Perucetus is known from only thirteen vertebrae, four ribs and part of the pelvis, so its total length depends on how many vertebrae it is assumed to have. Scaling to different relatives gives 17.0 or 20.0 meters. The study argued that whales reached their peak body size about 30 million years earlier than scientists had assumed, in coastal waters rich in food. Its heavy bones suggest it swam slowly in shallow coastal water, and as of November 2026 its weight estimate of 85 to 340 tonnes overlaps the heaviest blue whales and is still argued. Because so little of Argentinosaurus survives, scientists argue over how to read the bones, including where the vertebrae sat in the spine. As of November 2026, some say another titanosaur, Bruhathkayosaurus (110 to 130 tonnes), may have been bigger, though its remains are fragmentary too. The first Argentinosaurus bone was found in 1987 on a farm near Plaza Huincul, Argentina. A computer model built from a laser scan of a mounted skeleton had a weight of 83 tonnes.

As of November 2026, Patagotitan, from at least six young adult individuals, was first put at 37 meters and 69 tonnes. Later research lowered it to about 31 meters and 50 to 60 tonnes, similar to or smaller than Argentinosaurus. Argentinosaurus, meanwhile, is put at 30 to 35 meters long and 65 to 80 tonnes. Both sit well below the heaviest blue whales, at 190 to 200 tonnes, yet the ranking among the dinosaurs themselves is still debated. Most of what scientists know about giant squid comes from carcasses washed up on beaches or caught by fishermen, and from beaks in sperm whale stomachs, because the deep sea is so hard to study. As of November 2026, claims of 20 meters or more have not been scientifically documented, and recent guesses put the maximum body size near 5 meters from the fins to the arm tips. The colossal squid is the heaviest living invertebrate, reaching up to 495 kg, and has the largest eyes documented in any animal, 27 to 30 cm across.

Whale shark record lengths are not settled. As of November 2026, the largest reliably recorded one was 18.8 meters long, though the true maximum length is uncertain. Megalodon is a harder case: its fossils are fragmentary, so size guesses vary from 14.2 to 24.3 meters, with a typical length near 10.5 meters. A range that wide means that the record depends on which guess a writer picks. The lion's mane jellyfish gives a different record. The largest recorded specimen, measured off Massachusetts in 1865, had a bell 210 cm across and tentacles about 36.6 meters long. As of November 2026, a honey fungus in Oregon's Malheur National Forest covers about 2,200 acres and weighs as much as 35,000 tons. It is described as the largest living organism by weight, area and volume, but it is a fungus, not an animal.

Sizes sometimes get changed downward. As of November 2026, Argentavis, a huge extinct bird of Argentina, had a wingspan of about 5 to 6 meters and weighed up to about 70 kg, making it the heaviest flying bird known. Its weight and wingspan guesses have since been reduced. Aepyornis, the elephant bird of Madagascar, may have been the largest bird that ever lived. Leedsichthys, a Jurassic fish, was once guessed to be larger, and recent research lowered the biggest to about 16.5 meters. On land, elephants are the heaviest living animals, with no heavier animal on land today, as of November 2026. A big male African elephant can weigh about 6 tons, and the biggest ones about 7 tons. Among living birds, the wandering (snowy) albatross has the greatest wingspan, up to about 3.7 meters, with an average near 3.1 meters.

Which living thing is biggest depends on the measure. Pando, a grove of aspen trees, is one organism: about 47,000 stems that are genetically identical and linked by roots over about 106 acres. It is counted as the largest tree by weight, but classification systems differ. As of November 2026, the blue whale is the biggest known animal, but a single honey fungus is described as the world's largest living organism.

Words to know

• vertebrae: backbone bones
• fragmentary: in pieces
• invertebrate: no backbone
• specimen: one example