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The jaws/thumb thing actually has a pretty simple (potential)
explanation, I think.
You just have to remember that spinosaurs are like fishermen.
A fisherman uses one means to snag the prey, usually a line with a
hook, and brings it close in. This isn't really analogous to what
spinosaurs did which is something like spearfishing but the next point is
the important one.
If you've fished fairly big stuff, like salmon or halibut, you
know that you do not just pull your fish on board the boat. If you try
this with a rod and reel on a salmon it may break your line and if not it
may break your pole, because the whole weight of the fish, unsupported by
water, is now on it. For the same reasons, trying to pull a several
hundred pound halibut onboard a boat using the longline alone would be a
stupid thing to do- chances are good it will just bend the hook straight
and slip back into the water.
The thing is, spinosaurs have decently long necks, and very long,
slender jaws. If they were going after really big fish, say several
hundred pounds, it isn't going to have the leverage and strength to lift
it clear of the water with the jaws alone- any more than you will try to
lift a big fish onboard using a rod and reel, or a 300 lb. halibut onboard
with just a hook and line. The length of the jaws gives it great reach but
absolutely lousy leverage, also their slenderness makes them not
particularly strong against bending in the dorsoventral plane.
Basically, there is an initial capture of the fish, but once you
get it in closer, you have to secure it and make sure it doesn't get away
and then haul it out of the water, and unless you're fishing small stuff,
that requires a different technique.
Now, fishermen sometimes use nets to haul their catch clear of the
water, spinosaurs probably couldn't evolve nets. What they *could* evolve
is the other strategy employed by fishermen:
When that struggling 300 lb halibut comes thrashing to the
surface, a bunch of fishermen run up and hook a bunch of steel gaffs into
its head and haul it onboard the boat.
the strategy spinosaurs used may have been this: wading after the
fish, they would have darted out the long head, seized a fish, and dragged
it closer. Once within range, the arms could be deployed. The arms are
absolutely massive, and so are the coracoids. However, they aren't really
that long. That means that their leverage and the force they could have
exerted would have probably been tremendous. Considering that the arm
bones would easily put any human weightlifter's to shame(and almost any
theropod's), despite being about the same length, it doesn't seem
ridiculous to imagine that each arm could support a couple hundred pounds.
In other words, easily enough to secure a fish against escaping from the
jaws, and then lift a very large fish entirely clear of the water, and
haul it off somewhere to be devoured on land.