Ancient Fishes I: Origin and Evolution

This set of vocabulary flashcards covers key terms and concepts related to the origin and evolution of ancient fishes, including jawless fishes (Agnathans), jawed fishes (Gnathostomes), ray-finned fishes (Actinopterygii), and lobed-fin fishes (Sarcopterygii), as presented in Lecture 5 Fall 2025.

Neoteny

The ability to reproduce while retaining larval morphology.

Urochordata (Tunicates)

Subphylum of organisms with free-swimming larvae possessing chordate characteristics like a hollow dorsal nerve tube, ventral heart, and palate clefts.

Cephalochordata (Lancelets)

Subphylum typified by lancelets (Branchiostoma), possessing chordate characteristics.

Agnathans

Jawless fishes, generally small, probably detritivorous, with gill openings as pores, unreplaced notochord, two semicircular canals, and no pelvic fins.

Myxiniformes (Hagfishes)

Extant order of jawless fishes; all marine, no paired fins, no eyes.

Pteraspidiformes

Extinct order of jawless fishes with anterior plates over the body, no paired fins, and a hypocercal tail, reaching up to 1.5 meters.

Thelodontiformes

Extinct order of small jawless fishes with denticle-like scales instead of armour, a hypocercal caudal fin, and lateral fins.

Petromyzontiformes (Lamprey)

Extant order of jawless fishes; mostly freshwater, some migratory (diadromous).

Diadromous

Describes migratory fish species that move between fresh water and saltwater.

Gnathostomes

Jawed fishes; evolved to allow a wider selection of food.

Placodermi

Class of extinct jawed fishes characterized by seven fossil orders.

Arthrodiriformes (Arthrodires)

Order of Placodermi with heavily armored head and forebody, tusk-like jaw plates, and biting structures; up to six meters in length.

Actinopterygii

Ray-finned fishes; characterized by ganoid, cycloid, or ctenoid scales (not cosmoid), usually absent spiracle (present in chondrosteans), pectoral radials attached to the scapulo-coracoid complex, and absent internal nostrils.

Chondrostei

Subclass of Actinopterygii, with a singular triangular dorsal fin, forked heterocercal tail, paired fins with narrow fleshy bases, dermal bones lacking cosmine layer, and ganoid scales with an anterior peg-like process.

Sarcopterygii

Lobed-fin fishes; possess fleshy lobate fins and a variously shaped caudal fin (heterocercal, heterodiphycercal, or diphycercal); ancestral forms had two dorsal fins and cosmoid scales.

Coelacanthiformes (Actinistians)

Order of Sarcopterygii, homogeneous group from Middle Devonian to Upper Cretaceous, lacking cosmine, with two dorsal fins and a gas bladder; extant Latimeria is 2m long.

Latimeria

An extant species of Coelacanthiformes, measuring 2 meters long and weighing 80 kg.

Infraclass Dipnoi (Lungfishes)

Group of six modern species with significant Devonian diversification across all continents.

Ceratodontiformes

Order including the living Australian lungfish (Neoceratodus), characterized by large scales, paddle-like paired fins, an unpaired lung, and a cartilaginous endocranium, without an aestivation nest.

Neoceratodus

The living Australian lungfish, up to 2m, with large scales, paddle-like fins, an unpaired lung, and a cartilaginous endocranium, no aestivation.

Lepidosireniformes

Order including extant South American and African lungfishes (Lepidosiren paradoxus and Protopterus aethiopicus), highly dependent on atmospheric oxygen, capable of aestivation, with modified filamentous paired fins.

Aestivation

A state of dormancy during hot, dry periods, similar to hibernation.

Osteolepiformes

Extinct Devonian fishes believed to be the most likely sister group of modern tetrapods, possessing two dorsal fins, cosmine coverings, articulated skulls, and replacement teeth.

Tiktaalik roseae

The 'missing link' fossil discovered from the late Devonian of Arctic Canada, representing an intermediate form between fish and tetrapods.