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eggs that could be obtained in the Eocene brood seasons is in sharp contrast to the
Campanian brood in which the number of eggs was limited by the number of pits in the
brood chambers, and hence the number of hatchlings even if all the eggs were fertile. The
interpreted reduction in egg size did not affect the size of the mature crustaceans as attested
to by the similar range of diameters of the Campanian (D=14-17 mm) and Lower Eocene
(D=10-15 mm) galleries. This reflects a change in breeding strategy that is neither the result
of changes in body or population size, nor of food shortage. It reflects a transition to a more
economic mode of life to which the Lower Eocene (or earlier) crustacean population had to
adapt under ecological pressure within the same pelagic habitat of their Late Cretaceous
ancestors. The reduction in egg size increased the number of eggs that each female laid and
hence the overall quantity of eggs in each breeding period. It probably increased the number
of hatchlings despite some egg loss due to reduced brood-care. This interpreted need for
more hatchlings must have compensated for the loss of individuals being killed while
swimming outside the underground tunnel network to look for food. This fatal threat of
predation has profoundly increased during the Late Cretaceous as evidenced by other
faunal groups (discussed herein).
The evolutionary trend expressed by changes in crustacean burrowing systems can be
extended to Early Pleistocene times as exemplified by a burrow system of
Spongeliomorpha
sicula
D'Alessansro & Bromley (1995) from Sicily, Italy. Cylindrical vertical shafts and
horizontal galleries about 10 mm in diameter bear longitudinal fine ridges replicating
scratches. Plum-shaped chamber casts 30 mm high and 25 mm in diameter with similar
longitudinal striations occur at gallery junctions every few centimeters (Fig. 6F). They are
associated with much shorter cylindrical inflations. The plum-shaped chambers were
interpreted as microbial gardening sites (D'Alessandro & Bromley, 1995). Following the
study of the Campanian and Lower Eocene burrow systems, we are inclined to refer these
chambers to brood and nursery chambers as in the Lower Eocene example. However, in the
Pleistocene example each chamber has its own entrance from the lower side, whereby the
chamber content is not jeopardized by chamber-crossing crustaceans as in the Eocene
example. The associated sediments indicate shallow marine environments which were rich
in food sources and should not require production of special nourishment in specially
constructed gardening chambers. On the other hand, eggs and larvae shed into the shallow
marine water were subjected to rapid consumption by many predators. Thus keeping the
hatchlings until they were capable to defend and feed themselves would have been needed
to protect the species. This interpretation coincides with the evolutionary trend in this group
of burrowing crustaceans from the Late Cretaceous to almost present times. The interpreted
care for the brood and the young attests to communal organization at least until the Early
Pleistocene and probably might be detected in extant
Spongeliomorpha
species. It seems that
these burrowing Crustaceans have maintained communal organizations from times when
they inhabited deep water bottoms exposed to predators, and continued to experience the
benefits of this co-operation in shallower marine environments.
3. The ecological affinities of the upper part of the Cretaceous Period
MacLeod (2005) summarized the characteristic affinities of the Cretaceous Period which
ranged between 145.5-65.5 Myr and is generally divided at the Albian-Cenomanian
boundary (99.6 Myr) into the Lower and Upper Cretaceous. The warm equable climate
(warmer than today) extended into the high latitudes, and the poles were probably without
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