Owner Comments:
In 2016, the U.S. Mint issued this amazing gold Standing Liberty Quarter, along with two other coins, to commemorate the 100th Anniversary of the designs of the coins. What drew me to this coin was that its reverse featured an image over 100 million years old, that of a theropod dinosaur in powerful flight.
The ability to fly is one of the most amazing traits that life on Earth has ever evolved. Over the entire history of the Earth, only four groups of animals are known that evolved members who had or have the ability to truly fly. These animals are insects, the extinct pterosaurs, theropod dinosaurs and bats.
However, with three of the four groups, we have absolutely no idea how the ability to fly evolved and what path it took. Flying insects, pterosaurs and bats all simply appear in the fossil record with their flight equipment and ability intact and fully evolved. This is because the fossil records for these groups are highly incomplete, especially regarding the transitional forms leading to the ability to fly. Theropod dinosaurs are different. Thankfully, they have left us much of the story of their path to the domination of the skies preserved in the rocks of the Earth. This coin is perfect to discuss the three dominant theories behind the evolution of flight in theropod dinosaurs (birds are not the only group of theropods to evolve the ability to fly) and a few of the interesting forms of flighted theropods that are different from today’s surviving birds.
There are three main models proposed for the evolution of flight in theropods. They are called the “Pouncing Proavis” model, the Cursorial model and the Arboreal model. The “Pouncing Proavis” model theorizes that flight evolved from wing flapping that feathered, non-flying theropods would use to stabilize themselves when performing a prey-riding technique similar to that used by the modern Golden Eagle and that this flapping eventually, over time, evolved into true flight. This theory has much to recommend it. It allows for a slow build and many different forms of flying dinosaurs, which matches the fossil record. It also accounts for an animal like Archaeopteryx, a theropod dinosaur from the Late Jurassic period which was skeletally quite similar to terrestrial theropods but whose feathers were moving in a direction to allow for the evolution of flight. Archaeopteryx is considered by many to be one of the first birds, though it is not a true avian.
The Cursorial model, also known as the Running model or the Ground-up model, proposes that flight among theropods evolved from running leaping behavior which slowly evolved into flight. This theory is actually the least likely to be correct and is a poor match for the fossil record but it cannot be completely discounted due to a behavior known as Wing Assisted Incline Running (WAIR) which is exhibited by modern birds. WAIR is used by many precocial species of birds from the day they hatch to help them run up slopes and inclines. In this behavior, birds use their wings to make themselves more aerodynamic when moving up inclines. As many of the birds that use this behavior when young eventually learn how to fly, this one behavior allows for the Cursorial model to remain as a valid option.
The last model is the Arboreal model. This model states that flight among theropod dinosaurs has its origin in small arboreal theropods who glided from tree to tree and this behavior eventually led to true flight. As with the “Pouncing Proavis” model, there are examples from the fossil record which strengthen this theory. One of these is Microraptor, a small dromaeosaur who had feathered wings on all four limbs. It is uncertain if Microraptor could truly fly, but those four wings certainly allowed it to be a proficient glider. Some interpretations of Archaeopteryx find it to be a glider and not a true flier as well. Yet another four-winged theropod, Anchiornis, was thought to solely be a glider for years but a recent analysis found that juveniles of the species would have been able to generate enough lift to achieve true flight.
And then there are the bizarre Scansoriopterygids, which were availans from the Jurassic Period found in what is now China. These dinosaurs took a completely different route to gliding and flying than any other group of theropods. Instead of feathered limbs that formed wings, Scansoriopterygids had wings made of membranes of skin and enlarged fingers much like those found in bats and are a compelling example of convergent evolution, where two unrelated groups evolve similar features due to occupying a similar ecological niche. This kind of wing would be almost impossible to evolve from terrestrial prey-riding and flapping behavior. Though the Scansoriopterygids would prove to be an evolutionary dead-end, they did find their way to the skies in a novel way.
So what do I think is the correct theory? Actually, I think both the Arboreal model and the Pouncing Proavis model are correct. Flight evolved among several different theropod groups at different times. Many different Maniraptoran families had flying members; the Euornithes and Enantiornithes are just two of the avialan families confirmed to have members who flew and its strongly suspected that there were fliers among the Dromaeosaurs as well (Rahonavis and Microraptor are the main contenders here) and the aforementioned Anchiornis is suspected to be a Troodontid, though different researchers are debating this placement currently. And, of course, Avians, who are part of the Euornithe family, fly as well. I strongly believe that some theropods gained fight through prey-riding and others gained it through gliding. To me, this is the only way that the Scansoriopterygids could have evolved alongside the many other Avialans who flew. Also, in considering the incompleteness of the fossil record (it is estimated that only 1% of organisms ever fossilize and we only find 1% of those fossils), it is entirely possible that the Cursorial model is correct for some species of flying dinosaurs who either never left a fossil behind or have left fossils that we haven't found. WAIR among existing species makes this a very real possibility.
However it happened, the evolution of flight in theropod dinosaurs is amazing. Flight is the most difficult behavior that animals undertake, and the fact that theropods were largely able to take features that they already had and utilize them in different ways to take over the skies is absolutely fascinating. It speaks volumes to their intelligence and adaptability. And the next time you look at a Standing Liberty Quarter, please take the time to turn it over and look at an amazing rendition of an image which has graced the skies for over 100 million years.