As a group, invertebrates make up roughly 97% of animal species on Earth. In total, there may be more than 1.3 million individual species of invertebrates alive today, and countless more that are now extinct. It’s no surprise, given such diversity, that invertebrates comprise some of, if not the strangest animals that have ever lived.
The term invertebrate refers to any animal that doesn’t have a backbone, and this includes many groups that, together, we might loosely call ‘bugs’. If bugs give you that weird, funny feeling, then look away now; it’s only going to get stranger from here on out…
Weirdest prehistoric bugs
Uraraneids
They may have eight hairy legs, a handful of eyes, and a pair of fangs, but uraraneids are not spiders; or at least not spiders as we know them.
These extinct, spider-like bugs belong to the same overarching group as spiders - arachnids - though they’re recognised as their own distinct family. They’re also a lot older than modern spiders, having originated in the Middle Devonian (~385 million years ago) and roughly 66 million years before their living relatives emerged in the Late Carboniferous (~319 million years ago).
What sets uraraneids apart from spiders are the long, jointed ‘tails’ at the end of their abdomens. These are features they share with living whip scorpions, or uropygids - a sister group that appeared in the period between uraraneids and spiders. Another peculiar feature of uraraneids is their ability to produce silk without the use of spinnerets. This means that they probably wouldn't have been able to weave delicate webs like spiders do. Instead, uraraneids likely used their silk to line burrows and/or wrap up eggs.
There are several different species of uraraneids, but all of them are roughly spider-sized (no more than a few cm across) and are thought to have behaved quite similarly, scuttling through woody undergrowths in search of small insects to eat.
Arthropleura
At 2.6m in length and nearly 50kg in weight, Arthropleura is widely regarded as the largest bug to ever walk the planet. This double-duvet-sized monster belongs to a family of creepy crawlies known as myriapods - the group that includes millipedes and centipedes.
Like today’s millipedes, Arthropleura was probably a detritivore that fed on both decaying plant matter and the remains of dead animals - on the rare occasions that it would stumble across them.
It was long thought that Arthropleura lived in densely forested swamps - a particularly expansive environment during the period that it lived in (344 to 292 million years ago) - but it’s now suggested that it lived in far more open environments, preferring sparse woodlands, floodplains, and even coastal environments.
The sheer size of Arthropleura has puzzled palaeontologists ever since its discovery in 1849. Some think this prehistoric millipede was able to grow so large due to higher levels of oxygen in the atmosphere. However, recent fossil finds suggest Arthropleura appeared before the major peak in atmospheric oxygen during the Carboniferous Period.
Instead, a more likely driver of gigantism in Arthropleura was simply a lack of serious competition. It was only in the last few million years of its reign that it started to compete with tetrapods - the animals that ultimately gave rise to amphibians, reptiles, and mammals.
Isotelus
From one giant to another, Isotelus is perhaps the largest trilobite currently known to science. As a group, trilobites are incredibly diverse yet most are less than 10cm in length. Isotelus, a genus that includes aptly named species such as Isotelus maximus, Isotelus gigas, and Isotelus rex, is known to reach lengths of more than 70cm.
Isotelus lived from the Middle Ordovician (487 million years ago) to the Late Ordovician (443 million years ago) and is known from fossils found across North America, particularly in the US state of Ohio where it’s recognised as the official state fossil.
Isotelus lived in warm, shallow seas and is thought to have fed on worms and other soft-bodied animals that lived on or just below the seafloor. To get at its prey, Isotelus used its shovel-shaped head to dig burrows into the seafloor. From finds of such burrows, researchers think Isotelus may have often crawled inside them, leaving only its large, bulbous eyes exposed to the surface.
While it may have been a giant of its time, Isotelus was likely prey for other, larger marine invertebrates such as Cameroceras - a 6m-long cephalopod that’s distantly related to today’s octopuses, squids, and cuttlefish.
Read more
- Deep in the Malaysian forest, I found a prehistoric-looking insect that's left scientists puzzled for hundreds of years
- “If you’re stung, you must have sex within 24 hours or you’ll die” – the legend behind the insect that looks like a peanut
Sollasina
There are animals that look strange and those that look downright otherworldly. Sollasina falls into the latter category; with its 45 tentacle-like tube feet it looks more like something from the film franchise Alien than an animal from Earth.
Sollasina comprises three distinct species, one of which was described as recently as 2019. This species was named Sollasina cthulhu after H.P. Lovecraft’s horrifying fictional creation. It measures 3cm across, meaning it was no giant and roughly the same size as a £2 coin.
Like modern sea urchins, Sollasina’s mouth was located on the underside of its body and specially designed to scrape algae and other plant matter from rocks and other surfaces on the seafloor. To get around the seafloor, Sollasina walked on its tube feet, just like its distant relatives starfish do today.
Sollasina lived ~430 million years ago, just after a time when the first widely accepted echinoderms (the group to which Sollasina belongs) appeared. A lot of these early echinoderms were strange, like the boot-shaped Cothurnocystis, but Sollasina was arguably the strangest.
The recently described species, Sollasina cthulhu, was found in fossil-rich rocks belonging to the Coalbrookdale Formation located in Herefordshire, England.
Machaeridians
While they may be one of the most diverse groups of invertebrates around today, worms (or annelids) don’t exactly come in a lot of different varieties - almost all of them, living or extinct, look just like the common earthworms we used to collect as children.
That said, there is one extinct group of worms - machaeridians - that look unlike any others. These worms lived from the Early Ordovician (~485 million years ago) to the Late Carboniferous (~300 million years ago) and are known from fossils found across the world, from Sweden to Australia. The fossils of these strange worms look like leaves or birds’ feathers but they come from an animal that lived on the seafloor.
Since they were first discovered more than 150 years ago, machaeridians have confused researchers. Based on their strange appearance, they’ve been grouped with barnacles, echinoderms, and molluscs. It’s now understood that they’re a type of worm and that the peculiar structures that cover their backs are in fact armour plates made of calcite.
Machaeridians lived alongside another strange, enigmatic group of creepy crawlies known as stylophorans and are thought to have fed on their waste, as well as other organic detritus that accumulated on the seafloor.
Hibbertopterus
This giant bug looks like a cross between a woodlouse and a horseshoe crab, but it actually belongs to a group of extinct aquatic arthropods known as eurypterids, or ‘sea scorpions’. Contrary to their name, eurypterids aren’t technically scorpions, nor did they exclusively live in the sea - many later forms lived in brackish or freshwater.
Hibbertopterus lived during the Middle Palaeozoic, crawling along the seabeds that covered parts of what is now Europe, Africa, and North America from 380 to 304 million years ago. Unlike other enormous eurypterids that were predominantly predatory, Hibbertopterus was a filter feeder that used a pair of specialised, rake-like appendages to sweep through silt and filter out small invertebrates.
Measuring 2m from head to tail, Hibbertopterus was indeed a giant, but it’s not quite as large as the largest eurypterid ever found, Jaekelopterus. This behemoth is thought to have reached lengths of up to 2.5m!
While Hibbertopterus may not be the largest of its kind, its broad and compact body likely make it the heaviest. Despite its bulk, it’s thought Hibbertopterus may have been able to drag itself out of water and ‘walk’ on land, based on some fossilised trackways found in Scotland and South Africa.
Hallucigenia
Looking a lot like a child’s drawing, Hallucigenia is perhaps the most bizarre-looking bug on this list. It was discovered in the early 1900s in British Columbia, Canada, in the Burgess Shale - a series of rocks famous for preserving soft-bodied animals, like Hallucigenia.
In 1977, after several decades of close study, it was determined that Hallucigenia had seven pairs of stilt-like legs on its underside and seven pairs of mouth-tipped tentacles on its topside. A few years later and following the discovery of some more fossils, this interpretation was flipped on its head… literally. The ‘legs’ were in fact spines and the ‘tentacles’ were actually legs, tipped with a pair of claws rather than a mouth as was originally thought.
Another round of studies in the 2000s confirmed that the longer end of Hallucigenia was its head, or at least the part where its microscopic mouth and two simple eyes were located. From this point to its tail, Hallucigenia measures just 0.5 to 5.5cm.
Hallucigenia lived during the Middle Cambrian (~520 million years ago) and is best known from full-body fossils found in Canada and China. It likely lived on the seafloor and ate organic detritus.
Beorn
You’d need a microscope to see Beorn in all of its glory. Measuring just 0.3mm across, this extinct critter is by far the smallest on this list.
Beorn belongs to a family of miniature animals known as tardigrades, or water bears or moss piglets. They are short and plump, like their namesakes, and they have four pairs of legs, each tipped with claws or sticky pads.
Named after a shapeshifting character from J.R.R. Tolkein’s The Hobbit, Beorn was discovered in 1940 - its whole body encased in honey-coloured amber. It was the first ever fossil tardigrade found and dated to the Late Cretaceous (~84 million years ago); it’s also one of the oldest. While the origins of tardigrades are thought to stretch back more than 500 million years, Beorn and another Late Cretaceous-aged tardigrade known as Milnesium swolenskyi represent the oldest tardigrades found in the fossil record.
Tardigrades are among the most resilient animals known to science, and Beorn was likely no exception. In testing, tardigrades have survived extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, starvation, and even exposure to outer space. When they’re not resisting death, tardigrades can be found clumsily crawling through mosses and lichens.
Leanchoilia
Leanchoilia is another strange, soft-bodied bug found in deposits of the Burgess Shale. It measures 5cm in length and looks somewhat like an armoured centipede wearing many pairs of fluffy boots. It also had two long, whip-like flagella extending from the frontend of its carapace and a pair of eyes located on either side of its head.
Leanchoilia lived during the Middle Cambrian (~520 million years ago) - a time when life on Earth was very strange and entirely confined to the oceans. Other strange animals around at this time include the aforementioned Hallucigenia and the Leonchoilia look-alike Anomalocaris. This supersized shrimp was the superpredator of its day and likely preyed on soft-bodied critters like Leanchoilia, which spent most of its time foraging for small invertebrates on the seafloor.
Last year, an incredible fossil of a Leanchoilia was found completely covered in pyrite, more commonly known as ‘fool’s gold’ for its gold-like appearance. This exceptional level of preservation revealed a lot about this relatively enigmatic family of aquatic bugs, chiefly that their flagella probably served a sensory function rather than a grasping function.
Aethiocarenus
This big-eyed bug looks just E.T., though it’s certainly from our planet. Aethiocarenus lived during the Middle Cretaceous (99 million years ago) and at the same time as several iconic species of deadly dinosaurs such as Spinosaurus and Argentinosaurus.
Aethiocarenus was described in 2017 from a single, 4.5mm-long specimen found trapped in a drop of amber. This amber preserved the insect in immaculate detail, so much so that researchers were able to tell it was a female. That said, the features preserved were unlike those seen in other insects, prompting said researchers to place it in an entirely new, eponymously named order - Aethiocarenodea.
Aethiocarenus has big eyes and a strange, triangular-shaped head that’s positioned point-down on its long and slender neck. These features would have allowed Aethiocarenus to turn its head 180 degrees and literally look behind itself, say researchers. It also had a pair of glands on its neck that may have secreted some kind of predator-repelling chemical.
It’s thought Aethiocarenus lived on the trunks of trees and crawled through cracks in the bark as it hunted mites and worms, or searched for other foodstuffs such as fungi.
Discover more prehistoric creatures