#but we did not have trilobites or anything during the ediacaran when we had the silly leaf looking biota

[starswallowingsea]

THIS

is the Ediacaran period and THIS

is the Cambrian period. Got it?

[jurassicsunsets]

Sponges in the fossil record

Welcome back to another fun-filled week here at palaeofail-explained, which is apparently sponge central now. Last week I wrote a bit about sponges and why they’re great. You might wanna read that now, because this post is going to build off of that!

(Image: The fossil demosponge Astylospongia, a roughly spherical sponge with a hollow in the middle. It’s been broken to reveal the interior. [Source])

Sponges are some of the first animals to evolve (Perhaps the first - a controversy that’s made a bit more complicated by the ctenophore problem, which we’ll hopefully get to in a future installment) (Oh, and they might make up a paraphyletic grade in the way that monkeys and fish do, meaning that we might be just advanced sponges!).

Where was I? Ah, yes. Sponges are some of the first animals to have evolved, and consequently, are some of the earliest animals to appear in the fossil record. Or perhaps i shouldn’t say appear, because the earliest sponge fossils we have are not of sponge bodies at all!

What’s preserved are steranes. Steranes are a type of organic compound—specifically, a type of lipid, or fat. They are a highly degraded form of sterols, which are organic molecules found in many living things, especially eukaryotes (things whose cells have a nucleus). Examples of sterols include cholesterol, as well as testosterone and estrogen.

(Image: Estrogen, a sterol. It consists primarily of a bunch of carbon atoms bound together in four rings. Put this in a rock for a few million years and you get steranes. Put it in a semi well-known palaeontology blogger and you get gender euphoria. [Source])

The particular sterol that’s of interest to us (besides estrogen) is called 24-isopropylcholesterol (gesundheit), and today it is unknown in any living organisms other than demosponges. What does it do? I don’t know. Ask a sponge chemist. 

I mentioned before that sterols, when degraded, turn into steranes, and so we sometimes find examples of 24-isopropylcholesterANE in the rock record. This makes it an example of a molecular fossil. And what’s most exciting is that these molecular fossils are the earliest known evidence of animal life on the planet!

See, back about, oh, 700 million years or so ago, during the cryogenian period, the earth was covered in ice. Like, I mean, ALL OF IT. It’s a time in the history of the earth that we call “snowball earth”, and it’s a time when the Earth turned into the ice planet Hoth. But instead of tauntauns, there were only microbes living on Earth. And apparently, also sponges, as evidenced by molecular fossils.

(Disclaimer: there remains some controversy over these molecular fossils, and some possibility that they were produced by non-animals. I think they were most likely made by sponges, but time may tell.)

The earliest body fossils of sponges come from the next period after the cryogenian - the ediacaran, aka the age of pancakes.

(Image: Not a cellphone in sight, just rangeomorphs living in the moment. An Ediacaran scene with the pancake-like Dickinsonia in the middle, fern-like Charnia next to it, and a bunch of other things I don’t recognise around them. [Source])

This is all before the Cambrian Explosion, which you might have heard of before. The pancake guys lived a little less than 600 million years ago, but we’re not actually focusing on them. Rather, we’re focusing on the small shelly fossils that are around them. These are scientifically termed the Small Shelly Fossils. 

(Image: Scanning electron microscope photos of apparent sponge spicules in the Small Shelly Fossils. These particular ones are actually from the early Cambrian, but still before the Cambrian Explosion. [Source])

As you might have guessed if you read that caption, the Small Shelly Fossils of the Ediacaran and early Cambrian include sponge spicules, which if you didn’t read the sponge overview, are what make up sponge skeletons. So that’s something that can’t be so easily contested as molecular fossils.

One early group of...animals?? that appear in the fossil record are the archaeocyaths. These are the kind of things you might have seen at a natural history museum, looked at for 0.5 seconds, and then moved on to look at the pretty trilobites. These displays don’t do them justice, because they’re pretty interesting little guys.

(Image: Stapicyathus, an archaeocyath from the Early Cambrian, in top view (left) and side view (right). It’s shaped a bit like an ice cream cone, or like a bunch of sticks bundled together. [Source])

If you have heard literally anything about archaeocyaths it’s likely that they are, quote, “the first reef builders”, and indeed they were. It’s rare to see them in the form I showed above; normally, you just get cross-sections through them. 

Archaeocyaths are known exclusively from the Cambrian, and they occur on literally every continent. They’re used a lot for Cambrian biostratigraphy - in other words, correlating the fossils between different sites to determine how old they are. 

What did archaeocyaths do? It’s hard to know for sure, but it seems like they had a hole in the middle and porous walls—a lot like sponges, so it’s assumed (with good reason, I think) that they filter-fed like modern sponges. They also built skeletons out of calcite (limestone), hence their reputation as “reef builders”. They seem to have been common in shallow, warm-water environments, where you might expect filter feeders to live. 

One thing that’s peculiar about archaeocyaths, though, is that they start out entirely lacking pores, and occasionally will grow their skeletons to the point of entirely blocking off their pores. That’s strange for something that’s ostensibly living like a sponge, and it’s led some (i.e. the folks at Palaeos) to speculate that the structure evolved as a way for the earliest animal-like cells to live together, providing scaffolding for the cells to prevent them from being smothered. They make the case far better than I could, so it’s worth giving a read if you’re interested.

(Image: Generalised anatomy of an archaeocyath. Most of the details aren’t really important - what matters is they’re shaped kind of like an ice cream cone, with a hollow middle and porous walls. [Source])

Now we get to the tricky part: What are archaeocyaths?? They’re generally assumed to be animals, and sponges, but this is based mostly upon people not really being sure what else they could be. Having not studied them in depth, I myself have no clue, and I’ve gathered that the state of the literature seems to be “we have no clue”. They might be sponges. They might be some other group of animals. Heck, they might not even be animals. Maybe one day this’ll be illuminated with molecular fossils! It could be you! Whatever. I’m putting them in the sponge post for now.

(Image: A bunch of plus sign-shaped spicules clumped together on rock. [Source])

Once the Cambrian Explosion came in the mid-late Cambrian, actual real for-sure sponges got bigger and more recognisable. Those spicules in the image above belong to Protospongia, a sponge that lived in the Burgess Shale with your other favourite critters. It’s been referred to the hexactinellida, or the glass sponges, but I’m not sure how accurate that is due to spicules being notoriously convergent in different sponge groups.

(Image: Diagram of a stromatoporoid, looking like a lumpy lasagna. [Source])

Continuing the trend of “things you saw in a display cabinet and walked past” are the stromatoporoids, a group sponges from the middle palaeozoic ( about 300-400 million years ago). In fossils they’re normally seen in side view, but sometimes in top view as well. What’s more, we’re pretty sure they’re a group of demosponges. They made calcite skeletons, but palaeozoic forms lacked spicules. Mesozoic forms apparently had silica (quartz) spicules. 

Anyway, they’re the next “reef builders” after archaeocyaths died out, though there were also corals doing their thing at this time. They did...normal spongy things, it seems. Just your normal sponge fare.

And....”normal sponge fare” is pretty much the rest of the sponge fossil record. They make different shapes, different skeletons, different sizes, live in freshwater, live in saltwater, live under antarctic ice, and generally just do...spongy things. I’ll leave you with one interesting example of a sponge fossil—evidence of the Boring Sponge Cliona, preserved as the holes it bored in the shell of a mollusc.

Many people look past sponges as being too “simple” or less interesting than the more familiar fish or mammals or dinosaurs, but I think that just because something is a Boring Sponge, doesn’t mean it’s necessarily a boring sponge.