BONES & STONES

The Dawn Horse: A Tiny Fossil with a Monumental Story

Nestled in the limestone of Wyoming’s Green River Formation, a remarkably preserved fossil—no larger than a fox terrier—tells one of evolution’s most extraordinary stories. This “dawn horse,” scientifically known as Hyracotherium(once called Eohippus), was discovered in 2003 by amateur paleontologist Jim Tynsky and is now a crown jewel of the Smithsonian National Museum of Natural History. With three delicate toes on each hind foot and four on each front, it was perfectly adapted for darting through the dense, humid forests of the Eocene Epoch, about 50 million years ago. This ancient equid marks the starting point of a 55-million-year evolutionary journey that leads to today’s powerful, single-toed horse.

Frozen in Time: A Moment from the Eoceney

What makes this fossil so extraordinary isn't just its completeness—it's the story captured in stone. The dawn horse was found in the ancient lakebed of Fossil Lake, where fine-grained sediment, oxygen-poor water, and just the right conditions allowed for the rare preservation of not only the horse, but also several fish encircling its body. It’s as if this tiny mammal and its aquatic companions were caught mid-moment, preserved in a prehistoric tableau. After its discovery, the fossil was safeguarded in a bank vault before catching the attention of Smithsonian scientists at a gem show in Tucson. Today, it’s displayed in the museum’s Deep Time Hall, allowing visitors to gaze into a lost world, frozen for eternity.

From Forest Dweller to Plains Runner

This petite browser, standing only about 10–15 inches tall at the shoulder, looked more like a dog than a modern horse. It had a short face, padded feet, and low-crowned teeth—perfect for nibbling soft foliage rather than chewing tough grasses. Over millions of years, the horse family evolved dramatically in response to changing climates and environments. Forests gave way to open plains, and the dawn horse’s descendants grew taller, their toes fused into a single hoof for efficient running, and their teeth became high-crowned and ridged for grinding grass. Thanks to fossils like the one at the Smithsonian, scientists can trace this entire transformation in vivid, fascinating detail—making the horse one of the clearest and most complete examples of evolutionary change in the fossil record.

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The Devonian Period: The Age of Fish

The Devonian is famous for the thousands of species of fish that developed 395 million years ago. In addition to fish, brachiopods also reached their zenith in number and diversity. Early brachiopods (mussels and clams), pelecypods (early oysters) and gastropods (snails), dominated the sea floors when land was submerged by warm, shallow seas.

These bottom dwellers fed on food particles that they would filter out of the water. There are over 12,000 fossil species of brachiopods (and about 300 modern species). The abundance of species makes them useful for dating the rocks and associated fossils.

Evidence from Devonian supports the idea that the present continents were being united in a single continental mass called "The Old Red Sandstone Land Mass", uniting Gondwana, Laurentia and Baltica.

How did life make a comeback after the dinosaur-killing asteroid?