Vertebrate Zoology Chapter 3 Agnathans and Gnathostomes PDF

Agnathans and Gnathostomes Jawless Fish and the Evolution of Jaws Geologic Time & The Fossil Record Earth formed many, many years ago Each period or epoch is distinguished by change – either in landmasses, types of organisms, or asteroid impact “-zoic” means animal, so we first started...

Agnathans and Gnathostomes Jawless Fish and the Evolution of Jaws Geologic Time & The Fossil Record Earth formed many, many years ago Each period or epoch is distinguished by change – either in landmasses, types of organisms, or asteroid impact “-zoic” means animal, so we first started seeing animal life at the end of the Precambrian Ordovician, Silurian, and Devonion – “Age of the Fishes” Permian, Triassic, Jurassic, and Cretaceous - “Age of the Dinosaurs” Tertiary – Mammals start to show up Quaternary – Genus Homo appeared – Pleistocene – Cavemen – Holocene – Modern man What Did the Earliest Vertebrates Look Like? Pikaia from the Burgess Shale beds in British Columbia Invertebrate that clearly possessed a notochord & V- shaped myomeres, which place it in the clade Chordata Possibly a cephalochordate?? Early Vertebrate Evidence A small fish-like animal, Haikouella, was recently described from early Cambrian beds of Haikou, China It had chordate synapomorphies (dorsal nerve cord, notochord, pharnyx) Also had several vertebrate synapomorphies (pharyngeal muscles, paired eyes, enlarged brain) BUT, no cranium, no distinct forebrain – not a vertebrate! Possibly a sister group to Vertebrata?  For decades geologists found odd tooth-like fossils in Paleozoic marine sediments Conodonts In the 1980s, whole fossils were found that showed myomeres, teeth, paired eyes, notochord, etc. Almost certainly members of clade Vertebrata, although position is uncertain  Two lines of evidence which are thought to support a marine origin hypothesis for vertebrates Environment 1. Earliest fossils were found in marine sediment. of Early 2. All nonvertebrate chordates and Vertebrate deuterostome invertebrates were from marine origins and had body fluid Evolution concentrations same as their surroundings. Hagfishes also have concentrated body fluids which likely represent the original vertebrate condition.  * * * * Simplified Phylogeny of Early Vertebrates * Asterisk indicates current groups † Daggers indicate extinct groups  Cyclostomes – Extant Jawless Fish Agnatha (means no jaw) includes hagfish and lampreys Lack internal ossification Lack paired fins Lack scales Lack jaw Lack complex reproductive structures Single nostril Velum “pump” - pharynx Have rudimentary vertebral precursors – arcualia Single circulation Muscular “tongue” with keratinous teeth  Myxiniformes - Hagfishes ~75 species ~0.5m long Marine cold water Scavengers Slime glands (90-200) Single opening on the front of their head that allows for water intake (olfaction and gill ventilation) Gill openings have been moved posteriorly (bury head in prey) 5-16 pairs of gill openings Females outnumber males 100:1 https://www.youtube.com/watch?v=lZq4Dme7wi4  ~40 species, both freshwater and marine Petromyzontida Free-living and - Lampreys parasitic species 10 cm to 1 m length Temperate waters  Lampreys have 7 pairs of gill pouches Their mouth is located under the oral hood Larval lampreys take water in through their mouths then pump it over their gills (flowthrough ventilation) Adult lampreys use tidal ventilation where water is both drawn in and expelled through their gill openings  Larval lampreys use their gills to filter feed Most adult lampreys are parasitic – Suction to the body of another vertebrate (usually a fish) and use their tongue and oral hood to rasp a shallow wound – Don’t kill their host, but leave them weakened with an open wound  Lamprey Spawning Cycle Adults migrate from lakes or ocean into small streams in late winter/spring (anadromous) ♂ build nests & are joined by ♀; fertilization external Eggs hatch in ~ 2 weeks, releasing small larva called ammocoetes Ammocoetes remain in nest until ~ 1 cm in body length, then burrow into substrate & filter feed; remain in larval stage = 3-7 yrs, then rapid metamorphosis to adult Free-living species don’t feed as adults, & die after spawning (within a couple of months of metamorphosis) Parasitic species migrate back to lakes/ocean & live as parasites on other species of fishes  Humans vs. Lampreys The sea lamprey (Petromyzon marinus) moved up the St. Lawrence River into Lake Ontario Niagara Falls was a barrier until construction of Erie & Welland Canals in 1913- 1918, this species entered Lake Erie first, then sequentially invaded Lakes Huron, Michigan, & Superior. In combination with over-fishing, lampreys caused a collapse of the Lake Trout fisheries by 1950 Lampreys then moved on to other species, and went into decline with depletion of food sources, and the implementation of control measures Ostracoderms 1st unambiguous vertebrate fossils Jawless, possess true bone Median fins (dorsal, anal, caudal) Heavy armor-plating Gnathostomata  Jaws and teeth – firm grasp, greater food resources Two sets paired limbs – pectoral and pelvic; better movement Jointed gill arches – double-pump ventilation Gnathostome Mouth and gill arches create an internal pump, Derived and have different structures that create an external pump depending on the clade Characters Vertebral column with segmented vertebrae Nerve fibers have myelin sheaths Epaxial and hypaxial muscles in trunk Ribs between myomeres (helps locomotion) Heart has conus arteriosus in addition to sinus venosus, atria, ventricles Gnathostome Body Plan  Chondrichthyan – tooth whorl rests on jawbone, not embedded Some bony fishes, modern amphibians, Extant and lizards – teeth set in shelf of inside jawbone Gnathostome Most bony fishes and Teeth some reptiles – teeth fused to jawbone Mammals – teeth set in sockets and attached by periodontal ligaments  Origin of Jaws Gnathostome jaws formed from the mandibular arch, with the hyoid arch forming the jaw supports HOX genes duplication event allowed for more complex structures like jaws When Animals Learnt to Bite, the Evolution of the Jaw Bone - YouTube Cyclostome vs. Gnathostome Gill Placement Vertebrate Nose Cyclostomes have a single nostril (monorhiny) Gnathostomes have double nostrils (diplorhiny) This is important for brain shape – The nasal capsules (olfactory) are very close to the forebrain in cyclostomes, making it short, but in gnathostomes the nasal capsules are more anterior, allowing the brain to be longer Origin of Paired Appendages Pectoral fins were the first paired appendages Fish swim by flexing their bodies and tail back and forth. They stretch or expand their muscles on one side of their body while relaxing the muscles on the other side. This moves them forward through the water. Gnathostomes have distinct head region between head and neck. Advantages of Fins