I'm going to introduce you to either a fascinating question or a throbbing headache in evolution, depending on how interested you are in peculiar details of arthropod anatomy (Mrs Tilton may have just perked up, but the rest of you may resume napping). The issue is tagmosis.
The evolutionary foundation for the organization of many animal body plans is segmental—we are made of rings of similar stuff, repeated over and over again along our body length. That's sufficient to make a creature like a tapeworm or a leech (well, almost—leeches have sophisticated specializations), but there are further steps involved in making a fly or a spider or a human. There is an arrangement of positional information along the length of an animal, so one segment can recognize whether it is near the head or the tail, and the acquisition of new patterns of gene expression based on that positional information that cause the development of specialized structures in different segments. That process of specializing segments is called tagmosis. It's how a fly forms mouthparts in head segments, legs and wings in thoracic segments, and no limbs at all in abdominal segments.
The relationships between segments and how they are specialized are key features in identifying patterns of descent in the arthropod clade. An analysis of those elements in an obscure group, the pycnogonids, has uncovered a surprising relationship—they seem to be related to well known Cambrian organism. You'll have to read through to the end to discover what it is.
Continue reading Pycnogonid tagmosis and echoes of the Cambrian (on Pharyngula)