60: Animal evolution before vertebrates

Question 1

What does Phylum Chordata encompass?

Answer 1

Vertebrates and their kin, characterized by having a notochord at some developmental stage.

Question 2

Which organisms are included in Phylum Echinodermata?

Answer 2

Marine organisms like sea stars, sea urchins, and sea cucumbers.

Question 3

What does Phylum Hemichordata consist of?

Answer 3

Acorn worms and graptolites.

Question 4

What creatures are in Phylum Xenoturbellida?

Answer 4

Worm-like creatures with simple structures.

Question 5

What is the notochord in chordates?

Answer 5

A flexible rod along the dorsal side of the gut, present at some stage in all chordates.

Question 6

What happens to the notochord in adult chordates?

Answer 6

It provides embryonic support and is replaced by vertebral disks in adults, remaining as intervertebral disks.

Question 7

What is the function of the dorsal nerve cord in chordates?

Answer 7

It develops beneath the dorsal surface and differentiates into the brain and spinal cord post-gastrulation.

Question 8

What is the role of pharyngeal slits in chordates?

Answer 8

They connect the pharynx to the exterior and are functional as gills in aquatic chordates, present in embryonic stages of terrestrial animals.

Question 9

How does the postanal tail develop in chordates?

Answer 9

It extends beyond the anus during embryonic development and regresses to become the coccyx (tailbone) in humans.

Question 10

What is segmentation in chordates evident in?

Answer 10

The arrangement of muscles and vertebral columns.

Question 11

How did the thyroid gland evolve in chordates?

Answer 11

From a ventral groove in the pharynx of primitive chordates, and in vertebrates, it develops into the thyroid gland, regulating metabolism and growth.

Question 12

What is the origin of neural crest cells?

Answer 12

Cells derive from the lateral edges of the neural plate during neurulation.

Question 13

How are neural crest cells formed?

Answer 13

These cells form the neural folds, which then produce the neural crest between the neural tube and epidermis.

Question 14

What influences the functionality of neural crest cells?

Answer 14

Molecular signals from adjacent mesoderm and ectoderm.

Question 15

What do neural crest cells become and what do they migrate into?

Answer 15

Neural crest cells become mesenchymal, migrate ventrally, and differentiate into:
- peripheral nervous system ganglia
- pigment cells
- adrenal medulla
- various endocrine tissues
- cartilages of the visceral skull.

Question 16

What does the axial mesoderm form?

Answer 16

The notochord.

Question 17

Where is the paraxial mesoderm located and what does it form?

Answer 17

Adjacent to the notochord and neural tube, subdivides into somites as neurulation progresses.

Question 18

What are the two layers of the lateral plate mesoderm?

Answer 18

Visceral (Splanchnic) Mesoderm (inner layer) and Parietal Lateral Mesoderm (outer layer).

Question 19

What are the functions of the lateral plate mesoderm?

Answer 19

Lines the body cavity, develops into the circulatory system, forms smooth muscles of the digestive tract, and contributes to the bulk of the connective tissue.

Question 20

What does each somite divide into?

Answer 20

Dermatome, Myotome, and Sclerotome.

Question 21

What is the function of the dermatome in somite subdivision?

Answer 21

Forms the dermis of the dorsal part.

Question 22

What is the function of the myotome in somite subdivision?

Answer 22

Develops into skeletal muscles.

Question 23

What is the function of the sclerotome in somite subdivision?

Answer 23

Generates the vertebrae and ribs.

Question 24

What is the monophyletic origin of chordates?

Answer 24

Chordates derive from a single common ancestor.

Question 25

What is the fossil evidence for the earliest chordates?

Answer 25

Earliest chordate fossils are from the Early Cambrian Chengjiang Fauna.

Question 26

What remains a mystery about the evolution of cartilage?

Answer 26

The initial development of cartilage remains unclear.

Question 27

What do comparative studies of cartilage in cuttlefish and horseshoe crabs suggest?

Answer 27

Similarities with chordate chondrocytes, suggesting a shared evolutionary origin.

Question 28

What is a key skeletal structure in vertebrates?

Answer 28

The dermal skull.

Question 29

What are the two types of bones in the vertebrate endoskeleton?

Answer 29

Endochondral bones and dermal bones.

Question 30

How do endochondral bones form and develop?

Answer 30

They form initially as cartilage and later ossify.

Question 31

How do dermal bones develop?

Answer 31

They develop in the dermis through membranous ossification.