A tiny skull of a wee predator that lived 99 million years ago is in all likelihood that of the smallest species of dinosaur found. Picture a hummingbird. With fangs.
But what remarkable remains they are: Minus the snout, the skull measures about seven millimeters long (about a quarter of an inch). This dino head could rest, with area to spare, on the cap of a Triple-A battery.
“It’s smaller than the skulls that we find in hummingbirds,” said study author Lars Schmitz, a paleobiologist at the Keck Science Center in California. Birds are living dinosaurs, of which bee hummingbirds are the tiniest. But, extrapolating its body size from its skull, this newly discovered dinosaur could compete in size with the bee hummingbird.
“If it’s indeed a dinosaur, it’s definitely the smallest known extinct dinosaur,” said Ohio University paleontologist Lawrence Witmer, an expert in dinosaur heads who was not involved with the research. Oculudentavis “rivals the smallest avian dinosaurs — birds — known today,” he said.
Its large eye, domed skull and tapered, slender snout are characteristics of dinosaurs — and, more specifically, ancient birds. The researchers’ analysis placed the animal “fairly deep in the origin of birds,” Schmitz said, but with a “lot of uncertainty.” Without a skeleton to study, the scientists do not know whether the dinosaur could fly.
The fossil has a strange mixture of lizard- and birdlike traits. “It’s so tiny that it must be miniaturized, and evolutionary miniaturization can mess with the anatomy,” Witmer said. He added: “It would be super-helpful to know what kind of body was attached to that weird skull.”
In the past, when describing fossils, paleontologists have mistaken young animals for unusually small species. (A recent reexamination of leg fossils, once proposed to belong to a mini tyrannosaur species named Nanotyrannus, tilts the evidence heavily in favor of a different explanation: They are the legs of teenage T. rexes.) But Oculudentavis’s bone structure makes a convincing case for maturity.
“I accept that it’s an adult and not a young animal,” Witmer said, “which makes it all the more intriguing.”
The skull’s bony plates have stitched together, indicating this dinosaur was an adult, or nearly so. “We can look at the suture, the lines between different bone elements, and how well-fused they are,” Schmitz said. “It really looks like it’s an almost fully grown animal.”
Schmitz first met Oculudentavis as a digital scan of the dinosaur’s skull. He was astounded. “I was like, holy moly, this is really interesting,” he said. The fossil has an incredible amount of detail in three dimensions, said Schmitz, an eye expert, who was taken with its “very beautifully preserved eye.”
“We definitely can say that it’s a visually oriented animal,” Schmitz said. The size of the skull’s pupil hole suggests the dinosaur hunted during the day.
Unlike hummingbirds, which eat nectar, Oculudentavis was out for flesh. Each jaw sprouted about 30 sharp teeth per side. The dinosaur probably ate insects and other invertebrates, Schmitz said.
The skull was preserved when a gummy substance called resin, oozing from a tree, smothered the head. Over time, the resin hardened into amber, which was dug up in Myanmar.
“Amber has huge potential to preserve very small life,” Schmitz said. This dinosaur head joins an amber-trapped menagerie: a prehistoric spider frozen while attacking a wasp; very old frogs; even the nib of a feathered dinosaur tail. (The “Jurassic Park”-like scenario of extracting millions-of-years-old DNA from bloodsuckers in amber remains improbable.)
“It blows my mind,” said ReBecca Hunt-Foster, a park paleontologist at Dinosaur National Monument in Utah who was not a member of the research team. Miniature bones such as this are “so delicate,” she said, and would “not have a chance” to survive the petrifying process — which occurs when organic matter turns to stone under layers of sediment — that creates large dinosaur fossils.
Paleontologists expect small dinosaurs would have lived alongside the massive animals that stomped across prehistoric landscapes. Oculudentavis helps flesh out the diversity of dinosaur life, Hunt-Foster said.
Although it was but little, perhaps it was fierce. What small animals lack in mass, as Hunt-Foster pointed out, they often make up in aggression. Hummingbirds, for instance, will bully other birds away from flowers and feeders.
“A little hummingbird-like critter with teeth!” Hunt-Foster said. “Can you imagine a flock of these guys?”