Chordates are Bilaterian Deuterostomes

Phylogeny (cont.) 5 Classes (before tetrapods) 1.Agnatha Jawless fish; hagfish and lampreys 2.Chrondrichthes Sharks, skates and rays 3.Osteichthes Bony fish 4.Actinopterygii Ray- finned fish 5.Sarcopterygii Lobe-finned fish Sea Squirt Amphioxus Lamprey Hagfish

Chordate Origins Chordates developed during the Cambrian period, around 560 Ma Evolutionary issue There is a poor fossil record for the origin of chordates, since they were most likely soft-bodied creatures Primitive chordates (like the sea squirt) are studied closely, along with Hemichordates, to try and piece together their origins

Chordate characteristics: 1.Dorsal, hollow nerve cord 2.Notochord 3.Pharyngeal slits 4.Post-anal tail

1. Egg becomes fertilized, then divides repeatedly (cleavage) 2. Cells arranged into a round, hollow ball (called a blastula) with a fluid filled cavity (called the coelom) 3. One side of the ball indents and grows inward (gastrulation) until an opening is formed (called the blastopore) 4. Indentation continues until it breaks through the wall on the other side, creating a second opening

The notochord forms out of the mesoderm made up of a hard, fibrous material The dorsal, hollow nerve cord is derived from the ectoderm (through ‘invagination’) eventually forms the brain and spinal cord The Pharyngeal slits develop after the mouth and pharynx form, creating holes or slits into the pharynx may become gills for respiration used to filter food or eject water while eating The post-anal tail is created when the body elongates Embryonic Dev. (cont.)

Considered to be “primitive” chordates (“proto” means first) Larvae are generally planktonic, while adults may be sessile, benthic or they may burrow into the substrate May be solitary or colonial, and feed by means of cilia and mucus Male and female gonads are either found in a single individual (monoecious) or in separate individuals (dioecious) Includes subphylums: Hemichordata Urochordata (tunicates) Cephalochordates

Thought to be an intermediate group of species between the echinoderms and the chordates Acorn Worm Pterobranch Graptolites (extinct) Has pharyngeal slits and a dorsal nerve cord (may not be hollow). However, they do not appear to have a notochord (merely a stomochord) and have no post-anal tail

Acorn Worms (Enteropneusts - Hemichordata) # Species: Evolved: Ma Size: can reach over 1 meter Nutrition: Filter feeders or normal ingestion Lifestyle: Planktonic larvae, burrowing adults Support: Stomochord Musculature: Circular or longitudinal orientation Non-segmented muscles Reproduction: Dioecious or segmentation Chordate Characteristics: Pharyngeal slits and dorsal nerve cord

Acorn Worms (cont.) (Enteropneusts - Hemichordata)

Pterobranch (Hemichordata) # Species: about 20 Evolved: Ma Size: can reach 1 mm Nutrition: Filter feeders (ciliated tentacles) Lifestyle: Planktonic larvae & sessile, colonial adults Support: Stomochord and rigid, tube housing Musculature: Circular or longitudinal orientation Non-segmented muscles Reproduction: Dioecious or asexual Chordate Characteristics: Pharyngeal slits and dorsal nerve cord

Graptolites EXTINCT (Graptolithina - Hemichordata ) # Species: thought to be hundreds Evolution: Ma (extinct Ma) Lifestyle: Colonial and sessile or benthic These were tiny, tubular creatures, which when fossilized may resemble a saw blade.

Over 20,000 species - thought to be the oldest living chordates Includes: Tunicates (aka Sea Squirts; Ascidiacea) Larvacea Thaliacea

Tunicates (Sea Squirt) (Ascidiacea - Subphylum Urochordata) # Species: about 2,000 Evolved: 540 Ma Size: (small) Nutrition: Filter feeders (complex straining apparatus); eat mostly plankton Lifestyle: Planktonic larvae & sessile adults Support: Notochord and tunic Musculature: Reproduction: hermaphroditic or asexual

Tunicates (cont.) (Ascidiacea - Subphylum Urochordata)

Larvacea and Thaliacea (Subphylum Urochordata) Larvacea look more like tunicates during adulthood Thaliacea retain their tadpole-like body plan from larval state to their plankton-like adulthood (benthic) Some are sexual and others are asexual, skipping the larval state altogether May live solitary or colonial lives

About 25 species This is the first group of chordates to retain all 4 traits into adulthood Includes: Lancelets (or Amphioxus)

Lancelets (Amphioxus) (Subphylum Cephalochordata) # Species: about 20 Evolved: 520 Ma Size: up to 2 inches Nutrition: Filter feeders - plankton and algae Lifestyle: Burrowing adults Support: Notochord (extends all the way to snout) Musculature: Segmented (myotomes) Reproduction: Dioecious

Evolution of Vertebrates Debate DNA confirms neither of these diagrams are feasible Why? Larval Development basisPharyngeal slit development basis

The evolution of heads! The evolution of the head (or skull) occurred around the same time that vertebrates evolved. All species in the Subphylum Vertebrata convert their notochord into a backbone (except for the hagfish)

The notochord is now converted to vertebral plates. With the inclusion of a head (or skull), species developed a brain, eyes and more complex nervous systems (the nerve cord becomes the spinal cord). Includes: Hagfish Lampreys Cartilaginous, lobe-finned and bony fish

Hagfish (Subphylum Vertebrata) # Species: about 72 Evolved: 330 Ma Size: about 30 inches Nutrition: Parasitic or scavengers; eat mainly worms or dead creatures Lifestyle: Live in the muddy bottom of the ocean There is no known larval state Support: Notochord Reproduction: Dioecious (has both male and female gonads, but not hermaphroditic)

Lampreys (Subphylum Vertebrata and Craniata) # Species: approx. 3 Evolved: approx. 330 Ma Size: 5-40 inches Nutrition: Filter feeders as larvae & parasitic adults (feeds on the blood of prey) Lifestyle: Burrowing larvae & free swimming adults Support: Cartilaginous skeleton Reproduction: Dioecious (has both male and female gonads, but not hermaphroditic)