A 540-million-year-old fossil known as Salterella, which puzzled scientists for decades due to its unique double-layered shell, has been linked to the cnidarian family, including jellyfish and corals. This discovery from the early Cambrian Period reveals innovative skeleton-building methods among early animals. The findings, led by Prescott Vayda of Virginia Tech, were published in the Journal of Paleontology.
The Cambrian Period, from roughly 538 to 506 million years ago, marked an explosion of skeletal innovation among Earth's animals. Most creatures built mineralized skeletons using either an organic framework, like human bones, or by binding environmental minerals into a hardened covering. These methods persisted successfully for over 500 million years.
Salterella, a small cone-shaped organism abundant in early Cambrian rocks, broke this pattern. It constructed a conical outer shell and then filled the inner cavity with selected mineral grains, creating a rare double-layered structure. This anomaly made Salterella an index fossil for dating rock layers but complicated its evolutionary placement. "It makes Salterella difficult to place on the tree of life," explained Prescott Vayda, a geosciences graduate student who led the study.
Previously classified with squids, sea slugs, jellyfish ancestors, worms, and even in its own category alongside Volborthella since the 1970s, Salterella's affinities remained elusive. Vayda, along with University Distinguished Professor Shuhai Xiao, analyzed samples from Death Valley, the Yukon in Canada, and Wythe County, Virginia. Collaborating with institutions including Johns Hopkins University, Dartmouth College, the University of Missouri, and the Denver Museum of Nature and Science, they examined shapes, mineral compositions, and crystal structures.
The fossils avoided clay minerals, tolerated quartz, and preferred titanium-rich grains, suggesting selective material use for stability or feeding. Evidence hints at small appendages for arranging grains. The team concluded that Salterella and Volborthella belonged to the cnidarian group, relatives of modern corals, jellyfish, and sea anemones.
"Finding the right place for these fossils is important for our understanding of how animals evolved skeletons and shells," said Vayda. This reconnection illuminates early evolutionary innovations in body structures. For Vayda, the work underscores "truly learning where we come from and the history of life on Earth, which is an amazing and beautiful thing."
The study appears in the Journal of Paleontology (2025; DOI: 10.1017/jpa.2025.10164).