Monday, September 17, 2012

Why Is It Hard To Classify Archaeopteryx?

For the last few days I have had noticeable traffic to my blog via the search term " Why is it hard to classify Archaeopteryx?". And all of it from the United Kingdom! Maybe there is a college essay competition going on there on fossils and evolution..?

Coincidentally, I wrote a post more than two years ago on Archaeopteryx in response to a new analysis of Archaeopteryx bone growth which cast doubt on whether Archaeopteryx was an early bird. More recently, Xu using a wider selection of character traits suggested that Archaeopteryx is not a basal bird i.e. it is not an early representative of the lineage of birds but rather may belong to a different now extinct branch of feathered dinosaurs.  So there is genuine uncertainty on how to classify Archaeopteryx.

I'm reproducing the post below with some new additional comments. Image below sourced from  Wikimedia Commons:

How birdlike was Archaeopteryx?

Beak - Yes

Feathers - Yes
Wishbone - Yes

Growth pattern of blood vessels - Not quite there yet

By comparing the structure of the bone of Archaeopteryx  with modern birds and fossil dinosaurs, researchers found out the growth pattern of blood vessels in Archaeopteryx was more like dinosaurs than modern birds. Archaeopteryx individuals seem to have taken a longer time to mature than modern birds do.

Archaeopteryx lived about 150 million years ago and is the earliest bird-like creature to have been found yet.  A younger bird-like fossil -
Ichthyornis dispar - dated about 94 million years ago shows several characteristics of quick growth, giving us an idea of the timing of the various changes in morphology and physiology that were taking place within the Maniraptor dinosaurian clade as they evolved  characteristics that are recognized as bird-like.

By the way this is not some earth shattering find. The basic relationship that  small carnivorous dinosaurs evolved into birds still holds. But the study highlights how difficult categorizing a creature which is a composite of ancestral and derived traits can be.

It doesn't matter in the larger view if Archaeopteryx is classified as a dinosaur or a bird. The value is in demonstrating that evolution of a complex suite of characters takes place in fits and starts, some features evolve earlier than others preserving the historical trajectories that major transitions in evolution have taken.

here for the lighter version.

Additional comments: Recently, South Korea under pressure from creationist groups was mulling excising mentions of Archaeopteryx from science textbooks because the uncertainty regarding its relationship to birds meant according to creationists that the theory of evolution is an inadequate theory to answering questions about the history of life and to the problem of the evolution of birds. Thankfully better sense has prevailed and Archaeopteryx will continue to have a place in South Korean science education.

As evolutionary biologist Ryan Gregory recently pointed out, there are many aspects to evolution. There is evolution as a fact, evolution as a mechanism, and evolution as a path. Archaeopteryx not being a bird does not in any way threaten the factual basis of evolution, nor does it undermine natural selection or random genetic drift as the principle mechanisms of evolution. Its taxonomic status is a specific hypothesis about how two groups of organisms are related, i.e. evolution as a path. This aspect of evolution is concerned with the historical patterns of life, such as, when did a particular lineage originate, how are lineages related, do species originate geologically suddenly and then show almost no morphological change during their lifetime.

Was Archaeopteryx an early bird is a question of historical detail.  That initial assessment could turn out to be wrong. That does not mean that birds did not evolve but instead were spontaneously created as some creationists would insinuate. Rather, it means that our understanding about one particular ancestor descendant relationship is incomplete. It means that we don't yet have enough data to reconstruct with confidence the unfolding of one particular historical pathway of life.


  1. As a Wikipedia editor interested in birds, it is so common place for someone to come and put a random edit like "birds are dinosaurs" to the point that the the article has been put on semi-protection. The whole problem with a lot of the Linnean classification system is that it is based on what we have at the current time. If you think of the extant species as the tips of branches of the evolutionary tree it becomes hard to group them. What is a branch or clade? What is a stem group and so on. With more lost branches being uncovered, the only thing to question is the old system of naming - indeed what part of the tree is classified "birds" will remain a definitional problem and one that is created by taking into account the longer evolutionary history. I suspect that the situation will only get worse if more evidence of horizontal gene transfer is found and the tree analogy moves into a more reticulate framework. Perhaps it is fortunate that our evidence in this area will be restricted to the extant forms.

  2. Shyamal - thanks for that perspective.. regarding horizontal gene transfer and the tree analogy.. genes may be transferred occasionally across species resulting in a more reticulate gene tree, but that does not deny the reality of lineages having separate evolutionary histories i.e. speciation and divergence(species trees). Indeed to recognize horizontal gene transfer branches with distinct gene pools need to occur in the first place.

  3. As a footnote on tree thinking in classification - the Wikipedia article on cladistics is now quite a good introduction.

    You are right about the idea that the term "horizontal gene transfer" requires one to recognize the dominance of "vertical" or gene transfer along the branches. Our ideas of evolutionar are based on trees constructed on the basis of molecular phylogenies of certain well-recognized sequence fragments, typically those that are being expressed. Accepting reticulate evolution makes evolution more exciting but presumably makes evolution tougher to grasp. After all a reconstruction of the history of a cell-phone that links it to an older wired telephone might be easier to grasp but misses out on the independent evolutionary paths of a thousand other little ideas that come together in a modern cellphone.

    Anyway one should consider it a success story in education if Archaeoptery is now the inexplicable problem that creationists like to toss about. At least that would mean the 4000 year-old Earth idea has died out.

    PS: completely enjoyed the "Earth Story" series that gives a great introduction to geology with a very firm historic thread binding ideas. Highly recommended -

  4. thanks Shyamal on that interesting discussion.. will check out the Earth Story series..regarding the 4000 year old earth idea dying out.. just visit the Creationist museum in Kentucky.. that idea is well and alive and making a lot of money for some rich creationist entrepreneurs :)