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