Mammalian generic diversity and turnover in the late Paleocene and early Eocene of the Bighorn and Crazy Mountains Basins, Wyoming and Montana (USA)

ingerich b, Gregg

Abstract

pkrterns of mammalian generic turnover, richness, and fw~~al composition were investigated for faunas from I7 bicstratigraphic zones in middle Paleocene through early Eocene deposits of the Bighorn and Clarks Fork basins of northern Wyoming and the Crazy Mountains Bash of wrath-central Montana. Significlxe of turnover was evaluated ( 1 1 by comparison of observed turnover to expected turnover (calcuiated from the mLitipie regression OI turnover on zone duration and generic richness), and (2) by compariq,)n of observed turll,3ver to a bootstrapped turnover distribution. Dattcrns of turnover and rich:?:ss slso * wtl~ *tS.,cssedin light of relative sampling quality ofeach f’aunal zone. The an:r:ysis identitied t;,,trr inter’wls of‘sigwific;t 11.1tiiunal cltarige: the Tm-eiorikn TiffaiGn tr’;!n!si!4xi, :hc I;:tv Tilfimian. the earliest Wasatchian, and the middlt:-to-lute Wasatchian. The first ‘interval w;jc ,harac~rrhed by ;I high number of la~t.._pxw~nc~~in the latest Torreionian, resulting in a decrease in standing gsneric richness in the ear!iest Tiffanian, be:@no major changes in c?!dinal composit’,on. During the next interval of sigmicant turnover. the late TifLniatj, higher-thaal-expected first occurrences resulted in an increase in standing *dress and ct ch:rnge in fauna! composition, most probably reflecting the immigration of taxa from outside North America. I’he third, und mo!*t dramatic, interval of significant generic turnover took pL.+:e in ;he earliest Wasatchian and was distinguished by a high number of first occurrences, but relatively few last oczwrrences. This led to a marked increase in generic richness. a pattern similar to that for the early Wasatchian ot’ North America as a whole. The major change in fauna1 composition, as in the late Tiffanian, was largely composed of immigrants from other continents. The pattern of fauna1 change during the early Wasatchian of the Bighorn Basin. along with evidence for globa! warmi.ng at the Paleocene-Eocene boundary, supports previous interpretations associating this episode in mammalian evolution with the opening of high latitude intercontinental dispersal routes. During the fourth interval of interest. the middle to iate Wdsa$chian, the Bighorn Basin fossil record shows a drop in generic richness. This dithers from the overall North American pattern, and may be, in part, an artifact of still inadequate sanigling for the latest pdrt of the stratigraphic seq!lence in the Bighorn Basin.

003l-01 Q/95/$9.50 0 1995 Elsevie: Science B.V. All rights reserved SSDI 0031-0182(94)OOI 1 I-i

1. Introdactim The Palecigene fossil record of the northern

Rocky Mol_mtain region spans an important interval in mammalian history. The period from the middle Paleocene through the early Eocene is of particular interest: during this time, many of the archaic forms that dominated mammalian communities at the beginning of the “Age of Mammals” disappeared and the first representatives of modern groups appeared in the North American fossil record. This inter\r;jl is represented by strata in the Bighorn Basin of northern Wyoming and the Crazy Mountains Basiq of south-central Montana. The quality of the mammalian fossil record within these strat:. allows us to address questions about the nature of fauna1 turnover, its effects on diversity, and its probable causes within a regional as well as global context. A number of previous studies have examined early Cenozoic diversi.ty and turnover patterns on a global or a continental scale (Simpson, 1937a; Van Mouten, 1945; Simpson, 1947a,b; Lillegraven, 1972: Savage and Russell, 1983; Gingerich, 1?81*, 1987: Krause and Maas, 1990; Stucky, 1990, 1992; Archibald, 1993; Maas and Krause, 1994). Other studies have focused on local or regional sequences and their relationships to geographically broa&r patterns, but have concentrated on only a relatively short time period (e.g., Rose, 198la,b: Badgley and Gingeri&, 1988). ‘This analysis expands on previous work by examining the record of mammalian turnover and diversity in a geographically constrained region with a long and relatively complete stratigraphic record. The study is designed to address three questions: ( 1) What was the pattern of mammalian generic turnover in the Bighorn-Crazy Mountains region during the middle Paleocene to early Eocene? (2) What were the effects of this turnover on the diversity and conipositi: in of the mammalian fauna? (3 ) Were turnover and changes in diversity as~ncititcd with environmental change otl a global, continental, or rt$onal scale? The Paleogene record of the northern Rocky Mountain region IS particularly well represented in the two topographir basins th;if are the focus of this study, the Bighorn Basin (including the

contiguous Cfarks Fork Basin) in northwestern Wyoming, and the Crazy Mountains Basin in south-central Montana. These hasms are part of a structurally continuous Crazy Mountain-Bighorn trough that represented, during the Paleocene and

early Eocene, part of a single depositional province (Gingerich, 1983). Here we refer to the middle Paleocene through early Eocene stratigraphic sequence in the Bighorn and Crazy Mountains Basins as the Bighorn-Crazy Mountain;; sequence. We restrict our study to the Bighorn-Crazy Mountains sequence because of stratigraphic conqidt:rations. However. it should be Iloted that during the late Paleocene alld earliest Eocene. prior to the definitive uplift of the OY! Creek Mountains to the south and structural elevation or the southern Bighorn Mountains to the southeast, the Bighorn Basin was likely to have becTtat least partially open to the northern part of the Wind River Basin and the southern part of the Powder River Basin (e.g., Bown, 11979,1980; Q’ing and Bown. 1985; Lillegraven and Ostresh, 1988). The North American Paleocene-Eocene stratigraphic record is subdivided into a series of nine land-mammal ages. Four of these are especially well-represented in the Bighorn-Crazy Mountains sequence. These are the Torrejonian (middle PaPeocene), I‘i~anian ( late Paleocene;, Clarkforkian ( latest Paleocene), and Wasatchian (early Eocene) Land-Mammal Ages. ,Although thGre is some difference of opinion regarding placement of the Paleocene-Eocene boundary in th1: Bighorn Basin (e.g., Wing, 1984; Koch et al., 1992). here we place the Paleocene-Eocene boun& y at the beginning of the Wasatchian Land-Mammal Age. The Paleocene and Eocene land-mammal ages are further subdivided into fauna1 zones (e.g., Gingerich, 1976, 1983, 1991; Rose, 9980; Archibald et al., 1987: Krishtalka et a!., 1987; Stocky, 1992). The 17 late Torrejonidni to late Wasatchian fauna1 zones represented +dithin the Bighorn-Crazy Mountains sequence include the IX::.t;lvo zones of the Torrejonian (To3-To4), fiv; Tiffanian zones (Til-TiS), three ClarkforkiFn zones (Cfl--Cf3), and the first seven zox)$s of the Wasatchdan ( WaO-Wd61 ( Fig. 1). 7’he zone designated here as “Cf 1” actually includes two interva; subzones, Ti6, the P!~.U@I~.Y~~~zge~icr’tilP,Idenlla Interval Sub-

EPOCH

WASATCHIAN

CLARKFiXMIAtd

58

Y-r----

TORREJONIAN

Fig. 1. Middle Paleocene through early Eocene fawxtl zones represewed in the Bighorn-Crazy Mountains sequence. Correlation of North American land. nwnmal ages and fauna1 zones follow AnL%a!d et al. (1987) and Gingerick, f 1983. 1991 ). Gwchronologacpl correlation (in my.) is from ,l,ubry et a1. i 1988 ). Set text l;w discussion.

done, and CT 1, i.1~ RoJenri;l P/M&@s c :d-~ i hterw~ Sub-zone, and thus spans the ?%ihnianClackforkian boundary (Archibald et al., l987 ). Because Ti6 is known from only one locality in the Clarks Fork Bwsin! and because it includes genera known from Cf 1, it is here considered \nith the Clarkforkian fauna1 zones. The use of biostratigrapbically-defined fauna1 zones as temporal units for analysis of fauna1 turnover might be criticized as circ!r!ar, since fauna1 change both defines the zones and is the subject of analysis. However, most Paleocene and Eocene fauna1 zones are not defined by wholesale fauna1 change, but by first appearances (or particular abundance) of one or two taxa, usually genera or species. The late Torrejonian, all of the Tiffanian, and the early and middle Clarkforkian zones are based on first appearances within single evolving lineages of mammals (taxon-range zones or lineage-zones). The youngest Clarkforkian zone (Ci 3) is an acme zone (based on the simultaneous

liIX%geSof pliJ?latc;S (Git$gxich. 1976, 1983), !]aqe been redefined by Schankfer ( 1980) and by Gingerich ( 1991) on the basis of first appearances of unrelated taxa (interval zones, or range zones). Land-mammal age boundaries, in contrast to fauna1 zones, are defined by major change in fauna1 composition (Wood et al., 1941; Archibald et al., 1987; Krishtalka et al., 1987) and therefore we expect land-mammal age boundaries to show high turnover. The zones To& Ti 1, and Ti:! are best represented by localities in the Crazy Mountains Basin, while the other zones (To4 and Ti3-Wa6) are best: known from localities in the Bighorn Basin and Clarks Fork Basin (Bown, I979, 1980; Gingerich et al., 1980; Rose, 1981b; Gingerich, 1983; Bown and Rose, 1387: Hartman et al., 1989; Bown et al., 1994). Within the Bighorn Basin, Paleocene localities arc largely restricted to the Ciarks Fork Basin arca of the northern Bighorn Basin ( Rose, 198 1b), but Eocene localities are lmown from the central and southern Bi&m Basin as well. Patterns of diversity and fauna1 turnover in the 17 late TorreJonian lo late Wauatchiasl fauna1 zones in t!.~eBighorn Craq Mountains squence are the fxu&; of thi$; study.

2. Materials almdmethods

Our data consist of fauna1 lists of mammalian: genera from localities of Pate Torrejonian through late Wasatchian age (To3-Wa6) in the BighornCrazy Mountains sequence (Table 1). Generic ranges are used to calculate parameters of turnover and richness for each zone. The raw generic turnover data co!lsist of ta!!ies of first and last occurrences, without distinguishing among types of tLnnever, such as immigration and emigration. trilc extinction or pseudoextinction (Archibald, 1993). Generic ranges were taken from the literature and from extensive field work, including as yet ‘unpublished repor’& of occurrences or range extensions. For fauna1 zon& To3, TiI. Ti2, and Ti3, the

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unpublished reports include specimens from 1982--1991 SUNY-Stony Brook and joint SUN?‘ Stony Brook-Duke expeditions to !he Crazy Mountains Basin. Additions to generic lists fot zone To4 and zones Ti3 through Wa5 have been made from recent University of Michigan Museum of Paleontology ( UM ) c*ollcctions from the northcm Bighorn Basin. Fau~lal lists for zones Wa 1 through Wab wcrc updated from informatiuu gcuerously provided from U.S.G.S.-Johns Hopkins University expeditions to the central and southern Bighorn Basin ( Bown et al., 1994). Major localities and sources for each fauna1 zone are listed in Table 2.

Quality of a sample is a measure of how complete a record of the biocoenosis is contained in a fossil assemblage. Quaiity may be affected by the techniques used in acquiring the fossil sample or by the history of accumulation and preservation of the assemblage, As a resuh of ihcse factors. the full range of localities (and habitats) may not be sampled and certain sizes or shapes of spccimcns may be preferentially accumulated. Other stochas-

tic factors also may infiuence sampling quality. For example, rare taxa or taxa with short temporal ranges have a lower probability than common or lot:s-ranging taxa of being included in a given fossli assemblage, particularly if samples arc small ( Badgley and Gingerich. 1988: Badgley, 1990: Stucky, 1990). Incompletely sampled intervals will underestimate total number of taxa (richness) and turnover. Underestimation of richness can be corrected in part by including in total richness those taxa not known for a particular interval but whose ranges extend before and after the interval. This method, known as the minimum-census technique ( Hilborn et al., 1976) or range-through method (Cheetham and Deboo, 1963 ), is commonly applied in analyses of mammalian diversity (e.g., Stucky. 1990; Barry et al., 1990; Maas and Krause, 1994). It assumes that if a taxon is known from before and after but not during a particular interval, its absence is due to di!!I?:ential preservation of habital? in the fossil rec(Brdor to collecting bias (rather thau a real disappe&ante from the local biota). This method will still result in underestimates of richness for intervals at the beginning and end of a sequence, since there are fewer adja(‘ent zones in

190 Table 2 Major localities in the Bighorn--Crazy Mountains sequence. ?Aajor sources rA’rr to published Fdunal lists or major 3 ;stcmatic revisions. See text for sources of unpublished data -_-_l_l__ Age

Locality

Location

To3

Gidley Quarry Silberling Quarry Hartman Quarry Friday the 13th Quarry Willow Creek Quarry Rock Bench Quarry Bangtail Quarry Douglass Quarry Bingo Quarry Glennie Quart Scarritt Quarry Locality 13 Cedar Point Quarry Divide Quarry Princeton Quarry Little Sand Coulee Bear Creek Various UM localities Paint CifXk Holly’s Microsite Franimys Hill Phil’s Hill Krause Quarry Foster Gulch Upper Sand Draw Gmnger Mountain Rainbow Valley Various UM localities SC-67 Various U M localities Various tlM localities Various UM localities Various U M localities Various USGS localities Various UM !ocaiities Various USGS localities Various UM localities Various USGS locali ties Various USGS localities

Crazy Mountains Crazy Mountains Crazy Mountains Crazy Mountains Crazy Mountains Bighorn Basin Crazy Mountains Crazy Mountains Crazy Mountains Crazy Mountains Crazy Mountains Crazy Mountains Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Eghorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Uasin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin Bighorn Basin

TO4

Til

Ti2 Ti3 Ti4 Ti5 Cfl

Cf2

Cf3

WaO

Wtl5 Wtl6

which to detect a rare taxon (e.g., Rosenzweig and Taylor, 1980; Boltovsky, 1988). Nevertheless, inclusion of range-through gene:*a (genera known from before and after an interval. but not during it) allows a more realistic estimate of’ generic richness for many otherwise inadequately sampled zones. Correction of turnover patterns for the effects

Major source Basin Basin Basin Basin Basin Basin Basin Basin Basin Basin Basin

Simpson, 1937b Simpson, 1937b; unpublished data Unpublished data Unpublished data Unpublished data Rose, 1981b Gingerich et al., 1983 Krause and Maas, 1990 Unpublished data Unpublished data Krause and Maas, 1990 Simpson, 1937b: unyublished data Rose, 1981b; unpublished data Rose. 1981b; unpub!ished data Rose, 1981b Rose, 19810 Rose, 19Slb Rose, 1981b; unpublished data Krause, 1986 Krause, 1986 Rose, 1981b Rose, 19Slb Rose, 19Slb Rose, 19Slb Rose, 1981b; unpublished data Rose, IOSlh Rose, 19Slb RN. I YKI b (iingerich, 1989 Gingerich. 1989; unpublished data Badgley, 1990; unpublished data Badgley. 1990; unpublished data Badgley, 1990; unpublished data Bown et al., in press; unpublished data Badgley. 1990; unpublished data Bown et al., in press; unpublished data Badgley, 1990; unpublished data Bown et al., in press; unpublished data Bown et al., in press; unpublished data

of inadequate sampling is more difficult. This is because an inadequate sample will not only underestimate first and last occurrences for the interval in question, but also will affect the records of first and last occurrences before and after the incompletely sampled interval. Thus, first OccurrenceI not recorded in the inadequately sampled zone will appear to have taken place in a better-samplec

urrc11ces

wil

ear

zone. Some assessment of the relative quahEy of sampling among intervals is therefore desirable. Sampling quality can be evaluated in a number of ways, incl ng taphonomi ties (e.g., inkler, E983; 1986) and counts of total specimens per sampiing unit (e.g., Badgley and Gingerich, 1988; Badgley, 1990; Barry et al., 1990 ). The latter ttchnique allows comparison of sampling quality among localities but does not assess the degree to which the localities represent the original diversity of taxa for a particular interval. A simple alternative is tu estimate relative sampling quality from the proportion of range-through taxa. The proportion of range-through genera in a zone can be used to derive an index of relative generic complelznzss ( Krause and Maas, 1990; Stucky, 199& Maas 2nd Krause, 1994). In these studies, a fauna1 completeness index (U) was calculated as: CI = [ pJ,/‘ Nt ( f N,, )] x 100

where N, is the tota! number of genera actually known from the fossil record of an interval (first and last occurrences, plus genera known hefcre, during, and after the interval ) ard N,, is the number of range-through genera. Mere we also apply a more conservative completeness index, CIbda * that is based only on genera whose ranges extend through the interval, and thus excludes first and last occurrences: clbdo

= ENbda/tNbdo

+ Nrt )I x 1 O”

where Nbdais the number of genera actually known before, during, and after the interval, and thus excludes first and last occurrences. C&da theoretically represents a more realistic estimate of completeness, because it makes no arsumptions about the factors (other than sampling) that might influence numbers of first or last occurrences. In both cases, the assumption is that the relative quality of sampling improves as the index approaches 100 (the record includes few rangethrough genera, and the numerator and denominator approach equality). We consider intervals where the completeness index is below 70 (thus

of the estimated total number of genera are from the fossil record of the in. ai ) to be less than adequately sampled. The ‘6 GUI-off is somewhat arbitrary, but it reflects a in the distributions of CI and c&da for a*carOn:;. The minimum-census technique serves as a partial correction for poor sampling, but only for richness estimates. Turnover for zones with low completeness indices should be evaluated with caution. There is an important limitation to the usefulness of the completeness index. In cases where taxa whose ranges extend before and after the interval represent a small proportion of the total fauna, the completer:.ess index may give a distorted measure cf interval completeness. A special case of this is the beginnL)g of a stratigraphic sequence, and there the cancel t of “first ozcurrczee” is applied differently: betause there are no intervals sampling an earlier time, virtually all records appear to be first occurrences. A similar situation obtains at the end of the stratigraphic sequence, where all genera are necessarily last occurrences. In these cases, other criteria should be used to evaluate the quality of the sample.

Turnover patterns can be influenced by a variety of factors other than the recorded number of first and last occurrences. Two such fxtors are diExences in interval duration and differences in taxonomic richness among stratigraphic units (e.g., Gingerich, 1983; Barry et al., 1990; Stucky, 1990; Archibald, 1993; Maas and Krause, 1994). Rate quotients (Gingerich, 1987) are measures of turnover that take the effects of duration and richness into account. They are calculated as the ratio of observed turnover to expected turnover. Expected turnover is calculated from the multiple regression of log-transformed numbers of first or last occurrences (the dependent variable) on log-transformed interval durations and richness (the indeperldent variables). The “expected turnover” therefore is predicated on the assumption that turnover occurs at some constant background rate, and that the number of first or last occurrences varies only

according 19 the number of taxa within an interval and the length of time represented by the interval. Fauna1 zones with CI’s less than 70 were excluded from the multiple regression, since they are likely to significantly under-represent turnover. The multiple regression equation supplies two partial regression coefficients (one for duration and one for richness) and an intercepr that are used to calculate an expected first or last occurrence rate (R,) for each zone. For the Bighorn-Crazy Mountains sequence, expected first occurrences were calculated as:

+ 0.006 In (richmxs)

+

2.2 191

and expected last occurrences were calculated as: R, = exp [ 0.566*1n(drrvrtior~) +

1.408*ln (richness) - 3.3301

The first- and last-occurrence rate quotients (RQ) then were calculated as:

RQ = RJR, where R, is the observed number of first or last occurrences and R, is expected first or last occurrences. Interval durations for Torrejonian, Tiffanian, and Clarkforkian fauna1 zones are based on Archibald et al. ( 1987), who correlated fot:sil localities bisstratigraphically and magnetostrztigraphitally, and calibrated their ages according to the geochronology of Berggren et al. (1985). We have incorporated revisions to the Berggren et al. timescale from Aubry et al. ( 1988). Durations of Wasatchian zones are based on estimated sedimentation rates for measured sections in the Clarks Fork Basin (Gingerich, 1989, 1991). The first and last fauna1 zones of the BighornCrazy Mountains sequence (To3 and Wa6) were ckcluded from calculations of rate quotients for firsi and for last occurrences, respectively, to avoid skewing the rate-quotient distributions. AS discussed above, first-occurrence records are inflated for the first interval of a stratigraphic sequence, and last-occurrence records are inflated for the last interval. Rate quotients greater than 1.0 indicate that

observed turnover M’;~shigher than expected, whereas values less rL;tn 1.0 demonstrate lower than expected ttirnover. Rate quotients were not tested for statistical significance, but a chi-square goodriess-of-fit test was used to evaluate overall significance of the departure of the observed turnover pattern from the expected turnover distributiotii. A number of previous studies have used chisquare tests to assess the significance of turnover patterns with respect to a null hypothesis of a uniform distribution of turnover among intervals or clades ( Raup and Marshall, 1980: Barry et al., 1990; Maas and Krause, 1994). Although such tests can assess overall deviation from an expected distribtltion in turnover, and can be used to identify intervals contributing the most to the overall deviation, tests of significance for a single interval or taxon often are limited by the sample-size requirements of the statistical tests. Moreover, it has been pointed out that the assumption that observed probabilities should correspond to a chisquare distribution may not be valid (Hubbard and Gilinsky, 1992). Bootstrapping offers an alternative that makes n(v assumptions about the shape of expected distributions bul instead uses the data set itself to generate a test distribution to which an observation can be compared (Gilinsky, 1991). Here we use bootstrapping to test the null hypothesis that first and last occurrences are ran-

domly distributed among fauna1 zones. For each zone, we compare the observed number of first or last occurrences to a bootstrapped test distribution. The test distribution for each fauna1 zone consists of expected numbers of first or last occurrences generated for each of 10,000 simulations. For each simulation, generic ranges are shufned randomly among zones, and the first and last occurrences are cc untzd [we apply the minimum-census technique to generic ranges, thus assuming that each genus has only one first and one last appearance). Ranges are not allowed to be truncated (e.g., a genus that spans four zones cannot bc shifted to first occur in Wa5, where its range would be artificially truncated after two zones-Wa5-Wa6; likewise it could not be shifted to last occur In Til, since its range, To3-Ti 1, would span only three zones). The method is illustrated in Fig. 2.

193

Genera Genus A

Genus B Genus C Genus 0

R

Fauna1Zones

(TotaiLast

1 O(lj010t3

(2 11

Observed#

cl

0 123 4 5 67 Bootstrapped # Last Occurrences Zone 6

Fig. 2. Simplihed example of bootstrapping method for testing significance (two-tailed test, cc= 0.01) of fauna1 turnover. A. Observed first and last occurrences of seven genera (A-G) in seven zones ( l-7). B and C illustrate two randomizations of range data. ‘{‘he randomization program clces not allow truncation of ranges at the beginning or end of the stratigraphic sequence. The randomization of generic ranges is repeated 10,000 times for each zone, and a frequency distribution of first and iast occurrences is generated. D. Bootstrapped frequency distribution ior Zone 6 based on 10,000 randomizations of generic ranges. The arrow ndicates the observed number of last occurrences (4). 12.5% of the total distribution (solid black) falls to the right of the observed number, indicating that this number of last occurrences is not significant.

The null hypothesis is that the number of first or last occurrences is not significantly different from that expected if generic ranges were distributed randomly among intervals. To test this, the observed turnover for each interval is compared to the expected distribution for that interval. In our two-tailed test (a = 0.01 ), observed first or last occurrences that fall in the upper 0.5% of the bootstrapped distribution are considered significantly high, while values falling in the lower 0.5% are considered significantly low. 2.3. Generic richness and taxonornic composition The most widely used measure of biotic: diversity is the total number of taxa, or taxonomic richrless (e.g., Whittaker, 1977). Generic richness typically includes all genera whose ranges extend before and after a zone, plus all genera having their first or last occurrence within that zone. This number may overestimate actual richness for any single time

within the zone, unless one assumes that all first occurrences take place at the beginning of a zone, and all disappearances at the end of the zone. An alterr?ative method

is to estimate

rishness

at a

single point of time. One such estimate is standing richness. Standing richness estimates generic richness at the midpoint of an interval ( Harper, 1975; Maas and Krause, 1994), a concept analogous to the running mean (Webb, li 969; Marshall, I98 1). Standing richness (N,,) is calculated as; N,, = N~,da+ N,, + 1/2(N~+ N, -

No,,,)

where Nbdais the number of genera known before, during, and after a zone, N,, is the number of range-through gtinera (those known before and after but not during the zone), Nf is the number of first occurrences in the zone, N, is the number of last occurrences in the zone, and A’Onlyis the number of genera known only from that zone (krhich therefore is included in tabulations both of firs\”and of last occurrences). Standing richness

assumes a constant rate of first and last occurrences throughout the interval (which may or may not be a valid biological assumption for all intervals).

3. Results

Both fauna1 completeness indices, Cl, and CZbda, yield similar values (Fig. 3). Both show that three zones have fauna1 completeness indices less than 70, and thciefore are considered inadequately sampled: Ti2, Ti4, and WaO. In the Bighorn-Crazy Mountains sequence, Ti2 is known almost entirely tiom a single locality, Scarritt Quarry. Although Scarritt Quarry has been worked extensively and is now represented by a large number of specimens (n = 1083), it probably is biased taphonomically ( Rose, 1981b; Krause and Maas, 1990). The second interval with a low CI, Ti4, is known from several localities in the Clarks Fork Basin (e.g., Airport, Croc Tooth Quarry, Divide Quarry), but these faunas have yet to be fully described and all are relatively small samples. Clarks Fork Basin localities of Ti4 age also lack genera known from

stern Interior (e.g., other parts of the northern Russell, 1967; Krishtalka al., 1975; Krause, 1977, 1978: Fox, 1990). This suggests that sampling is indeed inadequate for this zone, since, in general, little North American Paleocene localities biogeographic provinciality (Anthony a daas, 1990; Stucky, 1990). The third interval with a low CI is WaO. Since WaO localities appear to represent a rarely sampled environmental setting in the Bighorn Basin, that of a high-floodplain (Gingerich, 1989), they almost certainly do not adequately sample the range of environments represented in the Bighorn Basin during this interval. In any event, the sample is indeed biased in the under-representation of small mammals (Gingerich, 1989). Fauna1 Tones with high completeness indices are not necessarily free of taphonomic or sampling biases. Most Paleocene quarry sites, and even many Wasatchian levels undoubtedly sample only a limited range of environments. However, in these cases, the greater diversity of localiiies in the zone or more extensive sampling of single localities has resulted in a more complete representation of the biocoenosis, especially of rare taxa. Tht.s, although some fauna1 zones with high completeness indices

Fig. 3. Mammulionhunal completeness indices for 17 middle Paleocene through early Eocene fauna1 zones in the Bigkorn-Crazy Mountains sequence. CI=(N, - N,,)/N,. where IV1includes genera known before. during, and after a zone, plus genera hkiving their first or last occurrences in the zone. and N,, includes genera known before and after but not during the zone. C’Ibda= &dI, /V&d3 + N,, 1. where NM:, includes only genera known before, during, and after a zone (thus excluding first and last occurring genera).

may represent a biase

icture of some as

1lt.t

number

species, they offer a rst ap~ro~j~l~atjo~~ of malian generic rich through early Eocene of the Mountains sequence. We expect that both turnover and standing richness are underestimated for the three zones with low completeness indices, Ti2, Ti41qand WaO. As a consequence, last occurrences may be overestimated for the preceding zones (Til. Ti3, and Cf3 ), and first occurrences may be overestimated for the subsequent zones (Ti3, Ti5, and 1 ). II11 addition, because the faunai zones prior to To3 and following Wa6 are not well sampled in the Bighorn-Crazy Mountains sequence, the ranges of genera from To3 asd Wa6 are likely to be artificially truncated. This accounts for the inflated

3”.h.

O!’last occurrences

in

TiI?‘II01Y7i’, ~id1rws.Y ctmi tasmomic

compositions

Table 3 summarizes turnover, richness, and results of x2 tests and bootstrap analysis of turnover for the late Torrejonian (To3) through late Wasatchian ( Wa6 ). Both expected turnover and observed turnover are given. Turnover and standing richness are presented graphically in Fig. 4, which illustrates both the observed numbers of first and last occurrences and first and last occurrence rate quotients. Tests of both first and last occurrences yielded highly significant &i-square values (x1 = 59.55 fb: first occurrences, and x2 =66X for last occurrences, d.f. = 15, p ~0.001). The early Wasatchian

Table 3 Mammalian generic turnover and richness for 17 middle Paleocene through early Eocene f,iunal zones of the Bighorn- CraL> Mountains sequence. Zone durations are based on Archibald et al. ( 1987) and Gingerich ( 1991) Zone

To3 To4 Ti 1 Ti2 Ti3 Ti4 Ti5 Cfl Cf2 Cf3 WaO Wal Wa2 Wa3 Wd4 Wa5 Wa6

Duration (my. 1

0.5 0.5

0.84 0.63 0.68 0.68 0.74 0.68 0.68 0.61 0.25 0.30 0.26 0.66 0.24 0.24 0.95

N,,

28.0

37.5 31 .o 32.5 34.0 33.0 36.5 38.5 43.5 41.0 43.0 57.5 63.0 59.5 53.5 48.5 35.5

Total genera

52 49 40 36 43 38 48 45 49 47 50 68 70 68 62 53 50

Rangethrough

Known only

5

7 4 7

12 3 18 7 9 3 3 16 6 4 3 7 8 1

0 4 1 6 2 2 1 1 2 1 1 1 4 1

BDA

7

L

19 17 17 22 10 18 23 35 32 20 41 52 49 38 36 20

First occurrences Obs.

Exp.

48+ s-

13 311 6 15+ 11 8 3 13 20+ 5 8 3 6 4

Last occurrences _--. _x’!

Ohs.

n/a

fl lil

7.4,s 8.86 8.04 8.26 8.25 8.50 8.26 8.26 7.96 5.89 6.25 5.95 8.21 5.84 5.83 9.25

0.:14 1.93 3.16 0.91 0.61 4.97 0.91 0.08 3.09 8.58 30.25 0.64 0.76 : 38 0.c: I _ 2.98

8 22+ 7 4 11 5 14-I4 5 10 2lb3 9 15 726

Exp.

x’

7.75 7.75 7.15 5.25 7.04 5.91 8.61 7.50 8.46 7.51 4.99 8.4’I 7.97

0.0 1 3x .40 0.0 1 0.30 2.23 0.14 3.37 1.63 1.42 O.S3 1.79 0.52 3.53

13.22 6.65 5.33 n/a

i .35 10.49 0.52 n/a

BDA: number of genera recorded b-fore, during, and after a ttirlc. R~~ge-~.h:~u&: genera known before and after but not during a zone. Known only includes genera known from a single fauna1 zone, Known only genera alho are include/i in counts of both first

and last occurrences. IV,,: standing richness (see text). Total genera: includes Range-through, BDA, and First and Last Occurrences. For First and Last Occurrences, observed (Obs.), expected (Exp.), and x2 values are given for each zone. Lxpected turnover values are based on multiple regression of First Occurrences (To4-Wa6) or Last Occurrences (To3-!%5) on fauna1 zone durations and generic richness. Significantly high observed turnover is indicated by “+” and significantly low obverved turnover is indicated by “-I’, based on comparison with bootstrapped distributions (two-tailed test, p co.01 ). See text for further explanation.

Observed= Expected

1 To3

To4

lrl

TM

Ti3

Ti4 Ti5

Cfl

Cf2

Cf3 WsO Wal Wa2 Wa3 Wa4 We5 Wa6

1

Observed Expected

1 To3

To4

Tll

Ti2

Ti3

Ti4

Tl5

Cfl

Cf2

Cf3 Wall

We1 Wa2 Wa3 Wa4 Wa5 Wa6

WASATCHIAN

PALEOCENE

I

EOCENE

1

a0 and Wa1 ) turnover conrributed the most to the first occurrence &i-square value whereas the latest Torrejonian ( 3X) and middle Wasatchian (Wail) account for the greatest part of the hst occurrence chi-square value. As discussed above, because the Paleocene and Eocene land-mammai ages are defined on the basis of fa-,lnal change involving a number of unrelated taxa, and most fauna1 zones are defined on the ba& of change in a sing’ie lineage. we expect the firs; fauna1 zone of a land-mammal age to be haracterized by high first occurrences, the last to have a large number of last occurrences, and zones at neither the beginning nor end of a land-mammal age to show less pronounced turnover. Moreover, if, for example, interval duration and taxonomic richness; were the primary constraints on turnover, first and last occurrences might track one another (e.g., Archibald, 1993). Turnover in the BighornCrazy Mountains sequence corresponds to these expectations in many respects-first occurrences are higher than last occurrences in Til, Cf 1, and WaO, and last occurrences are higher than first occurrences in To4 and Cf 3. The high turnover in To3 (first occurrences) and Wa6 ( last occurrences). neither of which is the begimling or end of a landmammal age, reflect the artificially truncated ranges at the bektinning and end of the styatigraphic sequence. In other instances, deviations from the expected pattern call for more detailed consideration. For purposes of discussion, we have grouped the 17 fauna) zones of the Bighorn-Crazy Mountains sequence into six intervals. These generally correspond to intervals traditionally recognized in the literature: late Torrejonian (To3-4), early (Til -Ti2), middle (Ti3) and late Tiffa(Cf 1-Cf 3 ), early nian ( Ti4-Ti5 ), Clarkforkian ( WaO-Wa2). middle (Wa3-S), and late Wasatchian ( WaG), but do not necessarily correspond to periods of significant turnover events based on the data presented here.

Lcrlc 7k~~~joirkr~l ( IW-4) . ootstrap analysis indicates significant turnover in both hte Torrejonian fauna! zones. As discN_issedabove, the slguificantly high number of first occurrences in To3 is most probably an artifact of poor representation oi previous intervals in the stratigraphic sequence. To4. however, is a well-sampled zone and is preceded *md followed by well-sampled intervals (see Table 3 ). Therefore, the significantly LW number of first occurrences and the significantly high number of last occurrences in To4 are likely to reflect the real pattern of t’aunal turnover at the end of the Torrejonian. To4, as ihe last fauna1 zone of the T&rejonian Land-Mammal Age. is expected to have a high m_unber of last occurrences, but the number of last occurrences is extraordinarily high--To4 represents the largest contribution in the entire sequence to the significant &i-square value for last occurrences. Differences in numbers of first and last occurrences result in net changes in richness. However, if first or last occurrences represent an unusually large proportion of the fauna, standing richness (which includes only half of first and last occurrences) will be much lower than total richness. This is the case with To3, where first occurrences

represent an artilkially high proportion of the total genera, and the net increase in St; nding richness between To3 and To4 ( Fig. 4). the~*efore, is considered an artifact. In contrast, because To4 and Til are well-sampled, we have conf’adencethat the diminution in standing richness betwee To4 and Til, a rcstlii of ths: high nuiaber of last occurrences in To4, more closely approximates the true generic pattern. Generic-level analyses are frequently used as surrogates for species patterns, although one might not always expect generic and species richness to coincide exactly (Archibald, 1993). However, the drop in generic richness across the TorrejonianTiiffanian boundary differs from the pattern of

Fig. 4. A. Standing richnessness and first and last occurrences (number of genera) of the mammalian

fauna frotii 17 middle Paleocene through early Eocene fauna1 zones represented in the Bighorn-Crazy Mountains sequence. See text for explanation of standing richness. B. First occurrence rate quotients for fauna1 zones To4-Wa6. Rate quotients are the ratio of observed to expected turnover, where the expected turnover is calculated from multiple regression of turnover on zone duration and generic richness. for zones with completeness indices greater than 70. C. Last occurrence rate quotients for fauna1 zones To3-Wa5. Rate quotients Of 1.0 (dashed lines in B and C ) indicate that observed turnover is equal to expected turnover.

species-level fau ;lal change reported by Krause and Maas (1993) and by FOX (1990). These authors interpreted the lack of change in species richness across the Torrejonian-Tiffanian boundary as evidence of ecological stability. In this case, where the species and generic patterns differ appreciably, we can turn to other aspects of diversity. The lack of significant change in ordinal composition of the fauna across the Torrejonian-Tiffanian boundary (Fig. 5) provides some support for the idea of ecological stability, despite the drop in generic richness. Early Trjlftocrnia~z (Til-Ti2) . Turnover was not significant in the earliest Tiffanian (Til ) despite the high first-occurrence rate quotient (Table 3, Fig. 4). First occurrences exceed last, as expected for the beginning of a land-mammal age. In Ti2, the number of first occurrences was significantly low according to bootstrap analysis, but this most likely reflects the still inadequate sampling for that interval (and thus underestimation of turnover). Niddk T#hiau (Ti3,J. As might be predicted from the probable underestimation of first occurrences in Ti2, first occurrences were higher than (though not significantly) in Ti3.

Sampling may also account for the higher than expected number of last Ljccurrences. Ti3 last occurrences are likely tc( include genera whose ranges actually extended into the less well-sampled Ti4. Thus, when sampling influences are consid= ered, there is no strong evidence for unusually high or low turnover during the middle Tiffanian. Throughout most of the early and middle Tiffanian ( Tii -Ti3 ), first and last occurrences tracked one another closely. Concomitantly, standing richness remained remarkably stable ( Fig. 4A). There also was little change in the ordinal composition of the faunas during this time ( Fig. 5). Late T@mim l Ti4-Ti5). The latest TilTanian (Ti5 ) was characterized by significantly high first and last occurrences. The significantly high number of last occurrences is not surprising, since Ti5 represents the end of the Tiffanian. In contrast, the ul;expectedly h&h number of first occurrences in Ti5 is likely to reflect the effects of sampling, in this case roll-over of generic first occurrences from the more poorly sampled Ti4. Despite the high number of first occurrences in TiS, there was only a slight net increase in generic standing richness during the late Tiffanian primates Artlodactyla /Perlssodactyla Redentia Creodonta Mesonychia Carnivora LlpotyphPa/ Marsuplalla Pantodonta / Taeniodonta / Tlllodontia / Dinocerata / Arctostylepida ! Palaeanodonta Plesladaplformes “Proteutherla” / Plaglomenldae / Leptlctlda / Mlcrochlroptera Multltubercutata Arctocyonla “‘Csjndylarthra”/ Phenacodanta

Fig. 5. Ordinal composition of the mammalian fauna of 17 middle Paleocene through early Eocene fauna1 zones from the BighornCrazy Mountains sequence. Orders are presented as proportions of standing richness. Arrows indicate well-sampled zones characterized by significant turnover. See text for discussion.

relatively complete sampling for Wd3 through W-i5 suggests that tht: drop in richness rL:presents the actual pattern of fauna1 change during this interval. Schankler ( 1980) Identified two important species-level turnove-l,- events, Biohorizon B and Biohorizon C, in the ~~~~dd~e-~~satc~~ial~ record of the BighGTnBasics. Biohorizon B is indicated by a high number of last occurrences and Biohorizon C is marked by high first occurrences (e.g.. Schankter, I980 ). The interpretation of these events is complicated, however, by recent work demonstrating that, in the south-central Bighorn Basin. thi: two biohorizons are acrl:ally much closer stratigraphically than previously thought, and may r+;esent 8 single turnover pulse ( Bown et al., I?!.Il; Bow11 and Kraus, 1993). However, in of the northern Bighorn Basin, University Michigan field parties have found that the two biohorizons are distinct: Wa4 precedes Biohorizon B, Wa6 follows Biohorizon C and the two are separated by a stratigraphic interval more than 200 ni thick I V&S). In any event. as was the case with the early WahGlt&ln Biuhorizon A, nciehcr Biohorizon B nor Riohorizon C coincides with signilicant goneric 1urnovcr. Ltrtc Ctirsdu’~tr~ ( Wir(i~. As discuss& above, the Bighorn Cl +\zyMountains sccpmx dwrihcd hv :Irf ifit-h IIy t runcutcs guncric ra ngcs :It Wi6, thus inflating the number of last occurrcnocs, Moreover. the late Wasatchian is poorly known in the Bighorn Basin, in comparison to the earlier Eocene. Faunas from WaG( Lysitean) and. particularly. Wa7 ( Lostcabinian), currently are reported from relatively few localities. although both kveis art‘ under study and may prow far richer than is now appreciated (e.g., Bown et al.. 1994). Because of this. the drop in richness after WaS is considered artificial, In sm. we have identified four intervals within t 11~Bighorn Crazy Mountains stx~ut.~~~e where the pattern of turnover and consequent changes in gcncric richness are robust with respect to sampling. These are the latest Torrejonian (TOM), the latest Tiffanian (Ti5). the earliest Wasatchian ( WA. but possibly ?VaO), and the middle to late

Wasatchian ( Wa4) (see arrows in Fig. 5). Other apparently significant turnover events and related changes in standing richness (To3-To4, TiZ-Tij,

Ti4--TiS, Wd%Wd6) are more likely to refiect sampling artifacts. Such artifacts include both under-estimations of turnover and richness in the inadequately sampled zones and concomitant overestimation of turnover in the preceding or following zones.

4. Discussion 4. i. ~}ltc,‘!~l’~~t~(tioII c~f’ttrrwowr~ put tcrrls

Determination of turnover and richness patterns is a first step in formulating and testing hypotheses of ci)mmunity esohition. tiowever, the patterns alone provide little insight into the nature of fauna1 change and its effect on the mammalian community. In part, this is because tallies of first or last occurrences can encompass a variety of evolutionary processes. For example, last occurrences may reflect the effects of true extinction, when a taxon disappears leaving no direct descendants, as well as pseudoextinction, when a taxon is yeplaced by its phyletic descendants through anagenesis or cladogcncsis (e.g., Stucky. 1990: Archibald, 1993). III other cast’s, Iast occurrences rccordcd in one slratigraphic scyuencc may represent local disappcxmxs rcsul t ing lYoni cmigra t ion to 01 ha regions or to habitats as yet unsampled within the same region. Archibald ( 1993 ) notes that this “Lazarus effect’” of disappearance and reappearance can have a major impact on extinction patterns. Just as last occurrences result from dihereht mechanisms, tabulations of first occurrences may ctlcolr~pilss different evolutionary processes, including pseudo-originations (the counterpart of pseudoextinction) and first appearances CIlle tn immigration. Distinguishing amon-;! pseudoextinction, true extinction. and emigration. or among pseudoorigination, true origination, and immigration is particularly diflicult when yhyletic relationships are poorly understood, as is the case for many taxa currently classified in paraphyletic Paleocene and Eocene groups (e.g., “Proteutheria” and “Condylarthra”). Interpretation of turnover is further complicated by the fact that pseudoextinction and pseudo-origination can result from either ana-

genesis or cladogenesis, each of which have different implications for n~~~~~l~la~ia~l ~o~~~~~~u~~jty structure. In the case of anagenetic pseudol_ASt OCCUlXlicSli wre significantly high at tile turnover, the number of taxa would not change, end of the Torrejoman, resulting ill a drop in whereas cladogenetic models of pseudo-turnover standing richness bet ween the Torrejonian and would result in a net increase in number of taxa. TXd~liall. At the SCm-~ iiins, hi&r-level tax@Such an increase potentially could change the nomic composition was relatively stable. In c;tllel relative pioportions of adaptive types, and thus words, generic last occurrences were evenly distrib-

the ecolog id structure _ of the mammalian community. The effects of turnover patterns on community ecology analyses should over represents or introduction

are not addressed here, but such take into account whether turnreplacement by ecological vicars of new adaptive types (e.g., Maas

et al., 1988; Maas and Krause, 1994). An additional confounding ilactor in interpreiation of the biological significance of basin-wide patterns of richness and turnover in the earl;! Eocene is the evidence for local, geomarphologitally controlled environmental variation, both between and .within stratigraphic levels in the Bighorn Basin (Schankler, 198 I : Winkler, 1983; Wing and Bown, 1985; Gingerich. 1989; Bown and Beard, 1990; Thewissen, 1990). The influence of this local envirc~siineiital variation on generic richness is uncertain, but presumably could vary among strat~graphic levels. For example, A~A.u

measures of total regional diversity could reflect zither a low alpha diversity (within-habitat divcrsity), and high beta diversity (between-habitat diversity) or high alpha diversity coupled with low beta diversity ( Stucky. 1990 ). Clearly conclusions concerning the ecological significance of basinwide changes in diversity and fauna1 composition must be tested and elaborated by detailed analysis of the facies distribution of contemporaneous early Eocene localities ( Winkler, 1983; Bown and Beard. 1990). Such a detailed study of lateral facies change has not been applied t,:, the Paleocene

record, but similar considerations should influence Paleocene diversity patterns. With these caveats and lin3ations in mind, we can begin to consider the po ,xible biological correlates of Paleocene-Eocer:k: turnover in the Bighorn-Crazy Mountains sequence. We focus on the four intervals (see above) where turnover was both significant anti robust with respect to sampling.

uted among orders, rather than representing decimation of a single group. As has been argued elsev;here. both Torrejonian species lastoccurrences and TiCi‘dnianspecies fir&occurrences seem to be largely the result of endemic anagenesis 3r cladogenesis ( Krause and Maas, 1990). Moreover. the composition of mammalian It‘aunas ill the Bipl?,3rn-Crazy ~+∫iins sequence wan similar to tkat of contemporaneous taunas from other regions of North America (Anthony and Maas, 1990), supporting the interpretation that Torrejonian-Tiffanian turnovei ill tJ12 Bighorn-Crazy Mountains sequence reflects endemic evolution, rather than the etf‘ects of dispersal from outside the Bighorn and Crazy Mountains Basins. ( But see Sloan., 1969 and

Gunnell, 1989 for discussion of biogeographic difT’erencesin some components of Torrejonian and Tifl’;ini;ln faunas). This 3s c~.,nsistcntwith the idea, based on av;tilable paicoclimatic information and q:construction of’the distribution oJ nlan~nlalian body sizes and trophic types, that the Torrejonian-TiKanian transition was not associated wit!?..n?;ljol environmental change ( Krause and Maas, 1990; Maas and Krause, 1994; see also Gunnel1 et al., this; issue: Morgan et al., this issue).

III

the

latest Titt’anian (Ti5 ). standing generic

richness increased, concomitant with the first occurrence of groups unknown or poorly represented in the earlier Tiffa:iian. At least some of those taxa (e.g., Pl’oh~EtlT!‘b~~)Si.s, Arc*to.~tjvk)pk~) appear to have originated outside North America (Gingeri&, 1985; Thewissen and Gingerich, 1987; Cifelli et al., 1989; Krause and Maas, 1990 ). The increase in standing richness began in the latest Tiffanian and continued into the Clarkforkian, although Ihc rate of t~~r~owr sw; not sigilikallt.

Clarkforkian first occurrences include representatives of higher taxa ( Rodent& Tillodontia, and possibly Coryphodontidae) that appear to have originated in Asia (e.g., Li, 1977; Chow and Wang, 1979; Gingerich and Gunnell, 1979; Rose, 1981b; Sloan, 1987; Krause and Maas, 1990). ‘Thesemay have entered North America by way of a highlatitude filter bridge ( McKenna, 1975, 19830). Dispersal across such a filter bridge would have coincided broac’ly with the global warming trend that began in the late Paleocene and continued into the early Eocene (e.g., Wolfe, 1978, 1985, 1987; McKenna, 1983a; Shackleton, 1984; 1986; Wing, 1987). Among modern faunas, differences in mammalian species richness and community trophic and size distributions are associated with temperature differences (e.g., Fleming, 1973; Andrews et al., 1979; Andrews, 1992). Whether the increase in generic richness and change in fauna1 composition in the late Tiffanian and Clarkforkian reflects such environmentally driven changes in community structure awaits more detailed analysis of basin paleoecology, including the structure of the mammalian community, and paleobotanical and geomorphological evidence.

increase in standing generic richness in the early Wasatchian ( WaO-Wa2) was not only more pronounced than during the late Paleocene, it also was associated with major changes in higher-level taxonotnic composition. Of particular note are first occurrences of artiodactyls, perissodactyls, and primates, as well as didelphin marsupials, and hyaenodontid creodonts. All of these first occurrences are likely to represent dispersal into North America from other continents (e.g., Simpson, 1937a; Van Houten, 1945: Rose, 1981b; Gingerich, 1989. 1990; Krause and Maas, 1990). Recent analyses of centers of origin for these major groups have focused on equatorial continental regions, including Africa (e.g., Gingerich, 1986, 1989, 1990) and the Indian subcontinent (Krause and Maas, 1990 ) Other Wasatchian first occurrences include various “proteutherians” and lipotyphlans, and the problematic genus W~~okste.~.Relationships of many Of these genera are uncertain: some genera The

??

appear to be immigrants from other northern continents, while other genera may have evolved from lineages known within the Bighorn-Crazy Mountains sequence. An equally remarkable aspect of the early Wasatchian turnover pattern is the low number of last occurrences. This suggests that the Paleocene-Eocene transition in the Bighorn-Crazy Mountains sequence resulted not in decimation, but in expansion and diversification of the mammalian community. That the early Wasatchian mammalian radiation was associated with a gradual climatic warming has long been recognized (e.g.. Sloan, 1969; Gingerich, 1980; Bown and Kraus, 1993), and recent studies have confirmed a significant temperature increase at the Paleocene-Eocene boundary (Rea et al., 1990; Koch et al., 1992). It is reasonable to hypothesize that global climatic warming served to reduce the filtering capacity of high latitude intercontinental land bridges and thus facilitated land-mammal dispersal (e.g., Van Houten, 1945; Gingerich, 1980; Godinot, 1982; McKenna, 1983a; Krause and Maas, 1990). It also is possible, as Koch et al. ( 1992) speculate, that climatic warming was associated with an increased rate of morphological change in mammals, although this Ilab yet to be demonstrated. In any event, it is clear that the early Wasatchian of the Bighorn --C’rakyAMountains sequence was characterized by high first occurrence rates and important changes in fauna1 composition. The implications of these changes for the ecological structure of the mammalian community are considered elsewhere (Maas and Krause, 1994; Gunnel1 et al., this issue; Morgan et al., this issue).

Evidence for decline in richness during the middle and late Wasatchian is more problematic than that for the early Wasatchian increase in richness. Wa3 through Wa5 faunas are relatively well-sampled, but are still less well-studied than those of the Clarkforkian and earlier Wasatchian. Thus the middle Wasatchian drop in richness may or may not represent a real change in the mammalian fauna. The late Wasatchian, particularly Wa7, is not well known in the Bighorn Basin and thus

the current ret lrd i the actual regiona, possibility that dificrences in fauna1 c!,mposition and siiucture between intervals arc at least in part due to facies differe es among major localities cannot be ignored ( wn and Beard, 1990). In this vein, it is important to note that coeval faunas from other regions of North America do not show the same decline in richness. Rather, generic richness in North America as a whole increased throughout the Wasatchian and into the Bridgerian Land-Mammal Age (Stucky, 1990 ).

5. Conclusion Overall, the pattern of late Paleocene to early Eocene fauna1 turnover in the Bighorn-Crazy Mountains sequence is similar to that for the North American continent as a whole (Gingerich, 1980; Stucky, 1990; Maas and Krause, 1994). The major difference is the somewhat lower generic Bighorn-Crazy Mountains richness in the sequence, as would be expected in a more limited area (e.g., Flessa, 1975; Koch, 1987;Studky. 1990). The continental record, like that of the BighornCrazy Mountains sequence, shows a decline in standing richness between the late Torrejonian and early Tiffanian (but see combined species richness and relative abundance measures in Krause and Maas, 1990). Likewise, both regional and con+.inental patterns show a gener;L;lequilibrium between first and last occurrences during the Tiffanian and, consequently, a steady level of generic richness. Both records show an increase in richness during the early and middle Clarkforkian, as first occurrences exceeded last occurrences, and both show a more pronounced increase in generic richness in the early Eocene, spurred by high first occurrences coupled with low last occurrences. The BighornCrazy Mountains pattern departs from the continental pattern only in the later Wasatchian. In the Bighorn Basin, last occurrences exceeded first occurrences, leading to a decline in generic richness. The apparent magnitude of this decline is quite likely to reflect the still poorly known late Wasatchian in the Bighorn-Crazy Mountains sequence. For the North American continent as a

eneric

rences nlaiKltai~~edthe richness throughout the early

The overall similarity of the continental and basin patterns reflects the low fauna1 provinciality that characterized Paleocene and early Eocene North American faunas ( Stucky, 1990; Anthony and Maas, 1990). Although relatively few North American Paleocene or early Eocene mammals are known from outside the Western Interior, taxa that are known show affinities with other North American forms ( Simpson, 1932; Schoch, 1985; Beard and Tabrum, 1990; Krause and Maas, 1990). In any event, there is little evidence to suggest that patterns of fauna1 turnover and richness in the Bighorn-Crazy Mountains Basins were shaped by regionally-distinct evolutionary circumstances. The primary causes of fauna1 change in the Bighorn and Crazy Mountains Basins, as for the North Americ an continent as a whole, appear to have been the coordinated effects of climatic warming and intercontinental dispersal. The role of climate may not have been direct: late Paleocene through early Eocene climatic warming would have gradually allowed dispersal of a broadet range of mammals between continents via highlatitude filter corridors.

4cknowledgments We thank the AK. Behrsnsmeyer and C. Badgley for the opportunity to participate in the symposium “tong Records of Land Biotas: A Comparison of Wyoming--Montana Paleogene and Siwalik Meogene Sequences”. This paper benefited from discussions with C. Badgley, J.C. Barry, T.M. Bown, K.D. Rose, and J.G.M. Thewissen, and we thank C. Badgley, J.C. Barry, K.D. Rose, and R.K. Stucky for their critical reviews. T.M. Bown and K.D. Rose generously allowed us io use unpublished fauna1 lists for the central and southern Bighorn Basin. This research was supported by NSF BSR 87-22539 to DWK, DEB 92-11243 to DWK and MCM, and EAR 89-18023 to PDG.

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