Lecture 6: Origin of Chordates Flashcards
Pelagic
Relating to deep ocean, Planktonic
Benthic
Relating to the bottom of a body of water, Sessile
Paedomorphosis
Retention of juvenile features in descendant adults. This is a key discovery in explaining the development of chordates
Synapomorphy
An evolutionarily derived character-state that indicates sharing taxa are each other’s closest relatives
Which are the 3 most prevalent species?
- Arthropoda
- Nematodes
- Mollusca
What kind of evidence can we use to help us find where chordates originated from?
- Paleontologic evidence - based on fossil evidence
2. Neontologic evidence - based on living animals
Complications with Paleontologic evidence
- Many early species did not leave behind fossils
- No retention of soft tissue
- Lack of embryonic development
Complications with neontologic evidence
- Current living animals also underwent continual evolution
- May not reflect what was in the past
What was the original theory regarding the origins of chordates?
Originally, it was theorized that chordates originated from arthropods. This theory was based on a comparison of analogous structures rather than homologous ones. The ventral nerve in arthopods vs dorsal nerve in chordates.
Auricularian hypothesis
Another chordate origin theory. It hypothesized that chordates originated from echinoderms through the retention of juvenile features during embryologic stages. These eventually would lead to the known chordate characteristics.
What is the CURRENT theory regarding the origins of chordates?
Radial evolution of deuterostomes (prechordate).
Do protostomes and deuterostomes have bilateral or radial symmetry?
Both have bilateral symmetry
What are characteristics of protostomes?
- Means “first mouth”
- Blastopore forms mouth
- Spiral cleavage
- Coelom develops from mesoderm separation
- Ectodermal skeleton
Which groups are protostomes?
Molluscs, annelids, and arthropods
What are characteristics of deuterostomes?
- Means “second mouth”
- Blastopore forms anus
- Radial cleavage
- Coelom develops as outpouching of primitive gut
- Mesodermal skeleton
Which groups are deuterostomes?
Echinoderms, hemichordates, chordates
Do protostomes and deuterostomes share the same evolutionary line?
No, they come from two distinct and independent evolutionary lines
What are the 5 chordate characteristics?
- Notochord
- Dorsal Nerve Chord
- Pharyngeal slits
- Endostyle (or thyroid gland)
- Post-anal tail
Notochord
An elastic, fluid filled structure that provides support for the body and facilitates animal movement (swimming) by accentuates muscle actions.
Pharyngeal slits
The digestive tract chamber just posterior to the mouth. In early chordates, these facilitated feeding mechanisms. They are present in all chordates now, but are not always functional.
Not gills –> but modifications will support gills in future vertebrates.
Protochordates
- All marine animals
- Feed by cilia and mucus (filtration)
- Larval forms generally pelagic or planktonic
- Adult forms generally benthic or sessile
What are the 3 protochordate groups?
- Hemichordata - “acorn worms”
- Urochordata - “sea squirts”
- Cephalocordata - Amphioxus
How are hemichordates similar to chordates?
- Deuterostomes
- Pharyngeal slits
- Invagination of nerve chord
- Lack notochord (stomochord not equivalent) , tail
What are hemichordates’ phylogenetic affinities?
- Links to chordates and echinoderms
2. Connection between chordate ancestry and invertebrates
