Image: Fossil of Eurypterus remipes, a sea scorpion that lived 420 million years ago. © Millard H. Sharp 2017
How far back can you trace your family tree? A couple of generations to maybe a couple of hundred years? How about 460 million years? Because that is when the granddaddy of arachnids swam in the earth’s oceans, making the arachnid family tree truly ancient. And with more than 100 000 identified species living today, their family tree is not only ancient, but gigantic.
These progenitors of the arachnid lineage were called eurypterids or, informally, sea scorpions. These extinct arthropods are the earliest known chelicerates, the group to which all arachnids, both living and extinct, belong. The subphylum Chelicerata, which gets its name from the Greek words χηλή, khēlē “claw” and κέρας, kéras “horn”, refers to the mouthparts common to all chelicerates. In most modern species, the mouthparts are modified into tiny grasping claws, although in spiders these have been modified into fangs that deliver venom. Eurypterids themselves were aquatic predators that lived during the Ordovician period, 467 million years ago. They averaged around 30cm in length, though via extrapolation from fragmentary fossils, Jaekelopterus rhenaniae is estimated to have reached 2.5 meters in length, making it the largest arthropod to have ever existed.
True arachnids – class Arachnida – first appeared about 420 million years ago, and were some of the first terrestrial animals. Their exact origins are unknown and their ancient history is poorly understood and difficult to study due to the scarcity of arachnid fossils. This is mainly due to the fact that many arachnids are relatively soft bodied, with fossilization of soft tissues being extremely rare. Early fossil arachnids can be found in lagerstätte sedimentary deposits that have remarkably preserved not only their more resilient exoskeletons, but also contain imprints of soft tissue structures.
The earliest recognizable arachnid fossils include a scorpion from the Silurian period (420 million years ago) and an acarid form from the Devonian (380 million years ago). The fossil of Proscorpius (meaning dawn scorpion) was found in the Lagerstätte Bertie Formation in New York, and true to their aquatic ancestry, Proscorpius still had gills and hunted in ancient oceans around 420 million years ago. One of the oldest known scorpion fossils is that of Parioscorpio venator (meaning progenitor scorpion), a 430 million year old arachnid. The fossil itself is of great importance as it demonstrates the first use of a scorpion pulmonary-cardiovascular system similar to those found in modern scorpions. Scientists believe that the progenitor scorpion may have had the ability to breathe air and would venture out on land to mate and spawn, like modern horseshoe crabs. By this time these ancient arachnids already possessed the instantly recognisable scorpion morphology which would remain virtually unchanged until modern times.
Fossils belonging to true spiders, meaning spider-form arachnids with spinnerets, date back to about 380 million years ago. These primitive spiders had segmented abdomens, as opposed to the unsegmented ones in modern spiders. They started off as ground-dwelling hunters, but by the mid Jurassic period (180 million years ago) spiders like Mongolarachne jurassica were already spinning standalone webs to capture prey.
Fossil Camel spiders (solifugids) are also found around this time period, although most known fossils occur in amber and date to around 100 million years. Due to their soft bodies, fossil solifugids are extremely rare.
Although early arachnid fossils are scarce and often fragmentary, there exists another form of fossil that has the ability to not only preserve a specimen whole, but also its three-dimensional form in the minutest morphological detail. It is also well known for being the method in which scientists were able to extract dinosaur DNA and clone the animals back to life in the Jurassic Park movies. We are talking of course about amber. Amber is fossilized tree sap and is unique in its preservation of ancient creatures in that it forms a stable matrix that does not distort, break apart or compress the preserved creatures it trapped when originally excreted by its host tree. This has led to the discovery of arachnid fossils with amazingly preserved details, such as the fossil pseudoscorpion below.
Amber fossils have also led to the discovery of some truly bizarre arachnids, like Chimerarachne yingi. This tiny terror was a ground-dwelling predator that lived 100 million years ago alongside dinosaurs and belonged to a now extinct group of arachnids called uraraneids. Overall spiderlike in appearance and with the ability to produce silk, they had the curious addition of a flagellum-like tail at the end of the abdomen.
One of the most important lessons that the fossil record of ancient arachnids teaches us, is just how resilient the arachnid lineage truly is. Over their 420 million odd years of existence they have survived four mass extinctions, including the Devonian extinction (365 million years ago), Permian-triassic extinction (250 million years ago), Triassic-jurassic extinction (210 million years ago) and the infamous Cretaceous-tertiary extinction of 65 million years ago that ended the reign of the dinosaurs. That said, not all ancient arachnid groups managed to survive into the modern era. Three entire groups of these arthropods are now extinct.
The were an order of arachnids that superficially resembled modern-day harvestmen and were first recorded from the early Devonian period. Fossil specimens have been found in coal beds in northern Europe and North America. Trigonotarbida were spider-like arachnids that did not possess the ability to produce silk. They inhabited Europe and parts of North America 350 million years ago. These guys had a triangular cephalothorax (head) and a segmented abdomen, with some of the seventy known species having had reinforced spikes on the armoured segments of the abdomen for protection. Lastly, the U were spider-like arachnids that lived from 350 – 100 million years ago. They had strange flagellum-like tails at the ends of their abdomens, spiderlike mouthparts and the ability to produce silk. Unlike modern spiders that have spinnerets, the silk-spinning spigots of these arachnids were located on the ends of two abdominal plates.
Although tricky to decipher, the fossil history of the class Arachnida gives us a glimpse of their amazing evolutionary history. Starting off as marine predators and then some of the earliest pioneering land animals, they have adapted to almost every habitat niche on earth. The extant orders of arachnids, namely spiders, scorpions, harvestmen, vinegaroons, pseudoscorpions, camel spiders, whip spiders, micro whip scorpions, schizomizids, uraneids, mites and ticks have remained virtually unchanged from their ancient ancestors, though they have expanded from their predatory nature in that modern arachnids can be found as predators, parasites, herbivores and even detritivores.
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