Unraveling the identity of the largest fish to have ever lived requires a journey through the fossil record and a clear understanding of what defines a “fish.” This exploration reveals a dynamic history of aquatic giants.
The Ancient Giant: Leedsichthys problematicus
Among the strongest candidates for the largest fish by overall size is Leedsichthys problematicus, an enormous filter-feeding bony fish that swam the Middle Jurassic seas approximately 165 million years ago. It is estimated to have reached lengths of 50 to 70 feet (15 to 21 meters), though some estimates place it closer to 54 feet (16.5 meters). Its immense size was supported by a diet of plankton, which it filtered from the water using specialized gill structures, much like modern baleen whales.
The discovery of Leedsichthys began in 1886 when amateur paleontologist Alfred Nicholson Leeds unearthed its fragmented remains in England. The species was named “problematicus” due to its incomplete and cartilaginous skeleton, which does not fossilize easily, making reconstruction challenging for scientists. Despite these challenges, ongoing research, including the study of specimens like “Big Meg,” has refined understanding of its growth patterns and colossal dimensions.
The Apex Predator Contender: Megalodon
In contrast to Leedsichthys, Otodus megalodon (often referred to as Megalodon) was the largest predatory fish known to exist. This shark dominated oceans from the Miocene to the Pliocene epochs, roughly 23 to 3.6 million years ago. Megalodon could reach estimated lengths of 60 to 79 feet (18 to 24 meters), possessing teeth the size of a human hand and an incredibly powerful bite force.
As an apex predator, Megalodon preyed on a variety of marine animals, including whales, dolphins, seals, and other large fish. Its diet was more flexible than previously thought, adapting to the availability of different prey. While Megalodon was a dominant ancient predator, Leedsichthys likely surpassed it in total mass due to its broader, filter-feeding body plan.
Distinguishing Fish from Other Marine Giants
The discussion of the largest fish often leads to confusion with other enormous marine creatures that are not fish. Animals like blue whales, the largest animals on Earth, and ancient marine reptiles such as Ichthyosaurs or Mosasaurs, lived in the ocean but do not fit the biological definition of a fish. Fish are aquatic vertebrates that typically possess gills, fins, and are usually covered in scales.
Blue whales are mammals, characterized by breathing air with lungs, giving birth to live young, and nursing them with milk. Ichthyosaurs and Mosasaurs were marine reptiles, belonging to a different class of vertebrates entirely. These distinctions are fundamental to biological classification and clarify why, despite their immense size and aquatic habitats, these creatures are not considered the “biggest fish.”
The Largest Fish Alive Today
Turning to the present day, the largest living fish is the whale shark (Rhincodon typus). These sharks can reach lengths of up to 61 feet (18.8 meters), although individuals typically measure around 45 feet (13.7 meters) and weigh approximately 41,000 pounds (18.5 metric tons). Whale sharks are found in tropical and warm temperate oceans worldwide and, like the ancient Leedsichthys, are filter feeders.
Their diet consists primarily of plankton, small crustaceans, and schooling fish, which they filter through their large mouths. The whale sharkâs filter-feeding strategy and immense size draw parallels to Leedsichthys, demonstrating a successful ecological niche that has persisted across vast evolutionary timescales. This shows how some of the largest creatures, both past and present, have adapted to thrive on the ocean’s smallest inhabitants.
How Scientists Measure Ancient Giants
Estimating the size of extinct marine animals from their fossilized remains presents unique challenges for paleontologists. Often, only fragmented evidence is available, such as isolated teeth for Megalodon or scattered bone fragments for Leedsichthys, as complete skeletons are rare. Scientists overcome this by employing techniques like comparative anatomy, which involves comparing known fossil parts to the corresponding bones of living relatives or similar species with known sizes.
This method allows researchers to extrapolate the full body size based on the proportions observed in modern animals. Biomechanical modeling also plays a role, using engineering principles to simulate how an animal’s skeleton and muscles would have functioned, providing insights into its overall dimensions and mass. These size estimates are based on scientific inference and represent a range of possibilities rather than a single, definitive number, reflecting the inherent uncertainties of working with an incomplete fossil record.