Field of Science

Agathoxylon, the Wood Morphogenus Previously Known as Araucarioxylon

We've known for a couple of years now that the name Araucarioxylon, used for the majority of the petrified wood from the Chinle Formation is illigitimate. Savidge (2007) reexamined the type specimens of Araucarioxylon arizonicum, and assigned it to a new genus, Pullisilvaxylon arizonicum. Problematic is that the type specimen is from the Black Forest Bed, which is much younger than the main log bearing horizon in Petrified Forest National Park where thousands of colorful logs have been referred to A. arizonicum for over a century.

Complicating this problem is that the majority of this wood has been almost completely agatized, obliterating the cellular structures used to make taxonomic assignments.  Furthermore, Savidge (2007) found that there are actually several wood taxa at this horizon, in fact almost every specimen sampled turns out to be new.  Thus, park staff are at a loss when asked what the name the main type of fossil wood in the park is, and most just simply still continue to use the invalid name Araucarioxylon.

Philippe, in this new paper, suggests that a better name for the morphogenus is Agathoxylon.  However, I would be hesitant to refer the large amounts of currently unnamed wood to this genus as he also suggests that eventually it should be restricted to a single species.  Furthermore, because of the restriction of the type of P. arizonicum, the majority of Chinle wood lacks a direct assignment to a species-group name.  This is a mess that probably will never be resolved because of the poor preservation of the wood structures, thus the majority of Chinle wood will simply be referrable to Araucariaceae indeterminate (or maybe even a more inclusive clade or rank?).

Philippe, M., in press. How many species of Araucarioxylon? C. R. Palevol (2011), doi:10.1016/j.crpv.2010.10.010

Abstract - Fossil wood, similar to that of modern Araucariaceae, has been known for a long time, and is usually called Araucarioxylon. More than 400 morphospecies have been described, whereas this wood type displays few characteristic features. This taxonomical profusion is compounded by nomenclatural problems, Araucarioxylon being an illegitimate name. The status of the wood morphogenus, the infrageneric structure and the names that apply to the taxa designated for fossil woods of the Araucarioxylon-type are discussed. A database with 428 morphospecies designated for Araucarioxylon-type of wood is analyzed. The name Agathoxylon Hartig seems to be the most appropriate for the corresponding morphogenus. Albeit theoretically several hundred morphospecies could be recognized within this group, it is at least as probable that only one should be retained.

REFERENCES

Savidge, R.A. 2006. Xylotomic evidence for two new conifers and a ginkgo within the Late Triassic Chinle Formation of Petrified Forest National Park, Arizona, USA, 147–149. In Parker, W.G., Ash, S.R. & Irmis, R.B. (eds) A century of research at Petrified Forest National Park: geology and paleontology. Museum of Northern Arizona Bulletin 62.

Savidge, R. A. 2007. Wood anatomy of Late Triassic trees in Petrified Forest National Park, Arizona, USA, in relation to Araucarioxylon arizonicum Knowlton, 1889. Bulletin of Geosciences 82:301–328.

Savidge, R.A. & Ash, S.R. 2006. Arboramosa semicircumtrachea, an unusual Late Triassic tree in Petrified Forest National Park, Arizona, USA, 65–81. In Parker, W.G., Ash, S.R. & Irmis, R.B. (eds) A century of research at Petrified Forest National Park: geology and paleontology. Museum of Northern Arizona Bulletin 62.

5 comments:

  1. There is still enough wood in some of the Sonsela Member wood-bearing beds that are not completely agatized, from which we could potentially get some anatomical information. So I don't think all is lost.

    I doubt however that most identifiable remains can be referred to "Araucariaceae" - my understanding is that much of the wood does not belong in this clade, but in other gymnosperm groups.

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  2. Agreed. One of the harder things to get people to understand is that we will pretty much have to sample almost every tree to be able to assign them to various taxa.

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  3. Come on, folks -- monospecific genera! As soon as you start assigning multiple fossil species to a aingle genus, you make your nomenclature a slave to your phylogeny. Here we are in 2011 -- how does anyone still not get this?

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  4. As you know - quite a few folks disagree with you Mike. Whats the point of having a binomen if all your genera are monospecific?

    I mean, I would like to get rid of the binomen and just have species + clade names. If we're stuck with the binomen, lets just treat genera as clades. I know I'm not going to convince you Mike, but just wanted to reply.

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  5. Randy asks: "What's the point of having a binomen if all your genera are monospecific".

    No point whatsoever. But we're stuck with it for now, so until that changes, we put a space in the middle our mononomials. "Xenoposeidon proneneukos" is a single word, it's just spelled ... D, O, N, space, P, R, O ...

    I honestly don't understand how anyone can possibly disagree with this, not when we're dealing with animals from which if we're LUCKY we have 10% of the skeleton. It simply is not realistic to think that, based on specimens like this, we can recover the phylogeny with enough confidence to tie our nomenclature to it (by means of binomials with many species forming monophyletic genera). For people who work on extant beetles, who have numerous complete specimens of each species, maybe. For sauropod workers who typically have a single very incomplete partial skeleton of each species, no.

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