| Pal�os: |  |
Protostomia |
| METAZOA | Pancrustacea |
| Parent Unit: Ecdysozoa |
Sister Unit: Nemata |
Sister Unit: Scelidophora |
| Child Unit: Crustaceamorpha |
Child Unit: Hexapoda |
Cladogram |
The term Pancrustacea "All the Crustacea" is used to designate the clade uniting insects and Crustacea. This is the opposite of the Atelocerata hypothesis, and while still a minority position, is coming increasing into favour
As with the Mandibulata and Atelocerata, the evidence for a Pancrustacea is from neontological (molecular analysis and soft part anatomy) than paleontological grounds (Friedrich and Tautz 1995; Dohle 1998; Kraus 1998). For example
"The homology of characters brought forward in favour of a close relationship between myriapods and insects (absence of second antennae, formation of malpighian tubules, postantennal organs and tracheae) cannot well be established. These characters may be due to convergence. This is especially true for the tracheae.
In contrast, the...characters combining insects and crustaceans, malacostracans in particular, ( axonogensis in early differentiating neurons, formation of neuroblasts, fine structure of ommatidia, expression patterns of the engrailed gene) seem to be much better substantiated."
and
"Despite the diversity of their limbs and lifestyles, the nervous systems of insects and crustaceans share many common features both in development and in function....Examples include the basic ganglionic architecture and the strikingly similar pattern of GABAergic inhibitory enervation of leg muscles in Crustacea and insects. These and other examples...argue against the idea that these two groups of arthropods evolved independently from separate non-arthropod ancestors....Myriapods are often regarded as being more closely related to the insects than are the crustaceans. However, we find that myriapods show substantial differences, for example in the pattern of early axon growth, to the common pattern of early neural development seen in the Crustacea/Insecta."
Suggestive as these facts may be, they are not totally persuasive, for the Uniramia hypothesis is also built on factors of similarity in embryology and physiology. Clearly, there has been a great deal of parallelism and convergence in the evolution of the arthropods.
The main problem with the Pancrustacea theory is the lack of transitional forms, especially in the fossil record, where one would expect to find them most. Not surprisingly, Conway Morris and Hou & Bergström, who adopt a wholly paleontological approach, retain the old taxon Uniramia.
Presumably (assuming insects do derive from Crustacea) primitive hexapods evolved from marginal marine and amphibious crustaceans during the late Silurian or early Devonian
An ironic twist, if this hypothesis does turn out to be true, will be that Crustacea will then become a paraphyletic taxon, and hence invalid under the cladistic system, perhaps to be replaced by a confusing array of smaller clades. Or insects will have to be called "crustacea", just as many vertebrate paleontology enthusiasts now refer to birds as dinosaurs, thus confusing the fact that birds, while evolving from dinosaurs, really have become a distinct form of animal. The same can be said of insects and crustaceans.
Pancrustacean Phylogeny |
<==o ARTHROPODA
|-- CHELICERATA
`--o MANDIBULATA
|-- MYRIAPODA
`--o PANCRUSTACEA
|-- CRUSTACEA
`-- HEXAPODA
Pancrustacea as a sister clade to the Myriapoda and daughter clade of the Mandibulata (see rival phylogenies)
<==o ARTHROPODA
|--+-- MYRIAPODA
| `-- CHELICERATA
`--o PANCRUSTACEA
|-- HEXAPODA
`-- CRUSTACEA
Pancrustacea as sister group to the Myriapoda + Chelicerata (see rival phylogenies)