In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous. This has the effect of making the bones light enough so that it is easy for the animals to swim back to the surface after a deep dive. The outer shell of the bones was also porous in the ichthyosaur, an extinct prehistoric marine reptile. We can conclude from this that ichthyosaurs were deep divers. Which one of the following, if true, most weakens the argument? ['Some deep-diving marine species must surface after dives but do not have bones with porous outer shells.', 'In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous.', 'In addition to the porous outer shells of their bones, whales have at least some characteristics suited to deep diving for which there is no clear evidence whether these were shared by ichthyosaurs.', 'There is evidence that the bones of ichthyosaurs would have been light enough to allow surfacing even if the outer shells were not porous.']