You might not know this about me, but I adore puzzles.

So I guess my career makes sense since zoology is like one big, wonderful jigsaw puzzle. Well, except that there are no helpful edge pieces.

And lots of pieces can fit in more than one place. Oh, and we don’t have the picture on the box as a guide towards the right answer, just some blurry snapshots of different sections from fossils. It’s fun!

Today in 2021 we’ve gotten pretty far in solving the zoology puzzle and have a general idea of what the metazoan family tree looks like, but it’s not perfect. Some species were -- and probably still are -- misidentified, and there are a lot of gaps and missing details. But that’s part of what makes zoology so thrilling -- all the times when evolution has stumped or surprised us.

The mysteries we’re still trying to solve. All the animals that defy our human brains and refuse to be put into a neat little box labelled “carnivora” or “perissodactyla” or whatever. There’s so much we have to learn.

So grab some fossils, fire up the DNA sequencer, and bring your best hypotheses -- it’s time to solve some phylogenetic mysteries! I’m Rae Wynn Grant, and this is Crash Course Zoology! You might remember from our first episode that phylogenies or phylogenetic trees are diagrams that scientists use to describe the evolutionary relationships between different animals.

But they can also show where there are gaps in what we know. Like there are lots of cases where we don’t know, or only very recently found out, exactly how a group of animals with a common ancestor, or clade, fits on the metazoan phylogeny. We call these phylogenetic mysteries -- animals whose traits could place them in multiple groups, or in groups that aren’t actually related to each other.

And studying and solving phylogenetic mysteries tells us not just about that particular animal’s evolutionary history, but also about how we’ve understood and studied animals in the past. So being a zoologist can be like being a detective. We get pieces of evidence, like what an animal looks like or what genes it has, and then have to figure out the series of evolutionary events that explains that evidence.

But we have to be really crafty, like Sherlock Holmes-level crafty, because that evidence isn’t always super reliable. Lots, maybe most animals have wildly different body shapes and lifestyles from their larval form early in their lives to their adult form. And it can be really hard to match the two if there aren’t a lot of opportunities to observe it’s whole life cycle.

Like maybe that animal is hard to find in the wild, or maybe it can’t be kept in captivity. These are called within lifetime phylogenetic mysteries. And one of the most enduring examples of baby animals going undercover and fooling zoologists is the Case of the Missing Baby Eels!

Let’s go to the Thought Bubble. People have been catching and eating eels for a long time. Like back in medieval England, monks often paid their landlords with eels.

Which is very on brand for monks -- eels were Church-approved because at the time, no one had ever seen an eel engage in the type of scandalous “carnal relations” that happen when animals make other animals. Instead zoologists thought they formed from mud and rainwater, or that adult eels just popped out of other eel-looking animals, like eelpouts. And this eel mystery went on for a long time -- even famous psychologist Sigmund Freud got involved, dissecting hundreds of eels in the spring of 1876, trying to find their gamete-producing organs to prove eels used sexual reproduction.

But the baby eels had been there all along. They were tiny, transparent, flat fish called Leptocephalus. A decade after Freud’s futile attempts, zoologists managed to grow some captive Leptocephalus into juvenile eels, and others observed the same thing in the wild.

The case of the missing baby eels was solved! To be fair, eels have a really complicated life cycle with five forms that look and act so different it’s pretty easy to mistake them as separate species. PLUS, eels move around a lot, so you don’t see babies and adults together.

It wasn't until 1904 that we figured out European eels breed in the Sargasso sea, thousands of miles away from their adult homes. Thanks, Thought Bubble! Eels are just one example of an animal that goes through big changes and lives in unknown places as part of their life cycle.

This type of knowledge gap is really common in ocean-living creatures, because the ocean is so big. It’s difficult to not only know how well their populations are doing, but also how to better conserve them for future generations. But for the eels, at least, the case is closed.

Which brings us to across sexes phylogenetic mysteries. With some animals, the differences aren’t so much due to age, but sex. Scully and Mulder, Cagney and Lacey, Agent K and Agent J -- detective media loves to have two very different people work the same case for drama.

But at least they’re both clearly people. Some animal pairs are so different that they don’t even seem to be part of the same species. Sexual dimorphism is when different sexes of the same species have physical differences beyond what gametes they make or sex organs they have.

Sometimes it’s straightforward, like the female being much larger or the male being more brightly colored. And sometimes the differences between sexes aren’t so much in how they look, but how they act and behave. Which can lead to one sex being a lot less well known than the other.

Like in the Case of the Lonely Spiders, subtitle: 8 legs but only 1 sex. Of the 46,000 species of spiders described so far, almost 50%! of them are known based on a single sex. Probably because one sex tends to get itself into situations where it’ll get scooped up by a zoologists -- like wandering around looking for a mate.

And when we finally do get a specimen of a different sex, it can totally change our understanding of a species. Like in Solifugids AKA camel spiders. Some species have females that are a darker color, have different shaped mouthparts, and differently arranged hairs than males.

They look so different that they were described as separate species for years, until a group of researchers in Iran put things together. Another case closed! Beyond developmental or sexual diversity in animal form, another frequent cause of phylogenetic mysteries is that some animals are just so bizarre compared to everything else we’ve seen before.

Like sometimes a behavior or an adaptation is just so amazing that early zoologists decreed it couldn’t have possibly evolved more than once, so that all animals with that trait must be related. But as we’ve seen several times in this course, convergent evolution happens all the time, even for fairly complex traits. One example is termites, which are eusocial and live in mounds with a reproductive queen and non-reproductive soldiers, workers, and other highly specialized roles.

For a long time, they were grouped alongside ants and bees in the order Hymenoptera, because eusociality requires so many co-ordinated changes in an animal’s body and behavior that early zoologists figured it probably only evolved once. But starting in the 1930s, bits of evidence suggested termites weren’t quite like ants or bees. They had different microorganisms in their guts, and baby termites look a lot like a very different insect.

By 2008, genetic evidence confirmed that termites weren’t closely related to bees or ants at all, and were actually highly social cockroaches! Recategorizing the taxonomy of termites really brought into question the idea that a trait could be too complicated to evolve more than once. Amazing what evolution can do with a few hundred million years to experiment!

Even if everything is secretly trying to evolve into a crab. One of the many kind-of crabs -- horseshoe crabs -- is a key figure in another mystery: the Case of the Land-Living Arachnids. Horseshoe crabs first appeared in the fossil record about 450 million years ago… and zoologists are still arguing about who their closest relatives are.

Until recently, the case seemed closed. Horseshoe crabs are definitely an arthropod, and their mouthparts and body plan put them somewhere in the class Chelicerata, which would make them distant cousins with the more famous chelicerates: arachnids, like spiders and scorpions. At one point in evolutionary time, all chelicerates lived in the water, like horseshoe crabs do today.

But, since all arachnids live on land, their ancestor must have undergone one of life’s monumental evolutionary events: leaving the sea once and for all. So horseshoe crabs must’ve split off from the chelicerate family tree beforehand, putting them in their own, very ancient, order, Xiphosura. But in 2019, a long overdue genetic study turned the case upside down.

Horseshoe crabs weren’t distant cousins of arachnids -- many parts of their DNA matched that of hooded tickspiders and camel spiders, which means rather than cousins, they are arachnids! So if horseshoe crabs didn’t diverge from arachnids before they moved onto land, that means either that arachnids evolved to live on land more than once, or that the ancestor of horseshoe crabs were on land once, and then re-evolved all the adaptations to live back in the ocean! We finally had horseshoe crabs figured out... for like 2 months.

And then another group of researchers published another genetic study that pulled an epic zoology Uno reverse card. They made some “corrections” and poof! horseshoe crabs were definitely back out of the arachnids, and never lived on land. So again, we finally had horseshoe crabs figured out… for like 4 months.

Then double reverse! Yep, another, bigger genetic study, that for the first time tossed in three truly obscure arachnids: microwhip scorpions, a rare order of mites, and short-tailed whip scorpions. Horseshoe crabs were aquatic arachnids once again!

And this back and forth goes on and on and on. No one really agrees what a horseshoe crab is or if it once lived on land, but plenty of zoologists are still working on the case. Being a scientist really does take some finely tuned sleuthing skills: you take all the evidence you can get and use it to unmask the truth.

In zoology, we make our best guesses about how something evolved or why it works the way it does. Solving these types of phylogenetic mysteries sometimes takes a lot of time, and new technology, and underscores the power of evolution. Next episode, we’ll talk about another common culprit in phylogenetic mysteries, unexpected gene sharing between different animals, and what that means for our understanding of how we define what a species even is.

Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE. It’s shot on the Team Sandoval Pierce stage and made with the help of all these nice people. If you’d like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.