Where did the fish shaped reptiles go?
During the Triassic and Jurassic fish shaped reptiles (ichthyosaurs) ruled the sea for 157 million years as effective and powerful predators, however their legacy only exists in the fossil record. a recent article published in Nature Communications explains how such a successful and diverse group of reptiles managed to go extinct 28 million years before the next major extinction event and whether or not they were the only group of organisms to become extinct in the same period and if so how can a major group of animals to just disappear, and is it likely to occur again?
During the early Triassic ichthyosaurus began to grow as a group, however it is widely speculated when they actually became extinct; recent analysis has determined that it was likely to have occurred at the end of the Cenomanian. Previous studies have incorrectly assumed that the ichthyosaurs had low richness in the Cretaceous which had been declining since the Jurassic however we know thanks to recent studies that even up until a few million years before their extinction that the ichthyosaurs were a diverse group of organisms with many very different species existing within the group, a group of this size and diversity should have been able to withstand a large amount of selection pressure without going extinct under ordinary conditions however a large unexplained decrease in diversity in the Early Cenomanian
There were previously two hypotheses as to what caused this extinction event, they were either that the ichthyosaurs were out competed by other marine reptiles or fish or that there was an extinction event which killed off ichthyosaurs sole food source of soft cephalopods, both hypotheses claim there was a minor biological cause for the extinction of the ichthyosaurs as opposed to a major event.
Competition with Fish
It has been suggested that the ichthyosaurs were not diverse enough to cope with an explosion of biodiversity caused by the appearance of fish however this has been contradicted by more recent data which suggests that the ichthyosaurs were as diverse as the fish that were present prior to their extinction. There are other issues with the hypothesis that fish out competed the ichthyosaurs as the fossil record fails to show enough evidence to support this idea. Fish which were large enough to successfully compete with the ichthyosaurs for prey don’t appear in the fossil record until 3 million years (link http://www.ncbi.nlm.nih.gov/pubmed/19923126) after the last ichthyosaurs do and the two are unlikely to have inhabited the same waters, this means that it is unlikely that was any competition between ichthyosaurs and fish. The first fish also occurred in different region to the
Scarcity of food
The idea that starvation alone caused the extinction of the ichthyosaurs is unlikely to be correct however starvation is likely to have been a major factor in this event as starvation is compatible with recent data. Ichthyosaurs are unlikely to have starved as they would not have had a sole food source in which they’d have been completely dependent on so would be virtually unaffected by the extinction of one food source.
Previous studies have been incompatible with the data regarding the decline of the ichthyosaurs claiming that only biological factors caused their extinction when evidence suggests an extinction even occurred in the Early Cenomanian which caused a large decrease in diversity leaving only the apex predators. These declines are not specific the ichthyosaurs and recorded in many other groups around the same time such as phytoplankton and reef building organisms providing evidence that the extinction of ichthyosaurs was not an isolated event but a part of a much broader series of extinction events.
Why does this affect us?
Since the industrial revolution humans have changed the conditions on earth, there is more carbon dioxide and other greenhouse gasses in the atmosphere, the oceans are becoming slowly more acidic and many species that have long since been established on earth are going extinct. Understanding why and how certain groups of organisms have gone extinct in the past can help us to identify individual species or larger taxonomical groups which are threatened now and can help to minimise the damage we do to the diversity we have on earth, especially in the oceans where we know very little about what manner of species exist within them yet alone the impact we have on the species that do live there.
It also holds importance as we can see conditions that historical extinction events have in common and use them to predict if another extinction even is coming and how we can prevent ourselves from causing another one as we cause global climate conditions to change through human caused climate change.