Fossilized Skin Fragments From China Reveal the Secret Dawn of Insects’ Earliest Ancestors

Some 535 million years ago, in the shallow seas covering what is now southern China, a quiet revolution was underway. Animals were learning to dig, to swim, and to wear armor. A new study of tiny fossilized fragments of skin and cuticle from the lower Cambrian Yanjiahe Formation, published in the Journal of Micropalaeontology, offers one of the most intimate glimpses yet into that pivotal moment, revealing that the ancestors of today’s insects, crabs, and roundworms were already experimenting with a dazzling array of body plans long before the Cambrian explosion reached full throttle.

The research, led by Lei Zhang of Chengdu University of Technology together with an international team spanning China, France, and Sweden, focused on a class of fossils that most people would walk right past: microscopic scraps of the outer body coverings, or cuticles, of soft-bodied animals. These fragments were extracted from roughly 2,000 kilograms of phosphatic dolostone collected at the Muyangxi section in the Three Gorges area of Hubei Province. The team dissolved the rock in weak acetic acid over two months, sieved the residue, and then examined the surviving fossils under a field emission scanning electron microscope after coating them in gold-palladium alloy.

What emerged from the acid bath was extraordinary. The fossils belong to a preservation style known as Orsten-type, in which early diagenetic phosphatization replicates delicate cuticular surfaces in three dimensions with fidelity down to subcellular scales. Many specimens exceed two millimeters in length even in fragmentary form, and some hint at original animals approaching centimeter scale, a striking size for creatures this early in Earth history. Based on distinctive patterns of ornamentation, spines, and sclerites, the team recognized six forms representing at least four distinct ecdysozoan taxa, the group of molting animals that includes scalidophoran worms and panarthropods.

Ecdysozoa are, by any measure, one of evolution’s greatest success stories. The clade encompasses arthropods, the most species-rich animal phylum on Earth, along with nematodes, priapulid worms, kinorhynchs, and their relatives. Molecular clock analyses consistently place the origin of the group in the late Ediacaran Period, but their earliest fossil record has been frustratingly sparse and controversial. The best evidence comes from the slightly older Kuanchuanpu Formation, also in South China, which has yielded a remarkable menagerie of phosphatized worms and sac-like creatures, including the famous Saccorhytus coronarius, once touted as the earliest known deuterostome and now firmly repositioned within the ecdysozoan stem group.

The new Yanjiahe material adds crucial texture to this picture. Among the six forms, several stand out. Form C, the most abundant in the collection, bears at least four distinct spine types, including hollow tubular spines with dome-shaped bases and coniform spines with depressed funnel-shaped bases, alongside smaller hook-like spines with what the authors describe as cowboy-hat-shaped basal expansions. The interspinal cuticle displays a hierarchical reticulated pattern of small polygons superimposed by larger orbit-like meshes. These features invite comparison with the spiny introverts of priapulid worms and the armored bodies of early kinorhynchs such as Eokinorhynchus rarus.

The most provocative specimens, however, are Forms A and B, which carry lateral branches that may represent appendages. Form A is a vermiform cuticle lacking spines entirely, instead showing broad sinuous ridges running obliquely across the body and finer striations just a few micrometers wide. Its most distinctive feature is a series of paired, tubular projections extending laterally from the main cylindrical body. These structures resemble the lobopodous limbs of early panarthropods, the soft-legged ancestors from which arthropods, velvet worms, and tardigrades all descended. If the interpretation holds, Form A would rank among the earliest direct body fossil evidence of appendages in the entire ecdysozoan lineage.

The timing is significant. Trace fossils such as Rusophycus avalonensis and the scratch marks of Monomorphichnus, found in Fortunian-age rocks of southern China, Kazakhstan, and Newfoundland, have long hinted that appendage-bearing animals were moving across and through sediments at this time. But a frustrating gap persisted between these tracks and the first unambiguous body fossils of euarthropods, which do not appear until Cambrian Stage 3, millions of years later. Form A, with its annulated trunk and paired lateral appendages, could help bridge that gap, suggesting a Fortunian expansion of motile, appendage-bearing bilaterians whose soft anatomy was simply too fragile to fossilize in most settings.

The researchers are careful to note the limits of the evidence. The appendages of Form A lack the transverse annulations and terminal claws typical of walking lobopods, and their lateral rather than ventrolateral insertion suggests they may have served for paddling or sediment manipulation rather than weight-bearing locomotion. The team also considered and rejected alternative identities for the branches, including annelid parapodia and palaeoscolecid setae. Form B, with its clearly segmented trunk of three annuli and densely spinose surface, might instead represent a stem-group kinorhynch, its branches interpreted as specialized spines rather than true limbs. Definitive taxonomic placement, the authors concede, will require additional fossils.

Beyond the individual forms, the assemblage tells a broader ecological story. The three fundamental ecdysozoan body architectures, sac-like forms, vermiform worms, and appendage-bearing panarthropods, all appear to have been established during the initial phase of the Cambrian explosion. The widespread development of spines and sclerites across these lineages points to their adaptive importance, likely serving for protection, anchoring in sediment, and other ecological functions. Meanwhile, the complex burrow systems of Treptichnus pedum at the Ediacaran-Cambrian boundary record the so-called Substrate Revolution, when vermiform animals began churning seafloor sediments in earnest, reshaping entire ecosystems from the bottom up.

The Yanjiahe Formation itself, dated radiometrically to between roughly 540 and 526 million years ago, sits squarely within this transformative interval, coinciding with a major carbon isotope excursion and faunal turnover. Its lower beds, previously underexplored compared with the celebrated Kuanchuanpu deposits, are now proving to be a treasure trove in their own right. As systematic excavation of Orsten-type fossils continues, each acid-dissolved kilogram of ancient rock promises to reveal more about how Earth’s most successful animal groups, from mosquitoes to lobsters, took their very first evolutionary steps in the shallow seas of the earliest Cambrian.

Subject of Research: Early Cambrian ecdysozoan microfossils from the Yanjiahe Formation of South China

Article Title: Cuticular microfragments from the lower Cambrian Yanjiahe Formation, China: insights into ecdysozoan biodiversity at the dawn of animal radiation

Article References: Zhang, L., Zhai, F., Wu, Y., Chang, S., Ye, Y., Lang, X., Pang, Y., Hu, L., Feng, Q., Forel, M.-B., Danelian, T., Yong, Y., & Vannier, J. (2026). Cuticular microfragments from the lower Cambrian Yanjiahe Formation, China: insights into ecdysozoan biodiversity at the dawn of animal radiation. Journal of Micropalaeontology, 45(1), 455-474. https://doi.org/10.5194/jm-45-455-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-455-2026

Keywords: Ecdysozoa, Cambrian explosion, Yanjiahe Formation, Fortunian Stage, phosphatization, Orsten-type preservation, panarthropods, scalidophorans, lobopodians, microfossils, South China, animal evolution