The Subclass Elasmobranchii is widely considered nowadays to be the sister group of the Subclass
Holocephali, although chimaeroid fishes were originally classified as elasmobranchs along with modern
sharks and rays. While this modern systematic treatment provides an accurate reflection of the
phylogenetic relationships among extant taxa, the classification of many extinct non-holocephalan
shark-like chondrichthyans as elasmobranchs is challenged. A revised, apomorphy-based definition
of elasmobranchs is presented in which they are considered the equivalent of neoselachians, i.e.
a monophyletic group of modern sharks and rays which not only excludes all stem and crown
holocephalans, but also many Palaeozoic shark-like chondrichthyans and even close extinct relatives
of neoselachians such as hybodonts. The fossil record of elasmobranchs (i.e. neoselachians) is
reviewed, focusing not only on their earliest records but also on their subsequent distribution patterns
through time. The value and limitations of the fossil record in answering questions about elasmobranch
phylogeny are discussed. Extinction is seen as a major factor in shaping early elasmobranch
history, especially during the Triassic. Extinctions may also have helped shape modern lamniform
diversity, despite uncertainties surrounding the phylogenetic affinities of supposedly extinct
clades such as cretoxyrhinids, anacoracids and odontids. Apart from these examples, and the supposed
Cretaceous extinction of ‘sclerorhynchids’, elasmobranch evolution since the Jurassic has
mostly involved increased diversification (especially during the Cretaceous). The biogeographical
distribution of early elasmobranchs may be obscured by sampling bias, but the earliest records of
numerous groups are located within the Tethyan realm. The break-up of Gondwana, and particularly
the opening of the South Atlantic Ocean (together with the development of epicontinental
seaways across Brazil and Africa during the Cretaceous), provided repeated opportunities for dispersal
from both eastern (European) and western (Caribbean) Tethys into newly formed ocean
basins.
Holocephali, although chimaeroid fishes were originally classified as elasmobranchs along with modern
sharks and rays. While this modern systematic treatment provides an accurate reflection of the
phylogenetic relationships among extant taxa, the classification of many extinct non-holocephalan
shark-like chondrichthyans as elasmobranchs is challenged. A revised, apomorphy-based definition
of elasmobranchs is presented in which they are considered the equivalent of neoselachians, i.e.
a monophyletic group of modern sharks and rays which not only excludes all stem and crown
holocephalans, but also many Palaeozoic shark-like chondrichthyans and even close extinct relatives
of neoselachians such as hybodonts. The fossil record of elasmobranchs (i.e. neoselachians) is
reviewed, focusing not only on their earliest records but also on their subsequent distribution patterns
through time. The value and limitations of the fossil record in answering questions about elasmobranch
phylogeny are discussed. Extinction is seen as a major factor in shaping early elasmobranch
history, especially during the Triassic. Extinctions may also have helped shape modern lamniform
diversity, despite uncertainties surrounding the phylogenetic affinities of supposedly extinct
clades such as cretoxyrhinids, anacoracids and odontids. Apart from these examples, and the supposed
Cretaceous extinction of ‘sclerorhynchids’, elasmobranch evolution since the Jurassic has
mostly involved increased diversification (especially during the Cretaceous). The biogeographical
distribution of early elasmobranchs may be obscured by sampling bias, but the earliest records of
numerous groups are located within the Tethyan realm. The break-up of Gondwana, and particularly
the opening of the South Atlantic Ocean (together with the development of epicontinental
seaways across Brazil and Africa during the Cretaceous), provided repeated opportunities for dispersal
from both eastern (European) and western (Caribbean) Tethys into newly formed ocean
basins.
