Saccorhytus coronarius, a fossil organism, emerged during the early Cambrian period, around 535 to 540 million years ago. Its name, “crowned wrinkled bag,” describes its tiny, bag-like appearance. This creature, measuring about a millimeter, lived burrowed between sand grains on the seabed. It has been central to scientific discussion about the origins of complex animal life.
The Initial Discovery and Interpretation
The initial discovery of Saccorhytus coronarius was reported in January 2017 from the Kuanchuanpu Formation in Shaanxi Province, central China. Researchers isolated 45 specimens. Under an electron microscope, scientists observed its millimeter size and a globular body.
The creature possessed a large mouth relative to its body, surrounded by a ring of tridentate protuberances and folds. Several conical openings were also noted on either side of its body. These were interpreted as pharyngeal openings, or “gills,” allowing water to pass through. This led scientists to classify Saccorhytus as the earliest known deuterostome, a potential distant relative to vertebrates, including humans.
A Second Look and Reclassification
Subsequent research, published in August 2022, re-examined Saccorhytus coronarius using advanced imaging techniques. An international team collected hundreds of new fossil specimens from the same region in China, many of which were better preserved. They employed synchrotron X-ray tomographic microscopy, allowing for three-dimensional reconstructions.
This analysis revealed the supposed “pharyngeal openings” were not true openings. Instead, they were taphonomic artifacts—holes where conical spines and sclerites had broken off. The researchers confirmed these conical features had closed tips.
Saccorhytus also lacked an anus, with its single opening likely serving for both ingestion and excretion. This evidence, combined with a thick, two-layered cuticle and a non-ciliated epidermis, led to its reclassification. Saccorhytus coronarius is now classified as an early ecdysozoan, a diverse group including arthropods like insects and crustaceans, and nematodes.
What Saccorhytus’s Reclassification Means for Evolution
The reclassification of Saccorhytus coronarius alters our understanding of early animal evolution. Its removal from the deuterostome lineage means the fossil record for the earliest ancestors of vertebrates, including humans, becomes less clear. This creates a mystery regarding the deep ancestry of our own branch on the tree of life.
Saccorhytus now holds significance as one of the earliest known ecdysozoans. Its unique sac-like body challenges the traditional “worm-like” body plan of early members, expanding the morphological diversity of early Cambrian ecdysozoans. This shift provides insights into the ancestral body plan and early diversification of this large animal group. The scientific journey of Saccorhytus coronarius illustrates the scientific method. Hypotheses are formulated, and new technologies allow them to be tested and revised, advancing our knowledge of life’s history.