Arthropods Flashcards
What are arthropod exoskeletons made of?
Chitin
How many species of arthropod are there?
Approx. 180,000
What percentage of all extant organisms are arthropods?
1/2
The arthropod group is hugely diverse. What percentage of past arthropod diversity is now extinct?
99%
From what period do we have the best fossil record for arthropods?
The Cambrian when rapid diversification occurred.
Define a trace fossil.
A fossil that shows evidence of arthropod locomotion, shows their tracks.
Most arthropod fossils are trace fossils. True or false?
True.
What are the 4 major subphyla of arthropod?
- Chelicerata
- Myriapoda
- Hexapoda
- Crustacea
What organisms does the chelicerata contain?
Spiders, scorpions, horseshoe crabs and sea spiders.
What organisms does the myriapoda contain?
Millipedes and centipedes.
What organisms does the hexapoda contain?
Insects, silverfish and springtails
What organisms does the crustacea contain?
Crabs, lobsters, shrimp, barnacles etc.
Define the tracheata.
All arthropods that breathe via a tracheal system.
What is another name for the tracheata?
Atelocerata.
Which groups does the tracheata contain?
Hexapods and myriapods.
How do a) chelicerates and b) crustaceans breathe?
a) Book lungs
b) Gills
Define pancrustacea.
A clade that proposes the hexapods and crustaceans are a sistergroup.
What is another name for pancrustacea and why?
Tetraconata: based on the ommatidia, which are the optical units of the compound eye. Both hexapods and crustaceans have 4 cone cells. Plus the axons in the neuropil (dense areas of nerves) cross over - this is unique to hexapods and crustaceans.
What is pancrustacea based on?
Molecular data: ribosomes, HOX genes and novel microRNA.
There are 2 competing hypotheses for the phylogenetic placement of arthropod groups. What are they?
- Paradoxopoda
2. Mandibulata
Explain the paradoxopoda hypothesis.
Proposes that the arthropods are composed of two sister groups: pancrustacea is one and the myriapods and the chelicerates form the other.
Explain the mandibulata hypothesis.
Proposes that the pancrustacea and myriapods form a clade called the mandibulata. The chelicerates are proposed as a sistergroup.
What similarities between the myriapods and chelicerates serve as support for paradoxopoda? Give 2 reasons.
- Mandibles of myriapods anf chelicerate fangs share a structural identity
- Neural tube undergoes post-mitotic division to form the ectoderm in the same way
Why is the paradoxopoda theory now widely dismissed?
It is though similarities are due to convergent evolution, not shared ancestry.
Explain why the structure of the head serves as support for the mandibulata hypothesis.
In the mandibulata (hexapods, crustaceans and myriapods), the limb on the third head segment has been internalised to form the jaws. In the chelicerates the fangs are external appendages.
Describe the structure of the jaw in the madibulata.
The mandible is the crushing mouthpart formed from the appendage on the third head segment. The labrum is the chewing chamber. They chew using the dentate and grinding plate apparatus.
What is dachshund and why does it support the mandibulata hypothesis?
1 of the 4 main genes in the arthropod leg: in mandibulata it is expessed at the tip of the leg. In chelicerates it is expressed proximally (near the body).
