The Carapace and its Evolution

This is the dorsal covering of the prosoma on which the eyes are found. Its shape can be variable although it is always longer than wide. As with other parts of the exoskeleton this can be ornamented. Common ornamentations are granules (whether covering the whole surface or just the margins) and carina. Whilst granules are usually round bumps on the surface, carina are long angular ridges, usually running from anterior to posterior and there may also be a thin median depression (sulcus) in the same orientation.

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.

Prosoma of Stoermeroscorpio delicatus. Reconstruction found in Kjellesvig-Waering's 1986 monograph

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.

Liocheles waigiensis Showing the inverse-Y suture on the carapace. Although formerly Ischnuridae the Hemiscorpiidae has a complicated history (see the 'Scorpion Files' for an overview). Image courtesy of the 'Scorpion Files'-

1 comment:

Sam W. Heads said...

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