Savory - Arachnida 1977

120 III. PROLES ARACHNES
DISTRIBUTION
Scorpions are found only in the warmer parts of the world (Fig. 32). In
the northern hemisphere they occur in the countries bordering the
J\Iediterranean, and are to be found in the south of France (five species)
and in southern Germany. In America they reach the west coast, cross
into Canada and are recorded from British Columbia, Alberta and
Saskatchewan. In the southern hemisphere they are also widespread,
but are absent from ='lew Zealand, Patagonia and the oceanic islands.
Frc. 32. !\Iap showing tht' distribution of scorpions.
PALAEONTOLOGY
Fossil scorpions range from the Silurian to the Oligocene. The earliest
known scorpion is the Isle of Gotland species, Palaeophonus nuncius,
described by Thorell and Lindstrom (1885). This scorpion differed
from all living forms in the shape of its tarsi, which were sharply pointed
and possessed no terminal claws, a fact which has been taken by some to
support the hypothesis that these Silurian scorpions were water-dwellers.
In any case, Petrunkevitch has placed Palaeophonus in a separate suborder,
Protoscorpionina, which, besides the family Palaeoscorpionida
also contains the families Dolichophonidae and ~Iaxonidae, with three
genera and three species from Scotland and Korth America.
Carboniferous scorpions, with claws on their tarsi and therefore
placed in a sub-order named Euscorpionina, are fairly numerous, and
occupy 22 genera with 38 species in six families:
13. THE ORDER SCORPIONES
Palaeoscorpiidae
Archaeoctonidae
Eoscorpiidae
Cyclophthalmidae
lsobuthidae
Cen tromachidae.
One of the most interesting of these is the species Gigantoscorpio willsi
from Scotland, described by St0rmer ( 1963). Its great size of 36 cm long
is far in excess of that of any other known scorpion, and may be taken to
suggest that an arachnid of such dimensions would need the support of
water. In other respects the Carboniferous scorpions showed all the
features characteristic of living species, and it appears that the order
really reached its acme during this epoch. The specimens have been
discovered in Bohemia, Britain, Pennsylvania, Illinois and New
York.
The Mesozoic era has yielded remains of a family of Triassic scorpions,
known as the ~Iesophonidae. This contains the genus l\!Iesophonus,
with six species, all from the Lower Keupcr Sandstone of
Lanark, and Spongiophonus, with one species from the Midlands.
Berland points out that the rarity ofTriassic scorpions is to be expected,
for animals which prefer a dry habitat are not likely to die in circumstances
favourable to preservation. This idea cannot, however, be made
to account for the almost complete absence of fossil Arachnida from
any other :Mesozoic strata, and makes all the greater the interest of
Mesophonus and the Palpigrade Sternarthron.
A few Tertiary scorpions have been found. One of these, Tityus
eogenus, preserved in amber, is to be included in the recent family
Buthidae, so marked is its resemblance to living forms.
Two considerations of more than usual interest have followed from
the study of fossil scorpions.
In the sub-order Euscorpionina, mentioned above, the family Eoscorpiidae
shared a super-family, Scorpionoidea, with the seven families of
living scorpions. In other words, of all the Carboniferous families one
only has survived in the form of scorpions of today. Eight other superfamilies
have left no descendants, suggesting that during the period
evolution and competition were rapid and intense.
Vachon, comparing the characteristics of the extinct families with
the distribution of the existing ones, points out that the African deserts
have not always been the arid, infertile tracts of the present time, but
that they have undergone alternating wet and dry periods. Scorpions
react to physical changes by adapting to specialized habits and restricted
habitats, from which it again follows that existing scorpion fauna is
to be regarded as a relict group, the survivors of inorganic evolution that
destroyed the majority.
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