Xenacanthida is an order of prehistoric shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s that appeared during the Lower Carboniferous

Carboniferous

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period. The family includes the families Xenacanthidae, Diplodoselachidae, and Orthacanthidae. The most notable members of the group are the genera Xenacanthus

Xenacanthus

Xenacanthus is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end of the Triassic, 202 million years ago. Fossils of various species have been found worldwide....

and Orthacanthus

Orthacanthus

Orthacanthus is an extinct genus of fresh-water shark from a family of prehistoric sharks known as Xenacanths. Members of the genus had a long spine growing from the back of their skull and a very long dorsal fin, which ran all along its back giving it an eel-like appearance.About 260 million years...

. Some Xenacanthus

Xenacanthus

Xenacanthus is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end of the Triassic, 202 million years ago. Fossils of various species have been found worldwide....

may have grown to lengths of 4 m (13.1 ft). This group of sharks inhabited freshwater environments. Some forms had large serrated spines extending backwards from the neck. Xenacanthus had characteristic teeth. The base or root of each tooth had a pair of hook-like cusps. Most Xenacanthus died out at the end of the Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 in the Permian Mass Extinction, with only a few forms surviving into the Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 period.

Taxonomy

• Family: Diplodoselachidae Dick, 1981
  • Genus: Diplodoselache Dick, 1981
  • Genus: Dicentrodus Traquair, 1888
  • Genus: Hagenoselache Hampe & Heidkte, 1997
• Family: Orthacanthidae Heyler & Poplin 1989
  • Genus: Orthacanthus
    Orthacanthus
    Orthacanthus is an extinct genus of fresh-water shark from a family of prehistoric sharks known as Xenacanths. Members of the genus had a long spine growing from the back of their skull and a very long dorsal fin, which ran all along its back giving it an eel-like appearance.About 260 million years...
    
    Agassiz, 1843
• Family: Xenacanthidae
  Xenacanthidae
  Xenacanthidae is a family of prehistoric sharks in the order Xenacanthida....
  
   Fritsch, 1889
  • Genus: Plicatodus Hampe, 1995
  • Genus: Triodus
    Triodus
    Triodus is an extinct genus of xenacanthidan shark that lived in the Permian. It was a freshwater shark, and fossils have been found in Germany.-Sources:*The Rise of Fishes: 500 Million Years of Evolution by John A. Long...
    
    Jordan, 1849
  • Genus: Xenacanthus
    Xenacanthus
    Xenacanthus is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end of the Triassic, 202 million years ago. Fossils of various species have been found worldwide....
    
    Beyrich, 1848
• incertae sedis
  • Genus: Anodontacanthus Davis, 1881