Everyone knows that dinosaurs are extinct. As children, many of us gazed in awe at the fossils of these magnificent beasts. As adults, a lot of us still do!
Except that dinosaurs aren’t extinct, at least based on the most recent interpretations of the fossil record and analyses of DNA. The collective evidence points to a conclusion that once seemed improbable: birds are dinosaurs. I still recall the awe and wonder that beheld me when I learned this in college, and when I’ve taught it to children (all of whom are bonafide dinosaurs experts), I can see the same amazement on their faces.
So what evidence is there in the fossil record for this supposed ancestor – descendant relationship?
Prior to the Triassic, there were a lot of reptilian looking animals that no longer exist today. One such animal, Protorosaurus, has been found in rocks dating to about 260-251 million years ago (Ma). Typical of its contemporaries, it walked around on four limbs, each of which terminated in five fingers or five toes, possessed a long tail and teeth, and was almost certainly covered in scales. At this point in time, Protorosaurus and its fellow reptiles had seemingly little in common with today’s birds.
Not too much later, about 245 million years ago, animals like Asilisaurus appear in the fossil record. Though this species likely walked on all fours, it had shorter arms, suggestive of an increased ability to walk and/or stand on its hindlimbs.
In another nine million years, we see animals like Marasuchus (236-234 Ma): clearly reptilian in form, very dinosaur like, and notably bipedal, just like birds.
Eodromaeus and its kin are among the earliest true dinosaurs, popping up a mere five million years after Marasuchus (231.4-229 Ma). One of its typical dinosaurian traits is a hip socket with a hole in it (perforate acetabulum). What’s additionally notable about this species and some of its contemporaries is how its fingers have changed. Modern birds do not have fingers, but their wing bones terminate in what appears to be the remnants of three fingers. Starting this trend toward digit reduction, Eodromaeus has five fingers, but the ring and pinky are very reduced in size.
Fast forward about 30 million years, and we have more modern-looking dinosaurs on the scene. Coelophysis (203-196 Ma) is typical of the early carnivorous dinosaurs (theropods), and continues the march towards birdiness. The pinky finger is practically non-existent at this point, and the toes have also reduced in number. Whereas earlier dinosaurs and other reptiles have five toes, Coelophysis has only four, with a tiny remnant of the fifth high up on the foot. Notably, birds have four toes, three in the front and one in the back, so this trait had already appeared at least 200 million years ago.
Whereas Coelophysis had four fingers on its hands, Sinosaurus (201-196 Ma), appearing two million years later, only has three fingers, having lost the ring finger altogether.
While other bird-like traits accumulated over time, perhaps the most significant change is found in rocks that date to 50 million years after dinosaurs like Sinosaurus. Archaeopteryx (150.8-148.5 Ma), the first documented transitional fossil, has a striking mix of bird- and reptile-like traits. Perhaps most significantly is the appearance of feathers in this species.
There is reason to think that feathers appeared prior to Archaeopteryx, however. Soft tissues like feathers don’t typically preserve well as fossils, but over the last 20+ years, a number of exceptionally preserved specimens have demonstrated that plenty of non-flying dinosaurs had feathers. This suggests that these structures didn’t appear for the purpose of flight, but rather for a simpler function, such as thermoregulation. Just like hair keeps us and other mammals warm, feathers provide a similar insulating layer for birds.
Archaeopteryx clearly had wings, though, suggesting some ability to glide or perform flapping flight. It had another bird-like feature, known as the furcula. Also known as the wishbone, the bone frequently broken apart as a Thanksgiving dinner ritual in the United States, this bone formed by the fusion of the two clavicle bones present in earlier species like Coelophysis. This fusion is thought to be important in withstanding the stressors caused by flight, but it is also present in many other carnivorous dinosaurs, suggesting it initially appeared for a different reason.
Despite these innovations, Archaeopteryx is still not as bird-like as you’d think. For one, it still very clearly had three fingers, along with claws, on its hand. It also had a long bony tail, a structure reduced to a nub (pygostyle) in modern birds. Last but not least, Archaeopteryx had teeth, whereas birds have lost their teeth entirely in favor of a beak made of a protein called keratin.
After another 15 million years, other dinosaurs, like Sinornis (135 Ma), were just a few steps away from modern birds. The tail bones had finally become shortened and fused into a pygostyle in Sinornis, providing a structure tail-feather attachment. The sternum also became keeled increasingly keeled, allowing for the attachment of more powerful flight muscles, suggesting that Sinornis was a better flyer than Archaeopteryx. Despite these innovations, Sinornis still retained teeth and three distinct clawed fingers, traits that are not present in modern birds.
40 million years later, when tyrannosaurs and pachycephalosaurs were still roaming the earth, some extremely bird-like animals appear in the fossil record. At first glance, Ichthyornis (93-83.5 Ma) may be difficult to discern from a modern seabird. Many of the limb bones, including the fingers have become fused in Ichthyornis, resulting in a skeleton that was probably well-adapted for powered flight. Nonetheless, as one of the final holdouts of its reptilian past, Ichthyornis still had teeth. Interestingly, however, its jaw tip appears to be covered by an incipient beak, suggesting the transformation is nearly complete.
After most dinosaurs disappeared from the fossil record, coinciding with geological evidence for a disastrous meteor impact and intense volcanism, birds persisted and began to really thrive. Among the earliest species is Waimanu (60 Ma), which appeared just five million years after this mass extinction event. Not only does Waimanu have the appearance of a modern bird, paleontologists think it was the earliest known penguin, suggesting the surviving bird species have already begun to resemble some of their modern forms.
Over a period of about 200 million years, the fossil record appears to document the gradual appearance birds from reptilian forebears. With the advent of bipedalism, the reduction in fingers and toes, derivation of feathers and wings, reduction of the tail, and loss of teeth, fossils seem tell the story that birds are, in fact, dinosaurs. Do not hesitate to remember this the next time you feed a duck or eat a chicken wing!
Question for Creationists
Where did all of these fossil animals go? Could they not fit on Noah’s ark? Wouldn’t species with some capacity for flight, such as Sinornis, Archaeopteryx, and Yi qi, be able to avoid the Flood? Why does the fossil record appear to document a transition between reptilian animals and birds? If Noah’s Flood is responsible for the placement of these fossils, why did they appear in this particular sequence? Is it just a coincidence that the fossil record appears to document birds descending from ancestors with teeth and birds have remnants of tooth genes in their genomes?
Protorosaurus, chicken, Asilisaurus, Marasuchus, Eodromaeus, Eodromaeus hand, Coelophysis, Coelophysis hand, Sinosaurus, Archaeopteryx, Sinosauropteryx, Beipiaosaurus, Sinornithosaurus, Microraptor, Psittacosaurus, Epidexipteryx, Similicaudipteryx, Anchiornis, Changyuraptor, Yi qi, furcula, Sinornis, Waimanu