DEVORE, MELANIE L.1*, JOHN J. SKVARLA2, ZAIMING ZHAO3, and ROBERT K. JANSEN3. 1Department of Biology and Environmental Sciences, Georgia College & State University, Milledgeville, GA 31061; 2Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019; 3Section of Integrative Biology and Plant Resources Center, University of Texas at Austin, Austin, TX 78712. - Comparisons of Tubuliforidites and pollen types within Asteraceae and related families: How do you recognize Asteraceae in the fossil record?
A number of dispersed Cretaceous and Tertiary pollen grains with exine
morphologies suggestive of Asteraceae have been assigned to the form
genus Tubuliforidites. Zavada and de Villiers (2000) carefully
examined the ultrastructure of two species of Tubuliforidites
with Anthemoid type morphology from the Paleocene-Eocene of South
Africa. The fossil grains, as well as Calyceraceae, Barnadesioideae,
and Mutisieae, have a reticulate wall layer. Unlike
Tubuliforidites, members of Barnadesioideae have thick
columellae that extend through the exine. In Mutisieae a distinct
intertectum is present. Like Barnadesioideae, some members of
Mutisieae have basal columellae that extend through the exine. The
sister group to Asteraceae, Calyceraceae, and a second family
Valerianaceae, exhibit wall structure types most similar to that of
Tubuliforidites. It is possible that the reticulate layer in
Barnadesioideae and Mutisieae is not homologous to the condition found
in Calyceraceae, Valerianaceae, and Tubuliforidites. Grains of
Calyceraceae, Valerianaceae, and Tubuliforidites appear to have
a reticulate exine layer that is derived from divisions of the upper
portion of columellae. In considering both wall structure and
phylogenies of Asteraceae and closely related families, it appears
that the Anthemoid pattern has evolved multiple times both within
Asteraceae and in related families. The presence of this pattern in
fossil grains is not a good character to use to recognize Asteraceae
in the fossil record. The challenge for the paleopalynologist
interpreting the ultrastructure of grains believed to have affinities
with Asteraceae is to determine if the reticulate layers are not
derived from distal divisions of the columellae. Also, the presence of
columellae extending through the wall would also suggest an affinity
to Asteraceae.
Key words: Asteraceae, Barnadesioideae, Calyceraceae, Mutisieae, Pollen, Tubuliforidites