Rays and sharks hit hard by the last mass extinction 66 million years ago
The last mass extinction event in the evolution of life occurred 66 million years ago (Ma), marking the Cretaceous/Paleogene boundary. While this biological crisis is known to have caused dramatic global extinctions and wiped out large groups of vertebrates such as the dinosaurs, the consequences of this extinction on marine biodiversity are still the subject of intense debate. We have just published a study in the journal Science on the impact of this crisis on the diversity of elasmobranchs (sharks and rays), a major group of marine vertebrates that went through this mass extinction. Our work indicates that this crisis was brutal and that it hit elasmobranchs in a heterogeneous way, both in terms of the groups affected and the geographical distribution of species.
Guillaume Guinot, University of Montpellier
Previous estimates suggest that this crisis eradicated over 40% of genera and 55-76% of species. However, a growing body of evidence indicates that the magnitude of this event would have varied across groups, ecologies (e.g. diets, lifestyles), and geographic areas.
However, global estimates of diversity loss at this time have been extrapolated primarily from data for marine invertebrate groups that cannot alone reflect the complexity of extinction events during this crisis. Marine vertebrates, because of their higher position in the food chain, could therefore provide new information on this extinction and on the post-extinction recovery of faunas. But these groups must have survived!
Among these marine vertebrates, elasmobranchs are an emblematic group of predators that were already an important component of marine ecosystems in the Cretaceous and had developed a wide range of ecologies. Belonging to the class of cartilaginous fishes (chondrichthyans), these organisms have a skeleton that rarely fossilizes. However, they are represented by an abundant fossil record, mostly composed of teeth that they lose and replace throughout their lives and whose morphology allows to identify the species. Thus, by the quality of their fossil record, their presence before and after the extinction event and their position at the top of the food chain, sharks and rays are a very good case study to analyze the impact of this crisis on marine vertebrates.
Using the fossil record, our goal was to accurately quantify the extent of the extinction, the profile of victims and survivors, and the consequences of this crisis on the evolution of shark and ray faunas after the extinction.
Over ten years of data compilation
We first compiled all the data from the fossil record for all elasmobranch species over a time interval of about 40 million years (from -93.9 to -56 Ma), including the extinction event. This long term work was spread over more than a decade and consisted in making an inventory of the species of sharks and rays present on the Upper Cretaceous-Paleocene interval, but also of their occurrences: all the times where fossils were found for each of these species. This information is available in a disparate way in several hundred scientific works published since the nineteenth century until today, which had to be compiled.
A species can thus have several occurrences, and each occurrence corresponds to an age as well as to distinct geographical coordinates. We were able to inventory more than 3,200 occurrences for 675 fossil species, but we had to verify the identifications and geological ages attributed to each of these occurrences in the scientific literature. Indeed, the classification of species (taxonomy) is a discipline in constant evolution and it was first necessary to update the classification of each species and sometimes to correct erroneous identifications. Moreover, the ages of the geological formations having delivered fossils can also be reevaluated by new studies, and it was necessary to update this information. This tedious but crucial work of expertise represents the basis of the analyses that we conducted for this study.
Once the data were compiled, we used statistical models to estimate ages of appearance and extinction for each of the 675 species. This heavy analytical work is essential because the fossil record includes a number of preservation and sampling biases. It is therefore necessary to take into account the spatial and temporal heterogeneity of the fossil record to try to estimate the life span of fossil species. These models, of which Fabien Condamine (co-author of the study) is also a specialist, can then be used to estimate speciation and extinction rates (number of extinctions or appearances per million years per species) for the group studied.
Sharks and rays were not affected in the same way
Our results show, with a fine resolution, that 62% of elasmobranch species disappeared during this crisis and that this extinction was "brutal" on the geological time scale since it was restricted to a period of 800,000 years.
But were the different groups of elasmobranchs affected equally by this extinction? To answer this question, we evaluated extinction rates between sharks and rays, and between different groups of sharks and rays. Our results indicate that skates were more heavily affected than sharks (72.6% extinction vs. 58.9%). The selective nature of this crisis is also marked within rays and sharks. Some groups of sharks still represented today (orectolobiformes, lamniformes) were more strongly impacted, and some groups of rays (rajiformes, rhinopristiformes) even came close to complete extinction, although they now include several hundred species.
Paleodiversity studies provide only a partial view of the consequences of a crisis on the structure and functioning of ecosystems. We therefore needed to evaluate the impact of this crisis on the different ecological groups represented in elasmobranchs. We were therefore interested in the diets of the shark and ray species most affected by the extinction by studying the morphology of their teeth. We were able to separate species that are "durophagous" (feeding on hard prey, such as bivalve shellfish represented today by oysters, clams, mussels, and other scallops) from other species (non-durophagous) and analyzed the magnitude of this crisis on these two ecological categories. Our results indicate that species of sharks and toothed rays that specialize in a durophagous diet were more heavily affected (73.4% extinction) than the others (59.8%). This is an interesting point because it has been shown that this extinction has strongly impacted the first links of marine food webs (plankton) and organisms directly dependent on them (e.g. bivalves). Our results suggest a cascade of events that caused a huge loss of diversity of durophagous elasmobranchs. We thus have here a second type of selectivity, ecological this time, against species feeding on shell prey.
Our analyses indicate that sharks - and particularly non-durophagous species - recovered to pre-crisis levels of diversity more quickly (within a few million years) than rays, the latter not having fully recovered even 10 million years after the extinction. Moreover, this crisis had a major effect on the composition of the elasmobranch faunas that survived the extinction by deeply reshaping the diversity of this group. These modifications are particularly marked in the case of rays for which we observe a diversification of a group called Myliobatiformes (stingrays, eagle rays, etc.) which probably took advantage of the ecological niches left vacant by the extinction to diversify.
Finally, we tested the effect of species' geographic distribution on their probability of surviving this crisis. To do this, we compiled the geographic range of all species that went extinct or survived the extinction. Our results show that species that had a wide geographic distribution had a higher survival rate than others. More interestingly, species that lived at lower latitudes were more heavily affected, suggesting geographic selectivity.
The causes of this crisis are debated and certainly multiple (asteroid of course, but also volcanism, climatic cooling, lower sea levels). Although our study does not propose a direct answer to this debate, it provides clues as to the possible mechanisms that played during this crisis, in particular our results on the highest extinction at low latitudes.
Today, one third of shark and ray species are threatened with extinction and it is important to understand how the evolutionary history of this group has been impacted by previous extinctions and how this group has survived these extinctions. Our study proposes a kind of standard profile of extinction victims for the last mass extinction and also gives an idea of the time needed for post-extinction recovery. A time that can be counted in millions of years.
Guillaume Guinot, Paleontologist, University of Montpellier
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