South America was an isolated continent for much of the Cainozoic, connected only to Antarctica during the earlier part of the period and not forming a land bridge to North America until the Pliocene. This enabled the continent to develop a unique fauna and flora unlike that of other parts of the world, including creatures such as giant ‘Terror Birds’, Sloths, large carnivorous Marsupials and a number of now extinct groups of large herbivorous Mammals (‘Ungulates’); the Xenungulata, Notoungulata, Litopterna, Pyrotheria and Astrapotheria. The relationships of these extinct groups has remained controversial, with some palaeontologists suggesting they are related to other ‘Ungulates’ - Artiodactyls (Deer, Antelopes, Camels, Pigs etc.) plus Perissodactyls (Horses, Tapirs and Rhinoceroses) – while others suggest they are more closely related to the South American Xenarthrans (Sloths, Armadillos etc.).
In a paper published in the Proceedings of the Royal Society: Series B, Biological Sciences on 1 April 2015, Michael Buckley of the Faculty of Life Sciences at the Manchester Institute of Biotechnology presents a study of evolutionary relationships within Mammals based upon collagen protein sequence analysis, which includes two South American ‘Ungulates’ the Litoptern Macrauchenia patachonica and the Notoungulate Toxodon platensis.
The last two decades has seen a revolution in taxonomic classification based upon the use of DNA analysis. Loosely speaking, the DNA of animals and plants contains large areas of non-coding DNA, which do not apparently serve any function and therefore are not subject to evolutionary pressures. The mutation rate in these areas of DNA is therefore thought to be a steadily paced ‘random walk’, enabling DNA-based phylogenies to be build up which are not subject to convergent evolution (structures being subject to similar evolutionary pressures and therefore ending up looking the same), as is the case with phytogenic trees based upon morphological comparisons.
DNA-based studies have led to Therian (Placental) Mammals being divided into four main groups, the Euarchontoglires (Primates, Rodents and Lagomorphs), the Laurasiatherians (Bats, Carnivores, Perissodactyls etc.), the Afrotherians (Elephants, Elephant Shrews, Hyraxes etc.) and the Xenarthrans (Sloths, Armadillos etc.), with the Euarchontoglires and Laurasiatherians being considered sister-groups, sometimes referred to collectively as the Boreoeutheria and different studies resulting in either the Afrotherians or Xenarthrans being the closest relatives of this combined group.
DNA sequencing techniques have also been applied to the remains of a variety of ancient organisms, with positive results being obtained from material as old as the early Pleistocene in cool regions (claims of DNA recovery from older material, particularly Mesozoic Dinosaurs, are at best fanciful). However much of South America has a tropical or subtropical climate, severely limiting the potential of DNA recovery from many of its unique fossil groups somewhat unlikely.
Collagen is one of the toughest and most widely used proteins in vertebrate bodies, being found in cartilage, ligaments, tendons and, significantly, in bone. The mineral structure of vertebrate bone is essentially laid down over a framework of collagen, encasing this protein in a mineral matrix which can preserve it from environmental damage long after the death of the animal. Significantly collagen has a somewhat flexible composition, and away from binding sites can contain any amino acids without apparent harm. This means that like DNA, the amino acid structure of collagen is subject to an evolutionary random walk, which can be used to trace evolutionary relationships within vertebrate groups.
Specimen of the Notoungulate Toxodon platensis on display at the Bernardino Rivadavia Natural Sciences Museum. Wikimedia Commons.
Buckley extracted collagen samples from two specimens each of the Litoptern Macrauchenia patachonica and the Notoungulate Toxodon platensis from Pleistocene sites in Buenos Aires, Argentina, and used them to build a collagen proteomic family tree which also included representatives of the Euarchontoglires, Laurasiatherians, Afrotherians, Xenarthrans, Marsupials and Monotremes.
His results recovered the existence of the Boreoeutheria, comprising the Euarchontoglires and Laurasiatherians. The sister group to the Boreoeutheria was the Xenarthrans, with the Afrotherans being the sister group to all other placental Mammals. Within the studied animals, the Litoptern Macrauchenia patachonica and the Notoungulate Toxodon platensis were the closest relatives of eachother, and both were more closely related to the Perissodactyls than any other group.
Phylogenetic analyses of Toxodon and Macrauchenia collagen sequences matched by LC–MS/MS rooted to the duck-billed platypus (Ornithorhynchus), showing maximum-likelihood analysis using PhyML with 10 000 bootstraps (less than 50 not shown except for Toxodon and Macrauchenia). Buckley (2015).
This study suggests that at least two of the extinct South American Ungulate groups, the Litopterns and the Notoungulates, were closely related to the Perissodactyls, leading to a distinct possibility that the other three groups, the Xenungulates, Pyrotherians and Astrapotherians were also. This also suggests that the ancestors of these animals were able to make the crossing between Laurasia (Eurasia plus South America) some-time in the early Cainozoic or possibly the End Cretaceous, suggesting that a land bridge between North and South America must have existed at this time.
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