The Extent of the Cretaceous
The Cretaceous Period began about 144 million years ago and terminated with the well-known asteroid impact some 66 million years ago. Geologists subdivide it into 12 stages, each defined by particular rock formations, fossils and sediments at a specific locality called the type area. Several of these type areas are located in France (e.g. Cognac, France, is the type area for the Coniacian Stage). These stages lasted for several million years each, and much geological, climatic and evolutionary change took place within each of them. Figure 1 gives the time frames of the twelve Cretaceous stages.
Figure 1: The Cretaceous and its twelve stages
The Cretaceous stages vary in duration but average somewhat less than seven million years. The Aptian stage has the greatest duration, at about 12 million years; while the shortest stage is the Santonian, at below three million years. The final stage is known as the Maastrichtian (approximately 6.1 million years in duration), the stage that ended with the famous asteroid impact and mass extinction of dinosaurs and other life forms. Significant rock formations or the earliest appearance of particular organisms define both the lower and upper boundaries of these stages. For example, one marker for the base of the Maastrichtrian is the earliest occurrence of a marine mollusc, the ammonite Pachydiscus fresvillensis.
Figure 2 shows a fossilised Pachydiscus fresvillensis from Madagascar, dated at about 69 million years.
Figure 2: A Pachydiscus fresvillensis fossil, about 69 million years old
The Cretaceous lies within the Mesozoic Era, which we can think of as the age of dinosaurs. The Mesozoic encompassed the Triassic, Jurassic and Cretaceous periods, and lasted for almost 180 million years. In turn, the Mesozoic era lies within the Phanerozoic eon, the expanse of time since the beginning of the Cambrian period to now (about 541 million years in total) and that is characterised by teeming life on Earth). This eon comprises the Palaeozoic, Mesozoic and Cenozoic eras. The Cretaceous was followed by the Paleogene Period, lasting to approximately 23 million years ago.
Figure 3 shows the configuration of Earth’s land and oceans in the late Cretaceous (94 million years ago).
Figure 3: The configuration of Earth’s land and oceans in the late Cretaceous
We see that Africa and South America were much closer than they are now, but Europe and North America were much closer too. Over the period of approximately 79 – 80 million years of the Cretaceous, the general configuration of today’s continents emerged, the Pacific and Atlantic oceans were formed and the Indian Ocean emerged from the Tethys Ocean. Previously, the Earth’s land had consisted largely of two continents, Laurasia in the north and Gondwana in the south, a configuration that persisted into the Jurassic (the period immediately preceding the Cretaceous). These continents were almost completely separated by the Tethys Ocean, and Laurasia and Gondwana had already started to separate, a process known as ‘continental drift’.
Though drift rates may seem slow over a human lifetime, when nature has millions of years to play with, the distances moved by continents become enormous. For example, the current drift is approximately 2.5 cm per year, or 2.5×10-5 km per year. This does not seem a lot but, if we consider a very long period of time, such as the duration of the Cretaceous (80 million years), the distance grows in this case to 2000 km. Drift rates have varied over geologic time but, during particular phases of the Cretaceous, have reached as much as 17 cm per year in certain parts of the ocean basins.
Earth’s magnetism was relatively stable during the Cretaceous. In particular, magnetic reversals involve flipping of the Earth’s magnetic field, such that the north and south poles essentially swap places. Generally, this phenomenon occurs somewhat randomly every few million years. Currently, the magnetic north points roughly towards the geographic north pole – known as ‘normal polarity’. However, no magnetic reversals are observed in the geomagnetic record from about 120 million years ago to about 83 million years ago. This phase of extended normal magnetic polarity is known as the Cretaceous Normal.
Cretaceous Sea Levels and Reduced Currents
The world’s sea levels were much higher than at present and, for much of the Cretaceous, land took up only approximately 18% of the Earth’s surface, compared with 28% today. However, despite a reduced area of land, the total habitable area of the Earth may have reached its greatest during the Cretaceous because a warm world fostered widespread life.
The oceans were about 100 to 200 metres higher in the Early Cretaceous (145 – 100.5 million years ago) and about 200 to 250 metres higher in the Late Cretaceous (100.5 million – 66 million years ago) than at present. High sea levels may have resulted from water affected by the growth of mid-oceanic ridges. Ocean building was widespread at this time of splitting of the mega-continents. The production of new oceanic crust over the last 80 million years is estimated at approximately 20 million cubic kilometres per million years. However, about 120 million years ago, the production of new crust rose to approximately 35 million cubic kilometres per million years and remained at about this level for the next 40 million years. Thus, an array of new and expanding ocean ridges and their additional volume of rock displaced water and caused sea level rise.
The atmosphere was much warmer than today and it was warm even at the poles. The temperature gradient from the equator to the poles was much less than now, so that there was less seasonality and less atmospheric circulation (winds) than today. Reduced wind led to weaker oceanic currents than today, reduced up-welling and long periods of relatively stagnant deep oceanic waters. That relative stagnation led to anoxia (oxygen deprivation), exactly the conditions for the formation of dark shales that today yield oil (e.g. oil deposits of Saudi Arabia and the wider Middle East). Shale is a fine-grained sedimentary rock, made of clay minerals and other minerals, including quartz and calcite. Dark shales are dark in colour because they comprise significant amounts of unoxidized carbon (organic matter that yields oil) and are often deposited in stagnant waters with little oxygen.
Seasons and Climate
During the Cretaceous the Earth began to experience more well-defined seasons, involving colder winters and warmer summers. Both fossil evidence and computer models of the Cretaceous climate suggest that at times the global average temperature was up to 15 degrees Celsius higher than today. Such changes in temperature gave rise to rapid evolution of plant life. The climate was generally warmer and more humid than today, probably because of very active volcanism associated with high rates of seafloor spreading. Seafloor spreading releases the greenhouse gas, carbon dioxide, the warming effect of which is greater than the cooling effect of the release of sulphur dioxide, which partially blocks the transmission of sunlight. Thus, the poles were covered by forest rather than ice sheets. Evidence of forests in both polar regions is extensive and the existence of large forests cloaked in darkness, in some places for nearly six months every year, is a very interesting scenario and so different to what we have today.
The climate of the Cretaceous was possibly warmer than at any other time over the 541 million years from the Cambrian Period to now. In addition, the climate was more even in that the difference between the temperature at the poles and the Equator was about 50% that of the present. Evidence from fossil plants suggests that tropical climates pertained as far as 45o North. In addition, temperate climates (climates with moderate rainfall over the year, mild-to-warm summers and cool-to-cold winters) characterised the poles.
Models of Earth’s climate for the mid-Cretaceous take account of the positions of the land masses and the distribution and extent of the existing oceans. They indicate weaker atmospheric circulation than today. Though the Early Cretaceous was somewhat cooler than the Late Cretaceous, for most of that period the Earth’s surface was a greenhouse environment.
Cretaceous Carbon Cycles and Climate Change
Estimated Cretaceous carbon dioxide levels are very consistent with climate variation from the geologic record. Mid-Cretaceous atmospheric carbon dioxide levels were at times up to five times the present level, indicating a surface temperature of 20–21 degrees Celsius. The warmth of the Cretaceous resulted from tectonism which emitted additional carbon dioxide. However, the Cretaceous also saw enhanced organic carbon burial (sequestration), which tended to decrease atmospheric carbon dioxide levels. Burial involved the formation of carbonates such as limestones and chalks, and organic carbon in the form of oil, coal, gas and organic-rich rocks. These carbon reservoirs took carbon from the atmosphere and influenced the long term Cretaceous climate significantly. For much of the Cretaceous the organic carbon burial rate was greater than burial rates since then, and included three major peaks that correspond to sustained phases of anoxia (oxygen depletion) in the oceans, leading to the formation of oil and coal. Thus, the Cretaceous climate reflected an interplay between emission of carbon dioxide from tectonic activity and burial of carbon. Factors that influenced this interplay included the relative positions of the continents, a lack of mountain ranges by comparison to today, runoff from rivers, the radiation of flowering plants and their eventual predominance over conifers, and the radiation of plankton and other carbon-bearing forms into deep waters.
Cretaceous Land Animals
The Cretaceous is well known for its array of dinosaurs (including T-Rex) but other kinds of fauna are of interest. Many life forms that had arisen in the Jurassic, or beforehand, proliferated during the Cretaceous (e.g. marine molluscs such as ammonites, and rudists, which were box- or tube-shaped marine bivalves). Many animal groups assumed their modern form, including birds, reptiles, molluscs (especially clams and snails), placental mammals, snakes and lizards, modern teleosts (bony fishes) etc. In particular, many modern ants, wasps and bees made their appearance in the Cretaceous. Fossil bee nests from Patagonia show that bees and certain flowering plants co-evolved around 110 to 120 million years ago during the Early Cretaceous, though the oldest known fossil bee is much more recent, at about 72 million years.
Figure 4 shows an early Cretaceous fossilised wasp from the Araripe Basin of Brazil.
Figure 4: An early Cretaceous fossil wasp
Figure 5 shows a member of the Orpthoptera (grasshoppers) from 125 million years ago. It is similar to the modern katydid and its wings still display colour banding. It derives from the family Haglidae, which are found only in particular rock formations in China.
Figure 5: A fossil grasshopper from 125 million years ago
Figure 6 shows a spider attacking a parasitic wasp that was caught in the spider’s web, about 100 million years ago.
Figure 6: A spider and parasitic wasp from 100 million years ago.
Both spider and wasp were preserved after becoming trapped in amber.
Of course, dinosaurs and marine reptiles flourished and birds radiated everywhere. Figure 7 shows a Cretaceous bird, Gansus zheni; from 120 – 130 million years ago from north-eastern China.
Figure 7: A Cretaceous bird, Gansus zheni
The morphology of Gansus zheni indicates that it had adapted to an amphibious lifestyle, possibly similar to that of a modern duck.
Figure 8 shows another Cretaceous bird, Confuciusornis sanctus, also from 120 – 130 million years ago from north-eastern China.
Figure 8: A Cretaceous bird, Confuciusornis sanctus
Confuciusornis sanctus is considered to be the most primitive example of a beaked bird and displays very long feathers. Its beak was toothless, though relatives of today’s birds had teeth. Thus, loss of teeth may have evolved separately in Confuciusornis and modern birds. Though it had a rather small wing amplitude and somewhat slender and weak feathers, it may nevertheless have been a competent flyer. Its wing shape suggests a dense forest habitat and probably it was a good glider. Initially thought to be herbivorous, Confuciusornis fossils containing fish bones have been found.
Cretaceous Fish and Marine Reptiles
Figure 9 shows a Cretaceous fish, Paleobalistum goedeli, of the Cenomanian stage between 100 – 93 million years ago, from Lebanon.
Figure 9: A Cretaceous fish, Paleobalistum goedeli
Figure 10 shows a fossil fish Pseudoberyx syriacus, of the Middle Cretaceous, Cenomanian Stage (about 95 million years ago), also from the Lebanon.
Figure 10: A mid-Cretaceous schooling fish Pseudoberyx syriacus
It may have been a schooling fish. It probably died out during the Late Cretaceous and it is thought to have left no descendants.
Cretaceous seas are famous for their large reptilian predators such as the mosasaurs, plesiosaurs and ichthyosaurs. Certain mosasaur species, such as Mosasaurus hoffmannii and Tylosaurus Proriger, reached very large sizes – certainly 13 – 15 metres in length and possibly up to 17 metres in the case of hoffmanii.
Figure 11 shows the skull of Mosasaurus hoffmannii, from approximately 68 million years ago.
Figure 11: The skull of Mosasaurus hoffmannii
This specimen is 13 metres in length and is held at the Natuurhistorische Museum in Maastricht, the Netherlands.
Marine reptiles needed to be large because they shared the seas with large sharks such as the ginsu shark, Cretoxyrhina Mantelli. Cretoxyrhina refers to a genus of mackerel sharks that lived from approximately 107 to 73 million years ago, of which Cretoxyrhina Mantelli was one of the largest. In competing with the existing large sharks, some mosasaurs evolved rapid growth to very large sizes, and they changed from land-dwelling creatures the size of monitor lizards to those extreme sizes in only about five million years.
Figure 12 shows a fossilised Cretoxyrhina Mantelli, from approximately 100 million years ago.
Figure 12: Cretoxyrhina Mantelli, approximately 100 million years old
Cretoxyrhina Mantelli could reach over eight metres in length and, at those lengths, had a body mass of 3 – 4 metric tonnes, much bigger than today’s great white sharks. Experts in biomechanics judge that a large specimen was well capable of killing and feeding on a mosasaur of considerably greater body length. The Cretaceous fossil record shows that sharks predated on and consumed all but the biggest mosasaurs in large numbers. Possibly, these sharks were the apex predator in Cretaceous seas (or equal to the large mosasaurs), and even the biggest mosasaurs were not safe from them.
Molluscs and Small Marine Life-forms
Most chalks were deposited during the Cretaceous. Chalk is a soft limestone composed mainly of the hard plates of coccolithophores, floating algae (phytoplankton) that proliferated during the Late Cretaceous. The most famous chalks are those of the Cliffs of Dover, on the coastline of Kent in England, dating to approximately 70 million years ago (late Cretaceous) when Britain and much of Europe lay under an ocean. White deposits deriving from coccolithophores and other creatures, formed sediments that accumulated up to 500 metres thick in some places. Eventually, these sediments compacted and became chalk. In addition, diatoms (single-celled algae with cell walls made from transparent silica) emerged and radiated into the oceans and, after the Cretaceous, moved into fresh waters.
Figure 13 shows a Cretaceous fossil clam from Tulear, Madagascar, approximately 110 million years old.
Figure 13: A Cretaceous fossil clam from Madagascar
Ammonites are extinct marine molluscs, related to today’s squid, cuttlefish and octopuses.
They arose during the Devonian period, and became extinct in the Cretaceous–Paleogene extinction. They were very common in the Cretaceous, probably living in the open sea, rather than at the sea bottom, and fossil evidence suggests that they may have used ink to avoid predators. Figure 14 shows a fossil ammonite, mantelliceras, from Upper Albian Strata, Morocco. It is approximately 100 million years old.
Figure 14: Cretaceous fossil ammonite
Cretaceous Plants and Flowers
Flowering plants (angiosperms), which have seeds enclosed within an ovary (often a fruit), arose at the start of the Cretaceous and became more abundant as time progressed.
Figure 15 shows a modern water lily.
Figure 15: A modern water lily
We believe that water lilies and their relatives were among the first flowering plants. They may have evolved from gymnosperms (flowerless plants that produce cones and seeds which are not enclosed by any kind of covering), between 125 million and 115 million years ago.
Figure 16 shows the oldest known flowering plant.
Figure 16: The oldest known flowering plant
It is an aquatic plant, Montsechia vidalii, found in limestone from the Pyrenees in Spain, and is dated to between 125 and 130 million years old. It is thought to have lived and pollinated in freshwater lakes. It had no petals, but had seeds enclosed within a fruit. However, evidence exists for flowering plant pollen in fossils that are even older – around 140 million years old.
Figure 17 shows a 125 million year old Eudicot (a flowering plant, Leefructus mirus), from China. It is related to the modern buttercups. A Eudicot is a dicotyledonous plant (i.e. having an embryo with two seed leaves), that displays grooved pollen and other characteristics.
Figure 17: A 125 million year old Eudicot
At the time of this Eudicot the flowering plants were just beginning to diversify and radiate, initially from the low latitudes and then towards the polar regions. We think that they could have originated from a single common ancestor. Today, Eudicots comprise about 75% of all angiosperms, including peas, apples, peaches, sunflowers and roses.
Figure 18 shows a Tropidogyne pentaptera flower, preserved in amber, from Myanmar, about 100 million years old.
Figure 18: A Tropidogyne pentaptera flower from Myanmar, about 100 million years old
It is a newly-discovered species of petal-less flower and is very finely preserved in amber. What appear to be five petals are in fact sepals. A sepal is a part of the flower which usually serves as protection for the flower during budding, and can provide support for the petals when the flower is in bloom. This flower is about four millimeters across and may have fallen from an araucaria tree into resin which then hardened and was preserved. We see that the flower has spreading, veiny sepals but it also has a ribbed ovary.
As the Cretaceous progressed, more and more flowering plants (angiosperms) appeared. They radiated all over the Earth’s land and became a major ground cover. Of course, the evolution of flowering plants was stimulated by the emergence of bees that co-evolved with the plants to pollinate flowers.
Figure 19 shows a Sapindaceae fossil plant from Lebanon, dated at 93 million years ago. It is from the Soapberry family, whose fruits contain saponin, a substance that is sometimes used in the manufacture of soaps.
Some Cretaceous plants were already in existence during the Jurassic period or before, including mosses, ginkgos (large trees that appeared around 270 million years ago and that are represented by only one species today – Ginkgo biloba), ferns, horsetails (primitive vascular plants that reproduce by spores, similarly to ferns), cycads (seed plants, with a cylindrical woody trunk, that look like ferns and palms but are not closely related to either) and conifers. However, flora such as oak trees, hickory trees, maple, walnut and beech trees arose and radiated all over the world. Grasses (and the related bamboos) evolved during the Cretaceous and also radiated everywhere. Other plants that first appeared in the Cretaceous include the Eudicots (similar to our primulas), Ceratophyllum (hornworts), Monocotyledons, flowering plants that have a single cotyledon (an embryonic leaf) in their seeds, and ancestors of the Magnolia.
Figure 20 shows the fossilised trunk of a conifer tree from approximately 120 million years ago.
Figure 20: The fossilised trunk of a Cretaceous conifer tree
Figure 21 shows a fossilised Viburnum lesquereuxii leaf, from the Dakota Formation of Ellsworth County, Kansas, USA. It is from the Upper Cretaceous, approximately 93 – 99 million years old.
Figure 21: A Viburnum lesquereuxii leaf, chewed by insects
This leaf shows insect damage, possibly from caterpillars or beetles, that fed on the tissue between the veins. Viburnum is today a common group of plants that still undergo significant insect damage. The Viburnum group arose in the early Cretaceous, about 140 million years ago, and began rapid diversification and radiation at about the time of this fossil leaf.
Inferring Past Temperatures from Fossil Leaves
In modern forests we find a strong correlation between the average annual temperature and the percentage of the forest species that display untoothed leaves (smooth edges). The estimated percentage of species with untoothed leaves in plant and tree leaf fossils enables us to estimate the average annual temperature and rainfall in which ancient forests grew. Though we do not understand very well the relationships between leaf morphological characteristics and climate, we infer that the average annual temperatures of the Cretaceous were much higher than today. For example, leaf morphology allows us to infer an average annual temperature of Coniacian stage (about 90 million years ago) floras of the Arctic at approximately 13 degrees Celsius. Today the average winter temperature in the Arctic is minus 34 degrees Celsius, while the average summer temperature is 3-12 degrees Celsius.
The fossil record of Cretaceous leaves suggests reduced stomata density because of increased carbon dioxide concentrations. Stomata are openings on leaf surfaces (sometimes also on stems) that enable exchange of gas between the plant and the surrounding air. As the atmospheric carbon dioxide content increases, some flora reduce the size of their stomata. No longer must they open their stomata as wide to enable diffusion of carbon dioxide for photosynthesis. In addition, many flora display reduced density of stomata on their leaves when carbon dioxide levels increase. Rapid evolution of flowering plants took place during the Cretaceous when leaves developed smaller, but in fact more numerous, stomata, enabling them to use carbon dioxide more efficiently, and therefore adapting easily to any decline in carbon dioxide levels. Cretaceous leaves suggest that carbon dioxide levels did indeed rise and fall at different times of the Cretaceous, and the enhanced gas exchange capability of angiosperms enabled them in some environments to outcompete conifers that previously had been predominant species.
Mass Extinction at the End of the Cretaceous
Today the asteroid impact is credited with causing the mass extinction at the end of the Cretaceous (the Cretaceous-Paleogene extinction). The so-called Chicxulub crater is buried beneath the Yucatán Peninsula, Mexico, near the town of Chicxulub. The impact occurred in shallow water (100 metres – 200 metres) on a carbonate platform, thus releasing carbon dioxide and sulphur dioxide. The crater is buried beneath sediments that were laid down during the Tertiary Period (post-Cretaceous time, between 66 million years ago and 2.6 million years ago), up to a kilometre thick.
Evidence for the impact is provided by extensive Iridium-rich layers at many sites around the Earth, and particularly shocked quartz, which indicates an impact rather than mere volcanism (i.e. extremely high pressures are required for shocked quartz – much higher than provided by volcanic activity). Sites where these deposits have been found include South America, many in North America, Asia and Europe. In addition, shocked quartz of the correct age has been retrieved from sediments collected in the Pacific Ocean. On the basis of argon-argon dating, the Berkeley Geochronology Center has dated the impact at 66.043 ± 0.011 million years ago.
The first attempts at modelling the impact concentrated on ejection of dust and consequent cooling of the atmosphere because of interception of sunlight. More recent models also account for the additional effect of the release of sulphur dioxide but also the warming effect of release of carbon dioxide.
Approximately three quarters of plant and animal species disappeared after the impact. It is estimated that 80 – 90% of all marine species, such as ammonites, many coccolithophores and other small marine species, vanished completely, though a very incomplete fossil record makes such estimations uncertain. Evidently, extinction was selective, and 90% of freshwater species survived the extinction event by comparison to 12% of land-dwelling vertebrates. Creatures that lived on the sea floor were more likely to survive the extinction than those living higher up the water column.
Possibly, many groups were in trouble a few million years before (e.g. flying reptiles, marine reptiles such as ichthyosaurs and, in fact, many bird types). The intense volcanism of those times, especially that relating to the Deccan Traps, impacted on many species and ecosystems. These traps involved extreme basaltic volcanism of the Deccan Plateau of west-central India, starting about 67 million years ago and persisting for an extended period, estimated variously at a minimum of 30,000 years but up to five million years. They embody many layers of basalt that are over two kilometres deep in some places and cover an area of approximately 500,000 square kilometres, with a volume of about 1,000,000 cubic kilometres.
Since the 1980s it has been held that this episode could have led to the extinction by itself, though the modern view is that the Deccan volcanism released sufficient carbon dioxide to increase the atmospheric temperature by only one or two degrees Celsius -not sufficient to cause such a severe mass extinction. In any case, the fossil record suggests reductions in biota (animal and plant life) towards the end of the Maastrichtian, so that declines in ecosystems were well under way. The asteroid impact may have caused the extinction or else may simply have provided the coup-de-grace on species and ecosystems that were already struggling. In the end, the mass extinction of so many life forms paved the way for the coming age of mammals and, eventually, of man.
I wish to thank both Torin O’ Neill and Noelle O’ Neill (students at Muritai School in Eastbourne, Lower Hutt), Quincy Buckley (student at Muritai School, Eastbourne), Michael Sheridan, Dr. Brian Jones and Dr. Lucy Forde for feedback on an early draft.
My interest in the Cretaceous is long-standing but was stimulated recently by reading The Cretaceous World by Peter Skelton, Robert Spicer, Simon Kelley and Iain Gilmour (Cambridge University Press; ISBN-13: 978-0521831123). I thoroughly recommend this book, covering the major geological, biological and climatic evolutions of the Cretaceous.
References to Figures
Figure 1: retrieved from https://www.britannica.com/science/Cretaceous-Period/Major-subdivisions-of-the-Cretaceous-System
Figure 3: retrieved from https://www.britannica.com/science/Cretaceous-Period
Figure 4: retrieved from http://www.fossilmuseum.net/Fossil_Sites/Santana-Formation/wasp/wasp.htm
Figure 6: retrieved from https://www.vice.com/en_us/article/mgb3ga/a-tour-of-ancient-life-still-trapped-in-amber
Figure 7: retrieved from https://www.sciencefriday.com/articles/from-china-a-flock-of-fossils/
Figure 8: retrieved from https://www.sciencefriday.com/articles/from-china-a-flock-of-fossils/
Figure 9: retrieved from http://www.fossilmall.com/EDCOPE_Enterprises/fish/fishfossils74/fish-fossils-74.htm
Figure 10: retrieved from http://www.fossilmall.com/EDCOPE_Enterprises/fish/fishfossils115/fishfossils115.htm
Figure 11: retrieved from https://www.nu.nl/wetenschap/2914550/reusachtig-fossiel-maastricht-van-mosasaurus.html
Figure 12: retrieved from https://www.sciencephoto.com/media/815387/view/cretoxyrhina-mantelli-shark-fossil
Figure 13: retrieved from https://www.fossilera.com/fossils/polished-fossil-astarte-clam-cretaceous–45
Figure 14: retrieved from https://moorabool.com/product/fossil-ammonite/
Figure 15: retrieved from https://blog.everythingdinosaur.co.uk/blog/_archives/2012/09/10/cretaceous-flowers-in-your-pond.html
Figure 18: retrieved from https://www.earth.com/news/flower-discovered-100-million-year-amber/