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Inevitable simultaneous extinction?
The extinction of the dinosaurs (for whatever reason) could have
triggered the demise of mosasaurus and other thermophilac sea life. The
immediate aftermath of dinosaur extinction probably witnessed a major
increase in terrestrial plant biomass in the absence of megaherbivores to
suppress it. An increase in vegetation could have caused a reduction in
atmospheris CO2, absorbed by plants, a reduced greenhouse effect, and cooler
temperatures which, coupled with regression, could have dealt the coup de
grace to thermophilac marine reptiles.
Since the Triassic, dinosaurs were probably the key element in a warmer
Gaian regime they "invented" and maintained. Suppressing vegetation with
their appetites, dinosaurs kept CO2 levels and global temperatures high,
which facilitated recuperative growth (especially after angiosperms appeared)
and thus maximized the productivity of the land, supporting huge dinosaurian
sizes and populations, even with elevated metabolic rates.
If this view is correct, the existence of dinosaurs was essential to the
maintenance of Mesozoic warmth. Sea reptiles were beneficiaries of
dinosaurian dominance and could not survive without them.
A very important point is that the near-simultaneous extinction of
dinosaurs and sea reptiles need not imply a common agency of extinction. The
dinosaur extinction may have caused marine extinctions indirectly, made them
inevitable. Generally, it is assumed that the lack of a common biological
agency of extinction, on land and at sea, implies a common physical agency
e.g. an asteroid. But a biological agency or agencies could have
extinguished the dinosaurs AND, indirectly, the sea creatures, for the latter
may have needed the dinos.