Embedded Files
Southern
Southern
Towering conifer forests covered much of what is now Australia in Gondwana. Smaller plants such as ferns, gingkoes, cycads, clubmosses and horsetails created an understorey. The first flowering plants had begun to bloom. Fossilised pollen indicates a wide variety of plants was present in South America with hornworts liverworts ferns and several gymnosperms including conifers and the extinct gnetophytes...and now also the angiosperms or flowering plants.
Northern
Northern
They shared many plants in the early part of this period, but the pine genus originated in Laurasia in the Early Cretaceous c. 130 Mya in competition with faster growing flowering plants. Pines adapted to cold and arid climates in environments where the growing season was shorter or wildfire common, between 31° and 50° north. This and resulted in a split into two subgenera: Strobus adapted to stressful environments and Pinus to fire-prone landscapes. By the end of the Cretaceous, pines were established across Laurasia, from North America to East Asia
gnetophytes
Strobus
With the exception of grasses, most of the plants we know today would have been around then.
Flowers, the reproductive organs of angiosperms, are the most remarkable feature distinguishing them from the other seed plants. They provide the means to have a more species-specific breeding system. This enabled a way to evolve more readily into different species without the risk of crossing back with related species. Faster speciation enabled these Angiosperms to adapt to a wider range of ecological niches. It means flowering plants became dominate terrestrial ecosystems, comprising about 90 percent of all plant species.
Abominable Mystery
Abominable Mystery
Charles Darwin wrote in 1879, about "an abominable mystery". He described it as "a most perplexing phenomenon". Twenty years after the publication of his seminal work The Origin of Species, there were still aspects of evolution that bothered the father of evolutionary biology. Chief among these was the ‘flower’ problem.
Suddenly there were flowering plants. To Darwin this was an enigma. Flowering plants from gardenias to magnolias, water lilies to wheat belong to a large and diverse group called the angiosperms. Angiosperms are vascular plants with stems, roots and leaves, but their main development was that their ‘eggs’ were fertilized in an ovary by pollen landing on the stigma. The developed inside the flower and became protected by fruit outside. All our favourite fruits, like plums, apples and oranges are examples angiosperms where the fruit surrounds seeds..
Burst
Burst
What worried Darwin was that the very earliest samples in the fossil flower records all dated back to around 100mya - the middle of this Cretaceous period. The earliest angiosperm fossils appear early on (133-125 mya) (Maggalan et al 2015), the last major group of land plants to appear. The oldest confirmed fossil flowers are no older than 130 Mya (Sauquet et al 2017) By the Middle Cretaceous (~115-100 mya), a burst of diversification had produced most of today’s existing lineages. They diversified extensively between 150-120 mya to became widespread by 120 million years ago, and replaced conifers as the dominant trees from then to the end of this period 66mya.
Molecules
Molecules
Perhaps, the molecules can shed some light. Identifying genomic changes with the origin of angiosperms could be the key to unravelling to the biological innovations that contributed to their fast rise to ecological dominance. The Amborella genome suggests that the first angiosperms probably appeared when the ancestral gymnosperm underwent a 'whole genome doubling' event about 200 million years ago.” Genome structure and phylogenomic analyses indicate that the ancestral angiosperm was a polyploid with a large constellation of both novel and ancient genes that survived to play key roles in angiosperm biology. (Albert et al 2013 ) Another says flowering plants evolved 100my earlier than fossils suggest.
They came in a bewilderingly wide variety of shapes and sizes. This suggested flowering plants had experienced an explosive burst of diversity very shortly after their origins – which, if true, threatened to undermine Darwin's entire model of gradual evolution through natural selection. While Darwin believed in slow and gradual, there are some periods that do not fit the bill. Stephen Jay Gould coined the term 'punctuated equilibrium' in the early 1990 to describe this sort of event..
Recent research reveals that angiosperms evolved relatively gradually after all. Yet this still leaves a number of key questions. About 350,000 known species of flowering plants make up over 90% of all living plant species. Without them, we would have none of our major crops including those used to feed livestock. How, where and when did they originate? And why did they become so spectacularly successful? ( )
Well Charles, I think we have something to offer. Perhaps a better understanding of the environment impacts may help. Plant evolution was slow for a over a hundred million years, dominated by conifers and ferns. Yet these were overtaken in this period by flowering plants. Was this because of the warm dry conditions? These created deserts that only new plant forms were able to colonise, perhaps with some help from soil crusts? The new mixing of the key elements of earth, air and water make us what we are today. What was it in this environment which gave us such a profusion of flowers ?
Wretchedly poor
Wretchedly poor
Darwin did suggest a solution, however. Angiosperms, he said, may have evolved gradually in a remote region of the world as yet unexplored by scientists. By the middle of the Cretaceous, something caused them to spill out of their homeland and rapidly spread across the world.
Aware of the lack of evidence to back up his theory, Darwin described it as "wretchedly poor".
What are the main contenders to explain Darwin's abominable mystery?
- Faster Growth
What were the environmental impacts of these flowering plants? Annual and deciduous angiosperms create higher chemical weathering rates than do perennials and conifers and may have led to long-term climate cooling (Volk 1989).
The plants which preceded the angiosperms - the pines and spruce and cypresses - were conservative when it came to using and loosing water, so that the air was dry - meaning less rain. The angiosperms took up more water, for more souped up photosynthesis. Their faster growth knocked out the pines and took over the tropical regions of the globe. As they spread they bought their own rain with them. This cools off the forest too. Hence the Amazon is not as hot as a pine forest, and a lot cooler than a desert like the Sahara.
High growth
High growth
“Early angiosperms appear to have been limited to disturbed, aquatic or extremely dry sites, suggesting that they were suppressed in most other places by the gymnosperms that still dominated the plant world…
We propose that angiosperms due to their higher growth rates profit more rapidly from increased nutrient supply than gymnosperms, whereas at the same time angiosperms promote soil nutrient release by producing litter that is more easily decomposed” (Berendse & Scheffer 2009)
2. Pollen
2. Pollen
Was it the pollen? The smaller size of the – male - pollen reduces the amount of time between pollination — the pollen grain reaching the female plant — and fertilisation. In gymnosperms, fertilization can occur up to a year after pollination, whereas in angiosperms, fertilization begins very soon after pollination, enabling angiosperms to produce seeds earlier after pollination than gymnosperms, giving them a distinct evolutionary advantage.
The transfer of pollen grains to the female reproductive structure, pistil in angiosperms, is called pollination. There are various agents for this to occur. The one which would have always been there is wind. Plants relying on wind are called anemophilous, literally wind-loving and includes all the conifers previously existing.
There would have been a lot more nutritious powder flying around, some of which will fall to ground.
Darwin saw the success of angiosperms was due to something about the flowers. Could the flowers themselves help explain why the angiosperms became so diverse? The function of the flower is to ensure fertilisation of the ovule and development of fruit containing seeds. But may it not have something to do with the lucsious leaves, now a lot fleshier than the old pine needles?
Pollen ID
Pollen ID
It is interesting that paleobotanists can use pollen to identify the plants at this time. This is the rise of fertilised seeds, where pollen from one flower finds its way to the ovum of another flower. Sometimes that has to be on a different plant. Pollen is like a powder which covers the male cells (gametes). Pollen grains have a hard coat made of sporopollenin that protects the gametophytes during the process of their movement from the stamens to the pistil of flowering plants, or from the male cone to the female cone of gymnosperms. Pollen is produced in the microsporangia in the male cone of a conifer or other gymnosperm or in the anthers of an angiosperm flower.
3. Cotyledons
3. Cotyledons
There are two groups of angiosperms, those with two cotyledons (Dicots) and those with only one - monocotyledons (Monocots). Did Darwin’s ‘abominable mystery’ actually refer only to Dicotyledons?
There is one theory that thinks he did His mystery "has been misunderstood ever since it came into the public sphere in 1903. This is because it has been assumed that by "higher plants," Darwin meant "angiosperms"; that is, all flowering plants". The editors of Darwins letter 1903 clearly believed that he meant angiosperms when he said 'higher plants'.
Yet the Darwin Correspondence Project have published letters between Darwin and two palaeobotanists Oswald Heer (in 1875), and Gaston de Saporta (in 1876). In these letters, it is clear that Darwin believed that it was dicotyledons that appeared suddenly in the Cretaceous, not angiosperms as a whole.
Which came first?
Which came first?
Moncots in Darwin’s time were presumed to prefigure the ‘sudden rise’ in Dicots. I wonder what Darwin would think now we know the following:
The Monocots are now said to have evolved from one source – and that was among the Dicotyledonous plants . The divergence between the two groups is put at about 120mya according to fossil evidence. The oldest fossils that are unequivocally monocots are pollen from the early Cretaceous about 120-110 million years ago.
This split between monocots and dicots is major event in higher plant evolution, yet the date of its occurrence remains unknown because of the scarcity of relevant fossils. As usual the molecular clocks predates the fossils considerably. One reconstruction of the family tree from DNA sequences found the divergence of the monocots, maize, wheat, and rice at about 200 mya (+/- 40my!). “These results imply that the angiosperm lineage emerged in Jurassic-Triassic time, which considerably predates its appearance in the fossil record (120 Myr ago). We estimate the divergence between cycads and angiosperms to be -340 Mya, which can be taken as an upper bound for the age of angiosperms" (Wolfe et al 1989) However Nature Plants recently published (Harendeen et al 2017) a thorough debunking of all claimed pre-Cretaceous angiosperms, by some of the world's most authoritative experts in the palaeo-flora of the lower Cretaceous.
The cotylden is the seed leaf within the embryo of a seed. Moncots are where there is a single seed leaf which grows from the seed. If it is a pair of leaves then it is categorised as Dicots. This little distinction at the beginning of the plant’s life cycle causes each plant to generate big differences. The roots and stems of monocots do not possess a membrane under the bark (cambium) and cannot increase in diameter, whereas dicots possess a cambium and have the ability to increase in diameter Monocots also have narrower rather than broad leaves.
A few examples of monocotyledons are garlic, onions, and most of our major crops like sugar cane, bamboos, wheat, corn and grass, while dicots include beans, cauliflower, apples and pear, etc.
Monocot
Primary root of short duration, replaced by adventitial roots forming fibrous or fleshy root systems
Dicot
Roots develops from the radicle. Primary root often persists forming strong taproot and secondary roots
4. Nitrogen Nodule plants
4. Nitrogen Nodule plants
Could new N-fixing nodules on plants help produce a wide variety of flowers? They certainly helped some plants to grow.
5. Co-evolution with flying insects
5. Co-evolution with flying insects
The 19th century French palaeontologist, Saporta, was the first to suggest that flowering plants and their insect pollinators ‘co-evolved’ together. He argued that this 'co-evolution' drove both groups to diversify unusually rapidly.
Entomophilous, literally insect-loving, plants produce pollen that is relatively heavy, sticky and protein-rich. Insect pollinators are attracted to their flowers by various signals and the nectar offered. They accidentally distribute the pollen. Many insects and some mites are specialized to feed on pollen, and are called palynivores.
The concept of coevolution was picked up and developed by Darwin. “He used it to explain how pollinators and food-rewarding flowers involved in specialized mutualisms could, over time, develop long tongues and deep tubes, respectively. He famously predicted that Angraecum sesquipedale, a long-spurred Malagasy orchid, must be pollinated by a hawkmoth with an exceptionally long tongue. Darwin’s idea of a coevolutionary ‘race’ was championed by contemporary naturalists, including Alfred Wallace…However, strong empirical support for the mechanism behind Darwin’s co-evolutionary model has been forthcoming only in the past two decades. It is now established that selection often strongly favours plants with floral tubes that exceed the length of their pollinator’s tongues. There is also evidence that pollinators gain an energetic benefit from having tongues that enable them to consume most or all of the nectar in deep tubular flowers. (Johnson & Anderson 2010) .‘Co-evolution’ could lead to geographical diversification.
Problems with Co-evolution
But the theory runs into trouble today, according to Augusto (Augusto et al 2014). Early angiosperms may have had flowers, but we now know from fossils that those first flowers were very plain - and probably not that attractive to pollinators. By the time the big, bold flowers that entice insects appeared, the angiosperms were already diverse. However, I would point out that white flowers may have invisible colours to us, and there may have been other forms of attraction, which led to the bright flowers we know today. Rather than contradicting the coevolution, this may give a direction of travel.
6. The answer lies...
6. The answer lies...
Did the soil animals have anything to do with the abominable mystery?
If the co-evolution theory is largely true, then where did the flying insects come from? Most bees, wasps and ants - the Hymenoptera - responsible for much pollination/fertilisation would have had nests in soil. "Ants now found in terrestrial ecosystems the world over, apparently began to diversify only about 100 million years ago in concert with the flowering plants.“ Many flies, Diptera, like fungus gnats, bibionids and crane flies are also widely responsible for pollination, albeit not as specific; and their larvae grow in soil.
It is not hard to imagine how much tastier the new luscious leaves (many now grown as agricultural crops) would have been compared with the dry pine needles previously. These would be more easily decomposed by the worms now on the surface and mites, with their 'new' humification processes in their guts, to produce a load of nutrients which the flowering plants respond to better than the pines.
Soil would be needed to support plants, and as time went on they were able to produce flowers and thus improve fertilisation and its benefits. Flowers and seeds need minerals – which they would get from soils. Again we are looking at change and movement in an environment – soil - over a period of time, rather than thinking flowering plants just arrived. We’ve seen flowering plants need insects to help pollinate/ fertilise them and thereby produce the seeds, but the flying insects need places to feed.
Those seeds – wrapped by fruit – drop to the soil. Unlike seeds of conifers – naked – they have that layer of food helps them on their way. These fruit ion the ground would have provided more food for surface and soil animals When they drop onto soil, either their own fruit feed or the excrement of the animal helps them grow. This would turbo-charge the soil, enriching it further. Again we probably take this relationship between plant fruits and the soil for granted, yet this richness was not always there. It would have evolved due to change, ionic charges and movement, over this period of time.
Were these the components that helped flowers and insects co-explode across the earth?
The components for flowers and insects to expand lay in the soil.
We saw in the last period how Darwin was a great fan of earthworms. If only he had realised the role they play in seed dispersal, he may have found the answer to his own abominable mystery. There would have been no mystery at all.
By exploring how the various roles of these newly emerging characters and organisms, we can get a better sense of the dynamics going on. The evolutionary lists and tables in those scientific papers can come to life as we work out how the various characters- leaves, roots, fungi insects and mites interact to encourage flowers
Page updated
Google Sites
Report abuse