: Changing Views J. David Archibald Department of Biology San Diego State University, San Diego, CA 12 David Archibald is f you played a word association game with people and asked them to re- Professor of Biology and spond with the first word that popped into their heads, more than likely, Curator of Mammals at the response for “fossil” would be “dinosaur.” If you asked them “what San Diego State University. Ikilled the ” more than likely they would say “an asteroid.” Just as He received his Ph.D. from there are many unanswered questions about how dinosaurs lived, so too there the University of are many unanswered questions about how they died. Before we can address California, Berkeley in the specific question of dinosaur extinction, we must examine the general topic of extinction. 1977. He has written numerous articles, essays, Extinction: No One . . . it appears that reviews, and monographs Gets Out of Here Alive on the systematics and marine regression, evolution of early Before the beginning of the 19th century the idea that species could become extinct was not an asteroid impact, mammals, biostratigraphy, widely believed. In the western culture, it was faunal analysis, and believed that all species of plants and and massive extinction. His fieldwork were perfectly created in a matter of days. Cer- volcanism each has taken him from the tainly a creator would not allow his creations to American West to Middle disappear from Earth. The work of the re- probably played a Asia. His 1996 book nowned French paleontologist, Georges Cuvier soon showed that some species have disap- significant role in Dinosaur Extinction and the peared. He compared the teeth of the two spe- End of an Era: What the cies of living elephants with those of fossil what is the best Fossils Say (Columbia elephants and their relatives. The teeth of fossil University Press) known mass mastodons and mammoths were so different documents what we know from those of the two living species that Cuvier extinction in Earth’s of the fossil record at the concluded that the mastodons and mammoths time of dinosaur no longer existed. Very soon the work of Cuvier history. and others showed that not only had extinction extinction. occurred, but that it had been a very common Dave Archibald event throughout Earth’s history. Scientists even began to use extinction to measure geological time. Probably best known for this kind of work is the English geologist Charles Lyell. Lyell compared collections of modern marine invertebrates (clams, snails, etc.) to collections of fossil marine invertebrates from different aged rocks and places in Europe. He found that the older and more different the collections of fossil inverte- brates were from modern invertebrates, the more extinction had occurred.

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Lyell was measuring the fact that, over long each species lasts about 12.4 million , and periods of geological time, as species evolved that life originated 3.5 billion years ago, we ar- others became extinct. rive at the staggering figure of over 2.8 trillion Today, we know that not only does extinc- species that have lived on Earth! If only 10 mil- tion occur, but that of all the species that ever lion are alive today, this means that only one in lived, well over 99 percent are now extinct. This every 280,000 species that has ever lived is figure is not an exact percentage, but rather it is alive today. Thus, 99.99 percent are extinct. an approximation, based on three factors: (a) the This total may seem like a shocking number, age of the Earth, (b) the number of species alive but it is clear that the fate of most species is ex- at any given time, and (c) an estimate of how tinction rather than further evolution. Because long the average species exists. most species become extinct, only a small per- a) Paleontologic evidence points to the ori- centage of species provides the future diversity. gin of life on Earth at about 3.5 billion years. Extinction is so common that it is the rule b) For the number of species alive at any rather than the exception. Just as evolution is given time, we can use estimates of E.O. Wil- an ongoing process adding new species, extinc- son, who placed the number of known species tion is an ongoing process that eliminates spe- alive today at 1.4 million, while judging that cies. the number may be anywhere from 10 to 100 Armed with this information, we might million.1 To be conservative, we can use the think that the extinction of species that we hu- lower estimate of 10 million for the number of mans are causing today is normal or even com- species alive at any given time. This number mon. Such an assumption would be wrong. The overestimates the number of species alive early great many occurring today because of human activity fall into an extremely rare category known Fig. 1. as mass extinction. In the past The five major 540 million years, during the mass extinctions Phanerozoic Era, there have in the past 540 been five times when the num- bers and rates of extinction be- million years as came so high that they stand demonstrated by out from all other times in the extinctions of geological past (Fig. 1). A sixth marine inverte- mass extinction is now under- brates and verte- way, but this time humans are the culprits. brates. From In Fig. 1, each point on Archibald the graph represents the num- (1996) after ber of extinctions per million Raup and years for families of marine in- vertebrates and vertebrates for Sepkoski that particular interval of geo- 1 (1982). in the history of life, but underestimates the logical time. The solid line is called a regression number of species alive later in geological time. line and can be thought of as showing the aver- c) In the late 1980s Niles Eldredge esti- age extinction rate through geological time. The mated how long a variety of species survived. dashed lines surround 95 percent of all the An average duration for a species based on his points shown on the graph. Each of the five various estimates was 12.4 million years.1 spikes falls well outside the 95 percent interval, We now need to make a few calculations. indicating much higher rates of extinction five If we use the estimate that the number of spe- times in the past. These five spikes are generally cies at any given time is about 10 million, that recognized as times of mass extinction.

100 D inosaurs: The Science Behind the Stories Chapter 12—Dinosaur Extinction: Changing Views

When we add up all the extinctions that 32 to 19 during the last 10 million years of the have occurred in the past, we find that mass ex- Late . In other words, some 40 per- tinctions probably account for no more than cent of dinosaur genera were lost in this region about 10 percent of all extinctions. The other during that time. Whatever killed the last spe- 90 percent or more are normal or background cies of dinosaur, the record shows that they extinctions that are the counterpart to evolu- were declining during the last 10 million years tion. Although the five mass extinctions com- prise a relatively small percentage of total extinctions during Earth history, each repre- sents a major reorganization of the Earth’s biota. The severest of the big five reorganiza- tions occurred at the end of the some 250 mya (million years ago). There was over 90 percent species extinction, although this is not obvious from Fig. 1. The most famous mass extinction, how- ever, is the one that included the last of the dinosaurs, the terminal Cretaceous mass ex- tinction 65 mya.2 This mass extinction wiped out the dominant land vertebrates and opened the evolutionary way for mammals that until that time were no larger than a small cat. Al- though the terminal Cretaceous mass extinc- tion is the most famous, we still cannot say what happened with certainty in part because of the relatively poor record of its best-known victims—the dinosaurs. The K/T Extinction The Late Cretaceous (from about 100–65mya) is not only the last geological time interval from which dinosaurs are definitely known, it is also one of the best sampled for dinosaurs. The name K/T comes from a combination of the names Cretaceous, in which we find fossils of dinosaurs, and Tertiary, the time in which we see the first appearance of modern groups of mammals (the K comes from Kreide, the German word for “chalk,” deposits of which are common at this time in Europe; Cretaceous means “chalk-bearing”). Around the world there are literally hun- dreds of Late Cretaceous dinosaur sites, including such seemingly unlikely places as Antarctica and New Zealand (Fig. 2, A). One Fig. 2. A. Important dinosaur sites (black stars) for the Late Cretaceous (100 of the best-known areas for Late Cretaceous to 65 mya). B. Important vertebrate faunas spanning the K/T boundary. dinosaurs is in the northern part of western More recently discovered possible K/T boundary vertebrate faunas are indi- North America. In this area, we know that the cated by a question mark. From Archibald (1996)1 using data mostly from number of genera of dinosaurs dropped from Weishampel (1990) and map after Smith et al. (1994).

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of the Cretaceous, at least in the western part only 19 are dinosaurs. You may also notice that of North America. (flying relatives of dinosaurs) and Unfortunately, as one examines the rocks are not included in this table, even representing the end of the Cretaceous, the fos- though fossil evidence indicates their existence sil record of dinosaurs gets worse. Only a at this time.3 These two groups are not in- handful of places in the world have a nearly cluded because their fragile skeletons make continuous fossil record of vertebrates across them poor candidates for fossilization com- the K/T boundary. The one area where we do pared to most other vertebrates. Of the 107 re- have an adequate record of dinosaurs very near maining species of vertebrates, 49 percent (52 the K/T boundary is in western North species) survived, or 51 percent became ex- America. Recently discovered sites in China, tinct.1 Even though only about half of these South America, and India offer hope in the fu- species of vertebrates became extinct, the over- ture for an even better K/T record of verte- all biological effect on the land was profound. brates (Fig. 2, B). Dinosaurs had been the dominant land verte- What this means is that the vertebrate brates throughout the Mesozoic. Although (and dinosaur) fossil record at the K/T bound- mammals had been around for almost the same ary is far poorer than is usually realized. Most amount of time, it was only after the dinosaurs importantly, this means that at least for now, were gone that mammals truly began to we cannot say anything about how fast dino- flourish. More importantly, we can use the sur- saur extinction occurred. The record is simply vival data in Table 1 to test the various theories too poor to address this issue. Fortunately, the of dinosaur extinction. record is good enough that we can say some- thing about how many species of dinosaurs The Three Most Popular and other vertebrates became extinct and how Theories of Dinosaur Extinction many survived. We can also say something Soon after the discovery and naming of the about what is called the selectivity of these ex- th tinctions. This means that we can ask whether first known dinosaurs in the early 19 century, all kinds of species of vertebrates from bony people began to speculate on what had hap- fish to turtles were equally or unequally af- pened to them. By the 1980s, there were over fected during the change from the Cretaceous 80 dinosaur extinction theories, more than for to the Tertiary. In order to ask such questions any other group of animals. With so many we need a fossil record of vertebrate species theories, it is no wonder that they ranged from the highly reasonable to the absurd, e.g., over- other than dinosaurs. Indeed, we have such a 4, 3 record, once again in the western portion of hunting by aliens. Given so many theories of North America. As we examine this record, extinction, only the three best explained and keep in mind questions such as: Why did this testable of these will be discussed here. All dinosaur extinction occur? Was it only the di- three theories, to one degree or another, argue nosaurs that went extinct? Which other groups that changes in climate, whether sudden and of animals survived? drastic or slow and cumulative, caused the ex- tinctions. It is important to keep in mind that none of the three theories is mutually exclu- Other Vertebrate Species sive, that is, any combination of the three may Living with the Last Dinosaurs have happened. The most important question Our K/T sites for vertebrate fossils are found is which theory, or part of a theory, is best sup- in eastern Montana. It has been well docu- ported by the fossil evidence. Before testing mented that at least 107 species of vertebrates these theories against the vertebrate fossil existed here during the closing million or so record, they first must be described. They are years of the Cretaceous. These species belong described in the order of the length of the time to 12 major groups, which are listed in Table 1. intervals over which they are thought to have It may be surprising that of these 107 species acted, beginning with the longest.

102 D inosaurs: The Science Behind the Stories Chapter 12—Dinosaur Extinction: Changing Views

The Deccan Traps Table 1. Survival and extinction of vertebrate species across the Massive eruptions of flood basalts on the In- K/T boundary in the western part of North America. dian subcontinent, called the Deccan Traps, occurred at the time of the K-T boundary. Sharks and relatives 0/5 (0%) Bony fish 9/15 (60%) Flood basalts flow from great fissures and vol- Amphibians 8/8 (100%) canoes with moderate amounts of explosive Mammals power. They are more like the lava flows of the Rodent-like multituberculates 5/10 (50%) Hawaiian Islands than the explosive eruption Placentals 6/6 (100%) of Mount St. Helens, which literally blew the Marsupials 1/11 (9%) mountain apart. However, the Deccan Traps Turtles 15/17 (88%) erupted over four or more million years and 3/10 (30%) produced enough lava to cover both Alaska Crocodile-like champsosaurs 1/1 (100%) and Texas to a depth of 2000 ft (610 m). The Crocodilians 4/5 (80%) effects of such massive volcanism have not Dinosaurs been as well studied as the effects proposed in (-hipped) 0/10 (0%) (-hipped) except birds 0/9 (0%) other extinction theories. It is clear, however, that such eruptions would greatly increase the Total number and percent survival 52/107 (49%) amount of very fine-grained material in the at- The first number is the number of species that survive the K/T mosphere. This “dust” would decrease the boundary extinction. The second number is the number of species amount of sunlight reaching the Earth’s sur- known from the Late Cretaceous. Percent is percent survival. face, which would in turn lead to long-term global cooling. Both cooling and a decrease of sun reaching the Earth’s surface would, over this long time interval, change the vegetation cies shrank and became more distant from one and thus, affect the animals feeding upon it. another, population sizes would have de- creased. Furthermore, as land emerged from Marine Regression the sea, land bridges were exposed, such as the Bering land bridge between Asia and North The second theory relates extinction to marine America. This new land would allow migration regression, which is the process whereby very of terrestrial vertebrates and the potential for shallow seas that covered much of the low-ly- increased competition among previously sepa- ing areas of continents drain away, back into rated species. River systems that had once the deeper ocean basins.1 One of the greatest flowed over relatively short distances grew in such marine regressions is recorded in rocks length as the shoreline receded further and fur- near the end of the Cretaceous Period, some 65 ther and provided greater habitat for many mya. Unlike the eruption of the Deccan Traps, fresh water organisms. As new land areas were which took place over millions of years, the exposed with the regression of the great inte- terminal Cretaceous marine regression oc- rior seaways, the climate cooled and climatic curred over a much shorter period of time, extremes increased, further stressing an already only tens or hundreds of thousands of years. stressed environment. Estimates suggest that 11.2 million square miles of land were exposed during this interval, more than twice the next largest such addition Asteroid Impact of land during the past 250 million years. The The argument presented in this theory is that a landmass that was exposed is approximately 6-mile wide asteroid struck Earth 65 mya, equal to the area of modern day Africa. As spewing very fine material high into the atmo- these continent-sized shallow seas drained sphere where it spread around the globe.5 The away, great areas of low, coastal habitat were major result was the blockage of many of the fragmented into smaller and more isolated ar- sun’s rays. This blockage greatly reduced or eas. As these habitats for terrestrial coastal spe- possibly stopped photosynthesis. Many indi-

D inosaurs: The Science Behind the Stories 103 D inosaurs as Living Animals

vidual plants would have been stunted or Using the Vertebrate killed, and many plant species would have be- Record To Test Theories come extinct. Herbivorous dinosaurs and other of Dinosaur Extinction vertebrates that fed upon these plants would have disappeared, which in turn would have The most obvious pattern of extinction among caused the extinction of the carnivorous dino- the major vertebrate groups listed in Table 1 is saurs that fed upon the . This pro- that extinctions were concentrated in only five cess appears to have taken only a few thousand of these groups: sharks and their relatives, liz- years at the most. ards, marsupials, ornithischian dinosaurs, and Evidence for the asteroid impact comes saurischian dinosaurs. Species in these five from three sources: groups account for 75 percent of the extinc- The crater The probable crater for this im- tions. This pattern demonstrates that the K/T pact has been located near the tip of the extinctions were highly selective and any Yucatan Peninsula in Central America. It was theory of extinction must account for this se- appropriately named Chicxulub, which means lectivity. Because the biotic effects of volcanic the devil’s horns in a local dialect. At 110 miles eruption have not been explored extensively, (180 km) across, it was originally thought to but the effects are considered to be similar to be the second largest such structure on Earth. the effects of an asteroid impact, these two theories will be discussed together under the More recent studies argue that it is more on 1 the order of 60 mi (100 km), or possibly even asteroid impact. We will start with the marine smaller. regression theory. An increase in the element iridium at the Cretaceous/Tertiary (K/T) boundary Marine Regression A high level of iridium found in rocks from Global marine regression began in the last few this time period in many places on the Earth is million years of the Cretaceous. As this oc- a strong indication of an extraterrestrial source curred, tremendous new tracts of dry land were for the iridium, such as from an asteroid strik- added. Dinosaurs may well have lived away ing the Earth. Iridium, a very heavy element, from the seacoast near the end of the Creta- like gold, is rare at the surface of the Earth ex- ceous, but their habitats would not have been cept where concentrated in a small area by very affected. The well-known dinosaur-bearing deep volcanoes. vertebrate localities near the K/T boundary, Shocked quartz Quartz grains, showing however, come from coastal plain habitats. shocked lamellae (or parallel layers) in two di- Thus, it is from this information that we rections at a microscopic level, indicate a great should draw our inferences of what may have amount of rapidly applied pressure—such as occurred. might be caused by the impact of an asteroid. As indicated in Table 1, the fossil record Following the publication of the asteroid shows a 0 percent survival for both dinosaurs impact theory in 1980, a number of other pos- and sharks and their relatives. With marine re- sible consequences were suggested, such as acid gression, the coastal plain habitats were being rain, globe wildfire, sudden temperature in- drastically reduced, stranding dinosaurs in creases and/or decreases, tsunamis, and super- ever-smaller areas—this is similar to what hu- hurricanes. Each of these is argued to have had mans are doing to the habitats of large mam- consequences on the world’s plants and ani- mals in Africa today. The loss of habitat mals.5 Some of these consequences can be stressed the dinosaur populations, setting them tested using the fossil vertebrate record, but up for any other biotic insults such as that others cannot, notably tsunamis, and super- from even a small asteroid impact or from mas- hurricanes. We now turn to examining and sive volcanism. At the same time, the coastlines testing the various theories using mostly infor- were retreating away from the Western Interior mation from the fossil record of vertebrates. taking the sharks and relatives with them.

104 D inosaurs: The Science Behind the Stories Chapter 12—Dinosaur Extinction: Changing Views

Sharks could follow freshwater courses up to a lated events as acid rain, sharp temperature de- few hundreds of miles or kilometers, but not crease, and global wildfire.5 thousands of miles or kilometers. Their marine a) We know from work on living species connections were severed. Much larger and and habitats that among vertebrates, acid rain longer rivers replaced the small coastal streams, hurts aquatic organisms most, killing both which continued to support many species of adults and eggs laid in the water. Among the freshwater fish, turtles, amphibians, and croco- aquatic organisms, however, only sharks and dilians. This too is supported by the evidence their relatives show very high levels of extinc- shown in Table 1. In fact, freshwater species tion. Other aquatic species did very well did very well, with descendants such as through the K/T transition, thus acid rain was paddlefish, sturgeon, gar, snapping turtles, and probably not a major factor. alligators still plying the Missouri-Mississippi b) If a sharp drop in temperature had oc- river systems.1 curred, the species that should have been most The lowering of sea level reconnected once affected would have been cold-blooded (ecto- separated landmasses, such as eastern Asia and thermic) vertebrates that spend at least part of western North America. The fossil record their time out of water—this is why today we shows that the earliest relatives of what would see far fewer species of amphibians and reptiles later evolve into hoofed mammals and whales (except warm-blooded or endothermic birds) in the far northern and far southern regions of probably reached North America at this time the world. Yet, most of these ectotherms, ex- (65 mya)—their possible ancestors being pla- cept lizards, did well through the K/T bound- cental mammals known in Asia 20 million ary. Whether dinosaurs should be considered as years earlier. These new North American pla- endotherms, ectotherms, or as having another cental mammals had teeth that resembled kind of physiology remains controversial (see those of the opossum-like marsupials living in de Ricqlès, page 79). North America at this same time. The marsu- c) Finally, a global wildfire is argued to pials arose in North America over 100 mya and have consumed 25 percent of all above ground were very common for at least the 20 million burnable material. Geological evidence for years leading up to the K/T boundary 65 mya. wildfire has been presented based on large It seems likely that the appearance of these new amounts of carbon and other compounds oc- placental mammals in North America spelled curring at the K/T boundary. Some paleon- competitive doom for the marsupials. Interest- tologists argue that such a global wildfire ingly, when both groups appear in South would have transported great quantities of de- America a few million years after the K/T tritus (very small fragments of plants and ani- boundary, they do not compete. Rather, the mals) into the streams, which would have placentals became more strictly herbivorous, favored the survival of aquatic animals that eat while the marsupials became omnivorous and such material.4 Other scientists argue that a carnivorous, including large saber-toothed global wildfire would have been an equal op- marsupial cats. The one group whose fossil portunity killer: terrestrial creatures would record cannot be explained by marine regres- have been burned on land, and aquatic verte- sion is the lizards, which underwent a drastic brates would have suffocated from all the reduction, at least in western North America. burned material dumped into their environ- A possible explanation is that when the climate ments. Thus, depending upon how the fossil became wetter in this area following the K/T information is interpreted, global wildfire boundary, the more dry-adapted lizards could could have been either a significant or unim- not tolerate the changes. portant event at the K/T boundary.

Asteroid Impact Volcanism and Asteroid Impact We can start testing the asteroid impact theory One probable result of an asteroid impact or by examining the effects of such possibly re- high levels of volcanic activity would have been

D inosaurs: The Science Behind the Stories 105 D inosaurs as Living Animals

the blocking of sunlight, either around the Summary globe or in more restricted areas. Blocking sun- When using the vertebrate fossil record to test the light would have caused the reduction or even various theories of dinosaur extinction, it appears cessation of photosynthesis among green that marine regression explains more of the plants. The fossil record of land plants in the highly selective pattern of extinctions and surviv- northern part of western North America sug- als through the K/T transition in western North gests an extinction of at least 80 percent, which America than do either an asteroid impact or tends to support the hypothesis that photosyn- massive volcanism. When combined with evi- thesis was suppressed. Reduced photosynthesis dence from plants and marine species, it appears would have had a devastating effect on large that marine regression, an asteroid impact, and herbivores, especially if they were already massive volcanism each probably played a stressed by other events such as marine regres- significant role in what is the best known mass sion. Unfortunately, more recent studies have extinction in Earth’s history. These three theories, questioned just how much dust really would plus other causes that we still do not know, each have been spread around the world from such may have been necessary, but clearly were not an impact. Thus, more studies are necessary enough individually to cause the extinctions that before we can resolve this question. we see at the end of the Cretaceous.

R eferences

1. Archibald, J.D. 1996. Dinosaur extinction and the end of an era: what the fossils say. New York: Columbia University Press. 2. MacLeod, N., and G. Keller (eds.). 1996. The Cretaceous-Tertiary mass extinction: biotic and environmental changes. New York: W.W. Norton and Co. 3. Dingus, L., and T. Rowe. 1998. The mistaken extinction: dinosaur evolution and the . New York: W.H. Freeman and Co. 4. Fastovsky, D.E., and D.B. Weishampel. 1996. The evolution and extinction of the dinosaurs. Cambridge: Cambridge University Press. 5. Alvarez, W.L. 1997. T. rex and the crater of doom. Princeton, NJ: Princeton University Press.

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