Questions for Further Thought
- After having studied the material on cycads in this week's lab including slides of cycad ovules and mature seeds, as well as cycad wood -- would you argue for an alliance with Medullosan pteridosperms? Also read the chapter on cycads in Taylor and Taylor and analyze the features that are listed: Which are good homologies, which are potentially homoplastic? Explain. Make a list!
- There has been some discussion about an evolutionary "trend" of stelar morphology within the Medullosan clade. Some workers argue that the simple eustele is primitive, with the complex steles of Medullosa represent a specialization. Others believe that the simple stele type has formed through reduction of a primitive "polystelic" condition. Does the choice of hypothesis affect where we place the cycads in relation to the Medullosans? Explain. (Hint: compare Medullosan and cycad vascular structure).
- Many paleobotanists want to see an evolutionary trend from the simple leaves such as Taeniopteris and Nilssonia, to the more complex pinnate leaves of e.g. Crossozamia. What problem does this present to the idea that cycads are derived from a Medullosan stock? Refer to Taylor and Taylor (1993) for a discussion on this topic.
[Geologic Time Scale]