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What fossils can—and cannot—show
A fossil is evidence left by an organism or its activity, preserved in rock. Different forms of evidence answer different questions: a body fossil may show shape, a burrow can record movement without preserving the animal, and a chemical signal may suggest biological identity or feeding. The geological layer and conditions of burial help establish when and where that evidence formed.
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No one kind of fossil provides a complete account. Researchers compare anatomy, traces, chemistry, age, and geological context, and distinguish direct observations from interpretations about an organism’s relationships. A fossil’s first known appearance marks the oldest evidence currently found—not necessarily the moment its lineage originated.
How scientists read different kinds of evidence
| Evidence | What it can reveal | Important limit |
|---|---|---|
| Body fossils | Impressions, compressions, and preserved hard parts can show an organism’s shape and, in unusually favorable deposits, fine anatomical details. | Soft-bodied organisms are less likely to fossilize, and an unfamiliar body plan may be difficult to classify. |
| Trace fossils | Trails, feeding marks, and burrows record movement and activity, even when the animal’s body is absent. | A trace may not identify its maker precisely. |
| Chemical evidence | Preserved molecules and their breakdown products can provide clues to biological identity, tissues, or diet. | Chemical traces can be ambiguous and need interpretation alongside other evidence. |
| Geological context | The rock layer and its setting help establish a fossil’s age and the environment in which it was buried. | Different preservation conditions can make sites of similar age appear to contain different communities. |
What the Ediacaran record says about life before the Cambrian
The Ediacaran Period came before the Cambrian and contains a diverse record of organisms, including soft-bodied forms that often do not resemble familiar living animals. Many are known from impressions, and their relationships to modern animal groups remain uncertain. That uncertainty is a reason to be careful with classification, not a reason to ignore the evidence.
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Body impressions and chemical clues
Dickinsonia, a large, soft-bodied Ediacaran fossil, is known from impressions. Steroid evidence has been interpreted as supporting its identification as an animal, but its precise evolutionary placement remains an open question. The study describing this evidence is available through PubMed.
Kimberella is often described as mollusc-like and is associated with grazing tracks. The Natural History Museum’s account of a study says chemical evidence is consistent with an internal gut and feeding on algae or bacteria, while noting that some traces cannot be identified with certainty: the museum’s explanation of the evidence. “Mollusc-like” is not the same as proof that Kimberella was a member of the modern mollusc lineage.
Burrows and tracks record behavior
Burrows and trails preserve actions rather than bodies. The Smithsonian describes end-Ediacaran burrows as evidence that worm-like animals were excavating the seafloor. Such traces support the presence of animals capable of moving through and disturbing sediment before the Cambrian, even when their bodies were not preserved at the same site. See the Smithsonian’s overview of animal origins.
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Preservation is selective. Soft bodies decay readily, and fossilization depends on burial and local conditions. A group may therefore be absent from a particular fossil site because its members were not preserved there—not because the animals did not yet exist. This means that a sudden increase in fossils can reflect both biological change and improved visibility in the rocks.
The Cambrian Period is commonly described as a major diversification of animal life. The Natural History Museum places it at about 539–485 million years ago and describes its explosive phase as roughly the first 20 million years of that period. The Smithsonian gives a range of 541–485 million years ago. These are source-specific geological dates, so their slightly different boundaries should not be treated as a contradiction or combined into a single exact range. The Natural History Museum’s Cambrian overview and the Smithsonian account explain the transition.
A 2019 synthesis describes a rapid increase in animal diversity and abundance from approximately 540 to 520 million years ago, while arguing that animal diversification unfolded through successive radiations rather than one isolated burst. In that view, the Cambrian “explosion” is a useful name for a particularly rapid phase visible in the fossil record—not the instant when animals appeared from nowhere, nor proof that every modern animal phylum originated at once. Read the Nature Ecology & Evolution synthesis.
How the Cambrian record adds anatomy and ecology
Cambrian deposits preserve a wider range of recognizable animals, including organisms with hard parts, and often reveal more anatomical detail than the earlier soft-bodied record. Fossils also document ecological change: burrowing, active feeding, and interactions among organisms show that animals were altering seafloor environments as well as diversifying in form.
Trilobites are one useful example because they occur abundantly in Cambrian rocks. The Natural History Museum reports that they appear with substantial diversity around 521 million years ago and summarizes research suggesting an early burst in the evolution of their features, followed by a more stable rate. Their record helps scientists study change within a group; it does not by itself date the origin of all animal life.
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What the 2026 Jiangchuan fossils add
In an April 2026 report, the Natural History Museum described a fossil assemblage from Jiangchuan, in Yunnan, China, containing more than 700 fossils dated to 554–539 million years ago. The fossils were preserved as carbon-rich films, a mode of preservation that can retain anatomical details rarely visible at other Ediacaran sites, including feeding structures, digestive systems, and organs associated with movement.
The assemblage includes forms previously known only from Cambrian rocks, as well as early bilaterians, possible comb jellies, and possible early relatives of deuterostomes. These are interpretations reported by the museum, not settled classifications for every fossil. Associate Professor Ross Anderson, a co-author of the study, said the findings indicate that the apparent absence of complex animal groups from other Ediacaran sites may reflect differences in preservation rather than true biological absence. The Natural History Museum’s April 2026 report describes the discovery and its significance.
The key contribution is not simply that some forms may be older than their previous fossil records suggested. Jiangchuan’s preservation offers a way to test whether apparent gaps at other sites reflect missing organisms or missing evidence. Dr Imran Rahman of the Natural History Museum, who was not involved in the study, called the assemblage exciting because it includes taxa previously known only from the Cambrian and “therefore serves to push back their origins deeper in time.” That phrasing concerns origins inferred from fossil evidence; it does not establish an exact date when those lineages first evolved.
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- Ask what was preserved. A body impression, burrow, hard part, or chemical signal supports different conclusions.
- Check the setting and age. The geological context helps place evidence in time and environment; dates should be read with their source and timescale.
- Separate observation from inference. A fossil’s visible structure is evidence; assigning it to a living animal lineage is an interpretation that may remain debated.
- Look for evidence that converges. Body form, traces, chemistry, and ecological context can strengthen an interpretation when they point in the same direction.
- Treat absence cautiously. A missing fossil at one site may reflect preservation conditions, especially for soft-bodied organisms.
Why the transition remains an active question
Many Ediacaran organisms are hard to place in familiar animal groups, and evidence types do not always resolve the same question. A trace can demonstrate activity without identifying the animal; a chemical signal can support an interpretation without settling anatomy; a body fossil can preserve shape while leaving evolutionary relationships unclear. Researchers therefore distinguish the oldest known fossil evidence from inferred origins and treat proposed affinities for enigmatic organisms as hypotheses.
For a broad account of early animal history, the most accurate picture is a transition: diverse Ediacaran life and evidence of animal behavior precede a pronounced Cambrian rise in fossil diversity, followed by further evolutionary radiations. The boundaries and pace depend partly on what organisms existed and partly on which environments preserved them.
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