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.