Chordates Ch23

Chordate definition

Animals in phylum Chordata characterized by a notochord at some stage of development

Major chordate subphyla

Tunicata, Cephalochordata, Vertebrata

Notochord

Rod-like flexible structure providing skeletal support for swimming muscles

Dorsal tubular nerve cord

Hollow nerve cord dorsal to the gut that develops into brain and spinal cord

Pharyngeal pouches and slits

Openings in the pharynx used for feeding or respiration

Endostyle / thyroid gland

Mucus-secreting groove that evolved into the thyroid gland in vertebrates

Postanal tail

Extension of body beyond the anus primarily used for locomotion

Chordate developmental mode

Deuterostome development with anus forming from blastopore

Difference between homologous and analogous structures

Homologous share ancestry; analogous share function only

Subphylum Tunicata characteristics

Invertebrate chordates with tunic; adults retain few chordate traits

Sea squirt larvae significance

Larvae possess all five chordate characteristics

Unique tunicate heart feature

Heart periodically reverses blood flow direction

Subphylum Cephalochordata

Lancelets that retain all five chordate traits throughout life

Amphioxus importance

Most closely resembles ancestral vertebrate body plan

Vertebrata defining trait

Presence of vertebral column and/or cranium

Early vertebrate adaptations

Musculoskeletal, physiological, and neural innovations

Neural crest cells

Embryonic cells forming skull, jaws, teeth, and cranial nerves

Ectodermal placodes

Epidermal thickenings forming sensory organs

Tripartite vertebrate brain

Forebrain, midbrain, and hindbrain

Vertebrate sensory advances

Vision, hearing, smell, taste, lateral line, electroreception

Early fossil chordate example

Pikaia from Burgess Shale

What are the 3 famous early vertebrate fossils

Myllokunmingia, Metaspriggina and Haikouichthys

Ostracoderms

Armored jawless fishes of Ordovician–Devonian periods

Placoderms

Early jawed vertebrates with heavy armor

Evolutionary significance of jaws

Allowed active predation and exploitation of new food sources

Origin of vertebrate jaws

Derived from mandibular (first gill) arch

Gnathostome definition

Jawed vertebrates with paired fins