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Abstract Detail


Doyle, James A [1].

Stem, crown, and real and apparent conflicts between fossils and molecular dating.

There have been many claims that fossils and molecular dates for taxa contradict each other, but some conflicts are more apparent than real. All clades recognized in molecular dating analyses are crown groups (living representatives of a line, their most recent common ancestor, and extinct derivatives of this common ancestor), whereas fossils may represent stem relatives of crown clades (forms on the stem lineage connecting the crown group and its common ancestor with its living sister group, and extinct branches of this lineage). Fossil members of a crown group should have all the synapomorphies of the living members, but if all these synapomorphies in preserved organs arose early on the stem lineage, modern-looking stem fossils may be much older than the crown group. Fossils can only be assigned to a crown group if they share synapomorphies with one of its subgroups. For example, Early Cretaceous seeds resembling extant Ephedra were initially thought to contradict molecular analyses that dated the radiation of Ephedra as Tertiary. However, analysis of characters of the seed envelope implied that the fossils were probably stem relatives of Ephedra. Hence they do not conflict with a Tertiary radiation; instead, they provide evidence for morphological stasis through the Cretaceous. Similarly, Early Cretaceous flowers with Asteropollis pollen are essentially identical to modern Hedyosmum (Chloranthaceae), which molecular analyses date as radiating in the Tertiary. They have no features that place them in the crown group, but none that are recognizably more plesiomorphic than those of the crown group either. They could therefore be branches of a stem lineage that remained static in known characters until the Tertiary, but data from new organs or new characters of the flowers or pollen are needed to test this. The Early Cretaceous flower Monetianthus was originally compared with Nymphaeaceae, which would contradict a molecular analysis that dated Nymphaeales (represented by Cabombaceae and Nymphaeaceae) as Tertiary, and this was taken as evidence that the fossil was a stem relative of the order. However, phylogenetic analyses firmly place Monetianthus within crown Nymphaeaceae, and character optimization indicates that a stem relative of Nymphaeales would look quite different. This case therefore presents a more serious conflict between fossils and molecular dating, due either to failure of phylogenetic methods or to incorrect assumptions of molecular dating analyses.

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1 - University Of California Davis, DEPT OF EVOL & ECOLOGY, One Shields Avenue, Davis, CA, 95616-8537, USA

Molecular dating

Presentation Type: Oral Paper
Session: 23, Paleozoic and Mesozoic Paleobotany
Location: 102/Savannah International Trade and Convention Center
Date: Tuesday, August 2nd, 2016
Time: 10:30 AM
Number: 23009
Abstract ID:148
Candidate for Awards:None

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