16 November 2011

What Is and Is Not a Stem Group

In recent years, I've noticed a trend: the prefix "stem-" is becoming more and more popular for stem groups. For those who don't know what a "stem group" is:

  • A crown group is the last common ancestor or two or more extant taxa, and all descendants thereof.
  • A total group is the first ancestor of a crown group that is not also ancestral to any other extant taxa, and all descendants thereof.
  • A stem group is a total group minus its crown group. (Which means, of course, that a total group is a crown group plus its stem group.)
Or, to put it more simply, an extinct organism is a stem-X if it does not belong to X, but it shares more ancestry with X than with any extant organisms outside of X. Real-life examples:

Velociraptor mongoliensis, a stem-avian.
Illustration by myself (Mike Keesey).
  • Stem-mammals: Dimetrodon, Moschops, Cynognathus, Castorocauda.
  • Stem-avians: Marasuchus, Psittacosaurus, Plateosaurus, Tyrannosaurus, Velociraptor, Archaeopteryx, Hesperornis.
  • Stem-humans: Ardipithecus(?), Australopithecus, Paranthropus, Homo habilis, Homo erectus.
  • Stem-cetaceans: Ambulocetus, Pakicetus, Maiacetus, Basilosaurus.
  • Stem-felines: Proailurus, Smilodon.
  • Stem-pterygotes Stem-neopterans: Dictyoneura, Lithomantis.
This is a great convention. It's consistently useful in every area of the Tree of Life. It's concise. It communicates instantly the general area we're talking about, and sets us up to make proper phylogenetic inferences (when the fossil data is lacking).

So I'm glad this trend is becoming more popular. Unfortunately, I've also noticed another trend: rampant misuse!

Case in point:
  • CABREIRA & al. (2011). New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil. Naturwissenschaften (online early). doi:10.1007/s00114-011-0858-0
This looks to be an excellent paper on a very interesting find, so it's unfortunate that there's a glaring error in the title, but there it is: "stem-sauropodomorph". There is no such thing, because Sauropodomorpha is not a crown group. It doesn't even include a crown group (sadlyit'd be very cool if it did). Rather, all sauropodomorphs are part of the avian stem group.

Panphagia protos, a stem-avian
(not a "stem-sauropodomorph").
Photo by Eva K.
Used under the GFDL.
I see a lot of people making this mistake. I think what's happening is that they're using the basic concept of a stem group, but replacing "total group" with "some large clade" and "crown group" with "an interesting subclade". In this case, Sauropodomorpha is "some large clade" and Sauropoda is "an interesting subclade". (And in that case, the usage is even wronger, because it should at least be "stem-sauropod".)

This misuse is unfortunate because it is subjective, while the proper usage is objective. One could make the argument that the real "interesting subclade" of Sauropodomorpha is Titanosauria, or Neosauropoda, or whatever, and then the terminology would mean something very different. By contrast, e.g., "stem-crocodylian" very clearly indicates a particular paraphyletic group.

So, please, people, use the "stem-" prefix, but use it correctly!


  1. Do you have refs for Lithomantis and Dictyoneura being stem-pterygotes?

  2. There was a recent graptolite phylogeny paper that attempted to define 'stem' and 'crown' groups in a manner that meant they could be applied to extinct lineages. IIRC, a 'crown group' was a node for which two or more daughter branches contains a number of species, while the 'stem group' contained the pectinate part of a tree in which nodes subtended only one speciose daughter branch. So not entirely arbitrary, but still problematic (I didn't like it myself). From memory the paper was:

    Maletz, J., J. Carlucci & C. E. Mitchell. 2009. Graptoloid cladistics, taxonomy and phylogeny. Bulletin of Geosciences 84 (1): 7-19.

    (I don't have a copy right on hand to check.)

  3. Tim, not offhand. Quite possibly I am talking through my anal fan on that one. (But I was under the impression that "paleodictyopterans" are a major part of the pterygote stem group.)

    Christopher, so if one node had one subbranch with two species and another with a thousand, it would be a "crown"? That does sounds awfully problematic.

    I do appreciate the basic problem, though: people who work entirely within stem groups (like stem-avians, or stem-pterobranchs) want a clear, easy convention like total/crown/stem. Well, maybe there's a good idea out there, but I really don't see that twisting the existing convention is a good solution.

    Perhaps it's worth noting that the total/crown/stem convention is tied to a particular time (the present). If you tied it to a different time (say, the J/K boundary), it would work differently. So maybe you could say that, e.g., Panphagia protos is a J/K-stem-eusauropod. (That's rather unwieldy, though.)

  4. Wait, or was Neosauropoda a crown group at the time of the J/K transition? Yes, that, not Eusauropoda.

  5. so if one node had one subbranch with two species and another with a thousand, it would be a "crown"?

    I presume that they intended that both sides should have had a reasonable radiation, but yes, that was basically my objection as well.

    I was under the impression that "paleodictyopterans" are a major part of the pterygote stem group.

    Nope, they're probably in the crown group. They may be stem-neopterans, though. With the possible exception of Rhyniognatha, there aren't any likely stem-pterygotes known. The Russian school identifies paoliids as stem-pterygotes, but everyone else thinks they're polyneopterans.

  6. Fixed.

    So the reduction from 6 to 4 wings happened independently in neopterans and paleopterans? That's interesting.

  7. What reduction? There was never such a thing as a six-winged insect. Some palaeodictyopterans did have very large pronotal paranota, but they were never hinged like a pair of wings.

    Such large paranota are found in a number of Palaeozoic insect lineages, but they were almost certainly independent acquisitions in light of their scattered distribution. Lemmatophorids, in particular, were probably stem-plecopterans and therefore well nested within the neopterans.

  8. I must have gotten confused by the fact that they are sometimes referred to as "winglets". Are they not homologous to wings at all?

  9. On the basic level, no. Wings may (or may not) be ultimately derived themselves from modified paranota, so questions of serial homology and genetic co-option are not entirely irrelevant, but that's a slightly different matter.

  10. My mistake—that's what happens when you step outside your domain!

    But then again, this discussion not only informed me but further illustrated the usefulness of the "stem-" convention! See how he uses terms like "stem-neopteran" and "stem-plecopteran" and I, someone largely (but not entirely) ignorant of insect systematics, instantly know what he means? :)