A routine field survey in a California redwood forest has led to an extraordinary glimpse into the Ice Age. A scientist searching a San Mateo County creek for signs of habitat conditions for threatened California red-legged frogs spotted an oddly shaped object that initially appeared to be an ordinary rock. Instead, it turned out to be a remarkably well-preserved tooth belonging to an extinct Pacific mastodon. The fossil is estimated to be at least 10,000 years old and is being described as an exceptionally complete specimen from the region.According to the Midpeninsula Regional Open Space District (Midpen), geomorphologist Brigid Lynch discovered the fossil while working on a habitat enhancement project for California red-legged frogs. Paleontologists later confirmed that the object was an intact molar from an adult Pacific mastodon, a giant herbivore that once roamed parts of western North America.
From a strange-looking rock to an Ice Age fossil
Lynch, who works with Balance Hydrologics, was surveying a creek bed when she noticed the unusual object. Its shape was distinctive enough to catch her attention, but there was initially no indication that it had survived from a prehistoric animal. Paleontologists later examined the object and identified it as a molar from a Pacific mastodon. The condition of the fossil made the discovery particularly significant. Wayne Thompson, a paleontologist and owner of Pacific Paleontology, described it as the most complete mastodon molar from the area and among the most impressive he had seen.The exact location of the discovery has not been disclosed. Midpen officials said the information was being kept private partly to prevent people from attempting to search the area themselves for fossils.
The fossil is estimated to be at least 10,000 years old (Image Credit: Midpeninsula Open Space District)
A giant herbivore from California’s past
The fossil belonged to Mammut pacificus, commonly known as the Pacific mastodon. Although mastodons are often associated with the better-known American mastodon, scientists recognise the Pacific mastodon as a distinct species. The species was formally recognised in 2019 following research on mastodon remains from Southern California. Pacific mastodons differed from American mastodons in several physical characteristics, including narrower teeth, thicker leg bones and differences in their tusks.These animals were large herbivores that lived during the Pleistocene, an epoch that began roughly 2.5 million years ago and ended about 11,700 years ago. They ranged across parts of western North America, with Pacific mastodon remains identified in California, Idaho, Oregon and, more recently, parts of Mexico. Unlike mammoths, which primarily grazed on grasses, mastodons were browsers that fed on leaves, shoots and other vegetation. That makes the newly discovered tooth more than just an unusual fossil. It is a physical remnant of a landscape that existed thousands of years before modern California took shape.
What the tooth could reveal
The fossil has now been donated to Stanford University’s Doerr School of Sustainability for further study. Researchers plan to use carbon dating to determine its precise age rather than relying solely on the current estimate of at least 10,000 years. Scientists also intend to create a three-dimensional scan of the tooth. Midpen plans to use the resulting digital model and a replica for educational and public outreach purposes.According to the researchers, fossils such as this can help reconstruct past ecosystems and biodiversity. By studying what lived in a region thousands of years ago, scientists can gain a better understanding of how landscapes and species have changed over time. The discovery also creates an unexpected link between modern conservation and ancient history. Lynch was searching the landscape to support the habitat of a threatened frog species when she encountered evidence of an animal that disappeared thousands of years ago.For a moment, the creek bed offered a glimpse of a very different California, one where enormous mastodons moved through forests and other now-extinct animals shared the landscape. Today, that same environment is being studied and protected for species that are still here, making the accidental discovery a reminder of how dramatically ecosystems can change over time.
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