Euanthial vs paleoherb theories
|There are two main theories about the first angiosperms: were they small
trees or woody shrubs, the euanthial theory, or ‘weedy’ herbs, the palaeoherb
theory? Both theories assume a terrestrial origin.|
The euanthial theory is based on the assumption that Magnoliales are most primitive; this would make the first flowers numerous, large and complex and spirally arranged on a cone-like axis, belonging to woody trees or shrubs with thick branches and large leaves, and living below the forest canopy.
The paleoherb theory (sometimes known as the ‘sneaky herbs’ hypothesis) relies on the assumption that angiosperms evolved in response to disturbed soils, for example sunny stream-sides and flood plains. This implies that the first angiosperms were rapid colonizers that tolerated disturbance, thanks to an herbaceous nature which does not require so much maintenance, and by having leaves capable of high photosynthetic rates and small simple flowers to reduce the cost of producing them.
Evidence from the fossil record fails to pin down one of the hypotheses as the more probable. Promoters of the euanthial theory maintain that most features of the Magnoliales, such as the long axis of the flower, the woody stem and simple oval-shaped leaves alternating on the stem, and the simplicity of the seed coat and pollen grains, are all primitive features and that compound leaves and complex structures are derived. Herbs are said to be more advanced in structure compared to woody taxa; indeed no gymnosperm is herbaceous, so evolving herbs from the gymnosperms involves changes that later had to be reversed to produce the non-herbaceous forms. The flowers of Magnoliales are also often compared to and associated with the reproductive structures of cycadian gymnosperms.
Other evidence is better explained by the paleoherb theory. Angiosperm wood is rare in the Early Cretaceous compared to gymnosperm wood, and angiosperm leaf fossils have been found in deposits interpreted as sunny habitats. Fossilized seeds attributed to the angiosperms are small with thin seed walls, features generally associated with small weedy plants with rapid life cycles. The earliest flowers from the Late Cretaceous are small and numerous along an axis, similar to those found in the living pepper family, the Piperaceae, which are herbaceous. Fossilized flowers similar to the Magnoliales are not found in the fossil record until about 30 million years after the piperoid type.
At present, a third hypothesis is gaining support: that of an intermediate form as the most basal ancestor, a herbaceous, weedy, small shrub, much like Amborella, developing in the tropical, damp understory.