Friday 27 April 2012

What a 4.6 million-year-old Three Toed Horse can tell us about the climate of Mid Pliocene Tibet.

The Indian Plate has been pushing into the Eurasian Plate from the south for about 55 million years, creating uplift in the Tibetan Plateau and Himalayas in the process. This has had a profound effect on the global climate and the development of monsoon rain patterns in Asia, and is therefore of great interest to scientists. The majority of this uplift is thought to have happened within the last 5.5 million years (the Himalayas are very young mountains) but the rate at which this happened is far from clear.

In a paper published in the Proceedings of the National Academy of Sciences on 23 April 2012, a team of scientists led by Tao Deng of the Key Laboratory of Evolutionary Systematics of Vertebrates at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and the Department of Geology at Northwest University discuss the functional morphology of the Mid Pliocene Three Toed Horse, Hipparion zandaense from the Zanda Basin of the southwest Tibetan Plateau, and the implications that can be made about the local climate, and therefore elevation, from this.

Maps showing the location where Hipparion zandaense was found. Deng et al. (2012), supplemental material.

Hipparion zandaense has enlarged trochlear ridge on its femur, an adaptation which enables modern Horses to lock their knee joints, enabling them to stay erect for long periods of time, but which is absent in other Three Toed Horses. It also has reduced side-toes and an elongated central tie compared to other Three Toed Horses; these animals are believed to have run using only their central toe, but to have stood on all three toes when stationary or walking. H. zandaense (like modern Horses), stood permanently on a single toe, so that it was always in running mode.

The skeletal anatomy of Hipparion zandaense, preserved bones in grey, white bones are inferred. (a1) The foot of Hipparion zandaense, showing the shortened side toes. (a2) The foot of the closely related H. primigenium with longer side toes. (b1) Central hind toe of Hipparion xizangense. (b2) Central hind toe of H. zandaense. (b3) Central hind toe of the modern Wild Horse Equus caballus. (c1) Femur tip of Hipparion primigenium, lacking an enlarged trochlear ridge. (c2) Femur tip of H. zandaense with an enlarged trochlear ridge. (c3) Femur tip of the modern Wild Horse Equus caballus, with an enlarged trochlear ridge. Deng et al. (2012).

Hipparion zandaense also has high tooth crowns compared to other Three Toed Horses, which suggests a diet of grasses rather than leaves, and isotopic studies of these teeth further suggests that they were feeding on cold adapted C₃ grasses rather than warm adapted C₄ grasses.

Grasses are less nutritious than leaves, requiring grass feeders to remain on their feet grazing for more hours per day. Such animals also need to be alert for predators, which is best done from an upright position; modern Horses sleep on their feet, and Hipparion zandaense appears to have been well adapted to doing the same. The best strategy for a grasslands grazer faced with a predator is to try to outrun it, whereas woodland grazing animals will often try to hide.

Taken together these adaptations strongly suggest that Hipparion zandaense was a grassland animal, almost certainly living in a cool climate.

The most likely explanation for this is that the portion of the Tibetan Plateau on which Hipparion zandaense was living had already been elevated above the permanent tree line by 4.6 million years ago, when it was alive. The Mid Pliocene climate was on average 2.5°C warmer than today, which would have made the tree line about 400 m higher than it is now. This suggests that the Mid Pliocene Zanda Basin was at least as high as it is today.

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