145 - 66 mya Cretaceous

Palaeosols Dinosaurs Flowering plants Leaves Root Nodules Mycorrhiza Animals Mites

Dear Charles,

We know that you had concerns which you called the ‘abominable mystery’ - about how flowers spread across the earth so quickly…

We could take note of a fictitious gardener called ‘Arthur Fallowfield, in an old BBC comedy programme, whose answer to any question was:
‘The answer lies in the soil’

Cheers Charlie

Chalk

This period goes on a long time, for nearly a hundred million years. Cretaceous means ‘chalky’ and refers to the river bed where the age was discovered and described, with extensive chalk deposits in Europe, and many parts of the world. The deposits from the Cretaceous are of marine limestone , a rock type that is formed under warm, shallow marine conditions. Chalk is a very specific type of marine limestone, made from coccoliths, microscopic discs from the armour plating of tiny amoeboid protists that thrived as part of the phytoplankton population in the warm shallow Cretaceous seas - and is the bedrock under agricultural soils in many parts of the world.

Cretaceous chalk have few impurities because sea levels were very high, so there was little land exposed to supply other sediments, and as the continental margins were flooded most land was far away.

One important impurity led to an advanced technology. There are extensive layers of clay mineral rich chalk and silica in the form of flint that is very common in places and formed the basis for a continent wide extractive and advanced technological industry across Stone Age Europe.

Cracks

This is when the supercontinent Pangea broke up and formed continents much as we know them today. All those continents had all the creatures and characteristics we have been unearthing up till now. However, as soils now drift apart, the more recent soil inhabitants' like earthworms, evolve further in the isolated areas, in their case north and south. It is hard to see that much soil could be carried from one continent to another now – either by air or water, enabling divergent evolauion..

The continents were similar as now. They are quite distinguishable at this period. While many of the soils’ components a have grown up together over the past 250 million years, there are new opportunities for species of plants and animals to now evolve differently on different continents.

The soil, at the end of this period would house a similar biodiversity in many parts of the world as it does today.

Climate

The climate 100mya was due to a 2-10X increase in CO2 compared with today, when we know higher values of CO2 result in additional climate problems. Iceland would be like Florida, and much more CO2 but similar amount of oxygen, as today (Barron & Washington 1985). The climate was warmer than present, although throughout the period a cooling trend is evident. The tropics became restricted to equatorial regions, and northern latitudes experienced markedly more seasonal climatic condition.

If you could visit Earth as it was 100 then, you wouldn't recognise it. Compared with our temperate world, this was a hothouse world of dense jungle and Sahara-like desert. It was overrun by dinosaurs and the air was full of life with various insects buzzing around them. The plants grew more and became dependent on the flying insects, as they relied on them for fertilisation. But to do so, they needed the soil.

Several Enigmas to Cretaceous climate (Hay et al 2018)

'Proper' Terrestrialisation

But what was going on underground? I believe this was the period of 'proper' terrestrialisation. The common use of the word terrestrialisation' talks about plants and animals 'conquering the land'. But they could not have; they would have soon perished in the dry stark conditions. The idea that they could have survived and evolved does not fit with the prevailing conditions. To make that monumental move requires a stable environment to live in. That requires 'complex terrestrial ecosystems to be established' (Seldon 2001). And this was the period when that occurred ' As the plants and animals and other organisms came out of the sea there was something to worm, burrow into or grow in.

There was more digestible leaves, a lot more roots, and more dead bits of plants and animals, and loads - shed-loads, of dinosaur dung. These all fed a lot more soil organisms - micro organisms (bacteria), meso organisms (mites) and macro organisms (worms), and encouraged a new diversity.

The soil would have been getting deeper and darker, as the soil processes that were present in the previous period now began to radiate into the new worlds caused by Pangea's breakup. They took with them the three decomposition processes - mineralisation, de-lignification, and varying abilities of humification. The soil structures now based on micro- and macro-aggregates were strong enough to support the weight of those dinosaur treads.

This period of 'proper' terrestrialisation, ie using the soil as a new living space, was helped as the tasty leaves piled up, creating new structures for inhabitants to mix with underlying minerals to provide more nutrients and structure and support. It may look wasteful, but leaf decomposition provided a faster recycling of the necessary nutrients for plants - provided there is enough air about for aerobic decomposition by bacteria and fungi. I propose that woodlice were the first proper terrestrialisers.

Added to which was nutritious pollen and accompanying creatures and organisms that fall from trees back to earth regularly providing more food for the soil grubs. This together with the new rhizospheres around the new rooting systems, would have increased the volume for aggregates and pores to provide life ,

Anoxic Event

The Cenomanian-Turonian oceanic anoxic event around 90mya is considered to be the most recent truly global oceanic anoxic event in Earth's geologic history. There was a large carbon cycle disturbance signified by a large positive carbon isotope excursion and also large disturbances in the ocean's nitrogen, oxygen, phosphorus, sulphur, and iron cycles. Later on, when anoxia became widespread, the production of nitrous oxide, a powerful greenhouse gas, drastically increased because of elevated nitrification and denitrification rates. This powerful positive feedback mechanism is what may have enabled extremely hot temperatures to persist in spite of the supercharged organic carbon burial associated with anoxic events.
But what was the role of soil in all this?
It seems much of the carbon burial was at sea. Enhanced phosphorus recycling, evidence in seafloor sediment would have resulted in an abundance of nitrogen fixing bacteria, increasing the availability of yet another limiting nutrient and supercharging primary productivity through nitrogen fixation. (Monteiro et al 2012)

Mixing machine

The new inhabitants, like ants and insect larvae did not contribute much to the soil structure, instead feeding off the soil inhabitants and roots. The soil gave them an environment, but they contributed little to its formation. Meanwhile the earthworms added the massive new component of earth moving. They could mix & move the earth and all its constituents to new levels. The dung beetles ate dinosaur dung to recycle it. They go on to become important links between later herbivores and the grass in the Cenozoic Era.

It is noticeable that earthworms and dung beetles, which evolved relatively recently, then radiated out into separate strands as the new continents broke away. These new continents would produce new species. Those creatures that had existed for a hundred million years already, did not evolve much after the break-up. On reflection, this makes sense. The oldies were already colonising the soil which ended up all over the world , whereas the upstarts had not fully explored their new environments, so when new opportunities opened, they adapted more.

There was not only a lot of dung about, but there would also have been a lot of carcasses to be mixed in to the soil.

Burying beetles

Burying beetles, are an ancient group of beetles belonging to the family Silphidae, that appeared around 100 mya

Fossil Evidence

Fossils of burying beetles, like sexton beetles, are quite rare, primarily because these beetles live in environments (such as soil and decaying matter) that are not ideal for fossilisation. However, fossil evidence of burying beetle families (Silphidae and its relatives) suggests that they and carrion beetles (which include Nicrophorus) have been around since at least the Cretaceous period, approximately 100 mya. These beetles likely evolved to take advantage of dead animals as a food source around the time larger terrestrial vertebrates (including dinosaurs) were common. A notable fossil discovery of a carrion beetle from the family Silphidae was found in Cretaceous amber, dating back about 99 mya.

Molecular Evidence

Molecular phylogenetic studies provide studies use comparisons of DNA sequences from living beetles to estimate when different groups diverged from common ancestors. Based on molecular clock analyses, divergence times suggest that sexton beetles likely diverged from other carrion beetles in a later period, round 66 - 23 mya (Paleogene)..

We know that at the end of this period the dinosaurs became extinct. But did the richness of the soil community survive? Many dinosaur-dependent insects survived the Cretaceous/Tertiary mass extinction, but had to adjust to life without the big guys (Chin 2008). How did the soil get on in the next era - the Cenozoic Era

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