The fossil record, despite its extremely fragmentary nature remains the best scientific tool available for learning about life in the past. It is far from complete and it can only provide a limited amount of information about organisms, ecosystems and palaeoenvironments, but it has provided evidence of extinctions and five major mass extinction events have been identified in the immense time period known as the Phanerozoic.
A Selection of Shark Teeth Fossils
Picture Credit: Everything Dinosaur
An Extinction is Forever
Notwithstanding the technological developments heralded by advances in genetics, an extinction is finite. Extinctions represent the complete, world-wide end of the line for a species. There are no individuals representing that species to be found anywhere. Non-avian representatives of the Dinosauria, the long-necked Sauropods for example, are extinct, the very last of these animals, collectively termed Titanosaurs, died out at the end of the Cretaceous, some 66 million years ago.
However, it is important to distinguish local extinctions, whereby an organism becomes extinct in a region or area, from true, global extinction. A species or genus may die out in one part of the area where it is distributed, but it might be thriving, or at least surviving everywhere else. Identifying local extinctions, especially in an incomplete fossil record, where many of the fossils have been transported long distances and with a record of moving continents (tectonic plate theory), is extremely challenging.
The correct scientific term for a local or regional extinction is “extirpation”, an organism may cease to exist in one area but could still be found in other areas. Palaeontologists usually use the term extinction in its correct sense, noting the complete disappearance of an organism. Thanks to the vagaries of the fossil record, identifying extirpation events in deep time is extremely difficult. The Liaoning Province of northern China has provided scientists with numerous examples of feathered dinosaurs. Their remains are often beautifully preserved, a result of the way in which these animals may have died . Corpses were deposited in lakes and sank to the muddy, still bottom before being rapidly buried by fine ash deposited over the region by the nearby volcanoes. Whether some of these animals drowned, or whether their deaths were directly attributable to the volcanism is difficult to say for certain in most cases.
Zhenyuanlong Fossil (Zhenyuanlong suni) from Liaoning Province
Picture Credit: Chinese Academy of Geological Science
Unfortunately, whilst a devastating deposit of volcanic ash, perhaps a pyroclastic cloud or the release of toxic carbon monoxide fumes could have led to the deaths of many animals within a habitat, it is very difficult to determine whether such events led to a local extinction (extirpation). In the case of the Liaoning fossils, the stratigraphic record would indicate numerous volcanic episodes but whether a single episode or a series of catastrophic events led to the demise of an entire taxon in the region it is impossible to say. However, the forest ecosystem with its large lakes would have suffered a loss of individuals and probably a reduction in diversity over time.