From a distance, the ocean might seem the same every day. However, there are continuous activities and evolution beneath the waves. The oceans as we know today weren’t the same as, say, 70 million years ago. In the same breath, marine life has evolved to what we have today.
Over 3 billion years ago, ocean life forms were microscopic. Then, there came bizarre and alien-like creatures who ruled the ecosystem. Interestingly, even the marine creatures known to us, such as whales, octopuses, and sharks, have a long and storied history. This piece attempts to describe ancient marine life, highlighting key groups and periods.
Ancient Marine Life during the Precambrian Period
Also known as the “Age of Early Life,” the Precambrian period refers to when continents were formed and the atmosphere as we know it developed. It is when early life started and flourished, creating soft-bodied creatures such as jellyfish and worms. Scientists estimate Precambrian to be between 4.6 billion and 541 million years ago.
During the Precambrian period, most of the marine life was simple, microscopic organisms. They included:
Prokaryotes
Scientists believe prokaryotes were the earliest marine life organisms. They are single-celled microorganisms, which include archaea and bacteria. It is thought that the cyanobacteria oxygenated Earth’s atmosphere during the Great Oxygenation Event, which happened 2.4 billion years ago.
Eukaryotes
Like prokaryotes, eukaryotes are single-celled. However, they have more complex cells with a nucleus. They first appeared around 1.8 billion years ago, long after prokaryotes had made their mark. Algae, an example of early multicellular eukaryotes, formed the foundations of simple food chains.
Ediacaran Biota
The Ediacaran biota first existed between 635 and 541 million years ago. They are the first known large, multicellular organisms. With their soft bodies, these marine creatures thrived in the later Precambrian. They included dickinsonia, charniodiscus, and spriggina. As expected, most of the marine life that existed at the time is now extinct. They also do not have any direct modern analogs.
Microbial Mats and Biofilms
Biofilms refer to super thin cohesive microbial communities that form extensive mats in shallow marine environments. Like the others listed here, microbial mats and biofilms dominated the Precambrian ecosystems.
Marine Life during the Cambrian Explosion
Cambrian explosion was a time when rapid evolution occurred. Experts are convinced the event might have been triggered by increased oxygen levels in Earth’s atmosphere and shallow ocean waters. It is estimated that the Cambrian explosion happened between 541 and 485 million years ago.
Regarding marine life, the Cambrian explosion facilitated the diversification of aquatic organisms. It was when many of the major animal phyla we know today first appeared in the fossil record. Unlike most of the marine life in the Precambrian period, which had soft bodies, the ones in the Cambrian explosion had hard parts.
The Cambrian marine animals had shells and exoskeletons that protected them. Some evolved to be predators and occupied diverse niches. Cambrian explosion established the foundation of modern marine ecosystems.
Examples of marine life that existed during the Cambrian explosion include:
- Arthropods like trilobites and anomalocaris
- Echinoderms
- Brachiopods
- Mollusks like halkieria
- Cnidarians, including early jellyfish and primitive forms of coral-like creatures
- Chordates
- Sponges and early reef builders
- Opabinia
- Hallucigenia
- Wiwaxia
Marine Life during the Ordovician and Devonian Periods
The Ordovician and Devonian periods existed between 485 and 359 million years ago. Individually, the Ordovician period lasted between 485 and 443 million years ago. It immediately followed the Cambrian period, often called the “Great Ordovician Biodiversification Event.”
Many marine animals developed new adaptations during the Ordovician period, and their ecosystems expanded. Invertebrates like trilobites, branchiopods, and cephalopods emerged and thrived. Reef builders, echinoderms, graptolites, jawless fish, and plankton flourished.
On the other hand, the Devonian period, also called the “Age of Fishes,” lasted between 419 and 359 million years ago. Significant advancements in marine life and ecosystems happened at the time. Fish like placoderms, sharks, lobe-finned, and ray-finned fish ruled the waters. Invertebrates like ammonites, trilobites, and brachiopods were also present.
Marine Life from the Carboniferous to Permian Periods
While the Carboniferous (359-299 million years ago) is famous for lush terrestrial environments, its marine ecosystems were equally impressive. It was when invertebrates like crinoids, brachiopods, corals, and ammonoids thrived. Fish and early vertebrates also existed in more complex forms. The fluctuations in oxygen levels in the oceans ensured that more marine animals could thrive.
The Permian period (299 to 252 million years ago) was marked by the rise and development of more modern marine life. They included:
- Invertebrates like ammonoids, brachiopods, bivalves, gastropods, and gastropods
- Fish and vertebrates
- Reef builders
- Conodonts
- Eurypterids
Unfortunately, the majority of the progress was wiped away, thanks to the Permian-Triassic Extinction, the largest mass extinction in Earth’s history.
Mesozoic Marine Life Revolution
After about 90% of living organisms were wiped off the face of the Earth during the Permian-Triassic Extinction, the Mesozoic era took over. Sometimes referred to as the “Age of Reptiles, the Mesozoic era lasted between 252 and 66 million years ago.
When talking about marine life, the Mesozoic period is further divided into the Triassic, Jurassic, and Cretaceous eras. Together, they saw the rise of iconic marine reptiles, the diversification of mollusks and fish, and the establishment of modern reef ecosystems.
Notable marine animals that existed during the Mesozoic era include:
- Sharks
- Sea turtles
- Crocodyliforms
- Plesiosaurs
- Ichthyosaurs
- Belemnites
- Teleosts
Generally, the Mesozoic era was a time for recovery and diversification, reef evolution, and the ending of the Cretaceous mass extinction.
Cenozoic Era
We are currently in the Cenozoic era. It started 66 million years ago and is sometimes called the “Age of Mammals.” However, it is also a transformative period for marine animals and ecosystems. Notable marine organisms in this period include:
- Whales
- Sea cows
- Diversified sharks like megalodon
- Sea lions
- Tuna and other pelagic fish
- Expanded coral reefs
So far, the Cenozoic era has been characterized by marine mammal evolution, reef expansion, plankton dominance, and human impact.
Conclusion
Ancient marine life is as ancient as it can be. Its history dates back to over 3 billion years ago, even though most organisms were microscopic at the time. Over time, these organisms have evolved, gone extinct, and evolved again to become what we know today.
