Science

Enigmatic Ediacarans

The Ediacaran Period (635-542 million years) represents a turning point in the history of life with the advent of the first large and complex multicellular soft-bodied organisms. These include sponges and cnidarians, as well as a number of problematic groups represented by both macrofossils and microfossils. Some of these fossils have traditionally been regarded as the remains of forerunners to Cambrian (and modern) animals, while others have been seen as a completely extinct kingdom.

The exact affinity of these organisms is still debated, but many researchers agree that they display a wide range of morphologies, suggesting they might belong to different groups at the base of the animal tree of life.

Graphic showing origins of different Ediacarans

Possible positions of various types of Ediacarans at the base of the animal tree of life. Dotted lines represent the probable range of particular groups of animals. Solid lines represent fossil evidence. Extinct groups (taxa) are represented with a circled cross. (modified after Xiao and Laflamme, Peterson et al and Dunn et al.).

Perhaps the most iconic of these enigmatic fossils belong to a group known as the rangeomorphs, found in late Ediacaran (575-542 million years ago) rocks. These are feather- or bush-shaped and show self-repeating (fractal) growth patterns that resemble the outlines of some modern fern fronds. They are not, however, related to plants - rangeomorphs lived deep in the sea, far below the depth where light could penetrate to allow photosynthesis. The rangeomorphs lack any evidence of a mouth or gut - or indeed any other complex internal organs typical of most animals. Their affinity remains ambiguous. Some researchers have suggested rangeomorphs could represent a grade of organization that evolved before the sponges, making them very primitive metazoans.

Fossil of large, fern-like Ediacaran

Bradgatia, a bush-shaped rangeomorph from Mistaken Point, Newfoundland. Width of specimen = 9 cm.

© Royal Ontario Museum. Photo: Jean-Bernard Caron.

Rangeomorphs were either fixed to the seabed by anchor discs or lay flat on the seabed, stuck on or in the mud. How these organisms obtained nutrients is a mystery. They were traditionally regarded as suspension feeders, filtering tiny food particles from water currents. Recent studies suggest they might have absorbed dissolved nutrients from seawater directly into their bodies (a process called osmotrophy).

Some Ediacarans grew to over a metre in length, but most were less than 10 cm in size. Exceptionally well-preserved fossils of these mysterious organisms have been found at more than 40 sites around the world, suggesting they were the dominant large (i.e., visible to the naked eye) life forms on the planet at the time. Canada's Mistaken Point biota in southeastern Newfoundland represents not only the earliest (at 575 million year old) but also one of the best-preserved Ediacaran communities known. The Mistaken Point biota is dominated by rangeomorphs that were preserved under thin layers of volcanic ash.

Fossil traces in rocks along seashore

General view of several fossil surfaces along the coastline, Mistaken Point, Newfoundland

© Royal Ontario Museum. Photo: David Rudkin.

Researcher studies fossils on large slab of rock

Detail of one of the best-preserved fossil surfaces at Mistaken Point, showing hundreds of rangeomorph fossils.

© Royal Ontario Museum. Photo: David Rudkin.

Close-up view of two fern-like fossils in rock

Several rangeomorphs (Fractofusus) and Charniodiscus (middle top) from Mistaken Point, preserved under a thin layer of ash (darker areas). Diameter of lens cap = 6 cm.

© Royal Ontario Museum. Photo: Jean-Bernard Caron.

Other Ediacaran deposits preserve fossil organisms which might have evolved more complex feeding strategies. For example, trace fossils suggest the Ediacaran Kimberella (found in Australia and Russia) could move and graze on the sea bed. This is probably the most convincing bilaterian fossil animal found in Ediacaran rocks and might represent an extinct early branch - a stem group (see also crown group) - of the evolutionary tree that includes today's molluscs (snails, clams, squids, and their relatives).

Left, oval fossil impression in rock; Right, traces in rock

Kimberella quadrata, a putative bilateral animal from Rawnsley Quartzite, Flinders Ranges, Australia. Left, complete specimen (Length = 8 cm); right, putative resting trace of the animal (the large ovoid structure, length = 9.5 cm) showing "tooth-marks" (small parallel structures). The marks have been interpreted as evidence that Kimberella possessed mouth parts comparable to modern molluscs.

© The Museum Board of South Australia. Photos: J. Gehling.

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