The earliest fossil evidence associated with some sort of euarthropod presence are trace fossils dating to only a few million years after the start of the Cambrian (~537 million years ago), and the group’s common ancestor is estimated to have split off from the radiodont lineage no more than 550 million years ago in the end of the Ediacaran.
Horseshoe crabs are famous examples of “living fossils“, having changed their external appearance very little over hundreds of millions of years. But some fossil species were much more varied in shape than their morphologically conservative modern relatives, such as Austrolimulus fletcheri here.
Living in freshwater environments in what is now New South Wales, Australia, during the Middle Triassic (~247-242 million years ago), Austrolimulus had incredibly long spines on each side of its head, reaching a span of around 18cm (7″) – wider than its total body length!
The function of these spines is unclear, but they may have acted like a hydrofoil in fast-moving currents, or they may have served a defensive purpose by making Austrolimulus‘ carapace too wide and unwieldy for some predators to deal with.
Did you know that in all the time I’ve been posting paleoart here, I’ve never actually done a proper full illustration of the most popular name in all of paleontology?
I think it’s finally time to fix this glaring oversight.
Thylacocephalans were a bizarre group of extinct marine arthropods that often looked like tiny alien creatures – and whose evolutionary relationships are still uncertain. Despite existing in oceans around the world for at least 350 million years, their fossil record is rather spotty and their internal anatomy is often poorly preserved, making it difficult to figure out anything more specific than “probably some sort of crustacean“.
Possible thylacocephalans are known from as far back as the Cambrian, but one of the earliest definite members of the group was Thylacares brandonensis.
Living during the early Silurian, around 436 million years ago, in the region that today is part of Wisconsin, USA (found in the same fossil formation as last week’s Venustulus), this species measured up to about 7.5cm long (3″). Its body was enclosed by a large bivalved carapace, with protruding stalked eyes and what may have been a pair of antennae, along with smaller raptorial limbs than its later relatives.
While it was less specialized than other thylacocephalans it was probably a similar sort of swimming predator, catching prey with its spiny limbs.
Venustulus waukeshaensis was one of the earliest known synziphosurines, living in what is now Wisconsin, USA during the early Silurian, about 436 million years ago. It grew to around 8cm long (~3.2″) and had six pairs of appendages on the underside of its body, with the first pair modified into chelicerae and the rest being walking legs.
It also appears to have been blind, lacking any evidence of eyes despite its fossils being fairly well-preserved – suggesting it lived in conditions where vision wasn’t much use, such as dark murky water or burrowing around in seafloor sediment.
Ants first evolved sometime in the Late Jurassic or Early Cretaceous, but only really began to diversify about 100 million years ago in the Late Cretaceous after the rise of flowering plants.
One of their evolutionary experiments around that time was a group called the haidomyrmecinae – also known as the “hell ants”.
Known from Asia, Europe, and North America, hell ants had bizarre-looking heads, possessing huge upward-curving scythe-shaped mandibles and a horn-like projection between their antennae.
They were fast-moving arboreal predators that would have fed mainly on other invertebrates such as soft-bodied beetle larvae, and unlike most modern ants their workers were probably solitary hunters. They were capable of gaping their mandibles by almost 180°, and when they got close enough to their targets the long sensory hairs around their faces triggered their jaws to snap vertically upwards, impaling their prey against their horn in a unique trap-jaw mechanism.
Some species also reinforced the exoskeleton of their horns with metal particles, strengthening them against impacts from both struggling prey and their own powerful jaws.
Ceratomyrmex ellenbergeri was one of the oddest-looking of all known hell ant species. Known from a few specimens preserved in amber, with adult workers up to 6mm long (~0.25″), it lived during the Late Cretaceous of Myanmar about 100-94 million years ago.
It had an especially pronounced horn and very long mandibles, which may have been adaptations for tackling significantly larger prey items than other hell ants.
And due to this being a species known from Burmese amber, sadly we also have to address the controversy surrounding these sorts of specimens. This amber is currently mined in incredibly dangerous conditions, often using child labor, with sales of both jewellery and paleontological specimens directly funding the ongoing violent conflict in the region.
Trilobites were one of the most successful groups of early animals, existing for over 300 million years – and during that time they developed a huge diversity of weird heads, with various arrangements of spines, horns, eyestalks, and even long snouts and tridents.
But perhaps one of the oddest was the genus Odontocephalus, known mainly from the early-to-mid Devonian and represented here by Odontocephalus aegeria.
Living about 390 million years ago in northeast North America, this trilobite grew up to around 9cm long (3.5″). And although it wasn’t overall very elaborately ornamented, the front margin of its head had a row of extensions that flared out to meet at their tips, forming something resembling the cowcatchers used on trains.
The actual function of this structure is unknown. It might have been purely used for visual display since trilobites had excellent vision – but Odontocephalus was also a fast-moving bottom-dweller, and its “cowcatcher” may have served the same sort of purpose as its modern equivalent, deflecting small obstacles in its path as it trundled along the seabed.
Located in the South Atlantic Ocean, Saint Helena is a small volcanic island that formed several million years ago on the Mid-Atlantic Ridge. It’s one of the most remote islands in the world, and with no native land mammals and no large predators it developed an ecosystem with several uniqueflightlessbirds, and hundreds of endemic invertebrates and plants.
The Saint Helena giant earwig (Labidura herculeana) was, as its name suggests, an enormous species of earwig. Growing as large as 8.4cm long (3.3″) it was the biggest of its kind in the world, and was completely flightless with no hind wings.
It lived in deep burrows in the arid plain and gumwood forest regions of the island, and only came out at night during the summer rains. Since it was probably descended from the shore earwig (Labidura riparia) it was likely a similar sort of opportunistic carnivore, eating smaller invertebrates and carrion.
Humans didn’t reach Saint Helena until the early 1500s, and it wasn’t until 1798 that the earwig was noticed by naturalists and given its scientific name. Then it was more or less forgotten about, and scientific interest in it didn’t start to resume until the 1960s.
But by then it was just very slightly too late.
Extensive habitat destruction and predation by invasive cats, rodents, and centipedes had taken a huge toll on the earwig, and it had become incredibly rare. The last sighting of a live individual was in 1967, and attempts to locate more for potential captive breeding programs in the 1980s and 1990s failed to find any at all.
The last traces of the species were some isolated subfossil pincers found in the mid 1990s, and the giant earwig was officially declared extinct in 2014.
Flightlessness is very rare in the long-legged fly family (Dolichopodidae), however, with only about 12 out of over 5000 species known to have lost functional wings — and eight of those are endemic to the Hawaiian islands.
The Koʻolau spurwing long-legged fly (Emperoptera mirabilis, sometimes classified as Campsicnemus mirabilis) was found only on Mount Tantalus in the southern Koʻolau Range of Oʻahu, close to Honolulu. About 2mm long (>0.1″), its wings were reduced to thin stiff spines, and it moved around by walking and hopping in leaf litter in the moist cool forest at elevations of about 300m (~1000ft).
Like most other long-legged flies it would have been predatory, hunting other tiny invertebrates.
The Koʻolau spurwing was actually still common on Tantalus as recently as the early 1900s, but multiple searches since the 1980s have failed to find any more of them at all. The species is most likely completely extinct, probably due to a combination of predation from invasive ants and habitat destruction from feral wild boar rooting up the forest floor.
Of the other flightless Hawaiian long-legged flies several other species are now possibly extinct — only one out of the five known Emperoptera species still definitely survives on the highest slopes of Mount Kaʻala, and one of the three Campsicnemus is either very rare or also extinct.
The Hawaiian islands also have three endemic species of flightless crane fly in the genus Dicranomyia, all of which are incredibly rare.
