Evolutionary patterns of Mediterranean Oligocene-Miocene reef corals

Tracing evolution of reef-coral genera (zooxanthellate-like scleractinian corals) during the Oligocene and Miocene in the Mediterranean region is certainly a main goal to achieve in order to understand a complex palaeogeographic and climatic history, with stepwise transition towards cooler climates, biotic turnovers and the almost complete disappearance of zooxanthellate corals near the end of the Miocene.The dataset used for this work is based on our REEFCORAL database, that currently hosts data on more than 3000 occurrences of coral genera from 285 Oligocene and Miocene localities in the circum-Mediterranean regions, including the Mediterranean, the Paratethys, and some Middle-East and Eastern Atlantic areas. Up-dating and homogeneization of stratigraphical data associated to each coral occurrence, together with some needed systematic revision for some scleractinian genera, were made before entering the related information in the database. Although some stages or areas appear less represented than others, because of poor investigation, or limited stratigraphic resolution, or even non availability of outcrops, the broad geographic and temporal distribution of the different coral localities is considered to be satisfactorily representative of the whole “Mediterranean”region both in time and space.Evolution of reef coral generic diversity during the Oligocene and Miocene is traced by analyzing richness, extinction and origination curves for genera over this period.The Mediterranean generic pool of zooxanthellate-like corals during Oligocene-Miocene time includes 102 genera. Temporal changes of generic richness show an increase from Rupelian to Chattian followed by a marked decrease in the Aquitanian. The number of genera is increasing again in the Burdigalian and is then characterized by a continuous decline towards the end of the Miocene.By the production of a stratigraphic range chart, number of originations and extinctions per stage have been also calculated. Different curves have been produced in order to minimize potential biases, considering in particular the genera which are known from a unique locality in the Mediterranean Province and hence which can be related either to a true very rare occurrence in space and time or in some case to taxonomical-identification bias. Variation in the duration of stage also can introduce a biased pattern in coral evolution. Therefore, origination and extinction curves have been weighted by stage duration.The main characteristic of the evolutionary curves is that while originations are more important than extinctions during the Oligocene, this is clearly reversed from the Aquitanian onwards. During the Miocene, the number of originations is decreasing from the Burdigalian to the Serravallian while the extinctions remain broadly stabilized during the same time interval. This is followed by a drastic decrease of both originations and extinctions during the Upper Miocene. The marked increase of the extinction curve in the Messinian is due to the disappearance of the Mediterranean reef-coral fauna at the end of this stage. The Oligocene can be seen as period with important evolutionary turnovers as, both for Rupelian and Chattian, extinctions are in some way associated to originations, suggesting this period as characterized by good recovery conditions. This is in contrast with Miocene evolutionary patterns, also underlined by the comparison of the generic richness curve with the extinction and origination curves. In particular, the proportion of genera extinct compared to the Mediterranean generic pool at the same time is clearly increasing during the whole Miocene excepted during the Tortonian time.Moreover, while the evolutionary pattern during the Oligocene is in good accordance with that known in other reef-coral provinces, and at the global scale; the Miocene evolution of Mediterranean reef-corals appears mainly driven by regional environmental factors. These factors are certainly related to the gradual closure of the Mediterranean Basin during the Miocene, this limiting the genetic pool, and the general cooling of the region due to its progressive migration northwards, outside of the tropical belt.