The first 50 million years of neoselachian (Chondrichthyes, Elasmobranchii) evolution

Introduction

Living sharks, rays, and skates form a monophyletic group, the Neoselachii (= crown Elasmobranchii). Their evolutionary history is mainly based on isolated teeth and fin spines due to the low fossilisation potential of their predominantly cartilaginous skeleton and extends back into the Early Jurassic (ca. 200 Ma). The Toarcian (Early Jurassic) to Aalenian (Middle Jurassic) transition (ca. 175 Ma) is assumed to represent a first major diversification event in the evolutionary history of neoselachians resulting in a significant diversity increase cumulating in the Late Jurassic (ca. 150 Ma). Recent advances in identifying fossil elasmobranchs allow to re-evaluate the Early and Middle Jurassic elasmobranch fossil record to provide a better understanding about early neoselachian evolution.

Methods

A comprehensive data base of published Jurassic elasmobranch occurrences based on published records was compiled to establish diversity and diversification rates using the software package R.

Results

While stem group members (†Synechodontiformes, †Agaleidae) still dominated the elasmobranch associations during the earliest Jurassic, the transition from the Pliensbachian to the Toarcian (183Ma), represents a key time interval in the evolutionary history of neoselachians. It is marked by a major radiation event resulting in a significant diversity increase of crown elasmobranchs (Hexanchiformes, Heterodontiformes, Orectolobiformes, Squantiniformes, Rajiformes, †Apolithabatiformes, †Toarcibatidae). The remaining six crown elasmobranch orders known today appeared successively in the fossil record. Carpet sharks show the highest diversification rate across the Toarcian - Aalenian transition.

Conclusions

The results largely support previous interpretations but also allow to paint a much more detailed picture of the events and to better understand which group drove the overall diversity increase. Potential factors for the Early – Middle Jurassic diversification include biotic (declining hybodontiform shark diversity, evolutionary novelties in ecological adaptations) as well as abiotic factors (increasing seawater temperatures, variations in eustatic sea level).