Timing and Rate of the KT Extinction; What Else Died Out?
By Pete Goddard
What died out? ;In the Sea
In the Air
Recommended Further Reading
The event that marked the end of the Cretaceous period of geological time, the KT mass extinction, is most famous for its spectacular effect on the dinosaurs which hitherto had roamed and ruled the Earth. However, whatever destructive force it was that caused such devastation did by no means just select the dinosaurs for extinction. It is estimated that 65-70% of all species died out at the KT. In the sea, ammonites and plesiosaurs, so common in the oceans of the Cretaceous, disappeared whilst in the air the giant and terrifying pterosaurs also became extinct. On a smaller scale 90% of all algae species are thought to have died out. To appreciate the scale of the event it must be remembered that species only required relatively few individuals to survive, cross the boundary and multiply and so even those species that did survive would in all likelihood still have suffered huge losses. However the extinction itself was not the single dramatic event it is often thought to be. Many species were on the decline some time before the KT boundary itself, so, apart from the dinosaurs, who did die out and when?
What Died Out, and What Survived?
In the Sea
Surprisingly, we know least about the extinction of the marine vertebrates. Plesiosaurs and mosasaurs became extinct some time during the Maastrichtian together with the largest marine turtles, but very little is known about the exact timing of their extinctions. Ichthyosaurs, which were also present throughout the Mesozoic did not make it to the KT boundary, but died out during the Cenomanian, some thirty million years before the KT.
More common in the seas and the favourite prey of the mosasaurs were the ammonites. These hard-shelled invertebrates were abundant in oceans throughout the Mesozoic. This abundance combined with their hard shelled forms and marine habitats has led to a large number of fossil specimens and hence a great deal of knowledge about their extinction. It is widely accepted that the ammonites were in decline millions of years before the KT boundary. By the time of their extinction they were restricted to only certain areas of the world in contrast to their previous world-wide abundance. Only nine genera of ammonites have been identified in the topmost part of the Maastrichtian age. These nine genera died out at the boundary, bringing to an end the reign of one of the commonest groups of marine animals of the Mesozoic.
Plankton and Microscopic Life
Studies of these small, often microscopic life forms often give a great deal of information about events such as the KT. This is due to their abundance in the oceans. A study of a rock sequence in Tunisia, North Africa, has indicated that the extinction may not in fact have been the result of one single impact. The extinctions appear to have started before the boundary itself, and although this can be explained as a limited sample size (see later), the fact that the extinctions appear to have continued after the boundary is less readily explained. Larger, more complex species appear to become extinct first with smaller simpler forms surviving longer. It is thought that almost 80% of all species did become extinct at the KT. For those species that lived on the seafloor, as opposed to the plankton that floated freely in the water and near the surface, the crisis of the KT had a much less drastic effect. 50% of all bottom dwelling species of microscopic life became extinct at the Cretaceous/ Tertiary boundary. Those that did survive were found to be tolerant of low oxygen conditions, this points to possible ways in which the extinction was actually caused, which I do not want to mention here but can be found here.
Belemnites as with many other marine, especially free swimming, animals began to decline before the end of the Cretaceous. Further reduced in number by the extinction, they only survived into the Eocene before finally succumbing to an extinction probably caused by an environment they found increasingly hostile. Despite this survival just into the Tertiary, the KT is often credited with causing their extinction.
Brachiopods in some forms survived the KT, they are seen off our coastlines today. Many species did become extinct at the boundary however. Detailed studies of Maastrichtian and Lower Tertiary sequences in Danish chalk identify 27 species common in the uppermost Maastrichtian with only six of these crossing the boundary into the Tertiary. Furthermore it has been found that these six species have morphologically unspecialized forms and are therefore probably more tolerant of changing environmental conditions as would have been expected in such an event. The KT represented another blow to a group that had already suffered badly at the hands of a mass extinction. Abundant during the Palaeozoic ,they never regained their numbers after the mass extinction that ended the Permian.
The gastropods did not seem to be severely affected by the extinction, surviving it relatively unscathed they went on to benefit from the lack of competition in its aftermath to diversify, making the Tertiary their most prolific period. They are both marine and terrestrial.
The bivalves were another group who appear to have benefited from the extinction of competitors at the end of the Mesozoic. Relatively rare in the Palaeozoic they increased in number and diversity throughout the Mesozoic and from the early Tertiary onwards they have come to dominate the hard-shelled shallow marine fauna. This is not to say they were unaffected by the KT. Family Trigoniidae, characterised by enormous, complex hinge teeth were the dominant shallow-water burrowers of nearshore habitats during the early Mesozoic. Only one genus survived the extinction and is present today.
The Echinoderms including starfish and echinoids or sea urchins
also survived the KT relatively unscathed as their abundance in the shore habitats of today testifies as did the small and often delicate Bryazoans.
After stating that those forms of life that were free swimming in the sea were hardest hit by the extinction, it is interesting that the fish in general were not greatly affected. Sharks and rays were hard hit. Of five species found in the Maastrichtian of the western interior of North America none are found above the boundary. As for ray-finned species, the most common and diverse group of bony fish, out of the fifteen species seen below the boundary in the western interior, nine survived. This is a high survival rate amongst species of the Cretaceous.
To many people this is what the KT extinction is about, the extinction of the dinosaurs. This page aims to concentrate on what else apart from the dinosaurs died out. There has been huge amounts of work done on the extinction of the dinosaurs, work which cannot be done justice in the limited space on this page. Therefore......
Click here for "The Timing and Rate of the Dinosaur Extinction"
Mammals are well known for being probably the greatest benefactors of the extinctions of the KT. The mammals of the Cretaceous were small, rodent type animals not approaching the sizes of modern species and though there is clear evidence that significant faunal changes occurred around the time of the Cretaceous-Tertiary boundary, there is no sign of any dramatic event taking place at the boundary itself. The story is slightly different however for marsupial mammals. By far the most significant change in mammalian fauna was the drastic reduction in the number of species of marsupials, with basically only the opossum continuing through to the Tertiary.
Only one family of turtle died out towards the end of the Cretaceous, but this was towards the end of the Campanian and so before the KT anyway. Six other families survived well into the Tertiary and as we know there are many turtles and tortoises alive today. On a species level, seventeen species of turtles were present in the late Cretaceous, of these fifteen survived the event, resulting in a survival rate of 88%.
Work on the Western Interior of North America has identified seven species of salamander and one species of frog present in the late Cretaceous, all of these species survived the events at the KT boundary, yielding a survival rate of 100%.
Crocodiles, Lizards and Snakes
Crocodiles are almost unique in the fact that they are large reptiles who survived the KT extinction relatively unscathed. There does not appear to be any particular reason why an impact that could cause such devastating effect in wiping out all the dinosaurs should have little effect on the crocodiles. So for whatever reason, three families survived the Campanian, the Maastrichtian and into the Tertiary. Several families of lizard similarly survived into the Tertiary with no evidence of any change. These included iguanas and slowworms and other forms were to evolve and join them during the Tertiary such as skinks. Snakes also do not appear to have been greatly harmed by the KT, although only the Boas have spanned the entire time through the Campanian to the present.
Studies have been carried out on terrestrial plants in North America and these figures can be extrapolated to cover global extinction levels, although it must be remembered that North America may have been relatively close to the impact site at Chicxculub and so worse affected than other areas of the world. Taking this into account, studies of the leaf fossil record indicate species-level extinction as high as 75% in vegetation from the southern Rocky Mountains, with a northward decline to 25% in polar broad-leaved deciduous forest from central Alberta. This decreasing effect with increasing latitudes is due to temperate latitude vegetation being better adapted to extremes of climate combined with the presence of dormancy mechanisms, allowing seeds to remain dormant until conditions improved. Groups of both land and sea plants with well-developed dormancy mechanisms appear to have survived better than those without, this is more evidence for the extinctions being due to one major event and not a gradual extinction. However, as with many of the animal extinctions, many species of plants were in decline before the KT. Some groups of plants, despite declining in abundance at the KT survived into the Eocene only to then become extinct. This is more evidence for a general climatic change that appears to have begun at least by the Early Maastrichtian. Overall it should be concluded that the Kt had a limited effect in terms of extinctions on plant life.
In the Air
Pterosaurs existed for the same amount of time as the dinosaurs and it is a common misconception that they were in fact dinosaurs. Emerging in the Late Triassic period they began their existence as fairly small, winged reptiles. It was only in the Cretaceous that they became the huge creatures that they are often thought as, with some species up to 15m in wingspan. No species of pterosaur survived the extinction.
The rate and timing of the extinction is therefore not as straight forward as may have been previously imagined. It is certainly true that there was a long term climate change that had been proceeding from at least the beginning of the Maastrichtian. This climate change was resulting in a marked increase in the extinction rates of various groups of life. Species such as the ammonites and pterosaurs may have become extinct anyway and may never have continued to live on in the world today. The extinctions at the KT boundary itself however were very important and it can be said with a great deal of confidence that the impact at the KT boundary caused the extinctions of many species that would otherwise have survived. Furthermore it must be remembered that the last point in time when a species was alive is not necessarily the latest point it is found in the fossil record. Fossils thought to have died out before the boundary may have in fact reached the boundary but simply not been found as a preserved fossil. The more fossils that are found of any one species, the more the KT looks like one single catastrophic event.
Recommended Further Reading
Mass Extinctions by Stephen K. Donovan
Life Pulse by Niles Eldredge
Extinction: Bad Genes or Just Bad Luck? by David Raup
Basic Palaeontology by Michael Benton and David Harper
The Mistaken Extinction by Lowell Dingus and Timothy Rowe
The Geological Timescale
The Timescale of the Cretaceous
Click here to go to main page "The KT Event"
The University of Bristol, Palaeontology Home page