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“What morphological characteristics have contributed to the success of the
arthropods? How has our understanding of the relationships of the four major groups
of arthropods been changed by analysis of molecular data? Do arthropods form a
monophyletic group?”
Arthropods are widely cited as being the most successful group of animals on
Earth – and for good reason: they’re estimated to make up 80% of all known animal
species. Having first appeared over 500Ma in the Cambrian, they have continued to
diversify; progressing from their marine origins onto the land and then into the air,
until they can be found exploiting almost every habitat on every continent in the
Their success is probably attributable to several key features of their
- The invention of a rigid, protective exoskeleton. Most likely evolved to enable
more powerful jaws, and occupy predatory niches. It would in turn give
them protection against other predators; such evolutionary arms races will
have driven their evolution in the early Palaeozoic (particularly the extinct
Trilobites). The ability to add a waxy cuticle to this exoskeleton facilitated
waterproofing and the invasion of terrestrial ecosystems.
- Segmented body plan – segments are easily added/removed by the
modification of Hox genes. Confers adaptability.
- Small size means they have a huge variety of niche opportunities. Can
specialise to correspondingly small habitats.
These features are all shared by the four groups of extant arthropods: myriapods,
chelicerates, hexapods (insects), and crustaceans.
But are these features convergent and analogous, or derived homologies?
Originally, taxonomists presumed several distinct lineages had undergone
‘arthropodisation’. For example, the insects and myriapods were thought to
be part of the phylum Onychophora (velvet worms), and arthropods in
general were considered polyphyletic.
However, various studies involving both morphological parsimony as well as
molecular Maximum Likelihood techniques have indicated that the
arthropods are in fact monophyletic. The Panarthropoda hypothesis places
Onychophora as their sister group, with tardigrades branching off slightly
earlier. More recent studies (Dunn et al. 2008) ally the tardigrades more
closely with Nematoda and Priapulida.
What about phylogenetic relationships within the arthropods? Traditionally
morphology (especially uniramous/biramous appendages) closely linked the
insects and myriapods (uniramous appendages associated with being
terrestrial). Their similar tracheal gas exchange system and excretory organs
(Malpighian tubules) reinforced this idea. The next closest branch was
thought to be crustaceans (together all considered the mandibulate
arthropods) with chelicerates forming a sister group.
However – molecular data from rRNA challenges this view. The surprising
outcome was that all analyses pointed to the insects and crustaceans forming
a clade (Pancrustacea), with the myriapods and chelicerates as sister groups –
the order of which have been difficult to determine.
The idea of the Pancrustacea clade however is well-supported, for example
insects and crustaceans have very similar mitochondrial gene order. The
other arthropods are much more similar to onychophorans and priapulids in
this respect, making the pancrustacean gene order a derived characteristic.
Insects in fact are probably nested within the crustaceans; meaning
‘crustacean’ is a paraphyletic term. Insects must have secondarily lost
biramous appendages. A closer look at bristletails (outgroup to the insects)
reveals they seem to be uniramous but with a greatly reduced second branch
to their appendages – perhaps supporting this idea.
The tracheal gas exchange and Malpighian tubules must, quite surprisingly,
be convergent in the insects and myriapods. It would be interesting to take an
‘evo-devo’ approach to examine these features in more detail; one would
expect their development to differ if they have indeed evolved
The ambiguity here is where to place the myriapods and chelicerates in
relation to the Pancrustacea. The problem is exacerbated by the 500 million
years of noise that has accumulated since these groups’ rapid and early
divergence. Differing rates of evolution and long-branch attraction confound
the issue further. Comparative analysis of Hox genes suggests an early split
into two groups: one being the Pancrustacea and the other being the
myriapods and chelicerates. Other studies have placed the chelicerates as a
sister group to the other mandibulate arthropods.
The distinctive arthropod morphology has been the key to this phylum’s
success, but it is their genomes that have been of most use to understand the
relationships of its four extant constituent groups. Molecular data has radically
changed our view of their phylogeny; the most significant finding being that
insects appear to be a clade of ‘terrestrialised’ crustaceans. Although tentative
conclusions can be drawn about where the myriapods and chelicerates fit in to
this picture, further work on a greater variety of gene sequences is needed. Only
when we have a robust and complete phylogenetic tree can we begin to ask
meaningful questions about the evolution of arthropod diversity.