When it comes to fossils, arachnids are not a group that obviously springs to mind. However, with more than 100,000 described living species, Arachnida form the second most diverse group of primarily land-living organisms after the insects. And they probably made up a significant proportion of the Earth’s biodiversity in the past, just as they do in terrestrial ecosystems today. Despite this, arachnids have usually received only a cursory mention in palaeontology textbooks. In fairness, they are not as common as trilobites or brachiopods in the fossil record, and are usually found only under conditions of exceptional preservation. Yet, despite their rarity, we aim to show here that there are more fossil arachnids out there than is sometimes appreciated.
What are arachnids?
Arachnids are not insects and can easily be differentiated from them by the fact that they have eight legs and, in general, two principal parts to the body. Arachnids also lack both wings and antennae. In total, there are 16 arachnid orders (including four extinct) and all of them have a fossil record. Despite the advent of computer cladistic analysis and new molecular techniques, the relationships between the different arachnid orders continues to be debated and there is no universally accepted consensus regarding how they are all related to one another.
Arachnids as fossils
Fossil arachnids date back more than 400myrs to the Silurian period, making them one of the first animal groups to appear in terrestrial ecosystems. Most of the arachnid orders were established in a recognisable form by the time of the late Carboniferous Coal Measures. Tiny, poorly-sclerotized groups like schizomids and palpigrades are missing, which is unsurprising, although the absence of parasitiform mites during the Palaeozoic remains a puzzle. Towards the end of this era, the coal swamps dried out and were replaced by harsher, drier environments. There also seems to have been a shift in the arachnid fauna sometime between the Palaeozoic and Mesozoic. Four entire orders became extinct: Phalangiotarbida, Trigonotarbida, Uraraneida and Haptopoda. At least in groups like spiders and scorpions, the first anatomically modern representatives appear towards the end of the Carboniferous and begin to diversify during the Mesozoic into lineages that we would recognise today. Unfortunately, the arachnid fossil record is weakest during this crucial Permian–Triassic timeframe, but, hopefully, future finds from new localities will help to fill in the gaps.
Much of what we know about arachnid evolution is based on a surprisingly small number of ‘windows’ of opportunity such as Rhynie or Gilboa, or the late Carboniferous Coal Measures. Around half of all fossil spiders have been described just from Baltic amber and many of the published names come from only two workers: Alexander Petrunkevitch and Jörg Wunderlich. There is also an obvious historical bias towards more intensively studied sites in Europe and North/Central America; as compared to the ‘Gondwanan’ continents making up the Southern Hemisphere. Recent discoveries of ambers from East Africa, Australia and India may go some way towards balancing our view of global arachnid diversity in the past. Given its size, Asia has also yielded relatively few productive fossil arachnid sites, but, as well as the Cretaceous Burmese amber, important localities like the Jurassic Daohugou deposit in China are continuing to yield exciting finds.
Arachnid fossils can be preserved in a number of different ways, each of which requires particular techniques of preparation and study. Some fossils are simply preserved as flattened impressions in the rock, particularly shales and limestones; generally, the finer the grain of the stone, the better the quality of preservation. Such fossils tend to be fairly two-dimensional, but may retain something of their surface relief. Other fossils can be found in concretions and nodules. This is particularly common at the famous Carboniferous Coal Measures sites from Europe and North America. Here, the animal has rotted away, but leaves a three-dimensional impression of itself pressed into the inner walls of the ironstone nodule. With luck, the nodule splits in such a way as to reveal the upper surface of the animal in one half and its underside in the other. Another important source of arachnid fossils is amber (see Deposits, Issue 26, Biodiversity of fossils in amber and Issue 27, Preparation and study of fossils in amber), as well as its younger precursor copal (see Deposits, Issue 31 Sub-fossils in copal: an undervalued resource). In both cases, animals became trapped in sticky tree resin that has solidified and hardened over hundreds to thousands or millions of years. Specimens trapped in amber are known as inclusions and are often extremely well preserved.
More unusual types of fossilisation include the early Devonian Rhynie chert of Scotland. Here, an entire ecosystem of plants and animals was silicified and the hard, translucent – almost glass-like – chert preserves its contents in exquisite detail. Another useful method for obtaining specimens is macerating shales (that is, dissolving the fossils out of the rock). Under ideal circumstances, pieces of original cuticle from both plants and animals can be recovered. A downside is that the fossils tend to be fragmentary, but macerating the shales has, nonetheless, provided valuable insights into a number of fossil localities from the Silurian through to the Carboniferous, including Ludford Lane in England and Gilboa, near New York, in the USA.
In addition to copal mentioned above, arachnids can also be found as subfossils in other localities. These specimens are usually only hundreds or thousands of years old and tend to occur in peat bogs or at archaeological sites. Here again, it is the original cuticle that remains. Spiders (or at least components of their prosoma, such as the carapace or chelicerae) and the tough-bodied oribatid mites are sometimes recovered under such circumstances. Unusual subfossil finds include a tick, indistinguishable from the modern Dermacentor reticulatus, found in the auditory canal of a Pliocene woolly rhinoceros, and a subfossil of the extant tick species Ixodes sigelos from the late Holocene of Argentina. In the latter case, the tick was recovered from an owl pellet collected within a small mammal sequence in the Las Máscaras Cave in Catamarca. Based on bones also present in the pellet, the tick was most likely parasitic on a sigmodontine mouse that had been ingested by the owl.
Identification and palaeodiversity
Order: Scorpiones. Scorpions are immediately recognisable by their large, grasping pincers and long body with a narrow, segmented tail ending in a venomous sting. They are widely distributed across a variety of terrestrial habitats over all continents (except Antarctica), especially in the tropics and subtropics.
- Palaeozoic: 80 species in 32 extinct families (rock only).
- Mesozoic: 17 species in 11 families, eight of which are extinct (rock and amber).
- Cenozoic: 19 species in three extant families (rock and amber).
On current evidence, scorpions are the oldest known arachnids. The oldest unequivocal scorpion is Dolichophonus loudonensis from the mid Silurian Pentland Hills of Scotland (about 430myrs-old). The classification of fossil scorpions is problematic, with more than 30 fossil families and numerous superfamilies, each accommodating only a few (or a single) species. Current work is beginning to reject some apparently superfluous fossil families, but much remains to be done before a satisfactory classification of the fossils is achieved.
Order: Opiliones. Harvestmen are diverse arachnids, the most familiar of which are easily recognised by their small, rounded body, and extremely long and slender legs. However, some species may be small, cryptic, almost mite-like creatures, or large and often brightly-coloured and spiny, such as in the laniatorids. They occur in various habitats worldwide, but are particularly diverse in humid, tropical forests.
- Palaeozoic: Seven species, some in two extinct families (rock only).
- Mesozoic: Four species in two extant families (rock and amber).
- Cenozoic: 19 species in seven extant families (rock and amber).
The oldest fossil harvestman is the remarkably well preserved Eophalangium sheari from the early Devonian Rhynie chert of Scotland (about 410myrs-old). It has tracheae, which are remarkably similar to those of living species and were clearly adapted for breathing air. This shows that harvestmen were fully terrestrial back in the Devonian. From the early Carboniferous onwards into the Mesozoic, there were remarkably modern-looking ‘daddy long-legs’ forms which probably had a similar ecology to today’s living species.
Order: Phalangiotarbida. Phalangiotarbids form an extinct order of moderately large arachnids with a torpedo-shaped body and robust, rather crab-like legs. The construction of the opisthosoma is unique, with the first six tergites being very short and the remaining three tergites large and in some species all fused together into a single plate. They appear to have been quite widespread and abundant during Carboniferous Coal Measures times (UK, Europe and USA).
- Palaeozoic: 31 species in four extinct families (rock only).
- Mesozoic: No fossil record.
- Cenozoic: No fossil record.
The oldest phalangiotarbid is Devonotarbus hombachensis from the early Devonian of Hombach in Germany (about 410myrs-old). The youngest phalangiotarbid is Permian in age and comes from the Rotliegend (Ilfeld Basin) of Germany, dating the last known example of this puzzling group to about 295 to 299mya.