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.

Fig 1. Basic body plans of the fossil and extant arachnid orders. Note that the eye arrangement in Uraraneida is unknown.

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.

Fig 2. Scorpion: Eoscorpius sparthensis (Eoscorpiidae) in a nodule from the Carboniferous British Coal Measures of Sparth Bottoms (body length excluding tail ca. 30mm).

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.

Fig 3. Harvestman: Eophalangium sheari (family uncertain) from the Devonian Rhynie chert (body length ca. 4.5mm).

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.

Fig 4. Phalangiotarbid: Mesotarbus peteri (Architarbidae) from the Upper Carboniferous (upper Westphalian A) of Westhoughton, Lancashire, UK (body length ca. 13mm).

Order: Palpigradi. Micro-whipscorpions resemble miniature versions of either whip scorpions or schizomids. They have a body length of less than 3mm and possess a long flagellum with approximately 15 segments, each with 6 to 8 long setae. These can easily break off, so may potentially be absent in fossil specimens. Palpigrades live in dark, humid habitats, such as caves, soil, damp leaf-litter and even littoral environments, and are distributed worldwide, but are most diverse in the tropics and subtropics.

• Palaeozoic: No fossil record.
• Mesozoic: No fossil record.
• Cenozoic: One species, which cannot be identified to family level (rock only).

Despite their popular perception as primitive arachnids, palpigrades have the worst fossil record of any arachnid order. This is unsurprising given their poorly sclerotized body and often cryptic habitats. The only genuine fossil palpigrade is Paleokoenenia mordax, described from 17 specimens from the Neogene Onyx Marble of Arizona. The nature of the structure of the onyx formation suggests that it was laid down by subterranean waters in fissures, and the faunal assemblage that contains the only known fossil palpigrade species is strongly reminiscent of animals encountered in moist, subtropical caves today.

Fig 5. Micro-whipscorpion: Paleokoenenia mordax (family uncertain) in Neogene Onyx Marble from Arizona (body length ca. 2mm).

Order: Pseudoscorpiones. Pseudoscorpions, sometimes referred to as false or book scorpions, are tiny predators that look like miniature versions of scorpions, but without a tail. The body rarely exceeds 5mm in length and is typically oval or rounded in shape, with prominent pedipalpal claws. They occur throughout the world in a wide variety of different habitats, but are most diverse in the tropics and subtropics.

• Palaeozoic: One species in an extinct family (rock only).
• Mesozoic: Three species (plus partially identified forms) in four extant families

(amber only).

• Cenozoic: 39 species in 14 extant families (amber only).

The oldest pseudoscorpion, Dracochela deprehendor, is a remarkable find based on fragmentary material from several different juveniles from the mid-Devonian Gilboa deposit (390myrs-old). The remaining fossils are known only from amber. Pseudoscorpions sometimes disperse by means of phoresy (using another species to carry them from one place to the next). Rare examples of this are known in the fossil record, with specimens preserved attached to various hosts, such as Diptera (flies), Coleoptera (beetles), Hymenoptera (wasps), Lepidoptera (moths) and Opiliones (harvestmen).

Fig 6. Pseudoscorpion: Oligochernes bachofeni (Chernetidae) phoretic on a braconid wasp host (Hymenoptera) in Eocene Baltic amber.

Order: Solifugae. Camel spiders, or solpugids, are known by a wide variety of common names, including sun spiders and wind scorpions. They are diverse arachnids that can be easily recognised by their massive, two-segmented chelicerae, a soft but segmented opisthosoma and their large, leg-like pedipalps, which can give them the appearance of having ten legs. They also possess highly characteristic stalked, leaf-like chemosensory structures (the malleoli or racquet organs) on the coxae and trochanters of the fourth pair of legs. Most species occur in arid habitats in warmer regions of the world, although they are curiously absent from Madagascar and Australia.

• Palaeozoic: One or two species, neither currently assigned to family (rock only).
• Mesozoic: One species in the extant family Ceromidae (rock only).
• Cenozoic: Two species in two extant families: Ammotrechidae and Daesiidae

(amber only).

The camel spider fossil record is very sparse. The oldest unequivocal record – and historically the first fossil camel spider to be described – is Protosolpuga carbonaria found in an approximately 306myr-old ironstone nodule from the late Carboniferous of Mazon Creek, Illinois in the USA. Cratosolpuga wunderlichi is known from at least six well-preserved specimens from the early Cretaceous Crato Formation of Brazil (about 115myrs-old). They have also been described from Baltic and Dominican ambers, but are very rare as inclusions in fossil resins.

Fig 7. Camel spider: Cratosolpuga wunderlichi (Ceromidae) from the Cretaceous Crato Formation of Brazil (body length ca. 20mm).

Order: Acariformes. The acariform branch of the mites expresses an often bewildering range of both morphologies and ecologies. Apart from their (usually) small size, it is hard to make generalisations, although a number of detailed anatomical features are typical of the group. In general, they are tiny animals, often – but by no means always – with a somewhat oval to rounded body and frequently without an obvious division into separate regions. The legs can range from being short and stubby, to longer and more spider-like appendages. As a group, acariform mites occur worldwide in almost every conceivable habitat, sometimes in association with other animals. Some use larger animals to carry them around in a process called phoresy, which is also seen in pseudoscorpions. In some species, different stages of the life cycle may have different ecologies. Prime examples here would be the parasitengonid mites, where the larvae go through a parasitic stage. Even in amber fossils, a larval parasitengonid may be found attached by its mouthparts to another, larger arthropod.

• Palaeozoic: 15 species in 12 families, two of which are strictly fossil (rock only).
• Mesozoic: 19 species in 15 families, two of which are strictly fossil (rock and

amber).

• Cenozoic: 123 species in 56 extant families (rock and amber); plus around 130

Recent oribatid species identified as subfossils.

The oldest potential acariform mite is an oribatid from the Ordovician of Sweden. However, this is a controversial record, as it was originally placed it in a fairly modern-looking group (Desmonata), which is otherwise only known from the Jurassic onwards. If true, this fossil macerated from an approximately 475myr-old limestone would be the oldest arachnid known, but the suspicion remains that it could be a younger contaminant. The oldest unequivocal mite fossils are several species from the approximately 410myr-old Rhynie cherts of Scotland, where they occur silicified in three-dimensions. More than 60 different families have also been tentatively identified from the Eocene amber deposits of Rovno, Ukraine. A phoretic astigmatid mite on a spider in Eocene Baltic amber was recently described using X-ray computed tomography. With a body length of about 180μm, it is the smallest inclusion in amber to be reconstructed using the tomographic technique.

Fig 8. Acariform mite: parasitic juvenile Leptus sp. (Erythraeidae) in Eocene Baltic amber attached to its moth (Lepidoptera) host; normally such specimens are seen attached to flies (see photo in Deposits, Issue 26: p. 48, Biodiversity of fossils in amber).

Order: Parasitiformes. Parasitiform mites are the second major assemblage of animals traditionally grouped under Acari. As their name implies, they include many parasitic forms, particularly the ticks, although, as a group, they actually show a much wider range of lifestyles. Unique morphological characteristics applicable to all parasitiforms are hard to generalise and some of the features relate largely to internal anatomy, which is unlikely to fossilise.

• Palaeozoic: No fossil record.
• Mesozoic: Three species in one extant family (amber only).
• Cenozoic: 13 species in seven extant families (rock and amber).

Considering the abundance of mesostigmatid mites in soil habitats today, the fact that only a handful of fossil parasitiforms have been found is surprising. If they did split from the acariforms before the Devonian, then a Palaeozoic record would be expected. Instead, the oldest parasitiforms are Cretaceous ticks, namely Cornupalpatum burmanicum and Compluriscutata vetulum from the approximately 100myr-old Burmese amber from Myanmar. It has been proposed that the origin of ticks was during the early part of the Cretaceous (with both hard and soft ticks established by the mid-Cretaceous), and that their first hosts were probably reptiles or amphibians. Whether soft (bird) ticks also fed on feathered dinosaurs remains an interesting point of speculation.

Fig 9. Computed tomography reconstruction of an acariform mite (?Histiostomatidae) phoretic on a spider in Eocene Baltic amber (body length of mite ca. 180μm).

Order: Ricinulei. Ricinuleids are uncommon and unusual arachnids, sometimes referred to as hooded tickspiders. They have an extremely thick cuticle and are characterised by a granular body and a range of unusual anatomical features. These include a plate that folds over the mouthparts called the cucullus, a mechanism by which the two main body regions (prosoma and opisthosoma) ‘lock’ together, and longitudinally divided tergites in all living (and some fossil) species. Extant species are known only from West and Central Africa and the Americas. They are unique among the arachnid orders in that they were discovered as fossils before any living examples were found.

• Palaeozoic: 15 species in two extinct families (rock only).
• Mesozoic: One species, not formally named (amber only).
• Cenozoic: No fossil record.

All fossil ricinuleids come from the late Carboniferous Coal Measures of Europe and North America, the known specimens spanning a time interval from about 319 to 306mya. The first fossil example was described as Curculioides ansticii from the Coal Measures of Coalbrookdale in England and was misidentified as a fossil weevil. The stratigraphically oldest ricinuleid is Curculioides adompha from the Coal Measures of Hagen-Vorhalle in Germany.

Fig 10. Tick: Amblyomma sp. (Ixodidae) in Miocene Dominican amber (body length ca. 0.6mm).

Order: Trigonotarbida. Trigonotarbids, or trigs for short, are an extinct order of spider-like arachnids. They are characterised by an opisthosoma in which the tergites are longitudinally divided into median and lateral plates, a feature they share with ricinuleids. They ranged from the end of the Silurian through to the beginning of the Permian. Most trigonotarbids are found in Europe and North America, but there is also one record from Argentina. In general, their body plan is relatively simple. Early trigs were fairly small, but some of the later ones reached a few centimetres in length. In some cases, they were quite heavily armoured, with distinctive patterns of spines and/or tubercles.

• Palaeozoic: 65 species in nine extinct families (rock only).
• Mesozoic: No fossil record.
• Cenozoic: No fossil record.

The oldest known trigonotarbid is part of one of the oldest assemblages of terrestrial arthropod body fossils from the Late Silurian of Ludford Lane in England. It is about 419myrs-old and is the oldest non-scorpion arachnid. Tiny and coalified (and therefore nicknamed the ‘burnt cornflake’), it was formally described and named as Palaeotarbus jerami. The majority of the described trig species come from the late Carboniferous Coal Measures of Europe and North America, dating to approximately 300 to 325mya. These animals were apparently diverse, abundant and widespread at this time. There are more fossils of trigs known from this time period than spiders and they have been found at more Coal Measures localities than any other arachnid order. The youngest trigonotarbids come from the early Permian Ilfeld and Saale Basins of eastern Germany.

Fig 11. Trigonotarbid: Eophrynus prestvicii (Eophrynidae) from the Carboniferous (Moscovian) British Middle Coal Measures of Coseley, UK (body length ca. 30mm).

Order: Uraraneida. Uraraneids are an extinct group (described in 2008) of two spider-like arachnids that had a segmented opisthosoma, but which also retained a whip-like tail (or flagellum), similar to that of palpigrades or whip scorpions. Significantly, they could produce silk from spigots, but these structures were arranged along the ventral plates of the abdomen, rather than being consolidated into moveable spinnerets as in true spiders.

• Palaeozoic: Attercopus not assigned to a family; otherwise one species in one

extinct family (rock only).

• Mesozoic: No fossil record.
• Cenozoic: No fossil record.

Attercopus fimbriunguis, from the 390myr-old mid Devonian of Gilboa, was previously considered the oldest spider. It was assigned to this new arachnid order as the result of the discovery of further specimens from the adjacent and contemporary South Mountain locality, which showed the distinctive Attercopus cuticle associated with a whip-like telson. Permarachne novokshonovi, from the approximately 275myr-old Koshelevka Formation in the Ural Mountains of Russia and described as the first Permian spider, also belongs in this order. This order is evidently closely related to spiders, but probably became extinct some time after the late Permian.

Fig 12. Uraraneid: Permarachne novokshonovi (Permarachnidae) from the Permian of Russia (body length excluding flagellum ca. 5mm).

Order: Araneae. Spiders are without doubt one of the most diverse and important groups of predators in modern terrestrial ecosystems. They are ubiquitous, apart from Antarctica, and often occur in very large numbers. As a result, they are easily recognisable. They are the only arachnids to have paired opisthosomal appendages (spinnerets) used for producing silk, and the pedipalp of mature males is modified into an organ for transferring sperm to the female.

• Palaeozoic: 16 species in three extinct families (rock only).
• Mesozoic: 74 species in 34 families, nine of which are considered extinct (rock

and amber).

• Cenozoic: 1,052 species in 76 families, seven of which are considered extinct

(rock and amber).

With more than 1,100 fossil species, there has been more published on the palaeontology of spiders than of any other arachnid order. Until a few years ago, the oldest spider was thought to be Attercopus fimbriunguis, from the 390myr-old mid Devonian shales of Gilboa. However, this was transferred to the new arachnid order Uraraneida. Other putative records of Devonian spiders have turned out to be misidentifications. Therefore, the oldest spiders are those from the late Carboniferous Coal Measures of Europe and North America, which ranged from around 300 to 312mya and the taxonomy of these species is currently in the process of being revised. The supposed giant Carboniferous spider, Megarachne servinei, from Argentina, was recently demonstrated as actually being a fossil sea scorpion (Eurypterida).

Fig 13. Spider: undescribed specimen from the Cretaceous Crato Formation of Brazil preserved in lateral view (body length ca. 7mm).

Order: Haptopoda. This extinct arachnid order is known from only nine fossil specimens from the late Carboniferous Coal Measures of England. Haptopodids possess a unique combination of characters, including a divided sternum, a massive ventral genital plate (the megoperculum) and legs in which the tarsi are divided into six elements in the first leg and three in the remaining legs. They also have a unique pattern of ridges running along the carapace.

• Palaeozoic: One species in one extinct family (rock only).
• Mesozoic: No fossil record.
• Cenozoic: No fossil record.

This order contains only one species Plesiosiro madeleyi. The original Coseley locality from which these fossils originate is no longer accessible, so it is unclear whether any further examples will be discovered. Nonetheless, its status as a valid and distinct arachnid order has been confirmed and it most probably has close affinities to whip spiders and whip scorpions.

Fig 14. Whip spider: Phrynus resinae (?Phrynidae) from Miocene Dominican Republic amber (body length ca. 10mm).

Order: Amblypygi. Whip spiders – sometimes also called tailless whip scorpions – are dorso-ventrally flattened arachnids. They are easily recognised by their first pair of legs, which are extremely long, thin and whip-like, and give the group its common name. They are waved around in a similar manner to insect antennae and are used to detect prey, which is caught using the pedipalps. These form a ‘catching basket’ adorned with sharp thorns. In some species, the pedipalps are short and compact; in others they are extremely long with a spiny ‘hand’ towards the tip. Another distinctive feature is the kidney-shaped carapace. They are found throughout the tropics, especially in humid forest habitats.

• Palaeozoic: Five species, three assigned to one extant family (rock only).
• Mesozoic: One species in one extant family (rock only).
• Cenozoic: Three species in one extant family (amber only).

The oldest possible evidence for a fossil whip spider is Ecchosis pulchribothrium, based on some 390myr-old cuticle fragments from the mid Devonian of Gilboa. These include a patella with a trichobothrium, a feature which is otherwise only known from living Amblypygi. They are first recorded in a recognisable form in the late Carboniferous Coal Measures of Europe and North America. The only Mesozoic whip spider is Britopygus weygoldti from the approximately 115myr-old Cretaceous Crato Formation of Brazil. Cenozoic fossils are restricted to the Neogene, about 16myrs-old, Neotropical ambers of Mexico and the Dominican Republic, but are very rare.

Order: Thelyphonida. Whip scorpions are distinctive creatures, easily identified by their large, robust pedipalps and their long, slender, whip-like tail (or flagellum), which gives the group one of its common names. Similar to whip spiders, the first pair of legs is elongate (but not to the same extent) and they are used like antennae rather than for walking. They are found throughout the tropics (but are absent from Central and East Africa) and tend to occur in forest habitats, although at least one American species lives under arid conditions.

• Palaeozoic: Six species, not assigned to a family (rock only).
• Mesozoic: One species in the single extant family (rock only).
• Cenozoic: No fossil record.

The oldest known whip scorpion is Prothelyphonus naufragus from the Carboniferous (Namurian B) of Hagen-Vorhalle in Germany. This is about 319myrs-old; the other Coal Measures whip scorpions range from about 306 to 312myrs. Mesoproctus rowlandi has been described from the early Cretaceous Crato Formation of north-eastern Brazil, which is about 115myrs-old. Most of the known specimens are relatively small, but an incomplete fossil assigned only to Mesoproctus sp. is enormous, and with a carapace length of 32.5mm. It is at least as big as, if not bigger than, the largest living whip scorpions ever recorded. Other very large Crato specimens are also known, but remain undescribed.

Fig 15. Whip scorpion: Mesoproctus rowlandi (Thelyphonidae) from the Cretaceous Crato Formation of Brazil (body length ca. 17mm excluding flagellum).

Order: Schizomida. Schizomids resemble small whip scorpions, and are generally only a few millimetres long. They are weakly sclerotized, with a slender, flexible body and their flexibility is aided by having a divided carapace. They also have a slender first pair of legs, which they use as sensory organs. The pedipalps are large and are used to grasp prey. Some species have an enlarged femur in the last pair of legs, which enables them to jump. The opisthosoma ends in a short flagellum, which may have a bulbous tip in males.

• Palaeozoic: No fossil record.
• Mesozoic: Several undescribed specimens in amber (Myanmar).
• Cenozoic: Six species in two families, one of which is extinct (rock and amber).

The schizomid fossil record is poor, presumably a consequence of the small size, cryptic habitats and weakly sclerotized bodies of these animals. The oldest named species, ?Calcitro oplonis, was described from Oligocene oil well cores, about 30myrs-old, from the petroleum-rich Gubei district in Shandong Province, China. Unfortunately, the extinct family Calcitronidae was not defined on characters conducive to a meaningful comparison with the two living families. Slightly younger are Rowlandius velteni and Antillostenochrus pseudoannulatus from Miocene Dominican Republic amber dated at about 16myrs-old. The youngest locality yielding fossil schizomids is the Onyx Marble of Bonner Quarry in Arizona, USA, which has been dated as Pliocene in age, so somewhere between 1.8 and 5.3myrs-old.

What does the future hold for the study of fossil arachnids? Advances in imaging technology, such as the application of computed tomography to palaeontological specimens, means that we are now able to extract much more detail from fossils than ever before. Historical, enigmatic specimens can be re-evaluated to clarify their correct systematic placement and specimens that would previously have yielded no valuable data using traditional techniques now have the potential to be highly informative. The additional morphological data provided by CT scanning means that the divide between palaeo and zootaxonomy is getting smaller and, in many cases, the fossils can be reliably classified within the systematic framework of extant species.

One of the most valuable contributions that fossils can make towards modern studies of arachnid evolution is dating when groups first appeared. Fossils offer a minimum age for any given order, family or genus, and a regularly updated online list of all fossil arachnid taxa makes these data easily accessible and available. The fossil data and molecular-derived data were not always comfortable bedfellows, and molecular clock dates often predicted splits between animal groups much further back than the evidence shown in the fossil record. Of course, older specimens might be found in the future, but one of the most exciting recent developments has been molecular biologists sitting down with palaeontologists (or at least with fossil data) to try to calibrate their molecular trees, and determine when the major groups appeared and how the fossils fit into wider patterns of relationships.

Fig 16. ?Rowlandius velteni (Hubbardiidae) in Miocene Dominican amber; the top of the abdomen has been ground away during preparation.

About the authors

David Penney FRES is an honorary lecturer in the Faculty of Life Sciences (Preziosi Lab) at the University of Manchester, who specialises in research on amber palaeobiology and spiders. Jason Dunlop is the curator of arachnids and myriapods at the Museum für Naturkunde in Berlin. He researches fossils from all of the arachnid orders and is particularly interested in their evolutionary relationships. Together, they have recently (2012) published the first book to provide a broad overview of the arachnid fossil record.

Acknowledgements

For this article, we acknowledge the following for the images used in this article: the Bayerische Staatsammlung für Palaeontologie und Geologie, Munich (Germany), Henry Moseley X-ray Facility, University of Manchester (UK), Lapworth Museum, University of Birmingham (UK), Manchester Museum (UK), James Jepson (UK), Museum für Naturkunde, Berlin (Germany), Paul Selden (USA), San Diego Natural History Museum, San Diego (USA), Marius Veta (Lithuania), Pat Craig (USA), Westfälische Wilhelms-Universität, Münster (Germany) and Wolfgang Weitschat (Germany).

Further reading

Dunlop, J.A. (2010) Geological history and phylogeny of Chelicerata. Arthropod Structure and Development, 39: 124–142­­

Dunlop, J.A., Penney, D. & Jekel, D. (2012) A summary list of fossil spiders and their relatives. 261 pp. In: Platnick, N.I. 2012. The world spider catalog, version 12.5. American Museum of Natural History, online at http://research.amnh.org/entomology/spiders/catalog/index.html

Dunlop, J.A. & Penney, D. (2012) Fossil Arachnids, 192 pp. Monograph Series, Volume 2. Siri Scientific Press, Manchester (available from http://www.siriscientificpress.co.uk).

Penney, D. & Selden, P.A. (2011) Fossil Spiders: the evolutionary history of a mega-diverse order, 128 pp. Monograph Series, Volume 1. Siri Scientific Press, Manchester.

Table and legend

Arachnid order	Number of fossil species	Number of extant species
Araneae Pseudoscorpiones Scorpiones Solifugae Thelyphonida (Uropygi) Amblypygi Opiliones Actinotrichida (Acariformes) Anactinotrichida (Parasitiformes) Palpigradi Ricinulei Schizomida Phalangiotarbida Trigonotarbida Haptopoda Uraraneida	1,142 43 121 5 7 9 30 293 16 1 15 6 31 65 1 2	42,751 3,454 1,947 1,100 108 160 ca. 6,500 ca. 54,500* (*Actin+Anactin combined) 83 59 267 0 0 0 0

Comparative diversity of described fossil (including copal) and extant arachnids and their relatives. Fossil data based on Dunlop et al. (2012); note that some species known as fossils are extant.

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Filed under: fossils Tagged: Acariformes, Amber, Amber Inclusion, Amblypygi, arachnids, Araneae, Fossil, fossils, Haptopoda, Opiliones, Palpigradi, Parasitiformes, Phalangiotarbida, Pseudoscorpiones, Ricinulei, Schizomida, Scorpiones, Solifugae, Spiders, Thelyphonida, Trigonotarbida, Uraraneida
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