ECOLOGY and the EXT]NCTTON of the DINOSAURS Robert E. Sl

ECOLOGYAND THE EXT]NCTTONOF THE DINOSAURS

Leigh Van Valen Robert E. Sl-oan Dept. of Biolory Dept. of Geologl and Geophysics University of Chicago University of Minnesota Chicago, Ill . 6063T l4inneapolis, Minn. 551+55 Received November 3, I9T,

ABSTMCT: The first d.irect evidence of the process of d.inosaur extinction comes from a sequence of vertebrate faunas in Montana, spanning the Cretaceous-Pal-eocene boundary. Much of the-physical, environment can be reconstructed. The extinction here lasted. roughly 10" years and proceeded by ordinary community evol-ution. Botanical- evidence shows a decline in winter temperatures and a replacement of much of the original subtropicaf flora by a temperate fl-ora. This southward.- moving community l-acked dinosaurs (and. marsupials) and. replaced the subtropical terrestrial community, apparently by d.iffuse competition. Mammalswere conunon but subord.inate in energll flov in the subtropical community; the temperate community included early members of the large Paleocene placental radiation. About seven d.inosaur species persisted d.uring the infiltratlon, but they gradually became rarer and then disappeared". The process implies that dinosaur extinction occurred. somewhat later in the tropics than in Montana. Some slight evidence supports this. The contemporaneous major extinction in the sea was also gradual and perhaps as long. ft is also erplicable by diffuse competition in an increasingly rigorous environment. *lcli

Di-nosaurs were spectacul-ar animal-s. Althougfu some were probably feathered q.nd gave rise to bird.s (Ostrom, 1971+), dinosaurs as d.inosaurs had the misfortune to become extinct. Until recently (t) ttrere has been effectively no actual evi- d"enceon the cause of this extinction, and speculation has been unbounded. We hope to show that the subject is amenable to serious scientific study and that the naJor processn at 1east, was ord.inary connunity evolution.

Geology and geography

Fig. 1 is a rough map of North America and Eurasia just before the end of the Cretaceous. Europe and Asia were both probably connected to North America but South Anerica was not, and a d.eep and broad seaway that had l-inked. the Gul-f of Mexico with the Arctic Ocean was about to be bisected. by sediments eroded from the rising Rocky Mountains (2). In the Hell- Creek Fiel-d. of east-central Montana, where we workedo land en- croached into the seavay although it may not have reached the eastern boundary until after a brief (and. perhaps worldwide) rise in sea level- in the early Pafeocene. Fig. 2 shows the stratigraphy diagralnnatically. The Hel-l Creek Fiel-d contains one of the best knovn terrestrial sequences in the vorl-d before the Pl-eistocene (3). The Pal-eoeenebeds have much lignitic coal, and many cl-assic d.inosaur specimens have come from the Cretaceous there. Pl-ants have been stud.ied extensively from both pollen and l-eaves, and- the nearby and unde::lying marine sediments and their fossils have been the focus of detaifed study afso. Sedimentation vas cyclic and more or l-ess continuous. Single cycles can be traced for many mil-es, especially in the Fort Union, and"the coal geologists *t6 *

Evol-. Theory Z:37-6\ (February, \977) The ed"itors thank R.T. Bakker, W.A. Cl-emens,and N.D. Newell- for help in evalua- ting this paper.

@ 1977 The university of Chicago VAN VALEN and. SLOAN

Fig. 1. Map of the northern part of the worl-d in the early part of the late Maestrichtian, about 65 mil-lion years B.C. in polar stereographic projection. ' The continents are in their most likely configuration at that time and l-and is distinguished from ocean. The large circl-es are approximate paleolatitudes. The edges of the modern continental shelves are al-so shovnl mod.ern coastlines are onitted to avoid clutter and confusion. Turkey, Iran, and Ind.ia vere still separate from the Eurasian continent (and from each other), and. fall outsid"e the map area. Unfortunately tvo important l-and brid.ges are inadequately documentedl on the possible seaway across eastern North America see Jeletzky (1971b). The aata come frommany sources. Dotted area : l-and. Dashed lines : future or intra- continental plate boundaries. X : Hell- Creek Field. + : North Pol-e at 66 U.y.B.C. ? : Possible seaways. A: Afaska. AB : Arctic Ocean Basin. B : British fsles. ES : Eastern Siberia. G : Greece. GR : Greenl-and.. I : Iberia. J : Japan. K : Kazakhstan. M : Mexico. N : Newfoundland. S : Scandi-navia. T : Tethys Sea. *lf# ECOLOGYAND THE EXTINCTION OF THE DINOSAURS 39

MONTAN A, DAKOTA

o- f o ' .. IJ E ..LEBg . FORMATTON: z o lrj CJ o CANNONBALLFORMATION trJ J (L

J.....z-coALTBED 'HELi,.' CN cneEi. ronuarron :f, o lrl C) F lrJ BEARPAWSHALE E. C)

Fie. 2, Diagram of the stratigraphy in and near the Hell Creek Fiel-d.. Non- marine strata are dotted. C : lenses of Colgate Sand.stone. ? : Continuity of marine strata is uncertain. *tt* have done so extensively. A single cycle is approximately synchronous throughout the region; the corresponding beds of tvo successive cycles are appreciably separated in time even if one bed. is in one area and the other bed is elsewhere. Bentonites, the remains of vol-canic ash fal-ls, sometimes provide time markers even far into the marine sediments. The Bearpaw Shale represents the open sea and grades laterally into the near- shore d.eposits of the Fox Hil-ls Sandstone. In some areas l-and sediments are included in the Fox Hil-l-s, but often there is a beach-sand d.eposit, the Colgate Sandstone, that separates the Fox Hifls from the fluviatil-e Hell Creek Formati-on. The Hell- Creek consists of flood-pIain deposits in which are interspersed sand.y rernnants of aggrading channels and point bars. The channels prod.uce local dis- conforrnities. Carbonaceous shales, loca1ly coalIy, represent times of relatively high sea level- and l-ow strea.m gradient. The first regionally persistent coal, the Z bed of the coal geologists, con- veniently marks the end of the Cretaceous in this area, as cl-osely as a correlation can be mad.e. The l-ast dinosaurs occur a l-ittle bel-ov it. More than 20 potassium-argon d.ates from it and from stratigraphically related. coal-s and bentonites to the north give the Z bed an age of 6h + l mi.Lfi.on yea.rs (Folinsbee et aL, 1965, LgTO; Shafiqullah et ar., 1968; Lanbert, l97l- a, b). Sedimentation seems to continue regionally uninterrupted into the Paleocene Fort Union Group, l+0 VAN VALIN and SLOAN of which the Tu]lock Formation is the l-olrest part although locally there are ' disconformities near the boundary as at other times. There were no hil-ls nearby. Hill-s form either from uplift (incl-uding vo1- canism) or from erosion of pre-existing high land.. Neither process operates on a delta-fike wedge of sediment that is pushing its way into the sea. The land was lov and fl-at, with braided or meand.ering streams fil-l-ing their channefs and cutting new ones. Ihe streams flowed pred.ominantly northeast; their sediment came from mountains rising near Yel-l-ovstone Park. The height of land at any point can be estimated roughly by the vertical d.istance of that point above the Fox Hil-l-s. Regional subsidence and compaction of the sediments reduce this figure by a factor of perhaps 2, but linrits can be set from the minimum stream gradient and the distance to the contemporaneous coastl-ine. Sedimentation exceeded the joint effect of subsidence' compaction' and" erosion, while the sea fevel- presu:nably fell in the Cretaceous part of the sequence. From this analysis we pred.ict that the Gulf of Mexico d.id not retreat more than about 2OO or 300 km. from the site of the Fort Peck Reservoir in the HeIl Creek Fieldo until- well- into the Paleocene. A very brief greater withdrawal' at the Cretaceous-Pal-eocene boundary, is neverthefess possible. A longer with- d.rawal woul-d.prod.uce a large regional d,isconfornity by erosion below the earlier base fevef.

The original animal- communities

fhere are two more or l-ess distinct animaL communities in the l-ower part of the Hell- Creek Formation. Tlrese are known from correl-ative and. somewhat earl-ier strata from central- Alberta to southeast Wyorning and. South Dakota. Farther south there is a geographic replacement by ecol-ogically sinilar but taxonomically somewhat d.istinct communities. One community was aquatic or semi"-aquatie. Estes and Berberian (1970)' Estes, Berberian, and Meszoely (tg5g), and. others have found the following taxa in this conmunity: the bowfin @!g, two sturgeons, a gar and the garlike genus Bel-onostomus, a paddlefish, a tarpon and several other teleost fish, two freshwater rays, two freshwater sharks, three crocodiles (one, Brachycha"mpsa, apparently a moll-usc-eater), the crocod.ile-l-ike eosuchian Champsosaurus, various poorly preserved shorebirds, a half dozen frogs, a half d.ozen aquatic and semi- terrestrial salamanders, and a half dozen or more aquatic and semiaquatic turtfes. Various unstudied mol-luscs al-so occur, and as usual softer-bodied invertebrates are not preserved. This community had. been evolving slowIy for millions of years, and it continued to do so until the Eoeene. It underwent no appreciable change at the cl-ose of the Cretaceous (l+). The second" connunity, the Triceratops community, was terrestrial, and also had been evolving slowly for millions of years. It contained 15 or 20 species of lizards (snall- to very large), two primitive snakes, a small cursorial crocodil-e, seven nul-titubercul-ates ( smal-l-, primitive, herbivorous mammals), about nine moderately conmon opossums, and. four or five rare insectivorans. The Triceratops cornmunity also contained dinosaurs. Tricer*pE- and the aucttitt anatosalr",rs were most abundant (5). other tertiGffier. the ornitho- pod Thescel-osaurus, a bonehead, (perhaps PachycephaLosaurus), and an armored- ankylosaur or tvo. Two coelurosarrrs were present: Paronychod.on and Ornithomimus or a close rel-ative. A spur-clawed dromaeosaurid represents a newly recognized group (Ostromo f959). Gorgosaurus was a larger pred.ator, and the Hell Creek is the home of TSrrannosaurus.

Abundance and d.ominance

The calculation of rel-ative abund.ance is a hazardous procedure. Elements other than teeth are not often preserved in an id-entifiable condition and, ECOLOGYAND THE EXT]NCTIONOF'THE DINOSAURS l+f gnlike the teeth of ma.rnmal-s,those of reptiles are replaced throughout the life of the aninal. Some estimate must therefore be made of the mean rate at which dinosaur teeth were replaced. The val-ue we have used is once a year, which is reasonabl-e by analogl with recent reptiles (nd.mund.,1960) Uut is not val-id.ated. The mean (not maximr:n) tltespan of ind.ivid.uals ol,d. enough to have recognizable teeth must also be guessed; for Anatosaurus and. Triceratops we guess 2l years, although this value j-s irrelevant to some of the cal-cul-ations, and for the mammalswe guess 0.5 year. The number of teeth in a jaw and the relative nurnber in the sedinent are known (6). On this basis it is possible to estimate, with large error, that the marnmalsoutnumbered the d"inosaurs in the Triceratops cornmunity by something like 300 to 1, a reasonable figr.rre by analogy with recent ma^una1communities. This suggests that the rarity of Cretaceous mamrnal-sin most col^l-ections is due to col-lecting or preservational bias rather than low population density. Relative d.ominance ( control of trophic enerry: Van Va1en, l-.9T3b) is another matter. Ratios of weights based on the l-inear d.imensions of load-supporting l-inbs (gakXer, I9T2) give about the same values as more direct estimates based on modern mice and Colbertts (f952) weights for d.inosallrs. The more common dinosaurs in the HelL Creek weighed. about 5000 kg., or nearly 105 tines as much as the average mammal-underfoot. Dinosaurs seem to have been active endotherms (e.g. Bakker, I9T2; Ricqlbs, 197\). If we take their metabolic rate to be that of mammalsof the same size, the standard 0.75-power proportionality with body weight (..e., Kleiber, 1961-) gives a d.inosaur about 4000 times the metabolic energy requirement of a contemporaneous mammal-. From avian evidence this applies to active movement as wel1 as to basal- metabolism, but it excludes growth and reprocluction. By analogy with nodern ma:nmalian faunas (Bourl-ibre, l-96\), there were roughl-y 3 dinosaurs per square kilometer. Ihe analory is based on the assumption that gross trophic structure of a cormtunity type is stable, as suggested by similar trophic structure of communities of d.ifferent origins and by return to earlier trophic strueture but with taxonomically d.ifferent organisms, after dis- turbance (e.e. D.E. Wilson, I9T3; Lein, 1972; Walker, 1972; Heatwole and Levins, 1972. Paine , I}TI; Olson , 1965). With a mean lifespan of 2l years, dinosaur growth was at e. mean rate of roughly 6OO t

Evolutlon of the animal communities In the upper part of the Hel-J-Creek Formation there are three faunas (fig. 3) vhich contain the earliest record. of the great Cenozoic diversification of \z VAN VALEN and SLOAN

LEBO FORMATION

TULLOCK FORMATION

Z COAL BED .HH BCW'.taO KS' oCC oMKH HELL CREEK FORMATION

BEARPAWSHALE

Fig. 3. Stratlgraphic cross-section of the Hell Creek Fietd between 1060 and 10To W, a distance of 72 km. Vertical distances are in feet from the Z coal bed. From Sl-oan ana Van Varen (irg6>). BCA (Bue Creek Anthill-s), SCWGug Creek West), HH (Harbicht Hil-l) and PH (Purgatory Hill) are localities with the protunzul-atum community. Other dots d.esignate mammal localities r,rith only the Triceratops commrmitY. ;** the placental- marmnals. There are the first ungulates (not yet with hoofs), the first primatesr and new insectivorans as well- as new multitubercul-ates and poikilothermic vertebrates (Sl-oan and Van Valen, 1965; Van Valen and S1oan, 1ge>; Estes an6 Berberian, 1970). How did. their entry affect the Triceratops community? The first thing to note is that the invaders were not rare. In Bug Creek Anthil-l-s (8) , tfre earliest and richest in specimens of the three faunas' the invading ma.mmal-soutnrmrber the indigenes by 20 to I. However, almost every species in the Triceratops commwrity occurs at Bug Creek Anthill-s. tt is not clear whether the two groups were always physically separated. by habitat differences, but it is convenient to refer to the invaders as the Protunqulatum connunity. This conununity occupied only suitable parts of the HeIl Creek Field' as it is absent from Brorrnie Butte and some other faunas l-ater than Bug Creek Anthills. We wil-l- refer to the time from the deposition of Bug Creek Anthil-ls to that of the Z eoal bed as Bug Creek time. There were no dinosaurs in the Protungufatqm community, although several- of the original sal-amanders and nu.l-titubercul-ates (and the terrestrial- crocodile) changed in frequency like the invaders ancl so presumably vere adaptable in rel-evant ways. The new comrnunity also l-acked. marsupial-s. At Bug Creek Anthill-s placentals already comprise a major, although not preponderant, part of the fauna. In the unlikely event that the preserved specimens quantitatively represent those dying in the general region, the mammal-s were already more or l-ess as d.ominant as the d"inosaurs, and the placentals had a major rol-e in this increase. More fikely there is a local- bias toward the pJ-acentals and other newcomers, perhaps a large bias, but their role in the regional energf flow in the Bug Creek area was already large and beca.ne larger. The Protungulatgm community gradually changed through tine (Fig. l+). The two species of ungulates initially present, one exceedingly rare' were joined. by three others. These other species are cl-osely related to the original two, but we can only speculate as to vhere they originated. The first primate ECOLOGYAND THE EXTINCTION OF THE DINOSAURS 4J

IMMIGRANT INDIGENOUS DINOSAUR

lrj z. lrl o o lrl

Y UJ UJ 6ttJ o= (9F = cl

Fig. 4. Change in observed mmbers of species of different groups in Bug Creek time. The vertical axis is proportional- to thickness of sediments and so, roughly, to time. Many more specimens are known from Bug Creek Anthllls than from the l-ater faunas, so later absence of some rarer species may spuriously contribute to a declining trend in the ind.igenes. The overalf picture is, however, unambiguous for each group. Abbreviations as in Fig. 3. **lf appeared., as d.id a new nu-l-titubercul-ate. Two multitubercul-ates evolved. into new species, anil other lineages a]-so changed morphologically. This community was evolving quite rapidly. In fact it seems to have given rise to most later placental- mammals. Ttre surviving multituberculates al-so d.iversified in the first half of the Paleocene before becoming extinct (Van Valen and. Sl-oan, L966; Sloan, unpublished). At the nearby focal-ities Purgatory Hill and Garbani (Van Valen and S1oan, 19611 Clemens, I9T\), ttre former of vhich we bel-ieve to be of late early Pal-eocene (about niddle Danian) age on the basis of lineages of the condylarths and mul-tituberculates, a normal Paleocene fauna like those known from areas to the south and west coexists with some ancestral genera that survived from the Cretaceous. The community change apparently occurred by both irunigration and local evolution. While the Protungulatum community grad.ually extrland.ed.,the Triceratops community gradually contracted. A bone that may represent the l-ast known pterosaur occurs in the Lance Formation of eastern Wyoming (Estes, 796)+; cf. Lavson, f97r), which is at l-east faunally correfative with the lower part of the Hell Creek Formation. Two or three of the insectivorans seem to d.rop out, as d.o a1l- but perhaps one of the nine marsupials, some of the l-izards, and two or three mul-tituberculates . The top carnivore, TSrrannosaurus, was rare j-n the l-ower part of the Hell- Creek Formation and does not occur at al-l in the channel- of Bug Creek Anthills. l+! VAN VALEN and SLOAN

Hovever, one specimen has been found" by others in l-ater sediments. Ankylosaurs and. perhaps the bonehead"had al-so become at least local1y extinct. The other dinosaurs persist at least to Bug Creek Anthil-ls, and about seven species beyond it, but they become rarer. By the time of the latest of the Bug Creek faunas, Harbicht Hil-l-, the relative abundance of Triceratops wa.s a tenth that at Bug Creek Anthil-l-s. Mammal-sv'ere by then the doninant vertebrates in community energy flow. The increasing scarcity of dinosaurs is al-so observabl-e in the remains scattered in floodpLain sediments, at least in the Bug Creek area, and gives quantitatively similar results. It is apparent that the grad.ual change of these comrnunities immed.iately precludes all possible catastrophist explanations for dinosaur extinction. The change was geological-ly rapid, but ecologically very s1ow. The time involved canrt as yet be estimated directl-y, but as a first approximation ve can assrune that the net rate of sedimentation vas the same for the Hel1 Creek Formation and the overl-ying Fort Union Group. The fatter effectively spans Paleocene time, about 10 million years, and in the Hel-l Creek Fiel-d is about l+50 m. thick. Bug Creek time, or the d.uratj-on of d.inosaur extinction in Montana, was then, very roughl-y , \oo , OOOyears (9 ) .

Plants and climate

Several studies (tO) trave been made of the l-eaves and pollen of the HeIl Creek Fie1d., rith some attention paid also to fruits and wood. Appreciable work has al-so been done on the plants of stratigraphically related. bed.s in nearby areas. It is possibl-e to make a coherent picture of these studies, although there j-s some conflict aJnongthe interpretations of the original lrorkers. We must accept responsibility for the folloving synthesis, some aspects of which are new although based on published. data. Before Bug Creek time, the deltaic envirorunent of the Hel-l Creek Field. existed in an equable, moist, subtropical clinate, although there were maJor storrns (Erickson, 19?\). Shoemaker (tg5g ) nas compared"the fl-ora favorably vith the mid-montane forests of Costa Rica and Sabah. It i-s, howevern unknown hov open the canopy vas, i.e. where it lay on the forest-savanna continuum (ff). Both feaves and po1len show appreciabl-e differences anong nearby local-ities in their flora, even vhen studied. by the sa.ne workers. The nature of this local- hetero- geneity is uncl-ear. Larger-scale heterogeneity, includ.ing latitudinal grad.ients, also exj-sted.: palms reached their northern l-imit near the southern part of the Hell Creek Field, although they extended into British Columbia on the Pacific coast (re). In the transition to the Paleocene, this flora und"ervent major changes. At least in some local-ities the ffora in Bug Creek time is intermediate between the earlier and later floras. There are several ways of l-ooking at the changesn and. almost all give a single concl-usion. The diversity of pollen d.eereases markedly in the transition. There is more than a )0 percent turnover in the species of both pollen and leaves. G;rmnosperms(and. ferns) increase at the expense of both monocots and dicots. Most taxa are either poorly id.entified. with recent taxa or the l-atter occur in diverse environments, but of those for which some compari"sonis possibfe most are tropical or subtropieal indicators before Bug Creek time, while afterwards more than tvo third.s are temperate indicators. The proportion of species vith entire-margined leaves decreases, but the proportion vith large leaves increases. A fl-ora where most dicots were evergreen gives way to one where there is a preponderance of deciduous species among the remaining dicots. Except for leaf size, alf these l-ines of evidence favor a decrease in vinter temperature. fhat is, vinters became more severe. The continuecl occurrence ECOLOGYAND THE EXTINCTION OF THE DTNOSAURS ll\

1.,lI

Fig. 5. Hypothetical- reconstruction of a typical scene in the Hel-l Creek tr'ield before Bug Creek time. Note a lO-meter-long Trice.ratops skulking in the under- growth *** of rather large crocodiles, turtles, and charnpsosaurs in Montana and Alberta somewhat suggests the continued occurrence of winters vithout much frost. Stream d.rainage may also have become poorer. ,Because the northern l-imit of temperature-controfled d.istributions is set by the vinter ternperature and related effects, nothing can be said about any change in the sulnmer temperature from shifts in northern limits. Southern limits of temperatr:re-controlled. distributions are more ambiguous, but the imnigration of some more northerly species suggests, if anything, a l-over sutrmer temperature (Conolly and Dahl-, 1970). Therefore Axelrod and Bailey's (f968) concl-usion (on much less evidence than ours) ttrat the equability (13) d.ecreased is an unsupported possibility. Figs. 5 and6 depict visually the general habitat change. Ttrere is much other evidence, of diverse kind.s, that bears on cl-imatic change at the close of the Cretaceous (e.g. Saito and Van Donk, I9T\; Krassil-ov, 1975). We defer its presentation and eval-uation to our longer treatment, as ve d.o much else, but it predominantly supports the picture we give here and extends it vorl-dvide. Fig. 6, Reconstruction of an early Pal-eocene scene in the HeJl Creek Field. The spacing of trees is based. on a plane-table mapping of a petrifietl forest in the Tul-l-ock Formation. Dralrn to the same scale as Fig. 5. tc* *

Dinosaur extinction in Montana

So what happened.? Winter temperatures became more severe. Tlri.s caused the local extinction of many subtropical plants, and a temperate forest moved.south. This forest .l-acked dinosaurs, and- as it infiltrated the existing biota the dinosaurs found it progressively less easy to cope (lh) witfr their nev biotic environment. But why did the temperate forest l-ack dinosaurs? This is uncl-ear and perhaps will remain so until- the origin of this cornnunity is d.iscovered. We will nevertheless evaluate four hypotheses. Perhaps the lover productivity of a temperate forest could.nrt support dinosaurs. But the difference today is rel-atively smalt (Cotley , I9T2; Lieth, I9T3; Rod.in, Bazilevich, and Rozov, 79Tr), so some species should survive (ff). Perhaps the dinosar:rs cou-l-d.nrt eat the temperate vegetation. Moreovero the increase in decid.uous trees would make l-ess food avail-abte in winter. But the teeth of the herblvores comprised batteries anatomically effective for crushing, and Anatosaurus is known to have eaten coni-fer needles. Presumptive- ly decid.uous tress d.id.ntt come to d.ominate the vegetation. With respect to digestiono chemical defenses at least today seem to be more characteristic of tropical vegetation than of temperate (Levi-n, f9T6). Moreover, dinosaurs l-ived all- over the vorld, in widely d.iverse fl-oras, and had survived. without notice- able harm through one of the great floral turnovers of the history of the earth. About 30 million years earfier the angiosperms had d.iversified and spread" ECOLOGYAND THE EXT]NCTION OF THE DINOSAURS )+T

Vorldwid.e, replacing a worl-d fl-ora dominated. by cycads and other pre-existing groups. That d"inosaurs flourished. and. rad.iated exhuberantly throughout this transition makes implausible a d.ietary ertrllanation for their absence from a temperate forest. Perhaps dinosaurs were excluded from the temperate forest by a direct effect of temperature, an insuperable physiological threshold. Reptiles today, espe- eially ones too large to burrov, are concentrated. in warmer areas. But the probable endotherrny of clinosaurs makes an arguaent by analory suspect here. Although a few probably had. feathers (Ostrom, fgTL+), Just as pterosaurs had. hairlike insul-ation (Sharov, 197f), Anatosaurus, at leasto had neither, But neither d.o whales that live in icy water, which is a much better heat conductor than air. The presence of insul-ation like that of whales, or an analogous ad.aptation, cantt yet be tested for d"inosaurs (particularly for young ones, for whom it woul-d be most useful-). Although the Arctie is poorly known paleontologically, the d.inosaurs farthest north by Cretaceous lati-tudes seem to be from northeastern Yukon (Rouse and Srivastava, I9T2), at about T5o N paleolatitud"e (16). 75o N today is wel-l north of Baffin Land., Alaska, and all but the Taimyr Peninsu-l-a of Siberia. Late Cretaceous floras from Alaska and Siberia suggest that a moderately large area was temperate (e.g. Samoilovich, 7967; Zaklinskaya, 1970; Smiley, I9T2; Stanley, 1972; Krassilov, I9T5). However, the Cretaceous fl-ora of the Yukon and nearby was more similar to the Cretaceous flora of Montana than to the Montana Paleo- cene flora (Rouse and Srivastava, l-972; Bihl, 1973). The Triceratops community survived for an ecologically very long time on flat terrain in close proximity to the invading Protungulatum community with its flora. ff the invaslon was climatically caused.n the Triceratops community was able to tolerate the clinate causing that infiltration. And. the marsupials of the Triceratops cornmunity became almost as extinct as the d.inosaurs; some indigenous placentals and^mul-tituberculates also d.isappeared. Most inportantly, perhaps, the hypothesis of a direct effect of temperature cantt easily explain the extinction of dinosaurs in areas which remained tropical. A fourth and. final hypothesis is primarily historical rather than functional. Perhaps in the fornation of the Protungul-atu:m community no d.inosaurs happened. to participate even though they coul-d. have d.one so. In other word.s, their ex- clusion was d.ue to l-ocal- effects and what species were available rather than to overal-l properties of dinosaurs (17). This hypothesis is hard. to test in the absence of a record. of the earl-ier history of the Protungulatum community, and it lacks d.irect evid.ence. It nevertheless is the only hypothesis that seems plausible. A perhaps rel-evant iten is that all the placental mammals in the Protungu- ]atum cormunity seem cl-osely related to each other. fhis is true despite their being classified. into three orders because of the evol-ution of their d.escendants. By thenselves, the insectj-voran Procerberus, the primate Purgatorius, and the condylarth Protungulatum could justifiably be placed in the same family. It may be thatn a geologically short time earlier, a conmon ancestor of these placentals developed. some kind. of ad.aptation that pernitted more effective use of a temperate forest. Perhaps it vas chance that this happened. to a placental instead of to a marsupial, a multitubercul-ate, or a dinosaur. (n:e temperate cornmunity did share some multj-tuberculates r,rith the subtropical conmunity.) Or it may have been rel-ated. to a peculiarity of placentals, or to an immigration of a lucky species from the continent of Europe plus eastern North America or from elsewhere (f41. Nothing requires that every community contain d.inosaurs (19). A more effective use of the resource space by these smal-l ma,nmal-smay well have prevented the invasion of the corrnunity by other vertebrates and. may have l+B VAN VALEN and SLOAN caused. the extinction of some already present. Presr:mably such effects would" be by diffuse competition rather than by one-to-one face-offs. Such trophic competitive processes require that the ultimate regulation of the d.ensity of at least most of the competitors be by food. Hairston, Smith, and Slobodkin (fg6O) have argued. that this is unlikely for most herbivores, but we have given gen- eralJ-y applicable evidence that even the herbivore trophic l-evel- is ultimately regulated by food (Van Valen and Sl-oan, 1956; Van Valen, l9T3a). Just how conpetition ordinarily occurs arnongherbivores remains rgrsterious, and this is quite as true for the end. of the Cretaceous as it is for conmunities tod.ay. A major part of the flora canrt be adequately reconstructed., so reconstruction of d.iets on the basis of d.ental- adaptations by analogl with recent mammals is even more tenuous than usual- (eO). Most resource use should have no necessary relationship to body size; Webb (tg6g) tras in fact suggested a partial replacement of ungulates by rod.ents in the l-ate Cenozoic, Stone et a1. (tg>t) gave evidence for corrpetition between rats and Drosophila, and Brovn et al-. (fgTr) do so for rodents and ants (2t). Possibly competition with young dinosar:rs or even baby ones vas critical . If dinosaurs kept mammals from rad.iating vid.ely and from becoming d.ominant in energll flow d.uring their l3O-million-year coexistence (a piece of conventional wj-sdon which neverthefess seems likely) n this requires that the rwo groups were marginally in competition for that long interval-. Pred.ation is unlikely to have been critical because man:nal-scould have become energetically d.ominant whil-e remaining small-, as insects have d.one. There is therefore no conceptual d.ifficulty in thinking of mammals outcompeting dinosaurs when the mammalsbecame better by their rapid. evol-ution at the end of the Cretaceous. The extinction process rras similar to an ecological- succession. One community gradually infil-trated and replaced another. The replacement doesnft satisflr all of Odumrs (t959) or Margalefrs (f968) criteria of successions, but neither d.o many ordinary successions. No mysterious processes need. to be postu- l-ated. The extinction rates were much slower than those of the drastic extinction of large terrestrial vertebrates after the latest glaciation (Martin, l-968), sl-ower by a factor of perhaps 100 to 1000. The minimrur d.ensity consistent vith survival is a greater proportlon of the usual- density for large animals than for smal-l ones. Large animals are scarcer and breed more slowly. And so for the d.inosaurs: they coufd malntain only progressively sparser populations j-n the face of a community infiltratihg and. replacing the vegetation they had eaten for millions of years. Eventually some mi-nor crisis was too much. What crisis this wa,s may have been different for different speciesn or it may have been something like the coincidental super- nova postul-ated. by Russel] and Tucker (fgff) and others. It doesnrt matter muchl competition doomed their d.eath and any final executioner would. do.

Di-nosaur extinction elsewhere

The area of the present-day tropics obviously d.id.nrt lose its d.inosaurs in quite the same way Montana did. A temperate forest has never reached. Brazil or central Africa. As vith the origin of the temperate forest community, we wonft really knov vhat happened in the tropics until- someone find.s an appro- priate sequence there and studies it ecologically. But here, at least, there are some facts. One fact is that no dinosaurs have been found. in sed.iments definitely known to have been deposited after the end of the Cretaceous. Another fact is that all- over the worl-d., except on islands (ZZ), diverse pJ-acental mammal-soccur in the first post-Cretaceous sediments that contain terrestrial- vertebrates. And everywhere many of them are of kinds that seem descended. from members of the ProtunFulatum cornmunity. ECOLOGYAND THE EXT]NCT]ON OF THE DINOSALAS )+9

In most places, unfortunately, the gap between the end of the Cretaceous and the first Cenozoj-cvertebrates is large. The Hefl Creek Field is the only knovn area with & more or less continuous sequence, and even here the first Cenozoj-c faunas are not much less than a nill-ion years after the boundary. The gaps elselrhere betveen the l-ast known dinosaurs and the first knovn terrestrial Cenozoic vertebrates range from 1 or 2 nil-l-ion years in New Mexico and Utah, and not much if any longer in vestern Europe, to about 15 mil-lion years in all- of Africa (on which see Gevin et at., 1975). It is afmost necessary to infer that the rapidly evolving placentals escaped the lirnits of their temperate forest and expand.edworl-dwid.e in a geologi- ca1ly short time. In doing so, they may well have outcompeted. the dinosaurs in the d.inosaurs I home grounds. This hypothesis requires, and it may be taken as a pred.iction, that dinosaurs sr:rvived longer in the tropics than in Montana (e:). Two fl-imsy data support the prediction. The only hadrosaurian di-nosaur described from the Southern Hemisphere (Casamiqueta, L965) was found. in beds dated provisionally as Danian (early Paleocene). Cox (f97l+) tras noted that this is the only Southern Hemisphere record. of a d.inosaur of any of the several- characteristically Northern Hemi- sphere families. Other dinosaurs are, hovever, vell- known from the Southern Hemisphere. Many nonmarine molfuscs entered South America in the early Paleo- cene (Parod.iz, 1969) and a d.iversity of ungulates derivable from Protungulatum occur in the first definitel-y Cenozoic vertebrate deposits of South America a few mil-l-ion years later. The Bombacaceae arrived, apparently from North America (Wotfe, IgTr), ly the midd.l-ePal-eocene (Gerrneraad, Hopping, and Muller, f96B). The South American crustal plate was at the end of the Cretaceous probably overthrusting the Caribbean (e4), whose geotogical history is stil-l- very poorly known. The hadrosaur may well have entered South America from North America at this time of relative approach (and perhaps junction) of the two continents, together with the mol-luscs and marrnafs (e5). It d.id not survive long with the invading mammals. The second. flimsy datum concerns a mammalian fauna from Laguna Umayon Peru (Grarnbast et al., t96T; Sig6, 1958, l-.97I, f973). This fauna contains eggshells reported to be those of d.inosaurs, charophyte oogonia dated as late Cretaceous, several- marsupials, and. a placental. The placental, Perutheri-ura, is an ungulate decid"edly more advanced. than Protungul-atum; by North American standard.s it is of about the grade present in the late part of the early Pal-eocene or a l-ittl-e later. At l-east one of the marsupials is simil-ar to those that had. been present in North America for millions of years in the Tri-ceratops community and before (Fox,197fb; Sahni, L9T2), and Sig6rs id.entifications of the other marsupials support this view. Unl-ess there was a long-lasting or repeated l-and. connection between North and South America, vhich is unlikely on both geological and zoo- geographic groundso the ancestors of these marsupials immigrated to South America with the placentals. We prefer the evidence of Perutherium to the reportedly much more arnbiguous evidence of the charophybes in deternining the age of the Laguna Umayo Fauna, and so assign it a Pal-eocene age. ff the eggshells there really came from d.inosaurs, the dinosaurs were then Pal-eocene (26). The absence of marsupials from the temperate forest (and from knovn Creta- ceous faunas in Asia and Europe, unless Deltatheridir.rm really vas a marsupial), and their restriction to subtropical and presumably tropical- communities, helps explain the present d.istribution of marsupials. Unfess marsupials had. been evolving in South America (Tedford., L9T)+), as is possible if unlikety, they came from the geographically ad.jacent part of North America, where they were the predominant therian mansnal-s. The Austral-ian marsupiaf s are al-so d.erivable from those knovn in North America at the end of the Cretaceous (Clemens, 1968), 50 VAN VALEN and SLOAN

so a further er;lansion into Antarcticaustralia is plausible. ff separation from South America occurred shortly afterwards, as may have happenea (22), the placentals of South America were stil-l perhaps too rare or l-ocal to have followed., or the marsupials may have crossed. a vater gap. Possibly the placentals followed anlrway and were later exterminated before the first (about late Oligocene) known mammalfossils of Australia (27), but otherwise the d.ominance of marsupials in Australia is both provisionally explained" and relevant to the exbinction of dinosaurs there. An inmigrating vave of marsupials coul-d perhaps d.o as much damage to a coadapted fauna of dinosaurs as coul-d a wave of placental-s. But we wonrt knov what real1y d.id happen there, and vhen, until- evidence from the Austral-ian record is available.

Extinctions in the sea

The great dying affected also organisms that lived in the sea. In fact extinction here was at least as severe as that on land. About p0 percent of the specific lineages of coccol-ithophores (ttre dominant planktonic algae) became extinct (Bra"mlette and. Martini , 196+; Perch-Niel-sen , 1959, I9T2) . So did 13 of the 18 generic lineages of planktonic foraminiferans, the nexb l-evel- in the food. chain (original analysis). Most groups of marine reptiles and all ammonites completely vanished. And many other organisms did too, although not al-l- at quite the same time (Kauffinan, 1973b). Very roughly half the generic or famil-ial lineages of marine organisms disappeared. in the l-ast 5 million years or less of the Cretaceous. This extinction has some ecological regularities, each with exceptions. Ttre plankton and the whole pelagic food chain, which depends on them, suffered severely. So did fil-ter-feeders on the sea bottom. But sea-bottom pred.ators and detritus-feeders in the mud had l-ittfe l-oss n and groups usually resistant to physical- stresses fared. better than those r,rith a narrow tol-erance. The pattern differs from that of the l-ate Permian, when a restriction of the area of shallov seas seems to have been the main control- (Newell-, 1963; Ruzhentsev and Sarychevao 1965; Schopf, 797\; but see Boven, I9T5). Although some of the l-ate Cretaceous exbinctions took place over a few mil-l-ion years, both on land and in the sea there was a geologically short but ecologically long interval vhere a major crisis occurred. For the sea this is best shown in vork by Percivat (t9TZ; Percival and Fischer, L97T), who studied an apparently continuous section at, Zumaya j-n northern Spain (see al-so Hil-l-ebrandt, 1965, and Herm, 196r, for Zumaya foraminiferans, and Worsley, 1970, pp. 28-29, for a generally sirnilar result in Alabama). fhe tine scal-e is l-Oa or l-05 years,like that for the dinosaurs in Montana. The planktonic foraminiferans gradually d.ecreased.in size and" al-l- the larger speci-es grad.ually becarne extinct. The absolute number of planktonic foraminiferans, estimated from the ratio of planktonic to benthonic ind"ivid.uals ( eB) , decreased. grad.ually by a factor of l-000 if the nrimber of benthonic ind.ividuals remained. constant. Somevhat after the start of the foraminiferal decl-ine, there was a more rapid but gradual decl-ine in the number of species of coccolithophores. It is wrknown whether there ldas a concomitant or anteced.ent d-ecl-ine in the nrmber of individ.uals (Zg) . For both foraminiferans and coccolithophores, some of the species of the early Pal-eocenewere present but rare before the end of the crisis. Both groups took a nil-lion or more years to reeover, as vas the case in Montana for the re-establ-ishment of a more or l-ess balanced terrestrial_ cornmunity (fo; . ECOLOGYAND THE EXTINCTION OF THE DINOSAURS 5t

Synthesis

As normally happens d.uring a regression of the sea (Kauffman, l.]TO), there were many marine extinctions during the l-ast few mil-l-ion years of the Creta- ceous. About 105 years before the end, there was a geologically sud.den decline in temperatr:re, apparently worldwide. The cause of this is unknov-n although several possibilities exist (31). Other physical or chemical changes in the sea occurred at about the same time and may well be interrelated (tappan, t968; Wors]ey, I97O, f971). Perhaps cold bottom water reappeared briefly (SZ). The plankton and species dependent on the plankton vere drastical-ly affected; nost others in the sea were not. 0n land, the fal-l- in temperature caused a southward movement of cornmunities. One of these, which had placental mammals instead of dinosaurs for perhaps historical reasons, slowly replaced a d"inosaur-dominated community. Ttre diver- sifling placentals soon escaped and outcompeted. the dinosaurs everywhere. So how did. the vorld end? Not with a bang, not even vith a whimper, but with, of all- things, a slow plant succession, which pernitted. the placental mammal-sto d"iversify. And. soon they took over the earth.

NOTES

(f) fnis presentation is abstracted from a much longer and documented version, most of which was done by about l-?il. (fne lact of publication is due to the unavailability of a vay for persons without the right institutional- connections to publish monographs in this country. The problen nay possibly be solved now for this monograph. ) We have presentetl our results on many occasionso publicly and privately, in this interval. The only published ab- stracts are Sloan (fg5)+) and Van Val-en and Sloan (l]7Z), but a sunmary of or:r results (attributed to another paleontologist) vas given on the BBC television d.ocr.mentary The Dinosaur Hunters (ryfZ). Our work in Montana, at first ind.ependent but soon coal-esced, had two main purposes: to find. Cretaceous ancestors for the Cenozoic radiation of the placental manma"l-s,and to look, from the viewpoint of mod.ern ecolory, at what really happened. at the d.inosaur extinction. Both purposes were satisfactorily real-ized (cf . Sloan, 1970) . Many people have helped us in our work. We thank them all here, and vill- try to do so individually and more ad.equately in the longer version. (2) Pertraps other causes also operated, but Jeletzky (19T1+)has concluded. that local events predominated at least in Canada in the Cretaceous. ( 3) aspects of the geology of the Hell- Creek Fiel-d. and nearby areas have been given in the following publications, in addition to our or,rnwork and" papers mentioned elsewhere: Bauer (tgz\), B. Brorrn (rgo7, l-91-l+),R.w.Brown (1938, 1952), R.w. Brown and Lindvalt- (r9r3), collier (t9t]), colton (t)jr), Colton and Bateman (t956), Dobbin and Reeside (1929), ret-dman (l?Tz), Fox and otsson (t969), Frye (ry5f , 1959), Gerhard (1953, 196T), cit-t- and cobban (rgf:), Jensen (rg:r), Jensen and varnes (195\), Johnson and smith (r96t+), Knowlton (tgog ), Leonard (r9tr), lindbere (r9U) o Malt-ory (:-:972), Miner (rg:5), Perry (lgZS), stanton (1909), Thom and Dobbin (l9e\), waage (1958), and yen (fgl+B). References to several- dozen reports on coal geolory can be found in Bryson and Bass (rgtS). (l+) Sug Creek represents the last record.ed. occurrence of Belonostomus and its small- ord.er; this is ordinary slow community change. (5) Anatosaurus seems to have been partly aquatic, as shown by the webbed feet of mr:mmiesand. the d"uckl-ike bill in soft tissue as welf as bone (Morris, 52 VAN VALEN and SLOAN f970, 1971). However, the only reported stomach contents are conifer needles and other remains of land plants (KrH,usel, 1922), and Ostrom (fggh) nas gtven other evidence for en important, terrestrial component in its Life. (5) fire estimates are based on specimens collected. by washing from channel deposits. They are therefore subJect to the biases of stream sorting. Hovever, minute rnul-tituberculate teeth are much conmoner than the more consplcuous large bone fragnents, so the overall- blas here is presumably not large relative to other sources of error. (7) UnpuUlished calculations by Van Valen for various recent ma.unal communities give values on both sides of 10 for the ratio of enerry used by large ma.n- nals to that used by smal1 ma;rmals. This ratio was not used. in deriving that for the Cretaceous. (B) nug Creek vas na,med.by a local rancher after Barnun Brown took out t'bug" some big bug in 1906. The narnesake happens to be the mounted skel-eton of Brrannosaurus in the American Museum. Harvester ants (Pogononyrnex owvheei at Bug Creek) d.ecorate their anthills with snal1 mannnalteeth as well as other resistant obJects. The 20:1 ratio of invaders to indigenes is based. on the ratio of speeinens of invading marnmalspecies, to specimens of marnmal-species that are restricted to the Triceratops connunity. The data are in Sloan and Van Valen (tg5>) anil are for thousands of specinens from Bug Creek Anthill-s. The invaders are Stygi]nJs, Catopsalis, Procerberus, and Protungulatum, which constitute 53.32 p"r"""t. Th" i"dis."e" totarTlgS p"t"@ and cimexonvs, which occur in both conmunities, are excluded. ) ftre ratio is 18:1, which we round to 20:1 because of probable inaccuracy. (9) Aftfrough some Fort Union coal-s are thick and surprisingly free of lransported sedinentn a decrease in the estimated duration of Bug Creek tine by as rnuch as a faator of 10 is unlikely. Perhaps l-00r000 years would be a better estimate than l+001000; this is also more consonant with an estimate from marine strata (Worsley, 19?0). (tO) Sone relevant botanical studies of the Hell Creek Field are the folloving: R.w. Brown (tgZg, 1962), Dorf (tg\z), shoemaker (J:965), Ha1I and Norton (tg6t), Norton and Hat-t (tg6t, 1969), oltz and Hall- (rggA), ottz (tg6g, 1971), Tschudy (rgfr). (ff ) Wfrat the herbaceous stratum was at this'bime, asid.e from ferns and a few nongrassy monocots, remains a nystery. Most recent herbaceous angiosperns seem to have originated. later (Uutter, I?TO), and aL:nost none occur in the Hell Creek Field. Possibly the reputeclly extinct Normapolles, a diverse pollen group largely restricted to the contiguous land of Europe and eastern North Anerica, was largely herbaceous, but this d.oesnrt help much in Montana. Pos- sibly such exbinct genera as the abundant Aquilapollenites provid.ed the niss- ing stratum, but there is no positive evj.dence. Pollen of herbaceous plants ls normally rare in sed.iments. This may af fect Mul-l-erI s stratj.graphic records as well as the observed floras from Montana. Monocots were, however, well established, and their basic adaptation is to a herbaceous habit (Cronquist, 1974). (fa) me l-atitudinal- d.ifference between British Columbia and Montana lras gpeater than it is today, so the greater northward erbent of palns on the Pacific Coast inplies greater equability there in the Cretaceous as well as today. (rg) saileyts (1950) measure of temperateness or equability, based. on d.epar- tr:re from the mean temperatr:re of the earthrs surface in the first half of the twentieth century, is inappropriate for most biological applications. It can be replaceci by a direct measure of temperature variation, such as the annuaf variance. The annual mean and extremes are of cor:rse also often important, but d.ifferent organisms are adapted d.ifferently. ECOLOGYAND THE EXTINCT]ON OF THE DINOSAURS

(rh) Uven outside the Marshes. (f5) foaay the vertebrates of rain forests l-ive aLmost entirely on the production of the canopy rather than on that of the toxic understory (Bourlibre, l9T3). We d.onrt knov the situation in the Cretaceous, but ve suspect that Triceratops wasnrt arboreal. --llf,)-Df1osaurs occur in Spitzbergen in early Cretaceous rocks (ae lapparent, 1960, 1;962), but tectonic reconstruction places Spitzbergen at a latitude of 5Oo or 50o then. (fT) Wirat species vere available for inclusion in the temperate community coufd neverthel-ess be related. to their average properties. Both marsupials and dinosaurs may have been predominantly tropical and. subtropical, and" therefore l-ess likeIy to participate in a temperate community than placentals. There is, however, no positive evidence for dinosaurs being more important in tropical areas. And.marsupials, at least, are abfe to overcome such restrictions, vhich are at l-east for narsupials as a vhol-e ecological- rather than inherently physio- logicaI. (f8) ft is possible, if unlikely, that the water gap betveen North America and most of Asian the deep-water part of which now lies in the Verkhoyansk Mountains of Siberia, didntt close until the Cenozoic, although the orogeny there was in the early Cretaceous (on the latter, see Churkin, 1972, and Herron, Devey, and Pitman, 197\). Each possible result here lrould be incompatible vith some of the recent anti-ecological zoogeographical speculation on Cretaceous interchange across this region. (ft is also rel-evant to such specul-ation that the Turgai Straits in Kazakhstan vere only episod.ic in the early Cenozoic and so werenrt a compl-ete barrier fVinogradov, 1967, 1968, I97r]). Possibly the Okhotskian Belt of Nalivkj.n (t952), which lies farther east but extend"s farther south, is associated with later movement in conJunction vith the Pacific p1ate. The northern boundary of this plate seems to have shifted. south to the Aleutian Trench at about the end of the Cretaceous (Sctrott, Buffington, and Marlow, f9T5). Our reconstructed. plate movements, based on evid.ence from the Atl-antic, fit eastern Siberia awkward"ly. Its geographical- history needs fr:rther study, in- tegrated vith evidence from other regions. (f9) ffrat placentals were in some way preadapted to the temperate forest is suggested by a roughly simul-taneous diversification of placentals in southern Alberta. Such preadaptation coul-d be related to temperature, the lower pro- d.uctivity of northern floras vith less annual sunlight, possibly greater ad.aptability of placentals, or various other untestable causes. At least most of the lineages in Alberta differed" from those entering Montana, and- in this case also most of the l-ineages are interrel-ated (Lillegraven, 1969). They too diversified. into the Cenozoic. However, the putative ancestors of the Alberta rad.iation were already present in the Triceratops community earlier, and" they were abl-e to adapt rapid.ly to a possibly ehanging flora. (fne botanical evidence fron Alberta is too poor for a stronger statement, and- more work needs to be done there.) Censuses of specimens by locality arenrt avaifabfe for Alberta, but a total- count fron Lillegraven's paper gives 286 placentals, 322 marsupials, and" 255 mul-tituberculates. Prolably the rad.iation of placentals in Alberta occured before the ,Pro.!un@gm community inmigrated. Possibly it was a parallel response to the appearance of a temperate forest. This l-ine of speculation can be carried. further but seems too insecrirely based f,or elaboration. A montane origin for the temperate forest in Montana is unlikely because this forest occurs perhaps 100 km. from the eplcontinental seacoast before it occurs farther inland, on the basis of a few l-ocalities. Perhaps it came from Greenl-and.. The evidence for a southvard movement of the Protungul-atum conrnunity is the strong spatiotemporal coincidence of its appearance vith the appearance of a cooler flora. 54 VAN VALEN arrd SLOAN

(eO) Anus had.rosaurs and. ceratopsians vere dentally ad.apted.to resj-stant food., but Protungulatr:m and StyeiqVs might have been too because smal-l- mamtal-s have less need for a special ad.aptation here than d.o large ones, partly because of the all-ometric rel-ations between tooth size, body size, and food. intake, and partly because of their shorter l-ives (Van Valen, 1960). More l-ikely, the di.nosaurs were eating more resistant parts of plants than were the marunal-s. Ttre opal phytoliths that make grass leaves so abrasive presrulably were absent before grasses and sedges originated in the Cenozoic. If d"inosaurs emnched. branches or stems as wel-l as leaves, this may itself explain the difference in dental adaptation. Such a dlfference in diet woul-d be irrel-evant to competition if both groups ate the same plants. Such arguments as this can merely establ-ish plausibility rather than probability, but a plausible interpretation of super- ficially eontrary evid.ence is he1pfu1. (ef) Van Valen (tgl>) nevertheless gives evidence suggesting that, despite individual counterexamples and contrary theory, most interactions of at l-east marnmalsand foraminiferans are with species of a generally simil-ar size. (eZ) nustralia and Antarctica, still connected. to each other in the Paleo- ttisland.tt cene, were far from Asia and Africa. They were an in the sense that South America was one for most of the Ceaozoic, being isolated from evolution elsewhere. The time of the separation of Antarcticaustralia from South Amepica is still uncl-ear (Oatziet et al,, a9T3). South America and Antarctica are now farther apart than they were in the early Cenozoic and Cretaceous (Dalziet and Elliot, 1973). The initial- bending of the Scotia Arc, which joins these continents, was afso no earlier than l-atest Cretaceous (natziet and Elliot, fgrc). The presr:mptively earliest Cenozoic mammal-ianfauna of South Anerica, that of Laguna UmayonPeru (on which see Sig6, l-.973), consists largely of species of marsupials,'with an even greater prepond.erance in numbers of ind.i- viduals. (T}re faek of multituberculates here as in Australia is, however, striking. ) It is therefore plausibl-e but unproved that a path to Australia was open long enough for southern derivatives of the tropically concentrated. marsupials to come but closed before placentals diversified in South America. Sweepstakes dispersal via islands is al-so possible. Ttre Protunzul-atr:m corrnunity was very probably absent from the South Anerican Cretaceous. Apart from its not having been found", its descendants are d.iverse in Holarctica but in South Arneri-ca come from only one of its lineages until the Oligocene inmigration of primates. (e3) Stone and Langston (J-!lS) have Just reported a d.inosaur from southwest Texas that is associated. vith a Paleocene-l-ike pol1en flora. Ttris coul-d. well be a southern exbension of the flora that invaded Montana, but presumably is slightly l-ater. If so, dinosaurs survived longer even in Texas than in Montana. ttftre discovery of dinosaurian remains in Tertiary d.eposits, or of trilobites in the Permian, should give far l-ess cause for surprise than a positive an- nouneement that they d.id.nowhere so occurtt (Heitprin, 188J, p. 201+), (el+) fectonic references on the relative approaeh of North and South America at the end of the Cretaceous are Beets (lrgfZ), Bell (tglZ), Bellizzia (tgtz), Freeland and Dietz (;.glz), Harvey (tgtz), Mauait and Dinkel-man (r97e), and Marescfr (fgfh). Shagam(WfZ) in particul-ar notes that at the Cretaceous- Cenozoic boundary the tectonic envirorunent changed from tensional to com- pressional across the entire northern bound.ary of South America. The isthmus- forrning position of southern Central- America is unl-ikely to be simply chance, and a similar relationship might have appeared. at the end of the Cretaceous, perhaps in rel-ation to the col-liding borderland. of Maresch (197\). An Antil-lean connection isrhowever, a]-so possible. Fox and. Heezen (l]75) note that there was "significant plate consumptiontt north and south of the Caribbean until the Eocene, and that the whole Caribbean basin may have been epeirogenically ECOLOGYAND THE EXT]NCTION OF THE DINOSAURS qq uplifted to some extent in the l-ate Cretaceous and early Cenozoic. The najor orogeny in southern Central America began about the end of the Cretaceous (Dengo, I9T3). Scotese (tgt>), in a set of computerized. reconstructions of eontinental- segments, shol,rs a d.istance between North and South America at 50 mil-l-ion years simil-ar to that today. Kauffhan (1973a), finding a progressively increasing endemism of clams on each side of the present CentraL funerica as the Cretaceous neared. its end, postulated a landbarrier there. Andn if senrm albumin evolved at a constant average rate, the best estimate for the time of divergence of North and South Amerj-can hyline treefrogs is 55 nillion years (Maxson and Wilson, 1975), very cl-ose to the tirae suggested. for mi-gration of manmals and land molluscs. (e5) Haarosaurs may of course have been in South America for a long time, although they originated" near the time of the separation of South America from Africa. However, they are quite unknovn in the d.efinitel-y Cretaceous dinosaur faunas from South America. The partly archaic nature of the Edentata (includ.ing Pholid.ota), and their southern d.istribution, suggests that they may represent a southern radiation of primitive placental-s that started before South America l-eft Africa. If son some Palaeanodonta probably moved to North America from South America at the beginning of the Paleocene, perhaps even before the Xenarthra originated" from them in South America. Edentata thus may well- have inhabited. Australia before the marsupial invasion, and perhaps well into the Cenozoic. (e5) lccord.ing to current id.eas Ind.ia was rafting merrily along to the north by itself at the end of the Cretaceous, out of sight of other land, although the imense lava flovs of the Deccan Traps n of latest Cretaceous and. Paleocene age, might suggest some degree of external interaction. Unl-ess advanced maramals arrived from elsewhere before India col-lided. with Asia at about the mid.dle Eocene, we pred.ict that the known late Cretaoeous dinosaur fauna of India survived through the Paleocene into the Eocene. (Zl) I placental of the family Anthracotheriidae seems somehowto have arrived on Timor in the Eocene (von Koenigswald, f96T). Anthracotheres are unknown in South America, as are any other artiodactyls before the connection to North America in the Pl-ioceneo and Timor has been associ.ated. with Australia at least since the Paleozoie (e.g. Aud.ley-Charles,1973; Veevers and Evansn 1973). However, a water gap did appear to separate Tinor from Australia then as now, and the first col-lision of Timor with an fnd.onesian microplate reportedly was in the Paleocene (Ruatey-Charles, I9T3), before Australia had separated from Antarctica and while it was stil1 far from Asia. Ttris explanation is un- satisfactory, but so are others. (eB) fne ratio of planktonic to benthonic foraminiferans can al-so be affected. by dissol-ution of planktonic tests and by ad-verse bottom cond.itions (Sl-iter, f9T5). Because the carbonate compensation depth seems to have risen remark- ably high at the end of the Cretaceous, selecti-ve d"issolution must be consid.er- ed possibl-e until- the planktonic specimens are examined for evidence of partial d.issolution. Ho'wever, the rise itself may have been caused by a decrease in productivity of carbonate-secreting plankton, mostly foraminiferans. (e9) We predict that a decline in mrmber of ind.ividuals of coccol-ithophores occurred together with that of the fora.uriniferans, and that the algaen being more numerous and smal-ler, were able to maintain their fu11 conplement of species longer than could the protozoans. (30) n:ere are no known land vertebrates in the early Paleocene (or in the Protungulalum conmunity other than the cursorial erocrodil-e and ord-i-nary heron- like birds) that coul-d eas11y have preyed. on other vertebrates. Triisodontine arctocyonids may have d.one so ineffectively and soon evolved. into predaceous )o VAN VALEN and SLOAII mesonychids, and. crocodiles and. Champsosaurus coufd. perhaps forage on land sometimes. Ad.d.itionally, no known land vertebrate of the early Pal-eocene was more than a meter long, unlike the situations imrnediately earlier and later. Earl-y Paleocene land vertebrates are known only from North Anerica and Europe. Probably elsewhere there were effective pred.ators, such as sebecoid crocodil-es in South America and ratite bird.s on al-l- continents of the Southern Hemisphere. There is good if ind.irect evidence that these groups were present there through the Pal-eocene. (gf) fne geologically rapid cl-inatic change and the changes in movements of continental and oceanic plates may have been coincidental, but there are at feast two possible causaf connections. The more 1ikely is the ind'uction of unusual amounts of volcanism. As for the second, the postulated temporary Central- .A,mericanl-and bridge would have interrupted" any vorldvide equatorial fl-ow of "Tethystt warm water. This shoul-d. reduce the tenperature of the tropical seas, because their warmth comes from their residence time ln highly iso- l-ated regions. Howevero indirect effects woul-d be required for a red.uction in surface insol-ation and for the temperature clecrease at higher latitud.es total- ' and the maps of Smith, Briden, and Drevry (fgf:) suggest that Europe and Africa were close enough together in the Cretaceous to preclude a major worldvide flown although an epicontinental- flov may have been wid.er (Gordon' 1973). (le) ftre possibil-ity of Antarctic glaciation at the Cretaceous-Paleocene boundary can be tested by o:qggen-isotope measurements of benthic foraminiferans in a sufficiently complete deep-water section, if any exist. Perhaps the Zr:maya sediments were deposited. in deep enough water. The probable sudd.en eustatic regression of the sea at the bound.ary, culminating a more gradual but perhaps not worldwide retreat, is most easily explained by such a glaciation. Ttre temperature drop at the boundary would provide a cause. A glaciation vould, however, i"ncrease the d.ifficulty of marsupials getting to Austral-ia at this time' especially without Placentals.

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