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Fish may have evolved from an animal similar to a coral-like sea squirt (a tunicate), whose larvae resemble early fish in important ways.The first ancestors of fish may have kept the larval form into adulthood, as some sea squirts do today, although this path cannot be proven.
Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod". [76] [77] 365 Ma: Acanthostega: A fish-like early labyrinthodont that occupied swamps and changed views about the early evolution of tetrapods. [61]
Gars have been referred to as primitive fish or living fossils because they have retained some morphological characteristics of their earliest ancestors, such as a spiral valve intestine, and a highly vascularized swim bladder lung that supplements gill respiration for breathing both air and water.
Paddlefish have been referred to as "primitive fish" because the Acipenseriformes are among the earliest diverging lineages of ray-finned fish, having diverged from all other living groups over 300 million years ago. Both living and fossil paddlefish are found almost exclusively in North America and China.
Despite the large size alligator gar can attain, they are kept as aquarium fish, though many fish labeled as "alligator gar" in the aquarium trade are actually smaller species. Alligator gar require a very large aquarium or pond, and ample resources for them to thrive in captivity. They are also a popular fish for public aquaria and zoos.
These events of extinctions led to the disappearance of primitive tetrapods with fish-like features like Ichthyostega and their primary more aquatic relatives. [56] When tetrapods reappear in the fossil record after the Devonian extinctions, the adult forms are all fully adapted to a terrestrial existence, with later species secondarily adapted ...
Rhenanida ("Rhine fish") were flattened, ray-like, bottom-dwelling predators with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components—a mosaic of tubercles—as opposed to the ...
The study of prehistoric fish is called paleoichthyology. A few living forms, such as the coelacanth are also referred to as prehistoric fish, or even living fossils, due to their current rarity and similarity to extinct forms. Fish which have become recently extinct are not usually referred to as prehistoric fish.