Pseudofossils

Have you ever found a rock in the shape of a love heart? Or shaped like Australia? What about one that looks like a flower or a shell? Or a shape in a flat rock surface that looks just like a human footprint?

The forces of wind, water, and time constantly work to shape rocks into a range of shapes, sizes and colours — including ones that look like past life. Scientists call these features ‘pseudofossils’. Pseudofossils are formed by inorganic (non-living) processes but look like fossils or other signs of life.

How do we decide what is a fossil?

Detailed observations of the rock feature and its geological context are made, then compared to living organisms and known fossils. Many scientific tools, like microscopes, thin sections and synchrotron scans, help observation.

Chemical tests can help determine if a feature is a fossil, as the decay of biological material creates several specific chemicals, called biomarkers. Finding biomarkers can tell you whether the feature you are seeing was formed by life and can often tell you what sort of life — such as plant, vertebrate or crustacean — made the fossil. Biomarkers aren’t always preserved in rocks, as weathering and oxidation can remove them leaving only the observed shape to work with.

In some cases, it can be very hard to decide if a rock feature represents past life. This is true when the geological processes that have influenced a rock are extreme, removing most of the evidence that would identify whether a feature is organic. This is also true for the very earliest forms of life, which can look very different from what we recognise as life today.

Do your part to protect Western Australia’s fossils by staying up to date with the restrictions and responsibilities related to fossils and State geoheritage, following all laws and regulations, and spreading the word to others (including tourists).

Common types of pseudofossils

In addition to rocks sculpted by normal weathering processes into plant-like or animal-like shapes, there are some types of rocks or rock features that are commonly mistaken for fossils.

Concretions and nodules

Concretions and nodules form when water and chemicals move through sediment before it solidifies into rock. A speck or crystal can trigger mineral precipitation, creating a hardened core that grows over time. These mineral masses may eventually merge into larger formations.

This figure includes 6 examples of commonly found concretions which might be mistaken for fossils, with each photograph having its own scale bar for reference.

Color changes and banding can be from shifts in mineral chemistry, oxygen levels, or oxidation state over time. For example, iron-oxide minerals vary: hematite (red-brown) forms in oxygen-rich settings, magnetite (black-brown) in low-oxygen conditions, and goethite (yellow-brown) in wetter environments.

Concretions form as minerals precipitate in layers around a central nucleus, which can be a mineral speck (like pyrite) or an organic object (like a shell or leaf). Cracking one open can reveal a fossil, though the concretion itself isn’t a fossil. Typically round with distinct internal layering, they are often mistaken for eggs, marbles, or bowling balls. Septarian concretions, with angular cracks in contrasting colors, resemble fossil turtle shells. Pea gravel, common in Western Australia, is a type of concretion.

Nodules fill spaces in sediment rather than forming around a single point, giving them irregular shapes and little internal structure. Though they can form around fossils, this is less common.

Various minerals contribute to concretions and nodules, including silica (chert, jasper, agate, flint), calcium minerals (calcite, dolomite, ankerite), iron-rich minerals (pyrite, siderite, marcasite, hematite, goethite), phosphates (apatite), and sulfates (barite, gypsum).

Some so-called ‘thunder eggs’ are nodules or concretions with dissolved centers later filled with minerals or sand, sitting between concretions and geodes. True thunder eggs, however, are agate geodes formed in volcanic settings, not concretions.

Cone-in-cone structures and shatter cones

Both cone-in-cone structures and shatter cones resemble stacks of up-ended cones in cross-section and are often mistaken for microbialites, horn corals, or fossils.

Cone-in-cone structures occur only in sedimentary rocks, forming perpendicular to bedding at millimeter to decimeter scales. They feature ridges or grooves and are nested, with the central cone often detaching. Likely formed by pressures acting on fibrous minerals like gypsum or calcite, they result from mineral growth, phase changes, or environmental forces causing buckling and folding.

Shatter cones, in contrast, are shock-related fractures linked to impact events. They appear as individual or overlapping cones with ridged surfaces but are not nested or detachable. Found in various rock types and independent of bedding, their striations can resemble plant fossils like horsetails but vary in orientation across the rock.

Mineral growths

Minerals can form life-like shapes under the right conditions. These shapes depend on a mineral’s crystal habit—pyrite, for example, naturally forms perfect cubes resembling human-made objects.

Fossil-like mineral growths often involve manganese minerals, though gypsum and other salts can also be mistaken for fossils. Liesegang banding creates colored mineral rings in rocks, while mineral-filled fractures can resemble trace fossils such as tracks and trails.

This figure shows six examples of mineral growths that resemble fossils, each with a scale bar for reference. Parts A to C show different types of manganese dendrites, Part B commonly known as moss agate. Part D shows an example of liesegang banding and Part F is a mineral growth made of gypsum.

Dendrites and manganese forms

Dendrites are branching crystal formations, often mistaken for plant fossils. Typically composed of manganese oxide minerals, they appear darker than the surrounding rock and grow along cracks.

Unlike plant fossils, which are two-dimensional on bedding planes, dendrites extend in three dimensions and cross bedding surfaces. Their patterns are also more irregular than leaves. When formed in fine-grained siliceous rocks, they are known as moss agate (in chalcedony) or moss opal (in opal) and are highly valued in jewellery.

Gypsum and other salt features

Gypsum forms in salt lakes and hot, moist regions as water evaporates. Common in Western Australia, it occurs alongside other evaporite minerals like halite and barite, which are actively forming in salt pans and lakes.

Under arid conditions, these minerals create unusual shapes. Halite can form ‘salt straws’ resembling fossil roots or burrows, while evaporite crusts on rocks may mimic corals or microbialites. In deserts, gypsum, barite, and selenite produce ‘desert roses’—disc-like crystals that fan out into flower-like shapes, highly valued by mineral collectors.

Geodes

Geodes form when minerals grow inside voids created by processes like dissolution in sedimentary rocks or degassing in volcanic rocks. Mineral-rich water or gas condensation later deposits crystals, often differing from the surrounding rock.

If the void isn’t fully filled, well-formed crystals develop, with quartz being the most common. Amethyst, rose quartz, and smoky quartz geodes are especially prized. Fully filled geodes, including some ‘thunder eggs,’ are often mistaken for fossils like bird or dinosaur eggs, while those with unusual mineral coatings can resemble other fossilised forms

Western Australian pseudofossils

Several typically Western Australian rocks are regularly mistaken for fossils.

Ranford Formation and Brockman Iron Formation ‘jellyfish’

Rounded forms from the Neoproterozoic Ranford Formation near Mount Brooking and Lake Argyle were once misidentified as ‘medusoids’ and ‘jellyfish’ in 1965. This claim was later repeated by GSWA without re-examining the material.

Precambrian fossil specialist Preston Cloud determined these were disc-like mineral growths, likely gypsum, later altered by mineral replacement. GSWA geologists later confirmed they lacked the symmetry and smooth margins of true jellyfish, proving them to be pseudofossils.

Similar misidentified ‘jellyfish’ pseudofossils have been recorded in the Pilbara, particularly in the Brockman Iron Formation around Mount Bruce and Mount Brockman, before later being recognised as mineral formations.

Examples of the 'jellyfish-like' pseudofossils found in WA. Parts A to C show examples from the Ranford Formation. Parts D and E show examples from the Brockman Iron Formation. Scale bars are included in each example except for Part D where the geological hammer is 33cm long.

Banded iron-formation and zebra stone

These colourful, striped rocks are often mistaken for stromatolites or other microbial fossils, though they are not fossils. Found alongside stromatolite-bearing rocks, this adds to the confusion.

Banded iron formation (BIF) is a common rock in Western Australia, the primary source of the State's iron ore, and prized by collectors when polished. BIF consists of alternating layers of hematite-rich grey to black and jasper-rich red. It formed in ancient oceans as minerals settled out of the water during the Precambrian, primarily between 2500 and 1800 million years ago. The layers can vary in thickness, and features like folding or faulting may resemble stromatolites, but BIFs formed through chemical, not biological processes. A special type, tiger iron, contains silicified asbestos in cavities and is valued as a semi-precious stone.

Pseudofossil examples: Part A is banded iron-formation and Part B shows zebra stone examples. Compare to the stromatolite fossil in Part C. All three parts include scale bars for reference.

Zebra stone

Zebra rock is a fine-grained, clay-rich rock typically white or light-colored, with distinctive red-brown bands or spots. These patterns can appear as stripes or spots, depending on how the rock is cut and polished. The red color comes from small amounts of hematite, though the exact formation process is unclear.

Some believe the color formed during sediment deposition, while others think it resulted from later iron mineral movement or removal. Zebra rock is found only in the Johnny Cake Shale Member of the Ranford Formation, where 'jellyfish' pseudofossils are also located.