518-Million-Year-Old Fossil Rewrites Starfish Origin Story

Scientists have identified several fossils dating back approximately 518 million years that significantly alter the understanding of the evolutionary origins of starfish and their relatives. These fossilized animals, discovered in southwestern China, belong to a newly described species named Yujingia glutenotunica. The research team, which includes researchers from the University of Cambridge and Northwest University in China, suggests that these specimens are close to the last common ancestor of ambulacrarians. This group encompasses modern starfish, sea urchins, and acorn worms. The findings, published in the journal Current Biology, provide new insights into how these distinct animal lineages evolved from a shared ancestor during the Cambrian explosion.

The discovery challenges long-standing hypotheses regarding the anatomy and feeding mechanisms of early ambulacrarians. Previously, many researchers believed that the common ancestor of this group resembled a worm that lived in seafloor burrows and filtered food passively through its gills or pharynx. However, the Yujingia fossils indicate a different lifestyle. The animals possessed two distinct sets of appendages. One set consisted of feather-like feeding tentacles used to capture organic particles from the water. The second set was likely used to temporarily anchor the animal to the seabed, although the creature was probably capable of some degree of movement. This active feeding strategy suggests that the ancestor was more similar to modern starfish than to acorn worms.

The fossils were found in the Maotianshan Shales in Yunnan Province, China, a location known for preserving soft-bodied organisms from the Cambrian period. Most Cambrian fossils consist of hard-shelled creatures, but specific geological conditions in places like the Maotianshan Shales allowed for the preservation of delicate soft tissues before they could decay. The Yujingia specimens measure between 8 and 22 millimeters in length. They preserve the body, gut, and appendages, offering rare examples of ambulacrarians without a mineralized skeleton. This preservation is critical for understanding the soft-body anatomy of early deuterostomes, a group that also includes chordates, the phylum containing humans.

Lead author Kaiyue He from Northwest University described the physical appearance of the animal as having an elongated, bottle-like form with tentacles spreading above a rounded lower body. The outline of the fossil closely resembles the ritual vase, or yujingping, in traditional Chinese culture, which inspired the genus name. All specimens clearly preserve a pale, original membrane enclosing the visceral mass. This soft, gelatinous appearance is the species’ most immediately recognizable feature and inspired the specific epithet glutenotunica. The presence of this membrane provides direct evidence of the animal’s soft-bodied nature, contrasting with the hard skeletons seen in later echinoderms.

To determine the evolutionary position of Yujingia, the research team conducted a detailed phylogenetic analysis. Co-author Dr. Giovanni Mussini from Cambridge’s Department of Earth Sciences, along with He, assembled a large dataset comprising 505 morphological characters across 100 living and extinct taxa. They analyzed this data using Bayesian phylogenetic reconstruction. The results place Yujingia closer to the last common ancestor of living ambulacrarians than previously described fossils. This makes Yujingia a key transitional form linking early Cambrian tentacled animals with the distinct lineages of hemichordates and echinoderms. The study suggests that sophisticated feeding structures evolved early in ambulacrarian history, while echinoderm skeletons and other specialized features appeared later in evolutionary time.

The findings have broader implications for understanding the divergence of deuterostome animals. Chordates, echinoderms, and hemichordates represent the principal branches of deuterostome evolution, yet they differ profoundly in body organization and feeding strategies. Reconstructing the ambulacrarian ancestor is central to understanding how these lineages diverged. A longstanding hypothesis proposed that the ancestor was a tentacle-free worm that filtered food through its pharynx, resembling a modern acorn worm. The detailed anatomical study of Yujingia challenges this view with direct fossil evidence. The presence of tentacles indicates that the ancestor actively captured food, a trait that may have influenced the subsequent evolution of its descendant lineages.

Dr. Mussini noted that the fossil suggests the common ancestor of ambulacrarians might have actually fed with tentacles, making it more similar to starfish and their relatives than previously thought. This new perspective helps explain why two closely related lineages evolved such different body plans. The chordates became reliant on movement for finding food, leading to streamlined bodies and developed brains. In contrast, members of the starfish lineage remained relatively anchored to the seafloor and specialized in a particle-feeding lifestyle. The Yujingia fossil illustrates how ecology can drive different lineages onto very different evolutionary paths, resulting in distinct morphological specializations over millions of years.

Co-author Degan Shu, also from Northwest University, emphasized the importance of this discovery in the context of animal evolution. He stated that how the major deuterostome groups diverged during the Cambrian explosion is one of the central questions in the field. Yujingia helps connect primitive, tentacle-bearing animals living on the seafloor with modern echinoderms and hemichordates. The fossil offers a new way to understand the profound transition from bilateral symmetry to the fivefold radial body plan characteristic of echinoderms. By filling a crucial gap in the early animal tree of life, Yujingia provides a tangible link between early soft-bodied ancestors and the diverse forms of life that exist today. The research highlights the significance of soft-bodied fossil sites in reconstructing the history of animal life.

Subject of Research: Earth Science

Article Title: A star is born: half billion-year-old fossil rewrites the origin story of starfish

Article References: A star is born: half billion-year-old fossil rewrites the origin story of starfish. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Paleontology, Evolution, Cambrian Explosion, Starfish, Fossils, Deuterostomes, University of Cambridge, Northwest University, star, born, half, billion-year-old