Fish fossil record

The fossil record shows a gradual evolution of jawed and then bony fish from jawless fish (518 Ma). The evolution of the jaw took 80 million years. The jawed fish diversified into bony, cartilaginous, and lobe-finned fish by the Early Devonian Period, 20 million years later. Amphibians began moving to land after 60 million years at the end of the Devonian.

There were four major groups of fish in the Phanerozoic Eon. The jawless fish were the first fish but dramatically decreased in diversity in the end Triassic extinction (Figure 9‑18), which was caused by a flood basalt event that emitted sulfur dioxide and acidified the oceans. The first jawed fish were the placoderms, which appeared 430 Ma in the late Silurian Period and dominated the seas during the Devonian Period. They were wiped out by several Devonian extinctions and the Devonian-Carboniferous extinction. Sharks are cartilaginous fish, and they first appeared at the beginning of the Devonian. The major modern class of fish, the ray-finned fish appeared in the Devonian and gradually evolved to over 25,000 species in the modern oceans, lakes, and rivers. The lobe-finned fish also evolved in the early Devonian. They were the ancestors of amphibians and land tetrapods. In the Carboniferous, amphibians (tetrapods) migrated to land, beginning the evolution of reptiles, birds, and mammals.

Many classes of jawless fish (agnathans) evolved in the Ordovician, Silurian, and Devonian (Figure 9‑19). The conodonts included hundreds of species. Because of the unique teeth found among different species, their teeth are used for biostratrigraphy. The Triassic-Jurassic extinction event wiped out the conodonts (Figure 9‑20).

Figure 9‑20. Left. Life reconstruction of conodont. Credit: Nobu Tomura. Used here per CC BY-SA 4.0. Right. Conodont teeth (0.3 to 3 mm length) from different species of conodonts. Photo credit: Wikipek. Display at Geological Museum of the Polish Geological Institute.

In the late Silurian and Devonian, the next class of agnathans, the Heterostraci, evolved scales and heavy plates on the dorsal (top) and ventral (bottom) sides of their bodies (Figure 9‑21). They were successful, with dozens of families appearing in various strata.

The Ostreostraci (Figure 9‑22) included many families in the Silurian and Devonian evolved paired dorsal fins and pectoral fins on the side. They had a massive shield on the upper sides of their bodies but not on the ventral side.

The jaw evolved from the gill arches. Almost all modern fish are jawed fish, which are gnathostomes. Because they descended from jawed fish, all land vertebrates are gnathostomes. The armored placoderms (Figure 9‑23 and Figure 9‑24) were the first order of jawed fish (gnathostomes). They inhabited the Silurian and Devonian oceans. They had heavy plates, teeth and pectoral and pelvic fins. Their pectoral and pelvic fins became the paired appendages (legs and arms) in land animals.

There were three branches of the gnathostomes: cartilaginous fish, bony fish, and lobe-finned fish. The cartilaginous fish (Figure 9‑25), Chondrichthyes, first appeared in the mid Devonian. Sharks are cartilaginous fish with skeletons of cartilage rather than bone. Several orders evolved in the late Devonian and Carboniferous. Holocephali, rat fish, one of the fossils in Carboniferous strata, has survived until the present time in 10 families. However, other orders that appeared in the Carboniferous, such as Hybodontiformes (Figure 9‑26), went extinct. Several cartilaginous orders and families evolved since the Jurassic: skates (Figure 9‑27) (Squaliformes), sharks (Figure 9‑28) (Galeomorphes), and rays (Batoidea).

Bony fish evolved in the early Devonian. The two types of bony fish are ray-finned fish and lobe-finned fish. The earliest bony fish, Guiyu oneiros, in the Silurian fossil record (Figure 9‑29) dates from 419 Ma. It has a mix of features from ray-finned fish (Figure 9-30) and lobe-finned fish (muscular fins) although it is closer to lobe-finned fish.

The scientific name of the bony fish is Osteichthyes. Of the two groups of bony fish, the ray-finned fish (Figure 9‑30) have by far the most species (25,000) and include many interesting and spectacular fish. There was genome duplication event in which the number of Hox genes expanded from four to seven. Almost all fish in ocean, lakes, and rivers are ray-finned fish. The Zeus faber fish as a false eye on its side to scare away predators and confuse prey (Figure 9-31)

Ray-finned fish supply a large part of the world food supply. The largest modern order of ray-finned fish is the Percomorpha. One suborder of the Percomorpha is the Scombroidei, which includes the tuna, barracuda (Figure 9‑32), mackerel, and billfish. There are a dozen other extant orders of ray-finned fish.

Although the ray-finned fish are by far the most populous fish in the seas, the other group of bony fish, lobe-finned fish, are important from another perspective. Amphibians and land animals evolved from them. During the Devonian Period (416-359 Ma), the placoderm pelvic and pectoral fins were muscular and bony: (Figure 9‑24). These appendages (lobefins) would become the legs of amphibians and later tetrapods (four-legged animals). Recently, fisherman discovered what is called a living fossil (Figure 9‑33) because it resembles the ancient coelacanth lobe-finned fish from Silurian and Devonian periods.

Amphibians evolved from lungfish during the last half of the Devonian Period. Supporting the concept that lobe-finned fish evolved the capacity to live out of water, freshwater lobe-finned lungfish (Figure 9‑34) bury themselves in surface sediments of dried up lakebeds and hibernate (estivate) for years, waiting for a stream or lake to refill with water. During dry periods, these fish change their mode of respiration and breathe through their swim bladder instead of their gills. They die if held underwater during estivation.[1] Lungfish also have physical characteristics that link them with tetrapods (four-legged land animals that walk on land) including “tooth enamel, separation of pulmonary blood flow from body blood flow, arrangement of the skull bones, and the presence of four similarly sized limbs with the same position and structure as the four tetrapod legs.” [2]

References

[1] UCMP website, Introduction to the Dipnoi, http://www.ucmp.berkeley.edu/vertebrates/sarco/dipnoi.html

[2] UCMP, Dipnoi.

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Preserved coelacanth fish caught in the ocean near the Comoros Islands. Credit: Alberto Fernandez. Used here per CC BY-SA 3.0.