The Western Interior Seaway : An introduction to the general history, Paleogeography, and Ammonite record
Abstract: Throughout most of the Cretaceous, rapid sea floor spreading rates contributed to a rise in global sea levels culminating in a Phanerozoic all time high. Regionally this allowed flooding to occur in the lower lying regions of central North America in a predominately north/south axis forming the Western Interior Seaway.
Minor sea level fluctuations, tectonic events, and variations between deposition and subsidence rates formed complex cyclic deposits primarily composed of clastics (chiefly sandstone and shale) or carbonates (limestones). These fluctuations also acted as “gates” controlling entry into, or exit from the seaway of marine organisms by physical barriers or changes in ocean temperature and/or chemistry.
Ammonites left an impressive 50 million year record in the seaway and are the principal tools used in the regions biostratagraphic correlation. Additionally Ammonites are educational in evolutionary studies and are highly regarded by amateur & commercial fossil collectors alike.
Studies in progress are now unraveling their complex evolutionary history. Related websites are suggested as well as text. Selected specimens from the N.S. Brown collection are figured.
General History & Paleogeography
During the Cretaceous Period 145 to 65 million years ago, rising sea levels caused the flooding of central North America forming the Western Interior Seaway. This generally shallow waterway ebbed and flowed throughout the Cretaceous and at times reached from the Gulf of Mexico to the Arctic Sea, thus isolating the eastern & western landmasses of the continent (fig.1).
The ecosystems in around the seaway supported an enormous abundance of life. Dinosaurs and small mammals lived on the adjacent land areas while Angiosperms (the flowering plants) were displacing Gymnosperms and establishing themselves as the predominant land plants. Birds, Pterosaurs and Insects dominated the skies.
This warm seaway supported many forms of marine vertebrates such as fish, mosasaurs, pleisosaurs and aquatic birds. Additionally, the seaway teemed with invertebrates; crinoids, echinoids, crabs, lobsters, snails and clams thrived in great numbers. Most notably ammonites (extinct Cephalopod Mollusks related to squid, octopi & nautilus) abounded evolving into a great variety of forms.
Fluctuating sea levels formed cyclic layers of deposits composed primarily of Sandstone (nearshore), Shale and Limestone (farshore) thousands of feet in thick in some areas. Due to the chemical differences in these strata the preservation of the fossils differ. Invertebrate fossils found in the Carbonates (Limestone) occur as internal casts as the original shell has been dissolved (for example seeOxytropidoceras).
Those in shale and sandstone are often preserved in concretions in which the shell (aragonite or calcite) is preserved. Ammonites are used extensively to correlate these sediments and their zonation can be as fine as 1 to 2 million years. Vertebrate fossils are only relatively common in the Kansas chalk beds (Carbonate) and in the poorly oxygenated Sharon Springs Mbr. of the Pierre shale and are not generally associated with invertebrates due to their different preservation requirements.
To the west of the seaway the cordilleran region was mountainous with numerous active volcanoes (fig.2). Sporadic eruptions of volcanic ash rained on the seaway forming bentonite layers of commercial importance in some areas. Minor sea level fluctuations combined with high sediment loads from erosion of the steep western margins periodically restricted ocean circulation at the northern and/or southern regions of the seaway.
These restrictions had a major impact on the biota within the seaway due to the resultant changes in ocean chemistry (salinity, etc.), temperature and oxygination gradients as well as the blockage of migration into and from the seaway by these means or by actual physical barriers which periodically closed the northern region.
The result was periodic isolationism which led to the evolution of endemic (geographically restricted) forms which is well demonstrated in the ammonite fauna. To reiterate the seaway periodically allowed passage of biota into and out of the seaway (mixing) while at other times allowed little no mixing which led to the development of life forms unique to the area. For more on this the reader is referred to the detailed study of Kauffman (1984) for a stage by stage account.
Before turning our full attention to the ammonites some should be said concerning the latest Cretaceous when two events occurred which profoundly affected the Western Interior Seaway. First was the infamous latest Cretaceous mass extinction event (K/T) which was responsible for the eradication of as 50% of existing genera as well as the demise of the Dinosaurs, Mosasaurs, Pleisosaurs,Pterosaurs and Ammonites to name a popular few. Whole volumes have been devoted to this subject, and none is the final word !
Yes, there is ample evidence for a bolide impact at Chicxulub (Yucatan area), Mexico 65 million years ago and geologic documentation for this impact is found in many areas of the Western Interior basin as well (see link below). No doubt this impact had extremely detrimental effects, however, many geologists and paleontologists feel most ocean and terrestrial ecosystems were already stressed by other factors (there are many “smoking guns” although none in themselves are conclusive, for example excessive levels of volcanism or sea level fluctuations. For more on extinction events, evidence in the geologic and paleontological records and their possible causes the reader is referred to the exceptional works of Stanley (1989), and Hallam, A. & Wignall, P.B. (1997).
The second issue which affected the Western Interior Seaway during the latest Cretaceous and indeed caused it’s demise was a lowering of global sea levels. This was most likely was the result of a slowing in the ocean sea-floor spreading rate which is a controlling factor responsible for ocean basin depths. Thus as the spreading rates slowed the ocean basins themselves sank (in relation to the lighter rocks which form the continents), global sea levels fell, the seas withdrew from the western interior and the halves of North America were reunited into the familiar form we (as Homo Sapiens) now colonize !
The Ammonite record
Ammonites (fig.3) left a rich record in the Cretaceous rocks of the Western Interior Basin ranging from the Albian (some 120 million years ago) to nearly to the end of the Late Cretaceous. Sandy facies of the Fox Hills formation laid down just prior to the seaways complete retreat record the last ammonites dated as Mid-Maastrichtian, about 70 million years ago.
The area’s ammonite record occurs in a variety of lithofacies (rock types), representing many environmental conditions and spans 50 million years. Obviously ammonites had evolved, diversified and adapted to many an ecological niche to have enjoyed such successes.
The presumed diversity of ammonite habitats is a reflection of the phenomenal variety of shell forms utilized. While most forms coiled planespherically (in one plane), some are “thin” and sharp keeled while others may be “fat” and rounded. Further adaptations complicate shells more; ornament such as nodes, ribs, tubercles, spines, and other modifications add greatly to the number of possible shell forms.
Yet more bizarre are the Heteromorphs, which coil “loosely”, that is without the younger whorls in contact with the older ones, and with coiling in more then one plane. Juvenile ammonites were probably planktonic in nature, floating within the photic (light penetrating) levels of the ocean and dispersed by the warm ocean currents.
Presumably most adult forms were planktonic feeders (floating in the water column feeding on single celled organisms) while others such as the heteromorphs (for example Didymoceras) are best characterized as bottom dwellers. Other ammonites may have been carnivorous as are recent cephalopods. For a more in depth look at Ammonite biology the work of and Lehmann (1981) is highly recommended. Kennedy and Cobban (1976)also offer some very interesting insights on selected topics.
The study of Ammonites is important in several regards. As a highly successful group of biological entities in terms of abundance, longevity, and variety ammonites offer a wealth of information on evolutionary trends and extinction patterns. Additionally, ammonids (ammonites and their earlier kin) are the chief macrofossils used in correlating rock units of mesozoic age.
Ammonoid biostratagraphic zones allow stratigraphers a precise tool for startagraphic comparison regionally and in some cases throughout much of the world. Without such precise correlation historical geologists and paleogeographers would not be able to offer accurate accounts of world events or create accurate paleogeographic maps.
Phylogeny of the ammonites is itself an evolving entity, and great advances have been made over the years. Much of the framework (and field work) for these advances were made by Dr. William Cobban of the USGS (now retired, officially at least) over a career which spanned 40 years or more. Bill was the key player in developing the regions biostratagraphic zonation based in part on species of Baculites, a rapidly evolving, abundant, wide ranging and easily identified heteromorph ammonite.
With the biostratagraphic framework worked out by Bill, ammonite evolution, and phylogenic history can now be and is being clarified. Dr. Karl Waage (Yale University, retired) did much work on the stratigraphy and ammonites from the Maastrichtian Fox Hills formation of the Dakota’s and eastern Wyoming and his former student Dr. Neil Landman (American Museum of Natural History, N.Y.) is carrying on that work as well as on some Campanian forms.
Another key player is Dr. W. James Kennedy of Oxford University. Dr. Kennedy is the perhaps world’s leading Cretaceous Ammonite worker with worldwide experience. The list of papers co-authored by Cobban & Kennedy is enormous. In a model of cooperation between professionals and a commercial paleontological supplier (the Black Hills Institute of Geological Research, Inc. of Hill City, S.D.) ammonite research is advancing further. Those at the BHI (especially Neal Larson) have the ability to review massive amounts of ammonite material, note subtle variability’s and catalog unique specimens for their private collection or for the Museum they set up in Hill City.
These specimens then augment the large collections at the USGS and YPM (Yale) and are periodically reviewed by Kennedy, Cobban and Landman along with N. Larson of the BHI. Currently this group is busy revising the Later Cretaceous Scaphitoid Ammonites. Recently Larson et al. (1997) published a excellent book detailing currant knowledge and compiling information on Cephalopods from most of the Campanian and Maastrichtian stages. As most of the literature on these Ammonites is scattered throughout dozens of scientific journals, this is a book of choice for non-professionals.