You may have learned in biology class that every life form has a taxonomic designation, but that doesn't make it true. Many life forms, especially ancient ones, are so hard to categorize that scientists don't even know whether they are plant or animal. They call these organisms the "Problematica."

Dealing with Problematica is just a part of Carl Mehling's everyday life. He works with the paleontology collections at the American Museum of Natural History, and last week gave a group of museum visitors a taste of just how weird these things get. Standing in the museum's collection of mammoth and mastodon tusks, he pointed out that there are a few things we know for sure about these extinct megafauna. Mastodons have pointy teeth, while mammoths have teeth that look like big square blocks. Their tusks, also a form of tooth, came in a variety of lengths. So far, so good.

But these creatures had many relatives whose tusks defy explanation. Mehling pointed to a set of ancient tusks that looked like enormous shovels. They belong to an animal called an Amebelodon, which paleontologists believe may have looked like the creatures in the image above. But it's hard to know, because there is no modern analogue of this body part in any living creature. "We think they used these as scoops to get at food, but what do we know? We weren't there to observe their behavior." Grinning, Mehling added, "They could have been using them as lasers — all we have to go on are these teeth. The past is a black cloud and all we have are educated guesses."


Still, the Amebelodon actually has a taxonomic designation, and we know it was a mammal related to other megafauna like the woolly mammoth. Likewise, the 6-foot shrimp-like creature called an Anomalocaridid at the top of this post is a bizarre but categorized Cambrian creature. The Problematica are even weirder — "these are organisms that can't be placed in any group," Mehling said. Many of them are animals from very early in history, like the Cambrian and Ordovician periods, roughly 400-500 million years ago.

Take, for example, this example of Problematica. It's a tulip-like creature that lived during the Cambrian. The researchers who discovered it in Canada's Burgess Shale described it like this:

This stalked animal (reaching at least 20 cm in length), has a large ovoid calyx connected to a narrow bilayered stem and a small flattened or bulb-like holdfast. The calyx is enclosed by a flexible sheath with six small openings at the base, and a central terminal anus near the top encircled by indistinct openings. A prominent organ, represented by six radially symmetrical segments with comb-like elements, surrounds an internal body cavity with a large stomach, conical median gut and straight intestine. Siphusauctum gregarium was probably an active filter-feeder, with water passing through the calyx openings, capturing food particles with its comb-like elements. It often occurs in large assemblages on single bedding planes suggesting a gregarious lifestyle.

Gregarious yet sedentary, plantlike yet an animal — even the researchers who found it admitted that it sounded like an alien. The creature is related to no other known groups of animals.


Then there's the unidentifiable Gluteus minimus, a clam-like creature or possibly a fish tooth that is named for its distinctive butt-like shape. No, I am not kidding. Here it is:

And no I am also not kidding about the extreme lack of knowledge about what this is. It might be a tooth or a shelled animal.

Still, even when a creature isn't designated Problematica it can still be pretty mysterious. Graptolites are an animal that filled the seas during the Ordivician period roughly 470 million years ago. They were insect-like creatures with fluffy antennae who built vast hive-like structures out of what were probably bodily excretions — just like bees or wasps building a hive. Except these hives hovered just below the surface of the water, with the graptolites poking their little heads out to gobble up plankton. It's possible that the graptolites might be related to some modern-day worms — but they're nothing like worms that exist now.

And even today, there are animals like the California condor that defied categorization for many years. The Problematica are the most obvious examples of how life doesn't always fit into tidy categories — but there are many more such creatures. In fact there are so many that we needed to invent a category to define them as undefinable.

Annalee Newitz is the author of the book, Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.