Posted on 05 April 2023, by Kevin Mora
Which dinosaur, hypothetically, would be most likely to be still living today if a non-avian dinosaur were discovered?
The first thing we can say is that it wouldn't be a species that was present 66 million years ago because by that time it would have evolved or experienced some sort of genetic drift. Just take a look at the variety of birds that have existed over the past 66 million years. That being said, I could imagine another dinosaur lineage surviving under the proper conditions – one that was small, had a lot of offspring, and was able to hide.
And you know what? This is half true. Bird diversity is primarily a result of selective pressures from their environments as they occupied every available niche following the extinction of the pterosaurs. They also went back to land and in some cases reoccupied the carnivorous theropod niche. A comparable explosion of diversity was seen in mammals. All of this is very speculative, but I'd bet that if we gave any other adaptable lineage of non-avian dinosaurs the same ecological chance, they'd take it and run with it in a variety of different directions.
Because they fill a niche that hasn't altered significantly for millions of years, crocodiles haven't changed much over the years. But if you go back to the Triassic, their lifestyles were extremely varied because they had a lot more opportunities to fill specific niches. But at the end, everything comes down to the environment and the selective preassure of genes. We may even think that crocodiles are the closest to a non-avian dinosaur that is still alive, but crocodiles are endothermic ornithodirans from the opposing branch of the archosaur family tree than endothermic pseudosuchians. Take a look at what Triassic crocodilians and their close relatives did: they proved that given the ecological opportunity, they can change... a lot.
A small part of me also hopes dromaeosaurs survived – I'd be interested in learning how they fared. Theoretically, a lineage could have survived in a far-off region of the earth because they were of a size that allowed them to survive, and undoubtedly there is a chance that one of them could've reached T-Rex proportions, but we will never know.
To try to solve this question, first we have to take into account that it was much hot back then. As a result, the northern hemisphere can essentially be ruled out completely. Only South America, Africa, Australia, and a portion of Asia and the Middle East remain. They must reside in relatively unexplored areas. We would need to determine why they aren't included in the fossil record, but this is relatively simple given that bones in tropical rainforests decompose very rapidly, making it nearly impossible for such animals to have been preserved in fossilized form.
Microraptor is out of the question because we'd also need something with a pretty distinct niche that couldn't be filled by birds – it’d likely have to be quite a bit larger than that to compete. So we're looking at some sort of small to medium sized, forest dwelling dinosaur in the Southern Hemisphere. Therefore, only Archaeornithomimus remains as a viable option.
Assuming this is still not good enough of an assumption, we must make a little brake and state the following: creatures that were closely related to birds were probably the only species able to escape the mass extinctions. So, let's keep that in mind and move on to our next hypothetical scenario.
It could only have been otherwise something small, omnivorous, likely burrowing, and ideally able to hibernate in the Maastrichtian era, as the great filter of the Cretaceous–Paleogene (K–Pg) extinction event demonstrates. The animal's odds would also be higher the further away from the asteroid it was, so I'd bet on Antarctica, from which it might have migrated to Australia or South America as the glaciation destroyed its native habitat.
A small ornithopod, perhaps an elasmarian, has the greatest chances in my opinion. The ideal candidate might be a parankylosaur with an especially slow metabolism, a very small troodontid or buitreraptorine dromaeosaur, or both. This unexpected dinosaur must have remained small, unremarkable, and generalist throughout its existence in order to have lasted to the Holocene. It would most likely have lived in difficult to traverse rainforest terrain. If you didn't get a good look and know what to watch for, you might mistake it for another group of living animals.
For most people, it would be disappointing to discover the last non-avian dinosaur still alive and learn that it looks like a rabbit-sized horned toad with upright limbs and a turtle beak, or a downy pheasant with arms and fangs. Finally, a dinosaur group that separated from birds over 100 million years ago could be included in our phylogenetic brackets. Instead of forming assumptions based on bones, we could examine soft tissues, proteins, and genomes.
We would be able to answer questions that have been open for over 150 years just by looking at this creature and its behaviors. Guess we should send someone out to look for this creature?