From early origins to walking on land – Flashcards
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Vertebrates
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A subgroup of the phylum Chordata; they contain a notochord
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Notochord
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A stiff rod of cells that runs down the centre of the spine
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What places vertebrates in Phylum Chordata?
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The fact that they have a notochord
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What distinguishes vertebrates from invertebrates?
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The presence of a series of stiffened segmented elements (vertebrae) and the presence of a definite head
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What can skeletons of vertebrates be composed of?
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Cartilage (sharks) or the mineral apatite (phosphate minerals)
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What is a chordates that might be early ancestors of the vertebrates?
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Pikaia
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Pikaia
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A chordate found in the Middle Cambrian Burgess Shale; lived swimming above the ocean floor; had a proto notochord and myotomes; had tentacles at the head which caused people to think it wasn't a chordate
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Myllokunminga
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Early Cambrian; had gill pouches behind a poorly developed head and myotomes and a narrow dorsal fin
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Haikouichthys
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May have possessed a skull; characterized as one of the earliest fish but that is not widely accepted
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What were the most abundant early vertebrates?
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Conodonts
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Conodonts
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Common in marine sediments from the Cambrian to the Triassic; known to have evolved rapidly producing many species even though they used to be only known from the "conodont elements", tooth-like structures composed of apatite; possessed a series of different types of conodont element articulated into some sort of an apparatus; most were fairly small; occupied a scavenging niche
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What are conodont elements?
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Teeth; functioned to grasp, slice and then grind food
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What was found in 1982 in Carboniferous rocks in Scotland that make researchers believe it was a conodont?
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Found impressions of a creature with a notochord, myotomes, swimming fins, a clearly defined head, and large paired eyes along with conodont elements associated with the head
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What do conodonts not possess?
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A jaw; classified as a "jawless" fish
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Ostracoderms
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An Agnathan (Late Silurian); lacked a jaw; sucked sediment or small prey from the ocean floor; some forms do possess indications of lateral fin stabilizers that may have allowed greater swimming control
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When did jaws evolve?
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In the Silurian
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What did jaws evolve from?
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Developed from "gill arches" and may have initially been an adaptation to more efficiently pump oxygen over the gills
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After jaws evolved, what organism rapidly evolved because of this?
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Gnathostomes
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Placoderms
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Some of the first jaw bearing fish; large; did not have modern teeth; had bony plates over the heads and shoulders that made them formidable to predators to account for lack of teeth
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Chondrichthyes
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Devonian; cartilaginous fish; includes sharks
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Acanthodians
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Smaller fishes (50-500mm); had defensive spines on their fins and their undersides; possibly swam in schools (shoals) in open water
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Ray Fins
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Generally lightly built, small and currently dominate marine and freshwater environments; radiation of ray fins is often concurrent with pulses in radiation of the Chrondrichthyes (sharks) - suggests some kind of evolutionary arms race between the two groups
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Lobe Fins
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Have slower more powerful fin stroke than the Ray Fins; important for understanding later terrestrial vertebrates; include modern lungfish
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What are Devonian Lobe Fins regarded as? What are they a key piece of proof for?
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Regarded as being the ancestors of all the land animals that were to follow; key for the Darwinian concept of transitional forms
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Transitional forms
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Generally appear in the fossil record during times of environmental stress or change; the changes allow for mutants that would normally be outcompeted
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What do all tetrapods share? (5 things)
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1) A spine consisting of interlocking spurs 2) A pelvis attached to a backbone that supports the weight of the animal 3) All possess a rib cage to protect the heart and lungs 4) All breathe air through nostrils 5) They possess limbs that follow a pattern of 1 bone - 2 bones - many bones
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What do the common characteristics of tetrapods suggest?
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That they share a single common ancestor
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What is thought to be most closely related to tetrapods? Why?
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Lobe Fins called Rhipidistians, in particular Eusthenopteron; possessed a very similar bone pattern in its fins to the tetrapod limb plan of 1 - 2 - many; also shares many skull features found on early tetrapods; had nostrils and teeth
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What is the idea behind why fish went onto land?
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In the Devonian, it was very dry and drought stricken, so if fish were stranded in pools as rivers dried up they would have to drag themselves out of the drying puddles to search for deeper water; Lobe Fins used their strong lobe fins to drag themselves; fish that had the best limbs would be selected for and limbs would start to evolve
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What was the transitional form found in Greenland by Erik Jarvik?
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Ichthyostega; shared many characteristics of Eusthenopteron and possessed limbs adapted to walking and a foot with five toes
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What is the other idea behind how fish evolved to walk?
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A specimen called Coelacanth (a lobe fin) was found by Marjorie Latimer; it's lobe fins could have permitted it to walk on the ocean floor; but they found a live fish and it swam; wasn't a transitional form
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What did Jenny Clack discover in Greenland in 1987?
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Another early tetrapod, Acanthostega; not a transitional form; similar to Ichthyostega
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What was interesting about Acanthostega?
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It had 8 digits on one hand; the front foot could not bend forward at the elbow (couldn't be brought into a weight bearing position) - more suitable for paddling; the ankle (vital for locomotion) was not present; this creature was not equipped for walking
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What did Clack hypothesize about evolving limbs?
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They evolved in the water (because Acanthostega couldn't walk); it had jaws, teeth and a pectoral girdle that could support some weight - indicate that it was probably hunting close to the land/water interface; may have been able to access the land on a limited basis
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What else did Clack discover?
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A new specimen of Ichthyostega; had 7 digits on hands; still largely an aquatic animal; means that Jarvik interpreted his specimen incorrectly
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What was wrong about the idea of the Devonian? What did it actually contain?
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That is was in a mega-drought; it was wet and lush; it had the first forest-forming trees which formed a canopy some 10 m high; leafy ferns of many kinds were common
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What new environment emerged in the Devonian?
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The swamp
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What evolved in the swamp environment?
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Limbs and "hands"; limbs with fingers are more efficient at navigating a swampy tangled environment than a fish with fins
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What was an additional evolutionary incentive for limb development?
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(Devonian swamps) the ability to escape into the shallows and the tangle of the swamp; Lobe-Finned fish like Hyneria (up to 4 m) used this tactic
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What has still not been discovered?
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The "fishapod" - the animal that was in transition between a fish and a tetrapod
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What biological systems had to change in order for a fish to move from the water? (5 things)
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1) Respiration - new organs for air breathing 2) Feeding - more sophisticated biting skills required 3) Locomotion - new skills and appendages required 4) Musculoskeletal - greater head mobility required the development of the neck 5) Re-engineering of excretion, reproduction, etc. systems
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What were some differences between Lobe-Finned fish and tetrapods?
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Tetrapods had flattened heads like crocodiles rather than conical heads of the Lobe-Finned fish; Lobe-Finned fish had no necks; limbed animals have a head with flexible joints at the base of the skull
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When did the earliest land animal appear?
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363 Ma (Devonian)
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When did the Lobe-Finned fish appear?
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390 Ma Devonian
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How many years is the gap in which the "fishapod" could be found? What were the conditions like?
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27 Ma; Strata of deltaic facies (swampy areas) of the upper Devonian age
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What are the three possible areas where the "fishapod" could have been found?
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The Catskill range, East Greenland and the Canadian Arctic
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What was found in the Canadian Arctic by Shubin on Ellesmere island?
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5 skeletons of a fish with a flat head like a crocodile
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What did the Tiktaalik fossil Shubin found have as features?
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The fish varied in size from 45cm to 3m in length; had fish-like features - fins with fin webbing, scales, primitive jaw; tetrapod-like features - a neck with clear separation from the shoulder, wrists, flat head, expanded rib cage, lack of an operculum (flap covering gills)
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What was the Tiktaalik adapted to do?
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To do a kind of "push-up" with an elbow joint bent and extended with a "palm" flush against the ground; the fin webbing of the animal was present but reduced to allow movement of the "limb"
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What can explain the Tiktaalik's flat head?
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It was a benthic animal lying in shallow water
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Was the Tiktaalik a predator?
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Yes; it had sharp teeth
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What does warm shallow water contain very little of, and why would the Tiktaalik benefit from its evolved limbs for this?
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Oxygen; the ability to do push-ups may have helped the animal raise its nostrils into the air or close to the water/air interface so it could breath where other fish could not
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Hox genes
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Control the cascade of events that determine limb pattern and form; critical for placement of structures such as legs and eyes; specify position of head and segments of the body
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The coding for _____ are very similar to the coding for ____. What does this mean?
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Fins; limbs; no new genes had to be created as the development of limbs progressed
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How did the development of appendages in fish start? What are some examples of this?
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Via gene switching; mudskippers and the frog fish (walks on the bottom of the ocean) - each resulted in a different pattern of limbs that the tetrapods evolved from
