8.1 Fish evolution

Fish evolution marks a crucial chapter in vertebrate paleontology. From primitive chordates to diverse marine and freshwater species, fish have undergone remarkable adaptations over millions of years. Their development of jaws, paired fins, and complex body systems laid the groundwork for all vertebrate life.

This evolutionary journey showcases the resilience and adaptability of fish. Through major extinctions and environmental changes, fish have diversified into countless forms. Their fossil record provides invaluable insights into vertebrate evolution, revealing key innovations that shaped life on Earth.

Origins of fish

• Fish are the earliest known vertebrates, evolving from primitive chordates during the Cambrian Period
• The first fish were jawless and lacked paired fins, but possessed a notochord, a hollow nerve cord, and pharyngeal gill slits
• The evolution of fish marked a significant step in the history of vertebrate life on Earth

Early vertebrate ancestors

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• Primitive chordates, such as the lancelet (Branchiostoma), are considered the ancestors of vertebrates, including fish
• These early chordates possessed a notochord, a flexible rod that provided support and served as a precursor to the vertebral column
• The development of a hollow nerve cord dorsal to the notochord was another key innovation in early vertebrate ancestors

Emergence of jawless fish

• Jawless fish, known as agnathans, were the earliest fish to appear in the fossil record during the Ordovician Period
• Agnathans, such as ostracoderms and conodonts, had no jaws or paired fins but possessed a notochord and a cartilaginous skeleton
• These early fish relied on filter-feeding or parasitic lifestyles and were covered in bony armor for protection

Development of paired fins

• The evolution of paired fins was a significant adaptation that allowed fish to improve their swimming ability and maneuverability
• Paired fins, including pectoral and pelvic fins, are believed to have evolved from lateral fin folds
• The development of paired fins paved the way for the diversification of fish and their ability to exploit new habitats

Diversification of fish

• The evolution of jaws and paired fins led to a rapid diversification of fish during the Devonian Period, known as the "Age of Fishes"
• This diversification resulted in the appearance of numerous new fish lineages, each with distinct adaptations and ecological roles
• The Devonian Period saw the rise of both jawed and jawless fish, as well as the appearance of the first freshwater fish

Appearance of jawed fish

• Jawed fish, or gnathostomes, first appeared in the Silurian Period and quickly diversified during the Devonian
• The evolution of jaws allowed fish to become active predators and exploit new food sources
• Jawed fish are divided into two main lineages: cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes)

Placoderms vs ostracoderms

• Placoderms were a group of armored jawed fish that dominated Devonian marine ecosystems
• Ostracoderms were jawless fish that coexisted with placoderms but were less diverse and abundant
• The extinction of placoderms at the end of the Devonian Period paved the way for the rise of modern jawed fish lineages

Cartilaginous fish evolution

• Cartilaginous fish, including sharks, skates, and rays, evolved from early jawed fish during the Devonian Period
• These fish have skeletons composed of cartilage rather than bone, and their scales are modified into tooth-like structures called denticles
• Cartilaginous fish have a long evolutionary history and have survived multiple mass extinction events

Rise of bony fish

• Bony fish, or Osteichthyes, are the most diverse group of fish and include both ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii)
• Bony fish first appeared in the Silurian Period and diversified rapidly during the Devonian
• The evolution of bony fish led to the appearance of new morphologies, such as swim bladders and flexible fins, which allowed them to exploit new habitats and ecological niches

Key adaptations in fish

• Throughout their evolutionary history, fish have developed a range of adaptations that have allowed them to thrive in diverse aquatic environments
• These adaptations include morphological, physiological, and behavioral traits that enable fish to navigate, feed, and reproduce effectively
• Understanding the key adaptations in fish is crucial for interpreting their evolutionary success and ecological roles

Swim bladders for buoyancy

• Swim bladders are gas-filled organs that help fish maintain neutral buoyancy in the water column
• The ability to control buoyancy allows fish to conserve energy and occupy different depths without constantly swimming
• Swim bladders evolved independently in several fish lineages, including bony fish and some cartilaginous fish (e.g., coelacanths)

Fins for locomotion

• Fish have evolved a variety of fin types and configurations that enable efficient swimming and maneuvering in the water
• Paired fins, such as pectoral and pelvic fins, provide stability and help with turning and stopping
• Median fins, including the dorsal, anal, and caudal fins, are used for propulsion and steering

Gills for respiration

• Gills are the primary respiratory organs in fish, allowing them to extract oxygen from water
• Fish gills are composed of filaments that are rich in blood vessels, facilitating the exchange of gases between the water and the bloodstream
• The efficiency of fish gills varies among species, depending on their metabolic needs and the oxygen availability in their habitats

Scales for protection

• Fish scales serve as a protective covering, reducing drag and providing defense against predators and parasites
• There are several types of fish scales, including placoid (sharks), ganoid (sturgeon), cycloid, and ctenoid scales (bony fish)
• The arrangement and morphology of scales can provide important clues about a fish's ecology and evolutionary history

Freshwater vs marine environments

• Fish have evolved to occupy a wide range of aquatic habitats, from freshwater rivers and lakes to marine environments like coral reefs and the deep sea
• The physical and chemical properties of freshwater and marine habitats pose distinct challenges for fish, requiring specific adaptations
• Understanding the differences between freshwater and marine fish can provide insights into the evolutionary history and ecological diversity of fish

Challenges of each habitat

• Freshwater habitats are characterized by low salinity, variable temperatures, and fluctuating water levels, which can impact fish physiology and behavior
• Marine environments have higher salinity, more stable temperatures, and a greater variety of habitats, but also pose challenges related to water pressure, light availability, and predation
• Fish in both habitats must cope with specific challenges related to osmoregulation, pH balance, and dissolved oxygen levels

Adaptations for freshwater

• Freshwater fish have evolved adaptations to maintain osmotic balance in low-salinity environments, such as the ability to actively transport salts across their gills and produce dilute urine
• Many freshwater fish have developed specialized reproductive strategies, such as migration to spawning grounds or parental care, to ensure the survival of their offspring in variable environments
• Freshwater fish often have adaptations for navigating complex habitats, such as well-developed lateral line systems and flexible fins

Adaptations for marine life

• Marine fish have evolved adaptations to cope with high salinity, such as the ability to drink seawater and excrete excess salts through their gills and kidneys
• Deep-sea fish have developed adaptations to withstand high water pressure, low light levels, and scarce food resources, such as large eyes, bioluminescent organs, and slow metabolisms
• Reef fish have evolved a wide range of adaptations for feeding and defense, such as specialized dentition, venomous spines, and cryptic coloration

Major fish radiations

• The evolutionary history of fish is punctuated by several major radiations, during which fish diversified rapidly and occupied new ecological niches
• These radiations were often driven by environmental changes, such as shifts in climate, sea level, or tectonic activity, which created new opportunities for speciation and adaptation
• Understanding the timing and patterns of fish radiations can provide insights into the factors that shape aquatic biodiversity and the response of fish to global change

Devonian fish diversity

• The Devonian Period (419-359 million years ago) is known as the "Age of Fishes" due to the rapid diversification of fish lineages
• Devonian fish faunas were dominated by placoderms, a group of armored jawed fish, as well as early sharks, ray-finned fish, and lobe-finned fish
• The Devonian saw the appearance of the first freshwater fish and the evolution of key innovations, such as jaws and paired fins

Carboniferous fish fauna

• The Carboniferous Period (359-299 million years ago) was characterized by the continued diversification of jawed fish lineages, particularly sharks and bony fish
• Carboniferous fish faunas included a mix of marine and freshwater species, reflecting the expansion of fish into new habitats
• The Carboniferous saw the appearance of several modern fish groups, such as sturgeons and paddlefish (Acipenseriformes)

Permian fish communities

• The Permian Period (299-252 million years ago) was marked by the diversification of ray-finned fish (Actinopterygii) and the decline of placoderms and other ancient fish lineages
• Permian fish communities were impacted by the formation of the supercontinent Pangaea and the associated changes in climate and sea level
• The end-Permian mass extinction had a profound impact on fish diversity, with many lineages going extinct and others undergoing major radiations in the aftermath

Mesozoic fish expansions

• The Mesozoic Era (252-66 million years ago) saw the continued diversification of ray-finned fish and the appearance of several modern fish groups, such as teleosts (Teleostei)
• Mesozoic fish faunas were shaped by the breakup of Pangaea, the evolution of marine reptiles, and the appearance of new ecological niches, such as coral reefs
• The end-Cretaceous mass extinction had a significant impact on fish diversity, but many lineages survived and underwent renewed radiations in the Cenozoic Era

Extinction events and fish

• Fish have been impacted by several major extinction events throughout their evolutionary history, which have shaped the diversity and distribution of fish lineages
• These extinction events were often caused by global environmental changes, such as shifts in climate, sea level, or ocean chemistry, which disrupted marine and freshwater ecosystems
• Understanding the patterns of fish extinctions and survivorship can provide insights into the resilience and adaptability of fish to global change

End-Devonian mass extinction

• The End-Devonian mass extinction (359 million years ago) was one of the "Big Five" mass extinctions in Earth's history and had a significant impact on fish diversity
• The extinction event was likely caused by a combination of factors, including global cooling, sea level changes, and the spread of anoxic conditions in the oceans
• The End-Devonian extinction resulted in the loss of many ancient fish lineages, such as placoderms and acanthodians, and paved the way for the rise of modern jawed fish

Permian-Triassic extinction

• The Permian-Triassic extinction (252 million years ago) was the most severe mass extinction in Earth's history, resulting in the loss of an estimated 96% of marine species
• The extinction event was likely caused by a combination of factors, including volcanic eruptions, climate change, and the spread of anoxic conditions in the oceans
• Fish were severely impacted by the Permian-Triassic extinction, with many lineages going extinct and others undergoing major bottlenecks and subsequent radiations

Cretaceous-Paleogene extinction

• The Cretaceous-Paleogene extinction (66 million years ago) was another major mass extinction that had a significant impact on fish diversity
• The extinction event was likely caused by the impact of a large asteroid, which triggered global environmental changes and the collapse of marine food webs
• Fish were less severely impacted by the Cretaceous-Paleogene extinction than other marine groups, such as ammonites and marine reptiles, but still underwent significant losses and subsequent radiations

Survival and recovery patterns

• The survival and recovery of fish lineages after mass extinction events were influenced by a range of factors, including their geographic distribution, ecological specialization, and evolutionary history
• Fish lineages with cosmopolitan distributions, generalist ecological niches, and high reproductive rates were more likely to survive and recover from extinction events
• The recovery of fish diversity after mass extinctions often involved the radiation of surviving lineages into vacant ecological niches and the evolution of new adaptations

Fish in the fossil record

• Fish have a rich fossil record that spans over 500 million years of Earth's history, providing valuable insights into their evolutionary history and past diversity
• The preservation of fish fossils is influenced by a range of factors, including the environment of deposition, the anatomy of the fish, and the diagenetic processes that affect the fossil after burial
• Understanding the taphonomy and limitations of the fish fossil record is crucial for interpreting patterns of fish evolution and diversity through time

Taphonomy of fish fossils

• Taphonomy refers to the processes that affect an organism's remains from death to fossilization, including decay, transport, burial, and diagenesis
• Fish fossils are often preserved in fine-grained sediments, such as shales and limestones, which provide ideal conditions for the preservation of soft tissues and delicate structures
• The taphonomy of fish fossils can vary depending on the environment of deposition, with marine settings generally providing better preservation than freshwater or terrestrial settings

Exceptional fish preservation

• Exceptional fish preservation refers to fossil deposits that preserve soft tissues, such as skin, muscles, and internal organs, in addition to hard parts like bones and scales
• Exceptional fish preservation is rare but provides unique insights into the anatomy, physiology, and ecology of extinct fish
• Examples of exceptional fish preservation include the Devonian Gogo Formation in Australia, the Jurassic Solnhofen Limestone in Germany, and the Cretaceous Santana Formation in Brazil

Limitations of fish fossils

• Despite their rich fossil record, fish fossils have several limitations that can affect their interpretation and use in evolutionary studies
• Many fish fossils are incomplete or fragmentary, lacking key anatomical features that are necessary for taxonomic identification and phylogenetic analysis
• The preservation of fish fossils can be biased towards certain environments, time periods, or taxonomic groups, leading to gaps in the fossil record and potential misinterpretations of diversity patterns

Inferring fish paleobiology

• Inferring the paleobiology of extinct fish requires the integration of multiple lines of evidence, including morphological, functional, and geochemical data from fossils
• The morphology of fish fossils can provide insights into their feeding ecology, locomotion, and sensory abilities, based on comparisons with living analogues
• Geochemical analyses of fish fossils, such as stable isotope ratios and trace element concentrations, can provide information about their diet, habitat preferences, and environmental conditions

Evolutionary innovations from fish

• Fish have been the source of many key evolutionary innovations that have shaped the history of vertebrate life on Earth
• These innovations include the origin of jaws, paired fins, and internal fertilization, which have opened up new ecological opportunities and paved the way for the diversification of other vertebrate groups
• Understanding the evolutionary innovations that arose in fish is crucial for reconstructing the broader patterns of vertebrate evolution and adaptation

Transition to tetrapods

• One of the most significant evolutionary innovations to arise from fish was the transition to tetrapods, or four-legged vertebrates, during the Devonian Period
• The transition to tetrapods involved the modification of paired fins into weight-bearing limbs, the evolution of a neck and wrist, and the development of lungs for air breathing
• Key transitional fossils, such as Tiktaalik and Acanthostega, provide insights into the sequence of anatomical changes that led to the emergence of tetrapods from lobe-finned fish ancestors

Origin of jaws and teeth

• The origin of jaws and teeth in fish was a major evolutionary innovation that allowed fish to become active predators and opened up new ecological niches
• Jaws evolved from the modification of the anterior gill arches in early jawed fish, such as placoderms and acanthodians, during the Silurian and Devonian Periods
• The evolution of teeth in fish allowed for the exploitation of new food resources and the development of specialized feeding strategies, such as crushing, piercing, and shearing

Evolution of internal fertilization

• The evolution of internal fertilization in fish was a key innovation that allowed for greater reproductive success and parental care
• Internal fertilization evolved independently in several fish lineages, including cartilaginous fish (sharks and rays), some bony fish (e.g., guppies and livebearers), and lobe-finned fish (coelacanths and lungfish)
• The evolution of internal fertilization in fish involved the modification of fins into intromittent organs, the development of specialized reproductive tracts, and the evolution of mating behaviors

Development of live birth

• The development of live birth, or viviparity, in fish was another significant evolutionary innovation that allowed for greater offspring survival and parental investment
• Live birth evolved independently in several fish lineages, including some sharks, rays, and bony fish (e.g., rockfish and seahorses)
• The evolution of live birth in fish involved the retention of fertilized eggs within the female reproductive tract, the development of placental structures for nutrient transfer, and the evolution of maternal care behaviors