The Western Interior Cretaceous Seaway (or "WIKS" as all the cool kids call it, which is a way better acronym than the stupid sounding KWIS that I sometimes see...) was pretty darn weird, when you get right down to it. First of all, we've got us a huge seaway that, by the Late Cretaceous, connected the boreal arctic waters of the north to equatorial waters in the Cretaceous equivalent of the Gulf o' Mexico. The seaway followed the general trend of a big ol' foreland basin, a zone of subsidence adjacent to the Sevier fold-and-thrust belt. At it's greatest extent, the seaway's western shoreline ran south from Alaska, through western Alberta, Montana, Wyoming, Utah, and and down into Mexico, while it's eastern shorelines were ~ 1000 km away. North America was a pretty different place in the Cretaceous!
Because this seaway was on continental crust, it wasn't very deep. I've seen depth estimates as high as 700 m or so, but to be honest, I find that a little hard to swallow. On the basis of the geodynamics of thrust emplacement and the subduction response of the foreland basin, as well as some work that's been done on benthic foram assemblages, I think the evidence seems to point towards a shallower, 200-300 m depth range or so.
Regardless, you can imagine that the environment of the WIKS must have been pretty unusual. A long, shallow trough of seawater, connecting to very different portions of the open ocean. And remember, pCO2 estimates and minimal (i.e., no) evidence for high latitude glaciations suggests elevated temperatures in the Cretaceous. Shallow waters and high evaporation rates means that you've got an opportunity to develop some really salty, briny, and generally unpleasant water in the Western Interior. Interestingly, elevated global temperatures might result in considerably less thermal gradients between equatorial and polar water masses, meaning that Cretaceous oceans might have been considerably less thermally stratified. As an aside, it's largely thermally-driven density differences in water masses today that drive the large-scale conveyor belts of ocean currents, which is why anthropogenic climate change has a real potential for screwing up ocean circulation. In the Cretaceous, salinity-driven differences in ocean water density might have had a more profound role in driven oceanic circulation. For this reason, big huge inland seaways, like WIKS, might have been important sources of hot, briny water for driving halothermal circulation.
Of course, we have evidence for unpleasant WIKS environments from more than just paleoceanographic thought experiments. One of the most striking characteristics of WIKS sediments is the fact that the muddy portions of the basin are chock full o' carbon. These black shales can have some shockingly high TOC (Total Organic Carbon), and must have formed under conditions of VERY restricted oxygen availability, anything from complete anoxia all the way up to more gentle dysoxic conditions. In fact, some of these black shale intervals are so striking, and so laterally persistent, that they've been given names: Ocean Anoxic Events, or OAEs in the acronym-rich jargon of geology.
So, in general, we know that a lot of the Western Interior seaway was kind of a rough place to make a living. The impact of this salty, oxygen-starved seaway on biological communities within the seaway is seen in the sort of things that lived and died there. For one thing, ammonoids, those opalescent shelled cephalopods that everyone loves, where all over the western interior seaway. Interestingly, modern examples of shelled cephalopods in the form of Nautilus have exceptionally high tolerance for low-oxygen conditions. Apparently, modern nautiloids will flee predators by going into low-oxygen waters, closing up there shell to conserve oxygen. Predatory fish, who burn oxygen like crazy as part of their active hunting life-style, can't follow a potential Nautilus into those zones. It's a pretty neat trick, and maybe part of the reason why ammonoids were so successful in the WIKS.
But nifty cephalopods weren't the only things living in the seaway. A completely rad group of bivalves, called inoceramids, are present in a lot of the fine-grained, distal portions of the western seaway, where they serve as important biostratigraphic markers. These guys are clams, and the group is characterized by having valves that are both big and flat, with some examples getting to be well over a meter. The picture below is from the Cenomanian of Kansas; the hashy, craggly, flaggy parts (one is juts under the hammer) are inoceramids.
If you look a little closer, you can see the characteristic prismatic texture of the shell; this isn't a diagenetic effect, but rather how the inoceramus precipitated it's carbonate shell. The picture below shows this texture, particularly on the right hand side. Any sed geochem types out there know of any isotopic work done on these shells? Seems like it might be an interesting recorder of bottom water conditions.
Another characteristic thing about these clams is that, in plan view, the shells exhibit marked growth rings. This kind of crappy picture is from the type section of the Cenomanian-Turonian boundary in Colorado; you can see a bit of the ringed morphology of an inoceramid; this poor little guy got himself chewed up as part of a sandy turbidity current. Would that we all could go so nobly into that good night!
The large size of these clams has been explained as being a response to the low oxygen conditions of the WIKS; big clams have bigger gill surface area, and could maybe pull in more oxygen in those rough conditions. They're also some suggestions that these guys might have had chemosynthetic bacteria living symbiotically with them. What is kind of weird is that, for how big the shells are, there wasn't a lot of room for the squishy bits in the middle...in other words, these clams, though impressively massive, would have been disappointing eating, I suspect.
I also wonder how the chemistry of the WIKS water might influenced these guys. We know that there are phases of the Western Interior that are dominated by thick successions of chalk. Maybe these clams dealt with all these ions in solution by precipitating a bunch of CaCO3?
Anyway, they're pretty nifty fellows, and very characteristic of the fine-grained portions of the WIKS. Just goes to show you that, even in the most monotonous-seeming mudrocks, you can still find cool stories about the evolving earth!