Laelaps

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The skeleton of Inostrancevia, a Permian synapsid from modern-day Russia. From the American Museum Journal.

The science of paleontology has long been concerned with searching out the origins of modern groups of animals, but at the turn of the 20th century there were frustratingly few transitional fossils. That evolution had occurred was generally agreed upon, but where the transitional forms might be found, what they would look like, and what mechanisms drove their evolution remained disputed. Among the murkiest of these subjects was the origin of mammals.

In an 1898 letter published in the journal Science the American paleontologist O.C. Marsh reviewed the problems involved in studying the origin of mammals. Chief among the factors that aggravated paleontologists was a lack of transitional fossils. The origin of fishes, amphibians, reptiles, birds, and mammals (the five great vertebrate classes everyone learns about in elementary school) could not be determined without more evidence, and “The answer to each [group's origins],” Marsh wrote, “belongs to the future, and how far it may profitably be sought in the present limited state of our knowledge is a fair question in itself.”

Marsh could not simply set the question aside, however. The origin of mammals was a mystery but this did not mean that he could not review a few clues that might eventually be of benefit to ongoing discussions. In order to answer the question, though, naturalists would have to determine whether mammals constituted a single, natural group or if different groups of mammals had separate origins. The group as a whole seemed disparate enough in form that Marsh could not say with certainty whether all mammals had a single common ancestor or not.

Whether they had a single origin or several, however, mammals were most popularly considered to have evolved from reptiles. This was in spite of the fact the fossil record seemed to only contain creatures that could be called “mammals” or “reptiles.” If someone discovered a mammal with multiple bones in its lower jaw (a tell-tale reptilian feature) rather than a single one, that would be something, but Marsh claimed to have seen every extinct mammal ever discovered and found no such evidence. If mammals had evolved from reptiles then paleontologists had not yet found the intermediate forms to confirm the hypothesis.

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The skull of Cynognathus, an Early Triassic synapsid. From the American Museum Journal.

Perhaps paleontologists had been looking in the wrong place. Marsh suggested that the resemblances between reptiles and mammals were only “adaptive likeness[es]“, otherwise known as evolutionary convergence. Maybe they did not share a close evolutionary relationship after all. There was another group that showed some resemblances to both reptiles and mammals, though. These were the earliest tetrapods (what Marsh called the “earliest amphibians”), and they seemed to be better candidates for the progenitors of mammals. Marsh concluded;

In the amphibians, especially in the oldest forms, there are hints of a true relationship with both reptiles and mammals. It seems to me, therefore, that in some of the minute primitive forms, as old as the Devonian, if not still more ancient, we may yet find the key to the great mystery of the origin of mammals.

This implied a ghost lineage of mammal-like amphibians (entirely bypassing any reptile-like form) that had not yet been found, but it seemed like a potentially fruitful hypothesis if extinct reptiles failed to bridge the gap to more mammalian types. This would have to be confirmed or refuted by further research, but it at least offered a fresh start to a somewhat beleaguered debate.

Today much of what we know about the origin of mammals has changed, and Marsh’s hypothesis has not held up. There were no “mammal-like amphibians.” Instead mammals are synapsids, part of a larger group including stem-mammals like the sail-backed Dimetrodon. Indeed, during the Permian and Early Triassic there were many synapsids that looked a lot more like “reptiles” than living mammals, yet they were more closely related to us than lizards or crocodiles. Some of these creatures were just the sort Marsh was looking for and even known during the time he wrote his letter, but for one reason or another they were considered to be more “reptile-like” than “mammal-like.” Today we known better, but even now there are many questions about the evolution of “true” mammals from within the synapsids that survived the Permian mass extinction. Details of this transformation will rely on future fossil evidence.

Comments

  1. #1 pough
    May 8, 2009

    There’s a lot of talk about where birds, dinosaurs and mammals all converge upon reptiles, but what about reptiles themselves? How are older reptiles different from modern reptiles? Are they very different?

    Seeing as how I’m asking questions, I figure I might ask you a question I posed to Afarensis a long time ago. There is a common perception (especially among creationists) that all transitional fossils should be readily available. What they seem not to understand is the nature of fossilization. It’s not like every creature that ever lived submitted itself for fossilization at the end of its life. Also, due to the nature of tectonics, not all fossils manage to remain intact. And another thing, some that have been exposed are gone and some are way too deep to get to.

    So my question is, what do we expect to find, why do we expect it, and (to give an example that might hit home) how many extant species are “proven to exist” by their fossil record?

    I’m guessing shellfish are way overrepresented and animals like mountain goats just plain don’t exist at all. (Hey, if mountain goats existed we’d see them in the fossil record, right?)

  2. #2 resimler
    May 9, 2009

    1
    Not quite. The Mexican sequences are not in GenBank. The CDC deposited them in GISAID instead – in a database called EpiFluDB, which is free but only to scientists with an account (which includes me). The data are not freely available – you have to agree to restrictions in order to use them. GISAID was originally intended to be a conduit to GenBank – a brief “holding tank” for those scientists who wouldn’t otherwise share their data immediately. But then last fall, GISAID changed its rules, and now they no longer require contributors to release data to GenBank.

  3. #3 Lilian Nattel
    May 9, 2009

    You know, I never thought about the origin of mammals before. Now I’m curious to learn more about it.

  4. #4 pough
    May 13, 2009

    No? Not an interesting question to anyone else? Oh well. You don’t seem interested and Afarensis has disappeared. I’d look into it myself, but I haven’t got a clue how… Maybe I’ll bug David Ng. If he ignores me I can just drive over to UBC and ask in person. :-)

  5. #5 Laelaps
    May 13, 2009

    Sorry pough, I have been very busy this week.

    I am not sure how many extant species are preserved in the fossil record. You might have better luck in terms of genera, since correlating a fossil species with a living species might be difficult. This problem is compounded by the fact that many zoology texts do not include information on fossils relevant to living animals.

    In terms of what we expect to find, where, etc., it is all a matter of forming a hypothesis based upon things like biogeography. The discovery of Tiktaalik is a good example of how scientists used available information to hypothesize where relevant fossils might be found (check out Shubin’s ‘Your Inner Fish’).

    Unfortunately I don’t have time to go into more detail at the moment, but I hope that is somewhat helpful.

  6. #6 pough
    May 13, 2009

    Thanks. It is helpful because you pointed out something else that creationists tend to get confused about: what is a species. I think that most times the word “species” comes out of their mouths, it’s genus, family, order or even class that is in their head. They just don’t know it.

    If you hadn’t already guessed, I’m very interested in the common misunderstandings of evolution that build a barrier between the thousands of very intelligent people actually studying the subject and the millions of laypeople who are suspicious of them.

    When many people think of evolution, they still think of dogs giving birth to cats as speciation, they still think it happens on an individual level and as a conscious striving to improve and they still think that the fossil record should show everything (and the fact that it doesn’t is a sign of failure).

    I really enjoy your writing and research abilities, so I’m asking you about these things. I never went past high school for education, so I don’t really have the tools to do this kind of research on my own.

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