Fossils of strange looking aquatic creatures found in ancient rocks give us a glimpse into the bizarre and wondrous forms of early vertebrate life. If you’re curious to learn more about the fish that swam Earth’s seas and freshwaters over 300 million years ago during the Paleozoic Era, you’ve come to the right place.
If you’re short on time, here’s a quick answer to your question: During the Paleozoic Era (541 to 252 million years ago), jawless fish like ostracoderms and early jawed fish like acanthodians and placoderms ruled the seas and freshwaters before rising to dominance in the Devonian Period.
Ray-finned fish, lobe-finned fish, and early shark species emerged and diversified.
In this approximately 3000 word article, we will provide an in-depth overview of major groups of fish that lived during the Paleozoic Era. We will explore their anatomy, evolution, diversity, habitats, and extinction. Key topics covered include:
– The Cambrian explosion of life and origins of early fish
– Armored, jawless ostracoderm fish
– Jawed placoderm and acanthodian fish
– The Age of Fishes in the Devonian Period
– Ray-finned fish and fleshy-finned fish
– Early shark evolution
– Mass extinction events
The Cambrian Explosion and Early Fish Origins
The Cambrian Explosion
The Cambrian explosion refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains about 541-485 million years ago. This evolutionary event marked the beginning of the Cambrian period and saw a dramatic diversification of lifeforms on Earth.
During the Cambrian explosion, most modern animal phyla first appeared in the fossil record, including arthropods, mollusks, chordates, echinoderms, and many more. This rapid emergence of new life forms is quite remarkable – many designs of complex animals appeared and radiated quickly over the span of only 10-25 million years.
There are several proposed explanations for what may have driven this accelerated rate of evolution. First, increased oxygen levels in the atmosphere and oceans may have allowed for larger, more active animals.
Second, an increase in ocean nutrients could have spurred greater biological productivity and opportunities for diversification and speciation. Third, evolutionary arms races may have emerged as predators and prey evolved new capabilities.
And fourth, the development of the first paired appendages likely drove an explosion of new body plans as animals could actively seek out new ecological niches. Regardless of the causes, the Cambrian explosion marked a major turning point in the history of life on Earth.
Early Fish Ancestors
The first fish ancestors likely emerged from earlier chordates during the Cambrian explosion or in its immediate aftermath. Some of the earliest chordates include the soft-bodied Pikaia that swam using lateral folds, and the armored, bilaterally symmetrical Yunnanozoon.
These creatures had simple nerve cords rather than true spinal cords, but were an evolutionary step towards vertebrates.
Other significant early fish relatives were the conodonts – eel-like creatures that were among the first animals with hard mineralized skeletal parts. Their tooth-like structures provide the earliest records of vertebrates in the fossil record.
And the enigmatic Anatolepis from the Late Cambrian also provides possible clues about early fish evolution with its flattened head shield and fins.
By the Ordovician period, jawless fishes like Astraspis had emerged – armored filter feeders that were true early vertebrates. And by the Late Ordovician, heavily-armored and jawed fish like Arandaspis swam the seas.
These armored agnathans had elaborate head shields and fins, and would eventually give rise to early sharks and bony fishes.
Armored, Jawless Fish: Ostracoderms
Anatomy
Ostracoderms, meaning “shell-skinned”, were an early group of jawless fish that lived between the Ordovician and Devonian periods (485 to 359 million years ago). They had armored head shields and scales made of bone, lacking both jaws and paired fins (features that characterize modern fish).
Ostracoderms did have a muscular, sucking mouth, single nostril, and gill pouches along their pharynx for filter feeding.
One of the most unique features of ostracoderms was their strange method of propulsion. Lacking fins besides a tail, they wriggled along the seafloor using peristaltic movements of their body assisted by mucus secretion. It wasn’t pretty, but it worked!
Major Groups
There are two main groups of ostracoderms: heterostracans and osteostracans. Heterostracans were generally small, under 6 inches long, with armored head shields and scales. Well-known examples of this group include Pteraspis and Cyathaspis.
They thrived in shallow marine environments during the Silurian and Devonian periods.
Osteostracans grew much larger, some over 3 feet long like Cephalaspis. They had bony armor encasing the head and front of the body. Several species adapted to freshwater habitats, and some paleontologists think they were one of the first groups of vertebrates to leave the sea for land.
Extinction
Ostracoderms eventually died out by the Late Devonian, replaced by more modern jawed fish. So what led to their demise after 100 million years of armor-plated success? One major factor was that jaws allowed predatory fish to eat hard-shelled animals like mollusks, giving them an advantage over filter feeders.
Additionally, the evolution of paired fins provided speed and maneuverability that the wriggling ostracoderms simply couldn’t match.
While their days are long gone, ostracoderms played a critical role as some of the earliest vertebrates. They pioneered adaptations that allowed vertebrates to succeed on land and in freshwater. Even without flashy fins or toothy grins, these weird armored fish left quite the legacy!
Jawed Fish Emerge: Placoderms and Acanthodians
Placoderm Anatomy and Diversity
Placoderms were among the first jawed fish to evolve beginning in the late Silurian period over 420 million years ago. They had bony armor plates covering their head and thorax regions giving them a unique armored appearance.
There were over 200 genera of placoderms which showed incredible anatomical diversity. Some, like the gigantic Dunkleosteus, grew up to 10 meters long and were apex predators of ancient seas. Others were small bottom dwellers. Their head and thorax shields evolved into many shapes and patterns.
Inside the armor, placoderms had simple inner skeletons. Their jaws were composed of 3 elements as in modern jawed vertebrates. Placoderms lacked pelvic fins and differed from modern fish in the structure of their skull and vertebral column.
Placoderms suddenly went extinct at the end of the Devonian period about 360 million years ago. The reasons are still unclear but may relate to loss of reef habitat or competition from the first bony fish.
Acanthodians – The First Jawed Vertebrates
Acanthodians were among the earliest jawed fish appearing in oceans over 410 million years ago. For many years they were thought to be related to sharks but new evidence shows them branching off even earlier in vertebrate evolution.
There were over 200 species of acanthodian fish. Most were small, under 30 centimeters long, though some species like Acanthodes bronni reached over 1 meter in length. Their most distinctive feature was the sharp spines on the leading edges of their fins.
Acanthodians were the first vertebrates to have adaptive immune systems with lymphocytes that developed immunological memory. This complex defense likely contributed to their 100 million year reign.
Most paleontologists now place acanthodians as a transitional group between jawless fish and jawed fish. They went extinct without any modern descendants around 305 million years ago.
The Age of Fishes in the Devonian Period
Mass Diversification of Jawed Fish
The Devonian period, from 419 to 359 million years ago, is often referred to as the “Age of Fishes” due to the tremendous diversification and proliferation of fish species during this time. With the evolution of jaws, fish were able to become fierce predators and exploit new niches.
Armored placoderms and gigantic Dunkleosteus up to 33 feet long dominated the aquatic scene. Ray-finned fish, lobe-finned fish, and early shark species emerged and radiated into a dazzling array of shapes and sizes. By the end of the Devonian, there were over 200 fish families on Earth.
Rise of Ray-Finned Fish
Ray-finned fish really came into their own during the Devonian period. Their light, flexible fins allowed for greater maneuverability and speed. One subgroup, the Actinopterygians, became incredibly diverse and abundant as they adapted to both freshwater and marine environments.
Sturdy, triangular scales armor plated their bodies. Advanced lungfish could supplement their breathing with primitive lungs. Ray-finned fish came to populate rivers, lakes, and oceans worldwide. Genera like Cheirolepis gave rise to modern rays and skates.
Blade-like Eusthenopteron possibly led the way to teleost fish. The ray-fins were poised for great success.
Fleshy-Finned Lobe Fins
While the ray-fins ruled the waters, lobe-finned fish began adapting to life on land. Fleshy fins with articulating bones allowed limited movement across muddy shorelines and bottoms. Lungfish like Dipterus could breathe air through well-developed lungs and thrive in low oxygen waters.
Other lobe fins developed massive tails and pelvic fins to hoist themselves up on the land for brief periods. Their lobed pectoral fins could be used to prop up their front ends. By lifting their heads off the ground, they could look for food and escape danger on land.
Little did they know, they were pioneering some of the first tentative steps that would eventually lead to the conquest of land by tetrapods and the evolution of all land vertebrates including amphibians, reptiles, dinosaurs, birds, and mammals.
Early Shark Evolution
Cladoselache – The First Modern Shark
Cladoselache is considered the first modern shark that appeared about 370 million years ago during the Devonian period. This apex predator evolved a sleek, streamlined body shape and had symmetrical tails and fins that provided exceptional speed and agility for hunting prey.
According to the ThoughtCo website, Cladoselache represents a pivotal transition point in sharks evolving from slow-swimming ancestors to faster modern sharks we know today.
Some key features of Cladoselache that made it such an effective hunter include a lighter cartilaginous skeleton rather than heavier bone, increased jaw power provided by stronger jaw muscles plus upper jaw mobility, and a unique tooth structure featuring several rows designed for grasping prey.
This combination of adaptations allowed Cladoselache to capture and feast on a wide variety of fish, squid relatives called belemnites, and other marine creatures. It thrived as an apex predator for millions of years across the ancient Devonian seas.
Xenacanthus – Freshwater Sharks
Xenacanthus sharks first emerged around 390 million years ago during the Late Devonian timeframe. One interesting characteristic of these early sharks is that they primarily lived in freshwater lakes and river systems rather than marine coastal habitats typically associated with modern sharks.
According to Benjamin Burger of ThoughtCo, Xenacanthus and related freshwater sharks likely adapted to take advantage of abundant food sources and reduced competition from marine shark species.
Despite the unusual habitat, Xenacanthus sharks retained classic shark features like dorsal fins, an elongated body shape, and sharp teeth lining its powerful jaws. They ranged considerably in size from just 1 foot up to nearly 6 feet long.
While Xenacanthus cannot directly be described as the ancestor of modern sharks, this freshwater group did coexist with and eventually died out around the same time more recognizable modern sharks came to prominence after the Permian mass extinction event wiped out many aquatic species.
Mass Extinction Events
Late Devonian Extinction
The Late Devonian extinction was one of the major extinction events during the Paleozoic era, occurring around 374-359 million years ago. This catastrophic event led to the disappearance of over 75% of species on Earth, especially many fish groups that dominated the Devonian seas (1,2).
The main causes were climate changes, global cooling, sea level fluctuations, and the possible impact of a comet or asteroid (3). The extinction spanned over a period of 15 million years and occurred in two major pulses, eliminating important fish groups like placoderms, acanthodians, and lobe-finned fishes.
This mass die-off opened ecological niches that were subsequently filled by ray-finned fishes, sharks, tetrapods, and ammonoids in the later periods (4).
Carboniferous Rainforest Collapse
The Carboniferous Rainforest Collapse occurred around 305 million years ago in the late Carboniferous period. It was a minor extinction event that led to a major global climate change. The event eliminated rainforests that covered the equatorial region of Euramerica, which comprised dense tree-like lycopsids, giant horsetails and ferns.
The leading theory suggests that the evolution of tree roots compromised soil stability. Decomposing matter washed into rivers and oceans, depleted oxygen and released carbon dioxide through microbial activity.
This shifted the environment to drier and seasonal conditions, leading to a die-off of rainforest species. The collapse of lush rainforests also corresponded with a decline in atmospheric oxygen levels in the Permian period (5,6).
End-Permian Extinction
The End-Permian extinction approximately 252 million years ago was the most severe mass extinction event in Earth’s history (7). Also called the “Great Dying”, it eliminated over 90% of marine species and 70% of terrestrial species.
One of the major killers was massive volcanic activity in Siberia that released enormous amounts of greenhouse gases, leading to global warming and oxygen depletion in oceans. Additional factors like methane release from seabeds, anoxia, and hydrogen sulfide poisoning likely contributed to this catastrophe.
Life on land was heavily impacted, marked by the disappearance of insects and the synapsid group of proto-mammals. In the oceans, trilobites, corals and many shelled cephalopods went extinct (8,9). But the extinction paved the way for the rise of archosaurs and mammal-like reptiles in the Triassic period.
Conclusion
The Paleozoic Era hosted an explosion of early fish diversity, as jawless ostracoderms gave way to jawed placoderms, acanthodians, ray-finned fish, lobe-finned fish, and early sharks. While many lineages went extinct in devastating mass extinctions, others survived to evolve into the fish we know today.
The fossil record of Paleozoic fish gives us a window into over 300 million years of vertebrate evolution and adaptation as fish conquered ancient seas and freshwaters.