Transitional Fossils: How dinosaurs gave rise to birds

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.

asilisaurusx2
Asilisaurus

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.

Marasuchus_white_background
Marasuchus

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.

Sinosaurus_triassicus_white_background
Sinosaurus

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.

NGS Picture ID:422890
Archaeopteryx

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.

Furcula_evolution4

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.

sin-skeleton
Sinornis

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.

Ichthyornis_Clean
Ichthyornis

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.

i-f1cdf2d13781a91c89e9a9770eee8d25-waimanu
Waimanu

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?

Photo credit

Protorosaurus, chickenAsilisaurus, Marasuchus, Eodromaeus, Eodromaeus handCoelophysis, Coelophysis handSinosaurus, Archaeopteryx, Sinosauropteryx, Beipiaosaurus,  Sinornithosaurus, Microraptor, Psittacosaurus, Epidexipteryx, Similicaudipteryx, Anchiornis, Changyuraptor, Yi qi, furcula, Sinornis, Waimanu

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11 thoughts on “Transitional Fossils: How dinosaurs gave rise to birds

  1. TFBW

    Where did all of these fossil animals go? Could they not fit on Noah’s ark?

    What makes you think they weren’t? Do you suppose it’s impossible that they became extinct afterwards?

    Wouldn’t species with some capacity for flight, such as Sinornis, Archaeopteryx, and Yi qi, be able to avoid the Flood?

    The flood covered the entire planet for months. How good were these birds at surviving on open water? If they aren’t waterfowl, they’ll just drown if they land in water, right?

    On another subject, how goes the search for a way to make a car out of potatoes and ketchup? There are still a few days left on my $10 offer. Would you like a time extension?

  2. “What makes you think they weren’t? Do you suppose it’s impossible that they became extinct afterwards?”

    It is indeed possible that they went extinct afterwards, but this raises the question of where they went afterwards. If they all went extinct naturally or humans hunted them, why were the patterns of extinction non-random, e.g., why were the non-modern birds unfairly targeted? Also, why don’t we have recent bones or sub fossils of any of these species?

    “The flood covered the entire planet for months. How good were these birds at surviving on open water? If they aren’t waterfowl, they’ll just drown if they land in water, right?”

    Most probably weren’t but some, such as Ichthyornis and its Hesperornithes kin, appeared particularly well-adapted to an aquatic lifestyle.

    1. TFBW

      It is indeed possible that they went extinct afterwards, but …

      Your original question kind of implied that this wasn’t an option. Be careful with the loaded questions.

      Most probably weren’t but some …

      Meaning that the problem implied by your question was grossly exaggerated, and whatever subset might have been able to survive without the ark are then just a special case of “went extinct afterwards”, above.

      In general, though, the kind of questions you are asking here don’t make any sense as challenges to Creationists. If the flood is true, then the standard, modern interpretation of the fossil record is a huge mistake because it’s predicated on gradualism and the uniformity of history — i.e. the explicit denial that Noah’s flood happened. It makes no sense to even attempt to explain fossils as interpreted by the gradualist model if we are taking Noah’s flood as a given.

      1. “Your original question kind of implied that this wasn’t an option. Be careful with the loaded questions.”

        You really don’t believe when I say these questions aren’t meant to be rhetorical! If a person legitimately has an explanation for where these or other animals went, I want to hear their answer and discuss it with them! There is no implication that there are no possible alternatives: there may be dozens.

        You have to understand where I’m coming from: I am a former disbeliever in evolution, subsequently I became convinced of it, and now my career revolves around it. The more I read, the more research I do, the more I think about it, the more convinced I am that evolution is a real phenomenon. This does not mean that I’m correct, and it does not mean I’ve considered all options. If someone answers these questions and gets me thinking about things that I might have missed, who knows? Maybe I’d become a creationist.

        Regardless, for many people that disbelieve in evolution, they have not thought this through: where are these animals? Why are they extinct? What are the possible reasons and what are the most plausible of these?

        “Meaning that the problem implied by your question was grossly exaggerated, and whatever subset might have been able to survive without the ark are then just a special case of “went extinct afterwards”, above.”

        It wasn’t grossly exaggerated; it might just mean that it’s not a problem for some creationists.

        Either way, I don’t think it does it justice to say that it was just some subset of these organisms that went extinct: it’s entire groups of organisms that don’t have so much as a hint of having existed in recent history. That’s why I’ve devoted one of my six major themes to this topic:

        https://evolutionforskeptics.wordpress.com/category/where-did-they-go-2/

        1. TFBW

          And, as I said, this doesn’t make much sense as a question to creationists, because belief in the flood invalidates the entire standard interpretation of the fossil record, which is predicated on long-term uniformity. When you talk about extinctions, you say so based on an interpretation of the fossil record which simply can not be maintained if the flood actually happened. The models are fundamentally incompatible.

            1. TFBW

              “Where did all of these fossil animals go?” and the follow-up question “Also, why don’t we have recent bones or sub fossils of any of these species?” The questions assume things about the fossil record which aren’t true if Noah’s flood happened. Standard geology is based on long-term uniformity.

              1. Gotcha!

                Why don’t you think these are issues that need to be addressed if Noah’s flood happened? Entire groups of animals have disappeared: (1) some of these are terrestrial and, according to one Creationist interpretation, were on the ark. Why did these go extinct after they disembarked? If they weren’t on the ark, another model I’ve seen, why weren’t they selected?; (2) many of the extinct animals had the capacity to fly. Their abilities to disperse presumably should not have led to being hunted to extinction in a pre-firearm world; (3) tons and tons of animals were aquatic, but no longer exist; (4) entire groups of plants, which would be affected by the flood in different ways than animals, are also gone. Whether you believe in creation or evolution, the fossil record demonstrates they existed and demands an explanation.

                The question about recent bones and sub fossils raises the question of why, if these organisms survived the flood (which presumably many of them did), do we only find their remnants in fossil form, i.e., with the replacement of the original organic structures with minerals, etc. We never find any of the extinct species in any form other than as fossils, but if they existed post-flood, within the last 4000 or so years, we should find some actual bones and other typical organic matter.

                1. TFBW

                  Why did these go extinct after they disembarked?

                  Tons of stuff has gone extinct for one reason or another. Things are still going extinct. Ongoing extinction is an observable fact — much more so than any form of new life evolving (extrapolate that trend). Why should I need to supply a special explanation for a commonplace thing like extinction? Even if we assume that absolutely everything survived the flood, there’s still nothing special to explain. The standard evolutionary story is a vast history of extinction after extinction (with the occasional surprise “Lazarus taxon” which isn’t as long-extinct as the fossils suggest), and there’s no special need to explain the extinctions because dying is easy.

                  Seriously, what’s to explain? Things die for a great many reasons.

                  The question about recent bones and sub fossils raises the question …

                  If you’re going to raise questions about fossils, are you also prepared to accept something other than the long-age uniformitarian interpretation of the geological record? If you aren’t, then there’s no point in asking the question. If you are, then the basis for posing the question in the first place is likely to be rejected. That’s the point I was trying to make.

                  1. “Tons of stuff has gone extinct for one reason or another. Things are still going extinct. Ongoing extinction is an observable fact — much more so than any form of new life evolving (extrapolate that trend).”

                    I suppose it depends on how you’re defining “new life”, but I’m not sure that extinction is more observable than evolution. They’re probably pretty comparable. But involve field work sampling and resampling.

                    “Why should I need to supply a special explanation for a commonplace thing like extinction? Even if we assume that absolutely everything survived the flood, there’s still nothing special to explain. The standard evolutionary story is a vast history of extinction after extinction (with the occasional surprise “Lazarus taxon” which isn’t as long-extinct as the fossils suggest), and there’s no special need to explain the extinctions because dying is easy.”

                    The standard evolutionary explanation isn’t quite as simple as that. Yes, some extinctions are thought to be in essence random, but not for entire groups of organisms. Some examples includes (1) competition from individuals with adaptive innovations (i.e, novel features that make them better at eating a particular item than their relatives), (2) immigration of new predators or competitors that evolved in completely different biogeographic areas, (3) organisms with restricted geographic distributions wiped out by local catastrophes, (4) organisms with specialist niches not being able to handle change in climate, environment, etc. due to lack of lability, (5) some niches allowing for survival of catastrophes better than others (e.g., living underground is more thermally stable than living aboveground), (6) differences in dispersal ability in the case of catastrophes or climate change.

                    So back to the creationist model: what’s important is that it’s not as simple as species going extinct randomly. It’s the idea that entire groups of organisms (groups defined by shared characteristics) in the fossil record no longer exist. If you look at almost anything in any of the rocks that project to ages prior to, say, 20 million years ago (Miocene rocks), 99.999% of those organisms do not exist anymore (note*: I know the radio decay dates aren’t accepted by creationists, but the point is those rocks are projected to be those ages based on known decay rates and the composition of various isotopes, so it’s an observable characteristic of the rocks).

                    While some frogs are going extinct, and some beetles are going extinct, and some bats are going extinct, and a rhino is about to go extinct, and we can essentially observe these things happening, we don’t see ALL frogs, or ALL beetles, or ALL bats or ALL rhinos going extinct. By contrast, all trilobites, and all dinosaurs, and all triconodont mammals, and all birds with teeth, and all brontotheres, and all ediacaran animals, etc. etc. etc. are completely gone. It would be front page news if any of those were found, or if any of those were found with very recent looking bones, but the pure volume of extinct groups of species is something that I do not think can be ignored without consequence.

                    “If you’re going to raise questions about fossils, are you also prepared to accept something other than the long-age uniformitarian interpretation of the geological record? If you aren’t, then there’s no point in asking the question. If you are, then the basis for posing the question in the first place is likely to be rejected. That’s the point I was trying to make.”

                    Hmm, I’m not understanding what this has to do with finding bones that look fresh versus bones that have been replaced with minerals such that they are now rocks. Maybe you weren’t trying to respond to this though.

  3. Pingback: Molecular Phylogenetics: DNA points to reptilian ancestry of birds – Evolution For Skeptics

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