The tiny skull of a hummingbird-size dinosaur has been found trapped in amber, raising important questions about the evolution of birds and the surprisingly early trend toward miniaturization.
The Mesozoic is famous for producing gigantic animals, but as new research published today in Nature reveals, this era also featured animals of an astonishingly small size.
A 14-millimeter-long skull found trapped in amber represents a new species of bird-like dinosaur, which its discoverers named Oculudentavis khaungraae. The beaked animal lived 99 million years ago during the Cretaceous period of Myanmar, and it’s now considered the smallest dinosaur in the fossil record.
“It’s the weirdest fossil I’ve ever been lucky enough to study,” said Jingmai O’Connor, the lead author of the study and a paleontologist from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, in a press release. “I just love how natural selection ends up producing such bizarre forms.”
Over the years, paleontologists have collected amber fossils containing all sorts of organisms and biological material, including plants, feathers, ticks, flies, beetles, frogs, mollusks, and even bits of the occasional bird. Amber fossils are exceptionally valuable in that they preserve details of extinct organisms in ways that other fossils cannot. This is “particularly the case for tiny animals that lived in trees,” said Luis Chiappe, a co-author of the study and a researcher at the Natural History Museum of Los Angeles County, in a press release.
Using high-resolution synchrotron CT scanning, the researchers were able to study the fossil in exquisite detail, without having to crack it open. The skull measures just 14.25 millimeters in length, which is comparable in size to the skull of a bee hummingbird, the smallest modern bird living today.
“The discovery suggests that miniature body sizes in birds evolved earlier than previously recognized, and might provide insights into the evolutionary process of miniaturization,” wrote Roger Benson, a paleobiologist from the University of Oxford who wasn’t involved with the research, in a Nature News & Views article. “In this case, weighing perhaps 2 grams, Oculudentavis is about one-sixth of the size of the smallest known early fossil bird. This indicates that, only shortly after their origins late in the Jurassic period (which lasted from about 201 million to 145 million years ago), birds had already attained their minimum body sizes.”
Unlike modern hummingbirds, who use their beaks to sip nectar from flowers, Oculudentavis had a beak adorned with dozens of sharp teeth. In fact, with an estimated 29 or 30 teeth, Oculudentavis has more teeth than any other bird found in the fossil record. And yes, unlike birds living today, many birds from the Mesozoic had teeth.
To house all these teeth, Oculudentavis had an exceptionally long tooth row that extended all the way back to just under its large eye. So distinctive is this feature that the researchers chose to name it Oculudentavis, which means “eye-tooth” in Latin. The authors speculate that this Cretaceous bird used its many teeth to munch on various insects.
“This diet differs considerably from the nectar-based diet of the smallest living birds, and suggests that extinct and living birds took different paths to miniaturization,” wrote Benson, adding that it’s not clear how an animal’s diet might be involved in this evolutionary process.
Oculudentavis also featured an exceptionally large eye socket, one comparable to lizards. Its eyes would’ve bulged out sideways from its tiny head as it searched for food and strayed away from potential predators. The small aperture in its eye bone allowed for a limited amount of incoming light, which suggests it was active during the day.
That said, the researchers aren’t entirely sure how its visual system actually worked, as it’s unlike anything seen in living birds. Its eyebones are similar to those in owls, a nocturnal animal whose eyes face forward, but Oculudentavis had a gaze that looked sideways and was likely active during the day.
As to why certain birds were prone to miniaturization, the authors can only speculate, but as is so often the case in evolution, it’s the environment that matters most.
“Animals that become very small have to deal with specific problems, like how to fit all sensory organs into a very small head, or how to maintain body heat,” explained O’Connor in the press release. “This process—called miniaturization—commonly occurs in isolated environments, most famously islands. It is no wonder that the 99 million year old Burmese amber is thought to have come from an ancient island arc [in northern Myanmar].”
O’Connor said miniaturization is typically associated with such things as the loss of teeth and unusually large eyes, but “since Oculudentavis has more teeth than usual, it shows that evolution doesn’t always follow the rules,” she said.
Of course, Oculudentavis might not even be a bird—a possibility proposed by the authors—but its large eye socket, combined with its pointed beak, are features only found together in birds. So Oculudentavis was very likely a bird, also known as an avian dinosaur. As an aside, all birds alive today—whether ostrich or hummingbird or penguin or parrot—are avian dinosaurs, and don’t let anyone tell you otherwise.
As to Oculudentavis’s place within the evolutionary family tree, the authors slotted it in between Archaeopteryx and Jeholornis, two primitive bird species that lived between 155 million and 120 million years ago in China. That’s a bold decision, given that the Oculudentavis fossil was dated to 99 million years ago. Other paleontologists may take exception to this, demanding that more fossil evidence be collected from the many millions of years that separate Oculudentavis from these ancient bird species.
On a related note, the authors failed to include Fukuipteryx prima—the second-most primitive flying dinosaur in the fossil record—in their phylogenetic analysis. This creature lived 120 million years ago, but it likely emerged long before that, causing scientists to bump Jeholornis down a notch on the evolutionary family tree, making it the third-most primitive bird known in the fossil record. The inclusion of Oculudentavis now complicates this even further, with the authors claiming the bird as among the most primitive known to science.
As always, more fossil evidence would help clear up these debates. We just have to keep on digging and hope that, every once in a while, we find something as extraordinary as this most recent amber fossil.
Correction: Due to an inaccurate figure in press materials from Nature, a previous version of this article stated that the skull is 7.1 millimeters long; it is actually 14.25 millimeters long.