#perspicaris

Perspicaris – Middle Cambrian (508 Ma)

Today is a special day for a couple of reasons! 1) I get to talk about the Burgess Shale again, and, 2) Today’s animal is MSPTD’s very first requested animal! So, before I get into the writeup, I want to thank @futureimagineer843 for requesting this animal! This writeup was a lot of fun and I hadn’t actually heard of this one before xe mentioned it. Also, if you ever want me to talk about a specific animal, requests are something I am absolutely open to.

Our third trip to the Burgess Shale, the famous Cambrian fossil bed from British Columbia, examines a lesser-known and lesser-understood animal named Perspicaris. Using Perspicaris, we can really put into perspective how much of a treasure trove the Burgess Shale really is. This is one of the more rare animals from the Burgess Shale, and it’s known from only 202 specimens. For comparison, we have around 50 specimens of Tyrannosaurus rex, an animal found in significantly younger rocks and one of the better-known theropod dinosaurs. It might sound ridiculous to call it rare, but those 202 fossils of Perspicaris make up only 0.38% of the Greater Phyllopod Bed, where most of the Burgess Shale fossils have been found.

We don’t know much about Perspicaris. It’s a really weird animal. It’s really similar to a common Burgess Shale animal called Canadaspis (known from 4,525 specimens and making up about 9% of the organisms found), and we can extrapolate a bit about its appearance and how it might have acted from looking at it.

Perspicaris is a tiny arthropod. It was less than an inch in length (2-3cm), and bears more than a passing resemblance to shrimp and other modern crustaceans. We’re unsure of whether or not it was an early crustacean, a basal Euarthropod (the modern groups of arthropods), or from a family outside of that group that left no descendants. It’s definitely some kind of arthropod, but getting more specific is pretty hard. That’s the problem with the Cambrian fauna, and one of the reasons it’s so fascinating. This is when just about every modern phylum evolved, so when can we say they split for sure?

It’s also hard to say what the hell it was doing with that body plan. It had big eyes on the end of stalks and limbs that could have aided swimming. But, it didn’t have any claws or enlarged biramous limbs (limbs that branch into two different segments that are usually adapted to some special purpose), so if it was swimming, how did it eat? We know Canadaspis was a bottom-feeder, but don’t have any evidence for that in Canadaspis.

This brings up the question: How do we know all these things about prehistoric animals? We use a lot of methods to figure out all this. Since the Cambrian Explosion, most animals fill different roles in a given modern ecosystem. A lot of those ecosystems have parallels between each other. Let’s use the Great Barrier Reef and an African savannah as an example. I’ll simplify it, because food webs can be really complex and can make it hard to get what I’m getting at.

At the base of both ecosystems are vegetation. In the Reef, it’s algae and kelp. In the savannah, it’s grass. Then you have the herbivores who eat those things. So, dugongs/krill, and gazelles/wildebeests. Then you have the carnivores, which eat other animals, like tiger sharks and lions. A lot of animals have a parallel animal in other ecosystems, and we can apply that same logic to prehistoric ecosystems, too. We can figure out roughly where animals fall in prehistoric food webs based on the features they share with modern animals. Canadaspis has a lot in common with modern benthic (bottom-feeding) animals, so we can say pretty confidently that it was a bottom-feeder. But what do you do when you have an animal like Perspicaris, which has a mix of traits but nothing pointing definitively in any direction? You speculate. Throw stuff at your peers and see what sticks.

Perspicaris looks a bit like tadpole shrimp. They have plenty of differences, but in broad strokes they look alike. Now, tadpole shrimp are bottom-feeders, too, but doesn’t have eyestalks like Perspicaris. Our friend here shares that with internet celebrity called the mantis shrimp, which actively hunts larger prey. But it doesn’t have claws like the mantis shrimp, so…

You see the problem. That’s why paleontologists debate a lot. A lot of media likes to sensationalize these disagreements like they’re rap beefs or something, but no they’re usually just discussions about stuff where people don’t agree. You know, like, how science works.  

Also, the media tends to latch onto the more outlandish stuff. There are plenty of folks around who still correct people by saying stuff like, “Actually, they found out that T. rex was a scavenger,” even though it was a theory that people only really looked at because Jack Horner liked it, and Jack Horner is, putting it lightly, a big fucking deal. That being said, there’s a truckload of evidence against that, and most scientists brush it off because Tyrannosaurus was built like a predator. Maybe I’ll talk about that someday.

So, what’s the deal with Perspicaris? In short: ¯\_(ツ)_/¯. It’s a weird little arthropod with a really vague body shape, and it’s really hard to figure it out because it doesn’t really look much like anything around now. And the thing it does look like has specializations it lacks. They’re little mysteries in a field full of little mysteries.

P.S. I have to talk about this whenever the Burgess Shale comes up, but Perspicaris has really pretty fossils.