For graptolites, the lundgreni event was one of the most severe extinction events of the Silurian. There are many hypotheses for the cause of the lundgreni event, including bolide impact and the spread of anoxia in the oceans. If the spread of anoxia was the cause, at least some sections across the event should show a change from an oxic to an anoxic environment at the time of the extinction. My work in the Welsh Borderland has shown that the sea floor becomes oxygenated at the time of the extinction, having previously been anoxic. This finding does not support the hypothesis that the spread of anoxia was responsible for the extinction. Comparison with other sections across the event is needed to reveal whether or not the same pattern is seen in the rest of the world.

The lundgreni event has been intensively studied in many parts of the world, and it is well known which species survive and which go extinct. However, the question of why some species went extinct and others did not is unanswered. I have tested two hypotheses.

The first hypothesis is that species found over a wide area are more likely to survive than those that are geographically restricted. Results from the database I compiled from the literature show that there are no differences in the geographic ranges of survivors and of victims, so I conclude that geographic range does not affect extinction probability.

The second hypothesis is that K-selected species are more likely to become extinct than r-selected species. K-selected species are large, long-lived and have few offspring, most of which survive; r-selected species are small, short-lived and have many offspring, few of which survive. I have used survivorship curves and data on rhabdosome sizes to determine whether individual species are K- or r-selected. Results so far support the hypothesis, but further research is needed.

The world's longest graptolite

Graptolite parasites!