Ostracoderm

Ostracoderms are any of several groups of extinct, jawless, armored fish that lived during the Paleozoic Era. The term "ostracoderm" (shell-skinned) is derived from the Greek words "ostrakon" (shell) and "derma" (skin).

Overview

Ostracoderms were among the earliest vertebrate fauna, characterized by their heavy bony shields, which covered the head and most of the body. The remainder of the body was covered with small, bony scales. The mouth of ostracoderms was located on the underside of the head and could not be opened wide, as the jaw had not yet evolved.

Classification

Ostracoderms are divided into two main groups: the Pteraspidomorphi and the Cephalaspidomorphi. The Pteraspidomorphi, which includes the Heterostraci, are characterized by their large rostral plates, while the Cephalaspidomorphi, which includes the Anaspida, Thelodonti, and Galeaspida, are characterized by their large cephalic shields.

Evolution

Ostracoderms first appeared in the Ordovician period and flourished until the end of the Devonian period, when they were replaced by the first jawed fishes, the Placoderms. The evolution of ostracoderms is not well understood, as their fossil record is incomplete. However, it is believed that they evolved from a common ancestor with the Cyclostomata, which includes the living jawless fishes, the lampreys and hagfish.

Extinction

The extinction of the ostracoderms at the end of the Devonian period is believed to have been caused by the rise of the jawed fishes, which were more efficient predators and competitors. The last surviving groups of ostracoderms, the Heterostraci and Galeaspida, disappeared at the end of the Devonian period.

See also

• Paleozoic Era
• Ordovician period
• Devonian period
• Pteraspidomorphi
• Cephalaspidomorphi
• Heterostraci
• Anaspida
• Thelodonti
• Galeaspida
• Placoderms
• Cyclostomata
• lampreys
• hagfish

Ostracoderm Resources
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