Spider Tales 1: The giants from the Past

"Whatever your dream, your fantasy is, there's a scientist somewhere who's busy ruining it"
Boulet, 2009

A common misconception about fossils is that in prehistoric times, animals were much larger than their modern counterparts. There are several reasons why: 

1. A question of probability: even based on a model where biodiversity would be constantly increasing since the emergence of life on Earth, extinct species that came and went in the last 600 million years (including those we don't know about, i.e. the vast majority) necessarily outnumber those that are around today. The biodiversity living alongside us at the present is only a tiny slice of the long history of life on Earth, one thin page in a book that has billions of them. This makes it all the more likely for "extreme" forms, in a given taxonomic group, to have existed at some point in time and eventually gone extinct, than to exist today (particularly if that group has a long and rich fossil history). Of course, there are exceptions: modern blue whales, for instance, are the largest known Cetaceans (and mammals) of all times.

2. A taphonomical¹ bias: in continental (freshwater and terrestrial) environments, large and sturdy skeletal elements, from massive animals, are more likely to preserve and fossilise over time than smaller, more fragile remains, that degrade faster and more easily. Therefore, fossil assemblages from ancient terrestrial and freshwater ecosystems give off a biased image of the fossil fauna, with an over-representation of larger life forms.

3. A bias in representation: outstandingly large, "monstrous" organisms captivate our imagination, and tend to get most of the public attention. Giants are far more commonly represented and talked about than their smaller relatives. The dinosaurs that kids know and love, such as Diplodocus, Tyrannosaurus or Stegosaurus, are the largest representatives of their families. When Cenozoic animals are discussed as something else than simply food and foes for prehistoric humans, it's generally all about mammoths, sabre-toothed cats, giant ground sloths, or the giant shark Otodus megalodon. Few people, on the other hand, have ever heard about Zalmoxes robustus or Prolagus sardus. 

This over-representation of giants in the fossil record, and even more in our minds, quickly leads to generalise, and picture fossil representatives of every living group as larger than their extant relatives. 

Arthropods are no exception. The most famous fossil taxa (and basically the only ones ever talked about, outside of the inner circles of hardcore palaeontology enthusiasts) are the giants of the Carboniferous, such as the giant "dragonfly" Meganeura brongniarti and its 70 cm wingspan, the enormous Myriapods of the genus Arthropleura, which could sometimes reach lengths over 2 meters, and the Eurypterida or sea "scorpions", which, although not at the peak of their size and diversity anymore, were still represented in the Carboniferous by species reaching well over 50 cm in length.

A life-size, but not extremely accurate, reconstruction of Arthropleura sp.

 

In this gallery of monsters, would a giant spider seem out of place?
Such a thought probably crossed Dr. Mario Hünicken's mind as he laid eyes on a strange fossil from the late Carboniferous (305 Million years) of Argentina, encased in sediment that apparently used to be a swamp forest or an estuary. The fossil remains were those of a relatively well-preserved, heavily armoured Arthropod. 

Its 35 cm long body seemed to be made up of three parts: a shield-like and rounded back part, a large, triangular carapace directly in front of it, and finally, at the very front of the body, a flattened, shovel-like appendage. Triangular in shape, with a very obvious symmetry, this appendage was interpreted as a pair of flattened chelicerae.
The carapace was adorned with a pair of large, bean-shaped eyes, and was posteriorly crossed by a deep, straight furrow, almost dividing it in two distinct parts.
While most of them were broken off, three legs remained attached to the carapace. Comparatively short and slender, they gave the animal a 50 cm legspan.
X-ray analysis of the underside of the fossil showed what could be interpreted as potential fangs under the chelicerae, a spider-like sternum and arachnid-like coxae (the segments that attach the legs to the body).
Based on the general shape of the body, particularly the rounded, unsegmented posterior part, reminiscent of a spider's abdomen, and what was seen as a pair of flattened chelicerae in front, the animal was described, in 1980, as a very strange and gigantic spider: Megarachne servinei.

Reproduction of the Megarachne servinei holotype², described by Hünicken in 1980 (source)

 

A very peculiar spider indeed!
Many traits of this animal's anatomy are not found in any other spider, extinct or extant: the shape of its carapace and its wide connection to the abdomen (while in spiders, the connection is usually a narrow structure called the pedicel), its two large, bean-shaped eyes, the ornamentation of the exoskeleton, the deep furrow crossing the posterior part of the carapace, and the strange, shovel-like chelicerae, are unlike anything found among spiders or their close relatives.
Meanwhile, none of the attributes commonly regarded as the trademark features of spiders, such as the presence of spinnerets, or of male pedipalps modified into copulatory organs, are visible on the Megarachne holotype². Presence of fangs was considered possible, but not indisputable.
In addition to that, the proportions of the animal themselves are puzzling: how could its ridiculously small and slender legs have supported its comparatively enormous and armoured (and thus probably very heavy) body on land, and carried it at reasonable speed?
Not surprising, then, that this interpretation of Megarachne servinei raised doubts and controversy among the arachnological community. Not only was its classification as a spider challenged, but some doubted it even was an Arachnid.
 

On the other hand, a cat-sized monster spider was, of course, bound to be a big hit with the general public. It quickly became a star (as far as stardom goes for a fossil Arthropod). Copies of the holotype² were displayed in many Natural History Museums around the world, and dioramas of recreated Carboniferous ecosystems never failed to include, alongside Meganeura and Arthropleura, a reconstruction of Megarachne, generally adorned with a few speculative spider-like features, such as a fuzzy body, pedipalps, or eight small eyes, to make it look more convincing.

Reconstruction of Megarachne that used to be displayed in the Natural History Museum of Geneva, Switzerland. Speculative elements have been added, such as pedipalps, small simple eyes and a fuzzy body, while some, such as the size and position of the larger eyes, the shape of the chelicerae, and the length and thickness of the legs, have been modified, in order to make it look more spider-like. (source)

 

It was only a matter of time before our giant spider made a grand appearance on TV. It was about to happen in 2005, in an episode of the docu-drama Walking with Monsters: Life before Dinosaurs, by Tim Haines and Chloe Leland, produced and broadcasted by the BBC, which featured emblematic Palaeozoic life forms. Embellished again with speculative spider-like traits, such as massive venomous fangs, a tarantula-like cluster of small eyes, and the ability to produce silk, Megarachne was depicted as an active, fast-moving predator, hiding in ambush inside a burrow, but also able to actively hunt and run after small vertebrates.

Megarachne in BBC's Walking with Monsters saga, speculatively equipped with large fangs, and pictured as an active predator, hunting small vertebrates such as Petrolacosaurus (source)

Unfortunately for them, in March 2005, while the BBC series was in the making, a new publication came out.
Written by Paul Selden, José Corronça and Mario Hünicken himself, this article shared the results of their reinvestigation of the Megarachne servinei holotype², in light of a new fossil and a new, closely related species, Woodwardopterus scabrosus.
That new study explained why Megarachne looked so weird compared to other spiders: it wasn't a spider at all.
Actually, Megarachne servinei was the incomplete, broken body of an Eurypterid, a "sea scorpion".
The round, spider-like abdomen was the second tergite³, modified into a shield-like structure, of an elongated, segmented abdomen. The strange, flattened chelicerae were actually the broken remnants of a shovel-like projection in front of the carapace.
Its puny legs did not actually have to support the weight of its heavy body, as Megarachne was in fact an aquatic animal.
In short, it was only chance and taphonomy¹ that made a fairly strange Eurypterid look like a spider of gargantuan proportions.
Although, with a total length approaching 60 cm, that animal was a very large Arthropod, it was far from a giant among Eurypterids. The largest of these "sea scorpions", such as Jaekelopterus rhenaniae or Acutiramus spp. (circa 400 My), are famous for their immense size: some reached over two meters in body length.

The new, more accurate reconstruction of Megarachne servinei as an Eurypterid, by Selden, Corronça & Hünicken, 2005 (source)

The redescription of Megarachne as an Eurypterid was a catastrophic blow to the BBC documentary, which was set to be aired in December 2005. Suddenly, their giant spider, one of the main protagonists of the show's second episode, had become an entirely fictional creature.
Removing the spider from the episode would have meant rewriting and remaking a large part of it, and find another equally charismatic predator as a replacement. So late in the production, that was not possible.
The only other option was keeping it and changing its name: it was thus featured as an unidentified Mesothelae.
The problem is, neither its morphology nor its size fit any known representative of the suborder Mesothelae, extinct or extant. While some Mesotheles, such as the extant Liphistius spp. (armoured trapdoor spiders) from southeast Asia, are fairly large spiders (up to 3 cm in body length), they are very far from anything like Megarachne servinei. None of the (poorly known) early spiders and close spider relatives that lived alongside Megarachne were giants. The recently described Arthrolycosa wolterbeeki (310 My), a Mesothele currently regarded as the oldest known spider, was only about 8 mm long. The fragmentary remains of the other spiders and allies from the Carboniferous are in the same size range. It seems that the hugely diverse order Araneae had humble beginnings.

Unfortunately, Walking with monsters contributed heavily in consolidating the obsolete image of Megarachne. A simple Google search of its name is enough to confirm that the legend of the giant spider of the Carboniferous still lives on. 

The title of largest fossil spider is currently held by Mongolarachne jurassica, from the middle Jurassic (circa 160 My) of Inner Mongolia, in northwest China. Originally known by a female specimen, and described in 2011 as a member of the extant genus Nephila (giant orb weavers), the species was reclassified in an extinct genus, Mongolarachne, in the fossil family Mongolarachnidae, after a male, which looks nothing like a male Nephila, was discovered in 2013.
With a body length of about 25 mm and front legs about 50 mm long, the female was a respectably large spider, but not nearly as big as some extant species are.

Mongolarachne jurassica, male (left) and holotype² female (right), photo by Paul Selden (source)

Currently, the Amazonian "goliath" tarantulas in the genus Theraphosa, that can reach, as adult females, a body length over 10 cm and weigh over 100 grammes, are not only the biggest spiders currently living on Earth, but also, as far as we know, the biggest of all times.

A preserved female specimen of Theraphosa blondi from French Guiana

 

Arachnophobes, rejoice! It seems that spiders have never been bigger than they are today. Although the absence of proof must never be taken as proof of absence, it looks like gigantic spiders have never existed outside of our imagination.⏹

Glossary:

¹Taphonomy: The processes and events that happen to an organism's corpse after it dies, up until the moment it is discovered as a fossil, influencing what the fossil looks like (compared to how it would look as a perfectly preserved, unaltered corpse), and the science that consists in studying these processes.
As an example, let's take a dinosaur that was killed by a predator on the shore of a river: tendons, muscles and other soft tissues will rot away, and the skeleton will dislocate. Scavengers may break or take away some bones. The river may wash the remains away, tumble and roll the bones, break them and wear them out. What's left of the skeleton may deposit in a messy pile at the bottom of the river-bed, and eventually get covered in sediment. As they get buried by the accumulating sediment, the pressure from its weight will crush and crack the bones. Tectonic activity might even deform the sediment layer containing our fossil, and its contents.
All these processes (and many more) will significantly change what our dinosaur skeleton looks like at the end, when it is discovered as a fossil. Understanding these processes correctly is therefore critical to interpreting the fossil and reconstruct the skeleton, and the animal, as accurately as possible. That's what taphonomy is about.

²Holotype: the first specimen of a new species to be discovered and described. While it can be part of a series of different type specimens, there is only one holotype. This specimen must be deposited in an accessible collection, which must be explicitly mentioned (along with the number or code under which it is referenced in the collection) in the original publication describing the new species; this ensures any scientist knows where to find it in case they need to examine it. The diagnosis, the list of features considered typical and unique to the new species, is based on what is observed on the holotype. If there's some extent of variation in these features from one individual to another (for instance, if males and females look different), it is common to designate several specimens as a series of types, and one of them will be the holotype.
A new species must have a holotype to be considered valid. However, under some conditions (if the holotype is lost or destroyed, or if the species was described before it became mandatory), another specimen can be designated to replace it, but it will not be called a holotype.

³Tergite: dorsal plate of an Arthropod's abdominal segment. Each segment of an arachnid's abdomen is covered with two plates: a ventral plate called sternite and a dorsal plate called tergite, connected on the sides by a soft, flexible membrane.

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I do not own the rights on the images whose sources are mentioned in the caption as (source). When no source is indicated, it means the image is mine and is not free of use.