Final Report Summary - SHARKEVOL (Novelties and phylogeny in the evolutionary radiation of modern sharks and rays)
Evolutionary radiations are key components of the great tree of life, marking points when groups of organisms expand dramatically, and yet the reasons for such expansions are debated: are they driven by new 'key' adaptations that make their possessors more successful than their contemporaries, or do they result from major environmental changes that provide new opportunities? Sharks, skates and rays are hugely successful and they have been characterised as perfectly adapted killing machines, and yet the origins of major groups and their key novelties are unclear. New methodological approaches will provide key insights into originations of different major groups and strong hypotheses about the timing and rate of radiation, and the relative importance and timing of different morphological adaptations in their success.
Neoselachians are highly successful in modern oceans, conquering not only offshore and shallow marine environments, but also the open and even the deep-sea. In these environments, they occupy the positions of top and apex predators and hence are responsible for maintaining the balance and stability of ocean ecosystems and contribute to the regulation of food webs. In addition to this key position in recent as well as ancient ecosystems, modern sharks, skates and rays (collectively named Neoselachii) have a very long evolutionary history ranging back to 250 million years. Apparently neoselachians replaced precursor shark groups such as hybodonts and xenacanthids rather rapidly in the Mesozoic, however the exact timing and manner of this replacement is still obscure.
Although the reasons for neoselachian success remain largely unknown, living sharks and rays combine unique adaptations presumably precipitating their outstanding position and might be the result of the most successful marine body-plan evolution. Among others these are adaptations for swimming (e.g. facultative homoiothermy), feeding (varied jaw kinematics and persisting tooth replacement), reproduction (highest diversity of reproduction modes in vertebrates; e.g. egg-lying, placental viviparity) and highly sensitive and additional sensory organs (i.e. electroreception). New numerical comparative phylogenetic methods allow biologists and palaeobiologists to probe questions about the timing of the origins, key evolutionary novelties and to track morphospace evolution. Comparisons of apparent competitors, such as crown-group neoselachians and extinct cartilaginous fish groups occupying similar ecological niches and food web positions (e.g. synechodontiforms, hybodonts, xenacanthids), can be made in terms of rates of change in diversity and disparity. These methods enable assessing which characters were crucial in driving the initial radiation of neoselachians and the evolutionary steps cumulating in the perfect predator we see today, but also whether the groups might have competed with each other.
Completing this ambitious and comprehensive project several different subprojects are required. The basis is constituted by the most comprehensive morphological phylogeny of all known fossil skeletal remains of extinct but also living neoselachians. To achieve this, the majority of all known fossil skeletal remains were examined and in part taxonomically revised with surprising results. In addition to establishing a few new taxa (e.g. Pseudorhinidae, Crassonotidae), intraspecific sexual and ontological dimorphism in neoselachian teeth was substantiated implicating the determination of a few species, especially stem-lineage neoselachians, as invalid taxa. Furthermore, almost all skeletal remains of previously assumed hexanchiform sharks, that are the most basal group within the major calde Squalomorphii, could not be verified but rather identified as stem-lineage neoselachians (Synechodontiformes). Therefore, a detailed revision of hexanchiform sharks is currently performed including the analysis of their dental evolution. Due to these unexpected new and crucial results the project focus was expanded to study the timing of origination and adaptation events (e.g. Squaliformes), but most importantly to establish a novel method to analyse node age estimations of different clades (e.g. Squatiniformes).
Neoselachians are highly successful in modern oceans, conquering not only offshore and shallow marine environments, but also the open and even the deep-sea. In these environments, they occupy the positions of top and apex predators and hence are responsible for maintaining the balance and stability of ocean ecosystems and contribute to the regulation of food webs. In addition to this key position in recent as well as ancient ecosystems, modern sharks, skates and rays (collectively named Neoselachii) have a very long evolutionary history ranging back to 250 million years. Apparently neoselachians replaced precursor shark groups such as hybodonts and xenacanthids rather rapidly in the Mesozoic, however the exact timing and manner of this replacement is still obscure.
Although the reasons for neoselachian success remain largely unknown, living sharks and rays combine unique adaptations presumably precipitating their outstanding position and might be the result of the most successful marine body-plan evolution. Among others these are adaptations for swimming (e.g. facultative homoiothermy), feeding (varied jaw kinematics and persisting tooth replacement), reproduction (highest diversity of reproduction modes in vertebrates; e.g. egg-lying, placental viviparity) and highly sensitive and additional sensory organs (i.e. electroreception). New numerical comparative phylogenetic methods allow biologists and palaeobiologists to probe questions about the timing of the origins, key evolutionary novelties and to track morphospace evolution. Comparisons of apparent competitors, such as crown-group neoselachians and extinct cartilaginous fish groups occupying similar ecological niches and food web positions (e.g. synechodontiforms, hybodonts, xenacanthids), can be made in terms of rates of change in diversity and disparity. These methods enable assessing which characters were crucial in driving the initial radiation of neoselachians and the evolutionary steps cumulating in the perfect predator we see today, but also whether the groups might have competed with each other.
Completing this ambitious and comprehensive project several different subprojects are required. The basis is constituted by the most comprehensive morphological phylogeny of all known fossil skeletal remains of extinct but also living neoselachians. To achieve this, the majority of all known fossil skeletal remains were examined and in part taxonomically revised with surprising results. In addition to establishing a few new taxa (e.g. Pseudorhinidae, Crassonotidae), intraspecific sexual and ontological dimorphism in neoselachian teeth was substantiated implicating the determination of a few species, especially stem-lineage neoselachians, as invalid taxa. Furthermore, almost all skeletal remains of previously assumed hexanchiform sharks, that are the most basal group within the major calde Squalomorphii, could not be verified but rather identified as stem-lineage neoselachians (Synechodontiformes). Therefore, a detailed revision of hexanchiform sharks is currently performed including the analysis of their dental evolution. Due to these unexpected new and crucial results the project focus was expanded to study the timing of origination and adaptation events (e.g. Squaliformes), but most importantly to establish a novel method to analyse node age estimations of different clades (e.g. Squatiniformes).