Chordates

Chordates are Deuterostomes

Protostome vs. Deuterostome

Echinoderms are sister group with chordates Bilateral symmetry Deuterostome lineage

Four Distinct Chordate Characteristics Notochord Single tubular, dorsal nerve chord Pharyngeal pouches Post-anal tail

Notochord Flexible rod-like structure Attachment point for muscles Often forms bony or cartilaginous vertebrae

Dorsal Nerve Cord Dorsal to alimentary canal and notochord Forms as a tube derived from ectoderm Anterior end thickens to form brain

Pharyngeal Pouches Openings in pharynx to outside Early chordates – filter feeding Evolved into gills, ear canal, parathyroid gland Eustachian tube etc…

Segmentation and Postanal Tail Segmented muscles act on notochord Tail structure extends past anus Swimming

Subphylum Urochordata Tunicates

Tunicate Body Plan Adult Sessile filter feeder Pharyngeal slits used for capturing food in mucus Most Chordate features not exhibited in adult form

Tunicate Development

Competing hypotheses Cephalochordate hypothesis: vertebrates evolved from essentially lancelet-like ancestors Calcichordate hypothesis: vertebrates evolved from armored bilateral echinoderms which were early chordates, distantly related to modern echinoderms.

Cephalochordate hypothesis Lancelet-like organism is the common ancestor of all vertebrates Cephalochordate Pikaia Rare fossils that extend back 530 million years

Calcichordate hypothesis The calcichordate hypothesis Not widely accepted Calcichordates Echinoderm-like skeletons Ambiguous chordate-like features Chordate features may be homologous with echinoderm structures May be too young to be direct vertebrate ancestors

Calcichordate Has both echinoderm and chordate features? Probably a filter feeder

Tunicates and Chordate Evolution Garstang’s Hypothesis Paedomorphosis – larval form does not develop into adult

Subphylum Cephalochordata Amphioxus Free swimming filter feeder Has all basic chordate features

Conodonts Early Chordate with some vertebrate features Conodont teeth are very common fossils, known since early 1800s, but animal body only described in 1983

Conodonts May or may not be true vertebrates Vertebrate characters sense organs with capsules CaPO4 mineralization Non-vertebrate characters V-shaped myomeres notochord but no trace of vertebrae

Subphylum Vertebrata Features Cranium – brain encased in cartilage or bone Most have cartilage or bone endoskeleton Complex nervous system

Development of Head Structures Derived from neural crest cells Cells that migrate during early development and form various structures

Neural Crest Migrations Facial and skull bones Lower Jaw Hyoid Heart

Development of Vertebrae Cartilage or bone surrounds notochord and nerve cord Segmented to allow flexibility

Development of Gills Capillaries line pharyngeal slits Increases surface area of gas exchange

Development of Complex Nervous systems Anterior brain Anterior sense organs Vision Auditory Olfactory

Early Vertebrates Agnathans Gnathostomes Ostracoderms Jawless Placoderms Jawed

Early Vertebrates

Evolution of the Jaw