The Burgess Shale

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.

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
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.
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.

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.

Gould Slideshow

In 1990, noted palaeontologist Stephen Jay Gould spoke at the Royal Ontario Museum about the fossils of the Burgess Shale. While many of Gould’s interpretations have been challenged, his talk provides a snapshot of how the organisms were viewed then. (6:20)

So this is Marrella. I should say that arthropods are classified primarily by numbers of segments and patterns in their various body parts.

Photograph of Marrella
DESCRIPTION: Photograph of Marrella

And here’s Marrella, it’s an arthropod that doesn’t fit into any group. It has these two sets of spines… there it is. It doesn’t have any allegiance.

Drawing of Marrella
DESCRIPTION: Drawing of Marrella

So Whittington was puzzled when he first published on Marrella in 1971 but he went on and the next creature he studied was Yohoia.

Photograph of Yohoia
DESCRIPTION: Photograph of Yohoia

Looked like a shrimp, had been called one by Walcott, and again, as Whittington studied it with care, it just didn’t fit into any modern group. It looks like a shrimp superficially, but when you start counting the segments you don’t have anything like the crustacean body plan.

Drawing of Yohoia
DESCRIPTION: Drawing of Yohoia

For instance, up in the head you have this unique set of frontal appendages which have no homologue anywhere else in the arthropods. Whittington ended up calling them simply “the great appendages” because he didn’t know what to do with them.

Photograph of Odaraia
DESCRIPTION: Photograph of Odaraia

This is Odaraia, a creature that swims on its back and has a tail fluke that looks more like a whale than an arthropod, but again, not allied to anything.

Drawing of Odaraia
DESCRIPTION: Drawing of Odaraia

Looked vaguely like a swimming crustacean, but isn’t when you look at the segments and their patterns of the tail.

Photograph of Sidneyia
DESCRIPTION: Photograph of Sidneyia

This is Sidneyia, which was described by Walcott as a chelicerate, that is a member of the horseshoe crab, eventually the spider-scorpion group. And in some superficial sense that’s what it looks like. But in detail it isn’t.

Drawing of Sidneyia
DESCRIPTION: Drawing of Sidneyia

All chelicerates have six pairs of appendages on their head. Sidneyia has one pair. It’s not like anything… just these antennae… it’s not like anything else… it is just is what it is.

Photograph of Habelia
DESCRIPTION: Photograph of Habelia

This is Habelia, an odd creature…

Drawing of Habelia
DESCRIPTION: Drawing of Habelia

… with tubercules all over its body.

Photograph of Leanchoilia
DESCRIPTION: Photograph of Leanchoilia

This is Leanchoilia, my personal favourite for elegance, but not among the survivors.

Drawing of Leanchoilia
DESCRIPTION: Drawing of Leanchoilia

Again, these odd great appendages, as Whittington calls them, with their whiplash endings.

Photograph of Aysheaia
DESCRIPTION: Photograph of Aysheaia

This is Aysheaia.

Drawing of Aysheaia
DESCRIPTION: Drawing of Aysheaia

Now, this creature is probably an onychophore, that is it is a member of a modern group symbolized by the genus with the wonderful name Peripatus, which is a not very well known group, but it’s thought to be possibly intermediary between annelids and arthropods and may be the ancestor of the insect group. So here we may have a creature that is truly related to one of the surviving groups of arthropods.

Desmond Collins, former ROM Curator, holding fossil
DESCRIPTION: Desmond Collins, former ROM Curator, holding fossil

And here is a form that Des Collins found and initially gave a field name, following paleontological tradition…

Photograph of Sanctacaris
DESCRIPTION: Photograph of Sanctacaris

… he called it “Santa Claws”. And eventually named it Sanctacaris, which means much the same thing. Now again, does it look any different than the ones I just showed you?

Drawing of Sanctacaris
DESCRIPTION: Drawing of Sanctacaris

Would you have picked out this creature for success? Could you have predicted that this, by virtue of superiority would go on? Yet it looks as though Sanctacaris really is a chelicerate.

There are six pairs of appendages in the right place on the head so this animal may be at least a cousin to one of the successful lineages. Again, would you have known? Could anyone have known?

Photograph of Opabinia
DESCRIPTION: Photograph of Opabinia

This is OpabiniaOpabinia, I think, should stand as one of the great moments in the history of human knowledge.

Drawing of Opabinia
DESCRIPTION: Drawing of Opabinia

Because Opabinia, which was described as an arthropod, a shrimp-like creature, by Walcott, who shoehorned it into modern groups as he always did. Opabinia was the first creature re-studied by Whittington that broke the conceptual dam, so to speak, and gave insights into this new world.

Technical drawing of Opabinia
DESCRIPTION: Technical drawing of Opabinia

Because Whittington began his studies in the early 1970s on Opabinia thinking it would be an arthropod. He realizes, as Walcott did not, that there was some three-dimensionality in these creatures, that they were not just films on the rock.

Photograph of Opabinia
DESCRIPTION: Photograph of Opabinia

That he could therefore dissect through and find structures underneath. So he said “Now I can resolve this, I’ll dissect through the body and find the appendages underneath which will prove its arthropod nature. He dissected through and he found nothing. There are no appendages.

Technical drawing of Opabinia
DESCRIPTION: Technical drawing of Opabinia

And as he reconstructed Opabinia, he came to understand it is not an arthropod, it is some bizarre creature of its own unique anatomy. And in publishing a monograph on Opabinia in 1975 I think you have the breakthrough point in the new interpretation of the Burgess Shale.

Drawing of Opabinia
DESCRIPTION: Drawing of Opabinia

Here is Marianne’s picture of Opabinia, a bizarre creature with five-count them, five-eyes, this vacuum-cleaner like nozzle with a food-collecting device in front, this bellows-like apparatus behind, followed by a tail. I don’t know what it is. It’s just weird.

Photograph of Nectocaris
DESCRIPTION: Photograph of Nectocaris

This is Nectocaris, a peculiar creature that looks like a chordate behind, combined with a fin ray…

Drawings of Nectocaris
DESCRIPTION: Drawings of Nectocaris

… and more like an octopod in the front. Who knows?

Photograph of Dinomischus
DESCRIPTION: Photograph of Dinomischus

This is Dinomischus, a peculiar, stalked, stemmed creature…

Drawing of Dinomischus
DESCRIPTION: Drawing of Dinomischus

… with no known affinity to anything else.

Photograph of Odontogriphus
DESCRIPTION: Photograph of Odontogriphus

This is Odontogriphus, literally meaning “the toothed mystery” a good name.

Drawings of Odontogriphus
DESCRIPTION: Drawings of Odontogriphus

A flat, gelatinous, annulated creature with a row of tooth-like structures surrounding a mouth and a pair of sensory palps.

Photograph of a fossil originally interpreted as a jellyfish
DESCRIPTION: Photograph of a fossil originally interpreted as a jellyfish

Walcott described three separate genera which he allocated, as was his wont, according to the shoehorn, into three conventional groups.
This animal he called a jellyfish and called Peytoia.

Photograph of a fossil originally interpreted as a sea cucumber
DESCRIPTION: Photograph of a fossil originally interpreted as a sea cucumber

This creature he called a sea cucumber and called Laggania.

Photograph of a fossil originally interpreted as a fossil shrimp
DESCRIPTION: Photograph of a fossil originally interpreted as a fossil shrimp

And this, which had been described before and looks like the body of an arthropod, he called (it had been named before) Anomalocaris, meaning “the odd shrimp”. Well I think that you’ve guessed it already.

Fossils of a complete Anomalocaris
DESCRIPTION: Fossils of a complete Anomalocaris

It turns out that all three go together. They form a single creature which is one of the weirdest of all the odd animals of the Burgess.

It’s also the largest Cambrian organism. Some specimens are almost a metre in length.

The so-called jellyfish is the mouth of this creature, working on a circular, nutcracker principle rather than the jaw of vertebrates principle.

The Anomalocaris itself turns out to be one of a pair of feeding appendages, and the so-called sea cucumber is the body of the whole animal.