Transitional fossils: Are there any, really?

Perhaps the most damning criticism of Charles Darwin’s theory of evolution by natural selection was based on fossils. If organisms evolve such that their descendants adopt radically different forms than their ancestors, why don’t we see evidence of this in the fossil record? As if on cue, two years after Darwin outlined his theory in On the Origin of Species, scientists uncovered Archaeopteryx, a beautifully preserved animal seeming to have mixture of reptilian and bird-like characteristics. And so the first ‘missing link’, or transitional fossil, was discovered.

Archaeopteryx

150 years later, skeptics of evolutionary theory will frequently say that Archaeopteryx does not actually qualify as a transitional fossil [1], that there are practically no other putative transitional fossils to speak of [2], fossils are always found fully formed with no realistic progenitors [3], and the fossil record is complete, so we should not expect to find any other alleged missing links [2].

I think that the first point is the most important and is really worth considering: what constitutes a convincing transitional fossil? The first quality such a specimen must absolutely possess is the presence of characteristics intermediate between two other fossil forms. Archaeopteryx most certainly qualifies on this point. For example, birds have feathers and wings, whereas modern reptiles do not, and modern reptiles have tails, teeth and claws, whereas modern birds do not. Archaeopteryx has all of these traits, proving that it is transitional in form.

But having an intermediate form does not mean that Archaeopteryx descended from reptiles and gave rise to birds. Take for instance, the platypus. The platypus has hair and produces milk for its young, just like other mammals, but it lays eggs like a lizard. While biologists think that this provides evidence that mammals descended from reptile-like ancestors, nobody would argue that our modern day platypuses gave rise to the mammals that don’t lay eggs.

This is where the next piece of critical information comes in: the dimension of time. It isn’t enough to look like a medley of organisms. Rather there has to be a logical chronological sequence to their appearances. Geologists use a variety of information, including the clock-like radioactive decay of atoms, to estimate the ages of rocks. If Archaeopteryx appears in rocks that are more recent than those of the earliest reptiles, but also pre-dates the appearance of the earliest birds, then it constitutes a specimen consistent with it being a transitional fossil. (spoiler alert: it does)

And this is a testable, refutable hypothesis. If tomorrow scientists provide insurmountable evidence that modern bird fossils predated or lived alongside Archaeopteryx,  we can no longer say that Archaeopteryx meets the transitional fossil criteria.

But why aren’t there more fossils like Archaeopteryx? There are, but certainly not the millions and millions that must have existed if evolutionary theory is correct. So why haven’t we found them?

The simple answer is bias. There is a whole field of research (taphonomy) that seeks to understand when, where and how fossils are formed, and to be blunt, we think we’re missing a lot of information. The general idea is that you have to die in the right place, at the right time, with the right conditions in order to preserve information to last millions of years. The fossils that researchers tend to find are almost always extremely fragmentary and limited. For instance, fossil teeth are extremely abundant. Fossil brains and skin? Not so much. Whole organisms? We wish!

Then there’s a more human problem: paleontologists tend to focus on the best fossil-bearing rocks that they’ve discovered so far, yet even those are limited. Not only do the fossils have to actually land in the right place and the right time, but the earth has to shift and erode in such a way to expose the fossils. How many fossils might be under your feet right now, but you’ll never know because you’re never going to dig under your house? How many fossils are buried in the sea? Under antarctic ice? What are we missing out on that we could find only if we could CT scan the entire earth?

Taphonomic bias

So the fact that there are only so many fossils we can find, and we only find them at a certain rate, and we usually only get to look at parts of bones and not the squishy parts of the bodies that presumably also were evolving, how much information are we missing out on here?

Which brings us to the third common point: if the fossil record is complete, how could we ever find any other transitional fossils? I’m not sure who first conveyed this idea, but boy were they wrong. As just one simple example, consider the fossil record of Mesozoic mammals, i.e. mammals that are found in rocks that date between 251 and 66 million years ago. Between 1830 and 1979, researchers found 116 general kinds of mammals that date to this era. Between 1979 and 2007, that number climbed to ~310 [4]. Far from being complete, it appears that paleontologists are just getting started!

You can bet that as paleontologists keep digging, researchers will uncover more putative transitional fossils, many of which I document here in this blog. Keep an eye out for them, and ask yourself: do these fossils meet the criteria for being transitional? And if so, what does this mean for Creationism?

References

1. https://www.icr.org/article/321/

2. http://www.icr.org/article/should-we-expect-find-transitional-forms-fossil-re/

3. http://www.icr.org/fossils-stasis

4. Luo, Z. X. (2007). Transformation and diversification in early mammal evolution. Nature450(7172), 1011.

 

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4 thoughts on “Transitional fossils: Are there any, really?

  1. I think it’s a bit misleading to present platypuses as appearing to meet the criteria of transitional forms by saying:
    “The platypus has hair and produces milk for its young like mammals, has a beak like a duck, produces venom like a snake, and lays eggs like a lizard. Does this make the platypus a transitional animal that connects reptiles and birds to mammals?”
    The “has a beak like a duck” comment is particularly simplistic because while a platypus snout superfical resembles a beak, it’s actually nothing of the sort, and it’s would never be mistaken for a bird beak by any anatomist or taxonomist.

    I would also contest your definition of “transitional form” slightly. I would tend to define it along the lines of “a body form that is transitional”, rather than “a body form that is transitional and is good candidate for being ancestral to the derived group”. For example, Tiktaalik is a great transitional form, despite the fact that it’s unlikely to be a direct ancestor of extant tetrapods given that we have indications of fully tetrapod footprints being made several million years *prior* to the dates assigned to Tiktaalik specimens. This sort of variation in ages of members of a stem group is to be expected due to the incompleteness of the fossil record – the dates will rarely line up perfectly along with a morphological transition, but they will approximately.

    1. Hi evograd, thanks for your comment!

      “I think it’s a bit misleading to present platypuses as appearing to meet the criteria of transitional forms by saying:
      “The platypus has hair and produces milk for its young like mammals, has a beak like a duck, produces venom like a snake, and lays eggs like a lizard. Does this make the platypus a transitional animal that connects reptiles and birds to mammals?”
      The “has a beak like a duck” comment is particularly simplistic because while a platypus snout superfical resembles a beak, it’s actually nothing of the sort, and it’s would never be mistaken for a bird beak by any anatomist or taxonomist.”

      Essentially in that passage, I’m trying to relate to how many creationists criticize the concept of transitional forms. Essentially, they often look at putative transitional fossils as simply being organisms that combine multiple traits, going off of the idea that the Creator could have simply swapped out multiple ‘parts’ to design whatever It wanted. The platypus is just a very salient example of an such an organism (even though we think many of those characters I listed are not homologous).

      “I would also contest your definition of “transitional form” slightly. I would tend to define it along the lines of “a body form that is transitional”, rather than “a body form that is transitional and is good candidate for being ancestral to the derived group”. For example, Tiktaalik is a great transitional form, despite the fact that it’s unlikely to be a direct ancestor of extant tetrapods given that we have indications of fully tetrapod footprints being made several million years *prior* to the dates assigned to Tiktaalik specimens. This sort of variation in ages of members of a stem group is to be expected due to the incompleteness of the fossil record – the dates will rarely line up perfectly along with a morphological transition, but they will approximately.”

      Absolutely. I don’t fully agree with the definition I’m using. But when discussing the issue with creationists, I think it’s most convincing to have fossils that are (a) transitional in form and (b) predate the next step in the series of fossils. Platypuses have “a body form that is transitional” given that they have a mix of derived mammalian traits and multiple ancestral amniote characters, but I don’t think this sort of example is as useful for discussion (at least how I’m approaching it on my blog).

      1. Thanks for the reply, and sorry it took me a month to get around to responding.

        I understand the broader point you were making with the platypus, leading into the discussion of the importance of timing, but I still can’t help but think that it could easily be misinterpreted. If I were to rephrase it, I would simply add in the caveat of “superficially” in a few places, and perhaps include an additional sentence or postscript about how these traits (e.g. the platypus “beak”) don’t *really* resemble these other organisms.

        I definitely agree that it’s “more convincing” to creationists to have transitional forms in a neat chronological order, but I would just say something to this effect rather than make it out as though the extremely narrow definition of “transitional fossil” you presented here is the “meaningful” definition. I think it’s important to try and drive home the point that evolution isn’t “ladder-like” in this neat sense – stem groups are often messy but that doesn’t take away from the evolutionary transitions they document. Coming back to the Tiktaalik example, I’ve lost count how many times creationists have tried to argue to me that it has been “debunked” as a transitional fossil in the light of recent data, and I’m getting really tired of trying to explain what a stem group is and how it works to them.

        1. “Thanks for the reply, and sorry it took me a month to get around to responding.”

          I figure if you’re an evograd, you probably don’t have a lot of time to respond!

          “I understand the broader point you were making with the platypus, leading into the discussion of the importance of timing, but I still can’t help but think that it could easily be misinterpreted. If I were to rephrase it, I would simply add in the caveat of “superficially” in a few places, and perhaps include an additional sentence or postscript about how these traits (e.g. the platypus “beak”) don’t *really* resemble these other organisms.”

          That’s a good point. I’ll add a qualifying phrase in there to just minimize confusion for others.

          “I definitely agree that it’s “more convincing” to creationists to have transitional forms in a neat chronological order, but I would just say something to this effect rather than make it out as though the extremely narrow definition of “transitional fossil” you presented here is the “meaningful” definition. I think it’s important to try and drive home the point that evolution isn’t “ladder-like” in this neat sense – stem groups are often messy but that doesn’t take away from the evolutionary transitions they document.”

          I should have added that these aren’t just more convincing to creationists but more convincing period. If all of our “transitional” fossils were contemporaneous with the more ‘modern’ forms, then I wouldn’t buy the transitional fossil argument for a second. Stem fossils can be very useful once you assume evolution is true, to get an idea of the polarity of characters, understands rates of diversification, etc. But a stem fossil that is 200 million years removed from its presumed ancestor is little different then calling a platypus a transitional form (not my silly bird, lizard example, but as a transitional form between early amniotes and therian mammals).

          So what you’re getting at is ultimately the frustration in explaining how species having a mosaic of primitive and derived traits are precisely predicted by evolutionary theory, but they’re not easily understandable as ‘transitional’ to someone with no expertise. This is why I try to generally avoid these sorts of fossils: if they’re hard to explain to a nonspecialist or a non-upper division biology student (or hell, even a typical upper division bio student!), then they aren’t going to be useful in talking to laypeople who are simply passing by to glance through my blog. At least that’s my take on it.

          In regards to fossils like Tiktaalik, even if they aren’t exactly older then the next ingroup, they are pretty darn close in age. So I think it’s still appropriate to reference given that the fossil record documents a bunch of ‘fishes’ for millions of years and then suddenly you get fishy looking things that have some capacity for walking, and then you get not-so-fishy things that are committed to walking. Tiktaalik is just one of those forms that has a mosaic of fishy and tetrapod traits, and it’s certainly not contemporaneous with squirrels and iguanas! Who cares if it’s a few million years younger than the successive ingroup? That’s also a problem with the unpredictability of the fossil record: today you find a fossil dated to 400 million years old, then tomorrow you find it (or relative) in rocks that are 5 million years older.

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