Sharks and rays experienced a brutal but selective extinction 66 million years ago
Two researchers from the Institute of Evolutionary Sciences of Montpellier (UM/CNRS) show that sharks and rays experienced a sudden and strong extinction (62%) during the last mass extinction that caused the disappearance of dinosaurs 66 million years ago. Within this group, not all species were equally impacted. Two main factors favored or diminished their chances of survival: diet and range. A study published in the journal Science on February 24, 2023.
Sharks and rays belong to the elasmobranchs group. They are currently facing strong anthropogenic pressures with a high risk of extinction for most species. Understanding how this group has reacted to past biological crises could help identify the characteristics of the victims and survivors of these extinctions and better understand the evolution of current populations. Two researchers from ISEM, the Institute of Evolutionary Sciences in Montpellier, Guillaume Guinot and Fabien Condamine, have studied the victims of the last mass extinction that occurred 66 million years ago and the mechanisms of this crisis. The causes of this extinction, which occurred at the Cretaceous-Paleogene (K/Pg) boundary, are still debated and it seems that its magnitude varied according to groups, ecologies and geographical areas.
A sudden and strong extinction
In the marine world, estimates of extinction rates have so far been extrapolated from groups of marine invertebrates that do not reflect the complexity of this crisis. By focusing this time on large marine vertebrates such as elasmobranchs (sharks and rays), which occupy a higher position in the food chain, the two researchers from Montpellier hoped to provide new information on this extinction. In this study, Guillaume Guinot and Fabien Condamine used data from the particularly abundant fossil record for these species: they compiled in the scientific literature the occurrences of each fossil species (each time fossils were found for a given species). This data assembly work spanned more than a decade and represents about 2,600 occurrences for 675 species covering a period of 40 million years including the K/Pg crisis. Because the fossil record is incomplete, statistical models were used to consider different preservation biases, yielding estimated ages of appearance and extinction for each species. Their results indicate a "brutal" extinction on the geological time scale (spread over 800,000 years) and a strong (62%) extinction of shark and ray species during this crisis. Their diversity did not return to previous levels, even after 10 million years. Analyses indicate that not all groups were equal in the face of this extinction: rays show higher extinction rates (72%) than sharks (59%).
Survival factors
Even within groups, not all species were affected in the same way. To understand the mechanisms of this selectivity, the authors were interested in the diets of the shark and ray species most affected by the extinction. By studying their teeth, which represent the majority of the fossils found, Guillaume Guinot and Fabien Condamine observed that species feeding on hard prey, such as bivalve shellfish (known as a durophagous diet) were more strongly affected than others. By a cascade effect, the loss of plankton caused the weakening of the populations of organisms that feed on them, such as bivalves, and thus the weakening of the populations of rays and durophagous sharks. In addition to their diet, their range has also impacted their chance of survival in favor of species with a wide geographic distribution. Species living at low latitudes, i.e. near the equator, have been more strongly affected than species living at high latitudes.
Finally, the two researchers demonstrate that while some groups of sharks still represented today (orectolobiformes, lamniformes) have been more strongly impacted, other groups among the rays (rajiformes, rhinopristiformes) have even come close to complete extinction, even though they now include several hundred species.