Exercise 20.1

Phylogenetic Insights from Fossils: A New Whale Fossil Sheds Light on a Major Division in the Modern Whales

(This exercise is based on Fitzgerald, E. M. G. 2006. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales.Proc. R. Soc. B. 273: 2955–2963.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

In Chapter 20 we learned about the early evolution of the order Cetacea (whales and their relatives) and we saw how fossil evidence shed light on the transition of this group from animals that were semi-aquatic, perhaps living like hippopotamuses, to animals that were completely aquatic and lacked the ability to live on land. This exercise will demonstrate how fossil evidence can also be used to make further inferences about the ecology and evolution of Cetacea by looking at a finer division within that group—the division between the toothed and the filter-feeding whales.

The order Cetacea is the sister taxon to the Artiodactyla (the even-toed ungulates), which is composed of land-dwelling animals such as pigs, deer, and hippopotamuses. Cetacea includes whales, dolphins, and porpoises, all of which are superbly adapted for life in the water. Within Cetacea there are two clades: toothed whales, dolphins, and porpoises are in the suborder Odontoceti, while baleen whales are in the suborder Mysticeti. This exercise is based on a recent paper that discusses evolutionary relationships within Mysticeti and provides insights into those relationships based on a new fossil discovered in Australia.

Odontocetes are active hunters that use echolocation to find their prey. In contrast, mysticetes do not actively hunt or use echolocation. Living mysticetes are large whales that strain small organisms from the water through hairlike plates of baleen that hang down from their upper jaws. The evolutionary origins of this filter-feeding ecology are somewhat obscure. This paper describes an unusual new whale species based on a remarkably well-preserved fossil of its skull. This new species represents a so-called “missing link” between the odontocete and the mysticete whales.

QUESTIONS

 

Figure 1  The phylogenetic tree of the Mysticeti.

 

Question 1. Refer to Figure 1 above. What morphological traits does the new fossil, Janjucetus hunderi, share with the extant odontocetes?

 

Question 2. List three synapomorphies shared by the extant filter-feeding whales.

 

Question 3. Janjucetus hunderi was a small, toothed whale that was thought to actively hunt prey rather than filter feed. Why is it considered to be a basal mysticete rather than an odontocete?

 

Question 4. Discuss some of the aspects of whale feeding ecology that are revealed in their skull morphology. For instance, what features of the skull are associated with macrophagy (feeding on large prey)? What features of the skull reflect adaptations for filter feeding?

 

Question 5. Thinking from an evolutionary perspective, provide a hypothesis that could explain why there are no toothed mysticetes alive today?

 

Question 6. Describe a hypothetical fossil of a whale skull that would increase our knowledge of cetacean evolution if it were to be discovered. List the pertinent features of this imaginary fossil and suggest what questions about the evolutionary history of whales could be answered by studying it.

 

Figure 2  The close resemblance between the teeth of Janjucetus hunderi (a–d) and those of the currently living crab-eating seal, Lobodon carcinophagus (bottom image). (Seal image courtesy of Dr. Alistair Evans, Monash University, Australia.)

 

Question 7. Refer to Figure 2 above showing the teeth of the fossil whale Janjucetus hunderi. As you can see, the teeth are very complex and closely resemble the teeth of a seal living in Antarctica today. This seal, Lobodon carcinophagus, is called a crab-eating seal, but its main diet consists of krill, which it filters out of the water through its complexly cusped teeth. Krill are also a major component of the diet of modern Mysticeti. Why do the authors discount this similarity in tooth structure as evidence that Janjucetus hunderi was a filter feeder?