ePostcard #147: Darwin’s Megafauna Bestiary (Part 1)

by | Jan 23, 2022 | 3 comments

Image Credit: Courtesy of Sci-News ( This life reconstruction of Megatherium by Sebastián Rozadilla honors the latest discovery in Argentina of what is thought to be one of the oldest-known fossils of the giant ground sloth Megatherium.

Do not go where the path may lead;
Go instead where there is no path and leave a trail.

Ralph Waldo Emerson


South America’s richly varied landscapes, dramatic geology, and spectacular wildlife compelled Charles Darwin’s to devote many more months ashore exploring the continent’s interior than he spent aboard the Beagle. FitzRoy was himself a gifted amateur geologist and had requested that the Admiralty assign a talented naturalist with additional geological knowledge to the expedition to sustain him intellectually during the rigors and isolation of the voyage. Although FitzRoy’s orders were to improve the nautical maps of South America, he personally hoped that Darwin, described as a naturalist ‘savant’ by the Admiralty, could also be employed to gather geological evidence for the biblical flood, a phenomena considered real by most geologists at the time.

Ironically, as a welcoming gift, FitzRoy presented Darwin with a copy of Charles Lyell’s recently published “Principles of Geology,” never imagining the influence the book would have on the young naturalist. For Darwin, by the close of the 1830s, the arduous task of navigating Genesis and geology would begin in earnest. Unfortunately for Darwin and FitzRoy, the friendship that had meant so much to both men would ultimately not survive FitzRoy’s eventual embrace of biblical literalism and his absolute rejection of Darwin’s emerging theories of evolution.

Charles Darwin’s made a series of fossil giant mammal discoveries in South America during the Beagle’s voyage that would revolutionize his worldview, shaping his understanding of extinction, and helping persuade him of the reality of evolution. The extinct mammal discoveries that helped Darwin develop his theory included four different species of giant ground sloth (Megatherium americanum, Mylodon darwinii, Scelidotherium leptocephalum, and Glossotherium robustum), Toxodon (T. platensis, a rhino-like southern ungulate), gomphotheres (Notiomastodon platensis and Cuvieronius hyodon, shovel-tusked and elephant-like mastodons, Glyptodon (an armadillo-like giant), the remains of an extinct horse (Amerhippus neogeus), and Macrauchenia (a strange llama-like grazer in the Order Litopteran). Our travels “In Darwin’s Footsteps” over the years have highlighted several of Darwin’s most famous fossil discoveries—a bestiary of extraordinary creatures that added multiple branches to our understanding of the ancient tree of life.

Illustration Credit: Robert Bruce Horsfall (1913) The Megatherium ground sloths were up to 10 times the size of modern sloths, reaching weights of up to 4 tons—way too heavy to live in trees.

In September 1832, the expedition’s first year, the Beagle anchored near Bahía Blanca, a settlement at the head of a bay about 400 miles southwest of Buenos Aires. General Rosas, Argentina’s brutal dictator at the time, was waging a genocidal war against the indigenous people throughout the region, and Bahía Blanca stood as a fortified outpost, occupied mostly by soldiers. The Beagle and her crew took advantage of the security provided by the outpost, remaining in the area for more than a month to replenish provisions. Their viewshed from the Beagle was classic Argentine Pampas, with lush grassland giving way to grassy sand dunes along the coast.

To feed the crew, the ship’s hunters brought back deer, agoutis, and other game, including several armadillos and a large flightless bird that Darwin referred to as an “ostrich, a species known only from Africa. The big bird puzzled him and turned out to be a greater rhea, specifically Rhea americana, ostrich-like in appearance but endemic to South America and the heaviest bird on the continent. Darwin learned from the gauchos he rode with in the pampas, that a smaller version of the rhea lived far to the south, in Patagonia. Darwin would eventually, quite by accident, add that smaller rhea species to the growing list of species named in his honor.

Darwin and Fitzroy, searching for potential fossil sites, commandeered a small boat from the outpost to explore the rocky outcrops at Punta Alta, Despite the name, Punta Alta (“high point”) was not very high, the reddish mudstone cliff rising only about 20 feet above the sea. But if the headland wasn’t dramatic, the exposed fossils were: big shapes, unusual shapes, and abundant. “These are the first I have seen,” Darwin wrote, “and are very interesting from containing numerous fossil shells and the bones of large animals.”

While a bemused Fitzroy looked on, Darwin and a helper went to work on the soft rock with pickaxes. Between that session and later efforts over the next several days, Darwin harvested the remains of nine huge mammals from Punta Alta, all unknown or barely known to science. They were extinct Pleistocene giants, unique to the Americas. The most famous of them was Megatherium, an elephant-sized ground sloth originally named and described by the French anatomist Georges Cuvier in 1796 on the basis of an incomplete assemblage of large fossil bones found in Argentina.

Circling back to Cuvier’s naming of Megatherium in 1796, I was fascinated to learn that the discovery of giant ground sloths would eventually involve not only Alexander von Humboldt and Charles Darwin, but also the sitting president of the United States, Thomas Jefferson, whose interest in fossils and the emerging field of paleontology was already well known internationally. When workers discovered huge bones while excavating saltpeter in a West Virginia cave, Jefferson had already written about “mammoth” bones discovered in Kentucky and New York’s Hudson River Valley. He welcomed the chance to solve another fossil puzzle.

Basing his identification on a single foreleg and a paw with long claws, which was all he’d been given, Jefferson thought it to be a giant, lion-like animal and gave it the name Megalonyx (or “Giant Claw”). In 1799, just as his article on Megalonyx was about to appear in America’s first journal of paleontology, Jefferson spotted Cuvier’s 1796 article and noted the resemblance of the bones to those of Megatherium. Later analysis showed that the bones from Virginia were in fact those of a ground sloth, which was later named Megalonyx jeffersonii in Thomas Jefferson’s honor.

Illustration Credit: Courtesy of Yukon/Beringia Interpretive Centre. Jefferson’s ground sloth is a distant relative of the tree sloths that still live in Central and South America today. The sloth family originated in South America and migrated northward across the Isthmus of Panama some five million years ago. Several species of ground sloth lived in North and South America during the Ice Age, but only the one named after former U.S. President Thomas Jefferson made it all the way to the Yukon and Alaska.

Illustration Credit: This drawing of Megalonyx’s holotype specimen’s bones is courtesy of Ted Daeschler/Academy of Natural Sciences and is adapted by Tom Dunne from a drawing by Dennis Murphy/Academy of Natural Sciences.

Jefferson’s ground sloth fossils are commonly found in the western United States, the Great Lakes region, and Florida. These ground sloths probably used caves for protection from the elements and hungry predators. One of their distant relatives, the Shasta’s ground sloth (Nothrotheriops shastensis), is famous for the incredibly thick layers of dung they left preserved in caves of the American southwest. Another spectacular find from a cave in Tennessee contained a skeleton of Shasta’s ground sloth that was still attached by ligaments and cartilage. There was even a mummified claw sheath on the sloth’s finger!

I was fascinated to learn that when President Jefferson dispatched Meriwether Lewis and William Clark to lead a military expedition to explore the Louisiana Territory and the Pacific Northwest, the mission included an order to observe “all the animals of the country generally, & especially … any which are deemed rare or extinct.” After the expedition’s triumphant return, Jefferson personally financed sending William Clark to survey a significant fossil bone assemblage in Kentucky, which yielded several hundred mammal bones. Many of the largest fossil bones were sent on to the White House, where they filled the unfinished East Room, which was then referred to as Jefferson’s “Bone” or “Mastodon” Room. Jefferson’s White House had become a scientific nexus where botany, geography, and exploration were the preferred topics of conversation at dinner.

It is not surprising that the legendary explorer of South America, the German naturalist Alexander von Humboldt (1769-1859), arguably the most famous scientist of his time, would eventually find his way to Jefferson’s White House. In the summer of 1799, Humboldt and his colleague, the French botanist Aimé Bonpland, had landed on the paradisiacal tropical shores of what is now Venezuela . No other naturalist had been there before; all that lay before them was new and unexplored. In the course of their five-year expeditionary journey (1799-1804), they would travel through Venezuela, Brazil, Guiana, Columbia, Ecuador, Bolivia, Peru, Mexico and Cuba. Their collections included vast quantities of botanical, zoological, geological, and ethnographic specimens. Anxious to learn of their discoveries and to hear their tales of adventure firsthand, Jefferson wasted no time in inviting Humboldt and Bonpland to the White House for an extended stay.

Illustration Credit: Courtesy of Global Botanical Geography archives and the Library Of Congress Geography And Map Division

Jefferson had gathered every scrap of information he could about the Louisiana Territory and about Mexico, and with Humboldt’s encyclopedic notes he had more than he could ever have hoped for. A treasure trove of knowledge. During the week the men spent talking about nature and politics, Humboldt described the wonders he’d observed in Latin America but also the destruction wrought by Spanish colonialism at its most brutal. In Humboldt’s words, the Spanish had annihilated ancient civilizations and any indigenous tribes that stood in their way, and he was the first to link colonialism to the devastation of the environment. Humboldt’s warning was clear. Jefferson needed to be vigilant in identifying and defending his country’s border with Mexico and in keeping Spain’s ambitions in check. The most revolutionary idea that Humboldt presented to Jefferson was his conceptual vision of nature as a complex and interconnected global force that does not exist for the use of humankind alone. Fast-forward that vision to 2022!

Map Credit: Courtesy of WikiMedia Commons. This map from 1811 is by Alexander von Humboldt (1811), which he gave to President Jefferson. It shows the Spanish colonial Viceroyalty of New Spain in North America (present day Mexico and the American West), and the Southern U.S.

Nature was clearly Humboldt’s teacher, and would become Darwin’s as well. In the 1820’s, while a fledgling student at Cambridge University, Charles Darwin would read the first seven volumes of Humboldt’s “Personal Narrative of Travels to the Eqinoctial Regions of the Americas.” In fact, Humboldt’s first volume was one of the few books that Darwin took with him on the voyage of the Beagle, and it would serve as the model for his own travelogue, The Voyage of the Beagle. Humboldt’s relationship with Thomas Jefferson, with whom he enjoyed a life-long correspondence on natural history and politics, and the lasting influence of his writings on Darwin, Emerson, Thoreau, Muir and so many more remains relevant today in how we see the world around us. Ralph Waldo Emerson’s quote at the top of this ePostcard was most likely inspired by Humboldt’s many contributions to science.


The quote from Ralph Waldo Emerson at the top of this ePostcard has always resonated with me. My childhood dream was to become a famous paleontologist, exploring the far corners of the world for evidence of ancient life. Although I didn’t realize it until I migrated west to the University of Colorado, what fascinated me most about paleontology was not just finding fossils but the study of ancient ecology, the subdiscipline we now call paleoecology. I was fortunate to take a wonderful class from a pioneer in the emerging science of paleoecology, the late Dr. Judith Harris (Van Couvering). Her approach to understanding the ecology and structure of ancient plant and animal communities was to adopt an ecosystem perspective grounded in evolution and modern biological processes.

Dr. Van Couvering’s final exam, conducted in her laboratory, was one of the most challenging I’ve ever taken. We had to choose one marine and one terrestrial fossil assemblage from a group of six sites that she had recreated from the museum’s collections from important research sites. Each assemblage included stratigraphic diagrams of the site. We had an hour to write an interpretive narrative for each site, identifying the key indicator fossils, the site’s geologic and paleoenvironmental context, its approximate geologic age and, using the taphonomic tools we’d been taught, describe the method of deposition and fossilization. Passing that exam earned me a well-remembered smile from Dr. Van Couvering and the advice to balance biology and geology courses in creating my own path through the sciences. I took her advice.

It was a very exciting time to be a student at the University of Colorado and to have the personal freedom to choose from an array of classes, all taught by a remarkable group of professors positioned at the leading edges of their respective fields. My stalwarts in geology included Drs. Bill Bradley, Ed Larsen, Ted Walker, Peter Birkeland, Martin Lockley, Emmett Evanoff, Al Werner and, most especially, my late husband Jim Benedict.

In ecology and evolutionary biology, I benefited from master teachers-scholars-mentors at every turn, including Drs. Dave Armstrong, Yan Linhart, Marc Snyder, Boyce Drummond, Bob Pyle, Brian Linkhart, Richard Reynolds, Dee Boersma, and the late Alex Cruz, John Marr, and Olwen Williams. Nearly 50 years later, as a naturalist and science writer, the windows on the natural world they opened for me have inspired and enriched my life and work in countless ways. I’m certainly not unique in the multidisciplinary path I’ve chosen in science, but I’m profoundly grateful to all the people in my life that have given me the “wings” and confidence to fly.

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