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Hipparionini

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Hipparionini was an extinct tribe of three-toed equids within the subfamily Equinae. These animals were closely related to modern horses, though they possessed several distinct physical differences. They played a major role in the history of the Earth's landmasses for millions of years. Their existence helps scientists understand how animals migrate and adapt to changing environments. By studying their fossils, we can track how species move between continents.

Cormohipparion occidentale (sturdy three-toed horse) (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA) 4
Cormohipparion occidentale (sturdy three-toed horse) (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA) 4

The physical structure of a hipparionine was quite unique compared to modern equines. While modern horses typically have a single toe, these animals bore three toes on each foot. The central toe was the largest, but the two side digits were also functional. Fossilized trackways show that these side toes actually contacted the ground. They likely helped with joint stabilization and load bearing. These extra toes might have also helped the animals change direction more easily.

Cormohipparion plicatile leg FLMNH
Cormohipparion plicatile leg FLMNH

Hipparionines also showed incredible variety in their body sizes and dental structures. Some species were very small, with a body mass under 50 kilograms, making them similar to sheep. Other species were much larger, exceeding 500 kilograms. Their teeth were designed for durability. They had complex enamel patterns that provided high wear resistance. This was an important adaptation for their changing diets. Some species, such as Proboscidipparion, even had elongated snouts. Scientists think these snouts might have functioned like a tapir-like proboscis.

Cormohipparion plicatile leg FLMNH
Cormohipparion plicatile leg FLMNH

The evolutionary journey of this tribe began in North America about 17 million years ago. During the Middle Miocene, they were as diverse as the Equini tribe. By the Late Miocene and Early Pliocene, they actually became more species-rich than the Equini. A specific genus called Cormohipparion is thought to be the ancestor of all Old World hipparionines. Around 11.4 to 11 million years ago, members of this genus crossed the Bering Land Bridge. This allowed them to migrate from North America into Eurasia.

Cormohipparion occidentale (sturdy three-toed horse) (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA) 4
Cormohipparion occidentale (sturdy three-toed horse) (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA) 4

Following their arrival in Eurasia, the migration continued into Africa around 10.5 million years ago. In Africa, they did not begin to diversify significantly until about 7.5 million years ago. The tribe reached its maximum diversity in Afro-Eurasia between 7.6 and 6.8 million years ago. During the Pliocene, many Old World lineages developed larger bodies and higher crowned teeth. This process is called hypsodonty, which helps teeth handle abrasive food. However, their increase in hypsodonty was not as pronounced as that seen in New World species.

Cormohipparion plicatile leg FLMNH
Cormohipparion plicatile leg FLMNH

The ecology of these animals changed as the world changed. In North America, some species like Neohipparion eurystyle were mixed feeders. This means they both browsed on leaves and grazed on grass. In the Old World, they started as browsers or mixed feeders as well. Over time, a larger proportion of the group became pure grazers. In the western Mediterranean, researchers noticed niche partitioning. This means smaller species were mixed feeders, while larger species focused on grazing.

Cormohipparion plicatile leg FLMNH
Cormohipparion plicatile leg FLMNH

Survival for hipparionines was difficult due to both predators and new competitors. They were hunted by several large animals. These included sabertooth cats like Machairodus and Homotherium. They also faced threats from amphicyonids, which are often called bear-dogs. Around 2.6 million years ago, a major change occurred at the Pliocene-Pleistocene boundary. Modern equines from the genus Equus migrated into Eurasia from North America. These "stenonine" equines provided direct competition for the hipparionines. This competition contributed to their eventual decline.

Cormohipparion plicatile leg FLMNH
Cormohipparion plicatile leg FLMNH

The decline of the tribe was steady across different continents. In North America, they were eventually confined to southern latitudes before going extinct in the Early Pleistocene. In the Old World, the very last members of the tribe lived approximately 1 million years ago. These final species include Proboscidipparion sinense from China and Eurygnathohippus cornelianus from Africa. Their disappearance marks the end of a long and successful evolutionary era. Their legacy remains in the fossil record of the great continents.

Cormohipparion plicatile leg FLMNH
Cormohipparion plicatile leg FLMNH
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File:Cormohipparion plicatile leg FLMNH.jpg
Cormohipparion plicatile leg FLMNH.jpg
File:Cormohipparion occidentale (sturdy three-toed horse) (Ash Hollow Formation, Miocene, 11.83 Ma; Ashfall Fossil Beds, Nebraska, USA) 4.jpg
Cormohipparion occidentale (sturdy...
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