Mar 12 2020

Smallest Dinosaur Ever

Published by under Evolution
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We like extremes, partly because they help define the borders of reality. It helps our mental model of the world when the know the biggest, smallest, hottest, lightest, or fastest of something. It’s also just fascinating to see how extreme some things can get. For this reason there has been a fascination with which dinosaur is the biggest – how big did these animals get. The record is currently held by Argentinasaurus, a long-necked sauropod, weighing between 77 and 110 tons. Meanwhile, the record for the smallest known dinosaur is microraptor, a bird-like dinosaur only 40 cm long. Well, that is until the latest discovery.

Scientists report the discovery of the head of a bird-like dinosaur trapped in amber. The species has been named Oculudentavis khaungraae and is about the size of a bee hummingbird, the smallest living bird. The specimen is trapped in amber from Myanmar, and is dated at 99 million years old. The amber preserved some soft tissue, including its tongue. The specimen is interesting on multiple levels.

First, it reflects the extreme of vertibrate miniaturization. It’s difficult to cram all the sensory organs into a tiny skull, and species that evolve to become so small have to find solutions. In this case the eye socket anatomy appears different than hummingbirds and other tiny birds. Rather than a rim of bone, the socket is more spoon-shaped. The anatomy also suggests a small opening for light, which further implies the species was diurnal.

The mouth sports a surprising number of teeth, making the creature look like a predator. At that size is likely fed on insects. So we have a bird-like dinosaur the size of a tiny hummingbird that hunted insects during the day. The anatomy, such as fusion of the skull, also suggests this was an adult, so not just a juvenile specimen to explain its small size.

There are a couple other thins to point out here. One is that the fossil record is massively biased toward large creatures. Larger creatures are more likely to fossilize, survive for millions of years, and be found. Since our only view into the age of dinosaurs is through fossils, our view is therefore biased toward the huge. We think of dinosaurs as large creatures, but in reality they came in all sizes. This is still an active question for paleontologists who try to estimate the range of sizes and the average size of animals living in these ancient ecosystems. They have to also estimate how the fossil record is biased by size.

Amber represents a unique opportunity to see very small creatures. Oculudentavis would likely never have fossilized by normal means, so being preserved in amber may be the only way we get to know this species.

I also like to point out that Oculudentavis represents another branch on the bush of bird evolution. This species was not on the main line to birds, and it retains some relatively primitive features. But it further shows that the feathered and bird-like dinosaurs (the paravans) was a successful and diverse group. They were evolving in many different directions and filling all sorts of niches. This makes sense from the perspective that this branch of theropods hit upon an adaptation, feathers, that have many uses, including flight. They were also relatively small, which gave them a lot of room in which to evolve.

It’s important to reinforce this pattern of evolution, because it runs counter to the iconic and incorrect schematic of evolutionary history that has unfortunately been drummed into the public for over a century. We need to purge the image of linear evolution (like the iconic ape standing up into a human). Theropod dinosaurs were not evolving into birds. They were experiencing adaptive radiation in many directions. One branch of that complex bush survived as modern birds, but that was not the foreordained conclusion. The same is true of hominids and humans, or the horse family and modern horses.

Related to this concept is that fact that “primitive” is always a relative term. I like to point out that bacteria are just as evolved as humans. They are not more “primitive” – they are the end result of their own 4 billion year evolutionary history. So when we look at Oculudentavis and say it has “primitive” features, we mean relative to birds. This is arbitrary, and only useful because birds happened to survive. We use concepts like primitive, derived, analogous and homologous in order to help fit individual species into the puzzle of evolutionary relationships.

Further, because evolution is not linear, more recent species can retain features that are relatively primitive compared to other evolutionary lines. For example, a side branch of bird-like dinosaurs may have retained a feature, like teeth, that is primitive when considering the branch leading to birds. It is primitive – to birds – but not to other evolutionary branches. In fact, the term “side branch” is also relative – it’s just as much a main evolutionary branch as the one that lead to birds. This sometimes causes confusion. How can a “primitive” feature be seen in a species that is later than more “modern” species? The confusion completely results from an incorrect linear model of evolution.

Finally I love pointing out that a hundred years ago the evolution of birds was a major gap in our model of evolution. This was a gap that creationists loved pointing to – a major group without any obvious connection to any other major group. Evolutionary theory (specifically that of common descent) required that this gap would have to be filled in by something. It did not require anything specific, but if evolution is true then we should find specimens that are part bird and part something else. This prediction has been fulfilled, in spades. We now have many specimens spanning the gap from feathered theropod dinosaurs, to a host of bird-like dinosaur type things, to creatures clearly on the path to becoming birds. We have half wings, and feathers not ready for flight, and birds with teeth. None of this had to exist if evolution were not true. Something like this had to exist if evolution were true.

This story has also repeated many times. We are filling in the gaps between humans and apes, whales and their terrestrial ancestors – turtles, fish, and many other groups. I do have to wonder if creationists get the slightest bit uncomfortable when such obvious transitional specimens are discovered, but then I remember that the human capacity for self-deception knows no practical limits.

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