5 Rare and Extinct Fossil Sea Creatures

Ancient marine reptiles and peculiar fossils from the Paleozoic and Mesozoic periods offer a captivating view of prehistoric oceans’ wonderous and bizarre inhabitants.

Sep 21, 2023By Andrew Olsen
rare and extinct fossil sea creatures

Dive into the mysterious world of ancient seas and explore the fascinating stories of extinct and rare fossil sea creatures from around the world. These marine marvels from bygone eras offer a glimpse into the richness and diversity of prehistoric marine life. From formidable predators to bizarre-looking invertebrates, this article will take you on a journey through time to uncover the secrets hidden within fossilized remains. So, buckle up your imaginary diving gear and get ready to be amazed by the extraordinary creatures that once thrived beneath the waves.

The Paleozoic and Mesozoic Periods

Phanerozoic Period
Photo credit: Sam Noble Museum, Oklahoma

The Paleozoic era, known as the age of ‘ancient life’ (541–252 million years ago), marked the appearance of the Earth’s oldest animals just before the Ediacaran Period. These ancient life forms, though not included in the geologic timescale at the time of its creation, were yet to be discovered by scientists. The Paleozoic era was characterized by a primitive form of life comprising many invertebrates (animals without backbones) and the earliest fish and amphibians.

Traveling forward in time, we arrive at the Mesozoic era (252–66 million years ago), known as the age of dinosaurs; we explore sea creatures found in the Triassic, Jurassic, and Cretaceous periods. This epoch ended dramatically with a massive meteorite impact, causing a mass extinction that wiped out the dinosaurs and 80% of Earth’s life. It was a profound event shaping life for millions of years to follow.

1. Ichthyosaurus

Ichthyosaurus reconstruction
Photo credit: Nobumichi Tamura

The Ichthyosaurus, a fascinating marine reptile, lived in the ancient seas during the Mesozoic era, approximately 245 to 90 million years ago, primarily in the Triassic and Jurassic periods. It was an agile swimmer and skilled predator, measuring about 6.5 to 13 feet (2 to 4 meters) in length and weighing up to 1,100 pounds (500 kilograms). With dolphin-like features, they were agile swimmers and hunted fish and other marine creatures using their large, low-light adapted eyes. Equipped with flipper-like limbs and sharp teeth, they were also successful hunters. The Ichthyosaurus provides valuable insights into prehistoric marine life and adds to our understanding of the diverse creatures that once inhabited ancient oceans.

2. Megalodon

Megaladon shark
Photo credit: warpaint/Shutterstock

Prepare to be amazed by the legendary Megalodon, the mightiest and most fearsome shark to roam ancient seas around 23 to 2.6 million years ago. This apex predator ruled during the Cenozoic Era, with teeth up to seven inches (17.8 cm) long, preying on massive marine mammals. The megalodon’s colossal size of 40 to 60 feet (12–18 m) and a weight of 100 tons (90,718 kg) earned it the title of a true leviathan. As ancestors of modern sharks, these ancient predators captivate our imagination, leaving awe and fascination. Though long gone, their fossils remain a reminder of their once-mighty reign in the ancient oceans.

3. Maotianoascus and Ctenorhabdotus

Maotianoascus and Ctenorhabdotus
Photo credit: Quade Paul

The Maotianoascus and Ctenorhabdotus are prehistoric relatives of present-day comb jellies that thrived during the Cambrian Period. These disc-shaped creatures, measuring 0.4–0.8 inches (1–2 cm) in diameter, possessed eight rows of comb plates for swimming and a pair of tentacles for catching prey. Though much smaller than their modern counterparts, they adapted well to their environment and likely excelled as predators. These early ctenophores lived in the oceans approximately 500 million years ago and were among the first animals to develop comb plates. Their small size made them susceptible to predators, but their swift swimming abilities enabled them to escape danger effectively.

4. Opabinia

Photo credit: Quade Paul

Our second-last stop takes us back to the Cambrian Period, where we encounter the Opabinia (oh-puh-BIN-ee-uh), a remarkable creature from 505 million years ago that thrived during the Cambrian period. At just 2-2.5 inches (5—7 cm) long, it had distinct features that set it apart from its contemporaries, including five eyes in a circular pattern and a flexible proboscis for feeding. Its segmented body had swimming paddles on both ends, hinting at a bottom-dwelling scavenger lifestyle. The purpose of these unique traits remains a mystery, with theories ranging from aiding feeding to sensing its environment or defense against predators. Opabinia offers insights into an ancient world, raising questions about Earth’s history and the diversity of life during the Cambrian era, long before dinosaurs roamed the planet.

5. Wiwaxia

Wiwaxia Bakay
Photo credit: Michelle Bakay after Steve Kirk

Wiwaxia corrugata, a slug-like creature from the Cambrian period, had a unique body plan, measuring about 0.8 inches (2 cm) in length. Scaly armor plates and a double row of pointed spines protected it. The ventral surface featured a soft foot and a mouth equipped with a protruding radula, characteristic of mollusks.

Additional details about Wiwaxia include its existence in the oceans of the Cambrian period, between approximately 505 and 520 million years ago. It belonged to the extinct group of animals known as lobopods and was notably common in the Burgess Shale. Likely a filter feeder, Wiwaxia lived on the seafloor. Its distinctive body plan has piqued the interest of scientific researchers, and discussions about its exact relationships with other animals continue to be a subject of debate. However, the microstructure of its scales resembled that of polychaete annelids and may be likely related to today’s mollusks too.


De La Beche Painting_May Anning Discoveries
Photo credit: Oxford University Museum of Natural History

The world beneath the waves has seen an incredible array of extinct and rare fossil sea creatures, each telling a unique tale of life’s evolution throughout Earth’s history. From the Paleozoic and Mesozoic marine reptiles to the bizarre inhabitants of ancient seas, these remarkable fossils offer a fascinating glimpse into the wonders that once swam through prehistoric oceans. As we continue to unearth and study these marine marvels, we gain a deeper appreciation for the dynamic and ever-changing world of marine life and its vital role in shaping our planet’s biodiversity. So, let’s keep exploring, learning, and marveling at the incredible fossil treasures from the depths of time.

Andrew Olsen
By Andrew Olsen

Andrew is a proud cat dad of three spoiled cats, Schopenhauer, Poppy, and Empress Sisi. Growing up with various pets instilled a lifelong love for animals in him. His work in environmental management, particularly water governance, gave him first-hand experience with the delicate balance between humans and animals — domestic and wild. His favorite hobbies include browsing and buying cat toys, gardening, growing fresh catnip for his cats, drinking tea, and reading with a cat on his lap.