evolution in action: ceratopsians and the paleontological evidence for evolution

Nearly everybody familiar with evolutionary theory and anti-evolution movements is familiar with the old refrain claiming that “the fossil record does not support evolution.”  However, such claims are extremely misguided, and stem from a major misunderstanding of what the fossil record does, in fact, show.  While one could pick any one of hundreds of fossil lineages to examine, we’ll look at one that most people are familiar with, the ceratopsian dinosaurs. To get a partial idea of the diversity of this group, I’ll post some pictures before continuing.

chasmosaur skull on display at AMNH

triceratops on display at AMNH

Protoceratops display at AMNH

While I’ve only shown a few specimens here, the ceratopsian lineage itself is far more diverse.  However, for the sake of ease and sanity, we’ll only look at a few key ceratopsians for our comparison.  For some basic paleontological background, ceratopsians are a Cretaceous group, thus existing towards the end of the reign of the dinosaurs.  The group as a whole was relatively successful, with a distribution throughout Western North America, Asia, and possibly Australia and South America as well.  They were herbivores, and tended to live in large groups.  In phylogenetic terms, the ceratopsians shifted from a relatively lightly built, at least partly bipedal, basal form such as Psittacosaurus, through slightly more robust forms such as Protoceratops, and ultimately towards larger forms such as Triceratops. Now I don’t want to speak in terms of “progress” or “orthogenesis” here; I’m not trying to imply that there was something inevitable about how these forms developed. Rather, it’s just the way it happened. Selective pressures pushed towards that direction, and natural selection responded by building larger, more robust forms.  Let’s take a quick look at the basic sequence:

psittacosaurus on display at AMNH, public domain pic from wikipedia

another psittacosaurus picture, this time on display in Copenhagen. Also from wikipedia.

Protoceratops on display at Carnegie Museum of Natural History, from Wikipedia

Triceratops at AMNH, from Wikipedia

With these specimens, you can get a pretty good idea of how body plans shifted during the phylogenetic history of the Ceratopsians.  However, don’t get the wrong idea here; it isn’t necessarily so that Psittacosaurus evolved into Protoceratops evolved into Triceratops. This isn’t how the fossil record works.  Rather, an organism like Psittacosaurus evolved into an organism like Protoceratops, and so on.  These specific examples show us a basic picture of the transition that occurred. They aren’t the full story.  We’re looking at over a hundred million years between us and them.  We don’t have the full picture.  Look at today’s biodiversity, and compare it to what’s available in the fossil record.  The fossil record itself is like trying to understand a person’s life by looking at a photo album, most of the pictures missing, with a picture of an individual, we’ll call him Bob, as a toddler, then as a highschooler going to the prom, then maybe as a middle-aged man with another younger man (perhaps his son?), then a funeral announcement.  We can get a basic idea of how his life unfolded, but there are many, many things that we can’t answer about his life.  This is the way the fossil record works. In order to understand how evolution operates over time, one has to look at the basic patterns visible in the fossil record. Sure, you can’t get a complete, full, exact picture of every species-to-species transition, but you can get a pretty good idea of what actually did happen in an evolutionary sense.

Another thing to note about evolution, something which is often overlooked, is the fact that evolution can, in fact, transform entire bodies as a whole. The transmutation in body plan visible in our ceratopsian lineage is a complete one, with changes in cranial anatomy (just look back at some of the ceratopsian skulls shown to see it), body size, stance, and (as we’ll look at in a minute), sacral (a fancy word for the region of the backbone that runs through the pelvis) anatomy.  What you’re looking at is not natural selection working on just one trait at a time, but rather many traits, all interacting with each other, all being tweaked slightly through the differential reproductive success of different individuals, not as distinct parts, but as a whole.  People far too often think of natural selection as acting on just one trait at a time.  Sure, it can happen like that in rare cases, but a more full understanding of evolution implies that it is the body as a whole which is acted upon by natural selection.  Yes, individual traits play key roles in reproductive success. But it is the entire body, not merely one trait, which is the agent of reproduction.  Sure, you might have a reproductive edge because you have bigger horns than your opponent.  But you ultimately get to mate because you’re a complete organism, not just one sexy part.  Now that I’ve gotten your attention by talking about reproduction, let’s look at some hips:

psittacosaurus sacrum at AMNH

If you’re having trouble finding the sacrum in this picture, look to the right of the “gastroliths” arrow, right between the little guy’s (not sure if it’s male or female, but I digress…) hind legs. It’s the bone structure shaped more or less like this: )I(

Now onto the next specimen, Protoceratops:

Protoceratops sacrum, AMNH

Look in the same place on this one, right between the hind legs. I apologize about the crappy images with the last two pictures. I took both of them a few years ago, without this purpose in mind.  But I have them, so might as well use them as examples.  Now let’s check out a Triceratops sacrum:

Triceratops sacrum, AMNH

With these three sacrums, you can see how evolution has shaped one specific body part over time. However, after looking previously at the full bodies of these dinosaurs, its much easier to view this for what it is, natural selection tinkering away at one body part as it shapes the whole body.  Regardless, this series of sacrums at least helps to illustrate the relationship between these organisms.  But keep in mind, once again, that it is the entire body that evolves, not just one part.  What paleontologists look for in the fossil record is not “transitional forms”, but “transitional features”, such as our sacrum example, when trying to define evolutionary lineages (phylogenies).  This happens precisely because superficial traits (size, weight, etc) are relatively fluid.  In order to fully understand an evolutionary lineage, it is necessary to look at specific traits which are carried throughout a sequence.  While our sacrum example is not a perfect one, we can at least roughly view how one might find such a trait.  Yes, you can see it change through the lineage, but it is also possible to build a relatively complete picture of how the sacrum has changed throughout its evolution.  Therefore, it could be used to help understand phylogenetic relationships between ceratopsians. Thus, while natural selection does operate on entire bodies, specific traits are also extremely important in defining exact evolutionary sequences.  Both angles are necessary in order to fully define an evolutionary sequence.

Anyway, let me shut up before I make this post any longer/more painful to read.  So what exactly does our quick look at ceratopsians do?  Besides being a (relatively dull) way to kill a few minutes, it also provides an often overlooked example of an evolutionary lineage.  So read up on your ceratopsian evolution (I’ve tried to stay away from the boring, terminological stuff for the sake of a quick introductory glance; Donald Prothero’s Evolution: What the Fossils Say and Why it Matters has a really good discussion of ceratopsian evolution, and would make an extremely good starting point for looking at evolution in the fossil record in general), and be ready next time someone tries to explain away the basic examples of paleontological evidence for evolution like Tiktaalik, Ambulocetus, or Archaeopteryx.  Provide an example of a full lineage such as the ceratopsians, rather than one “transitional form” in a vacuum.  It’ll provide a much stronger proof if you’re ever trying to explain evolution to someone with an open mind that doesn’t know much about the subject.

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6 Comments on “evolution in action: ceratopsians and the paleontological evidence for evolution”

  1. jg Says:

    I haven’t read the full thing, but I will shortly. First I need to point you have a classic spellchecker error. Please start at the paragraph that begins with

    “With these specimens, you can get a pretty good idea of how body plans shifted…”

    and look carefully at you spelling of “organism” throughout.

    jg

  2. darwinaia Says:

    Hey, thanks a lot for catching that. Don’t know how that happened, but I guess that definitely shows the importance of carefully checking your typing, especially late at night!

    How have you been?

    • john g Says:

      Things have been good for me, thanks for asking. I’ve had an opportunity to refine and present two shows this summer, one on exoplanets and another of orbital connections to climate.

      Thanks for making the correction to your text. I can now give the article the serious attention it deserves.

      It’s good to see you back from your break.

      jg

      • darwinaia Says:

        That’s awesome with regards to your presentations! Congrats. The exoplanet stuff always fascinates me. No worries about the corrections. Thanks again for catching the issue; that’s the one bad thing about doing stuff late at night while I’m fighting to stay awake I guess. I’m glad to be back on here myself. Just had a hectic summer of work. Glad its over to be honest.


  3. […] While I”ve only shown a few specimens here, the ceratopsian lineage itself is far more diverse. However, for the sake of ease and sanity, we”ll only look at a few key ceratopsians for our comparison. For some basic paleontological … visit. […]


  4. […] While I”ve only shown a few specimens here, the ceratopsian lineage itself is far more diverse. However, for the sake of ease and sanity, we”ll only look at a few key ceratopsians for our comparison. For some basic paleontological … visit. […]


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