The origin of fishes Sole Mates: Evolution of Fishes can be traced to the first chordates (something like Pikaia Pikaia or the modern (Cambrian) Branchiostoma), which lacked a backbone but possessed a flexible rod of tissue called a notochord Like other chordates, these have the basic worm-like body plan, “Finding Nemo” muscle packs, and a pharynx Branchiostoma (Recent) Primitive cephalochordates: fish-like forms without backbone General Picture (but with well-differentiated head and body) Length of “spindles” in diagram represent time ranges of major groups Width of “spindles” in diagram Amphioxus denote diversity of species within each group Earliest fishes were jawless From the jawless fishes arose jawed fishes Note: The diversity of major fish groups peaked in the Devonian conodonts conodont animal Period. This period is therefore often referred to as “The Age of Fishes” 1 First True Fishes: Agnatha (jawless fishes): Modern Forms Jawless Fishes (Agnatha): Ancient, Armoured Forms (Ostracoderms) Followed by appearance of jawed fishes (all other groups have jaws) Range: Cambrian-Carboniferous Had heavy external armour, but soft, cartilaginous Representatives of the jawless fishes have survived to the internal skeleton present day, but none of these possess armour. Evolution of Jaws (Step 1) Evolution of Jaws (Step 2) The evolution of jaws is an example of evolutionary modification of existing structures to perform new functions. Jaws are modified gill arches ! Lose first couple gill arches and modify third in line into solid jaws (upper mandible is upper part of arch becomes attached Start with no jaws and many gill slits (supported by gill arches to skull, lower mandible remains free) or “skeletal rods”) 2 First Jawed Fishes (but some disagreement on this): Evolution of Jaws (Step 3) Acanthodians Distinguished by spines that supported primitive “fins” and slightly hardened internal skeleton Range: Silurian-Permian Modify next gill arch in line into secondary components of the upper and lower mandibles (red) May have been ancestors of bony fishes Cartilaginous Fishes: Chondrichthyes (Sharks, Rays, Skates) Jawed Fishes, cont’d: Placoderms Distinguished by jaws and thick plates of bony armour. Skin with “denticles” “Antiarchs” “Arthrodires” Range: Silurian-Carboniferous Distinguished by cartilaginous skeleton, exposed gill slits, and skin with imbedded denticles Range: Silurian-Recent 3 Bony Fishes: (Ray fins, Lobe fins) Rayfinned Fishes Fins supported by thin bones that radiate out from body Ray finned fishes Fins attached to body by fleshy lobe with complex internal bone structure. Forms one usually thinks of as Fins much more muscular “fishes” than in ray-finned fish Lobe finned fishes More diverse group of present- Range: Devonian- Recent day fishes, Lobe finned Fishes Lobe finned Fishes Lobe finned fishes were once fairly diverse (especially in late Paleozoic). Three major groups (Coelocanths, Lungfishes and Rhipidistians) Coelocanths A group once Only two major groups have survived to the present thought to have day: Coelocanths and Lungfishes gone extinct by the Cretaceous. Lungfishes Coelocanths (Latimeria) First living coelocanth caught off the coast of South Africa in 1938. Since then, several more have been caught and/or observed Rhipidistians 4 Lobe finned Fishes Lobe finned Fishes: Toward Tetrapods Lived in shallow ponds, probably had lungs (may have been as sister group to lungfishes) Similar skeletal structure to amphibians Rhiphidistians The only fish which had developed an Lungfishes Labyrinthodont teeth (with folded enamel) opening to the Have gills, but can also breathe using lungs provides close link with early amphibians nostrils inside the Various species live today in South America, Africa and Australia mouth - a feature These lead us to the next stage of vertebrate which is found in all During dry season, some lungfish can burrow into mud, make a “cocoon” out of skin evolution; the land vertebrates. secretions and breathe air through openings to surface. leaving the water… Why leave the water ? -new, and as yet unexploited, food resources in terrestrial realm -escape from predators -drying up of ponds (need to cross land to reach other water bodies) END OF LECTURE Challenges -support of the body in water is made possible by water, but difficult to support on land -need to strengthen shoulder and hip girdles (that supported limbs) -modify oesophagus into lungs to breathe out of water -develop ears to pick up sound waves in air 5.