Phylogeny 1028.docx

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University of Massachusetts Amherst
Craig Albertson

Phylogeny: 10/28/2013 9:12:00 AM Office hours tues/Wednesday 1:30-3:00 Ecdysozoa Nematoda Phylum Arthropoda: Jointed appendages ~ crab legs  “Jointed appendages”  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 peristalsis. 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: Class: Trilobita (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  Eight legs  2 main body segments: Defining feature prosoma (head) & opisthosoma (abdomen).  Some possess a telson but this is an outgrowth of the opisthosoma.  2 defining anterior appendages: chelicerae (red) and pedipalps (blue).  Poisonous fangs – Chelicerae Subphylum: Chelicerata  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 trajectory.  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. Subphylum: Chelicerata  Example: Arachnids  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 structure forms. 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 Hox expression.  Have mandibals  Mandibal turns on the same set of genes as the first leg of the chelicerae o The first leg turns into a feeding appendage, and the pedipals and cheliceae were turned into a antennae  Athropods a
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