Chapter 34 Vertebrates

Chordates Chordates describes animals that have bilateral symmetry and belong to clade known as deuterostomia Vertebrata is the subphylum of chordata Shared characteristics of chordates a. Notochord – flexible and fibrous tissue rod that is remnants of vertebrae, becomes cushioned disks in vertebral column of humans b. Dorsal, hollow nerve chord - primitive form of brain and spinal cord, can be dorsal to notochord c. Pharyngeal slits – water circulation; modified gas exchange d. muscular tail

Tunicates Also know as Sea Squirts Have 2 life stages : 1. larval stage -have bilateral symmetry -free swimming -notochord supports whole tail *stage may only last hours or days 2. Adult life stage – -lose tail, notochord, and most of nerve cord- which becomes cluster of cells called ganglion -water enters through oral siphon and exits atrial siphon -water will be ejected out of atrial siphon to clear feces or digested material -have circulation system with heart and blood vessels *Adults become sessile and can be solitary or colonial

Lancelets Adult and larvae stages, but no real change is body shape or motility Notochord runs full length of body Muscles in V-shaped segments that pull side to side for wiggly swimming motion Nervous system has hollow nerve cord that divides into segmented nerves that branch out and control muscle contraction Small sliver or arrow shaped organisms Inhabit sandy or muddy bottoms of warm ocean waters Outer body surface is nearly transparent and internal organs are visible Feed on diatoms or decaying food matter Characteristically hide in sand during day with mouths sticking out to feed, swim about at night

Craniates Next evolutionary checkpoint of chordates was the development of a head with eyes, sensory organs, and a nerve cord Two sets of Hox genes compared to one in tunicates and lancelets Neural crest – cells that appear near dorsal margins of closing neural tube - cells disperse and become parts of teeth, bones or cartilage of the skull, dermal layer of skin, and neurons or sensory capsules in the eyes Example: Hagfish – notochord, eyes, brain and ears, no vertebrae, no jaw

Vertebrates Branching from craniates, development of transcription factor genes ( Dlx) that formed the elementary nervous system and skeleton More extensive skull developed and vertebrae formed over spinal cord Lampreys are the earliest known vertebrates - cartilage skeletal network, jawless Conodonts – lamprey-like soft body vertebrate, large eyes, and early signs of mineralized dental tissues

Gnathostomes Jawed vertebrates with teeth - developed from modification of skeletal rods between pharyngeal gill slits Other features of gnathostomes: - additional duplication of Hox genes (4 cluster) - enlarged forebrains – enhanced sense of vision and smell Lateral line system – rows of microscopic organs sensitive to vibrations in the water Examples: sharks, rays, ray finned and lobe finned fish

Tetrapods 365 million years ago fins of lobe fins developed into limbs and feet Tetrapod – four feet Limbs supported body weight, feet had digits to transmit muscle forces on ground Pelvic girdle attached to hind legs and fused with backbone Appearance of one neck vertebra( up and down) then a second vertebra (side to side) No gill slits, pharyngeal clefts became ears, glands, and other structures Example: amphibians

Amniotes Group of tetrapods named for the amniotic eggs with 4 specialized membranes that protect and nurture developing embryo Other features include less permeable skin and ability to use rib cage to ventilate lungs Example: reptiles, birds, and some mammals

Mammals One step further in evolution, mammals have hair, fat layer to retain heat, and mammary glands to produce milk to nurse young Larger more developed brains Differentiation of teeth Synapsids – early group of mammals lacking hair, sprawling gate, and laid eggs - temporal fenestra – hole behind eye sockets

Mammals During Mesozoic era three lineages emerged: 1. Monotremes – egg laying, hair, produce milk, no nipples (Ex: platypus, spiny anteater) 2. Marsupials – mammals with pouch, live young develop in pouch, hair, milk, nipples (Ex: wombat, kangaroo) 3. Eutherians – placental mammals, hair, milk, longer gestation period (Ex: primates, lemurs, humans)

Primates Primates have hands and feet for grasping objects and climbing Flat nails on digits instead of claws Large brain, flat face, short jaws Forward looking eyes, close together Big toe on each foot separate from other toes Opposable thumbs- mobile and separate from other digits giving more precise manipulation and “power grip”

Hominoids Features: longer arms, short legs, no tail, larger proportional brain, and social organization

Hominids Homo sapiens – bipedal (upright on two legs) larger distinct divisions of brain – complex language, symbolic thought, and manufacture and use of complex tools reduced jaw bones and muscles shorter digestive tract Sahelanthropus tchadensis – oldest known hominid that resembled primitive characteristics of humans

Hominids Two common misconceptions: 1. Hominids are not necessarily chimpanzees, separate branch exists and each acquired own characteristics after divergence from common ancestor 2. Homonids are not an organized ladder leading to modern day Homo sapiens *several hominid species coexisted at one time, disorganized alignment of characteristics passed on and breaking away from other groups- one gave rise to homo sapiens others became extinct

Timeline of Hominids