The Antarctic land bridge was a land bridge connecting the continents of South America, Antarctica, and Australia that existed from the Late Cretaceous to the Late Eocene. The land bridge consisted of the entire continent of Antarctica (at the time unglaciated), as well as much narrower, now-submerged landforms that connected Antarctica to both South America and Australia (with the land bridge between South America and Antarctica sometimes called the Weddellian Isthmus or the Isthmus of Scotia [1] [2] ). At its greatest extent, the Antarctic land bridge allowed for a terrestrial connection between South America and Australia, allowing numerous animals and plants to disperse across both continents using it. [3]
The Antarctic land bridge came to an end during the Late Eocene or Early Oligocene, when the formation of both the Drake Passage and Tasmanian Passage cut off any further land connections of either continent with Antarctica. These openings also created the Antarctic Circumpolar Current, which eventually led to the glaciation of Antarctica, making the continent largely inhospitable to any terrestrial life. Signals of the Antarctic land bridge's influence are still present in both the genetics and distribution of many modern animals and plants. [3]
As components of the supercontinent Gondwana, the continents of Antarctica, South America, and Australia were connected to each other for much of the Mesozoic. However, the supercontinent started to rapidly fragment from the Early Cretaceous onwards. By the time of the Late Cretaceous (96 Ma), a shallow seaway had started to develop between Australia and Antarctica, with only a narrow strip along the South Tasman Rise, joined to Wilkes Land, still connecting both continents. Evidence of dinosaurs dispersing between the continents has been inferred from the Australian sauropod Diamantinasaurus , which closely resembles the South American Sarmientosaurus , suggesting that a common ancestor of both dispersed across Antarctica. [4] [5] [6] Australia and Antarctica finally broke apart around 45 million years ago, and the South Tasman Rise was likely already submerged by water a few million years prior to this point, forming the Tasmanian Passage. However, a steady circumpolar likely did not form between the continents until 30 million years ago. [3]
Meanwhile, the tip of South America stayed closely connected to the Antarctic Peninsula via exposed parts of the Scotia Plate, with both starting to gradually separate starting from 50 million years ago. [7] Continued seafloor spreading along the Scotia Ridge eventually separated both continents by 28 million years ago, forming the Drake Passage. [3]
Thus, a narrow connection between South America, Antarctica, and Australia existed between ~96 to 50 million years ago, and South America and Antarctica (but not Australia) remained connected up to 28 million years ago. The breakup of these three continents can be considered the final step in the fragmentation of Gondwana. [7] The narrower landforms that connected both continents to Antarctica may have been contiguous land during the Cretaceous, but by the Cenozoic, they had most likely turned into island chains (first separated by freshwater lakes, then shallow seas as ocean basins formed) that terrestrial organisms would have needed to island-hop across. [3]
The term "Amphi-Pacific distribution" is used for taxa that have a distribution consisting of southern South America and Australia. [3]
Taxa whose evolutionary history was influenced by the Antarctic land bridge include: [3]