Skates and sharks were hit hard by the last mass extinction 66 million years ago
The last mass extinction to affect 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 major groups of vertebrates such as dinosaurs, the consequences of this extinction on marine biodiversity remain the subject of intense debate. We have just published a study in the journal Science examining the impact of this crisis on the diversity of elasmobranchs (sharks and rays), a major group of marine vertebrates that survived this mass extinction. Our research indicates that this crisis was sudden and that it affected elasmobranchs in a heterogeneous manner, both in terms of the groups impacted and the geographic distribution of species.
Guillaume Guinot, University of Montpellier
Previous estimates suggest that this crisis may have wiped out more than 40% of genera and 55% to 76% of species. However, a growing body of data indicates that the extent of this event likely varied across different groups, ecologies (e.g., diets, lifestyles), and geographic regions.
However, overall estimates of biodiversity loss during this period have been extrapolated primarily from data on groups of marine invertebrates, which alone cannot reflect the complexity of the extinction patterns during this crisis. Marine vertebrates, due to their higher position in the food chain, could therefore provide new insights into this extinction and the post-extinction recovery of faunas. But these groups would have had to have survived!
Among these marine vertebrates, the elasmobranchs are an iconic group of predators that already constituted a significant component of marine ecosystems during the Cretaceous period and had developed a wide range of ecological niches. Belonging to the class of cartilaginous fish (chondrichthyans), these organisms possess a skeleton that rarely fossilizes. However, they are represented by an abundant fossil record, consisting mainly of teeth that they shed and replace throughout their lives, and whose morphology allows for species identification. Thus, due to 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 an excellent case study for analyzing the impact of this crisis on marine vertebrates.
Using fossil data, our goal was to accurately quantify the extent of the extinction, the characteristics of the victims and survivors, and the consequences of this crisis on the evolution of shark and ray faunas following the extinction.
More than ten years of data collection
We first compiled all fossil record data for all species of elasmobranchs over a time span of approximately 40 million years (from 93.9 to 56 Ma), including the extinction event. This long-term effort spanned more than a decade and involved cataloging the species of sharks and rays present during the Late Cretaceous–Paleocene interval, as well as their occurrences: every instance where fossils were found for each of these species. This information is scattered across several hundred scientific papers published from the19thcentury to the present day, and had to be compiled.
A species can therefore have multiple occurrences, and each occurrence corresponds to a specific geological age and distinct geographic coordinates. We were able to catalog 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, species classification (taxonomy) is a constantly evolving discipline, and it was first necessary to update the classification of each species and, in some cases, correct erroneous identifications. Furthermore, the ages of the geological formations yielding fossils can also be reassessed by new studies, and this information had to be updated. This expert work, tedious but crucial, forms the basis of the analyses we conducted for this study.
Once the data had been compiled, we used statistical models to estimate the times of first appearance and extinction for each of the 675 species. This extensive analytical work is essential because the fossil record contains a number of preservation and sampling biases. We must therefore first account for the spatial and temporal heterogeneity of the fossil record in order to estimate the lifespans of fossil species. These models, in which Fabien Condamine (co-author of the study) is also an expert, then allow us 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 high precision, that 62% of elasmobranch species went extinct during this crisis and that this extinction was “abrupt” on a geological timescale, as it occurred over a period of just 800,000 years.
But were the different groups of elasmobranchs affected in the same way by this extinction? To answer this question, we assessed extinction rates among sharks and rays, and among the different groups of sharks and rays. Our results indicate that rays were more severely affected than sharks (72.6% extinction rate versus 58.9%). The selective nature of this crisis is also evident within the ray and shark groups. Certain groups of sharks still present today (Orectolobiformes, Lamniformes) were more severely impacted, and groups of rays (Rajiformes, Rhinopristiformes) even came close to complete extinction, even though they now comprise several hundred species.
Studies of paleodiversity provide only a partial picture of the consequences of a crisis on the structure and functioning of ecosystems. We therefore needed to assess the impact of this crisis on the various ecological groups represented among the elasmobranchs. We therefore focused on the diets of the shark and ray species most affected by extinction by studying the morphology of their teeth. We were able to distinguish species classified as “durophagous” (feeding on hard prey, such as bivalve mollusks currently represented by oysters, clams, mussels, and scallops) from other species (non-durophagous) and analyzed the extent of this crisis across these two ecological categories. Our results indicate that shark and ray species with teeth specialized for a durophagous diet were more severely affected (73.4% extinction) than the others (59.8%). This is an interesting point because it has been demonstrated that this extinction has strongly impacted the first links in marine food webs (plankton) and the organisms directly dependent on them (e.g., bivalves). Our results therefore suggest a cascade of events that caused a massive loss of diversity among hard-feeding elasmobranchs. We thus have here a second type of selectivity—ecological in this case—against species feeding on shelled prey.
Our analyses indicate that sharks—and particularly non-hard-feeding species—returned to pre-crisis levels of diversity more quickly (albeit over a few million years) than rays, which had not fully recovered even 10 million years after the extinction. Furthermore, this crisis had a major impact on the composition of the elasmobranch faunas that survived the extinction, profoundly reshaping the diversity of this group. These changes are particularly pronounced among rays, where we observe, in particular, the diversification of a group called Myliobatiformes (stingrays, eagle rays, etc.), which likely took advantage of the ecological niches left vacant by the extinction to diversify within them.
Finally, we tested the effect of a species’ geographic distribution on its probability of surviving this crisis. To do so, we compiled the geographic ranges of all species that went extinct or survived the extinction. Our results show that species with a wide geographic distribution had a higher survival rate than others. Even more interestingly, species living at low latitudes were more severely affected, suggesting geographic selectivity.
The causes of this crisis are a matter of debate and are undoubtedly numerous (an asteroid, of course, but also volcanic activity, climate cooling, and falling sea levels). Although our study does not offer a direct answer to this debate, it provides clues as to the possible mechanisms at play during this crisis, particularly our findings regarding the most severe 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 shaped by past extinctions and how it has survived them. Our study provides a sort of profile of the victims of the most recent mass extinction and also gives an idea of the time required for post-extinction recovery—a time span measured in millions of years.
Guillaume Guinot, Paleontologist, University of Montpellier
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