Deck 15: Vertebrate Beginnings: the Chordates

Question 1

Urochordates, instead of amphioxus, are now considered to be the sister group of vertebrates. Why is amphioxus, instead of urochordates, still considered a better model for the prevertebrate body plan?

Answer 1

Urochordates are a group of animals which consists of 1600 species. They are mostly sea-dwelling, residing at various depths of the sea. In the adult stage they are usually sessile and free living. They are considered a sister taxon to chordates. During metamorphosis, however, the tail and the notochord disappear.
Amphioxus is a sub-phylum of the phylum cephalochordata. Their bodies are laterally compressed. They reside in sandy sediments of coastal regions all over the world. The nerve cord in amphioxus does not expand anteriorly into a well-defined brain. Special sensory organs that are paired as seen in vertebrates are also absent.
In spite of these marked differences the amphioxus is considered a better model to analyze the body plan of the vertebrate. This is because of the following reasons which include:
1. They possess a hollow dorsal nerve cord.
2. The dorsal nerve cord in amphioxus overlies a supportive notochord.
3. They have pharyngeal slits or pouches.
4. They also have an endostyle which is meant for filter feeding.
5. They possess a tail developed postanally for propulsion.
6. They possess a hepatic cecum, functionally similar to the liver and pancreas in vertebrates.
7. The musculature of their trunk is variegated s in vertebrates.
8. Their circulatory pattern also resembles the pattern in vertebrates.

Question 2

What characteristics are shared by the deuterostome phyla that indicate a monophyletic group of interrelated animals?

Answer 2

The characteristics that are shared by the three Deuterostome phyla that indicate a monophyletic group are listed below-
• They all show radial cleavage during development.
• Their anus is derived from the first embryonic opening that is blastopore.
• Their mouth is derived from an opening of secondary origin.
• Coelom formation is by Schizocoely.

Question 3

Explain how use of a cladistics classification for vertebrates changes the major important regroupings of traditional vertebrate taxa (refer to figure 15.2 ). Why are Agnatha and Reptilia, as traditionally recognized, inconsistent with cladistic principles?

Answer 3

The cladistics classification of the vertebrates produces important regroupings of the traditional vertebrate taxa.
Examples:
• Agnatha includes vertebrates which lack jaws, have features of anamniotes, that is, vertebrates which do not develop within a fluid-filled sac
• Tetrapoda include vertebrates that have four limbs. Now there are some tetrapods that are amniotes and some are anamniotes
• Amniotes include tetrapods with embryos that have extraembryonic membranes
• Bony fishes and tetrapods together compose teleostomi
Hence, this regrouping is possible by looking at the cladistics classification of the vertebrates.
Agnatha and Reptilia are inconsistent with cladistic principles because they fail to satisfy the cladistics requirement. They are paraphyletic due to which they are not accepted in cladistics classification. Paraphyletic refers to those which originate from different phylogenetic groups.
Hence, Agnatha and Reptilia are no longer recognized in cladistics usage.

Question 4

Name five characteristics shared by all chordates, and explain the function of each.

Question 5

In debating the question of chordate origins, zoologists eventually agreed that chordates must have evolved within the deuterostome assemblage rather than from a protostome group as earlier argued. What embryological evidence supports this view?

Question 6

Offer a description of an adult tunicate that would identify it as a chordate, yet distinguish it from any other chordate group.

Question 7

Both sea squirts (urochordates) and lancelets (cephalochordates) are filter-feeding organisms. Describe the filter-feeding apparatus of a sea squirt, and explain in what ways its mode of feeding is similar to, and different from, that of amphioxus.

Question 8

Explain why it is necessary to know the life history of a tunicate in order to understand why tunicates are chordates.

Question 9

List three groups of adaptations that guided vertebrate evolution, and explain how each has contributed to the success of vertebrates.

Question 10

In 1928, Walter Garstang hypothesized that tunicates resemble the ancestral stock of the vertebrates. Explain this hypothesis and evaluate its validity based on recent phylogenetic and developmental data.

Question 11

What is the phylogenetic placement of Haikouella, and what evidence supports its placement there?

Question 12

Distinguish between ostracoderms and placoderms. What important evolutionary adaptations first appeared in each group? What are conodonts?

Question 13

What is the currently favored explanation for the evolution of vertebrate jaws?