Chordates Overview

Deuterostomia
- Echinodermata
- Hemichordata
- - Enteropneusta
- - Pterobranchia
- Chordata
- - Urochordata (tunicates)
- - Cephalochordata (lancelets)
- - Craniata (craniates)
- - - Myxini (hagfish)
- - - Vertebrata

Summary of point made in lecture: There is some controversy about relationships among deuterostome phyla. Conventionally, echinoderms are sister taxon to (hemichordates + chordates). New molecular and morphological evidence would suggest instead that chordates are sister taxon to (hemichordates + echinoderms). This is an important distinction because it would change our reconstruction of the common ancestor of all deuterostomes from an echinoderm-like suspension feeder (like a crinoid) to more of a worm-like ancestor (perhaps like a chordate lancelet or hemichordate enteropneust worm). The classification above shows the relationship as an unresolved trichotomy (all three are regarded as sister taxa, at the same level of indentation in this unranked classification, without specifying which two of the three are closer relatives).

In lecture I also noted an inconsistency between the hand-out cladogram and the hand-out classification. The classification says:

Chordata
- Urochordata
- Cephalochordata (lancelets)
- Craniata

and the cladogram converted to a classification says:

(2) Chordata
- (3) Urochordata
- (4) unnamed clade
- - lancelets
- - (5) Craniata

In lecture I suggested that the cladogram could be changed to conform exactly to the hand-out classification by drawing a line from lancelets to node 2 and erasing the line connecting lancelets to node 4. This would make node 2 a trichotomy, as implied by the first of two classifications directly above. I also said that the cladogram is actually the leading hypothesis, and the cladogram in the text (fig. 25-3 or 10th: 26-3) conforms to this view. These details are only provided here to clear up confusion that might have resulted.

Chordate Terms

gill slits

dorsal hollow nerve cord

acorn worms (enteropneusts), pterobranchs

notochord, postanal tail, endostyle, thyroxin

tunic, pharyngeal basket, larvacean, salp

lancelet, ammocoete larva (lampreys)

Pikaia

Outline:

I. Overview of Chordate Relationships

- Chordate Classification Handout

II. Chordate Synapomorphies

III. Urochordates (sea squirts)

IV. Cephalochordates (lancelets)

V. Craniates (hagfish + vertebrates)

(lecture notes not included here)

Lecture on Aquatic Vertebrates:

Fig. 26-2 (10th: 27-2) revisions:

Delete Agnatha

Craniata Vertebrata,

"fleshy-finned" fishes includes Tetrapoda

Gnathostomes have what synapomorphy?

How did jaws evolve?

What sensory equipment do sharks have?

If sharks are members of Chondrichthyes,

- what are Osteichthyes?

What are two major subgroups of Osteichthyes?

- How are they different?

What specific group of Osteichthyes has the most species?

How do fish stay bouyant?

How do fish osmoregulate?

>Osteichthyes (bony fishes)

Osteichthyes
- ray-fins
- fleshy-fins (muscles in lateral fins)
- - coelocanths
- - Choanata (have choana)
- - - lungfish
- - - Tetrapoda (have 4 limbs)
- - - - Amphibia
- - - - - caecillians
- - - - - Batrachia
- - - - - - salamanders
- - - - - - frogs and toads
- - - - Amniota (have amniotic egg)
- - - - - Reptilia (or Sauropsida)
- - - - - Mammalia

Bony fishes - ray-fin and lobe-fin clades

ray-fins include teleosts (most vertebrates, 96% of fishes)

lobe-fins include coelocanths, lungfish, and tetrapods

Important fossil tetrapods

- - Ichthyostega, Acanthostega

What are tetrapod features and how do they reflect life on land?

Describe 3 types of respiration in a salamander.

What is paedomorphosis? Example: the axolotl

Rayfins have swim bladder to maintain bouyancy (p. 526; 10th: 516)

Two methods:

- simple (trout):

- - pneumatic duct connection to esophagus

- elaborate (diverse teleosts):

- - gas in from blood: gas gland

- - - network of blood capillaries ("rete mirabile")

- - gas removed from bladder: resorptive area

- - -

Gas gland is highly efficient:

- Example: fish living at depth of 2400 m

- - tremendous oxygen pressure differential:

- - - blood must be kept at sea surface pressure

- - - (0.2 atmosphere)

- -

- - - swim bladder must be kept inflated

- - - (> 240 atmospheres)

How does gas gland work?

- - secretes lactic acid into blood

- - forces localized release of oxygen from hemoglobin

- - oxygen diffuses into swim bladder

- - deep-sea fish have longer rete capillaries