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These notes explain how to run the ‘Highlighting Important Stages in Evolution’ activity ... - page 6 / 11





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The earliest vertebrates appeared at the point indicated by Z on the diagram. Vertebrates have an articulating backbone or spinal column.


How might having an articulating backbone made up of vertebrae provide a competitive advantage?

Increased animals’ repertoire of movement. Articulating structure presented greater strength so that animals could become terrestrial AND grow large.


Vertebrata is a sub-phylum of the phylum Chordata. The shaded region on the diagram highlights the members of the chordate phylum that are not vertebrates. Find out both what all members of Chordata share in common, and why members of this branch are not considered vertebrates.

They all possess a notochord – a rigid rod-like structure at some developmental stage (though in vertebrates it develops further into the vertebral column), a hollow dorsal nerve cord, pharyngeal slits and a post-anal tail. The invertebrate chordates either have a notochord during their larval stage only, or the flexible rod remains for the duration of their adult life.


The ancient pikaia, modern lamprey and hagfish show some unusual features that make them difficult to place into classification groups. Assigning some animals to taxonomic groups can be challenging for scientists. From our understanding of the principles of evolution by natural selection, why might we expect this type of problem to arise in trying to place living things into groups?

Classifying living things is artificial. Nature does not recognise the boundaries that we place around each group, which is one reason why scientists are constantly redefining taxonomic groups. Those living things on the borderline of evolutionary developments support the notion of evolution by natural selection being a continuum, but it is difficult to place them in a distinct group, since they often share characteristics of two groups.


Acanthostega was one of the earliest tetrapods to walk on land. Scientists believe that no living animal is descended from it. From this you should be able to locate Acanthostega in the Tree of Life and use the information provided to indicate why scientists have reached these conclusions.

Fossils of Acanthostega show that it had eight webbed digits on each limb, whereas no living tetrapods (amphibia, reptiles, birds and mammals) share this limb plan.


Locate Dimetrodon on the interactive (hint: Dimetrodon was a reptile that exhibited some of the characteristics of a mammal). Dimetrodon was a prehistoric animal that had a recognisable bony sail formed from projections formed from projections in its vertebrae and contained numerous blood vessels. How did Dimetrodon benefit from having the sail structure on its back? Dimetrodon was a ferocious carnivore during the daytime, but was unlikely to have hunted at night. How can scientists be fairly certain that it did not lead a nocturnal existence?

The sail structure was used to trap the sun’s heat energy and warm up it body during the day. Such a mechanism would have enabled Dimetrodon to possess a faster metabolism than other animals. This is because the higher body temperature would have allowed it to catalyse reactions more quickly. As a hunter, it would have been able to move more quickly than both its prey and competitors. However, unlike true mammals, which maintain constant body temperature independently of the sun’s direct energy, Dimetrodon was reliant on external heat from the sun.


Select two examples of species for each of the following: (i) amphibian, (ii) reptiles, (iii) birds, (iv) mammals (living or extinct species). For each, use a combination of the Tree of Life information and other sources to establish the following phylogenetic categories: kingdom, phylum, class, order, family, genus and species. Through clicking on one animal in the interactive and then another, you will see the point at which they diverged. State when your two species last had a common ancestor and write down the main

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