Origin of Chordates

Protostomes

Digestive Tract	Make their mouth first
Cleavage	Are spiral (off center)
Coelom	Schizocoelic: mesoderm splits
Skeleton	Ectoderm

Deuterostomes

Digestive Tract		Make their anus first
Cleavage		Are radial
Coelom		Enterocolic: outpocketing of the gut (endoderm)
Skeleton		Mesoderm

What are the Chordate characters?

1. Notochord: flexible cord 

2. Dorsal hollow nerve cord 

3. Pharyngeal slits 

4. Post-anal tail

5. Myomeres: segmented muscle blocks 

6. Endostyle (ventral food tract): collects iodine —> Will become Thyroid gland 

• NDPPME: Nate Does Phone Past My Eating Time 

Which Protochordate is most closely related (i.e., the sister taxon) to vertebrates? How do you know?

The Protochordate that is most closely related to vertebrates (i.e., the sister taxon) is the Urochordata due to their neural crest.

What taxa are referred to as “Protochordates” and what are their main features and/or significance?

The taxa that are referred to as “Protochordates” are single, invertebrate, filter-feeding marine organisms that have a close relation to vertebrates. The taxa include subphyla Cephalochordata and Urochordata, and the phylum Hemichordata. However, each chordate characteristic evolved as the organisms evolved and split into their subphyla, meaning they didn’t all have the same chordate characteristics. These are their main features, and the significance is that they evolved with chordate characteristics, but they lack a true skull:

1. Notochord 

2. Dorsal hollow nerve cord

3. Post anal tail

4. Pharyngeal slits 

5. Primitive filter feeders

No, Don’t Post Pictures, Please!

Why are Echinoderms thought to be close relatives of Chordates?

Echinoderms are close relatives of chordates because they are both types of deuterostomes, they’re bisymmetrical, and they have pharyngeal slits.

What are two theories about the origin of chordates? 

1. Unity of Plan: was the original theory of St. Hillcaire: Annel/Arthropod Theory: 

   1. All animals share a common body plan, meaning their homologous parts are organized similarly, but they vary in shape and size. He believed that the body plan of Annelids and Arthropods flipped upside down to create a dorsal nerve chord. However, this did not account for the fact that the nerve chord went from solid to hollow. There is no evidence to support this theory.

2. Garstang’s Theory: 

   1. Chordates evolved from echinoderms through paedomorphosis. 

      1. Paedomorphosis: the alteration in developmental timing