. In the case of Eocene mammals, this means working on Seymour Island, a large island located near the tip of the Antarctic Peninsula. This peninsula extends some 800 miles (1,300 km) north toward South America, with the result that Seymour Island is a mere 620 miles (1,000 km) from that continent’s southern tip, less than the distance between Atlanta and Miami in the US.

Fig 1 Seymour Island Map.jpg
Modern geography of southern South America and West Antarctica. Image by D. Croft, modified from Google Maps.

Traveling to Antarctica from South America requires crossing the Drake Passage, a legendary area of high winds, frigid temperatures, and rough seas. Nowadays, this is done by ship or plane, but in the deep past, the trip could have been done on foot. At the end of the Mesozoic Era and the beginning of the Cenozoic, around 70-50 million years ago, dry land joined Antarctica to both South America and Australia. These connections facilitated mammal dispersals to, from, and across what is now an ice-covered continent.

Fig 2 Paleogeography.jpg
Reconstructed paleogeographic relationships among Antarctica, South America, and Australia during the late Cretaceous and early Cenozoic (~75-50 million years ago). Note the land connection (in brown) between South America and West Antarctica, which includes Seymour Island. Shallow continental shelf areas exposed as dry land during certain periods are represented in light blue. Image modified from Reguero et al. (2013:fig. 1.1).

The Eocene mammal most commonly found in Antarctica is one that clearly came from South America: Notiolofos arquinotiensis. This endemic South American ungulate was quite large for its time – 900 lbs. or 400 kg, similar to a female bison – and is one of the few exceptions to the rule that South America’s native ungulates generally aren’t found outside of South America.

Notiolofos is a litoptern, the same group that includes a diversity of somewhat camel-like or deer-like species, but it belongs to the family Sparnotheriodontidae. (As far as I am aware, this is the second-longest name of any currently-recognized family of mammals*.) Sparnotheriodontids are uncommon in the fossil record, only being recorded at a handful of sites between about 50 and 45 million years in age. They are mostly known from isolated teeth, which are low-crowned and lophed and presumably were used to browse on twigs and leafy vegetation. An interesting aspect of Notiolofos arquinotiensis is that the species apparently underwent no significant evolutionary change over a period of more than 15 million years. This could be due to stable environmental conditions in Antarctica during this interval.

Fig. 3 Sparnotheriodon.jpg
A well-preserved lower jaw of the middle Eocene litoptern Sparnotheriodon epsilonoides (MACN 18225) from Cañadón Vaca, a site in Chubut, Argentina. This is the most complete specimen of a sparnotheriodontid yet discovered. Image from Soria (1980:fig. 1).

The other type of endemic South American ungulate that has been found in Antarctica is an early astrapothere, Antarctodon sobrali. Astrapotheres are typically among the largest mammals at Oligocene and Miocene sites in South America, often tipping the scales at a ton or more, but Antarctodon was a fraction the size of Notiolofus, similar to a sheep (65 lbs. or 30 kg).

The vast majority of mammal species discovered in Antarctica are marsupials. Like the continent’s endemic ungulates, these marsupials seem to have gotten there over dry land from South America. However, Antarctica didn’t act simply like a cul-de-sac for South American marsupials as it did for South American ungulates. Some marsupials traversed the continent and entered Australia, eventually giving rise to the great diversity of Australian species that we know today. From there, it seems that at least one lineage decided to head back home; the microbiotheres crossed Antarctica again and went back to South America. The only living members of this little-known branch of the marsupial evolutionary tree are three tiny species of monitos del monte. During the Eocene, the larger Marambiotherium glacialis, which was the size of a medium-sized opossum (5 oz or 150 g), inhabited Antarctica.

Monito_del_Monte_ps6.jpg
A modern monito del monte (Dromiciops sp.), the living representatives of the order Microbiotheria. Photo by José Luis Bartheld, from Wikimedia Commons. Reuse permitted under CC BY-NC-SA 2.0.

The most common marsupials in Antarctica are two species of the genus Antarctodolops, a member of the peculiar family Polydolopidae. Polydolopids are common at many Paleogene sites in South America, though despite their abundance, they are rather poorly known; only one or two skulls (depending on one’s definition of the family) have been described thus far. Polydolopids are easily recognized by their distinctive dentition, which includes a large, procumbent lower canine and a prominent, shearing, “plagiaulacoid” lower premolar.

Fig. 4 Kramadolops abanicoi.jpg
Illustration of the left lower jaw of the polydolopid marsupial Kramadolops abanicoi from the early Oligocene of Tinguiririca, Chile. Note the large, outward-pointing (procumbent) canine at the far left, and the tall, ridged, triangular, “plagiaulacoid” premolar in the middle of the jaw. From Flynn and Wyss (1999:fig. 1B). Scale bar equals 2 cm.

When did these trans-continental dispersals take place? With the caveat that we are still missing lots of information, it is thought that marsupials crossed at the very end of the Cretaceous (about 70-65 million years ago), whereas ungulates like Notiolofus and Antarctodon crossed slightly later, perhaps 60-55 million years ago. The Drake Passage probably did not open until 34 million years ago, but land dispersals between South America and Antarctica had probably ceased by about 55 million years ago.

As is often the case in paleontology, the fossils found in Antarctica raise as many questions as they answer. For me, a big one is: why haven’t notoungulates been found there? Remains of notoungulates are the most common fossils at most Eocene sites in Patagonia, and if litopterns got to Antartica, one would think notoungulates would have gotten there, too. It is true that the fossil sample is small and that we are ignorant of most of the mammal species that were living there in the Eocene, but it is curious that not a single notoungulate tooth has yet been found in Antarctica given their diversity and abundance in nearby southern Argentina at the time.

This is just one of many reasons to keep returning to Antartica to find more mammal fossils. As usual, I will be anxiously awaiting to hear what my colleagues have found on their most recent expedition!

*The longest mammal family name currently in use seems to be Palaeochiropterygidae, a group of extinct bats; thanks to Christian Kammerer for this taxonomic tip!

References:

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