EEB267 - Lecture 3

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University of Toronto St. George
Ecology & Evolutionary Biology
Deborah Mc Lennan

THE BASAL VERTEBRATA What the phylogenetic tree for Chordata looks like when we add one important fossil species: Cephalochordata Urochordata at least 540 my †
 Haikouella Vertebrata at least 530 my proto-vertebrae, true gills, two eyes, olfactory lobes at least 520 my dorsal hollow nerve cord, notochord, post-anal tail  complete notochord with irregular cartilaginous blocks in places (proto-vertebrae)   true gills (larger pharyngeal openings, cartilaginous bars in pharyngeal openings are much larger and stronger, muscles to pump water through the openings, cilia [seen on pharyngeal rods of Amphioxus and tunicates] replaced by stronger, longer filaments) Haikouella shows the first switch to a more active lifestyle in the chordates. These animals were capable of swimming for longer periods of time (true gills = better oxygen supply, higher metabolic rate) and selecting and tracking their prey via sight and smell.  many molecular characters The Vertebrata (vertebra [joint or articulation of body]) ... also known as the Craniata (from kranion [skull]) [ ----------------------------------- Vertebrata --------------------------------------- ] Myxiniformes Petromyzontiformes (hagfishes) (lamprey) Gnathostomata skull Representative outlines of the skull in a hagfish (top) and a lamprey. The skull of basal vertebrates is made of cartilage (bone hasn’t evolved yet!) Cyclostomata: kyklos [circle, round] + stoma [mouth]
 (Heimberg et al., 2010. MicroRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. Proceedings of the National Academy of Science 107:19379-19383) [ ----------------------------------- Vertebrata ---------------------------------- ----- ] [ ---------------- Cyclostomata ---------------- ] Myxiniformes Petromyzontiformes (hagfishes) (lamprey) lymphocytes (white blood cells) have unique antigen receptor genes skull Gnathostomata Grouping lamprey with hagfish was first suggested decades ago based on morphological characters. Since then there has been extensive debate about whether the Cyclostomata is monophyletic or paraphyletic, but the current combined data set (mitochondrial and nuclear genes and morphological characters) supports the original idea that the Cyclostomata is indeed monophyletic. Another rogue group: the Myxiniformes (myxa: slime) or Hagfish
 •~ 77 extant species, all are marine, temperate seas, shallow to deep waters (15-5000 m) •poorly developed eyes BUT well developed chemical (smell/taste) and touch senses •mainly scavengers (but with some predation on soft-bodied invertebrates), eat their way out of large carcasses, have the ability to absorb amino acids from the decaying animal directly across their skin/gills (***autapomorphy) For an amazing sequence of deep water hagfish feeding on a dead fish set up as bait go to: eepcam _at_2300m/index.html • tentacles around mouth are very sensitive to touch, tongue has two rows of keratinized tooth-like structures used to rasp away flesh. A hagfish protrudes its toothplates, grasps the food, and then draws the plates back into its mouth. • no metamorphosis!!! The eggs are huge and yolk-filled. The embryos develop inside the egg for 5-7 months before hatching out looking like a small adult. Details of reproduction are largely unknown: female fecundity is low, males appear to be rare, many eggs are unfertilized. ! •slime g
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