Phylogeny: 10/28/2013 9:12:00 AM
Office hours tues/Wednesday 1:30-3:00
Phylum Arthropoda: Jointed appendages ~ crab legs
MOST diverse phylum – >1,000,000 known species.
o 8/10 of all animals will be a arthropod
Estimated number of species – >>10,000,000!
80% of all known living or fossil animals.
Hard exoskeleton (chitin)
o Protects the organism, and allows for muscles to connect
o No need for hydrostatic skeleton – e.g., movement via
o Provides hard inner attachment sites for muscles – increased
mobility! More elaborate muscle system
o can move much faster than worms, can run and jump
Conspicuously segmented body axis Phylum Arthropoda:
(an early success story)
Originated in the Cambrian
Took over the ocean
Most fossils found will be Trilobita’s
Very diverse, basic conserve body plan, head region, segmented
trunk, with two appendages, one ventral and one dorsal oriented.
Subphylum: Chelicerata ~spiders
2 main body segments: Defining feature prosoma (head) &
Some possess a telson but this is an outgrowth of the opisthosoma.
2 defining anterior appendages: chelicerae (red) and pedipalps
Poisonous fangs – Chelicerae
Example: Horseshoe crab
o primitive arthropod prob looked like this
From the Ordovivian period (~450 mya)
Living fossils – changed little over time.
Similarities to trilobites?
o Possible early ancestry (e.g., Cambrian), but independent
evolution for ~450 my
o Trilobats and horsehoe crabs formed their own clade possibly,
the ancestor if real is ancient, independent evolutionary
How to measure evolutionary success? Horeshoe crab only 4
species today, all basically look the same but have never been
outcompeted in their niche, never needed to evolve, body is plan is
good for what they do, eat and reproduce.
Very successful terrestrial group
Most build webs with modified gills (i.e., spinnerets)! o spinnerets are modified gills, gills transpformed to lungs and
some to spinnerets
In other words, spinnerets (spiders) and book gills (horseshoe
crabs) are homologous.
We know this because the development of these structures, same
series of outgrowths on their abdomens
Observing developmental helps to clarify these relationships.
o Gene expression – e.g., what genes are turned on as a
o Darwin – “community in embryonic structure reveals
community of descent”
Each outgrowth turns on a set of genes as they grow
Clade Mandibulata = Subphyla Myriapoda, Crustacea and Hexapoda
Distinguished by mouth parts.
Chelicerae (and pedipalps) are homologous to antennae based on
Mandibal turns on the same set of genes as the first leg of the
o The first leg turns into a feeding appendage, and the pedipals
and cheliceae were turned into a antennae