Talk:Castorocauda lutrasimilis

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An entry from Castorocauda lutrasimilis appeared on Wikipedia's Main Page in the Did you know? column on 24 February 2006.

Contents 1 Tail 2 Not a mammal? 3 Subheadings 4 Closest relative to mammals?

[edit] Tail

If you look at the fossil itself, instead of to the fancy drawings made of it, is the tail really that similar in form to that of a beaver? ~:o\--MWAK 11:17, 24 February 2006 (UTC)

I was noticing that as well. The tail is dorso-ventrally flattened but nowhere near as wide as a beaver's. The beaver similarity mostly comes from it being scaly and flat. Though it seems to have more hair on its tail than a beaver. I think a scaly platypus tail might be a better comparison. --Aranae 15:24, 24 February 2006 (UTC)

I agree. And then even the flatness of the tail is incertain as the integument remains are rather 2D. Also I simply can't see the webbing of those presumed webbed feet. I get the feeling the animal was interpreted within the preconceived notion that it was semi-aquatic and that its qualities were not individually used to test this hypothesis.--MWAK 07:41, 25 February 2006 (UTC)

I also just noticed that the article compares the broadest of caudal vertebrae, C5, of Castorocauda to a narrow vertebra, C15, of Castor suggesting a similarity in adaptation.--MWAK 08:44, 26 February 2006 (UTC)

[edit] Not a mammal?

"Castorocauda is not considered to be a mammal by most authorities. The technical, cladistic, definition of a mammal is the group containing all living mammals (including monotremes, placentals, and marsupials) and their descendants."

So there's no such thing as an extinct mammal? That can't be right. If that was true, then mammoths, sabertooth tigers, etc. wouldn't be considered mammals. Does anyone who understands this better than me want to shed some light on this?

Since it has hair and tiny auditory ossicles, it sounds like a mammal to me. I'm not an expert here (I have only a minor in Biology), but I understand that there are three main characteristics that make something a mammal: fur, middle ear ossicles, and mammary glands. It would be hard to prove or disprove the presence of mammary glands from a fossil, but the other two are documented. Why isn't it considered a mammal?

Mammoths and sabertooth tigers are both members of Eutheria along with many modern mammals, so they're considered mammals. In other words: the genera are extinct, but the higher taxa are not. Order docodonta, on the other hand, is extinct with no modern descendants. Therefore, Castorocauda lutrasimilis is not technically a mammal. You can see one interpretation of the phylogeny here. bcasterline^talk 22:34, 24 February 2006 (UTC)

"containing all living mammals" This only means that all contemporary mammal-like mammals are mammals. There would have to be some species that are beyond classification, a Synapsids didn't give birth to a mammal, and a dinosaur never gave birth to a bird.

What was meant was: the ancestor of all living mammals and its descendants, as Liriodendron emended in the article. So it is a "crown group" definition. To use characteristics alone for a definition is problematic and this practice is being abandoned. Would you call a pterosaur with fur, milk glands and middle ear ossicles (tricky to evolve but stranger things have happened :o) a mammal?--MWAK 11:55, 25 February 2006 (UTC)

[edit] Subheadings

While this article is really interesting and all, I think that the subheadings ("Not quite a mammal", "Adapted to water") give a science magazine-esque feel to it. I can't really think of any replacements right now, but I think you get the point, so if anyone wants to go ahead and fix it, you have my support. Obli (Talk)^? 23:00, 24 February 2006 (UTC)

[edit] Closest relative to mammals?

The article currently says this:

Castorocauda is a member of the order Docodonta, which is a wholly extinct group that represents the next closest relative to true mammals (after the genus Hadrocodium).

This doesn't make sense. Both Docodonta and Mammalia are orders. If there is a group of animals closer to Mammalia than Docodonta is, it must also be an order, not a genus.

What I mean is, the genus Hadrocodium must be in some order itself. If this order is either Mammalia or Docodonta, then Castorocauda is no closer or further than Hadrocodium (in a Linnaean sense, though it might be further in a cladistic sense).

If it is neither, then it is a member of some other order X which lies "between" Mammalia and Docodonta. --Saforrest 04:04, 26 February 2006 (UTC)

I think the article means that Hadrocodium is more derived than any known docodont. Since it exhibits significant cranial features not usually associated with Docodonta or any other Mammaliaformes [1], it is usually considered a member of a different order. At this point, the order is incertae sedis [2]. They've only found one isolated skull. But you're right that comparing an order to a genus doesn't really make sense -- maybe someone can come up with a clearer way to put it. bcasterline^talk 05:33, 26 February 2006 (UTC)

The problem is caused by the fact that paleontologists really don't try any more to make the Linnaean system coherent as this is a hopeless undertaking anyway. It is therefore best to see the passage as a cladistic analysis still using some ranks, though not implying that these ranks could be brought into a coherent conceptual system. To fully understand the situation you should ask yourself why we would want to put Hadrocodium in an order in the first place. The answer for modern taxonomy can only be: in order to make it equal in rank to the order that is its sister group in the evolutionary tree. But in this case that sister group would be class Mammalia. Ouch. If an order can be a sister group to a class, why couldn't a genus be? If we accept an abundance of rank for Mammalia why not a shortage of rank for Hadrocodium?--MWAK 08:39, 26 February 2006 (UTC)

The relationships among these groups can be described in the following tree modified from the Ji et al. article:

  |-------------------- Birds, turtles, crocodiles, etc.
  |
--|  |----------------- Dimetrodon, other pelycosaurs, other synapsids
  |  |
  |--|  |-------------- Probainognathus, other "mammal-like reptiles"
  |  |  |
     |--|  |----------- Morganucodon
        |  |
        |--|     |----- Haldanodon, other docodonts
           |  |--|
           |  |  |----- Castorocauda, other docodonts 
           |--|
              |  |----- Hadrocodium
              |--|
                 |  |-- Monotremes and other Australosphenida
                 |--|
                    |-- Therians and other Boreosphenida

Mammalia is a crown based clade. It is defined as the group containing the most recent common ancestor of modern monotremes, placentals, and marsupials and all of its desendents. Basically if any taxon is more closely related to anything we would currently define as a mammal than that mammal is to some other living mammal then the taxon in question is a mammal. For example, the multituberculates are currently thought to be more closely related to the therians (marsupials and placentals) than any are to the monotremes. Multituberculates are therefore considered mammals. Monotremes and therians are more closely related to each other than to Hadrocodium or Castorocauda so neither Hadrocodium nor Castorocauda qualify as mammals.

Mammaliaformes is a more inclusive group and is defined by characteristics that include both mammals and some more distant relatives of mammals. Among the most important of those characters is that the jaw is made of a single bone (the dentary) and the dentary and squamosal meet at the point where lower jaw meets skull. These features are thought to be present in Morganucodon, the docodonts, and Hadrocodium. These three groups as well as all true mammals are in the clade Mammaliaformes. Probainognathus still retains the quadrate and articular as part of the jaw attachment and doesn't qualify as a mammailiaform. Mammaliaformes has no formal position in the Linnean hierarchy, but it would roughly correspond to superclass.

Synapsida would be an even broader group that is roughly a stem-based definition. In this case, anything that is more closely related to mammals than any other living vertebrate would belong to this group. In this instance, the "mammal-like reptiles" such as Probainognathus and some very different groups such as the pelycosaurs (e.g. Dimetrodon) qualify as synapsids because these "reptiles" are more related to mammals than to birds, lizards, turtles, or crocs (and because of characters that demonstrate such). Synapsida has no formal position in the Linnean hierarchy, but it would roughly correspond to Magnaclass or Parvphylum.

Putting Linnean levels to some of these intermediate taxa is pretty tough. Hadrocodium is a great example. Based on current knowledge of its limited remains, it is more related to the class Mammalia than it is to anything else, but it doesn't quite qualify as a true mammal and is excluded from that class. It is most conveniently considered to belong to the clade Mammaliaformes, but not formally placed in a class (incertae sedis). This use of incertae sedis in this case is not so much an uncertainty of its relationship, but an attempt to avoid having to erect a new class at every step more distant from mammals (one for Hadrocodium, another for docodonts, another for Morganucodonts, many more for the various stages of mammal-like reptiles, etc.). --Aranae 19:40, 26 February 2006 (UTC)