Coxosternal region in a range of scorpions. A: Archaeoctonus B: Praearcturus C: The extant scorpion, Diplocentrus. The asterix marks the sternum. Images edited from Kjellesvig-Waering, 1986.
In the modern scorpions the sternum shape is always derived from a pentagonal form, though the terms 'triangular' and 'transverse' are applied to some of these variations. In early scorpions the shape of the sternum was much more variable, but based upon an oval shape (needle like, pentagonal, triangular and a number of other terms can be applied to these variations). Particularly elongate (needle-like) sterna are found in a handful of extant and extinct scorpions (Praearcturus and Troglocormus springing instantly to mind.), scientists aren't too sure of the significance of this but it may be to aid spermatophore placement.
Over time, the sternum moved posteriorally, as first the anterior pair of coxae moved in front and fused, and then the second (and occassionally 3rd). During this progression lobes began to develop on the 1st (and much later, 2nd) pair of coxae. Known as coxapophyses, Stoermeroscorpio and Proscorpius are the earliest scorpions to show development of these coxal lobes. The 1st coxapophysis was well developed by the time of Praearcturus (whose 1st two pairs of coxae meet infront of the sternum; see 'B' in the figure above) and by the Mesoscorpionina the second coxae have maxillary lobes (just another term for the coxapophyses), which may, or may not, extend to the 1st maxillary lobes (as in 'C' in the figure above).
The trends in the evolution of the coxosternal (also known as the sternocoxal-) region are related to the change from particulate to liquid feeding and improved terrestrial movement. In Proscorpius and Waeringoscorpio the coxae may have been gnathobasic (a term meaning they might have been used in the crushing/tearing of food) as none of the four coxae appear fused. Although not occurring as a linear transgression to the more advanced state, the development of an oral tube made by the coxapophyses is indicative of improved liquid feeding (where the oral tube acts like a straw in sucking up the externally digested prey). Whilst it cannot be said with any certainty, it is possible that any anterior lobe may have helped with this. Likewise, any fusing of the coxae may have strengthened the scorpions ability for terrestrial locomotion.
Chelicerae are the name given to scorpion jaws (which just so happen to be chelate!). These are made of a coxa, tibia which has a fixed finger, and the tarsus forming a moveable finger (compare this to the segments/ podomeres of the legs). The ‘fingers’ of the chelicerae often have teeth, which can be basal, medial and/ or distal.
2 examples of the primitive jaw condition. A: Proscorpius B: Liassoscorpionides. Images are not to scale and both are edited from Kjellesvig-Waering (1986)
CHELICERAL EVOLUTION Several trends can be seen in the evolution of scorpion chelicerae. Kjellesvig-Waering initially thought that early scorpions had chelicerae composed of 4 segments which later reduced to 3, but Stockwell claims this to be unlikely as no extant chelicerate has more than 3 segments. Unfortunately the quality of preserved fossils means we cannot be certain who is correct. What can be said is that early scorpions possess MUCH larger chelicerae relative to modern forms. It is likely that this trend is an effect of the move from macerating (tearing food apart) food to terrestrial liquid feeding (see section on the Coxo-sternal Region). As far as we are aware, all protoscorpions were aquatic and had enlarged chelicerae, whilst all mesoscorpions had reduced chelicerae. From fossil evidence, the change can be said to have occurred in the Proscorpiidae, although there is no smooth transition and the Devonian proscorpiid Waeringoscorpio has small chelicerae whilst the Carboniferous proscorpiid Archaeoctonous has the more primitive form.
In later scorpion evolution, the number of teeth on the fixed finger can be important. The palaeopisthacanthids had 5 similar teeth, and a differentiation in size and reduction from 2 (in some chactoids) to 1 (in buthids) has been recorded. The importance of this trend is unclear.
At this point we're going to leave the prosoma, as having looked at the eyes, slit sensillae and trichobothria i'd like to stick with sensory structures of a scorpion and that brings us onto a structure unique to the group... the pectines
Sternum, genital operculi and pectines of the extant Gertschius crassicorpus. Image modified from Graham and Soleglad (2007).
Although pectine are sensory structures, their function is still a little unclear, however, they are larger in males and are thought to be used in selecting sites for spermatophores (see Stockwell, 1986). The pectines are made up of a basal plate onto which 2 articulated combs are joined. Just like normal combs, these have rows of teeth along a main branch (called a rachis), which is usually jointed. Each of the teeth have regions of special sensory structures called peg sensillae. There are literally thousands of microscopic peg sensillae on the pectens. These are often arranged into rows and work like antennae, picking up odorants and tastants on the substrate.
In front of the pectines are paired genital plates (OPERCULI); in scorpions they lack respiratory structures.
The overall anatomy of pectens has remained the same since the origin of scorpions. The earliest preserved pectens are found on the aquatic Palaeoscorpius and Allopalaeophonus and it is possible that the earliest scorpions laid their eggs in shallow nests in a similar manner to eurypterids and xiphosurans. If this were the case, than rather being used to position a spermatophore, they may have been used for digging (see Kjellesvig-Waering's monograph on fossil scorpions).
It is speculative to discuss how pectines arose, but it has been suggested that they may have been part of the genital segment and are homologous to (have the same evolutionary origin as) the median appendage of eurypterids, in particular the paired furcae at the end of the eurypterid genital appendage. If this is true, then a protoscorpion/ eurypterid ancestor would have lost the close association between the median appendage and the genital operculi, and later evolved true pectines.
Due to the microscopic structure of peg sensillae, little can be said regarding their evolution on the pectines, but many arthropods (both aquatic and terrestrial) possess peg sensillae in regions of the cuticle. Finally, there are no real trends regarding the number of teeth or divisions of the main branch (rachis). Teeth are very variable in both extant and extinct taxa, and a fully fused rachis can be seen in fossils such as Proscorpius and Branchioscorpio and recent species such as Megacormus.
As for the genital operculi- one morphological feature hinting at the ancestory of scorpions are the eurypterid deltoidal plates and like scorpion opercular plates, these are paired structures. In early scorpions the genital operculi were large and subquadrate (see Branchioscorpio) but circular (Anthracoscorpio), elongate (Kronoscorpio) and small subquadrate (Gigantoscorpio) varieties are present in Palaeozoic scorpions. When scorpion Infraorders were based upon sternal plates, the shape of opercular plates were important in understanding scorpion evolution, however recent systematic evidence shows that there is no real trend in their shape. Confusingly, the genital plates were originally fused, before separating completely. Today the genital operculi remain separate although in most bothriurids they have become loosely fused again.
Slit sensillae are thin, deep slices in arthropod cuticle leading to sensory neurons. Their roles are numerous, but rely on the deformation of the cuticle around the slit. Depending on where they are found they can function as proprioreceptors (sensing movement), georeceptors (measuring bending by the opisthosoma) and by measuring changes in air pressure can sense vibrations. It has also been shown that differences in vibration reception between the sensillae can lead to 2-dimentional prey location in scorpions. Readers should be aware that these slits can be found isolated or in parallel groups called LYRIFORM organs
SLIT SENSILLA EVOLUTION For a time, it was thought that slit sensillae were a derived feature of recent scorpions, as they were not present in fossil varieties, eurypterids or xiphosurans. To date, I am not aware of any slit organs in fossil specimens, however this may just be a preservational bias as recently they have been found in the Devonian trigonotarbid Palaeocharinus, and the Silurian, aquatic eurypterid Baltoeurypterus. Whilst those of Baltoeurypterus are wider than modern forms, their presence would make it far more parsimonious for all scorpions to have possessed them, and indicates that they served a purpose in aquatic habitats.
High resolution image of trichobothria. Image taken from www.answers.com/topic/trichobothria
TRICHOBOTHRIA All extant arachnids (and many other arthropods for that matter!) possess trichobothria. These are elongate, non-tapering hairs (unlike regular setae) which fit into a cup, allowing movement in most directions. This, coupled with their great sensitivity allows them to detect airborne vibrations. Trichobothria can either be full or petite and key trichobothria (regardless of size) are very important in the systematics of scorpions.
TRICHBOTHRIAL EVOLUTION Despite all extant arachnids possessing trichobothria, it is thought that they have evolved independently in each group as a result of terrestrialisation. As such, discussion of their evolution within the different arachnid groups is outside the scope of this site, however, they are very important in the systematics of modern scorpions.
The terrestrialisation of scorpions probably occurred within the Palaeoscorpiones and certainly by the Mesoscorpionina, yet true trichobothria are first found in the orthostern Palaeopisthacanthus. As trichobothria are obviously very difficult to preserve, these were inferred by the cup-shaped follicle which holds the trichobothrium. That said, the ‘prototypic palaeopisthacanthid’ Corniops mapesii is thought to have possessed smaller trichobothria whose follicles lacked the rimmed cup shape.
An awful lot of systematic work has been done based upon the trichobothria of the pedipalp and each of its segments (chela, patella and femur) and therefore the following may be quite hard to understand until you have read the sections on Systematics and General Evolution. Suffice to say, 6 major types have been identified. These are based on the number of trichobothria on each segment and the position of the major trichobothria. These are labeled P (because this pattern is found in Palaeopisthacanthids), F1 (for archaeobuthids), A (for Buthida), B (for Chaerilida), C (for Iurida) and D (for pseudochactids). To understand these patterns, a knowledge of trichobothrial terminology is needed, but this isn’t as scary as it first looks.
Basically, for each segment, trichobothria are numbered from the base (proximally) to the terminus (distally) and to make the numbers more manageable each segment is treated like a separate box, with a dorsal (PREFIXED with d), ventral (prefixed with v), external (e) and internal (i) surface.
A guide to help you visualize what is meant by the e, i, d and v prefixes in trichobothrial terminology
Deviations from this terminology
As well as this, on the patella and manus, it is traditional to group the dorsal and external trichobothria into basal (with a SUFFIX of -b), sub-basal (with a suffix of -sb), sub-terminal (-st) and terminal (-t) forms (surfaces of the palm of the manus being differentiated from those of the fixed finger by capitals, ie. The external, sub-terminal trichobothrium of the palm is Est, whilst for the fixed finger it is est).
With 18 trichobothria, the palaeopisthacanthids had relatively few compared with modern forms (see below). Admittedly there is some uncertainty as to this number and the images below are based on a composite of Palaeopisthacanthus and Cryptoscorpius, a method which has been criticised.
Trichobothrial arrangement in the Palaeopisthacanthid femur. Based upon Soleglad and Fet (2001)
Trichobothrial arrangement in a palaeopisthacanthid patella. Image based upon Soleglad and Fet (2001)
Trichobothrial arrangement in the manus and fixed finger of a palaeopisthacanthid. Image based upon Soleglad and Fet (2001)
With the exception of femoral trichobothria (which reduce from 4 and are completely lost in scorpionoids), all trichobothria increase during scorpion evolution. Archaeobuthids have a total of 27, pseudochactids have 34 and the Buthida have 39. Likewise the Chaerilida have 37 trichobothria and the Iurida (Scorpionoidea, Chactoidea and Iuroidea) possess 48. This increase in number most probably represents an evolution towards increased specialization in the use of trichobothria.
LEG EVOLUTION The legs of terrestrial arthropods are characterised by a differentiation in podomere length and an oval/compressed cross section in order to support the animals weight on land. We should be careful in using this feature as a sole indication of habitat though, as in very large aquatic eurypterids such as Tarsopterella this can also be seen - because they are so big they needed greater support!. It is generally thought that terrestrialisation of the scorpion lineage took place somewhere in the Palaeoscorpiones. If this is true then you might expect the aquatic protoscorpions to demonstrate the primitive condition. Sure enough, the four genera belonging to the Protoscorpiones (Allopalaeophonus, Palaeoscorpius, Palaeophonus and Dolichophonus) all have podomeres of roughly equal length (albeit decreasing distally) and tubular cross sections. Although Stockwell (In a very important, widely read but unpublished PhD thesis!) claimed the proscorpioids also showed this feature, Jeram (1997) codes these fossils as possessing the more typical, modern condition.
PEDIPALP EVOLUTION Chelate (pincer-like) pedipalps are found in ALL scorpions, but if scorpions evolved from mixopterid eurypterids (which also have chelate pedipalps) this is unsuprising. Scorpion pedipalps come in 2 main forms, GRACILE (with fingers longer than the palm) and ROBUST (with fingers shorter than the palm). The different pedipalp morphs are related to the mode of feeding in scorpions (Gracile morphs being found in much more toxic varieties and Robust form being used to crush prey in less toxic scorpions). It is of little importance in the systematics of fossil scorpions, but in recent scorpions it can be quite important as the Buthidae are all Gracile scorpions.
Image of the Robust clawed Tarsoporosus yustizi taken from the 'Scorpion Files'.
Cross section of a single ocellum. Taken from Brusca and Brusca (1990)
Prosoma of Proscorpius osbornii showing the primitive condition for scorpion eyes. Reconstruction based upon KjellesvigWaering's 1986 monograph
EYE EVOLUTION As with early scorpions, eurypterids are known to have possessed simple, median eyes and pairs of compound eyes. Whilst all of these are found on the dorsal surface in the Stylonurina and Eurypteracea; in the Mixopteracea and Pterygotacea the compound eyes are found antero-laterally on the carapace, as might be expected if scorpions evolved from a small mixopterid (see general evolution). However, unlike eurypterids, early scorpions had anteriorally placed median eyes and these only later moved to a more central location, a movement that can still be seen in scorpion embryology (See the 2005 paper by Farley in the reference section ). In many Palaeozoic scorpions, the median eyes were located on prominent eye nodes. In the Palaeoscorpionina and Mesoscorpionina (don't worry about the names, a full description will be provided in the General evolution and Systematics sections, suffice to say both groups are relatively derived), the median eyes/ eye node was so far forward it actually formed an anterior median process (see carapace evolution).
The posterior movement of the median eyes, and reduction of the lateral compound eyes into 10 or fewer isolated ocelli is a trend which begins with the Palaeosterni (this includes fossils such as Eoctonus, Buthiscorpius and Anthracoscorpius) and as such, is only present in neoscorpions (according to Jeram, 1994) With the Neoscorpionina being found from the Carboniferous, the trends in eye evolution seen within the scorpion lineage happened long after their terretrialization and may therefore be linked to a change in predatory behaviour towards a more modern approach. Today, the primitive number of lateral eyes is 3, a number shared with the fossil orthostern Palaeopisthacanthus. Interestingly, the lateral eyes of scorpions are not the same as the secondary eyes of spiders. Although both groups have lost their compound eyes, in spiders the secondary eyes have become inverted (the light sensitive cells facing away from the lens) whereas in scorpions the lateral eyes remain direct and it is thought that they evolved through the fusion of ommatidial rhabdomes into a single retina and the similar fusion of the lenses.
Loss of Eyes
The loss of/ extreme reduction of eyes in scorpions that live in caves (ie. Liocheles polisorum) is well known. Although researchers such as Kjellesvig-Waering have used this as a family level criterium, in modern varieties it is only important on a species level (just compare L. polisorum with other species of Liocheles). This may also be true of fossil forms, but because lateral eyes are hard to preserve, conclusive eye loss in fossil scorpions has only been shown in a few Mesophonids and their habitats remain obscure.
Reconstruction of the prosoma of Allopalaeophonus caledonicus. Adapted from Kjellesvig-Waering's 1986 monograph
CARAPACE EVOLUTION Ornamentation is of little importance in the higher classification of scorpions. In modern scorpions the shape of the carapace is likewize too variable to be of higher systematic importance, the same, however, can’t be said of fossil scorpions. One of the defining features of the Protoscorpiones (don't worry about names like this, they will be discussed in the sections on general evolution and systematics!) is an anterior margin of the carapace forming two bulbous areas. This unusual carapce is a derived feature of this group and not an ancestral condition, as neither eurypterids, nor any later scorpions possess it. To date, any functional significance of this shape is unknown.
In conjunction with the extremely anterior position of the median eyes in palaeoscorpions (Proscorpius and Stoermeroscorpio), an anterior median node evolved. This is not seen in the more derived group of palaeoscorpions known as the archaeoctinoids, but if these specimens represent only 1 genus, it is possible that this feature was later lost. Again, the function behind this structure is unclear, however in some derived, Triassic, mesoscorpions such as Mesophonus this protrusion is also found.
Moving onto Modern forms, the median sulcus (which has been lost several times) can be quite diagnostic, with an inverse Y suture having evolved in the Hemiscorpidae, but lacking in the Bothruridae and many Scorpionidae, beyond this it is of little use in suprafamily relationships.
Scorpions are ARTHROPODS, this means that they do not have bones, but instead have a segmented shell covering their jointed body and limbs. The Arthropoda includes spiders, ants, crabs and millipedes, each of which belong to a diffent group within the Arthropoda. The group of arthropods to which the scorpions and spiders belong is the CHELICERATA, this basically means that the body can be split into 2 major parts and instead of antennae (like all other arthropods), the 1st pair of limbs are modified into pincer-like jaws (chelae/ chelicerae).
The 2 main parts of a chelicerate are the PROSOMA (Head) and OPISTHOSOMA (Rest of the animal). However, if you look at a scorpion, you can see that the opisthosoma can in turn be split into a body (MESOsoma) and tail (METAsoma).
PROSOMA: This contains the chelicerae and the limbs that have been modified into pincers to help with catching food (PEDIPALPS). On the top (dorsal) side is the CARAPACE and EYES. on the bottom (ventral) side are all the LEGS and STERNUM.
MESOSOMA: This is made up from 7 segments. The BOOK LUNGS and GENITAL OPERCULUM and PECTENS are located on the mesosoma.
METASOMA: This has 5 tubular segments and the sting ACULEUS (just for confusion, this is also known as the telson). The anus is located on the 5th segment.