Savory - Arachnida 1977

11. PHYLOGENY: EVOLUTION 95
11
Phylogeny: Evolution
As has been foreshadowed in Chapter 2, the evolutionary problems
which the Arachnida pose are unusual in character and difficult to
solve, for the class is one of which the origin is uncertain and within
which the succeeding, progressive, changes are obscured. With as many
as 17 reasonably well defined and recognizably different orders, an
arrangement in a series of ascending steps from the earliest and most
primitive to the latest and most elaborate ought not to be impossible,
yet, in fact, it is.
The chief difficulty lies in the nature of the palaeontological record,
which does not present us with a series of fossils, dated, at least in relation
to each other, by the strata in which they have been found.
An arachnologist's first concern is with the Silurian period. Its
predecessor, the Ordovician, had been marked by a great increase in
the number ofliving things and simultaneously in the appearance of new
species. This was particularly true of the marine invertebrates, the land
being occupied only by plants and probably no insects. The Silurian
rocks were mostly deposited in the littoral and shallow waters of the
seas, and they show us how the evolution of the animal kingdom continued
with undiminished vigour. The seas were populated by Eurypterida
(Fig. 26), which now began to reach the great size for which
they are notorious; Xiphosura such as Hemiaspis abounded, and the
Ostracoderms foreshadowed the arrival of the vertebrates.
It was during this period that the scorpions appear to have left the
water and established their claim to have been the animals that began
the colonization of the land. They are usually regarded as descendants
from an eurypterid stock, and perhaps they were driven to their momentous
experiment by the rigours of life in the Silurian seas. Overcrowding
invariably leads to a state of pollution, such as we know only too well
today, and in general provokes a reaction which takes the form of
emigration. But whatever the stimulus and whatever the conditions of
survival, the order of scorpions has been consistently separated from the
FIG. 26. Ewypterus fisheri. After Holm.
other orders since the time of Ray Lankester's classification in 1905.
These pioneer scorpions may have been able to survive the traumatic
change of environment because of their chitinous exoskeleton, which
retarded the loss of water from the body and prevented a fatal desiccation
which even today is a constant danger to all land Arthropoda. The
necessary alteration of the respiratory system was relatively uncomplicated,
for book-gills which developed a covering would thereby be
converted into book-lungs. The close histological resemblance between
the gills of king crabs and the lungs of scorpions supports this idea. Two
points, however, remain to be considered.
The first is the revolutionary hypothesis, put forward by Versluys
and Demoll ( 1922), that the terrestrial scorpions were the first of the
Chelicerata and were the ancestors of the aquatic Eurypterida and their
allies.
The second point is the undeniably close resemblance between
Palaeophonus, the Silurian scorpion, and its modern counterparts.
True there is a peculiar difference in the ends of the tarsi, but a part from
this the earliest scorpions have the large well-formed chelicerae, the
segmented opisthosoma in which mesosoma and metasoma are clearly