Lecture 11.docx

Lecture 11: Vertebrates – Their Origins and Evolution Cambrian Explosion The full range of known phyla of animals appear Most particularly, the phyla with bilateral symmetry Bilateral animals come with two major body forms Protostomes Blastopore becomes mouth, 2nd pore become anus Nerve cord runs ve...

Lecture 11: Vertebrates – Their Origins and Evolution Cambrian Explosion The full range of known phyla of animals appear Most particularly, the phyla with bilateral symmetry Bilateral animals come with two major body forms Protostomes Blastopore becomes mouth, 2nd pore become anus Nerve cord runs ventrally Dueterostomes Blastopore becomes anus, 2nd pore becomes mouth Nerve cord runs dorsally A differentiation based on developmental styles, and two ways of organizing the nerve chord Ventral vs Dorsal Position of Nerve Chord Protostomia = Ventral Deuterostomia = Dorsal Urbilateria: Considered to be the last common ancestor of all bilaterian animals, which includes a vast array of creatures ranging from insects to humans. The term "Urbilateria" is derived from "Urbilaterian," meaning the common ancestor from which bilateral symmetry evolved. The first chordates (i.e., animals with a dorsal nerve chord and a notochord (stiffening rod made of cartilage)) Pikaia – The First Known Chordate Amphioxus (lancelet) – Similar anatomy as Pikaia Problem: The closest chordate relatives of vertebrates aren’t the lancelets, but things that look like sponges Tunicate Evo-Devo again Vertebrates arose from a neotenic tunicate Agnathan vertebrate – sea lamprey Basic chordate structure: Notochord Dorsal, hallow nerve cord Pharyngeal slits or clefts Muscular post-anal tail Ammocoete larvae of lamprey: Dorsal nerve cord Notochord From Chordate to Vertebrate Skeleton added to chordate body plan – why? Increased size, active predator Modern day agnathan – eating without a mouth Skeletal elements increase support for greater muscle mass and so improved movement (e.g., speed, force) Initially, the skeletal elements were cartilaginous Increased size also increases range of predators faced Sea Scorpions Early Diversity of Jawless (agnathan) fish Note the presence of external armour (dermal skeleton) Prompted deposition of external bony shields (and replacement of cartilage with bone in some vertebrate lineages) Whether cartilaginous or bony, the key feature of vertebrates is the vertebral column Trunk vertebra vs Caudal vertebra Trunk vertebrae are connected to the ribs and feature neural arches and spines, whereas caudal ones have no ribs. Most extant vertebrates have jaws But there is another important change in the chordate to vertebrate transition The Advent of the Brain From Whence Jaws? Early Jawed Fishes (Placodemrs) Gill arches one and two were lost; three became modified to form a hinged jaw Modern Jawed Fishes (Cartilaginous and bony fish) Gill arch four also became modified to form a heavier, more efficient jaw The structural supports of the gill slits From Whence Legs? Ray-finned fish (Actinopterygii) – Sockeye salmon (Oncorhynchus nerka) Lobe-finned fish (Sarcopterigii) – Coelacanth (Latimeria chalummae) Ray Fin Most bony fish are ray-finned. These thin fins consist of webs of skin over flexible spines. Lobe Fin Lobe-finned fish, on the other hand, have fins that resemble stump-like appendages Tetrapod’s are derived from lobe-finned fish How? Tiktaalik roseae Late Devonian (375 mya) sarcopterygian discovered in the Canadian artic. Even though a fish (scales and gills), it has some features akin to tetrapod’s (triangular, flattened head and limb-like fins) Like a land-living animal Neck, wrists, flat head, expanded ribs Like a lobe-finned fish Fins, scales, primitive jaw Modified boney elements of lobe-finned fish Water brain or land brain? Vision likely to be more useful on land compared to deep, murky or turbulent waters Sensing daylight (photoperiod) is only useful for shallow water or land-based animals The lateral line sensory canal only works in water and so this was lost in early tetrapods who left the water Balance/locomotion requirements differ between fish moving in 3D and tetrapods walking across flat ground. Dorso-ventral flattening of the head facilitates visual scanning above the water (Tiktaalik) Once the tetrapod limb evolved it could be modified to fulfill many different functions In an evolutionary tree, traits are passed from ancestor to descendant