If you look at the phylogeny (the 'tree of descent') on:
http://tolweb.org/tree?group=Arthropoda&contgroup=Bilateria
You'll see that trilobites (listed at the bottom) were a sister group
of just about everything else (incl. insects /hexapoda/, spiders,
horseshoe crabs etc.). 'Sister groups' is a term that means that the
groups diverged at a given point in time, and that the evolution of
the groups has taken place independently in those lines of descent.
Some groups (incl. horseshoe crabs) may have retained some of the
features that were common to the last common ancestor to both
horseshoe crabs and trilobites (the scientific term to look for is
'plesiomorphy' or 'plesiomorphic'). Which would make them more alike
in those retained features, but strictly speaking, trilobites are - if
the published phylogeny is correct - as closely related to horseshoe
crabs as they are to a common house fly or a millipede.
In a less strict wording, I don't *think* there are any other groups
that in life form, habit, etc. look more like trilobites than
horseshoe crabs do. |
