Palæos:		Arthropoda
INVERTEBRATES		Arthropod Phylogeny

Problems in interpreting the Arthropod Family Tree

Problems in Arthropod Phylogeny

As with many large and complex taxa in which the earliest forms are poorly known (if known at all), the arthropods present particular difficulties as far as phylogeny goes. Molecular evidence remains ambiguous, due perhaps to the fact that many clades diverged rapidly within a very short space of time (the Cambrian Explosion). Many fossil forms are known, but it is always difficult to make assertions regarding what little can be gleaned from the gross morphology preserved.

Polyphyly or Monophyly?

Arthropod polyphyly was popularized by Sidnie Manton. Although a popular view for a while, and still found in many zoology textbooks even today, as well as widely on the Web, it's basic premise - that of three distinct phyla (see above cladogram), is no longer considered tenable by most researchers in the field of high-level arthropod systematics. The consensus is now that arthropods are monophyletic, but the exact order of relationships is in dispute (ref). it seems like every possible alternative has been offered (left - from Giribet and Ribera 2000).

Rival Hypotheses

Many recent studies use molecular analysis to unite crustaceans with atelocerates (hexapods and myriapods) in the old established taxon Mandibulata, there are also many that align crustaceans with chelicerates and trilobites ( (as the "Schizoramia"). Supporters of Mandibulata can't agree whether Atelocerata is monophyletic and sister to Crustacea (the majority position), or whether crustaceans are sister to hexapods (the Pancrustacea hypothesis).

These conflicting results are influenced by the fundamentally different classes of evidence on which they are based, as well as reliance largely or wholly on recent forms to reconstruct ancient events.

The relative infancy of molecular systematics means that the data may not be adequate to the task, and produce misleading results. An rRNA study by Giribet and Ribera 2000 was inconclusive, as the high level of sequence heterogeny made placement of some taxa difficult and showed the importance of combining ribosomal gene data with other sources of information.

Complexes of Characters

In recent years, along with the rise of molecular systematics and interest in high-level arthropod phylogeny, there has been a revival of interest in fossil forms, as well as the discovery and description of several important Cambrian lagerstätten with their spectacular early arthropod faunas. This palaeontological research has resulted in cladistic analyses completely different to those based on neontological (recent taxa) evidence alone, and offers no support at all for popular taxa like the Mandibulata but suggest instead an Arachnomorpha that combines chelicerates and trilobites

But even here the situation is more complex and ambiguous than it may at first appear, due to the difficulty of interpreting often fragmentary and poorly preserved forms known only from thin carbon films or impressions in shale. Often interpretation of characteristics becomes a purely arbitrary, and the same taxon can be given totally different relationships, depending on how it is interpreted. As Professor Geoffrey Fryer (an advocate of arthropod polyphyly) argues:

Affinities among Cambrian arthropods, deduced by cladistic methods on the assumption of monophyly, are unreliable, and some are certainly erroneous. For example, Sarotrocercus, said at different times to be close to Limulus and a eurypterid, then to Yohoia and Sanctacaris, and most recently to Molaria and Leonchoilia, bears little resemblance to any of these very different animals and is in fact one of the most primitive arthropods yet discovered.

Single characters...can be misleading and, not surprisingly, are often less informative than complexes, which arthropods display in profusion and whose changes during the course of evolution can sometimes be followed in detail."

Geoffrey Fryer, Arthropod Polyphyly

For these various reasons it would seem to me advisable to take many of the cladograms dealing especially with earlier and less well-known taxa, with a small Siberian salt mine. The closer one gets to modern taxa, the more reliable findings may be, but even there is an element of uncertainty (as shown by the rival interpretations regarding myriapod relationships, left), which may remain for some time to come.

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