Today Earth is home to the heaviest animal that has ever lived: the blue whale. As far as we know, no past animal has ever weighed more. But some have been longer.

While the large dinosaurs tend to grab more than their fair share of attention, there have been many other giant animals that we will never get to see in the flesh. Some are the super-sized ancestors of creatures alive today, while others appear especially bizarre to us because they left no descendants.

The remains of yesterday's giants can help us understand how conditions on Earth have changed across the ages, because creatures often changed size in response to changes in the environment. Besides, there is something inherently fascinating about big animals that we can only see in our imaginations. Here are 10 particularly awesome extinct giants.

Aegirocassis benmoulae

What would the offspring of a whale and a lobster look like? If such a thing were possible it might look something like Aegirocassis benmoulae.

Reaching 2m (6ft 6 ins) long, it lived around 480 million years ago and belonged to an extinct family of marine animals called the anomalocaridids.

The alien-looking creature had net-like sieves, attached to appendages on its head, that it used to filter plankton from sea water to eat. It lived at a time when plankton were becoming more diverse, allowing it to take up a different lifestyle to most anomalocaridids, which were sharp-teethed predators.

This strange creature could help reveal how the limbs of arthropods – that's modern spiders, insects and crustaceans – evolved.

Based on earlier, less complete remains, anomalocaridids were thought to have just one pair of swimming flaps per body segment. However, A. benmoulae clearly had two pairs per segment.

In a paper published in Nature in March 2015, researchers showed that A. benmoulae's twin flaps correspond to the upper and lower segments of modern arthropod limbs. They re-examined other anomalocaridid fossils and found that they too had twin pairs of flaps. They concluded that in some species evolutionary pressures caused the flaps to fuse.

This suggests anomalocaridids were early arthropods. This has long been in question, thanks to their bizarre bodies. Until 1985, palaeontologists thought their spiny head appendages were the bodies of shrimps, their toothed mouths were jellyfish and their bodies sea cucumbers.

Jaekelopterus rhenaniae

Jaekelopterus rhenaniae is an arachnophobe's ultimate nightmare. At 2.5m long, this giant 'sea scorpion' has a claim to the title of largest arthropod ever to have lived.

Its common name is misleading. They weren't true scorpions, and probably scuttled about in lakes and rivers rather than the ocean. J. rhenaniae lived about 390 million years ago and spent its time chopping up fish.

It was described in 2008, after a spiked claw measuring 46cm was found in a quarry in Prüm, Germany. This was all that remained of the animal. However, the ratio between claw and body size is pretty constant in sea scorpions, so researchers were able to estimate that J. rhenaniae was 233-259cm long.

The discovery is another piece of evidence that arthropods were significantly larger in the past.

No one is sure why prehistoric creepy-crawlies were super-sized. Some suggest the answer lies in the atmosphere, which at times contained more oxygen than it does now. Others highlight a lack of backboned predators such as fish.

Arthropleura

Also in contention for the largest arthropod in history were the Arthropleura, a genus of millipedes up to 2.6m long.

They lived between 340 and 280 million years ago, and may also have benefitted from higher levels of oxygen in the air.

No-one has discovered a complete fossil. Partial remains 90cm (3ft) long were uncovered in south-west Germany, and trackways attributed to them have been found in Scotland, the US and Canada. It seems Arthropleura bodies were made up of around 30 jointed segments covered by side plates and a central plate.

Because the remains of Arthropleura mouths have never been found, it is difficult to say for sure what they ate. Researchers who have examined their fossilised excrement have found fern spores, suggesting they fed on plants.

Arthropleura have proved popular with film-makers, featuring in the BBC's Walking with Monsters in 2005 and First Life in 2010.

Meganeura

Giant arthropods were first linked to higher atmospheric oxygen levels in 1880, following the discovery of the first Meganeura fossil in France.

These dragonfly-like creatures buzzed about feeding on amphibians and other insects around 300 million years ago. With huge wingspans of up to 65cm, they were among the largest flying insects ever.

Strictly speaking Meganeura were griffinflies, because their bodies were subtly different to those of dragonflies.

Insect body sizes are limited by the way they carry oxygen from the air to their internal organs. They don't have lungs, and instead use a system of tracheal tubes.

During the Carboniferous Period, from 359 to 299 million years ago, as much as 35% of the air was oxygen. This may have allowed Meganeura to extract more energy from the same quantity of air, and thus to keep flying even when they grew huge.

The theory could explain why they did not survive into later periods when oxygen levels fell.

Sarcosuchus imperator

It's not just insects that have downsized over the years. Palaeontologists on a dinosaur hunt in Niger in 1997 were amazed to encounter fossilised crocodile jaw bones as long as a human.

They had stumbled upon the most complete specimen to date of Sarcosuchus imperator, a prehistoric giant that hunted in the broad rivers of tropical northern Africa 110 million years ago.

Also known as 'SuperCroc', it grew as long as 12m and weighed about 8 tons. That's twice as long and four times as heavy as the largest of today's crocodiles. It probably ate small dinosaurs as well as fish.

It had a narrow jaw 1.8m long, containing more than 100 teeth, plus vertically tilting eye sockets and a large bony protrusion on the tip of its snout. It would have resembled the critically endangered gharials of modern India and Nepal.

Despite its nickname, S. imperator wasn't a direct ancestor of the 23 species of modern crocodilians. It belonged to an extinct reptilian family called the pholidosaurs.

Other similarly huge crocs have been found, notably those in the extinct genus Deinosuchus. These were related to modern alligators and may have reached 10m in length.

Crocodiles were able to grow so big because they lived mainly in water, so they can float and carry more weight than would be possible on land. They also have strong skulls that give them powerful bites, so they can take on big prey.

Metoposaurus

It wasn't just crocodiles that prehistoric fish had to fear. Earth was also once home to giant carnivorous amphibians that looked like huge salamanders.

Fossils of Metoposaurus have been found in Germany, Poland, North America, Africa and India.

Most of the species were wiped out during a mass extinction 201 million years ago. This event wiped out many backboned animals, including large amphibians, and left the field open for dinosaurs to become dominant.

The newest species was described in March 2015 by Stephen Brusatte of the University of Edinburgh in the UK and his colleagues. They named it M. algarvensis, after the region of Portugal where it was found.

It was 2m (6ft 6ins) long and had a broad flat head that has been likened to a toilet seat, albeit one containing hundreds of teeth. Its small, weak limbs suggest it spent little time on land.

It was an ancestor of modern amphibians like frogs and newts. Despite appearances, it was only distantly related to today's salamanders.

Megatherium

What would an elephant-sized hamster crossed with a bear look like? Pretty odd, and perhaps a bit like Megatherium.

This genus included the largest of the giant ground sloths, which lived mostly in South America from 5 million to 11,000 years ago.

While not quite as big as dinosaurs or woolly mammoths, these impressive beasts were still among the biggest land animals. They were up to 6m (20ft) long.

They were part of a group that includes modern tree sloths, armadillos and anteaters.

Megatherium had had extremely robust skeletons. They were apparently built for strength and stability, but not speed.

They also had long arms and large claws. Most scientists believe they used these to reach up into trees and grab leaves and bark that were out of reach for smaller animals.

However, it's been suggested that Megatherium americanum ate meat. The shapes of their elbow bones suggest they could move their arms rapidly, which might have allowed them to swipe at prey.

Terror birds

In recent years, scientists have been trying to use gene editing tools to resurrect extinct species including the Pyrenean ibex, Tasmanian tiger, passenger pigeon and even woolly mammoths. We'd better hope they don't ever get their hands on any terror bird DNA.

More formally known as Phorusrhacids, these were a group of flightless birds up to 3m (10ft) tall. They could run at 50km/hr (30mph), and swallow a medium-sized dog in one gulp.

Their height and long necks would have given them a long reach and helped them spot prey from a distance, while their long, powerful legs provided speed and acceleration.

Terror bird beaks curved downwards, allowing them to rip flesh much like modern birds of prey such as eagles.

Most Phorusrhacid fossils have been found in South America, where they lived between about 60 and 2 million years ago. Some remains have also been found in North America. It was once claimed that they survived until 10,000 years ago, based on finds in Florida, but it turned out that these fossils were much older than first thought.

Their closest living relatives are believed to be South America's seriemas. These only grow to be about 80cm tall.

Megalodon sharks

You may have heard reports that there are massive sharks prowling the oceans, three times as long as a great white and 30 times as heavy. Relax: they're long since extinct.

They were called Megalodon, and no one is quite sure how big they were. Like all sharks, its skeleton was made of cartilage rather than bone, and so did not fossilize well. As a result, we only have teeth and a few bits and pieces of vertebrae to go on.

Recent estimates put it at 16-20 metres (52-65ft) long. That is significantly bigger than the largest fish alive today, whale sharks, which only reach 12.6 metres (41ft).

Megalodon's giant jaws contained over 200 serrated teeth, each up to 18cm (7in) long. It could bite with a force of 11-18 tonnes, four to six times that of a Tyrannosaurus rex.

The idea that Megalodon is still around was presented in the 2013 mock documentary Megalodon: The Monster Shark Lives, broadcast on the Discovery Channel. The programme was widely derided for including footage of actors pretending to be scientists and fake video footage.

Real scientists believe Megalodon lived from 15.9 to 2.6 million years ago. Afterwards, huge whales took its place as the largest animals in the ocean, according to a 2014 study.

Titanoboa cerrejonensis

Around 60 million years ago, shortly after the demise of the dinosaurs, a snake evolved that was twice as long as the biggest modern snakes.

Titanoboa cerrejonensis was 14.6m (48ft) long, and weighed in at more than a tonne. It was described in 2009, after fossilised vertebrae and skulls were found in a coal mine in Colombia.

Believed to be a distant relative of the anaconda and boa constrictor, T. cerrejonensis crushed its prey to death. Its victims may have included crocodiles.

Snakes rely on external heat to survive as they cannot regulate their own body temperature. T. cerrejonensis may only have reached its great size because Earth was warmer when it evolved.

A life-size replica of the slippery monster was put on display at the Smithsonian Institution National Museum of Natural History in Washington, D.C. three years ago.