The chordates Flashcards
What 3 main groups does chordata contain?
- Craniates
- Cephalochordates
- Urochordates
Of the 3 main chordate groups, which 2 are most closely related?
Craniata and urochordata.
What are 4 major synapomorphies of the chordates?
- Notochord
- Dorsal hollow nerve chord
- Myotomes
- Pharynx and pharangeal openings
What is the neural tube?
A structure homologous to the vertebrate spine, it does not expand at the end into a brain.
What is the notochord?
A stiffened rod of tissue that provides points for muscle attachment in locomotion. Homologous to the vertebral column.
What is a common example of a cephalochordate?
Amphioxus.
Give an example of a urochordate?
Sea squirts (tunicates).
What is Pikaia?
A fish-like organism from the Middle-Cambrian.
Why is Pikaia assumed to be the earliest known chordate?
It had a notochord and myotomes.
Why are the conodonts thought to be a sistergroup to the vertebrates?
They have bilateral symmetry, an enlarged brain and eyes.
From which group did the vertebrates evolve?
Craniata.
What are the 2 major groups of extinct agnathostomes?
- Cephalaspidomorphs
2. Pteraspidomorphs
What are the 2 major groups of extant agnathostomes?
- Hagfish
2. Lampreys
What are the 2 major divisions in the cephalaspidomorphs?
- Anaspids
2. Cephalaspids
What kind of tail did anaspids have?
Hypocercal tails.
What kind of tail did cephalaspids have?
Heterocercal tails.
What is a hypocercal tail?
The tail is weighted/stiffened at the bottom and flexible at the top. This pushes water backwards and up in locomotion, creating lift at the front of the animal.
What is a heterocercal tail?
The tail is weighted/stiffened at the top and the bottom is flexible. This pushes water backwards and down during locomotion, creating a downward movement. This is counteracted by pectoral fins that create lift at the front of the animal.
What 5 features are characteristic of cephalaspids?
- Dorsoventrally flattened
- Large head-shields
- Polygonal scales for electrical detection
- Evidence of a lateral line system
- Fleshy pectoral fins to counteract the heterocercal tail
What are 4 major characteristics of pteraspidomorphs?
- Heavy head armour and chainmail scales at the posterior for flexibility
- Hypocercal tail to counteract weight from heavy head armour
- No paired fins, presence of spines instead
- A single, posterior opening for all gill slits
How many species of hagfish are there and what kind of lifestyle do they have?
2, scavenging.
How many species of lamprey are there and what kind of lifestyle do they have?
7, parasitic.
What 5 synapomorphies do hagfish and lampreys have?
- No jaws
- No caudal fin
- Elongate bodies
- No appendages
- No true vertebrae (notochord instead)
Which group are the extant agnathostomes thought to be descended from?
Anaspids.
What are 3 major synapomorphies of vertebrates?
- A centralised nervous system whereby the spinal chord has enlarged at one end to form the brain.
- 3 paired sensory organs in the head; optic, olfactory and otic capsules
- Development of the vertebrae to form the backbone.
The neural tube and notochord in chordates are homologous to which structures in vertebrates?
The spinal chord and backbone respectively.
From which period of geological time do we begin to see well-preserved vertebrate specimens?
The Ordovician.
How do agnathostome gills work?
By tidal flow.
