Terrestrial or aquatic origin?
|There are two views about the ecology of the first angiosperms:
were they terrestrial or were they aquatic, something like water lilies?
From observation of the basal terrestrial lineages, a terrestrial origin would involve a woody tropical angiosperm with an unusual seedling development in a humid low-lit forest understory where the soil is often disturbed. Indeed, Amborella grows in two phases: it first establishes itself as a small ground-hugging seedling with a creeping root system and many shoots from the base of the plant, changing to a shrub-like growth once it is well established. Multiple low-lying shoots allow the plant to better resist possible breakage resulting from ecological catastrophes due to shifting sediments, while a developed root system enables better anchorage in unstable substrates such as the understory stream margins and upland forest washout zones where terrestrial ANITAs are currently found. However, although a phylogenetic reconstruction based on a terrestrial origin is the most parsimonious, there is no direct fossil evidence for primitive angiosperms occupying such habitats. In the case of upland slopes, this may result from a bias in the fossil record since these areas are rarely preserved; stream margins and channels are common depositional settings.
An aquatic origin predicts an herbaceous plant with extensive root systems and emerged leaves and flowers at the surface of the water. Seedlings establish at the bottom of the water column, either in low light conditions in stable sediments, later extending leaves to the surface to better photosynthesise or in clear streams where the sediment is constantly disturbed. Seasonal, ephemeral pools where most living water lilies are established today could explain the evolution of the angiosperm characteristics of rapid growth and accelerated reproduction which are thought to be the basis for their success.
Water lilies are primitive water-living angiosperms that may give us an insight
into what the first angiosperms looked like.|
Evidence supporting the aquatic origin hypothesis occurs in both living and fossil forms. Amborella, supposedly the most basal angiosperm, has vestigial gas exchange canals, useful in submerged stems and roots, and similar to those found in water lilies. A fossilised lily flower found in 2001 establishes the lineage as already existing in the Early Cretaceous, around 125 million years ago. Another fossil, Archaefructusfrom Chinese lake deposits dated at 124 million years ago, further proved that angiosperms had already colonized the water by the Early Cretaceous, as fossil fishes were found among the leaves.
Although there is clearly better grounding for this hypothesis, there are a number of questions. Firstly, all fossils except Archaefructus remained controversial in that none preserved enough detail for them to be assigned without doubt to the water lily family; Archaefructus itself cannot be classified unequivocally either as a common ancestor or as part of a derived lineage.
Secondly, demands on organisms differ greatly under water and in the air. Buoyancy provided by water supports the cells, exposing the organisms to forces such as currents and waves rather than gravity. It is hard to imagine how plants migrated from water to land and were able to evolve quickly enough to cope with gravity. In addition, aquatic angiosperms show strong convergence between different lineages in their mechanisms to deal with currents and the low diffusivity of gases in water impacting on their photosynthetic abilities. This suggests that a limited number of solutions are available, thus restricting the scope for widespread radiation and innovations which would lead to migration on to land. Fewer than two percent of angiosperm species are aquatic.
What remains certain is that angiosperms explored the aquatic habitat early in their evolution. Ideas about why this happened include escaping competition and overcrowding by gymnosperms on land; evolving in areas with environmental conditions similar to those under water such as low light levels under a tree canopy; and living in areas where there were frequent ephemeral seasonal pools, and so evolving an aquatic lifestyle.