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Re: What did Spinosaurus eat? New species of Lepidotes found

On Fri, Feb 25, 2011 at 9:53 AM, Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:

> Actually, that "immersion" argument of Amiot et al. is one of their 
> inferences that is, in my opinion, extremely weak. Diet's
> influence of p-del-18O is very strong. They dismiss diet as the cause of the 
> similarity between crocs and spinosaurs, and the
> disimilarity between spinosaurs and carcharodontosaurs, because spinosaurs 
> and crocs might have had some terrestrial food in their
> diet.
> Well, no kidding! Nevertheless, if crocs and spinosaurids both sampled a 
> greater fraction of fish in their diet than did terrestrial
> predators in the same environment, the prediction is that crocs and 
> spinosaurids would shift isotopically in the same direction.
> Which is exactly what we see.

Thanks Tom, that's very helpful.  In general, I'm not aware of the
relative contributions of diet vs immersion to the final oxygen
isotope ratio... but crocs would presumably be immersed most of the
time.  The isotope ratios of certain spinosaur remains are about the
same as contemporary crocs (Amiot et al., 2010) ... so if spinosaurs
spent less time immersed than crocs, does that mean that these
spinosaurs had a higher proportion of aquatic prey in their diet than
crocs (in order for the oxygen to shift isotopically to the same

I guess I was taking a conservative approach: spinosaurs and crocs had
comparable p-del-18O ratios because they had comparable lifestyles,
including the amount of time each spent immersed in water.