GSA Bulletin: Nonmarine Extinction Across the Cenomanian-Turonian

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Nonmarine extinction across the Cenomanian-Turonian boundary, southwestern Utah, with a comparison to the Cretaceous-Tertiary extinction event

Jeffrey G. Eaton* Department of Geosciences, Weber State University, Ogden, Utah 84408-2507 James I. Kirkland Dinamation International Society, 550 Crossroads Court, Fruita, Colorado 81521 J. Howard Hutchison Museum of Paleontology, University of California, Berkeley, California 94720 Robert Denton New Jersey State Museum, Trenton, New Jersey 08625-0530 Robert C. O’Neill } J. Michael Parrish Department of Biological Sciences, Northern Illinois University, Dekalb, Illinois 60115

ABSTRACT Cenomanian-Turonian boundary and suggests that some mechanism other than eustatic change played a significant role in the extinction. There is a marked, possibly stepwise, extinction of marine taxa across the Cenomanian-Turonian boundary. Across the boundary in south- INTRODUCTION western Utah, there is only minor species-level extinction of brackish- water taxa, and an actual increase in diversity of fully terrestrial organ- Extinction of marine taxa across the Cenomanian-Turonian boundary is isms; significant family-level extinctions are restricted to aquatic taxa well documented (e.g., Kauffman, 1984). Extinction of marine mollusks in such as fishes and turtles. the Western Interior of North America included ≈13% of genera and 51% It is not possible in the nonmarine setting to determine if this is a of species (Elder, 1987) and a global extinction of ≈30% of genera and 70% gradual, stepwise, or instantaneous extinction, or to what degree it cor- of species (Hut et al., 1987). Detailed study across this boundary reveals a relates to marine extinction events. Nonmarine faunas underwent no stepwise nature to the extinction event for marine mollusks in the Western major change during the transgressive phase of the Greenhorn cycle, Interior (Elder, 1985, 1987, 1991) and the nannoflora of Europe (Paul et al., and the loss of aquatic taxa along with displacement (but not extinc- 1994), suggesting widespread, possibly global controls resulting in the step- tion) of brackish-water vertebrates and some marsupial mammals is wise extinction pattern. first apparent in rocks deposited during regression in the Turonian. Data for nonmarine taxa (including terrestrial as well as brackish-water The loss of flood-plain habitat at maximum transgression may have taxa) were not included previously in discussions of extinction across the caused the extinction of some of the aquatic taxa. The absence but not Cenomanian-Turonian boundary. This paper provides data on the history of extinction of certain taxa on flood plains during the Greenhorn regres- nonmarine taxa across the Cenomanian-Turonian boundary and compares sion suggests that there may be some significant difference in trans- patterns to those of the better-known Cretaceous-Tertiary (K-T) extinction. gressive and regressive flood plains. Drawdown increases the gradients of rivers and results in incision along coastal margins. This restricts the Geologic Setting extent of brackish-water environments and may have had an impact on faunal compositions of riverine systems and contributed to extinction Southwestern Utah (Fig. 1) contains a relatively continuous record of within aquatic communities. deposition across the Cenomanian-Turonian boundary (Fig. 2). This bound- This pattern is quite different from that at the Cretaceous-Tertiary ary is marked biostratigraphically in marine rocks at the boundary of the (K-T) boundary. Aquatic taxa underwent relatively minor losses at that Neocardioceras juddii and the overlying Watinoceras coloradoense am- boundary, whereas terrestrial organisms underwent major extinction. monite zones (e.g., Elder, 1991). The stratigraphic sequence at the eastern It appears that much of the Late Cretaceous aquatic community was part of the study area (Kaiparowits Plateau region) contains nonmarine and restructured (mostly by exclusion of many taxa rather than extinction) brackish-water deposits of late Cenomanian age (Dakota Formation) over- and reduced in diversity during large-scale regression in the middle of lain by marine rocks of late Cenomanian through middle Turonian age the Maastrichtian before the end of the Cretaceous. This aquatic com- (Tropic Shale) (Eaton, 1991). The marine rocks represent the transgression munity was living in a rapidly expanding environment (overall regres- of the Greenhorn Sea (Kauffman, 1977a). The regression of the seaway is sion of marine waters) at the K-T boundary. The extinction of terres- recorded in the nearshore deposits of the Tibbet Canyon Member of the trial taxa at the boundary is unlike the pattern observed at the Straight Cliffs Formation of middle Turonian age and in the brackish-water and terrestrial deposits of the overlying Smoky Hollow Member of the Straight Cliffs Formation of middle or late Turonian age (Eaton, 1991). *E-mail: [email protected] The marine and marginal marine deposits thicken westward into a deep

GSA Bulletin; May 1997; v. 109; no. 5; p. 560Ð567; 5 figures; 2 tables.

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trough in the foreland of the Sevier orogenic belt (Eaton et al., 1987; Eaton and Nations, 1991) and these deposits grade westward into the predomi- Great nantly nonmarine Iron Springs Formation (Fig. 2), which represents proxi- Salt Lake UTAH mal debris shed from the Sevier orogenic belt. Brackish-water mollusks were recovered throughout the region (Fig. 2). Terrestrial and fresh-water faunas were recovered primarily from the Da- kota Formation and the Smoky Hollow Member of the Straight Cliffs For- mation in the Kaiparowits Plateau area. Terrestrial faunas of unknown age Salt Lake City (because of the lack of associated marine strata) were recovered from the

Uinta Straight Cliffs Formation on the Markagunt Plateau and from the Iron Basin Springs Formation in the Pine Valleys Mountains (Fig. 2).

BRACKISH WATER FAUNAS

Price Book Brackish-water taxa were recovered in great abundance, including brack- Cliffs ish environments along the western margin of the study area associated with

Plateau Wasatch maximum transgression (equivalent to the Tropic Shale to the east) and re- Green River gressive facies (lower part of the Straight Cliffs Formation, see Fig. 2). Brack-

Swell San Rafael ish-water localities were also abundant within the transgressive facies of the STUDY AREA Dakota Formation; they are not shown in Figure 2 because they are present in

➤ an almost continuous belt across the study area, and the recovered fauna is al- Hanksville Henry most identical to that already described by Kirkland (1983, 1990, 1996) and Basin 38¼ Cedar Fursich and Kirkland (1986) from the upper part of the Dakota Formation at Canyon Black Mesa, Arizona. Cenomanian occurrences of brackish-water taxa are Pine Cedar Valley City Kaiparowits based on the Black Mesa fauna, our collections in the study area, and exami- Mts. Markagunt Plateau Plateau nation of collections made in the study area by Gustason (1989). Plateau Paunsaugunt The Cenomanian-Turonian boundary is well established within the ma- Kolob Gunlock Terrace rine Tropic Shale in the Kaiparowits basin (Eaton et al., 1987), and brack- Kanab ish-water faunas in this area show little change across the boundary. Of more than 33 taxa, there are only a few (possibly only 2 or 3) extinctions at 111¼ the species level and virtually no loss at the genus or family level. A specific Figure 1. Cretaceous outcrop map of Utah showing study area. table of taxa is not provided at this time because virtually all of the groups

02010 km

50 Markagunt Plateau Cedar Canyon Pine Valley 100 Kaiparowits m Mountains Plateau

1230 Figure 2. Schematic east-west cross section showing formations, 1222,1223 Straight Cliffs Fm. (lower part) approximate stratigraphic thick- Iron Springs Fm. 1258 nesses, and fossil localities. Fossil 1231,1232 995 996 locality numbers are those of the Museum of Northern Arizona, Flagstaff; x indicates brackish-water invertebrate localities; triangles Tropic Shale indicate nonmarine vertebrate localities. C-T is Cenomanian-Turo- 1226,1225 C-T Boundary nian boundary.

1067,1064

Dakota Formation

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involved are in critical need of revision, and we would like to avoid con- Even with these taxonomic restrictions, certain patterns are clear (Fig. 3). tributing further to nomenclatural problems. We can recognize whether a There is no loss at the family level among either mammals or dinosaurs, and taxon crosses the boundary; however, it will take years of systematic revi- although there is considerable turnover of mammalian species during this sion to produce meaningful taxonomic lists. 2Ð3 m.y. interval, there is no indication that the turnover of species would It appears that the changes in the fauna (both extinctions and originations) be above the normal background rate for mammals over this length of time. occur in either a stepwise or gradual fashion, because it was not possible to Most of the families of marsupial mammals that are common throughout identify a distinct change in the brackish-water fauna at any specific strati- the Late Cretaceous make a first appearance in the Cenomanian, as do many graphic horizon. There are localities with distinctly Cenomanian faunas and dinosaur taxa. The Cenomanian appears to mark a period of diversification some with distinctly Turonian faunas, but localities near the boundary in the for terrestrial organisms that continues through the Turonian and into the western part of the study area that lack associated marine environments can- later Cretaceous of North America. not be confidently assigned to stage. The most biostratigraphically useful This pattern is similar to that seen in the pollen and spore record. There is taxon may be the common mollusk Craginia. In the Cenomanian, a species no extinction in either group across the Cenomanian-Turonian boundary, similar to Craginia turriformis Stephenson (1952) is commonly encoun- but rather a gradual pattern of extinction and origination of palynomorphs tered, but all the localities of certain Turonian age contain a carinate species (Nichols, 1994). Of 16 taxa found near the boundary, 14 successfully cross Craginia coalvillensis Meek (1873) (see discussion in Kirkland, 1996). The the boundary. This suggests that there was no major disruption within the placement in Figure 2 of brackish-water localities near the Cenomanian-Tur- plant community; however, Nichols’ study did not include a detailed exam- onian boundary is based on which form of Craginia is present; however, the ination of the boundary in order to detect temporary disruptions in plant transition to the carinate form may not correspond exactly to the productivity marked by a sudden increase in spores relative to angiosperm Cenomanian-Turonian boundary. pollens, as noted at the K-T boundary (Nichols and Fleming, 1990). The lack of a marked extinction among brackish-water taxa is not sur- The aquatic taxa are markedly different. Many turtles and fish common prising as long as there is not a biotic catastrophe. Kauffman (1977b, 1978) since the Jurassic vanish prior to the Turonian (38% of families, 53% of gen- predicted the high survival potential of taxa adapted to stressful and unsta- era). This suggests that although terrestrial vertebrate taxa were little affected ble brackish-water environments. by extinction across the Cenomanian-Turonian boundary, significant extinctions occurred among vertebrates in fresh-water and possibly riparian habi- Terrestrial Faunas tats. This represents a major reorganization of the fresh-water ecosystem. Another important pattern is apparent in the nonmarine faunal record Most of the terrestrial and fresh-water faunas were recovered from rock across the Cenomanian-Turonian boundary in the Kaiparowits region. Sev- units separated by 200 m of marine Tropic Shale, representing a gap in the eral taxa of marsupial mammals (e.g., Alphadon, [?]stagodontids) and terrestrial record of ≈2Ð3 m.y. (Obradovich, 1993). No attempt was made to sharks, which have post-Turonian records, are very rare or absent in the determine where within a stage a taxon either appears or disappears; only Smoky Hollow Member of the Straight Cliffs Formation (the coastal flood- its presence or absence is noted in Table 1. It would not be meaningful to plain deposits following the Turonian regression). This suggests ecological discuss whether the extinction in the nonmarine was stepwise, gradual, or displacement of many taxa rather than extinction. This may reflect differ- instantaneous. The taxa listed in Table 1 are based on Kirkland (1987), ences in the ecology of the coastal flood plain during transgressive and Cifelli (1990), Eaton (1993, 1995), and our unpublished data. regressive phases. The most practical basis for examining nonmarine extinction across the Cenomanian-Turonian boundary is at the family level regardless of the POSSIBLE CAUSES OF THE CENOMANIAN-TURONIAN inherent weaknesses of such an approach, and this is the same approach EXTINCTION used by Sheehan and Fastovsky (1992) in their study of the K-T boundary in Montana. In most cases, with the exception of mammals, the systematic The Cenomanian-Turonian extinction has been included in the 26 m.y. list presented in Table 1 is based on material inadequate for more refined cyclic extinction pattern, implicating the impact of an extraterrestrial object taxonomy. This is in part a result of the recovery of vertebrate material from (or objects; see Hut et al., 1987) as a cause of the extinction (Sepkoski, screen washing that produced extremely fragmentary fossil remains. Iso- 1982; Raup and Sepkoski, 1984, 1986). Although Orth et al. (1990) noted lated mammalian dental elements can often be identified to species, but for weak iridium anomalies in various CenomanianÐTuronian sections, there is most other vertebrates far more complete material is necessary for generic- little published support other than Hut et al. (1987) for a catastrophic bolide- or species-level identification. Another limitation on lower level systematics induced extinction in the marine realm at this boundary, and that pattern ob- for much of the material is the lack of well-studied middle Cretaceous ver- served here does not support such a scenario. tebrate faunas. There is no available equivalent to the abundant and well- Elder (1987) suggested that stepwise marine extinctions in the Western studied vertebrate material recovered from the latest Cretaceous of Wyo- Interior Seaway were adequately explained by oceanographic and climatic ming and Montana. changes associated with sea-level fluctuations. Arthur et al. (1987) sug- Extinction-survivorship percentages are shown in Table 2. Abundant ma- gested that an anoxic event was the primary cause of the Cenomanian- terial representing mammals, dinosaurs (identified on the basis of teeth), Turonian extinction; this view was challenged by Paul and Mitchell (1994), crocodiles, fish, and turtles was recovered as the result of washing several who suggested that a decline in coccolith productivity occurred as a result of metric tons of matrix. Virtually all taxa of the groups listed above could be the very sudden drop in sea level near the Cenomanian-Turonian boundary, listed as common occurrences (e.g., see hypodigms in Eaton, 1995). The and that zooplankton in turn were stressed by the decline in coccolith pro- weakest part of the database is among the amphibians and lizards (Squa- ductivity. Paul et al. (1994) correlated stepwise extinction patterns and de- mata). Although remains of these taxa are abundant, identifiable remains are tailed sedimentary records across the Cenomanian-Turonian boundary from relatively rare. Their records are included in the calculations because there England to Spain, and suggested that this correlation may reflect effects of is no discontinuity at the family level in these groups across the boundary. If global marine regression. family-level extinctions were claimed on the basis of these data, it would be The record of brackish-water taxa across the Cenomanian-Turonian difficult to justify. boundary provides little information about the cause of the extinction, but

562 Geological Society of America Bulletin, May 1997

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does not provide support for a catastrophic event. There is no detectable dis- boundary, it was highly selective and had little or no effect on either terrestrial ruption of the brackish-water environment or community. The only change vertebrates or brackish-water taxa. It appears more likely that the rapid ex- we noted is the lateral migration of the brackish-water environment and its pansion of the epicontinental seaway markedly reduced the available surface community (this can be seen locally along the outcrop). The record of ter- area for terrestrial habitation and the extent of riverine systems, resulting in restrial vertebrates is similar to that of the brackish-water faunas in that there increased stress on terrestrial faunas. Apparently, land-dwelling vertebrates were no family-level extinctions. If anything, the period around the responded to this stress by an accelerated rate of diversification rather than Cenomanian-Turonian boundary marks an acceleration of the appearance of by extinction accompanied by a decline in diversity. terrestrial taxa that came to typify the Late Cretaceous of North America. If The story is different for aquatic taxa. Turtles and fish common since the there was some kind of catastrophic event at the Cenomanian-Turonian Jurassic disappeared. Coastal streams and rivers were directly connected to

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MAMMALS

DINOSAURS

REPTILES

AMPHIBIANS

FISH

0 5 10 15 20 25 30 35 40 Figure 3. Percentage of nonmarine vertebrate families that became extinct across the Cenomanian-Turonian boundary.

oceans and may have markedly reduced salinity of their surface waters. Jewell (1993) suggested that the CenomanianÐTuronian seaway surface wa- the epicontinental seaway and responded both to eustatic and biological ters may have been diluted by fresh water by as much as 20% during peri- changes in the seaway. It is possible that the cause of the extinction in the ods of high precipitation. This may have resulted in unusually extensive off- marine realm also had an impact on riverine systems. There is no question shore areas of productivity for brackish-water organisms (salinity of the that the causes for any extinction are likely to be complex and related to the waters directly adjacent to the coasts probably remained normal; see Kirk- almost infinite factors that control food abundance, temperature, salinity land, 1990). During transgression, the seaway advanced by drowning adja- gradients, biologic productivity, and physical environment; however, it is cent coastal flood plains. The rivers draining into the epicontinental sea had possible to suggest certain impacts to riverine systems associated with eu- very low gradients (Fig. 4A) with the potential for extensive upstream mix- static fluctuations. ing of fresh and saline waters, particularly during episodes of low rainfall, Epicontinental seaways have more restricted circulation than open forming large estuarine areas. At maximum transgression (Fig. 4B) the

Sevier Brackish Influence Orogenic Belt Seaway Figure 4. Effects of eustatic rise and fall on the coastal flood plain of southwestern Utah and relative TRANSGRESSIONTRANSGRESSION extent of brackish-water influ- A ence. (A) Drowning of the coastal flood plain and low gradient streams by the brackish epiconti- Sevier Brackish Influence nental seaway. (B) Elimination of Orogenic Belt coastal flood-plain environment during maximum transgression. Seaway (C) Increased gradient and extent of rivers near coastal margins during regression accompanied HIGHSTANDHIGHSTAND by reduce influence of brackish B waters on the coastal flood plain. Coastal margins probably re- Brackish mained at normal salinity with Sevier Orogenic the brackish cap developed far- Belt ther offshore (Kirkland, 1990). Seaway gradient of incised rivers

C REGRESSIONREGRESSION

564 Geological Society of America Bulletin, May 1997

Figure 5. Comparison of the percentage of family-level extinctions of aquatic and terrestrial taxa across the Cenoman- ian-Turonian (C-T) and Cretaceous-Ter- tiary (K-T) boundaries. C-T data from this paper; K-T data from Archibald and Bryant (1990, Table 1).

coastal flood plain virtually vanished as brackish-water deposits extended Sea. It is unknown what other brackish-water taxa, such as crustaceans and into the Pine Valley Mountains (see Fig. 1), less than 50 km from the Sevier microorganisms, might also be excluded from the Turonian riverine sys- thrust belt that was forming significant mountains at that time (see Fig. 7C tems. It is possible that a shift in food availability caused by a sudden within Elder and Kirkland, 1994). This essentially eliminated the coastal flood- drawal of brackish elements from the riverine system may also contribute to plain environment while bringing brackish water and its associated faunas extinction, reduced diversity, or ecological exclusion of some elements of to the foothills of the Sevier orogenic belt. Lungfishes, semionotid fish, and the riverine fauna. certain turtles may have vanished along with their habitat at maximum transgression. During the rapid regression that followed (Fig. 4C), sea level COMPARISON WITH THE K-T EXTINCTION EVENT dropped markedly and streams became much more extensive and incised adjacent to the seaway (see Weimer, 1983). The incision of streams in- The pattern of nonmarine extinction at the K-T event is quite different creased stream gradients along the coast, thus limiting the development of from that of the Cenomanian-Turonian. Sheehan and Fastovsky (1992) estuarine systems and restricting upstream mixing of fresh and saline water noted this dramatic difference between the extinction of land-dwelling and dispersal of brackish taxa. species (88%) relative to the minor extinction of fresh-water taxa (10%) at The record of the sedimentary rocks marking the transition from coastal the K-T boundary. More recent evaluation of the data set in Archibald and flood plains (Smoky Hollow Member of the Straight Cliffs Formation) to Bryant (1990, Table 1) by Archibald (1996) indicates that 22% of aquatic the time-equivalent (Peterson and Kirk, 1977) shallow marine sequence species became extinct versus 72% for terrestrial species. Although the ex- (Ferron Member of the Mancos Shale) has been removed by late Cenozoic tinction at the Cenomanian-Turonian boundary is far less dramatic, it is erosion between the eastern margin of the Kaiparowits Plateau and the clear that the pattern is reversed relative to the K-T boundary in that fresh- Henry basin 70 km to the east (Fig. 1), leaving open the question of whether water taxa underwent greater losses than did terrestrial taxa (Fig. 5). the rivers were actually incised along the coastal margin. The lower part of Examination of fish extinction in streams from the late Campanian into the Ferron Sandstone in the Henry basin represents a regressive barrier the Maastrichtian (based on Archibald and Bryant, 1990) prior to the K-T sandstone, and the upper part represents mostly continental rocks (Peterson boundary event indicates that more than 55% of all fish genera and almost et al., 1980). About 100 km to the north, in the San Rafael Swell (Fig. 1), the all of the elasmobranch (shark and ray) genera were excluded (though most transition from swampy coastal flood plain to shallow marine sequence is did not become extinct) from the riverine system. This suggests that a sig- well preserved in the Ferron Sandstone. In this area, distributary streams nificant event restructured fresh-water communities prior to the K-T bound- that fed coastal deltas are deeply incised into backswamp deposits (e.g., ary. This event may be related to the late Campanian regression (R9 of Ryer, 1981) similar to those present in the lower part of the Smoky Hollow Kauffman, 1977a) or middle Maastrichtian regression (R10 of Kauffman Member. This incision of streams along the coastal margin may be compar- and Caldwell, 1993), both of which were more extensive than the relatively able to the equivalent age sequences in the study area; however, direct evi- smaller regressive events that occurred in the latest Maastrichtian. dence is lacking. The palynomorph record also suggests that there was a significant change Changes in sea level may be reflected by distinct changes in flood-plain in the flora in the middle of the Maastrichtian. Nichols (1994) divided the communities during transgression and regressive phases. The late Ceno- Maastrichtian of the Western Interior into two distinct palynostratigraphic manian riverine system (Dakota Formation) contains abundant rays and zones. The zones are divided by the extinction of six taxa and the appear- sharks. These taxa are often considered to be fresh-water rays and sharks, ance of eight new taxa, suggesting significant restructuring of the plant but they are absent in fossiliferous lake beds in both Cenomanian and Turo- community during the mid-Maastrichtian. nian rocks and probably required some contact with marine or brackish wa- By the time the K-T extinction event occurred, fresh-water systems had al- ters during their life cycles. These “fresh-water” taxa are almost completely ready been severely reduced in taxonomic diversity and, coupled with the vast absent (although they did not become extinct) in Turonian localities on the expansion of the riverine systems during the latest Maastrichtian, the impact flood plains (Smoky Hollow Member) during regression of the Greenhorn of a bolide at the end of the Cretaceous had little effect on riverine faunas.

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There are many factors that are too poorly understood to properly evalu- REFERENCES CITED ate nonmarine faunal turnovers near the K-T boundary. Included among these is the problem of the Western Interior Seaway isolating western and Archibald, J. D., 1996, Dinosaur extinction and the end of an era: What the fossils say: New eastern North American faunas on two distinct subcontinents for ≈30 m.y. It York, Columbia University Press, 237 p. is still unknown exactly when these long-separated faunas were reunited, Archibald, J. D., and Bryant, L. J., 1990, Differential Cretaceous/Tertiary extinctions of nonmarine vertebrates: Evidence from northeastern Montana, in Sharpton, V. L., and Ward, P. D., but it is possible that this could have occurred as early as the middle Maas- eds., Global catastrophes in Earth history: An interdisciplinary conference on impacts, vol- trichtian (the last ammonites occurred well before the close of the Creta- canism, and mass mortality: Geological Society of America Special Paper 247, p. 549Ð562. ceous; see Cobban et al., 1994) and certainly before the close of the Maas- Arthur, M. A., Schanger, S. O., and Jenkyns, H. C., 1987, The Cenomanian-Turonian oceanic anoxia event, II. Palaeoceanographic controls on organic matter production and preserva- trichtian (see Roberts and Kirschbaum, 1995). Faunal exchange of this tion, in Brooks, J., and Fleet, A. J., eds., Marine petroleum source rocks: Geological Soci- magnitude is associated with dramatic restructuring of communities (e.g., ety of London Special Publication 26, p. 401Ð420. Cifelli, R. L., 1990, Cretaceous mammals of southern Utah. III. Therian mammals from the Tur- North and South American late Cenozoic faunal mixing). To date, there has onian (early Late Cretaceous): Journal of Vertebrate Paleontology, v. 10, p. 332Ð345. been no recognition of this kind of event having an impact on the terrestrial Cobban, W. A., Merewether, E. A., Fouch, T. D., and Obradovich, J. D., 1994, Some Cretaceous vertebrate community during the Maastrichtian, although changes in fish shorelines in the western Interior of the United States, in Caputo, M. V., Peterson, J. A., and Franczyk, K. J., eds., Mesozoic systems of the Rocky Mountain region, USA: Denver, and plant communities noted above may in part reflect this event. This sug- SEPM (Society for Sedimentary Geology), Rocky Mountain Section, p. 393Ð413. gests that we are far from having a complete understanding of terrestrial Coccioni, R., and Galeotti, S., 1994, K-T boundary extinction: Geologically instantaneous or communities prior to the well-studied K-T boundary, and by implication gradual event? Evidence from deep-sea benthic foraminifera: Geology, v. 22, p. 779Ð782. Eaton, J. G., 1991, Biostratigraphic framework for the Upper Cretaceous rocks of the Kaiparo- conclusions based on the more poorly known CenomanianÐTuronian fau- wits Plateau, southern Utah, in Nations, J. D., and Eaton, J. G., eds., Stratigraphy, deposi- nas must be considered preliminary. tional environments, and sedimentary tectonics of the western margin, Cretaceous West- ern Interior Seaway: Geological Society of America Special Paper 260, p. 47Ð63. Eaton, J. G., 1993, Therian mammals from the Cenomanian (Upper Cretaceous) Dakota For- CONCLUSIONS mation, southwestern Utah: Journal of Vertebrate Paleontology, v. 13, p. 105Ð124. Eaton, J. G., 1995, Cenomanian and Turonian (early Late Cretaceous) multituberculate mammals from southwestern Utah: Journal of Vertebrate Paleontology, v. 15, p. 761Ð784. Adequate evidence exists to suggest a dramatic physical event at the K-T Eaton, J. G., and Nations, J. D., 1991, Introduction: Tectonic setting along the margin of the Cre- boundary that was coupled with widespread, relatively instantaneous ex- taceous Western Interior Seaway, southwestern Utah and northern Arizona, in Nations, tinction (e.g., Olsson and Chengjie, 1993; Coccioni and Galeotti, 1994). J. D., and Eaton, J. G., eds., Stratigraphy, depositional environments, and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway: Geological Society of This event apparently was particularly dramatic for marine microorganisms America Special Paper 260, p. 1Ð8. and angiosperm plants (e.g., Johnson, 1995), particularly at low latitudes, Eaton, J. G., Kirkland, J. I., Gustason, E. R., Nations, J. D., Franczyk, K. J., Ryer, T. A., and Carr, D. A., 1987, Stratigraphy, correlation, and tectonic setting of Late Cretaceous rocks in the but also had severe impacts on terrestrial vertebrates such as dinosaurs and Kaiparowits and Black Mesa basins, in Davis, G. H., and VandenDolder, E. M., eds., Ge- some mammals (particularly marsupials in North America). The event had ologic diversity of Arizona and its margins, Geologic Society of America 100th annual minor impacts on riverine communities. This is in direct contrast to the step- meeting field-trip guidebook: Arizona Bureau of Geology and Mineral Technology Spe- cial Paper 5, p. 113Ð125. wise nature of the Cenomanian-Turonian marine extinction, the lack of im- Elder, W. P., 1985, Biotic patterns across the Cenomanian-Turonian extinction boundary near pact on terrestrial animals, and the correspondingly higher extinction rate Pueblo, Colorado, in Pratt, L. M., Kauffman, E. G., and Zelt, F. B., eds., Fine-grained de- among riverine taxa across the Cenomanian-Turonian boundary. It seems posits and biofacies of the Cretaceous Western Interior Seaway: Evidence of cyclic sedimentary processes: Society of Economic Paleontologists and Mineralogists Field Trip unlikely that these two extinction events had an identical set of causes. Guidebook 9, p. 157Ð169. The nonmarine extinctions across the Cenomanian-Turonian boundary Elder, W. P., 1987, The paleoecology of the Cenomanian-Turonian (Cretaceous) stage boundary extinctions at Black Mesa, Arizona: Palaios, v. 2, p. 24Ð40. were most likely the result of environmental changes associated with eusta- Elder, W. P., 1991, Molluscan paleoecology and sedimentation patterns of the Cenomanian-Tur- tic rise and fall. The drowning of the coastal flood plain by the advancing onian extinction interval in the southern Colorado Plateau region, in Nations, J. D., and seaway was accompanied by diversification of vertebrate faunas. Maximum Eaton, J. G., eds., Stratigraphy, depositional environments, and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway: Geological Society of America transgression of the seaway virtually eliminated the coastal flood-plain envi- Special Paper 260, p. 113Ð137. ronment, possibly resulting in the extinction of some fish and turtle taxa. Elder, W. P., and Kirkland, J. I., 1994, Cretaceous paleogeography of the southern Western Inte- Regression of the seaway may have caused incision of streams along the rior region, in Caputo, M. V., Peterson, J. A., and Franczyk, K. J., eds., Mesozoic systems of the Rocky Mountain region, USA: Denver, SEPM (Society for Sedimentary Geology), coasts, vastly increased the extent of riverine systems, restricted upstream Rocky Mountain Section, p. 415Ð440. mixing of brackish and fresh water, and limited upstream availability of Fursich, F. T., and Kirkland, J. I., 1986, Biostratigraphy and paleoecology of a Cretaceous brackish lagoon: Palaios, v. 1, p. 543Ð560. brackish-water food resources, possibly further affecting riverine commu- Gustason, E. R., 1989, Stratigraphy and sedimentology of the middle Cretaceous (AlbianÐCeno- nities. Similar large-scale regression in the late Campanian or middle Maas- manian) Dakota Formation, southwestern Utah [Ph.D. dissert.]: Boulder, University of trichtian may have resulted in marked reduction in fresh-water taxonomic Colorado, 376 p. Hut, P., Alvarez, W., Elder, W. P., Hansen, T., Kauffman, E. G., Keller, G., Shoemaker, E. M., diversity prior to the bolide impact at the K-T boundary. and Weissman, P. R., 1987, Comet showers as a cause of mass extinctions: Nature, v. 329, The nonmarine and marine extinctions at the Cenomanian-Turonian p. 118Ð126. boundary are probably both related to eustatic changes, but were not neces- Jewell, P. W., 1993, Water-column stability, residence times, and anoxia in the Cretaceous North America Seaway: Geology, v. 21, p. 579Ð582. sarily synchronous. Marine extinction has been linked directly to the effects Johnson, K. R., 1995, Late Cretaceous and early Paleogene vegetation of the northern Rocky of regression (Paul and Mitchell, 1994), but the nonmarine extinction in Mountains: Significance for interpretation of the vertebrate fossil record: Geological So- ciety of America Abstracts with Programs, v. 27, no. 4, p. 17. southwestern Utah may be related to a combination of both high stand and Kauffman, E. G., 1977a, Geological and biological overview: Western Interior Cretaceous regressive eustatic phases. Basin: The Mountain Geologist, v. 14, p. 75Ð99. Kauffman, E. G., 1977b, Evolutionary rates and biostratigraphy, in Kauffman, E. G., and Hazel, J. E., eds., Concepts and methods of biostratigraphy: Stroudsburg, Pennsylvania, Dowden, ACKNOWLEDGMENTS Hutchison and Ross, p. 109Ð141. Kauffman, E. G., 1978, Evolutionary rates and patterns among Cretaceous Bivalvia: Royal So- This research was funded by National Science Foundation grant EAR- ciety of London Philosophical Transactions, ser. B, v. 284, p. 277Ð304. Kauffman, E. G., 1984, The fabric of Cretaceous marine extinctions, in Berggren, W. A., and Van 9004560 to Eaton and Kirkland. The cooperation of the Bureau of Land Couvering, J., eds., Catastrophes and Earth history—The new uniformitarianism: Prince- Management, the U.S. Forest Service, and the Utah Division of State History ton, New Jersey, Princeton University Press, p. 151Ð246. Kauffman, E. G., and Caldwell, W. G. E., 1993, The Western Interior basin in space and time, in are greatly appreciated. Reviews of the manuscript by J. David Archibald, Caldwell, W. G. E., and Kauffman, E. G., eds., Evolution of the Western Interior Basin: David E. Fastovsky, and Dale A. Russell were constructive and very helpful. Geological Association of Canada Special Paper 39, p. 1Ð30.

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J., Jr., 1982, Mass extinctions in Phanerozoic oceans, in Silver, L. T., and Schultz, Nichols, D. J., and Fleming, R. F., 1990, Plant microfossil record of the terminal Cretaceous P. H., eds., Geologic implications of impacts of large asteroids and comets on Earth: Geo- event in western United States and Canada, in Sharpton, V. L., and Ward, P. D., eds., Global logical Society of America Special Paper 190, p. 283Ð289. catastrophes in Earth history: An interdisciplinary conference on impacts, volcanism, and Sheehan, P. M., and Fastovsky, D. E., 1992, Major extinctions of land-dwelling vertebrates at mass mortality: Geological Society of America Special Paper 247, p. 445Ð455. the Cretaceous-Tertiary boundary, eastern Montana: Geology, v. 20, p. 556Ð560. Obradovich, J. D., 1993, A Cretaceous time scale, in Caldwell, W. G. E., and Kauffman, E. G., Stephenson, L. 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D., eds., Global catastrophes in Earth history: An interdisciplinary conference on impacts, volcanism, and mass mortality: Geological Society of America Special Paper 247, p. 45Ð59. Paul, C. R. C., and Mitchell, S. F., 1994, Is famine a common factor in marine mass extinctions? Geology, v. 22, p. 679Ð682. MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 30, 1995 Paul, C. R. C., Mitchell, S., Lamolda, M., and Gorostidi, A., 1994, The Cenomanian-Turonian REVISED MANUSCRIPT RECEIVED APRIL 26, 1996 boundary event in northern Spain: Geological Magazine, v. 131, p. 801Ð817. MANUSCRIPT ACCEPTED AUGUST 22, 1996

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