Elasmobranchii

Evolutionary history

The oldest unambiguous total group elasmobranch, Phoebodus, has its earliest records in the Middle Devonian (late Givetian), around 383 million years ago. Several important groups of total group elasmobranchs, including Ctenacanthiformes and Hybodontiformes, had already emerged by the latest Devonian (Famennian). During the Carboniferous, some ctenacanths would grow to sizes rivalling the modern great white shark with bodies in the region of 7 metres (23 ft) in length. During the Carboniferous and Permian, the xenacanths were abundant in both freshwater and marine environments, and would continue to exist into the Triassic with reduced diversity. The hybodonts had achieved a high diversity by the Permian, and would end up becoming the dominant group of elasmobranchs during the Triassic and Early Jurassic. Hybodonts were extensively present in both marine and freshwater environments. While Neoselachii/Elasmobranchi sensu stricto (the group of modern sharks and rays) had already appeared by the Triassic, they only had low diversity during this period and only began to extensively diversify from the Early Jurassic onwards, when modern orders of sharks and rays appeared. This co-incided with the decline of the hybodonts, which had become minor components of marine environments by the Late Jurassic but would remain common in freshwater environments into the Cretaceous. The youngest remains of hybodonts date to the very end of the Cretaceous.

Taxonomy

Elasmobranchii was first coined in 1838 by Charles Lucien Bonaparte. Bonaparte's original definition of Elasmobranchii was effectively identical to modern Chondrichthyes, and was based around gill architecture shared by all 3 living major cartilaginous fish groups. During the 20th century it became standard to exclude chimaeras from Elasmobranchii; along with including many fossil chondrichthyans within the group. The definition of Elasmobranchii has since been subject to much confusion with regard to fossil chondrichthyans. Maisey (2012) suggested that Elasmobranchii should exclusively be used for the last common ancestor of modern sharks and rays, a grouping which had previously been named Neoselachii by Compagno (1977). Other recent authors have used Elasmobranchii in a broad sense to include all chondrichthyans more closely related to modern sharks and rays than to chimaeras.

The total group of Elasmobranchii includes the cohort Euselachii Hay, 1902, which groups the Hybodontiformes and a number of other extinct chondrichthyans with Elasmobrachii sensu stricto/Neoselachii, to the exclusion of more primitive total group elasmobranchs, which is supported by a number of shared morphological characters of the skeleton.

• Total group Elasmobranchii
  • †Order Phoebodontiformes
  • †Order Squatinactiformes
  • †Order Xenacanthiformes
  • †Order Ctenacanthiformes
  • †Order Jalodontiformes
  • †Bandringa
  • Infraclass Euselachii
    • †Order Hybodontiformes
    • †Family Protacrodontidae
    • †Family Tristychiidae
      • Tristychius
      • Acronemus
    • †Artiodus
    • Division Neoselachii (Elasmobranchii sensu stricto)
      • †Family Anachronistidae
      • †Order Synechodontiformes (incertae sedis)
      • Subdivision Selachii (Selachimorpha) (modern sharks)
        • Superorder Galeomorphi
          • Order Heterodontiformes (bullhead sharks)
          • Order Orectolobiformes (carpet sharks)
          • Order Lamniformes (mackerel sharks)
          • Order Carcharhiniformes (ground sharks)
        • Superorder Squalomorphi
          • Order Echinorhiniformes (bramble sharks)
          • Order Hexanchiformes (frilled and cow sharks)
          • Order Squaliformes (dogfish sharks)
          • †Family Protospinacidae
          • Order Squatiniformes (angel sharks)
          • Order Pristiophoriformes (sawsharks)
      • Subdivision Batoidea (rays, skates, and sawfish)
        • Order Torpediniformes (electric rays)
        • Order Rhinopristiformes (sawfishes, guitar fishes, wedgefishes and relatives)
        • Order Rajiformes (skates and relatives)
        • Order Myliobatiformes (stingrays and relatives)

The 5th edition of Fishes of the World sets out the following classification of the Elasmobranchs:

• Infraclass Elasmobranchii
  • Division Selachii (sharks)
    • Superorder Galeomorphi
      • †Order Synechodontiformes
      • Order Heterodontiformes
      • Order Orectolobiformes
        • Suborder Parascyllioidei
        • Suborder Orectoloboidei
      • Order Lamniformes
      • Order Carcharhiniformes
    • Superorder Squalomorphi
      • Series Hexanchida
        • Order Hexanchiformes
      • Series Squalida
        • Order Squaliformes
      • Series Squatinida
        • †Order Protospinaciformes
        • Order Echinorhiniformes
        • Order Squatiniformes
        • Order Pristiophoriformes
  • Division Batomorphi (rays)
    • Order Torpediniformes
    • Order Rajiformes
    • Order Pristiformes
    • Order Myliobatiformes
      • Suborder Platyrhinoidei
      • Suborder Myliobatoidei

Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are a monophyletic superorder within Elasmobranchii that shares a common ancestor with the selachians.