EES 227: Paleobiology

Spring 2004

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Lecture 15: April 1

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Lecture Notes: Evolution of Plants

We will concentrate on the so-called 'higher plants' (living on land, having internal vessels for transport) ; and not look at many of the 'algae', a confusing term used for a mixture of unicellular and multicellular photosynthesizing organisms living in water. Note that some multicellular green algae (Chlorophytae) secreate carbonate skeletons (e.g., Penicillus, Halimeda), and the rather enigmatic fossil Receptaculites.

Land plants evolved much later than animals (Cambrian explosion): start in the Ordovician, few species in the Silurian,  more common in the Devonian, and first true forests in the Carboniferous. There is presently much discussion exactly how early plants became established on land, and some enigmatic fossils may push back the date of origin of life on land.

The establishment of forests must have had a major influence on global climate and on life in the oceans. If there are large forests  on land, much organic matter is captured on land (in soils, coal, and organic-rich materials). The organic material can not reach the oceans, which have become increasingly food-starved.
Life on land: problems, because surrounded by air, not water.

Many adaptations are needed by larger (multicellular) organisms that dwell on land as compared to life in the oceans.

Which organisms first became established on land?

In  Precambrian soils: Archaea and Bacteria  active in soil formation from at least 1900 Ma (1.9 Ga) . Prokaryotes thus probably were  the first land-dwellers, possibly joined early  by  symbiotic organisms such as 'lichens' (fungi and algae, with cyanobacteria involved as well).

Lichens: many thousands of different fungi live in these combinations, in which the combination has a genus and species name. There are only about 100 algae, (most common the single celled alga Trebouxia); including cyanobacteria.

Land plants , according to data on cellular structure and biochemistry, descended from green algae. Modern land plants ('higher' plants) are classified in the Embryophytes (see below), which are subdivided into the Bryophytes (non-vascular plants, such as mosses, liverworts, and hornworts), and the Tracheophytes (vascular plants, almost all familar plants). The non-vascular plants do not contain tissues to conduct fluids (containing nutrients and dissolved gases) internally. The vascular tissues are called xylem (lignin or wood) and phloem. Note that the evolution of wood was followed by a period of strongly enhanced burial of organic matter (coal): when wood first evolved there were no fungi and bacteria to digest it, and it thus did not rot much for millions of years.

Subdivision of EMBRYOPHYTES ('higher plants')

Bryophytes

Mosses

Liverworts

Hornworts

Tracheophytes

Sphenophyta (Horsetails)

Lycophyta (club mosses) 

Pterophyta (ferns)

Gymnosperms (naked seeded)

Conifers

Cycads

Cycadeoids

Ginkgo biloba

Pteridosperms (Seed ferns): extinct

Angiosperms (flowering plants)

Monocotyledons (grasses)

Dicotyledons (most flowering plants)

All plants have alternation of generations: a sexually reproducing generation (which is haploid: 1 set of chromosomes; the gametophyte) alternates with an asexually reproducing generation (which is diploid, 2n set of chromosomes; the sporophyte). It varies by plant group which of these generations is most important. In mosses the plant that we know is haploid, the diploid generation is small and hardly known.  Ferns and seed plants have a conspicuous diploid generation. In seed plants, the haploid (gametophyte) generation develops completely within the 'seed'. The Gymnosperms and Angiosperms together form the seed plants (Spermadophyta). 
Carboniferous forests were very different from today's forest. Most important trees (see site on Carboniferous forests of West Virginia):

Carboniferous forests were:

Compare -'zoic' with -'phytic': the boundaries between epochs for animals and plants

The major subdivisions of life as seen by plants were NOT at the same point in time as major subdivisions in animal life.

Note that at mass extinctions at end Permian and end Cretaceous there were ecosystem disturbances, but no major extinctions of plants.

 

Seed plants:

The most abundant Gymnosperms are the conifers, but there are also cycads (living fossils, which were much more common during the Mesozoic than they are now) and Ginkgo biloba, a group by itself and a living fossil, and the very strange group Gnetales, including such plants as Ephedra (herbal remedy with possible serious health effects) and the amazing desert plant Welwitschia mirabilis.

Flowers occur almost exclusively in the Angiosperms, with some flower-like structures among a few of the cycads. In the present world, the Angiosperms are by far the most abundant group of plants, with more than 250,000 species. They dominate almost everywhere on land, with the exception of high-latitude coniferous forests and lichen tundras.

Evolution of the Angiosperms had a major influence on Earth's climate:  deciduous trees produce many leaves, soft, easily degradable organic matter (in contrast to conifers which change needles less often, and which have needles with not by far so much soft organic material). Rotting of the leaves produces humic acid, which  causes faster weathering of the soil and rocks. Weathering takes up more CO2 from the atmosphere. Evolution of Angiosperms could thus have been a contributing cause of global cooling in the Late Cretaceous and Cenozoic.

What did Darwin mean with 'abominable mystery'?

Most recent gymnosperms are conifers (needle-shaped leaves), but people  looked for possible ancestors of the Angiosperms mainly among non-needle bearing groups. For several years the Gnetales were seen as the closest relatives of Angiosperms, and grouped together with the Angiosperms in the group called'Anthophyta'; the theory of this origin was called the 'anthophyte theory'. Genetic evidence, however, has shown that the Gnetales are more closely related to the conifers. We thus do not know which group contained the ancestors of the Angiosperms, possibly a now extinct group such as the Benetitales.

We do know that Angiosperms radiated rapidly in the early Cretaceous (Aptian, ~125 Ma). The oldest fossils widely recognzed as angiosperm is Archaefructus sinensis, from rathjer badly dated deposits (earliest Cretaceous or maybe even latest Jurassic). There are pollen that are clearly angiosperm in uppermost Jurassic deposits, so they existed at that time. There is a strong debate whether pollen dating back into the Triassic belonged to angiosperms.

What was the oldest angiosperm? The affinity of A. sinensus is not quite clear; it was a plant living in shallows along rivers in Southeastern China. Most people argue that it is close related to the water lilies. For a long time the following 3 groups were seen as candidates for the oldest angiosperm:

Sycamores would be more logical ancestors: small flowers sound more primitive than large flowers, and insect pollination much more complex than wind pollination. The latest evidence, however, favors the small-flowered water lilies, because of their possible relation to Archaefructus and because of their relation to the obscure living genus Amborella which occurs only in the New Caledonian islands (northeast of Australia), and on genetic evidence is closest of all living plants to the most primitive Angiosperm.

 

The Angiosperms rather suddenly increased enormously in abundance in the Cretaceous period called the Aptian, about 125-115 million years ago. Many theories have been proposed to work out why and from where. It has been assumed that they originated in the tropics in Southeast Asia (from which we have a poor fossils record), or in mountainous regions from which the record is extremely poor. The discovery of primitive Angiosperms in New Caledonia confirms such an origin, but also is in agreement with the theory that Angiosperm origins lay in China or southeast Asia, and that they originated at some time before the Late Jurassic (150-145 million years ago), at which time well developed Magnolia-like plants existed. Genetic information suggests that Angiosperms may have originated even earlier (in the Triassic period), but no fossil evidence of their existence has been found except for wdiely debated pollen evidence.
This radiation of the Angiosperms has been linked to various internal factors:

External factor:

Co-evolution with insects:

This appears extremely likely because of the close association between flowering plants and insects, but the insect fossil record is rather poor and we can not really prove such a pattern of co-evolution. New studies of insect-damaged fossil leaves may provide more evidence. Many insects whose presently-living relatives are pollinating evolved well before the angiosperms, and may have lived on seeds, pollen from gymnosperms.

Co-evolution with dinosaurs:

It does not seems unreasonable that huge herbivores influence plant evolution. Herbivorous dinosaurs show rapid evolution, more rapid, in general than carnivorous dinosaurs. One can thus suggest that plants were forced to adjust by evolution to changes in herbivorous faunas. The Angiosperms radiated after an extinction of many groups of dinosaurs at the end of the Jurassic. Jurassic dinosaurs (such as stegosaurs and brontosaurs) were probably high-feeders, browsing from high trees (10-12 m high). They have teeth for grabbing, not chewing, and may have swallowed stones for use in the 'gastric mill'. Plants (conifers and cycads) may have protected themselves by having alkaloid poisons, heavy oils, or spiny branches. The Cretaceous dinosaurs which evolved after the extinction of the Jurassic dinosaurs (we do not know what caused this extinction) include such groups as the iguanodons, hadrosaurs and ceratopsians, which were probably low feeders with good teeth for mashing plants, or with beaks. They browsed low to the ground, eating lower shrubs. Under the onslaught of such low feeders plants were at an advantage if they could grow fast, reproduce early, and have seeds that can be spread far: Angiosperms. Maybe the plant-eating dinosaurs opened up enough niches so that the angiosperms could use them. It has thus been proposed that 'dinosaurs invented flowers' by dinosaur paleontologist Robert Bakker.

There is much evidence that the earliest angiosperms lived in a 'dark, disturbed' environment, i.e., were small plants growing along roivers, where floras were commonly disturbed by flooding. Such plants must be small, grow fast and opportunistic, and be able to reproduce rapidly.

So what has been solved from Darwin's 'Abominable Mystery':

Angiosperms: