Yes, I think you maybe right.....if I remember correctly, wasn't there a hint on these pages a few weeks ago that there may be another giant out there apart from Archi and Messie? Or perhaps I just misinterpreted a comment (as I often do).
Well, as to your other point, you would need someone much wiser than me to explain the intricacies of ecology of the Palaeozoic oceans. However, if you are referring to 'soft-bodied cephs' as squid, octopi and cuttlefish then there was no real overlap with the nautiloids. The nautiloids really came to dominance way back in the Ordovician period where they certainly assumed the position of top predator. There was an evolutionary explosion of nautiloids at this time with no less than nine orders, most of these had cone shaped or even straight shells and were really quite diverse in their morphologies. Most of these nautiloid groups became extinct by the end of the Permian excepting just two groups, the Orthoceratina which lingered on to the end of the Triassic and, of course, the nautilida from which our modern nautilus is descended. The modern Nautilus
really is the sole surviving twig on what was once quite an exotic bush.
The gradual extinction of the nautiloids at the end of the Devonian extending in the case of some orders into the Carboniferous can (probably) partly be explained by the emergence of the ammonoidea in the Devonian, especially the goniatites. Why these should have come to dominate is a very good question, perhaps the ammonoidea were specialists and more adaptable or perhaps they had a faster growth rate and a shorter lifespan so that they could evolve quicker than the nautiloids. If anyone has any good theories, I'd love to hear them here!
Some of the nautiloids were adapted to deep water, indeed, one researcher, Westermann (1985), has established crush depths for these ancient creatures. It is estimated that one nautiloid, the Carboniferous Michelinoceras
, could probably withstand depths of 1125m. It seems that direct competition with the shallow-water ammonoids could not be sole reason responsible for explaining the demise of the nautiloids, there must have been other factors as well.
Anyway, going back to your origin point about soft-bodied cephs, the squid and octopi did not appear until the Jurassic era, by which time the only surviving nautiloid order, the Nautilida, had adopted the familiar form we have today and were probably already established in the deep-water niche they occupy today. So it seems likely that there really would not have been that much competition between the two groups. As an aside, I think the cuttlefish are first known from the Cretaceous and as shallow water creatures would certainly not have made much of a direct impact on the Nautilida.
Anyone still awake?