Skull of the smallest known dinosaur found in 99 million year old amber

By Lars Schmitz, Jingmai Kathleen O’Connor

In 2016, our colleague Xing Lida held up a small piece of polished, deep yellow amber. When sunlight fell through the old resin, Lida saw the outline of an immaculately preserved, amazingly small skull. There was a distinctive eye socket, a dome-shaped crown of the head, a long, tapered snout, and even small teeth. It was bird-like, but in a strange and ancient way.

The amber contains the skull of Oculudentavis khaungraae, a newly described dinosaur and one of the smallest ever discovered. Its tiny stature forces paleontologists to rethink the lower limits of body height in birds, and the nearly 100 million year old fossil questions the current understanding of when and how dinosaur giants shrank into today’s birds.

A mysterious transformation

Tiny Oculudentavis may have occupied a unique ecological niche in ancient times. Han Zhixin / CC BY-ND

The evolutionary transition from dinosaurs to modern birds is one of the most amazing changes in the history of life: large, bipedal and mostly carnivorous dinosaurs turned into small, flying birds. Famous discoveries like Archeopteryx and more recently, the Jehol Biota fossils in China have given researchers some evidence of the process. Finds from this evolutionary phase, which researchers believe started around 200 million years ago, are rare.

Paleontologists are far from having a complete picture of the evolution of birds and even further from a complete inventory of Earth’s ecosystems in the dinosaur age. Our research on the tiny Oculudentavis, published in Nature magazine, adds valuable information to the mystery of when, how and to what extent dinosaurs shrank.

Notes in the bone

With this high-resolution scan we were able to see the subtleties of a bone structure that have never been seen before in birds or dinosaurs. Xing Lida / CC BY-ND

Our team had to see the tiny details of the skull, and we had to do it without cracking or ruining the specimen – a difficult task with a skull encased in 99 million-year-old Myanmar amber. To do this, we scanned the skull with high-resolution X-rays and created a digital model with very fine anatomical details. What emerged was an image of a general bird-like anatomy. But in an interesting way Oculudentavis is different from any bird or dinosaur ever found.

The fossil’s obvious curiosity is its size: Oculudentavis competed with the smallest bird alive today, the bee hummingbird, and was probably no more than 4 centimeters from beak to tail. We considered whether the skull might belong to a very young animal, but the extent and pattern of bone growth and the proportional size of the eye indicated a mature bird.

With a total skull length of only 1.5 centimeters Oculudentavis presses against the lower limit of size in birds: the head had to keep functioning eyes, a brain and jaw. The small size is particularly surprising when you take that into account Oculudentavis lived at the same time as giant herbivorous dinosaurs like Argentinosaurus.

Small and specialized

The small size of Oculudentavis is striking, but there are other extremely unusual features for a trained eye.

At first the skull seems to be designed for strength. The bones show an unusual fusion pattern and the skull lacks an antorbital window, a small hole that is often found in front of the eye.

The eyes of Oculudentavis also surprised us. The shape of the bones found in the eye, the scleral ossicles, suggests that it probably had conical eyes with small pupils. This type of eye structure is particularly suitable for movement in bright light. While daytime activities can be expected for an old bird from the dinosaur era, the shape of the ossicles is completely different from all other dinosaurs and resembles that of modern lizards.

In addition to the list of unexpected features, the upper jaw has at least 23 small teeth. These teeth extend below the eye and are not in deep pockets, an unusual arrangement for most old birds. The large number of teeth and their sharp cutting edges suggest this Oculudentavis was a predator that may have fed on small beetles.

The sum of these features – a strong skull, good eyesight and a hunter’s teeth – suggests this to us Oculudentavis led a life previously unknown to old birds: it was a hummingbird-sized predator.

One of the earliest and smallest birds?

Place Oculudentavis in the tree of life is a challenge given its strange anatomy. Our phylogenetic analysis – examining their relationships with other dinosaurs – identified Oculudentavis as one of the oldest birds. Just Archeopteryx branched earlier.

Scientists consider the nectar-eating hummingbirds that emerged 30 million years ago to be the smallest dinosaurs ever registered. But if our placement of Oculudentavis It is true that miniaturization of dinosaurs may have peaked much earlier than paleontologists previously thought. In fact, the largest and smallest dinosaurs may have walked and flown on the same earth almost 100 million years ago.

Our work shows how little scientists know about the little things in the history of life. The scientists’ snapshot of fossil ecosystems in the dinosaur age is incomplete and leaves so many questions unanswered. However, paleontologists strive to answer these questions. What other tiny species were there out there? What was its ecological function? Was Oculudentavis the only visually guided bug hunter? In order to better understand the development of the diversity of life, we need more emphasis and recognition of the little ones.

Amber has great potential to close this gap. Maybe one day a scientist will hold up another piece and let the sunshine reveal a complete one Oculudentavisor even a previously unknown species. Further finds in amber will help illuminate the world of tiny vertebrates in the age of dinosaurs.

Reissued with permission from The Conversation.

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