Notes on Vertebrates and lower chordates

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Nancy Loadman

8:40 PM What is a Chordate? Shared Derived Characters: •Paired pharyngeal gill slits •Dorsal tubular nerve cord •Notochord • Post-anal muscular tail •Subpharyngeal gland that binds iodine (endostyle or thyroid) Pharyngula Stage in Vertebrate Development • pharyngula stage shows the basic Chordate characteristics present in all vertebrates Traditional Classification Phylum Chordata Subphylum Tunicata/Urochordata tunicates Subphylum Cephalochordata amphioxus Subphylum Vertebrata animals with backbones Cladistic Terminology •Outgroups/sister groups are the most distant relatives to the other species. •Sympleisiomorphy — shared primitive trait such as the vertebrae (backbone) of tetrapods. This represented by a node on the Cladogram. Every species on the right of the branch shares the character. •Synapomorphy - a shared derived trait found among two or more taxa and their most recent common ancestor, whose own ancestor in turn does not possess the trait. i.e. shared traits that have originated with the last common ancestor • Cladograms are hypotheses which use synapomorphies to establish relationships among clades. •Clades are referred to as monophyletic groups They contain a common ancestor and all the descendants e.g. mammals are monophyletic — contains all living and extinct mammals plus their ancestors •Parisomy- the least number of charges is most desirable when forming a Cladogram. •Polyphyletic - unrelated organisms descended from more than one ancestor. e.g. artificial group of birds and bats (both have wings) in which common ancestor of the two groups not included •Paraphyletic — some descendant groups are not included (e.g. birds and mammals not included in Reptilia) •Evidence for phylogeny (evolutionary relationships) comes from morphology, fossil evidence, molecular biology (e.g. RNA, DNA, proteins) and development (embryology and patterns in gene regulation “evo-devo”= evolutionary developmental biology). Hemichordates (e.g. acorn worms and pterobranchs); 80 species 2cm-2m •Pharyngeal gill slits •Deuterostomes, but not chordates •Filter feeders that burrow. Fill collar celum to anchor and use circular muscles of proboscis to move forward and tighten, pulling forward. •Proboscis is ciliated to make water and food particles enter mouth before proceeding to the pharynx where it will be covered in mucus before moving to the gut. •Lacks post-anal tail. •They have stomochord ("mouth cord"), composed partially of chitin. Extant Non-Vertebrate Chordates The Protochordates (“Lower Chordates”) • marine filter feeders — use cilia to create a current to bring in food particles which are then trapped on mucus strands produced by an endostyle Tunicates/Urochordates — 2000 species •Ascidians- sea squirts (1900 sp.) o Tunic: Tunicin and rare vanadium metal o Pharynx takes up most of body o Incurrent siphon and excurrent siphon for filter feeding o Gill slits are spirals to increase surface area. •Ascidian Larva o Has chordate characters o Dispersal phase in which larvae move to a new area o Adhesive cilia help them plant themselves to ground for adulthood. Ascidian • Ascidian Diversity o Solitary or Colonial (with a stalked configuration) o Carnivorous Ascidians with an incurrent siphon that acts as a trap. o Invasive Tunicates- lack of predators and warming has allowed numbers to explode. These tunicates attach to things and go from port to port quickly, entering new niches. They have become a problem for shellfish fishermen by out competing mussels. • Thalacians (e.g. salps) o Barrel shaped, see through, and supported by internal water. o Sometimes parasitic crustaceans will lay eggs and incubate them through two molts. o Pelagic tunicates e.g. salps o Derived from adult ascidian form o Solitary forms produce chains of salps asexually o The aggregate phase is when salps form a chain to reproduce sexually. o Salps Sequester Carbon • Swarms covering several km often last for months and are capable of sequestering several thousand tons of carbon a day • Antarctic salps have become increasingly common as sea ice declines, may outcompete krill o Pyrosoma- a bioluminescent colonial pelagic Thaliacean • Appendicularia (Larvaceans) o Tubular organism secrets gelatinous house to live in. They use their tail region to create a feeding current o They have a filter to prevent entry of large particles. o When they reverse movement of the tail, and thus the current, they blow out particles from the screen. o They may leave and create new house if the screen becomes too difficult to clean. o Bathochordaeus- produces gelatinous house that is 1 meter wide while organism itself is 3 cm long. Cephalochordates — 22 species • Amphioxus- a coastal marine filter feeder o Branchiostoma sp. 22 species (official genus) o Chordate characters are well represented in amphioxus • Notochord (with muscles that contract to stiffen), Endostyle, Pharyngeal gill slits, and nerve cord are present • Extreme rostral extension of notochord o Locomotion of Cephalochordates- myomeres, myoseptae are shared characters with vertebrates. When myomeres contract, notochord resists compression and body bends for locomotion and burrowing o Amphioxus Pharynx vs. Vertebrate Pharynx • Pharynx functions only in filter feeding, not gas exchange. • Cilary action brings food to mouth, not muscular pumping. • Epipharyngeal groove collects mucus strands plus food; endostyle produces mucus o Circulatory System • Dorsal and ventral aorta, one-chambered heart Early Chordates from the Cambrian • Pikaia is a stem chordate- Myomeres, notochord • Recent Pikaia Study- 114 Specimens: • Conclusion: Pikaia's places as a stem chordate is strongly supported • Conclusion: Pikaia is specialized and amphioxus is more basal Chengjiang fauna 522 mya • Vetulicolians Stem Deuterostomes • No apparent notochord • Pharyngeal gill slits • Endostyle? Yunnanozoans • Haikouella
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