The ancient oceans were home to creatures of immense size. Humans have long been fascinated by these prehistoric giants, pondering what colossal beings once swam in Earth’s waters. This exploration delves into paleontological discoveries to answer a compelling question: what was the biggest fish to ever exist?
The Reigning Giant of All Time
The title of the largest bony fish to have ever lived belongs to Leedsichthys problematicus. This massive filter-feeding fish inhabited the oceans during the Middle to Late Jurassic period, approximately 165 million years ago. Estimates suggest that Leedsichthys could reach lengths of up to 16.5 meters (54 feet) and weigh as much as 49 tons. These dimensions make it comparable to modern-day whales, despite being a fish.
This ancient giant was a passive filter feeder, much like today’s whale sharks, consuming vast quantities of plankton and small organisms by swimming with its mouth open. Despite its enormous size, Leedsichthys was likely a gentle creature, sifting microscopic food from the water. The “problematicus” in its name reflects the challenges paleontologists faced in reconstructing its full form from fragmented fossil remains. Its skeleton was partially composed of cartilage, which does not preserve as well as bone, making complete fossil specimens exceptionally rare.
Defining Fish and Biggest
Understanding the biggest fish requires clarifying what constitutes a “fish” in scientific terms and how “biggest” is measured. Fish are aquatic vertebrates, but this broad group excludes marine mammals like whales and marine reptiles such as ichthyosaurs or mosasaurs, which are not true fish despite their aquatic lifestyles and often large sizes.
Scientifically, fish are categorized primarily into two superclasses based on their skeletal structure. These are Osteichthyes, or bony fish, which possess skeletons made primarily of bone, and Chondrichthyes, or cartilaginous fish, whose skeletons are composed mostly of flexible cartilage.
When determining “biggest,” scientists consider both length and weight, although length is often a more consistent measure for comparing overall size across species, as an individual’s weight can fluctuate significantly.
Other Notable Giants Throughout History
While Leedsichthys holds its specific record, Earth’s ancient oceans were home to other impressive fish.
Among these was Dunkleosteus, an armored placoderm fish that lived during the Late Devonian period, around 358 to 382 million years ago. Dunkleosteus was a formidable predator, reaching lengths of approximately 3.4 to 6 meters (11 to 20 feet), with some estimates suggesting up to 10 meters. Its head and jaws were covered in heavy bony plates, and it possessed a powerful bite force rather than conventional teeth.
Another giant of the prehistoric seas was Megalodon, considered the largest shark to have ever lived. This cartilaginous fish roamed the oceans from the Early Miocene to the Pliocene epochs, roughly 23 to 3.6 million years ago. Megalodon could grow to an estimated 15 to 20 meters (50 to 65 feet) in length and weigh between 65 and 103 metric tons, making it an apex predator of its time.
Comparing these ancient giants with living species provides perspective on their immense scale. The largest fish alive today is the whale shark (Rhincodon typus), a cartilaginous filter feeder that can reach lengths of up to 18.8 meters (61.7 feet) and weigh around 21.5 tons. Its existence as a massive filter feeder echoes the feeding strategy of Leedsichthys, highlighting how this method can support tremendous body sizes in the aquatic environment.