It’s one of the most memorable scenes in the original Jurassic Park movie: the dinosaur Dilophosaurus spreads the frill around its neck and sprays deadly venom from its jaws. The frill (inspired by Australia’s frilled lizard) is pure Hollywood fantasy. But paleontologists did formerly speculate that Dilophosaurus spat venom, or at least had a toxic bite. Sam Welles, who described the Arizonan dinosaur in 1984, identified a structure in the jaw as a potential venom gland, and suggested that its jaws were too fragile to dispatch prey with brute force alone.
Today, more complete fossil evidence has rewritten these early assumptions. Scientists now believe that Dilophosaurus’s jaws were much stronger than previously thought, and what was thought to be a venom gland was just a misidentified part of the jawbone. So Dilophosaurus is no longer believed to have been venomous; but what about other dinosaurs?
In 2009, a small, feathered carnivore from Cretaceous China called Sinornithosaurus was also speculated to be venomous. Researchers noted grooves in Sinornithosaurus’s teeth that might have been channels for the flow of toxins. This claim initially generated media attention, but subsequent studies have called it into question. Today, most paleontologists do not believe there is sufficient evidence that Sinornithosaurus was venomous either. While some paleontologists think there could still be venomous dinosaurs out there, we only have evidence for venom in just a handful of prehistoric reptiles that lack the defining anatomical features of dinosaurs.
The difference between venom and poison
The terms “venomous” and “poisonous” are often confused with each other, but they actually refer to different ways that animals transmit organic poisons (toxins). Poisonous animals, such as poison dart frogs, passively deliver toxins when touched or bitten. Venomous animals have to actively sting or bite to deliver toxins, either to defend themselves, like bees, or to kill or immobilize prey, like spiders. While poisonous animals may store toxins throughout their bodies, venomous animals usually have specialized organs for producing and injecting venom.
How modern biology informs paleontology
When looking for evidence of venom production in prehistoric reptiles, paleontologists generally look for telltale venom-producing structures such as grooves or tubes in the teeth. “We have to use what we know in the modern world to inform what we can observe from the fossil record,” says Helen Burch, a PhD candidate in paleobiology at Virginia Tech University.
However, some modern venomous reptiles like the komodo dragon lack the clearly visible tube structures seen in snakes. Furthermore, “a lot of the reptiles that we see today have their venom glands positioned subdermally, or just below the skin,” says Burch, rather than in a depression in the bone. This means that “if we were looking for a venomous dinosaur, the structures that we would look for might not even be showing up in the bone,” she adds. So while there is no conclusively known venomous dinosaur, there’s a possibility that evidence simply wouldn’t show up clearly in the fossil record.
Burch explains that a prehistoric reptile called Uatchitodon, which lived in North America in the Late Triassic about 220 million years ago, “has a very definitive venom structure that looks pretty much exactly like what we see in modern snakes,” says Burch. “We have an opening at the base of the tooth and an enclosed tube, and then an opening at the tip of the tooth.” Without any remains of Uatchitodon beyond these remarkable teeth, we can’t say precisely where it fits on the reptilian family tree, so we can’t call it a venomous dinosaur.
However, it’s notable that “Uatchitodon had serrated teeth,” says Burch, and “at the time when we find Uatchitodon, the only animals that have serrated teeth are archosauromorphs,” a very large and diverse grouping that includes the dinosaurs, as well as pterosaurs, crocodilians, and more. So despite the venom-bearing structures in its teeth, Uatchitodon was likely more closely related to dinosaurs than to modern lizards and snakes, even though Uatchitodon was not a dinosaur.
Venomous prehistoric reptiles aren’t the same as venomous dinosaurs
Despite their incredible variety, all dinosaurs share certain skeletal characteristics. The presence or absence of these can help to diagnose a fossil as being from a dinosaur or not. For example, the legs of dinosaurs were positioned directly under their bodies, giving them an upright stance. In most other groups of reptiles, the legs sprawl out to the sides and the body may lie flat against the ground.
Burch was involved in the discovery of Microzemiotes sonselaensis, another of the few known reptiles with venom-producing features from the Mesozoic era, when the dinosaurs thrived. Microzemiotes‘ relation to other reptiles is unclear from existing remains.
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However, we know where to place another early venomous reptile, Sphenovipera: Rather than being a dinosaur, it belonged to a group whose only living representative is the tuatara, a lizardlike creature that lives on rocky beaches in New Zealand. The examples of these two species and Uatchitodon show us that venomous reptiles did live at the same time as the earliest dinosaurs, and that some fell within the same clade, or genetic grouping, as the dinosaurs themselves. But this doesn’t mean that there were actually venomous dinosaurs as Jurassic Park would have you believe.
Venom has evolved many times, in different ways
Burch points out that, while all modern venom-bearing reptiles are grouped together in the clade Toxicofera, “these animals that we’re seeing in the fossil record, that are reptiles that are hypothesized to be venomous, don’t fall inside this clade. So that is where it gets more interesting.”
The presence of venom structures across so many different groups suggests that venom has evolved multiple times in reptiles, just as it also evolved repeatedly in fish, mammals, and many other animals. Toxins are a useful evolutionary tool with many applications.
Burch points out that “we see venom used in super different ways” across different species, as well as a large variety of different toxins, including some meant to cause pain to an attacking predator, and others meant to immobilize prey long enough for it to be swallowed whole.
So, what’s the verdict on poisonous dinosaurs?
When it comes to the possibility of a venomous dinosaur, there’s not enough evidence to say for sure. “It is notable that we don’t have any modern birds which are venomous, which are the surviving lineage of dinosaurs,” says Burch. However, we do see dinosaurs—in this case, living birds—that store toxins as defensive poisons if we turn again to the modern world.
The jungles of New Guinea are home to several species of pitohui, the world’s only known poisonous birds. Pitohuis store built-up toxins from the insects they eat in every part of their bodies, even their bones and feathers, making them toxic enough to irritate the skin of humans who handle them. It’s certainly possible that some prehistoric dinosaurs did the same. In fact, genetic studies of pitohuis have shown that their toxic defense evolved multiple times independently, rather than once in a common ancestor. Perhaps it also evolved at least once in prehistoric dinosaurs.
While distinct structures like grooves in the teeth can point to an animal’s being venomous, no skeletal features can tell us if a creature was poisonous without organic material to examine. This means it would be impossible to know if an animal was poisonous “in the deep fossil record, when we lose all organic material,” says Burch. “I mean, we had frogs in the Triassic; we could have had poisonous frogs. But there’s simply no way to know, right?”
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