Question

Which geological period saw the highest diversity of fishes? List the three main groups of armoured fishes that coexisted and radiated in this period and their main characteristics. Provide some possible explanations for their extinction.

Answer 1

Which geological period saw the highest diversity of fishes?

Answer: The Devonian period also known as the Age of Fishes saw the highest diversity of fishes.The most important variety among the fishes were the armored placoderms,a group of fish with powerful jaws lined with bladelike plates that acted as teeth. Coiled shell-bearing marine organisms known as ammonites first appeared early in the Devonian.

When fishes started to develop they had no jaws and the support structure was made of cartilage.These fishes were known as Ostracoderms. Further development in fishes led to the formation of fishes with jaws,gills and paired fins. Sharks also developed during the Devonian period. Osteichthyes (known as the bony fish) appeared during the middle of the devonian period.

List the three main groups of armoured fishes that coexisted and radiated in this period and their main characteristics.

Answer: There are five or six major groups of Devonian placoderms.

One group, the Arthrodires, account for nearly 2/3 of the genera.

• distinguished by having two pairs of gnathal (tooth-like) plates extending from the upper jaw.
• The more advanced arthrodires have an unusual neck joint that considerably increases gape size.
• most advanced arthrodires had reduced armor and were probably adept swimmers.
• typically less than 1 m in length, but some (e.g., Dunkleostus) became the giants of their day.
• Two species of groenlandaspid arthrodires (Groenlandaspis pennsylvanica and Turrisaspis elektor) were found at Red Hill.

Antiarchs are the second most diverse group of placoderms.

• The jointed head and thoracic shields formed a heavily armored box with a small anterior opening for two protruding eyes.
• Each pectoral fin was completely encased in bone and in the more advanced genera it was jointed in a fashion more reminiscent of arthropods.

Peltalichthyida, Rhenanida, and Ptyctodontida are the three other main groups of placoderms.

• Peltalichyids (e.g., Lunaspis) are dorsoventrally compressed placoderms with shortened thoracic armor, expanded pectoral spines and dorsally oriented eyes.
• Rhenanids (e.g., Germendina) are even more dorsoventrally compressed placoderms with reduced thoracic armor and horizontally expanded pectoral fins that are reminiscent of skates and rays.
• Ptyctodontids (e.g., Cambellodus, Ctenurella and Rhamphodopis) have enlongate bodies, whip-like tails and reduced head and thoracic armor.

Provide some possible explanations for their extinction.

Answer:

• Placoderms lost 6 out of 14 Late Frasnian families during the Frasnian/Famennian Mass Extinction, an event which is estimated to have eliminated 20% of all marine animal families. Most of the remaining families persisted until the very end of the Devonian. This second extinction event also claimed some early sharks, ray-fins and lobe-fin fishes, but apparently had a modest effect on other fauna.
• Due to competition from the first bony fish and early sharks, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms.