Pygmy Mammoths and Sea Cows: The Fossil Record of Channel Islands National Park

Channel Islands National Park has the best record of fossil birds in the National Park Service, the most thoroughly studied microfossils of any park, pygmy mammoths, and the oldest human bones found in the Service. Justin Tweet, a paleontologist at the American Geosciences Institute working as an NPS partner, summarized it in the Park Paleontology News.

A view across Santa Rosa Island (NPS photo by Justin Tweet).

A park of five islands

The park is one of 18 National Park Service units whose establishing legislation mentions paleontology. All five of its islands have yielded significant finds, and fossil species have been named from all five. Rocks and deposits from the Late Cretaceous to the Holocene hold fossils from tiny single-celled plankton to pygmy mammoths, and they have been studied since the 19th century, with the first published mention in 1865. In 2019 to 2020 the NPS Geologic Resources Division's paleontology program made the park's first paleontological inventory.

Four islands, from west to east San Miguel, Santa Rosa, Santa Cruz and Anacapa, form a line about 100 km (60 miles) long off southern California. The fifth, Santa Barbara Island, lies about 65 km (40 miles) southeast of Anacapa. The area became a national monument in 1938 with only Anacapa and Santa Barbara, and the other three islands were added in 1980, when it became a national park.

The five islands of the park (NPS image).

An unusual geological past

The four northern islands were, until about 20 million years ago, part of an ocean shelf attached to California near San Diego. Plate tectonics wrenched a block of land that included them and what are now the Transverse Ranges and carried it north. It caught on the continent and rotated clockwise about 90 degrees. The tectonic activity also sent out large volumes of lava, which lie on all four northern islands, but the islands themselves are not volcanic. Santa Barbara Island has a different history but is also mostly volcanic rock.

The marine record

Because the islands lay below sea level for most of the history in their rocks, most fossils are marine: snails, bivalves, sea urchins, corals, barnacles and crabs among them. In the 1960s paleontologists tested many rock units for microfossils and documented hundreds of species of tiny marine organisms, a level of study no other park has had. Odder finds include bones of desmostylians, strange marine mammals like a sea cow with four stout hippo-like limbs and no tail fin, found on Santa Cruz Island in the 1960s and still undescribed, and of sirenians (sea cows). Bones of two Early Miocene sirenians turned up on Santa Rosa Island in the 2010s and are under study.

Ribs of an undescribed sirenian from Santa Rosa Island, in the Santa Barbara Museum of Natural History (NPS photo by Justin Tweet).

The Ice Age islands

The Quaternary began 2.58 million years ago, and the islands rose above water through uplift and changing sea level. When sea level was low, as in ice ages, the four northern islands joined into one large island called Santarosae. Some scientists once thought it also connected to the mainland, but the seafloor shows it did not, and the fossils agree: there are no plants or animals that would have had to spread over land. Everything that reached the island could have come by water or wind. Mammoths were strong swimmers and could reach Santarosae, but other large mammals could not.

Bird fossils are the best in the Service. They occur in caves, where birds of prey roosting concentrated them, and in sediment with the nesting burrows of small seabirds, including those of the extinct flightless sea duck Chendytes. Other animals were less diverse: a few mammals, mostly rodents including extinct "giant" deer mice, the extinct vampire bat Desmodus stocki, and a few species each of reptiles, amphibians and land snails. A hand bone of a short-faced bear on San Miguel Island was most likely carried there by a large bird of prey that scavenged it on the mainland. Plant fossils from Santa Cruz Island show a cooler, moister, more forested setting with stands of Douglas-fir, more like the area around Fort Bragg, 685 km (425 miles) to the north-northwest. On San Miguel Island, "caliche forests" are well-known: casts of the roots of lemonade sumac shrubs and trees.

A "caliche forest" on San Miguel Island. These are root casts, not trunks (NPS photo).

Pygmy mammoths and Arlington Man

The most famous fossils are the pygmy mammoths. They descended from full-sized Columbian mammoths that swam what was then a few miles to Santarosae, perhaps drawn by the smell of fresh vegetation, and their bodies shrank over time to suit a smaller landmass. Adults averaged 750 kg (1,750 lb) and 1.7 m (5.6 ft) at the shoulder. Mammoth bones had been reported since at least the 1870s, but the first nearly complete specimen was found in 1994, which greatly raised interest and led to years of surveys. Weathering and erosion work fast on the islands, so new finds must be documented quickly. A large skull of a "not-quite pygmy, not-quite mainland" mammoth was excavated in 2016 and is being studied to see how pygmy mammoth evolution happened.

A cast of the 1994 pygmy mammoth find, on display at the Santa Barbara Museum of Natural History (NPS photo by Justin Tweet).

The oldest human bones in the Park Service were found on Santa Rosa Island. "Arlington Man" lived between approximately 13,494 and 13,291 years ago, about when the most recent pygmy mammoth fossils date, so people and mammoths may briefly have coexisted. The archeological record is tied to the fossils; it has even been suggested that the island foxes and skunks were brought by early people.

The Arlington Man discovery site on Santa Rosa Island, in 2019, 60 years after the discovery (NPS photo by Justin Tweet).

Visitors who see a fossil in the park may photograph it but should not remove or disturb it, and should tell a park ranger.

Sources

• Justin Tweet, NPS partner paleontologist, American Geosciences Institute, Paleontology of Channel Islands National Park, Park Paleontology News, National Park Service; the article is by a partner author, so it is paraphrased in full; rewritten in hubnx's own words.