Equidae

• 1. Introduction

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  Equidae is the family of mammals that contains the horse, and it began as a fox-sized creature picking its way through Eocene forests on toes, not hooves. More than 50 million years ago, these small browsers fed on soft plants and were already built to run. Every horse, every ass, every zebra alive today belongs to a single surviving genus, Equus. Yet that genus is the last branch standing on a family tree crowded with extinct names. How did a multi-toed animal the size of a fox become a single-toed grazer? Why did so many of its relatives vanish, leaving only fossils behind? And what does the inside of a horse's gut have to do with the size it reached tens of millions of years ago?

• 2. From Four Toes To One

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  The oldest fossils placed in Equidae turned up in North America and date to the early Eocene, 54 million years ago. These early equids carried three toes on each hind foot and four on each front foot. They were herbivorous browsers, feeding on relatively soft plants rather than tough grasses. The complexity of their brains suggests they were already alert and intelligent. Over time, later species reduced the number of toes and grew teeth better suited to grinding tough plant food. By the Miocene, many species bore the main weight of the body on the central third toe, while the others shrank and barely touched the ground, if at all. This long reduction set the stage for the single-toed animals that would follow.

• 3. A Gut Built For Grass

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  The equids are hindgut fermenters, like other members of their order. They evolved specialized teeth that cut and shear tough plant matter to handle a fibrous diet. This digestion strategy looks inefficient, but it traces back to the animals' size when it first evolved. To be supported on such a strategy, they would already have had to be relatively large mammals. The diet and the body size are tied together in the fossil record. Teeth more suited for grinding up grasses and other tough plant food marked the shift away from the soft-plant browsing of the earliest forms.

• 4. The Rise Of Equus

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  By the Miocene epoch, the family had grown relatively diverse, with many new species appearing. By this time the equids looked more truly horse-like, having developed the typical body shape of the modern animals. The sole surviving genus, Equus, had evolved by the early Pleistocene and spread rapidly through the world. Equus originated in North America, the same continent that yielded the family's oldest fossils. The genus holds 22 species in the record, of which 7 are extant. Today every living horse, ass, and zebra falls under this one name, the last living chapter of a family that once branched in many directions.

• 5. Where Equidae Sits

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  Equidae belongs to the order Perissodactyla, the odd-toed ungulates. Its living relatives in that order are the tapirs, with four species in a single genus, and the rhinoceros, with five living species across four genera. More narrowly, Equidae sits within the superfamily Equoidea, whose only other member is the extinct family Palaeotheriidae. A naming knot lives in this corner of the tree. The oldest equid fossils were once assigned to the genus Hyracotherium, but the type species of that genus is now regarded as a palaeothere, and the other species have been split into different genera. The word equid covers any member of this family, including any equine, a reminder of how many extinct branches the single surviving genus now represents.