Where did all the Ediacaran fauna go?

Sorry, folks. I know it’s been an insanely long time since I’ve posted on Science Over a Cuppa. I’ve been drowning in teaching, packing, moving, and starting a new job. But I’ve made it, I’ve started at Loyola, I’m setting up the new lab, and I hope to be posting a little more regularly on SOAC.

I haven’t written much about it, but one of my favorite periods in evolutionary history is the Ediacaran. For those of you who know at least a little bit about the famous “Cambrian Explosion,” popularized especially by scientists such as Stephen Jay Gould, the Ediacaran is the oft overlooked older sister to the Cambrian. (They have a bit of a Venus-and-Serena-Williams kind of relationship: the Ediacaran’s a little quieter, a little shier, but no less remarkable and groundbreaking.) I love this period in particular because it’s so foundational to all the other incredible changes that would quickly follow. From a genetics perspective, this was undoubtedly a period of massive expansion and refinement of the core of the animalian genome.

Illustrations depicting Kimberella (left, front), Dickinsonia (middle, front), and various other fauna and flora of the Ediacaran world.

While the Cambrian displayed an astounding range of arthropods (aka, oceanic bugs) and later in the period led to the fascinating rise of our own clade, the vertebrates– it was during the Ediacaran that bilaterally symmetric organisms evolved and we see the first clear exploration of the bilateral bauplan (body plan).

For instance, there was the slug-like creature, Kimberella, which scraped along the hard surface of the ocean floor, likely eating bacterial and algal mats, leaving trace fossils for us to find more than half a billion years later. There was also Dickinsonia, a bizarre oval-shaped multicellular organism that recent research suggests may have also been an early animal (although there’s still ongoing debate about that) [1]. And there was even Tribrachidium, a now-extinct branch of the animal family tree that was tri-radially symmetric (trimerism), meaning they had three identical parts around a central axis. Nowadays, only pentamerism (5), tetramerism (4), and biradial symmetry (2) exist in animals. The trimeric animals are completely gone.

A fossil of the trimeric organism, Tribrachidium.

Although the Cambrian period displayed an explosive array of new animal forms, many of which ultimately didn’t make it out of the Cambrian, we nevertheless see some basic body plans that were formulated during this time and continue even to this day. We see continuity.

But one of the great mysteries of the Ediacaran period is that, despite the array of animal forms that evolved during this period, it’s difficult to find clear continuity between many of the animals(?) of the Ediacaran and those of the Cambrian. In short, we’re not exactly sure on who gave rise to whom.

In paleobiology, one can rarely be certain of direct ancestral lines, but at least scientists are capable of saying roughly what bushy branch of the family tree an organism resides in and what species are probably its closest relatives. But with the Ediacaran fauna, the vast majority of animals (if indeed some of them were even animals!) are a relative mystery. For example, the taxonomy of the slug-like Kimberella, though exhibiting features that are reminiscent of molluscs, is still hotly debated and the most paleobiologists seem to agree on is that it was most likely a bilaterally symmetric animal (which anyone can tell just by looking at its fossils, of course).

So, what happened to the Ediacaran animals and where are their relatives now?

While nobody can answer the latter question as yet (obviously, some of their predecessors must be around because the animal kingdom is well and thriving!), there are some tantalizing hints as to the former question.

At the End of the Ediacaran period, there was some kind of extinction event or events. As with most largescale extinctions, these tend to result in rapid loss of species, followed by radiation events of the remaining lineages quickly filling recently vacated environmental niches (Punctuated Equilibria at its finest!). But what were the causes?

Illustration of a priapulid worm, burrowing in the ancient sediment.

A leading theory concerns another early group of animals developing during the Ediacaran: WORMS.

Early oceanic worms, like many today, burrow within the ocean sediment. However, this behavior was a new evolutionary invention in the Ediacaran. Suddenly, we see trace fossils of burrows popping up during this period.

How did breaking up and stirring the ocean floor (known as “bioturbation”) affect other Ediacaran organisms? First off, that bioturbation probably led to some loss of the microbial mats, a major food source for many Ediacaran organisms. Breaking up the the microbial mats also likely affected some Ediacaran organisms’ ability to latch onto the ocean floor– a once-hard mat turned into a softer, more porous sediment. With that softer sediment, more particles may have also made it into higher levels of the water column, negatively affecting organisms that used suspension feeding. Essentially, they may have gotten “all choked up” and experienced significant interruptions in their ability to eat.

Breaking up and stirring the sediment may have also led to some lower-level oxygen conditions (anoxia), which could have unduly influenced some organisms more than others. And, finally, it’s entirely possible that this newly blossoming “wormworld” was composed of predators that found the traditional Ediacaran biota quite tasty [2]. We do know, later in the Cambrian for instance, that priapulid worms were adept predators, quietly awaiting prey to move near their holes before reaching out and striking. (If you wonder how groups of animals could be eaten into extinction, just think of the number of turtle species that were probably driven to extinction thanks to our early human relatives who had a predilection for turtle meat and an easy meal [3].)

Although it probably happened over a prolonged period, some researchers believe that the early development of this wormworld was the first firecracker that started off the Cambrian explosion. And our vertebrate lineage, which arose in the late Cambrian, was certainly a beneficiary of the altered environment.

So, the next time you’re working in your garden and run across one of the terrestrial cousins of those early oceanic worms, give them a hearty “thank you”– because you wouldn’t be here without them.

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