Transitional forms are fossils that demonstrate possible intermediate steps in the evolution of ancestral groups and their descendants. These fossils can exhibit both ancestral and derived traits and often show the gradual change of one group into another. Transitional species are important as they fill in gaps in the evolutionary tree of life and allow scientists to view the adaptations and modifications of body plans over vast amounts of time. These modifications and adaptations are often referred to as specializationsand can be tracked through the fossil record. The diagram below tracks the changing position of the nostrils in whales over time.
Transitional fossils are exceptionally rare in the rock record. It is estimated that of all life to have ever lived on Earth, only 1% has been preserved (Paul, 2008). Of those fossils, many are lost to geologic processes over time such as weathering and erosion, diagenesis or metamorphism, to name a few. Of these surviving fossils, fewer still show actual transitional forms between ancestral groups and their descendants. In some cases, where the fossil record is lacking, modern analogues can be used to reconstruct evolutionary history and make predictions as to the functions of special traits and characters.
- Explain the distinction between fossils and transitional fossils.
- What conditions favor preservation and fossilization?
- The Predictive Power of Evolution!
Paleontologist long speculated that tetrapods (four-limbed animals) arose from fish ancestors, specifically from sarcopterygians (the lobe-finned fishes). Late-Devonian fish and tetrapod fossil specimens included Acanthostega and Icthyostega respectively (figure 1). Acanthostega possessed weak fore-limbs that likely could not support its own weight on land. Ichthyostega possessed four limbs, stout leg bones, wrists and ribs. Icthyostega, while mostly an aquatic animal, could easily explore its terrestrial surroundings. Paleontologist Neil Shubin hypothesized that an evolutionary intermediate should exist somewhere between the small limbed Acanthostega and the sturdier Icthyostega.
In 2004, such a fossil was found in Canada. Named Tiktaalik, (pictured below) the animal has many features of both fish and tetrapods (thus being nick-named a ‘fishapod’). Tiktaalik possessed the ray fins of lobe finned fishes but also had complex shoulder bones which would have allowed Tiktaalik to briefly support its own weight out of water. Tiktaalik quickly became a classic example of an intermediate form. Tiktaalik had illustrated how knowledge of evolution can allow scientists to predict transitional fossil forms, before they ever actually find one!
- It is important to note that transitional fossils cannot be assumed to be direct ancestors of more recent groups. For instance, Tiktaalik is probably not the direct ancestor of tetrapods, but it clearly shares certain features that suggest it is closely related to the ancestral tetrapod. What characteristics indicate Tikaalik forms an important link between fish with fins and tetrapods?
- Why would a paleontologist argue that Tikaalik is not THE ancestor of all tetrapods?
- Tiktaalik is credited as a transitional fossil between fish and tetrapods, yet tetrapod tracks appear earlier in the rock record than Tiktaalik. Why is this?
The phylogenetic tree above traces horse evolution from Hyracotherium to Equus. Early horses had three toes while modern horses possess only one toe. Note that this tree is read from the bottom up.
- Make a few predictions about what traits or characters you might expect to see in fossil horses between Hyracotherium and Equus.
- Was Hyracotherium a direct ancestor of modern horses? Why or why not?
- What is meant by ancestral and derived traits? Give an example of each.
During the Miocene (23 mya), vast grasslands began to appear, which greatly reduced forested areas. Paleontologists noticed a transition to longer limb bones among hoofed animals and those that preyed on them.
- Why might long-legged animals have been successful in the grasslands? Explain your answer.
- How might you test your hypothesis?
- How do paleontologists find transitional species?
Before Neil Shubin and his team could find Tiktaalik, they had to know where to look. Shubin had to locate rocks of the right age, and rocks of the right type to preserve fossils. Furthermore, these rocks had to be exposed at the surface and relatively unexplored.
The image above is of a modern coelacanth. Coelacanths were thought to have gone extinct sometime in the Late Cretaceous, since no fossils are found after this time. However, coelacanths were discovered in 1938, alive and well off the coast of South Africa. Coelacanths are often and erroneously referred to as ‘living fossils’. As seen in the diagram below, modern coelacanths have changed quite a bit from their Cretaceous relatives!
- If you were to look for a hypothetical transitional species of coelacanth, what age rocks would you look in? (Give a range)
- In what type of rocks would you prospect?
- What evidence would you use to argue that the modern day coelacanth is not a “living fossil”?
- The dinosaurs in your back yard
Of the known transitional fossils, none may have been quite as dramatic or contentious as Archaeopteryx. Archaeopteryx is a Jurassic-aged bird, thought to represent a transition between small theropod dinosaurs and modern birds.
Archaeopteryx possesses a host of ancestral and derived traits including:
- Slow, dinosaur-like growth rate
- Forelimbs with three, unfused, clawed fingers
- Unserrated teeth in maxilla
- Long bony tail
- Fully developed asymmetrical flight feathers
- Fused furcula (wishbone) formed from joined clavicles
- Elongated pubis (turned backwards) not found in ancestral theropods
- How likely is it that Archaeopteryx represents the first bird? Why or why not?
- Dinosaurs before and after Archaeopteryx experimented with a variety of feather types. Some dinosaurs possessed wispy, hair-like proto-feathers, others displayed full pennate feathers with quills and veins. We can assume that modern birds descended from one of these groups of feathered dinosaurs. Where on the phylogeny below would you place Archaeopterix given ancestral and derived traits observed?
- The fossil record is spotty at best, and feathers are often poorly preserved. However, paleontologists have hypothesized that feathered dinosaurs used their feathers for mating displays and insulating nests. How did they come to these conclusions?
- The term ‘missing link’ is still used frequently, but it is an antiquated and misleading expression. Conjuring images of links in a chain, ‘missing links’ imply that evolution is a series of linear events, from species A to D. Why is this not correct?
18. We have only scratched the surface of transitional fossils! Find a transitional fossil (not mentioned in this lab) that interests you. Write a few sentences about why it is important, and which groups it represents a transition between.
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