The Early Jurassic represents a crucial time interval in the evolutionary history of elasmobranchs, because the Toarcian witnessed a first major diversification, suggesting a profound reorganization of ecological niches of chondrichthyans, probably accompanied by a subsequent diversity decline of hybodontiforms within marine environments. Potential factors underlying the Toarcian elasmobranch radiation event not only include evolutionary novelties in ecological adaptations of swimming, feeding, and reproduction, but also abiotic factors such as increasing seawater temperatures and variations in eustatic sea level associated with the Toarcian Oceanic Anoxic Event (T-OAE). These events might have played an important role in the Toarcian elasmobranch diversification event by regulating diversity dynamics through the availability of higher speciation and dispersal rates.
In attempt to better understand macroevolutionary patterns and processes of Jurassic chondrichthyans, we analysed the generic diversity of Pliensbachian to Aalenian elasmobranchs and hybodontiforms and explored their response to the T-OAE. In doing so, we calculated the estimated mean standing diversities (EMSD) using 10 time bins of approximately 2 Myr duration and evaluated the relationships between EMSD and variations in both seawater temperature and eustatic sea level to test whether these parameters affect the observed diversity patterns.
Our data indicate profoundly different diversity dynamics of elasmobranchs and hybodontiforms. The EMSD is low in Pliensbachian to Aalenian hybodontiforms, indicating an evolutionary stasis. Conversely, a constant taxonomic increase in elasmobranchs is recorded, spanning from the Pliensbachian to the end of the Toarcian, before reaching a diversity plateau in the Aalenian. These divergent patterns might suggest that hybodontiforms were not competing with elasmobranchs, but more likely are the result of still existing taxonomic misconceptions of Jurassic hybodontiforms, mainly caused by morphological characters that are either ambiguous or broadly distributed among these anatomically rather conservative chondrichthyans. Notwithstanding this, our results indicate that variations in seawater temperature and eustatic sea level changes associated with the T-OAE were not the primary drivers underlying the observed elasmobranch diversity patterns. Therefore, it might be possible that the diversification of elasmobranchs was opportunistic, benefitting from the appearance and subsequent radiation of new food resources, probably in response of enhanced surface productivity during the T-OAE. This hypothesis, however, needs to be tested, pending the inclusion of other time-equivalent marine vertebrate groups in future diversity analyses. Moreover, a detailed re-evaluation of Jurassic hybodontiforms will contribute to our understanding of chondrichthyan diversity dynamics across the T-OAE.
Comment on “Carbon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal)” by Hesselbo S., Jenkyns H.C., Duarte L.V. and Oliveira L.C.V.