Death of the Dinosaurs
Dr. Paul Olver FGS FRAS – 12th October 2011
This was the first talk in the autumn 2011 series and was given by Dr Paul Olver who has had a long teaching career in geology and astronomy. The event was very well attended by 78 members which is a record for a single meeting.
Over geological time the fossil record shows living organisms coming and going. Five major mass extinctions have occurred the first being in the late Permian and the last being in the late Cretaceous. By far the biggest of these was the first or ‘Great’ mass extinction where it is estimated that around 50% of all species were lost. The last of these was smaller in extent with about a 15% loss but is possibly the most well known because it saw, inter alia, the end of the Dinosaurs. It is also interesting to note that at the same time Ammonites also became extinct (see later).
A fossil of the first reptile was not discovered until 1980 and dates from the Carboniferous period. It was discovered in a rock in a garden wall and studied at the University of Edinburgh. It was initially thought to be an Amphibian but later recognised as the first true Reptile from which all Dinosaurs descend.
In Victorian times fossils were not originally thought to be associated with living things. However with the advent of Darwinism this changed and Gideon Mantle was the first to propose that fossils were the remains of animals.
Both Mammals and Dinosaurs descend from the original ‘Stem Reptiles’ in two principal branches distinguished by teeth! Mammals (and us) were on the branch that had ‘differentiated’ teeth (i.e. molars, canines, incisors) and Dinosaurs on the branch where the teeth were all alike. The Dinosaur branch also evolved into Crocodiles, Lizards and Birds.
There has been a long debate over whether Dinosaurs were hot or cold bloodied. Studies of the fossil record and comparisons with current mammals and reptiles favours Dinosaurs being warm bloodied.
In Britain most dinosaur fossils are found in Southern England where the ratio of carnivores to herbivores is consistent with what is seen today in the East African Plain with current species. This observation supports the view that carnivorous and herbivorous Dinosaurs were in a balance where large herds of herbivores were preyed on by much smaller numbers of carnivores. In this balance the much larger numbers of herbivores could survive the losses imposed by the carnivores and hence complete a food chain.
The speed at which large Dinosaurs could move has been estimated from footprints (‘bumprints’ have also been found showing that they also sat down!) giving stride length (e). Hip height (h) has been estimated from fossilised leg bones. The ratio e/h gives ‘Relative Stride’ and using Ostriches as a model it has been estimated that the larger Dinosaurs speed was in the range 4 – 10 mph. Herbivorous Dinosaurs like Brachiosaurus grew to extreme sizes dwarfing carnivores like Tyrannosaurus Rex and were just too big to be eaten. They could be up to 26 metres long, 16 metres tall (at the head) and weighing up to 88 tons.
Dinosaurs died out at the end of the Cretaceous period 65 MYA together with about 15% of all species. Over the years there have been many theories put forward as why Dinosaurs disappeared. Some of the less likely are:
However looking at the record of the five major extinctions there are good correlations with massive volcanic episodes or Large Igneous Provinces (LIPs). In particular the extinction of the Dinosaurs coincided with a LIP (The Deccan Traps in India). It has also been discovered that at about the same time there was a massive meteorite impact in the area of the Yucatan Peninsula and the Gulf of Mexico. The crater for this was discovered in the 1970s and named after the local town of Chixulub. The crater has long since been filled by sedimentary rocks but has been measured to be about 180 km wide. The size of the meteorite is estimated to have been about 10km possibly among one of the largest to hit the earth. The energy released by the impact was colossal and would have caused mega- tsunamis, firestorms, massive atmospheric pollution with ash and gases and possibly triggering earthquakes and volcanic activity. It is likely that this would have blocked out sunlight over most of the earth for several years and interfered with photosynthesis and plant life at the bottom of the food chain. The large amounts of Carbon Dioxide released into the atmosphere may also have led to longer term global warming.
The currently held view is that the extinction of the Dinosaurs was triggered by the combination of the Chixulub meteorite and the Deccan LIP. It is possible that the Chixulub meteorite alone would have done the job!
Further evidence for the Chixulub meteorite was the discovery of a thin layer of clay visible in many parts of the Earth (e.g. ‘Fish Clay’ in Copenhagen) deposited 65MYA at the Cretaceous - Tertiary (K-T) boundary. The K-T boundary sediments however cannot be found in the UK as they have long since been eroded away. This clay contains a very high percentage of the element Iridium not found at this concentration in any other rocks or deposits at other times. This concentration of Iridium is only found in primordial meteoric material which incidentally contains all the elements in the periodic table.
Meteoric impacts like this are thankfully quite rare although smaller meteorites are quite common. Pebble sized events are observed several times a year although most will fall into oceans and be unobserved. We can expect to see meteorites of around 100 metres every millennium. It is also the case that most meteorites have already been ‘cleaned-up’ by the planets and are therefore now less frequent..
There were several questions at the end of the talk:
Q - Did Dinosaurs make sounds?
A – Yes, as evidenced by bony sound chambers on the skull
Q - How long did Dinosaurs live?
A – Cold Bloodied up to 100 years; warm bloodied up to 30 years
Q – Was there an ‘Iridium Spike’ for the earlier ‘Great Extinction’?
A – No, so probably triggered by volcanic activity
Q – Why was the extinction selective with respect to species?
A – Mammals survived maybe because they were used to storing food and could burrow for protection. Ammonites and other marine species probably died out because their main food was Plankton which also went extinct
Dick Harris