Ancient “serpent star” fossils, Amphiura sanctaecrucis, not seen in the area since the early 1900s have been unearthed in the Santa Cruz Mountains and donated to the Santa Cruz Museum of Natural History.
By Kathleen Aston
November 26, 2024—We watch the sky for shooting stars, and might wait decades to catch the return of a comet. But not all stars are in the sky, and keeping a keen eye on the ground just led to the first sighting of the brittle star fossil Amphiura sanctaecrucis in over one hundred and twenty years. And we are delighted to announce they were donated to the collections at the Santa Cruz Museum of Natural History.
Bursting like a meteor shower from the soft brown stone of a Santa Cruz mountain hillside, these fossils are spectacular to behold. Incredibly detailed down to the fine lines of their body disks and the delicate spines lining their arms, they range in color from a blackened rust to a pale beige. They possess the radial symmetry, central disks, and snake-like arms we expect from brittle stars, also called ophiuroids from the Greek for “serpent stars.” The specimens are at once strikingly beautiful and somewhat like an illustration of monstrous, hand-sized aliens fighting a pitched battle.
They are also unlike anything else in our collections. While echinoid fossils like sand dollars are common in our Santa Margarita Formation collections, their relatives the brittle stars are rare in the local fossil record. For that matter, any kind of non-microscopic fossils are rare in the Santa Cruz Mudstone, the body of rock in which they were found. It is in part this rarity that first made Museum Collections Advisor and Pacific Paleontology founder Wayne Thompson wonder if they might be the reappearance of the long-forgotten Amphiura sanctaecrucis.
Known from only two fossils discovered near Scotts Valley in the early 1900s, the description of this species was published in 1908 by USGS paleontologist Ralph Arnold. Arnold worked for almost a decade on the central coast, identifying fossils and co-authoring the first detailed description of the geology of the Santa Cruz region. It was during this period that he also named a new species after local lighthouse keeper and Santa Cruz Museum of Natural History founder Laura Hecox. Fusus hecoxi is an extinct snail species that evokes Arnold’s appreciation of Laura’s generous legacy to local nature lovers.
Amphiura sanctaecrucis is also extinct, and to encounter it is therefore doubly special. It has no common name, although I would pitch the Santa Cruz brittle star, or perhaps the Santa Cruz burrowing brittle star. Based on comparing features of the first specimens to wet specimens of existing stars at the Smithsonian, Arnold and his colleagues placed these creatures in the Amphiuridae. Also called the burrowing brittle stars, this diverse group of stars contains over 200 species and can be found in oceans worldwide. Amphiura sanctaecrucis is found in the remains of an ancient offshore ocean that existed here between 7 and 9 million years ago. It is this ocean floor environment that over millenia put the mudstone in the Santa Cruz Mudstone, so to speak. As a point of reference, the splitting date between hominin and chimpanzee lineages is placed by some between 4 and 8 million years ago.
Recognizing the potential significance of this find, local paleontologist Wayne Thompson reached out to brittle star experts, and invited them to collaborate on the forthcoming paper related to the discovery, which, among other things will include the identification of a new species of fossilized clam. This clam is one of over a dozen mollusk species found nestled amongst the brittle stars. Also discovered were fish remains that appear to potentially be from a type of flatfish in the genus Lepidopsetta, which today lives in the ocean off our coast from British Columbia to Baja. Collaborating also with former USGS micropaleontologist colleagues, the paper will describe the tiny microscopic “glass plankton,” called diatoms, that were found mixed into the mudstone with the brittle stars, mollusks, and fish. And the specific diatom species that has been found allows the USGS scientists to precisely age-date our brittle star layer at between 8.4 and 8.6 million years old.
When asked about the discovery, Thompson highlights the rarity of finding so many complete specimens and such a wide variety of species so densely packed together into an ancient paleofauna. These creatures appear to have been buried swiftly, such that they did not have time to drift apart or be picked at by predators. This would be consistent with an underwater landslide, unfortunate for the stars themselves, but quite fortunate for us on this side of time. And while there is more to learn, Thompson emphasizes the excitement of finding a creature from the depths of the seas within a hillside full of redwood trees - an iconic look into the ancient and changing geology of the Monterey Bay region.
We look forward to learning more about what these stars can tell us about the history of this place, and what else might be found from keeping a keen eye to the ground.
Kathleen Aston is Collections Manager at the Santa Cruz Museum of Natural History.
Related resources:
- Attend the Museum’s A Family Fun First Friday: Dino December! event on 12/6/24
- Read “Fossil-Hunting Fun in Santa Cruz”
- Explore the Museum’s Guide to the Fossils of Santa Cruz County
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