Page 22 - Encyclopedia of Aquarium and Pond Fish, 3rd Edition

20 INTRODUCTION TO FISHKEEPING

Evolution and classification

Fish are the oldest of all vertebrates (animals with
backbones), with an ancestry dating back more than 500
million years. However, the earliest fish to appear in the
world’s oceans were very different from those seen today,
since they had no jaws or scales. They lacked specialized
fins, so they relied solely on tail movements to propel
Lungfish, forming the family
them through the water. Internally, their spine was Ceratodiformes, have changed very
little in appearance since they first
made of cartilage, rather than bone. evolved more than 400 million years ago.

The first scaly fish with movable jaws arose around 440 million there were recognizable forerunners of many of today’s
years ago, their jaws having evolved from the front gill arches. freshwater species, including Hypsidoris, a primitive catfish
These fish also possessed several pairs of spines along the lower with sensory barbels and protective spines on its pectoral fins.
sides of the body, from which paired fins later developed.
Fish did not colonize freshwater habitats until comparatively The fossil record shows that modern fish evolved from five ancient
groupings. Two of these groups are now extinct; of the remaining three, the
recently in their history, but by about 66 million years ago, Osteichthyes—the bony fish—are the most numerous and diverse today.
Millions of years ago
550 500 435 410 355 295 250 203 135 65 1.75 Present day
Class Diplorhina Agnatha (jawless fish)
These were the earliest fish. They had a
Class Cephalaspidomorphi boneless skeleton composed of cartilage.
Sea Lamprey
Lacking jaws, they had to rasp or suck at
Class Myxini
Hagfish their food, or swallow floating morsels.
Placodermi (extinct)
This lineage typically had broad plates
rather than teeth in their jaws.

Acanthodii (extinct)
The first jawed vertebrates, these fish
had a rather sharklike body shape.
Ratfish Chondrichthyes (cartilaginous fish)
Class Holocephali Order Chimaeriformes
Members of this class typically have a
Order Squaliformes Spiny Dogfish cartilaginous skeleton and toothlike scales,
but they lack a swim bladder. The pelvic
Class Elasmobranchii Order Lamniformes fins of males act as claspers, to hold the
White Shark
female during mating. The teeth are
Order Rajiformes continually replaced throughout life—a
Undulate Ray characteristic also shared by Osteichthyes.
Order Acipenseriformes Plated Osteichthyes (bony fish)
Class Actinopterygii Sturgeon A bony skeleton, overlapping scales, and a
Order Lepisosteiformes Longnose Gar gas-filled swim bladder characterize these
fish. There are two classes of bony fish: in
Order Perciformes Naso Tang
the Actinoptygii, or ray-finned fish, the
fins are supported by lengths of bone; in
Class Sarcopterygii Order Lepidosireniformes
Lungfish the Sarcopterygii, or lobe-finned fish,
Order Coelacanthiformes lobes of flesh support the fins.
Coelacanth

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