Fossil fish

The evolutionary development of fishes is a watershed event representing the ancestral line of all terrestrial vertebrates, including humans. Fish are divided into four main classes, three of which are represented throughout the fossil record to the present day. The oldest known fish species, only recently discovered, date from the Cambrian period, fifty million years earlier in prehistory than previously thought.

Evolution

The concept of evolution is commonly credited to Charles Darwin, who wrote of the idea in his book On the Origin of Species by Means of Natural Selection (1859). The idea of evolutionary progression, already being discussed by other scientists of Darwin's day, was an unpopular idea, particularly among creationists. The study of geology and the fossil record and advances in other fields, such as deoxyribonucleic acid (DNA) biochemistry, have provided ample substantive evidence for evolution as a dynamic process of adaptation. It should be realized that evolution is only a workable theory supported by evidence for the progressive development of species through time and not a definitive statement of the origin of life or the actual means by which new species appear. Other mechanisms for the appearance of new species may exist that have not yet been realized. The fossil record is an incomplete record at best, and even though millions of fossils have been discovered, billions of other creatures left no such record of their existence.

Evolution describes progressions of adaptation to environmental surroundings. Scientists believe that through time, individual characteristics favoring individual survival in a particular environment become epigenetically entrenched in DNA until the accumulated changes result in the generation of a new species. This remains a difficult concept because the fossil record has not yielded readily identifiable transitional forms between species in most cases. Nevertheless, the evolutionary progression of life forms can be envisioned as beginning with simple one-celled creatures slowly becoming more complex in time as different structural adaptations prove advantageous for survival. In this progression, water-dwelling creatures eventually acquired the form of fish in the late Cambrian and early Ordovician periods of geological time. Fish are the evolutionary ancestors of all living vertebrates, and they are now the dominant predators in freshwater and saltwater environments.

The Cambrian and Ordovician Periods

On the geological time scale created by the U.S. Geological Survey, the Cambrian period is the period from 540 to 485 million years ago, and the Ordovician period from 484 to 443 million years ago. These periods were followed first by the Silurian period (444-415 million years ago) and then by the Devonian period (415-355 million years ago). Scientists believe that during this span of time, the world consisted of the supercontinent Pangaea, the single ocean Panthalassa, and a large oceanic inlet called the Tethys Sea. This geology is thought to be the result of the process of continental drift, first proposed in 1906 and verified following the 1960s discovery of seafloor spreading. By tracing the movement of the present-day continents back in time, the approximate structure of Pangaea can be reconstructed. There is a great deal of uncertainty associated with the state of the planet before this time, however.

The Cambrian Period. The Cambrian period is important to the evolutionary progression because the fossil record of that time reveals the first creatures that can be identified as fish. The earliest examples are known from tiny bone fragments discovered in Wyoming dating to about 500 million years ago, which was near the end of the Cambrian period, and from samples discovered in China's Chengjiang fossil site. The fossil record of the Ordovician period is more expressive and has yielded numerous fossils of fish species.

The Ordovician Period. The Ordovician period was characterized by warm world temperatures, correspondingly high water levels, and a mass extinction event at its end that destroyed the aquatic life forms of the time, including most early fish. It is thought that the loss of ocean water caused by glaciation and ice formation lowered water levels sufficiently to lay bare the life-rich shallows of the ocean beds at the continental shelves and elsewhere. Fossils of jawless, armored fish called ostracoderms from this period represent some of the oldest-known vertebrate fossils.

The Devonian Period

The Devonian period is believed to have been another warm period. Reconstructions of sea surface temperatures from geological sources from that time indicate an average temperature of 30 to 33 degrees Celsius (about 86 degrees Fahrenheit). Water levels were correspondingly higher than during colder periods, encouraging the development of fish species in nutrient-rich shallow waters. The Devonian period is accordingly known as the age of fishes and is marked by the first appearances of ray-finned and lobe-finned fishes and by the proliferation of primitive shark species.

Several important developments in the evolutionary history of fish took place during the Devonian period. The armored, jawless fish known as the ostracoderms declined, while the jawed fish thrived in both fresh and oceanic environments. Fish species inhabiting the warm inland lakes and seas, which were shallow and had depleted oxygen levels, developed lungs and legs, enabling them to move onto land. Extinction events that occurred during the Devonian period almost completely eliminated the agnathan fishes in the middle of the period, while at the end of the period, a major extinction event eliminated most shallow, warm-water organisms. The jawless fishes were almost eliminated, and the placoderm fishes ceased to exist.

Primitive Fish

Fishes are the oldest of vertebrate species and are found wherever permanent water exists. Fishes have been divided into four general classes: agnatha, placodermi, chondrichthyes, and osteichthyes. All four classes are amply represented in the fossil record from the Devonian period to the present day, with the exception of the placodermi, which did not survive the extinction event at the end of the Devonian period.

The Agnathans. The agnathan fishes are characterized by the lack of a skeletal jaw structure; the fish feed either by ingestion of organic materials and suitable prey from bottom sediments and surfaces or by parasitic attachment to a living host. In the former case, manipulation of materials in the oral cavity separates edible from inedible materials, while in the latter case, toothed structures within the oral cavity are used to gain access to the flesh and blood supply of the host by grinding through scales and skin. A primitive class, few members of the agnathan class are known to exist.

The common aquarium denizen plecostomus, or “algae eater,” is of the bottom-feeding variety, while the parasitic type is typified by lampreys and hagfish. One of the oldest known fossil fish species is also believed to be a bottom-feeding agnathan called Haikouichthys. Fossil evidence of Haikouichthys was taken from the extraordinarily rich sedimentary strata near Haikou, in China, along with another species, Myllokunmingia, revealing it to be a small, flattish creature just 3 centimeters (1.25 inches), in length. Features visible in the fossil structure clearly identify it as a true fish rather than a fish-like organism and confirm that it is a member of the agnathan class.

The Osteichthyes. As might be expected, fish with bony skeletons are well-represented in the fossil record because of their appearance in the Devonian period. Bone is formed from calcium and other minerals, so it is not subject to the decomposition processes that erode soft tissue and cartilage before those materials can be fossilized. Among the many different recovered fossils of bony fish are numerous types of coelacanth. Long believed to be an important transitional species between fish and amphibians, the coelacanth disappeared from the fossil record along with the dinosaurs some 65 million years ago. Paleontologists deemed the creature extinct until living specimens were recovered in 1938 and again in 1952. It has since been determined that the extant species are viable populations and have been known to indigenous fishermen of South Africa since the seventeenth century or earlier.

Biologists have determined that these “living fossils” normally live at depths of some 300 meters or 1,000 feet, nesting in caves and having the low feeding requirements that typically prolong the life of species individuals relative to other carnivorous species of similar size. The Coelacanthus belongs to the order of sarcopterygians, which the modern-day coelacanth is believed to closely resemble. Devonian sarcopterygians were medium-sized predators 30 centimeters to 3 meters (12 inches to 10 feet) in length that probably fed on other aquatic vertebrates, such as the placoderms and the actinopterygians (ray-finned bony fish).

The modern coelacanth retains some features of its ancestors but has lost others because of its particular environmental niche. Living at depth, it can no longer use its lungs. It also does not definitively support the concept of its role as a transitional species. The fleshy fins of the creature, although containing bone structures that are homologous to the femur and humerus of tetrapod species, are not used for surface locomotion. Instead, they provide fine movement control, including vertical motion, in the aquatic environment.

The coelacanth is known to be ovoviviparous, retaining and hatching eggs slightly larger than a tennis ball internally, then expelling fully functional live young when those young have grown to about 30 centimeters (12 inches) in length. Because the fossil record of sarcopterygian sizes ranges from 30 centimeters upward, it is reasonable to assume that this birthing method was a common characteristic of those species, especially as some fossils show evidence of having had an attached yolk sac while most others do not.

Examples of ray-finned bony fish persist throughout the fossil record to present-day species such as perch and bass, representing about one-half of all known vertebrate species.

The Green River formation in Wyoming is known for the remarkable degree of preservation of the fossils found therein. Particularly fine examples of Priscacara liops have been recovered from the formation's lacustrine sedimentary deposits. There are two known species of Priscacara, called liops and serrata, with P. liops being smaller and much more common. The Priscacara are similar to sunfish in appearance but may be related to present-day Cichlidae. Fossils of P. liops larger than 150 millimeters (6 inches) have never been found, while fossil P. serrata have been found to be as much as 375 millimeters (15 inches) in size. The two species are believed to have become extinct during the Miocene.

The Chondrichthyes. It is difficult to know exactly when the chondrichthyes came into being because of the low fossilizing ability of the cartilaginous tissue that characterizes the skeletal structures of the class. Except in rare cases in which the cartilage has become somewhat calcified, typically, the only fossilized remains of the chondrichthyes are the teeth. These are found in abundance throughout the fossil record, indicating that sharks and their relatives have been a part of the aquatic realm at least since the Devonian period, contemporary with the placodermi. Certain fossil beds have yielded rare impressions of chondrichthyes’ soft tissue and cartilaginous remains, including the teeth, so it would be a mistake to think that there is no existing concept of the shapes and sizes of the animals.

A great variety of sharks and shark-like fishes are known from the fossil record, primarily based on the shapes and sizes of teeth found in different geological strata. Comparison of fossilized teeth with those of present-day sharks and rays indicates that similar species have persisted since the Devonian period, some with essentially no discernible changes. In other cases, such as the Carcharidae, teeth are found to be identical in every respect except size. The teeth of the present-day great white shark Carcharodon carcharias are identical to those of its ancient, extinct relative Carcharodon megalodon, for example, but are dwarfed by the size of the latter. The predatory abilities of sharks, in all likelihood, enabled them to overtake the ecological niche that had been occupied by the placoderms, perhaps contributing to their eradication in the extinction event that closed the Devonian period.

The Placodermi. The placodermi are perhaps the most intriguing of fossil fish species. This class is characterized by the absence of teeth and by the presence of external armor plating covering at least the front half of their bodies. Because members of the placodermi are known only from the fossil casts of these armor plates and not from skeletal remains, it is thought that they also had cartilaginous skeletons. Some rare fossils also show the presence of heterocercal tails and pectoral fins. Instead of teeth, placoderms had shearing surfaces on the edges of the bony plates surrounding their jaws. They are known to have existed in a great variety of shapes and sizes, indicating that they established themselves in every available ecological niche until they disappeared. First appearing during the preceding Silurian period, they reached their peak during the Devonian and existed for a span of only 50 million years, compared with more than 400 million years for sharks.

Hundreds of species of placoderms are known from the fossil record and classified into eight orders with two additional debated orders—Stensioellida, which contains only one species, Stensioella heintzi, and Pseudopetalichthyida. These orders include Arthrodira, Antiarchi, Brindabellaspida, Phyllolepida, Ptyctodontida, Rhenanida, Acanthothoraci, and Petalichthyida. The arthrodires constitute the most advanced and largest group, accounting for two-thirds of all known placoderm species. Their fossil skulls range in size from 10 centimeters (4 inches) wide in Bothriolepis to 1.3 meters (4 feet) wide in Dunkleosteus, the ocean's first super-predator that typically attained lengths of 6 meters (20 feet) and may have been as long as 15 meters (50 feet).

The oldest branch of the placoderms is the ptyctodonts. However, studies indicate that the ptyctodonts represent the oldest known examples of behaviors that have until recently been considered characteristic of evolutionarily advanced species. In 2008, fossil evidence was documented demonstrating that the ptyctodont placoderm Matripiscis attenboroughi, found in Australia, was viviparous. Fossils of the species Austroptyctodus gardineri taken from the same geological formation also contained embryonic remains, and the ptyctodontids have provided the oldest definitive evidence for vertebrate copulation in sexual reproduction. It also is known from fossil evidence that some placoderms had dermal pigmentation and were, therefore, probably also endowed with color vision.

Transitions, Tiktaalik roseae, and Further Research

The Devonian period also saw the appearance of the first amphibian species. Transitional fossils for these creatures have been notably absent from the fossil record, and while species such as Coelacanthus have traditionally been regarded as likely candidates for this role, the revelations provided by the present-day coelacanth have made this much less likely.

Careful research and theorization led paleontologists to the Canadian Arctic in 2004, where they discovered the 375-million-year-old remains of Tiktaalik roseae, the so-called fishapod, a creature that was clearly a fish. However, T. roseae had physical characteristics, such as robust pectoral fins designed to raise its body in the shallows at the water's edge. Tiktaalik exhibited many characteristics of fish (fins, scales, gills) and several characteristics normally associated with tetrapods, such as a flattened head with top-mounted eyes, functional neck, and supportive ribs. Later evaluation revealed a large pelvic girdle and hind fins with a prominent hip joint. Though these characteristics were not likely sufficiently articulated to promote a walking movement, T. roseae likely used its fins and leg-like appendages to move along riverbanks and mudflats in a way fish can not. This discovery marked the earliest known existence of tetrapod features in the fossil record.

Twenty-first-century findings continue to improve the fish fossil record. The discovery and analysis of tristichopterid fish fossils support the theory that these large, bony fishes lived across the ancient supercontinent Gondwana, not only in present-day Australia, as previously asserted. Researchers in South Africa discovered the largest and most complete fossil of this bony fish in 2023. The same year, scans of the Kolymaspis sibirica, a 400-million-year-old fish found in Serbia in the 1950s, revealed that the complex gill arches that supported the fish's breathing evolved over millions of years to become shoulder structures. This finding provided new evidence of the early evolutionary changes in the transition from fish to amphibians.

Principal Terms

agnathan: any member of the class of jawless fish characterized by the lack of any mobile jaw structure around the mouth

chondrichthyan: any member of the class of fish having cartilaginous skeletons

epigenetics: the effect of environmental influences on inheritable characteristics with their eventual incorporation into the structure of the affected deoxyribonucleic acid (DNA)

heterocercal: descriptive of a tail fin whose upper and lower halves have different radii, as is typical of the tails of sharks

lacustrine: descriptive of sedimentary deposits formed on lake bottoms

osteichthyan: any member of the class of fish having skeletons of true bone, which includes most species of fresh and saltwater fish

oviparous: reproducing by means of eggs that may be fertilized externally or internally but are incubated and hatched outside the body

ovoviviparous: producing offspring by the internal incubation and hatching of eggs followed by expulsion of live young that have developed to fully functional form

placoderm: any member of the extinct class of ancient armored fish, characterized by the presence of external skeletal plates enclosing the front half of the body

viviparous: reproducing by means of internal fertilization of an ovum, usually through sexual copulation, and delivery of live young that have developed internally

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