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Spinosaurus - the authors respond

The case for the affirmative...


It's also worth mentioning that evidence of an aquatic _Spinosaurus_
refutes a hypothesis put forward in the 1990s that non-avian theropods
were 'limited' in the potential niches they could exploit (including
aquatic/marine habitats), because they only possessed a single
locomotor 'module' (hindlimb + tail). By contrast birds, with three
separate 'modules' (wings, hindlimbs, tail) were far less constrained.
It is true that birds have exploited far more diverse ecological
niches than their non-avian counterparts... but the hypothesis that
non-avian theropods were anatomically precluded from becoming
semi-aquatic has been dealt a blow  by _Spinosaurus_.  I'd also be
surprised if _Spinosaurus_ was a 'one-off' in this respect.

This same hypothesis also proposed that the hindlimb of non-avian
theropods could not undergo any major modifications (such as for
aquatic locomotion) without severely compromising terrestrial
locomotion.  This would appear to hold water (so to speak).  In an
imponderable 'what if' of evolution, spinosaurs could have given rise
to obligate aquatic/marine predators similar to thalattosuchians or
mosasaurs or even penguins.