New study reconstructs the Eligocene to Pleistocene equine niche in East Asia

On February 15, the research team of Deng Tao from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences published the latest progress in the study of paleoecology of equines in the Linxia Basin of Gansu Province in the journal Paleogeography Paleoclimate Paleoecology. The study focused on a key stage in the evolutionary history of equines from the Pliocene to the Pleistocene, and used carbon-oxygen stable isotope analysis to reconstruct the ecological niche of five horse species in the fauna of Shilidun (early Pliocene, about 5.3 million years ago) and Longdan (early Pleistocene, 2.5 to 2.2 million years ago) in the Linxia Basin.

Deng Tao, one of the corresponding authors, said that combined with the ecological morphology, paleoenvironment and paleoclimatic analysis of equines, the study revealed for the first time the ecological niche differences of equines in East Asia and their significance in the evolutionary history of three-toed horses and true horses.

The paleoecological face of living horses

Skulls of primitive proboscis three-toed horses and living Malayan tapirs. Image source: Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences

There is only one genus of real horses, Equus, which is the only remaining representative of its prosperous population in ancient times.

More than a century after the publication of Darwin’s On the Origin of Species, the equidae became popular evidence for its textbook macroevolutionary history, and its widespread fossil material and significance in geological history made it the most studied category of mammalian fossils.

From the early Eocene Archae Hyracotheriinae to the Equinae after the Miocene, represented by the three-toed horse family Hipparionini and the True horse family Equini, horses gradually increased in size, crowned and reduced toe count. This evolutionary trend means that, overall, horses have changed their diet from soft leaves to rough herbaceous plants, and gradually adapted to the open grassland environment, until they have become capable of contributing to the history of human civilization.

Since their origins in North America, equines have roamed the vast old and new continents. Among its large family, the three-toed horse family is undoubtedly the most successful branch, with a high diversity of species and the richest fossil population. Different species of zebras and wild asses in the modern African grassland and different wild horses and wild asses in the Mongolian Gobi have symbiotic phenomena, and the coexistence of a variety of equine animals in geological history is extremely common.

In the third chapter of On the Origin of Species, “The Struggle for Survival,” Darwin hit the nail on the head: “Species of the same genus are always structurally similar… The struggle for survival between conspecifics and variants is the most intense.” Therefore, the symbiosis of multiple equines in the same ecosystem means that they need to use different resources and habitats to eliminate interspecific competition, which shows the important role that paleoecology plays in the evolution of equines.

But in East Asia, geologists and paleontologists often use horse fossils as tools to study strata and paleoenvironments, but pay little attention to their own paleoecological landscapes.

Answering questions about the ecological niche of the native Three-toed horse in East Asia

The study found that in the early Pliocene decay fauna, Proboscidipparion pater was more adapted to arid and open environments than symbiotic Cremohipparion licenti and Sivalhippus platyodus, and the ecological niche of the latter two basically overlapped and ingested more leaves than the primitive proboscis three-toed horse.

By the Pleistocene, the three-toed horse family declined sharply around the world, and only two species remained in East Asia, one of which was the direct descendant of the original proboscis three-toed horse, the Chinese proboscis three-toed horse. Among the dragon fauna of the Early Pleistocene in the Linxia Basin, the Chinese proboscis three-toed horse also occupies the most open ecological environment, ingesting more rough grass resources than the symbiotic large true horse Equus eisenmannae.

This conclusion goes against conventional wisdom, as the conventional view has always been that the recent true horse is the most typical “herbivore”. Nearly a century ago, in 1927, when European paleontologist Sefve first saw this three-toed horse from East Asia, he was surprised by its unique snout structure, so he named it the proboscis three-toed horse; And because of its morphological similarity with the skull of living tapirs, it is inferred that the proboscis three-toed horse may have lived in a swampy environment near water, feeding on soft leaves.

But the study found that both species of the genus Proboscis inhabit open environments, a finding that quantitatively answers a century-long unanswered question about the native East Asian three-toed horse niche.

The most survivable three-toed horse became extinct at the latest

After pooling data from other animal phyla in previous studies, the study found that carbon isotope data for two species of proboscis horses remained the most positive in the entire herbivore population, providing strong evidence that the genus occupies the most open ecosystems in their respective ecosystems.

Compared with other symbiotic three-toed horses, the two species of the genus Proboscis have larger body size, higher cheek tooth crown height index, and more complex fold morphology of the cheek tooth occlusal surface, and the mechanical analysis of limb bones also confirms the strong running ability of proboscis three-toed horses, all of which support the conclusion of stable isotope analysis from another perspective.

In addition, the carbon and oxygen isotope sequence sampling study of the high-crowned horsetooth horse teeth revealed that the early Pliocene East Asian summer wind signal in the Linxia Basin was stronger than that of the early Pleistocene, indicating that the climate of the early Pliocene was warmer and wetter. In this context, the proboscis three-toed horse evolved a pre-adaptation to the open environment and rough grass resources from the warm and humid Pliocene, which helped it successfully continue to the Pleistocene more than one million years ago.

The symbiotic Siwa three-toed horse and the weeping-nosed three-toed horse may not be able to adapt to the dry and cold and turbulent environment after the Pliocene because of the low proportion of grass resources in their food structure. The interspecific competition between these two medium-sized three-toed horses intensified in a deteriorating environment, causing them to lose their original ecological niche and become extinct around 3 million years ago.

At the beginning of the Pleistocene, true horses migrating from North America rapidly expanded in Eurasia, far exceeding the number of three-toed horses that had declined. The newly emerging Eyre’s horse weighs about 700 kilograms, which is larger than the symbiotic Chinese proboscis three-toed horse, and the number of fossils far exceeds that of the Chinese proboscis three-toed horse, but the latter, as an “indigenous” species in East Asia, seems to be tenacious to occupy its own shrinking territory.

Under the general trend of Quaternary global climate drying and cooling, the three-toed horse, a relict species of the Tertiary period, has disappeared in various continents. True horses took their place, adapted to the turbulent climate of the Quaternary and the effects of human activities, and continue to this day.

The research was also supported by the Natural Science Foundation of China, CAS Strategic Pilot Project B, CAS Youth Promotion Association and Special Research Assistant Program, and the Second Qinghai-Tibet Scientific Expedition. (Source: Cui Xueqin, China Science News)

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