Opsithrissops

Opsithrissops (Ancient Greek: "Aspect Fish Face") is an extinct genus of prehistoric bony fish that lived during the Thanetian stage of the Paleocene epoch. It is a small 100 centimetres (3.3 ft) nektonic opportunistic carnivore in the family Osteoglossiformes which includes other bony-tongues such as the extant species of Arowana and Arapaima. It hunted small invertebrates and fish similarly to today's trout and became a Paleogene keystone species in many habitats before its disappearance from the fossil record.

Appearance

Despite being part of the same family as more basal and primitive bony fish like the Arowana, the Opsithrissops had a body that was built for bursts of speed and powerful swimming, with a cleft caudal fin, and serrated pelvic fins. It had an elongated body with similarities to a large Herring, a durable head plated with thick bones and enlarged eyes. Its dorsal fin was placed closer to the caudal fin, and it had shiny leptoid scales all along the sides of its body. The lateral line is visible along the side and in small pockets along the fish's operculum.

Ecological History

Opsithrissops appeared in the fossil record following the Cretaceous-Paleogene extinction event and the decline of another possibly related species of osteoglossiform Thrissops. Due to the extinction of Mosasaurids, large Pachycormids and other Cretaceous marine megafauna which applied predatory pressure upon many small fish species, the Opsithrissops was one of the many species that thrived and spread to different aquatic environments.

Feeding & Diet

Opsithrissops feeding habits were similar to other freshwater bony fish of the Paleocene. Newborn fry fed upon Phytoplankton and hid near the bottom substrate amongst plant matter and debris until it grew large enough to hunt small invertebrates. As Opsithrissops juveniles grew in size they pursued more larger prey such as shrimp, arthropod larvae, mollusks (including gastropods and squid), and other small fish. Smaller members would congregate together for protection.

Natural Threats

While the Cretaceous-Paleogene extinction event caused the demise of the mosasaurs and pliosaurs, along with larger piscivorous ammonoids, and allowed Opsithrissops to spread to fresh water and salt water habitats, the ecological niche that was once filled by the massive Cretaceous predators was filled with the early members of the Lamnidae sharks, rapidly taking precedence in the ocean. Opsithrissops vanished from the fossil record following the rise of ancient whales (archaeocetes).^[1]

See also

• Prehistoric fish
• List of prehistoric bony fish

References