‘Spirit Whales & Sloth Tales’ tracks Washington fossils and their stories

Editor’s note: The following is an edited excerpt from “Spirit Whales & Sloth Tales,” a new book about Washington fossils written by Elizabeth A. Nesbitt and David B. Williams. The book will be published in October by the University of Washington Press in association with the Burke Museum, and will be available at local bookstores. The authors will hold an event at 7 p.m. Nov. 8 at the Burke Museum.

WITH MORE THAN a half-billion years of history, Washington state has an enviable diversity of fossils. Each is unique. Each is interesting. Each tells a story of natural and human history. Here are two such stories.

Tracking the Terrifying Birds

WASHINGTON STATE HAS no dinosaur tracks, but it does have the footprints of birds that could have made most large predators think twice about attacking. These footprints come from the flightless bird Gastornis giganteus, a species often called terror birds.

The 6-to-10-inch-wide tracks come from an animal that strode across muddy swamps 54 million years ago. Standing more than 6 feet tall, Gastornis had a massive skull and beak; a strong, relatively short neck; thick-boned, long legs; vestigial short wings; and a body that resembled that of a giant flightless turkey. For many decades, paleontologists depicted Gastornis as a predator on small mammals. They originally considered the huge beak suitable for a predatory carnivore, but now paleontologists conclude that Gastornis was herbivorous and used the beak for cracking nuts and seeds. The better understanding comes from chemical analyses of Gastornis fossil bones that showed that the birds fed entirely on plant material.

Gastornis tracks closely resembled carnivorous dinosaur tracks, with three elongated forward-facing toes bearing tiny triangular claws and a deep oval heel pad. (The name honors Gaston Planté, who was the first scientist to find fossil bones of the bird, in 1855, in Paris; they came from sediments that contained broken eggshells, long attributed to Gastornis because they are much larger than any other bird’s eggs.)

As happens in the world of fossilized tracks where no bones exist, paleontologists gave the trackways a specific trace fossil name, Rivavipes giganteus, meaning “the footprint of a giant bird on the river.” The big birds’ fossil bones are found in Europe and North America (so far, only in Wyoming and New Mexico). These bones originally were referred to as Diatryma steini, but paleontologists have found that this name is not valid. Researchers also assigned to crocodile tracks a trace fossil genus and species, Anticusuchipes amnis, which roughly translates as “ancient river crocodile footprint.”

Also fossilized with Gastornis in the Slide Mountain section of the Chuckanut Formation, near Bellingham, were footprints of other birds and a mammal very unlike those of modern Washington. These include heron tracks, each about the same size as a modern great blue heron, or around 4 inches long and wide. Additional tracks have been attributed to a duck and smaller shorebirds that trotted along the water’s edge.

Unlike the massive Gastornis, these birds probably had to be aware of a carnivore that left behind clawed footprints. These tracks might have been made by a creodont, a predatory mammal about the size of a house cat, which flourished in the Paleocene and Eocene in Europe, Africa and North America. Creodont fossils have long, narrow skulls similar in shape to a coyote’s, and carnassial molars, which cut through meat and bone like a pair of sharp scissors. Some modern carnivores also have carnassial teeth; evolution resulted in teeth adapted to do the same task for both groups, although they are not related. Creodonts became extinct in the Miocene.

Also walking in the wet mud was a species that produced abundant large squelch tracks. Most likely made by extinct Eocene grazers — either coryphyodonitids (large herbivorous mammals) or titanotheres — the big round hippolike tracks show that the animals traveled together. Titanotheres belong in the order Perissodactyla, along with rhinoceroses, tapirs and horses. Bones of these large grazing animals found in Oregon’s John Day area and in southern British Columbia confirm that titanotheres were in this region in the early Eocene.

Another set of tracks shows fore and hind footprints on either side of a sinuous line made by a tail dragging along the mud through very shallow water. Each footprint is between 2 and 4 inches long. This was a small crocodile, a testament to the warm and wet early Eocene climate, like central Africa today. They shared the water with an animal whose tracks suggest that it might have been afloat and punting across a mudflat.

What makes these trackways particularly interesting is that paleontologists have found only a single vertebrate body fossil in the Chuckanut rocks. It came from a soft-shelled turtle, or trionychid, which researchers think was the animal responsible for the punting tracks. None of the other animals described above left behind other evidence — either teeth or bone — of their lives. But clearly they lived in and trod across the broad coastal floodplain of meandering rivers and back swamps now preserved in stone.

Trace fossils such as these tracks illustrate another wonderful aspect of geology in that they record a single instant in an animal’s life or death. Geology is known as the scientific discipline of millions and billions of years, yet within that deep time of the Earth’s story, single moments — a dead rhinoceros covered in lava, an insect chewing a leaf, a mammoth defecating, a snail drilling a hole in a clam to get the meat within — are preserved, providing an unexpected and intimate insight into the past.

Further information about the Eocene ecosystem comes from the abundant leaf fossils. Plant remains in the Chuckanut sedimentary rocks near the fossil trackways reflect the subtropical rainforest ecosystem that covered the coastal region of Washington. The most common plants are the huge fronds of the Sabalites campbelli palm and Cyathea pinnata tree ferns. Other conifers in the fossil flora are the dawn redwood, Metasequoia, and swamp cypress Glyptostrobus. (Metasequoia still exist in the subtropical areas of China and Vietnam and have been transplanted around the world; numerous specimens are found across the state, including at the Seattle Art Museum’s Olympic Sculpture Park and the John A. Finch Arboretum in Spokane.)

Among the high diversity of flowering plants are fossils leaves and seeds of hydrangea, birch and plane tree (Platanus), as well as the huge leaves of the fossil sycamore genus Macginitiea. The name honors Harry MacGinitie, who collaborated with Estella Leopold on establishing the Florissant Fossil Beds National Monument in Colorado.

The Sea-Tac Sloth (Megalonyx jeffersonii)

THE WASHINGTON CONNECTION to Megalonyx began Feb. 14, 1961, at Seattle-Tacoma International Airport with the first known discovery of giant sloths in the state. Workers excavating a hole for a lighting tower saw bones in the bottom of their work pit. Alerted to the discovery, the Burke Museum sent a paleontologist and an archaeologist to investigate. Although flooding and collapsing walls made the dig difficult, the construction crew and scientists extracted the skeleton, which rested in a peat layer 13 feet thick, representing what was once a marshy wetland.

Most of the skeleton was intact, except for the skull, which was crushed and mostly missing. Based on the shape of the pelvis, which was 45 inches wide, as well as the limb bones and claws, the Burke paleontologist determined that the bones came from the extinct giant sloth, Megalonyx jeffersonii, or Jefferson’s ground sloth. Since the initial discovery, other isolated bones and claws of Megalonyx have been found in Eastern Washington Scablands’ megaflood deposits and dated at 12,100 years before present. Bones and teeth of a considerably older sloth, Megalonyx leptostomus, also have been found in Pliocene sediments in Eastern Washington, dated around 4.9 million years old.

Megalonyx is one of four ground sloth genera that inhabited North America during the Quaternary, 10,000 to 2.5 million years ago. In contrast to other large Quaternary mammals, such as mastodons and mammoths, which evolved in Europe or Asia and then traveled east, Megalonyx arrived from South America. They came after the Isthmus of Panama linked the two continents north to south, joining a northward migration that included opossum, armadillo and porcupine.

With their thick leg bones attached to wide-spreading hip bones and a sturdy tail, giant ground sloths could have stood upright on their hind legs to reach leaves high in trees. There is evidence from Quaternary South American ground sloths that they could stand on their hind feet, but there are no reliable tracks of them walking upright on two feet. In contrast, fossil trackways preserved in dried lake beds in Nevada and New Mexico show ground sloth footprints with all four feet on the ground. Like modern sloths, the animals placed their weight on the outer edge of their feet so that the top of the foot faced outward and the long claws were held off the ground. At present, the Sea-Tac sloth skeleton is on display at the Burke Museum.

Megalonyx jeffersonii holds a unique distinction in the annals of paleontology; Vice President Thomas Jefferson gave the animal its generic epithet in 1797. Jefferson, who had a deep passion for natural history, had received several bones, including the ulna, radius and claws, of an unknown animal. The bones had been dug out of a saltpeter (potassium nitrate, a commonly used fertilizer) mine in Greenbriar County, Va. (now West Virginia). In a March 10, 1797, speech in Philadelphia before the American Philosophical Society, Jefferson referred to a “large animal of the clawed kind,” which he named Megalonyx, or “great-claw.” He thought the claws belonged to a lion or tiger, though one that was three times larger than a modern variety. Jefferson also suggested that the Megalonyx might still have been alive in what was then the vast unexplored part of our continent, and some historians have proposed that Jefferson encouraged Meriwether Lewis and William Clark to look for Megalonyx on their expedition.

In the summer of 2002, workers constructing a pond on the western side of Orcas Island unearthed another Megalonyx jeffersonii specimen. These bones were found in a peat layer that had accumulated in a wetland, lying directly over a shelly marine clay layer deposited by glaciers melting into the newly formed Puget Sound. The Orcas Island sloth remains consist of an entire hind leg and a tooth, found with a few bones of two large bison and a mule deer, the shells of pond snails and pine cones. Other M. jeffersonii bones were found on Orcas Island, but their precise location is not clear. All of these sloth bones have been examined for evidence of breakage or cuts from human activity, but none was observed.

So how did these large animals reach Orcas Island? It could have to do with the dynamics of glaciation and sea level change. From about 12,000 to 14,000 years ago, sea levels in Puget Sound and around the world were low because of the immense volume of water tied up in the northern continental ice sheets. In addition, the massive glacier — 3,000 feet thick in Seattle and 4,600 feet thick in Bellingham — weighed down the coastal parts of the land. When the ice retreated, or melted, back to the north, the land responded by rising 300 feet surprisingly quickly (initially feet per year, then inches per year) over several thousand years — a process called isostatic rebound, similar to when a submerged bath toy rises after the pressure holding it down is released.

During the time of low sea level — soon after glacial retreat — islands such as Orcas were much larger, and water channels between them and the mainland were much narrower than at present. Alternatively, sloths could have island-hopped from Anacortes or Guemes Island, which at low sea levels would have been the same peninsula, to Cypress to Blakeley to Orcas islands.

Modern large mammals can, and do, swim to find new feeding grounds, new territories and empty spaces; sloths, for example, regularly cross lakes and rivers, albeit rather slowly. Therefore, it makes sense that sloths, bison and deer could have taken to the water and sought out new habitat in the past.

Over the couple of thousand years that the land rebounded, plenty of opportunity existed for large herbivorous mammals to migrate from the mainland and spread across the islands of the Salish Sea. Although we might never know the answer, there is a certain pleasure in simply considering how these large mammals crossed the water.

One reason they might have sought out virgin terrain was the new vegetation emerging on Orcas. In 2016, paleobotanist Estella Leopold published a study of pollen from the late Ice Age lakes on Orcas Island. Her data showed forested areas of pine, spruce and hemlock interspersed with grass-filled meadows and willow and poplar growing on stream banks, all plants that would have attracted large herbivores.