Exploring the Chordate Family Tree: From Sea Squirts to Birds, Slides of Biology

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Chapter 34a:

Vertebrates

(sea squirts

through birds)

Subphylum Urochordata

The urochordates include the tunicates, a.k.a., sea squirts

The tunicates are mostly sessile, filter feeding animals that look almost nothing like a chordate

However, their larval form possesses all of the basic characteristics of a chordate

Tunicate Larva

The notochord serves as a primitive internal support structure

The notochord is a stiffened but still flexible rod found between the ventral (front/bottom) gut and the dorsal (back/top) nerve cord

Subphylum Cephalochordata

Cephalochordates, the lancelets, retain a chordate body plan throughout life

In addition to the four defining chordate characteristics, lancelets, as well as tunicate larvae, have somites

Somites are blocks of musculature arranged in segments along the bodies of lancelets and fishes (as well as derivations of this segmentation found in tetrapods)

Class Cephalaspidomorphi

Class cephalaspidomorphi is represented in extant lineages by lampreys

Lampreys lack jaws, like their non-vertebrate, chordate ancestors and hence are known as jawless fish

Jawless fish were the vertebrate forerunners of the jawed fish

Lampreys in addition lack paired fins

Lamprey: Jawless Vertebrate

Lamprey have vertebrae (backbones) but no jaw

As vertebrates, lampreys are (just barely) members of subphylum Vertebrata Docsity.com

Superclass Gnathsomata

Jawed fish are referred to as gnathostomes (superclass gnathsomata) for their defining feature: jaws

Jawed fish also have skulls, vertebrae, and paired appendages (fins)

Early Gnathostomes

Vertebrate characteristics include a head, vertebrae, endoskeleton, and a closed circulatory system Many vertebrate characteristics are adaptations to vigorous swimming

Most also have ribs, jaws, and paired appendages (fins, legs)

Jaws, of course, aid in eating

Especially carnivorous eating…

Osteichthyans are Bony Fish

 The bony fish are all of the fish you are familiar with except the sharks, rays, and chimaeras  The skeleton of bony fish displays ossification, i.e., calcium phosphate in addition to the cartilaginous base  Bony fish additionally possess flattened scales (unlike those of sharks)  Bony fish also secrete mucus onto their skin to aid in reducing their coefficient of drag  Bony fish additionally possess a gas-filled swim bladder that allows them to adjust their buoyancy to match that of the water

• Sharks and rays lack a swim bladder and consequently sink when not moving
• The swim bladder is derived from lungs, an adaptation by early bony fish to shallow, fresh-water living Docsity.com

A Coelacanth

Lobe fins are supported by musculature and a bony skeleton; they may be employed for “walking” upon the bottom and other substrate found within bodies of water

Tetrapods

The tetrapods are the terrestrial descendants of some lobe- finned fish, starting approximately with the amphibians

Amniotic Egg

The amniotic egg is shelled, an adaptation to desiccation prevention

The amniotic egg employs extraembryonic membranes to transfer stored nutrients and water, exchange gasses, and remove wastes