Science

We Finally Know What a Dinosaur’s Butthole Looks Like

What a new fossil tells us about dinosaurs, paleontology, and us.

An illustration of Psittacosaurus with its posterior covered by a black box that says CENSORED
Photo illustration by Slate. Photo by Robert Nicholls/Wikipedia.

For the entirety of my career as a journalist covering paleontology, I’ve been wanting to know: What does a dinosaur’s butthole look like? When I wrote My Beloved Brontosaurus, a book about dinosaur biology, the chapter on reproduction required a lot of time imagining the nature of a Jurassic behind; one had yet to be found preserved. Even dinosaur models and sculptures often demur on the point of the dino butt, leaving the terrible lizards with terrible constipation.

Now I finally have a clearer view, thanks to a fossil of a horned dinosaur called Psittacosaurus, described in a paper online earlier this month. These dinosaurs, which lived over 100 million years ago in what’s now northeastern China, were odd little creatures. While belonging to the same branch of the dinosaur family tree as Triceratops, these Labrador retriever–size dinos walked around on two legs and had beaks like those of parrots, cheeks that were each adorned with a flared horn, and, jutting from the tail, a spray of featherlike bristles. Now we also know that they had buttholes like those of crocodiles.

It’s rare to get a look at something soft and fleshy on a dinosaur. We know most of what we know about Psittacosaurus the same way that we know things about most dinosaurs: from their bones. Durable skeletal parts are much more likely than skin and organs to survive the fossilization process, which involves burial and at least partial replacement of the original tissues. Most of the time, after a dinosaur dies, all the soft stuff just decays. But every now and then paleontologists find dinosaur “mummies” that preserve remnants of the soft bits either as impressions or geologically modified pieces of the original flesh. There’s no one way to make an exquisitely preserved dinosaur: Sometimes it happens when a dinosaur is quickly buried in ash; others dry out in the open for a while. For whatever reason, experts have uncovered several Psittacosaurus with preserved soft tissues. The fossilization in some of these specimens is so refined that we even know what colors these dinosaurs were, brown on top and lighter along the belly. The new fossil is one of the more detailed ones. It includes patches of skin and scales as well as the ornamental bristles on the tail. The most remarkable part is a patch of tissue between the hips and the base of the tail—aka a butt.

The actual description of the butthole, which appears in a paper that hasn’t yet been peer-reviewed, makes me have sympathy for a dinosaur that probably didn’t expect to have its posterior formally presented in the technical literature over 100 million years after its death. On the fossil, just below the tail, the butthole appears as a “blackish mottled ovoid area,” the paleontologists write (the image is on Page 4 of the PDF, found at this link). To the naked eye, the spot looks like a series of dark, stacked bands running between the base of the tail and hip bones, clearly different from the skin around it.

What those bands mean, the paper tells us, is that Psittacosaurus had a downstairs setup known as a cloaca. This is pretty different from our own mammalian plumbing. While we all know the old joke about “a playground next to a sewage system” in reference to the locations of our own sex organs and waste exits, we’ve got nothing to complain about compared with animals that have a cloaca. This orifice—its very name meaning “sewer”—is the only opening for the reproductive, urinary, and excretory systems. Something to keep in mind if books like Taken by the T-Rex are your jam.

By itself, evidence that non-avian dinosaurs had cloacae isn’t a huge shocker. While it’s possible that future peer review might interpret the fossil differently, the placement, color, and wrinkly texture of the tissue all seem to line up with what experts have long expected a dino butt to look like. Living dinosaurs—birds!—have cloacae, after all. Bird cloacae are round or square and normally covered up by feathers, unless you’ve ever seen an ostrich evacuate and wondered what the hell you’re looking at. Crocodiles and alligators, which are the closest living cousins of dinosaurs, have cloacae that are horizontal slits. According to a form of logic called extant phylogenetic bracketing, the fact that both birds and crocs have cloacae hints that the trait goes back to the last common ancestor of both groups—a creature called an archosaur that likely would have resembled a crocodile crossed with a greyhound—and would be present in the extinct animals that fall within those goalposts. A dinosaur butt, we’ve long known, should look something like a bird butt or a croc butt.

An educated guess is great, but firm evidence is better. Having a fossilized dinosaur cloaca actually available for study is like finally getting the pony you were always begging for on your birthday. The entire end goal of paleontology is to envision organisms as they were in their heyday. That involves drawing from what we know about modern animals and comparing that to the information we have so far on extinct dinosaurs and dinosaur relatives, generating cartloads of hypotheses about what a particular feature on a dinosaur might have looked like, and then testing them against what we are lucky enough to find preserved in the ground. This new fossil tells us that not only were we probably right about dinosaurs having cloacae, but that this species had cloacae that resembled those of crocodiles in particular. Though the drainpipes stashed behind the cloaca itself were not preserved—the penis in this setup, just so you can picture it, extends out of the butthole during mating—this fossil helps us imagine that dinosaurs might have had croclike genitalia, too. In crocodiles the female’s clitoris is so large and pronounced that the pink, tapering organs can easily be confused for the male’s penis.

Finding genitalia would go a long way toward filling in some dinosaur mysteries. Paleontologists are still perplexed by how dinosaurs mated with those big, thick tails in the way. Did they have to contort into difficult positions to align their butts, or did males have the necessary equipment to bridge the gap? Likewise, knowing more about the other tubes could make sense of the various splats dinosaurs left behind. Paleontologists have reported urolites, or pee fossils, from a few places, as well as hundreds upon hundreds of coprolites, aka poop fossils. One such fossil—a bone-filled pat attributed to T. rex—seems to have moved through the digestive system quickly. Having a fossilized excretory tract to study might help explain if a morsel’s rapid trip from mouth to cloaca was normal, or if the tyrant king was dealing with some dino-diarrhea. I’m having fun here, but even just a little more information about these organs could help experts better understand the physiology of creatures that lived a long, long time ago.

This is the reason I’ve been sharing Figure 1 of the paper, where the cloaca appears, with everyone I know: not just because of the information it presents, or the fact that it’s cheeky. The dinosaur butthole is a testament to how wonderful the sheer luck of fossilization can be, and how there’s so much more detail about dinosaur bodies that could be out there. For all the CGI renderings, toys, and even erotic imaginings of dinosaurs, so much of their basic biology—from body temperature to differences between dinosaur sexes and even what sounds they made—remains unknown. Every year, there seems to be another example of a one-of-a-kind fossil that gives us that much more information about dinosaur lives. Even familiar species can surprise, whether we’re talking about a fleshy little beanie on the head of Edmontosaurus, signs of parasitic infection in Tyrannosaurus, or, now, a Psittacosaurus booty. Surely we’re nowhere near the tail end of dinosaur discoveries.