Sponges of the New Caledonian lagoon - IRD

where its life forms were capable of dispersing widely across Gondwana. or
around the circumference of its shallow seas. During this time (the Late Cretaceous.
85 MYA). the earth was also virtually circled by a broad. tropical Tethys sea. with
the only barriers to sponge dispersal being the great distances between adjacent
land masses. This Cretaceous sponge fauna was relatively similar across the broad
expanse of the Tethys Sea. at least compared with the many different faunas found
in Recent seas.
The subsequent drift of continents and spreading of the sea floor over many millions
of years tore apart the austral land mass. and together with substantial volcanic
activity there were ridges pushed up from the sea floor. new islands and island
arcs emerging or submerging from the seas. and continents and coastlines were
reshaped into their present day forms. These events also produced the closure of
the once-contiguous Tethys Sea. isolating the sponge fauna initially into two components
(lndo-west Pacific and Atlanto-east Pacific regions). and later into many
more smaller regions with their own peculiar currents and climates in which separate
sponge faunas began to evolve in isolation from each other.
New Caledonia itself is one of the older land masses of the tndo-Australian plate.
thought to have been emergent the entire time since the Cretaceous. and was produced
by uplifting of a ridge through collision of three separate blocks or arcs on
this plate. These islands subsequently became isolated from the other fragments of
the Gondwanan supercontinent through the formation of the Tasman Sea and Coral
Sea. thought to have occurred from spreading of the seabed. This geographical isolation
restricted or perhaps completely blocked the recruitment of many species
from adjacent land masses and their coastlines. New species of animals and plants
subsequently evolved from these now isolated. once Widespread ancestors
through the mechanisms of natural selection and adaptation to the particular environmental
conditions now present on these islands and their coastal seas.
During the Tertiary period (60-1 .6 MYA). when New Caledonia became increasingly
isolated from other land masses. there were also marked global changes in climatic
conditions and sea levels. and local effects produced by changes in ocean currents
around New Caledonia obViously influenced the biological development of these
areas. In particular. cycles of glaciation. with associated fluctuations in sea levels.
undoubtedly had profound effects on the formation of marine habitats (such as the
development of reef structures). and biogeographic patterns (such as opening or
69
Sponge
diversity,
distribution and
biogeography