The reptile-mammal transition

May 27, 2015 • 1:07 pm

by Greg Mayer

Update: An alert reader, has objected to the theory presented below, or at least the specific evidence used; he has proffered what he contends is “much more pertinent evidence”, which I append below.

Jerry posted a couple of days ago on a specimen of an early tetrapod, Ossinodus, which seems to have had a partially healed injury to the radius of its right forearm. The authors who described the injured specimen interpreted the injury as a fracture that could only have occurred on land, arguing that Ossinodus therefore is the oldest tetrapod that can confidently be said to be terrestrial. (The first tetrapods, from the upper Devonian, are considerably older than Ossinodus, which is from the following Mississippian subperiod of the Carboniferous; but these earlier tetrapods, which had caudal fins and functional gills, may not have been terrestrial.) Ossinodus is thus potentially an important point in the transition from fishes to amphibians.

Another major transition in the history of vertebrate life was that from reptiles to mammals, which we have discussed here before at WEIT. As important as the morphological changes which can be seen in the fossils, are the changes in ecology and behavior, which, along with environmental changes, lead to changes in the extent to which one group or another dominates the ecosystems of its time. Although mammals originated in the mid-Mesozoic era, it was not until the Cenozoic (colloquially known as the “Age of Mammals”) that the mammals became the dominant terrestrial vertebrates. Most ideas on the rise of mammals to ecological dominance focus on the fate of dinosaurs and other large reptiles at the end of the Cretaceous, when the disappearance of the latter may have been caused or accelerated by the impact of an extraterrestrial body. I was recently forwarded another theory, visually expressed, about how it was that mammals replaced reptiles as the dominant land animals on Earth.
A mammal and reptile engage in a vigorous contest for dominance.

And now, the more pertinent evidence:

Leo battles velociraptor.
Leo battles velociraptor.

I will allow as the mammal in the new evidence does seem to have a more dominant position over the reptile.

h/t: C. Mayer, J.B. Losos

31 thoughts on “The reptile-mammal transition

    1. So I’m not the only one who noticed the stabbing with the upper canines martial arts move that the cat used.

    1. They could be part-time terrestrial. Mud skippers, walking catfish, etc. It’s not completely out of the question that some of the fossils are axolotls.

    2. I believe Tik had both lungs and gills. So the evolutionary process probably went something like this:
      Things with gills develop adaptations to move short distances on land.
      Some of those things develop adaptations allowing greater use of air, eventually resulting in lungs.
      Some of those begin more permanently inhabiting the land.
      The gills in these creatures no longer provide a competitive advantage and are thus lost to deleterious mutations.

      1. Yes, that seems about right, but since lungs are primitive for lobefinned fish (and rayfins, too, for that matter)– as you note, Tik probably had them– your third sentence should be more like the “lungs got better” or “lungs came to be more relied upon”. Gills were of course lost in reptiles and their descendants (birds, mammals), but modern amphibians generally retain them in some life stage, and some rather reptile-like extinct amphibians (e.g. Seymouria) still had larvae with gills.

        1. Greg: when I saw Jerry’s Ossinodus post, my first thought was “Why couldn’t a big fish have grabbed and broke the hapless creature’s leg?” Predators often grab and violently shake prey to subdue them, causing all kinds of damage. And who knows how old/big the individual was when the damage happened. The evidence given for terrestriality in Ossinodus is not at all convincing.

          1. Like you, I’m not convinced that a fracture could only have occurred on land; hence, I noted it was the authors’ interpretation. The absence of a caudal fin in Ossinodus might be better evidence.


      2. Lungs evolved early on in fish history, before the common ancestor of the lobe- and ray-finned fishes. Yes, some of the sardine ancestors had functional lungs.

        1. Were you there?
          Just kidding. Of course we can see indications of these things in early fish from fossils that preserve outlines of soft tissues.

  1. “it was not until the Cenozoic (colloquially known as the “Age of Mammals”) that the mammals became the dominant terrestrial vertebrates.”

    How so? Words like “dominance” are often loaded terms, and have a dubious history in biology, especially taxonomy.

    Among the amniotes, there are about twice as many “reptile” species as mammals,

    and, similarly, there are about twice as many species of birds (which of course nest wholly within the diapsid “reptiles”) as mammals.

    And until humans came along, I think it is safe to say that no North American mammal came close to the ecological abundance of the passenger pigeon.

    I suppose dominance = large size – but the most speciose order of mammals are rather small rodents.

  2. Awww…

    That is an Abyssinian kitty in the bottom photo.

    I love Abys, even if they are idiots. (My best friend is an Aby)

  3. The synapsid “reptiles” (current taxonomy restricts th reptile moniker to a branch of the diapsids) were the dominant land animals of the Permian period, squeeking through into the Mesozoic and by Triassic the Mammaliaformes and early mammals were in play, but by then the increasingly less temperate climate appears to have played a role in the rise of the dinosaurs (initially a minor player on the diapsid scene) where their extra-snappy jaws likely contributed to their rise, along with possibly their bipedal stance and grasping forarms (all the earliest dinosaurs had that layout until some specialized as quadrupeds). Mammals meanwhile appear to have cornered the nocturnal niche.

    I make a point of highlighting in my #TIP project ( the reptile-mammal transition as the big ticket slam-dunk example of “kind to kind” macroevolution when creationists demand evidence for same, both because the evidence for it is so clear (fossils, included predicted ones, developmental biology in the critical jaw layout, and more recently the genetics) but because antievolutionists do a particularly miserable job of reacting to it (as I note regarding all the current examples of YEC & ID redactors who veer onto the subject) and so have no uncountered apologetic legs to stand on.

  4. Since we are on the subject of using using stuffed animals to prove hypotheses about Great Moments in Evolution, I thought the WEITers would be interested in learning that stuffed animal biodiversity is rising. This is excellent news since we keep hearing about how meat animals are facing a new mass extinction.

  5. May I stress that what we see in the photographic evidence is not really a Velociraptor, but a Jurassic Park style “Velociraptor” which more closely resembles a Utahraptor or other large Dromaeosaurs, and with feathers removed.

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