METASOMAL EVOLUTION

TAIL

When discussing the macroevolution of scorpions, little of interest can be discussed about the tail! However, in lower level classification, keels, carinatation and other ornamentation can be quite important. One trend of note though is that in more toxic species (ie. buthids) scorpions have more gracile pedipalps and much thicker tails. Also, from Palaeopisthacanthus onwards the pre-anal segment (the anus is located just before the sting) is longer than the segment before it and this isn't seen in earlier scorpions.

STING

All scorpions possess a stinger. This can be split into the bulbous, venom gland- containing vesicle and the hypodermic needle which delivers the sting, called the aculeus. The venom that scorpions release is genereally neurotoxic, and is of variable potency, but the Buthidae are among the worst offenders. As a general rule however, those scorpions with fat tails and gracile claws (those where the fingers are longer than the palm) are likely to be the most toxic, whilst those with robust claws and thin tails are more likely to use their claws in killing their prey. That said, scorpions do possess 2 types of venom, one to merely stun their prey and another, stronger form, designed to kill.

STING EVOLUTION

Given a eurypterid ancestory, it is thought that the scorpion telson evolved from the telson of eurypterids. In most eurypterids, this was involved in propulsion or balance during swimming, however, in the Mixopteroidea it was very simialr in shape to that of scorpions and is believed to have been envenomated and held over-head like scorpions do. Little of use can be discussed regarding the evolution of the telson and venom, as venom does not preserve and there has been little change in the shape of the telson. That said, featurs such as the ratio of aculeus: vesicle, the presence of setae, Latero-basal aculear serations, granulation and carination are all of use in the lower (Family/Subfamily) systematics of modern scorpions. Some Palaeozoic oddities are of note, such as Palaeobuthus, which developed an incredibly long aculeus in the Carboniferous and it should also be made clear that the sub-aculear teeth (convergently evolved in a number of scorpion familes) are not known in any Palaeozoic or Mesozoic forms, possible only arising in the Tertiary.

STERNAL PLATES

Having dealt with the genital plates and pectines, let's move further along the opisthosoma. Here we find the remaining segments of the mesosoma contain the respiratory structures. In modern forms these are book lungs, so called because they comprise haemolymph ('blood')-filled lamellae that resemble the pages of a book. Each are separated by chitinous projections and the book lungs are stored in an air filled space (atrium) that communicates with the environment via Spiracles. Today the atrium is joined to the posterior margin of the sternite, but this may not have always been the case.

Spiracles are essentially holes in the sternites ( if you're struggling with the terminology at this point, you may want to check out the scorpion files at http://www.ub.ntnu.no/scorpion-files/) of the scorpion. The nature of scorpion sternites have changed through time, but just what a scorpion sternite is has been the subject of debate.

Image showing scorpion sternites (sternates) and spiracles. Original image taken from the scorpion files.

STERNITES vs ABDOMINAL PLATES

The difference between and structure of sternites and abdominal plates are historically very important. Kjellesvig-Waering based each of his Infraorders upon the structure of the abdominal plates and constructed his evolutionary history on these. According to Kjellesvig Waering, abdominal plates may have evolved from eurypterid blatfuss and were only sutured to the body at one margin, remaining loose. He felt that sternites were hidden behind these and only became visible when the abdominal plates were lost. Others have thought that sternites are merely fused abdominal plates. One thing that can be said is that the emphasis on their structure was unjustified; no recent study has shown any evolutionary significance of this structure and fossil holosternus and lobosternus forms can be found in almost every current (albeit extinct) Infraorder.

Hypothesised evolution of abdominal/sternal plated as proposed by Kjellesvig-Waering


BOOK GILLS- BOOK LUNGS: The evolution of scorpion respiration.

Evolution can, and has, thrown up many cases of convergence (when two groups evolve the same structure independently), just one of numerous convergences between spiders and scorpions associated with terrestrialisation are book lungs. Both spider and scorpion book lungs are thought to have evolved from very similar gill structures (handily known as book gills), but at different times. This process was probably easier than one might expect, as Stockwell has commented that, "all things being equal, the respiratory books would be far more efficient in air than water.

The timing of the book gill - lung transition is difficult to pin down. Fossil evidence differentiating these structures is equivocal at best, but Stormer identified book gills in Waeringoscorpio hefteri and Jeram claims that marginal stigmata found in mesoscorpions by Kjellesvig Waering were misidentified and only appeared with the Palaeosternina.

A FINAL POINT... HOW MANY STERNITES ARE THERE?

The reason for this final trend in the mesosoma is unknown, but for some reason eurypterids and early scorpions had 5 sternal plates, and the 1st sternal plate was lost by the time of Branchioscorpio and the mesoscorpions (in fact it may have been lost some time before this, but fossils preserving the relevent portion are very hard to come by).

As you can see from the second image in this section, Kjellesvig Waering believed bilobersternous plates gave rise to orthostern forms, and this has actually been supported by embryonic studies of Centruroides vittatus, where bilobostern plates develop into the orthostern sternites. However, only two fossil genera show the bilobostern condition (Branchioscorpio and Dolichophonus) and the holosternous condition may have been better adapted, as all of the relatively derived palaeosterns possess it and scorpions with this condition persisted into the Jurassic (albeit diversity had reduced to two known genera, Liassoscorpionides [a primitive scorpion of uncertain affiliation] and Mesophonus [a mesoscorpion]).

Now, it was once thought that all scorpions possessing abdominal plates had book gills, but fragments of indeterminate lobostern scorpions have revealed that book lungs were enclosed in a stigmate sternite behind the abdominal plates (which would have possessed marginal spiracles) in at least some species. This also indicates that Kjellesvig Waering’s hypothesis that abdominal plates reduced in size and were eventually lost, revealing modern sternites is wrong and that the orthostern condition is more likely to have evolved through the fusing of, and development of stigmata in abdominal plates… a concept supported by embryonic studies of Paruroctonus mesaensis and Centruroides vittatus.

Evolution and Importance of the Coxo-Sternal Region

If we were mean enough to flip a scorpion onto its' back (dorsal side) and took a look at the prosoma, we would see that all the legs join the body relatively close to one another (sometimes even fusing). In this region is a ventral plate known as a sternum. It is possibly made from 2 fused sternal plates (see discussion of the metosoma) but it may also be derived from the metasTomal plate seen in xiphosurans and eurypterids (I've written it in capitals as eurypterids have a sternum-like plate called a metasToma- it is neither mesosoma nor metasoma, confusing huh?!). The relative positions of the leg coxae and the sternum are among the more important features in the history of scorpion evolution.








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... and their Evolution!

One reason scorpions are so sensitive (not in the emotional way!) is to aid prey capture. Once this has been caught (through the use of the pedipalps and/or aculeus), it’s then down to the jaws.

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.

Pectines, Genital Plates and their Evolution

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.

PECTEN EVOLUTION

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.

Trichobothria, Slit Sensillae: Their Evolution and Significance

One of the many lyriform organs on a spider, scorpions possess very similar structures. This image was found at www.microscopy-uk.org.uk/mag/artjan00/spileg.html

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.

TRICHOBOTHRIAL TERMINOLOGY

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).

Trends
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.

Legs, the Pedipalp and their Evolution

If we turn the prosoma over so we are looking at the underside (ventral side), we can now see the sternum, pectines, legs and jaws. Being chelicerates, scorpions have eight pairs of walking legs and another pair of legs modified for manipulating food (the pedipalps). Each of the legs seen in scorpions are made of 7 podomeres (leg segments). Within the Arthropoda, these have been given various names and there is some confusion over their homology (a posh way of saying we don't really know which segments in the legs of crabs are the same in scorpions or moths etc.). Working our way to the tip of a scorpion leg, the segments are the coxa, trochanter, femur, patella, tibia, basitarsus and tarsus. The pedipalps of scorpions have reduced segmentation (6 segments including the moveable finger) and these are known as the coxa, trochanter, femur, patella and manus (occasionally known as the tibia). As with the chelicerae, the moveable finger is known as the tarsus. Whilst the limbs can be important in understanding scorpion evolution, equally important are the sense structures on them and these will be looked at in the next section.

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.

Photo of the extant, gracile scorpion Lychas suctilus taken from the 'Scorpion Files'.

Image of the Robust clawed Tarsoporosus yustizi taken from the 'Scorpion Files'.