Stem groups' of extinct clades

The notion of a 'stem group' is indespensible for a palaeontologist. Much used and abused, it is simply not possible to talk about the relationships of fossils to modern life without the use of the crown and stem group concepts. The crown group is a clade which is delimited by its living (extant) members. The stem group comprises those fossils which are closer to the crown group than to any other extant clade, but do not fall within the crown group. As a result, the stem group is paraphyletic, and thus not really a group at all. It is perhaps more useful to talk about a 'stem assemblage' than a 'stem group'.

While at this year's SVP (and at previous meetings), I was struck by some of the terminological abuses of the term 'stem group'. In various instances, it was used either to refer to the nearest sister taxa of an extinct clade, or it appealed to essentialist nomenclature. I comment further on these below the fold.

'Stem groups' of extinct clades:
When a clade is extinct is has neither a crown nor a stem. If we did not distinguish between extant and extinct clades when applying the crown group concept, then crown groups could be arbitrarily small and stem groups arbitrarily deep. Because nodes in a cladogram are rotatable, we could use any taxon (fossil or living) to be a stem taxon.

We already have a set of terms for this: sister group relationships. This is also what the crown group concept conveys. However, it's purpose is to convey the relationship of fossils to a particular living group. When we talk about fossil or extant clades, we can talk about the nearest sister taxa. When talking about fossils in relation to an extant clade, only then do we apply the crown group concept.

Arbitrarily deep stem groups
One abstract title at this year's meeting struck me, because it referred to the fossil Morganucodon as the earliest stem-mammal. This taxon is almost certainly a stem-mammal. Is it the earliest? Take a look at this figure (from Angielczyk, 2009) (you may have to click on it to see the full image):


Notice the placement of the node "Mammalia". It's a full two internodes displaced from the node that subtends the extant mammalian branches: monotremes, marsupials, and placentals. You'll also notice that the Triassic fossil Morganucodon is the nearest fossil sister group of the three extant mammal lineages. In other words, it's the nearest sister taxon (in this tree) of the mammalian crown group (which, strangely, is unnamed!).

This is a peculiar trait among palaeontologists: give the standard crown group name (i.e Mammalia, Aves, etc.) to some arbitrary node within the group's stem. For instance, Aves (birds) is often considered to be the clade delimited by the last common ancestor of all extant birds + Archaeopteryx.

What you should also notice in the diagram above is that the root node of this tree is called "Synapsida". This means it entire run of taxa in this tree from the Synapsida node up to (but not including) the unnamed mammalian crown group nodes are part of the mammalian stem assemblage. Yes, Dimetrodon is a stem mammal, as well as Morganucodon. This means that a host of Permian (and potentially earlier) forms are also stem mammals, leaving Morganucodon appearing fairly late in the game.

The utility of the stem/crown group concept comes in placing fossils in relation to living groups. When we do this, fossils can be used to build up knowledge of the sequence of acquisition of homologies where living forms provide no clues. Fossils can, in turn, help test hypotheses of homology by providing unexpected combinations of characters, as well as precluding or 'predicting' certain character combinations. It is important that these concepts are applied in the correct fashion, or else they (and fossils) will lose their meaning.

---

Angielczyk, K. 2009. Dimetrodon Is Not a Dinosaur: Using Tree Thinking to Understand the Ancient Relatives of Mammals and their Evolution. Evolution: Education & Outreach 2:257–271.

Taphonomy is the branch of research

There are good papers, great papers, and those clever little papers that make you say "I wish I'd thought of that!". Before I get to that, a little preamble:

Taphonomy is the branch of research that is interested in describing what happens to an organism between dying and ending up as a fossil (or even why it won't end up as a fossil). A lot can happen to an organism in that period of time, as the earth is a dynamic spheroid. The older a fossil, the more possible disturbances it can experience. Taphonomy can tell us a lot about the environment an organism was deposited in and it can provide important controls on the inferences we make about the environment we think a fossil organism once lived in. But taphonomy is also an important consideration in considering what an organism is. That is, the 'life' of a fossil after death, might have a profound impact on how we place that fossil in the tree of life.

Enter the experiments of Rob Sansom and colleague's experiments on lamprey larvae and the title organism of this blog, reported in this week's issue of Nature. Sansom et al. wanted to examine what happens to 'primitive' vertebrates that lack hard, mineralized tissues, the type of tissues that normally fossilize. I say "normally", because there are some 'abnormal' cases in which soft-bodied creatures with no bones, teeth, or hard cuticles actually form as fossils. Some such fossils have played an important role in understanding the timing and early origin of vertebrate animals.

For instance, this species known as Yunnanozoon (Chen et al. 1999) from the Cambrian of China. It represents one of the earliest known vertebrates or vertebrate-like forms.



Yunnanozoon is remarkably well preserved, but other Cambrian chordates can be even more incomplete. The problem with such fossils is that they're difficult to interpret because they're squished, and they're made of soft parts. We have no idea how much they might have decayed, apart from the fact that they seem to be an exception to the rule that soft parts don't fossilize. This usually implies some sort of exceptional conditions favouring preservation, but doesn't necessarily rule out decay or other types of disruption.

Sansom et al. let larval lamprey and lancelets rot in buckets of sea water and recorded the progress of the decay over the period of several months.

The impressive and startling results of watching fish decay are below the fold:



As the animals rotted away, Sansom et al. recorded details of their anatomy. Not just general features, but the types of characters that would be used to score an organism for a phylogenetic analysis. These include classically important features, like the gill filaments, cartilages of the gill arches, the type of heart, the shape of the body muscles, the dorsal rod known as a notochord, and so on. These are characters that have normally played a significant role in establishing the relationships of vertebrates and their nearest non-vertebrate relatives, such as the lancelet.



What this figure shows is the length of time each character survived as the animal rotted. What's striking is that the characters that lasted longer all tend to be characters that we consider phylogenetically more primitive. Characters such as a notochord and segmented axial musculature are all considered to be primitive features shared by the last common ancestor of lancelets and lamprey. On the other hand, features such as eyes, or a chambered heart are more derived features found in modern vertebrates.

This figure shows nicely how the decay features plot out in phylogenetic history. If you go back to figure above, there is a graph showing the relationship between phylogenetic rank and decay stage.



What we see is that the level of decay would lead one to think that the taxon was signicantly more distantly related to the vertebrates, much like the early chordates we find in the Cambrian.

Not only do these results provide a caution against how we interpret soft-bodied Cambrian chordates, but it illustrates a framework for studying the phylogenetic effects of decay. As decay is studied across a wider phylogenetic scope, the more we can determine about the generality of these types of patterns. That will have a profound effect on how we study and interpret the exceptional cases of soft-tissue preservation in fossils.

---

Chen, Y.-J., Huang, D.-Y., and Li, C.-W. 1999. An early Cambrian craniate-like chordate. Nature 402:518-522 link

Sansom, R.S., Gabbott, S.E., and Purnell. M.A.2010. Non-random decay of chordate characters causes bias in fossil interpretation. Nature 463:797-800 link

Briggs, D.E.G. 2010. Palaeontology: Decay distorts ancestry. Nature 463:741-743 link

the global scientific community.

I received the following message recently, via the Society of Vertebrate Paleontology. Please sign the petition.

The Department of Geological Sciences at Michigan State University is home to a nationally and internationally prominent, vibrant group of students, faculty and staff. As a response to current budget distress, the Board of Trustees and Provost Kim A. Wilcox are considering a proposal to close the Department, along with all of the graduate and undergraduate programs offered by the Department. The immediate effect will be the loss of three tenure-track untenured faculty positions and one technical and three office staff support positions. Longer term effects of such a closure would include the loss of formal geosciences as a fundamental part of undergraduate and graduate training, and would impair Michigan State University’s service to students, the local community, and the global scientific community.

To offer your support to geosciences at MSU and oppose closing the department, please add your signature to the petition located at:
http://new.ipetitions.com/petition/savegeosciencesatmsu/

Please also consider passing the petition along to others who would offer their support.

The Guardian has run a review

The Guardian has run a review by Richard Fortey of Richard Dawkins' upcoming book The Greatest Show On Earth. I won't get a free review copy, I'm sure... so I'm probably not going to spend time reviewing it myself. However, it looks like it's a feed of standard fare. I'm a bit comforted by this video, as it sounds like Dawkins doesn't waste much time with fossil apologetics. However, I'm wondering to what extent concepts such as homology, palaeontology, biogeography, and embryology are disjoined in their presentation. I wonder when someone is going to write a book about that for general consumption.

Anyway, here's the video plug:

Anyway, I'm off to Prague until Friday. Probably won't have much of a chance to blog again before that. Consider this an open thread.