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Dinosaurs, Extinction Theories For DINOSAURS, EXTINCTION THEORIES FOR n w J. David Archibald andw Norman MacLeod n San Diego State University and Natural History Museum I. Yet Another Dante’s Inferno saurischians Reptile-hipped dinosaurs such as the- II. Multiple Causes ropods. III. Long-Term Trends in Dinosaur Diversity IV. Short-Term Trends in Dinosaur Diversity V. Out with a Whimper, a Bang, or Both? VI. Conclusions POPULAR ACCOUNTS SAY ALL DINOSAURS DIED INSTANTLY from the impact of an asteroid, comet, or meteor 65 Ma. Arguments continue to be made that dinosaurs were at the height of their taxonomic diver- GLOSSARY sity at the time of their extinction. When examined in detail, the only good records we have of this extinction ammonites Shelled group of invertebrates related to in North America show that the number of species of octopus, squid, and nautilus. dinosaurs had declined by as much as 40% before their amniota Taxon of vertebrates including mammals extinction. Dinosaurs were not alone. Of the over 100 and reptiles (including birds). vertebrate species (including dinosaurs) known at archaic ungulates (or condylarths) Extinct group or the time of these extinctions in North America, B50% grade of mammals possibly giving rise to hoofed became extinct. Chances of survival were much lower mammals. if you were a large, terrestrial, amniotic endotherm chondrichthyeans Sharks and their relatives. rather than a small, freshwater, anamniotic ectotherm. ectotherm Animals that produce their heat from ex- Except for a recent hypothesis, none of the suggested ternal sources such as the Sun. ‘‘Dante’s inferno’’ events accompanying the impact can endotherm Animals that produce their heat inter- explain this pattern of survival and extinction. This nally through metabolic means. recent hypothesis argues that a pulse of intense ther- K/T Abbreviation for Cretaceous/Tertiary, usually in mal radiation killed any unsheltered organisms. While reference to the K/T boundary. it is intriguing, a scenario of multiple causes (marine nonavian dinosaurs Dinosauria excluding birds. regression, habitat fragmentation, volcanism, and im- ornithischians Bird-hipped dinosaurs such as horned pact) better explains the pattern of extinctions at the and duck-billed dinosaurs. end of the Cretaceous era, including that of nonavian palynomorphs Pollen and spores produces by plants. dinosaurs. Encyclopedia of Biodiversity Copyright & 2007 Elsevier Inc. All rights of reproduction in any form reserved. 1 2 ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ DINOSAURS, EXTINCTION THEORIES FOR___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Sixty-five and a half million years ago, a 10 km 100 diameter rock from space slammed into the Earth, almost instantly annihilating over 70% of the all spe- 76% cies, including all nonavian dinosaurs. Textbooks, the 75 66% press, and many scientists have accepted some version of this Cretaceous/Tertiary (K/T) mass extinction sce- nario for over 20 years. Yet, even as the popularity of 50 this single-cause explanation increased, nagging ques- tions continued to be voiced as to the possible biologic Species survival 28% 26% effects of such an impact event. This is particularly 25 true with regard to the record of plants and animals living on land and in freshwater habitats. For example, 0 12 clades representing about 107 species of better- Fresh- Land- Ecto- Endo- studied vertebrates known from very near the K/T water dwelling thermic thermic n = 49 n = 58 n = 61 n = 46 boundary in western North America suffered 51% extinction. Of these 12, however, only five— 100 chondrichthyeans, lizards, marsupials, ornithischians, and saurischians (without birds)—account for 75% of the extinctions. The obvious biological question is 75 what did sharks, lizards, opossums, and nonavian di- 57% 61% nosaurs have in common that made them collectively more susceptible to extinction, especially from the 50 44% environmental consequences of a large bolide impact? 30% Other statistically significant patterns emerge as well Species survival (Fig. 1). Chances of survival were much lower if you 25 were a large, terrestrial, amniotic endotherm rather than a small, freshwater, anamniotic ectotherm. None 0 of the myriad of Dante’s inferno events (e.g., sharp Small Large Nonamniote Amniote temperature increase, sharp temperature decrease, n = 74 n = 33 n = 28 n = 79 tsunamis, hurricanes, global wildfires, and acid rain) FIGURE 1 Differential patterns of vertebrate species survival at the alleged or modeled to have occurred in the wake of K/T boundary (Archibald and Fastovsky, 2004). w2 tests of compari- this impact have been able to explain these curious sons, except possibly the last, are significant (freshwater versus land- extinction and survival patterns. In fact, several of dwelling comparison and ectothermic versus endothermic comparison, P 0.005, small versus large comparison, 0.010 P 0.025, non- these so-called inferno events have been discounted oo o o amniote versus amniote comparison 0.150oPo0.100). completely (e.g., acid rain, because most aquatic spe- cies survive), or are regarded as unlikely (e.g., sharp temperature decrease, because ectotherms do well) would have been sheltered from the searing thermal when tested against the known K/T fossil record. pulse by a layer of water. This might explain the survival of most aquatic species. Terrestrial forms, however, would have suffered greatly unless they found shelter underground or in some sort of natural I. YET ANOTHER DANTE’S INFERNO cavity. At first blush, this mechanism appears to ex- plain why typically larger dinosaurian species faired A recently proposed postimpact environmental sce- more poorly than did some mammals. It may not ac- nario may be the first that shows some broad agreem- tually be large size itself that was being selected ent with the vertebrate fossil record (Robertson et al., against. Dinosaurs grew from a small size after hat- 2004). This scenario postulates that, following the K/T ching from eggs and some may even have been small impact, ejecta reentering the atmosphere created an enough after leaving any parental care to have survived intense blanket of infrared radiation that covered the in some sort of cavity. But in such cases, there would planet’s entire surface. Such a pulse of intense thermal have been enough survivors to form a viable postim- radiation would kill any unsheltered organisms. The pact population to ensure species survival. For the radiation scenario predicts that aquatic organisms smaller mammals, it is given that much larger __________________________________________________________________________________________________________________________________________ DINOSAURS, EXTINCTION THEORIES FOR________________________________________________________________________________________________________________________________________ 3 population sizes would have predominated. Indeed, seas. Thus, their absence in the earliest Paleocene of their numbers in the fossil record indicate this. By this region is no great mystery. Different shark species virtue of these larger population sizes, coupled with made a brief reappearance when the seas once again small physical size, more individuals may have been lapped interior North American shores for a short time able to shelter in some sort of cavity or underground, in the Early to Middle Paleocene. Aquatic species, ex- and then emerge to reassemble their population. cept the fore-mentioned sharks, generally did well This is all well and good, but how does the infrared across the K/T boundary as the freshwater systems radiation scenario explain the fact that placental mam- expanded in the wake of the sea’s retreat. mals faired much better than marsupial mammals in But what of the marsupials? One possibility is that North America, while in South America both groups the earliest archaic ungulates (or condylarths) that radiated in the Early Tertiary? Similarly, under this definitely appear in North America in the earliest radiation scenario, why would the clearly aquatic Paleocene (but probably were around by the latest sharks disappear completely? In the marine realm, this Cretaceous) may have out-competed the marsupials in scenario becomes even more unsatisfactory as it fails to North America. The geographic origin for these archaic account for the distinction between victims as dispa- ungulates is uncertain. More and more evidence, rate as nannoplankton, planktonic foraminifera, am- however, points to Asia. As sea level fell, archaic monites, and marine reptiles and survivors as disparate ungulates presumably crossed the perennially appear- as benthic foraminifera, sponges, corals, lophopho- ing Bering Land Bridge. In South America, instead of rates, and echinoderms. Many of these groups exhibit replacing the marsupials, local ungulate lineages shifted patterns of species-richness turnover that are not con- more and more to herbivory while the marsupials sistent with a single-event scenario located at the very remained omnivores or became more carnivorous. end of the Cretaceous. Indeed, even the timing of Latest Cretaceous dinosaurs, which are only known impact-related event scenarios
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