MARINE SPECIATION RATES DURING THE PALAEOCENE-EOCENE “REEF GAP”

We have now embarked upon a new geological era: the Anthropocene; in which anthropogenic climate change is causing a return to hyperthermal climate conditions, characterised by rapidly rising global temperatures, atmospheric CO₂ and ocean acidification. These hyperthermal conditions may have a particularly devastating effect on coral reef biomes, as evidenced by a global absence of coral reefs in the fossil record – a reef gap – from 42 – 57 Ma; a period of time coincident with the Palaeocene-Eocene Thermal Maximum (PETM) and early Eocene hyperthermals.

Here, a phylogenetic framework and time series analysis are used to explore how speciation rates in coral reef associated and non-coral reef associated species were affected during the Palaeocene-Eocene reef gap for four disparate clades – Scleractinia, Chondrichthyes, Anomura and Caridea. For each clade, marine species were split into coral reef associated and non-coral reef associated, where “coral reefs” were defined as those found in the tropics. Non-marine species were excluded. Separate speciation rate curves for coral reef associated species and non-coral reef species were extracted and changepoint time series analysis was used to identify significant changes in mean and variance for each speciation rate curve. Extinction rates could not be assessed as the phylogenies contained only extant taxa.

The analyses revealed differences in the speciation rates of coral reef associated and non-coral reef associated taxa during the Palaeocene-Eocene reef gap. Overall, there is a trend of decreasing speciation rates in coral reef associated species during this period, with little to no impact on speciation rates in non-coral reef species. Although it was not possible to test for changes in extinction rates, these decreased speciation rates in coral reef species during the hyperthermal conditions of the Palaeocene and Eocene may go some way to explaining the “reef gap” seen in the fossil record between 42 – 57 Ma. The results also suggest that coral reef biomes are at greater risk from hyperthermal climates than non-coral reef biomes and that anthropogenic climate change is likely to have an effect on coral reef biodiversity lasting for millions of years.