Episode 291: The Ediacaran Biota

This week let’s find out what lived before the Cambrian explosion!

A very happy birthday to Isaac!

Further reading:

Some of Earth’s first animals–including a mysterious, alien-looking creature–are spilling out of Canadian rocks

Say Hello to Dickinsonia, the Animal Kingdom’s Newest (and Oldest) Member

Charnia looks like a leaf or feather:

Kimberella looks like a lost earring:

Dickinsonia looks like one of those astronaut footprints on the moon:

Spriggina looks like a centipede no a trilobite no a polychaete worm no a

Glide reflection is hard to describe unless you look at pictures:

Trilobozoans look like the Manx flag or a cloverleaf roll:

Cochleatina looked like a snail:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

It’s the last week of August 2022, so let’s close out invertebrate August with a whole slew of mystery fossils, all invertebrates.

But first, we have a birthday shoutout! A humongous happy birthday to Isaac! Whatever your favorite thing is, I hope it happens on your birthday, unless your favorite thing is a kaiju attack.

We’ve talked about the Cambrian explosion before, especially in episode 69 about some of the Burgess shale animals. “Cambrian explosion” is the term for a time starting around 540 million years ago, when diverse and often bizarre-looking animals suddenly appear in the fossil record. But we haven’t talked much about what lived before the Cambrian explosion, so let’s talk specifically about the Ediacaran (eedee-ACK-eron) biota!

I was halfway through researching this episode when I remembered I’d done a Patreon episode about it in 2021. Patrons may recognize that I used part of the Patreon episode in this one. You’d think that would save me time but surprise, it did not.

The word Ediacara comes from a range of hills in South Australia, where in 1946 a geologist noticed what he thought were fossilized impressions of jellyfish in the rocks. At the time the rocks were dated to the early Cambrian period, and this was long before the Cambrian explosion was recognized as a thing at all, much less such an important thing. But since then, geologists and paleontologists have reevaluated the hills and determined that they’re much older than the Cambrian, dating to between 635 to 539 million years ago. That’s as much as 100 million years before the Cambrian. The Ediacaran period was formally designated in 2004 to mark this entire period of time, although fossils of Ediacaran animals generally start appearing about 580 million years ago.

Here’s something interesting, by the way. During the Ediacaran period, every day was only 22 hours long instead of 24, and there were about 400 days in a year instead of 365. The moon was closer to the earth too. And life on earth was still sorting out the details.

Fossils from the Ediacaran period have been discovered in other places besides Australia, including Namibia in southern Africa, Newfoundland in eastern Canada, England, northwestern Russia, and southern China. Once the first well-preserved fossils started being found, in Newfoundland in 1967, paleontologists started to really take notice, because they turned out to be extremely weird. The fossils, not the paleontologists.

Many organisms that lived during this time lived on, in, or under microbial mats on the sea floor or at the bottoms of rivers. Microbial mats are colonies of microorganisms like bacteria that grow on surfaces that are either submerged or just tend to stay damp. Microbial mats are still around today, usually growing in extreme environments like hot springs and hypersaline lakes. But 580 million years ago, they were everywhere.

One problem with the Ediacaran biota, and I should explain that biota just means all the animals and plants that live in a particular place, is that it’s not always clear if a fossil is actually an animal. Many Ediacaran fossils look sort of plant-like. At this stage, the blurry line between animals and plants was even more blurry than it is now, with the added confusion that sometimes non-organic materials can resemble fossils, and vice versa.

For instance, the fossil Charnia, named after Charnwood Forest in England where it was first discovered. In 1957, a boy named Roger, who was rock-climbing in the forest, found a fossil that looked like a leaf or feather. He took a rubbing of the fossil and showed his father, who showed it to a geologist. The year before, in 1956, a 15-year-old girl named Tina saw the same fossil and told her teacher, who said those rocks dated to before the Cambrian and no animals lived before the Cambrian, so obviously what she’d found wasn’t a fossil.

Tina’s teacher was wrong about that, of course, although he was correct that the rocks dated to before the Cambrian, specifically to about 560 million years ago. But while Charnia looks like a leaf, it’s not a plant. This was about 200 million years before plants evolved leaves, and anyway Charnia lived in water too deep for plants to survive. It anchored itself to the sea floor on one end while the rest of the body stuck up into the water, and some specimens have been found that were over two feet long, or 66 cm. Some researchers think it was a filter feeder, but we have very little evidence one way or another.

One common animal found in Australia and Russia is called Kimberella, which lived around 555 million years ago and might have been related to modern mollusks or to gastropods like slugs. It might have looked kind of like a slug, at least superficially. It grew up to 6 inches long, or 15 cm, 3 inches wide, or 7 cm, and an inch and a half high, or 4 cm, which was actually quite large for most animals that lived back then. It was shaped roughly like an oval, with one thin end that stuck out, potentially showing where its front end was, although it didn’t have a head the way we think of it today. The upper surface of its body was protected by a shell, but not the type of shell you’d find on the seashore today. This was a flexible, non-mineralized shell, basically just thick, toughened tissue with what may be mineralized nodules called sclerites embedded in it. All around its body was a frill that might have acted as a gill. The underside of Kimberella was a flat foot like that of a slug.

We know Kimberella lived on microbial mats on the sea floor, and it might have had a feeding structure similar to a radula. That’s because it’s often found associated with little scratches on its microbial mat that resemble the scratches made by a radula when a slug or related animal is feeding on a surface. The radula is a tongue-like organ studded with hard, sharp structures that the animal uses to scrape tiny food particles from a surface.

Kimberella displays bilateralism, meaning it’s the same side to side. That’s the case with a lot of modern animals, including all vertebrates and a lot of invertebrates too, like insects and arachnids. But other Ediacarans showed radically different body plans. Charnia, for instance, exhibits glide reflection, where both sides are the same as in bilateralism, but the sides aren’t exactly opposite each other. If you walk along a beach and make footprints in the sand, your trail of footprints actually demonstrates glide reflection. If you stand on the sand and jump forward with both feet together, your footprints demonstrate bilateralism since the prints are side by side. (This is confusing to describe, sorry.) Pretty much the only living animals with this body pattern are some sea pens, which get their name because they resemble old-fashioned quill pens. Many sea pens look like plants, and for a long time researchers thought Charnia might be an ancient relation to the sea pen. These days most researchers are less certain about the relationship.

A similar-looking animal that lived around the same time as Charnia was Dickinsonia. It looks sort of like a leaf too, but a more broad oval-shaped leaf instead of a long thin one like Charnia. It’s also not a leaf. Some are only a few millimeters long, but some are over 4 1/2 feet long, or 1.4 meters.

Dickinsonia may be related to modern placozoans, a simple squishy creature only about one millimeter across. It travels very slowly across the sea floor and absorbs nutrients from whatever organic materials it encounters. But we don’t know if Dickinsonia was like that or if it was something radically different. Until a few years ago a lot of paleontologists thought Dickinsonia might be some kind of early plant or algae. Then, in 2016, a graduate student discovered some Dickinsonia fossils that were so well preserved that researchers were able to identify molecular information from them. They found cholesteroids in the preserved cells, and since only animals produce cholesteroids, Dickinsonia was definitely an animal. But that’s still about all we know about it so far.

Spriggina is another animal that at first glance looks like a leaf or feather. Then it sort of resembles a trilobite, or a segmented worm, or a possible relation to Dickinsonia. It looks like all sorts of animals but doesn’t really fit with anything known. It grew up to two inches long, or 5 cm, and had what’s referred to as a head shield although we don’t know for sure if it was actually its head. The head shield might have had eyes and might have had some kind of antennae, and some fossils seem to show a round mouth in the middle of the head, but it’s hard to tell. The rest of its body was segmented in rings. What Spriggina didn’t have was legs, or at least none of the fossils found so far show any kind of legs. Some species of Spriggina show a glide reflection body plan, while others appear to show a more ordinary bilateral body plan.

Three Ediacaran animals have such a weird body plan that they’ve been placed in their own phylum, Trilobozoa, meaning three-lobed animals. They show tri-radial symmetry, meaning that they have three sections that are identical radiating out from the center. They lived on microbial mats and were only about 40 mm across at most, which is about an inch and a half. Tribrachidium was roughly round in shape although its relations looked more like tiny cloverleaf rolls. Cloverleaf rolls are made by putting three little round pieces of dough together and baking them so that the roll has three lobes, although Trilobozoans probably didn’t taste as good. Also, Trilobozoans were covered with little grooves from center to edge and had three curved ridges, one on each lobe. The ridges were originally interpreted as arms or tentacles, but they seem to have just been ridges. Researchers think the little grooves directed water over the body’s surface and the ridges acted as tiny dams that slowed the water down just enough that particles of food carried in the water would fall onto the body so that the animal could absorb the nutrients, although we don’t know how that worked.

Many other Ediacaran animals had radial symmetry like modern echinoderms and jellyfish, including the ancestors of jellyfish. Some Ediacaran animals even had shells of various kinds, and they’re generally referred to as small shelly fossils. They were rarely more than a few millimeters across at most and are sometimes found mixed in with microbial mats. Cochleatina, for instance, is less than a millimeter across and all we know about it is that it had a ribbon-like spiral shell like a really simple snail’s shell. It wasn’t a snail, though. We don’t even know if it was an animal. It might have been some kind of algae or it might have been something else. Unlike most small shelly fossils, Cochleatina survived into the Cambrian period.

We’re also not sure why most Ediacaran organisms went extinct at the beginning of the Cambrian, but it’s probable that most were outcompeted by newly evolved animals. There may also have been a change in the chemical makeup of the ocean and atmosphere that caused an extinction event of old forms and allowed the rapid expansion of new animal forms that we call the Cambrian explosion.

We can also learn a lot about what we don’t find in the Ediacaran rocks. Pre-Cambrian animals didn’t appear to burrow into the sea floor, or at least we haven’t found any burrows, just tracks on the surface. Most Ediacaran animals also didn’t have armored bodies or claws or so forth. Researchers think that predation was actually pretty rare back then, with most animals acting as passive filter feeders to gather nutrients from the water, or they ate the microbial mats. It wasn’t until the Cambrian explosion that we see evidence that some animals evolved to kill and eat other animals exclusively.

With every new Ediacaran fossil that’s found and studied, we learn more about this long-ago time when multi-cellular life was brand new.

You can find Strange Animals Podcast at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave us a rating and review on Apple Podcasts or Podchaser, or just tell a friend. We also have a Patreon at patreon.com/strangeanimalspodcast if you’d like to support us for as little as one dollar a month and get monthly bonus episodes.

Thanks for listening!

Episode 069: The Cambrian Explosion

This week let’s find out a little something about the Cambrian explosion, where the relatively simple and tiny life on earth suddenly proliferated and grew much larger…and definitely stranger.

The Burgess shale area: beautiful AND full of fascinating fossils:

Anomalocaris, pre-we-figured-out-what-these-things-are:

What anomalocaris probably actually looked like, plus a couple of the “headless shrimp” fossils:

More “headless shrimp” fossils because for some reason I find them hilarious:

Marrella. Tiny, weird, looks sort of like those creepy house centipedes that freak me out so much, but with horns:

Hallucigenia, long-time mystery fossil:

What hallucingenia probably looked like, maybe:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week’s topic is one I’ve been fascinated by for years but I’ve never read much about it: the Cambrian explosion. That refers to the explosion of life forms in the Cambrian period, which started about 540 million years ago. That was long before the dinosaurs, long before fish, basically long before almost all life on earth that wasn’t simple squidgy things living in warm, shallow seas.

To learn about the Cambrian explosion, let’s go back even farther first and learn about the first life on earth.

Obviously, the more recently an animal lived, the more likely we are to find fossils and other remains: footprints in fossilized mud, gastroliths and coproliths, and so forth. The farther back we go, the fewer remains we have. The earth is continually changing, with mountains rising up and continents moving around, volcanoes erupting, old mountains being worn down by wind and weather. That’s good for the earth and therefore for life in general, since nutrients are cycled through the ecosystem and habitats are continually renewed. But it’s bad when paleontologists are trying to find out what lived a billion years ago, because most of those rocks are gone, either weathered into sand long ago, melted into magma, or buried under the ocean or otherwise out of our reach.

The Earth formed about 4.5 billion years ago, oceans formed 4.4 billion years ago, and the oldest rocks we can find are about 4 billion years old. The first life on earth, single-celled organisms, dates back to about 3.8 billion years ago, maybe earlier. By 3.5 billion years ago, complex single-celled microorganisms had evolved—we know because we’ve found eleven microscopic fossils in rocks from western Australia. Researchers have concluded that the fossils belonged to five different taxonomical groups, which means that by 3.5 billion years ago, life was already well established and diverse.

By 2.5 billion years ago, the earth had continents roughly the same size as the ones today, although not anything like the same shapes or in the same places. Land also didn’t have dirt on it, just sand and bare rock, since dirt is largely decomposed organic matter and nothing was living or dying on the land yet. Not long after, 2.45 billion years ago, oxygen started to make up a large part of the earth’s atmosphere. That’s right, before then we literally could not have breathed the air. I mean, we could have, but we would die of suffocation because the air contained only trace amounts of oxygen. While having oxygen in the air sounds great to us now, the single-celled organisms living then couldn’t process it and died off—probably the greatest extinction event in the earth’s history. Only organisms that were able to evolve quickly enough to use oxygen survived and thrived.

One particular type of microorganism dating back 2.3 billion years, sulfur bacteria, again known from ancient rocks from western Australia, is still around. Modern sulfur bacteria live in the deep sea off the coast of Chile, and they literally have not needed to change at all in 2.3 billion years. That’s what you call success.

The earliest multicellular organisms date to around 2.1 billion years ago, or at least those are the oldest fossils we’ve found. Algae and fungi evolved soon after. The earliest animal fossils date from about 580 million years ago and include small jellyfish and sea anemones, but all the oldest fossils we’ve found are of specialized animals so they probably arose much earlier. At about the same time, fossils of more complex shelled animals start appearing in the fossil record, animals which may have been the ancestors of arthropods, echinoderms, and mollusks. We also have fossils of burrows made in the sea floor, although we don’t know what kind of animal made them—some kind of wormy creature, but none have been found, just their burrows. Clearly a lot was going on back then, but it was all on a small scale: tiny worms, colonies of bacterial mats, and shelled animals measured in millimeters.

Then came the Cambrian explosion, starting about 540 million years ago, where diverse and often bizarre-looking animals suddenly appear in the fossil record, proliferating at a rate unheard-of in the previous eras. We’re not completely sure why, but it was probably a combination of factors, possibly including increased oxygen levels, the development of an ozone layer in earth’s atmosphere that protects cells from lethal UV radiation, an increase of calcium in ocean water, and many other factors, large and small. As animals grew larger and more diverse, more species could exploit more ecological niches; and when all the available niches were occupied, competition grew even more fierce, leading to even bigger and more specialized animals.

The first Cambrian fossils found were those of trilobites, first described in 1698 but not recognized as extinct fossil animals until the 18th century. By the 19th century so many forms of trilobite were known that geologists used them to help date rock strata. While trilobites had probably been around before the Cambrian, during the Cambrian they evolved exoskeletons and became much larger and more common.

You’ve probably heard of the Burgess shale, and you’ve probably heard of it because of the book Wonderful Life, published in 1989 by paleontologist Stephen Jay Gould. The book is out of date now, but when it was new it caused a lot of popular interest in the Cambrian explosion in general and the Burgess shale fossils in particular.

Shale, if you’re not familiar with the term, is a type of sedimentary rock formed from mud containing a lot of clay, generally mud from slow-moving water, floodplains, and quiet lagoons. It’s common, generally gray in color, and splits into flat pieces that you can draw on with other pieces of shale like a chalkboard. People sometimes confuse shale with slate, but slate is actually shale that’s been hardened by pressure and heat within the earth into a metamorphic rock. Because shale is formed from fine particles instead of sand, it can preserve fossils in incredible detail, although usually flattened.

So the Burgess shale is a large deposit of shale some 30 miles across, or 50 km, and 520 feet thick, or 160 meters. The area was once the bottom of a shallow sea next to a limestone cliff, around 505 million years ago, right in the middle of the Cambrian period. When the Rocky Mountains were created by tectonic forces around 75 million years ago, the Burgess shale was lifted 8000 feet above sea level, or 2500 meters. It’s in Canada, specifically Mount Stephen in Yoho National Park in British Columbia, and it’s properly called the Stephen Formation.

In the late 19th century a construction worker found some fossils in the loose shale weathered out of the formation. A geologist working for the Geological Survey of Canada heard reports of the fossils and in 1886 visited the area. He found trilobites and told his supervisor. Eventually paleontologist Joseph Whiteaves took a look and collected some Burgess shale fossils he thought were headless shrimps. They weren’t, by the way. We’ll come back to them in a minute.

In a nearby section of the Stephen Formation, paleontologist Charles Doolittle Walcott set up a fossil quarry in 1910. He and his team worked the quarry intermittently for the next few decades, collecting more than 60,000 specimens. But he didn’t publish very much about his findings, and after his death no one was very interested in the Burgess shale until the 1960s and 70s, when a couple of paleontologists started poking through Walcott’s collection. Their findings are what Gould writes about in Wonderful Life. Since then, paleontologists have continued to find amazing fossils in the Stephen Formation, and research continues on Walcott’s collection.

Part of the reason Gould’s book was such a sensation, apart from the fact that he’s a great writer and fossils are just interesting, was that he suggested the Cambrian explosion was caused by an unknown event that forced new evolutionary mechanisms into play, leading to many animals that are completely unrelated to those living today. He and some of the paleontologists working on the Burgess shale animals in the 1970s thought many of them belonged to phyla unknown today. There are only 33 designated phyla, although they do get looked at and changed around occasionally as new information comes to light. Humans and all other mammals, as well as reptiles, birds, amphibians, and fish, belong to the Chordata phylum. Gould suggested that if the Burgess shale animals had continued to evolve instead of dying out, life on earth today might look radically different.

That brings us to Whiteaves’s headless shrimp. Its name is Anomalocaris, which means abnormal shrimp. If you’re familiar with shrimp—you know, the things you eat, especially with rice or grits and I am so hungry right now—you have probably seen a headless one. The heads are typically removed before shrimp are sold, even though the rest of the shrimp may be intact, including shell, legs, and those little finny bits on the tail. That’s more or less what the fossil Whiteaves found looked like, except that its legs weren’t jointed. It was a little over 3 inches long, or around 8.5 cm. Whiteaves described it as a type of crustacean in 1892.

But to find out what it really was, we have to look at a couple of other discoveries. Walcott discovered what he identified as a type of jellyfish, around two inches across, or 5 cm, a circular segmented creature with a hole in the middle that looks a lot like a fossilized pineapple ring. Walcott also found what he thought was a feeding appendage or tail of an arthropod called Sidneyia, but didn’t realize it was the same anomalocaris Whiteaves had described. And paleontologist Simon Conway Morris discovered another of Walcott’s pineapple ring jellyfish, preserved together with what he took to be a sponge.

Harry Whittington, a paleontologist working on the Burgess shale fauna in the late 20th century, finally realized all these fossils belonged together—not as a crustacean, a sponge, and a jellyfish, but as one large animal. The shrimp tail was its feeding appendage, of which it had a pair in the front of its head, and the unjointed legs were spines. The pineapple ring jellyfish was its round mouthpiece consisting of plates that it contracted to crush prey. The sponge was its lobed body, which was softer and didn’t preserve as well as its other pieces.

Whiteaves’s feeding appendage came from a larger species, Anomalocaris canadensis, which grew some three feet long, or about a meter. It probably ate soft-bodied animals. Peytoia nathorsti was much smaller and may have used its feeding appendages to filter tiny prey from the mud.

In the 1990s anomalocaris and its relatives were identified as stem arthropods, ancestors of or at least relations to modern arthropods like insects, crustaceans, and spiders, and not belonging to a new phylum at all. Another anomalocarid was found in rocks 100 million years younger than the Burgess shale, which means at least some of the strange Cambrian animals persisted well into the Devonian.

Another confusing animal is called Marrella, a common fossil in the Burgess shale. Walcott found the first one in 1909 and called it a lace crab, then decided it was a strange trilobite. It’s small, less than an inch long, or under 2 cm, and has long antennae and legs, and head appendages that sweep back into rear-facing spikes that may have protected its gills. It was probably a scavenger that lived on the bottom of the ocean, and we know some interesting things about it. We have one Marrella fossil that shows an individual partly moulted, so we know it moulted its exoskeleton periodically. We also have some specimens so well preserved that researchers have found a pattern on them that would have diffracted light. In other words, its exoskeleton was iridescent and colorful. Charles Whittington examined Marrella in 1971 and determined that it wasn’t a trilobite, wasn’t a crab or other crustacean, and wasn’t any kind of horseshoe crab. Instead, it’s a stem arthropod like anomalocaris.

Hallucigenia may be the most famous Burgess shale animal, although it’s also been found in fossil beds in other parts of the world. It was first described by Walcott as a polychaete worm. Simon Conway Morris redescribed it in 1977, pointed out that it definitely was not a worm, and gave it its own genus. But no one was really sure what it would have looked like when alive, how it would move around and eat, or what it might be related to. Fossils show a thin, flexible worm-like body with long spines sticking out along its length on one side, and flexible tentacles sticking out along its length on the other side. One end of the body is sort of bulbous and the other blunt, but it’s not clear which is the head and which is the tail. It’s small, only an inch or so long at most, or a few centimeters. Conway Morris thought the animal walked on its stiff spikey legs and the tentacles were for feeding, and that each tentacle might even end in a mouth. Other paleontologists suggested the fossil might be part of a bigger animal, the way Anomalocaris feeding appendages were initially thought to be separate animals.

But after more and better fossils were discovered in China, paleontologists in 1991 realized Hallucigenia had been reconstructed upside down and backwards by Conway Morris. The tentacles were paired legs and the stiff spines probably protected the animal from other things that wanted to swallow it. The bulbous end seems to be a head with two simple eyes and a round mouth, possibly with teeth. Its closest living relation is probably a caterpillar-like land animal called a velvet worm or lobopodian worm, although it’s not actually a worm.

Other Burgess shale animals include a bristle worm, an actual relative of modern shrimp, a relative of the horseshoe crab, something that may be related to modern mantis shrimp, a rare mollusk ancestor that was an active swimmer, and a fishlike animal with short tentacles on its tiny head that may have been a primitive chordate.

Most of the Burgess shale animals that have been studied are now classified as arthropod ancestors. But there are hundreds, if not thousands, of fossil species that paleontologists are still puzzling over, with more yet to be discovered in the Stephen Formation and elsewhere. It’s always possible that some animals that evolved during the Cambrian will surprise us as belonging to a completely new group of animals, and that we really will need to add a couple of phyla to the list.

Another exciting thing to remember is that because life on earth is common and arose relatively soon after the earth was formed, it’s almost 100% certain that some other planets also have life—maybe not planets in our own solar system, although we don’t know for sure yet, but astronomers have discovered lots of planets outside of our solar system. They estimate the Milky Way galaxy alone may contain 100 billion planets. In the past researchers have insisted that only planets similar to ours can support life, but that’s not the right approach. Only planets similar to ours can support life like ours. That’s because we evolved to fit our planet. Life on other planets naturally will evolve to fit those planets. Even here on earth we have extremophiles that survive in environments where most other organisms would be destroyed immediately. So next time you’re outside at night, look up at the stars and give them a little wave. Some curious creature might be standing on a planet’s surface untold light years away, staring into the sky and waving a greeting too.

You can find Strange Animals Podcast online at strangeanimalspodcast.com. We’re on Twitter at strangebeasties and have a facebook page at facebook.com/strangeanimalspodcast. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave us a rating and review on Apple Podcasts or whatever platform you listen on. We also have a Patreon if you’d like to support us that way.

Thanks for listening!