Introduction


Early Cretaceous


Mid-Cretaceous


Late Cretaceous


Palaeontological
Evidence


Sedimentological
Evidence


Isotopical Evidence


Summary


Glossary


Further Reading


Climate Change Home Page


KT Event
Links


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Palaeontological Evidence

Palaeontology, the study of ancient animals and plants preserved in rocks, can provide us with information as to what the climate was like in the Late Cretaceous. Observing how fossils differ in separate rock layers in one location can tell us how the climate changed there.

Animal fossils

On the land

Those that lived on land provide us with direct information on the climate.

The most famous creatures of the Late Cretaceous were the dinosaurs (click on the icon on the left to see two well known dinosaurs). They became extinct at the end of this period. Was this to do with a drastic change in the climate? Read on and see!

There has been a discovery of:

  • Campanian-Maastrichtian dinosaur fossils north of the Arctic Circle.
  • dinosaurs of Valangian-Albian age in south-eastern Australia. This region was thought to be near the south pole during this time.
Dinosaurs are generally thought to have preferred warm climates, but this suggests that some dinosaurs could either tolerate three months of winter darkness or were capable of seasonal migrations across a considerable distance.

The idea that dinosaurs coped with high latitudes (i.e. near the poles) for at least 65 million years through the Cretaceous suggests that cold and darkness may not have been the prime factors causing their extinction, as has sometimes been maintained. However, there are plenty of other theories for the extinction of the dinosaurs; some more plausible than others.

The presence of dinosaurs in the polar regions reflects the equable global climate during the Late Cretaceous; i.e. the climate at the poles was not drastically different from that at the equator.

In the oceans

The distribution of fossils of organisms that lived in the sea provide valuable information about the oceans and climate at the time the organisms were alive.

These creatures are very well preserved compared to those on the land. This is because when they died, they sank to the bottom of the ocean, away from the currents associated with shallow water and winds in the atmosphere.

It is known that the sea level at this time was substantially higher than it is today. This allowed many marine biota to spread, such as the ammonites and belemnites into north-western Europe.

The following trends have been observed in the early Late Cretaceous marine fossil record:
  • warm-water sea animals were widespread. For example, coccoliths are known to only live in warm waters. Distributions of these forams in the Late Cretaceous were widespread. This indicates that there was a wide tropical-subtropical zone, with cool water only found near to the polar regions.
  • coral reefs grew 5 to 15 degrees closer to the poles than they do now

This suggests that the ocean was warmer at the beginning of the Late Cretaceous and there was not such a variety in temperature from the equator to the poles as there is today. How did it change over this period?

Evidence for change

The tiny calcareous foram fossil, Nephrolithus frequens, which shifted towards the equator in the Maastrichtian , suggests global cooling occurred. This is supported by North Atlantic planktonic foram distributions which also trended away from the poles with time.

However, Late Maastrichtian poleward migration of several other planktonic forams directly conflict with evidence of cooling at this time. For example, the foram Pseudotextularia elegans is the only species having a high-latitude occurrence late in the Maastrichtian that can be correlated with independent isotopic evidence for climatic warming. Migration to both polar regions at the same time testifies to the global extent of a proposed latest Maastrichtian warming trend (66.9-66.6 Ma ago).

Cool-water assemblages of these fossils are found only in the very high southern latitudes (near the south pole) and are not found in such abundance around the palaeo-north pole. This suggests that the only true cold water in the Late Cretaceous occurred at the south pole. The majority of the Earth's oceans (including those at the paleo-North Pole) were relatively warm and temperatures did not vary considerably. This is completely different to what is observed today, where shallow ocean temperatures vary from 20 degrees C in the tropics to close to zero at the poles.

However, climatic evience from marine and terrestrial organisms does not always agree. Further investigation is required.

Plant Fossils

Data from terrestrial plants supports that from marine organisms in indicating greater climate equability:
  • Vegetation zones were displaced about 15 degrees poleward of their present positions.
  • Peat deposits that would give rise to widespread coal formations formed at high latitudes.
Vegetation, unlike marine organisms, is directly exposed to the atmosphere. The structure and composition of environmentally equilibriated vegetation is controlled by, and therefore reflects, climate. Vegetation types can be recognised in modern vegetation and correlated with temperature regimes.

As with the Early Mesozoic, plant productivity was evidently concentrated in middle and high latitudes.

Near the poles, conifer and deciduous plant fossils discovered suggest the winter temperatures were not as low as they are today. Perhaps these ancient forests were in coastal areas where freezing temperatures were avoided by the proximity of the ocean. In more continental high latitudes, winter temperature probably fell well below freezing, and the plant fossils reflect this.

At mid latitudes, open canopy woodlands and forests were dominated by a mixture of conifers and ferns. Large angiosperm trees were comparatively rare.

Low latitude (near the equator) vegetation tended to be xeromorphic and only patchily forested suggesting a hot arid environment.

It has been well established that the terrestrial plant record indicates an appreciably more equable world than today, with temperate forests extending to the polar regions. This supports the animal fossil evidence.

Flowering Plants

These evolved in the Early Cretaceous and are known collectively as angiosperms. They have different appearances across the climatic zones of the world. For example:
  • Leaf margins: In modern vegetation the ratio of toothed to smooth margined leaves changes with mean annual temperature. In the Cenomanian, leaf margin ratios correlated well with palaeolatitude.
  • Leaf size: This is related strongly to temperature, humidity/water availability, and light levels. Large leaves occur in humid conditions, and size decreases with decreasing temperature or precipitation.
  • Drip tips: Leaves with long tips occur most frequently in evergreen leaves in humid environments (e.g. tropical rain forests).
  • Leaf texture: Leathery leaves typically are evergreen and predominate in climates above 15 C. Thin leaves are typically deciduous and are most common in climates below 15 C.
The fossil evidence with respect to all these criteria suggests a predominantly tropical to subtropical climate throughout the Late Cretaceous. For example:
  • the first Late Cretaceous angiosperms were mostly weedy, pioneering shrubs. These were likely to have been of disturbed habitats in seasonally dry tropical or subtropical climates.
  • the later angiosperms from the same period had long, slender leaves suggesting a predominantly tropical climate.
The traditional approach has been confined to material visible to the naked eye, but increasing attention is paid in modern studies to spores and pollen.

Wooded Plants

In situations where climatic conditions vary frequently, wood produces rings as a consequence of variations in growth rate. Rings can reflect where temperature, light, or water availability fluctuates on a yearly cycle (for example, a cool, seasonal climate in the southern polar region during the Late Creataceous is indicated by growth rings in wood), or less regularly in environments with more erratic variations in growth conditions (e.g. sporadic droughts).

Evidence for change

North American palaeobotanical work suggests a temperature rise from the Albian to a Santonian optimum followed by a decline in the Maastrichtian. This decline is displayed by subtropical lush habitats giving way to strongly seasonal temperate conifer dominated mammalian habitats. At the very end of the Maastrichtian the climate change was relatively abrupt. Is it the signature of an impact winter.

Climate Change at the K/T boundary

A large bolide impact, such as that thought to have occurred at the Cretaceous/Tertiary (K/T) boundary, should produce large amounts of light-attenuating debris, thereby causing an 'impact winter'. Because of thermal buffering in the oceans, evidence for a brief impact winter would be found only in terrestrial environments. Aquatic leaves in the K/T boundary section at Wyoming, USA, suggest structural deformation due to freezing.

The impact winter is thought to have been significant in the northern hemisphere, but had little effect on the land biota at high latitudes of the southern hemisphere.

At present, climate is a major control on defining the location and diversity of plants and animals because of the marked temperature gradient from the tropics to the poles. In the Late Cretaceous, climate was less variable between regions. Instead, the rise and fall of sea level would tend to have more of a control. Therefore, palaeontology alone cannot be used to reconstruct the climate at this time. Sedimentology and isotope data from the Late Cretaceous need to be studied to provide a more complete picture.

 

 

| Intro | Early Cretaceous | Mid-Cretaceous | Late Cretaceous |

| Palaeontological Evidence | Sedimentological Evidence | Isotope Evidence |

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