Forces Driving Angiosperm Divergence IV

Dark, Damp and Disturbed Theory

Basal extant Angiosperms seem to be designed to go under the dinosaur herbivory radar. Ecophysiology and life-strategy of primitive extant angiosperms may throw light on why and how angiosperms became diverse and widespread.

ANITA Grade

A=Amborella

N=Nymphaeales

I=illiciales

T=Trimeniaceae

A=Austrobaileya

Need to understand their:

1. Morphology: Growth form. Seed size.

2. Ecology: Habitat. Seedling habitat. Adaption to disturbance.

3. Physiology: leaf anatomy. Photosynthetic performance.

Ancestral ancestors

Morphology (dark): Moderately woody (small shrubs and lianas). Seedlings colonized shady habitats.

Ecology (disturbed): plants adapted to disturbed habitats.

Physiology (damp): Adapted to high water availability and low evaporation capacity. Plants adapted to photosynthesis in low light conditions (dark).

Prolific respouters

Sprout= gave rise to new shoots/leaves. The ability for extensive clonal growth promotes survival, especially in disturbed areas. The morphology of fossilized wood, leaves, flowers and seeds show Cretaceous angiosperms to be herbs and small trees with early successional strategy. Early angiosperms were wood rhizomatous undershrubs growing in disturbed area of damp shaded forests and with the ability.

Due to their woody tendency basal angiosperms shortened the time needed to complete their life-cycle resulted in established quicker in these disturbed situations. Propagate before Gymnosperms with longer life cycle thus displacing them.

Amborellaceae

Consistently shown to be most basal living angiosperms (unisexual). Most theories try to take this into account.

Later evolutionary lines spread from amborella type plants into: aquatic habitats (nymphaea) drier habitats (chloranthaceae) woody liana’s (austrobaileya). Exception this this story is the water lily (Nymphaea) which is bisexual.

Wet and wild origin

Due to near basal position of waterlily (nymphaea) suggestions have been made that angiosperms have an aquatic origin.

Supported by early Cretaceous fossils such as Archaefructus (some analyses place it below Amborella in cladogram). Suggestion is that angiosperms ancestors entered water to evade competition on land.

Watery enviroment promoted

Herbaceous condition

Rapid growth

Rapid photosynthesis

Condensed reproductive axis (start of flower)

Not favored

Confusion surrounds the interpretation of Archaefructus reproductive structure-may not be basal to Amborella. Aquatic plants too specialized to evolve back into land. Loss of some functions (secondary growth) hard ro regain.