Spriggina is a group of early animals whose connection to modern animals is not fully understood. Fossils of Spriggina are found from the late Ediacaran period in what is now South Australia. Spriggina floundersi is the official fossil symbol of South Australia; it has not been found anywhere else.

The organism was about 3–5 centimeters (1.2–2.0 inches) long and may have hunted other animals. Its bottom had two rows of strong, interlocking plates, while one row covered its top. The front parts of its body were joined together to form a "head."

Scientists do not know exactly what kind of animal Spriggina was. It has been grouped into different categories, such as an annelid worm, a frond-like organism, a type of Charniodiscus, a proarticulatan, an arthropod (possibly related to trilobites), or an extinct group of animals. The absence of segmented legs or limbs, along with the presence of glide reflection instead of symmetrical body parts, makes it unlikely that Spriggina was an arthropod, even though it looks somewhat similar.

The Spriggina genus originally included three species—S. floundersi, S. ovata, and S. borealis. However, S. ovata is now considered the same as Marywadea ovata. The classification of S. borealis is still being studied and debated by scientists.

Description

Spriggina was about 3–5 centimetres (1.2–2.0 inches) long and had an oblong shape. Its body was divided into separate segments that were not joined together. Some segments were curved. The top of the organism had one row of overlapping plate-like structures, while the bottom had pairs of plates.

The first two segments formed the "head." The front segment was shaped like a horseshoe and had two depressions on its top surface, which might have been eyes. The second segment may have had antennae. Later segments had ring-like markings.

Some fossils show a possible circular mouth in the center of the head, but this is hard to confirm because the creature was very small compared to the large sand grains in the rocks where it was found. No legs have been preserved.

The body’s symmetry is not exactly the same as bilateral symmetry but resembles a glide reflection, where opposite segments are slightly shifted. In some fossils, the body segments tilt backward, forming V-shaped patterns, while in others, the segments are mostly straight. There are many different variations between these two patterns.

Discovery and naming

The genus was named after Reg Sprigg, who found fossils in the Ediacara Hills, part of the Flinders Ranges in South Australia. Sprigg supported the idea that these fossils belong to multicellular organisms. The only widely accepted species in this genus is Spriggina floundersi. The species name "floundersi" honors Ben Flounders, an amateur fossil collector from South Australia. Spriggina ovata has been placed in a new genus called Marywadea.

Spriggina fossils are found only in rocks from the Ediacaran period. Some fossils from the Vindhyan basin, accurately dated to about 1,650 million years old, were once classified as Spriggina. However, these are likely remains of simple, single-celled organisms rather than Spriggina.

Spriggina had a strong, but not calcified, body. This is shown by how the fossils are preserved: always as a mold on the bottom surface of the rock layer.

Classification

Spriggina, like many other Ediacara biota, is not clearly related to other groups. It looks somewhat like the living polychaete worms Tomopteris and Amphinomidae. However, it does not have bristles called chaetae, and other evidence shows it does not belong to this group. Scientists have also compared Spriggina to rangeomorphs, which are leaf-like members of the Ediacara biota. These rangeomorphs might belong to a completely different kingdom. Although its glide symmetry might indicate something else, some scientists, like Mark McMenamin, believe Spriggina could be an arthropod. Its appearance is somewhat similar to trilobites, which might mean it is closely related to this group. Some even thought it might be a predator. However, later studies do not consider Spriggina or similar Ediacaran biota like Parvancorina to be stem-arthropods. These organisms do not share key features with arthropods, and there is no clear evidence linking them to arthropods or other living bilaterians. This similarity to trilobites might be an example of convergent evolution, where different organisms develop similar traits independently.

Affinity

At first, Spriggina was believed to look like a polychaete worm, such as Nereis. However, when scientists closely examined its body segments, they noticed the segments did not line up along the middle, similar to Dickinsonia. In 1989, Seilacher changed the understanding, proposing that Spriggina might be a different type of sea-pen, with the "head" actually serving as a holdfast.

Some scientists have also suggested Spriggina might be related to arthropods because of some similar features to Cambrian trilobites. However, the absence of limbs and an exoskeleton makes this connection unlikely. Additionally, trilobites had broad pleural lobes that functioned as a rigid covering, protecting their legs as they moved through sediment to find food.

South Australia’s fossil emblem

On February 14, 2017, the Spriggina was chosen as South Australia’s fossil emblem because it has only been found in that region. This Ediacaran fossil, which is 550 million years old, was selected to represent the state’s geological and scientific achievements. Over 3,500 South Australians participated in an online poll to choose a fossil as the state’s new emblem.