DINOSAUR RENAISSANCE The dinosaurs were not obsolescent reptiles but were a novel group of "warm-blooded" animals. And the birds are their descendants by Robert T. Bakker he dinosaur is for most people the 10 kilograms or more are endothermic epitome of extinctness, the proto­ birds and mammals. Why? Ttype of an animal so maladapted The term "cold-bloodedness" is a bit to a changing environment that it dies misleading: on a sunny day.a lizard's out, leaving fossils but no descendants. body temperature may be higher than a Dinosaurs have a bad public image as man's. The key distinction between ec­ symbols of obsolescence and hulking in­ tothermy and endothermy is the rate efficiency; in political cartoons they are of body-heat production and long-term know-nothing conservatives that plod temperature stability. The resting meta­ HAIRY THERAPSIDS, mammal-like rep­ through miasmic swamps to inevitable bolic heat production of living reptiles is tiles of the late Permian period some 250 extinction. Most contemporary paleon­ too low to affect body temperature sig­ million years ago, confront one another in tologists have had little interest in dino­ nificantly in most situations, and reptiles the snows of southern Gondwanaland, at a saurs; the creatures were an evolutionary of today must use external heat sources site that is now in South Africa. Anteosau- novelty, to be sure, and some were very to raise their body temperature above big, but they did not appear to merit the air temperature-which is why they much serious study because they did not bask in the sun or on warm rocks. Once seem to go anywhere: no modern verte­ big lizards, big crocodiles or turtles in a brate groups were descended from them. warm climate achieve a high body tem­ Recent research is rewriting the dino­ perature they can maintain it for days saur dossier. It appears that they were because large size retards heat loss, but more interesting creatures, better adapt­ they are still vulnerable to sudden heat ed to a wide range of environments and drain during cloudy weather or cool immensely more sophisticated in their nights or after a rainstorm, and so they bioenergetic machinery than had been cannot match the performance of endo­ thought. In this article I shall be present­ thermic birds and mammals. ing some of the evidence that has led to The key to avian and mammalian en­ reevaluation of the dinosaurs' role in ani­ dothermy is high basal metabolism: the mal evolution. The evidence suggests, in level of heat-producing chemical activ­ fact, that the dinosaurs never died out ity in each cell is about four times higher completely. One group still lives. We in an endotherm than in an ectotherm of call them birds. the same weight at the same body tem­ perature. Additional heat is produced as Ectothermy and Endothermy it is needed, by shivering and some other special forms of thermogenesis. Except Dinosaurs are usually portrayed as for some large tropical endotherms (ele­ "cold-blooded" animals, with a physiol­ phants and ostriches, for example), birds ogy like that of living lizards or croco­ and mammals also have a layer of hair diles. Modern land ecosystems clearly or feathers that cuts the rate of thermal show that in large animals "cold-blood­ loss. By adopting high heat production edness" (ectothermy) is competitively in­ and insulation endotherms have pur­ ferior to "warm-bloodedness" (endother­ chased the ability to maintain more near­ my), the bioenergetic system of birds ly constant high body temperatures than and mammals. Small reptiles and am­ their ectothermic competitors can. A phibians are common and diverse, par­ guarantee of high, constant body tem­ FEATHERED DINOSAUR, Syntarsus, pur­ ticularly in the Tropics, but in nearly all perature is a powerful adaptation be­ sues a gliding lizard across the sand dunes habitats the overwhelming majority of cause the rate of work output from mus­ of Rhodesia in the early Jurassic period land vertebrates with an adult weight of cle tissue, heart and lungs is greater at some 180 million years ago. This small dino· 58 © 1975 SCIENTIFIC AMERICAN, INC rus (right), weighing ahout 600 kilograms, had bony ridges on the and the knowledge, from several kinds of data, that therapsids were snout and brow for head·to·head contact in sexual or territorial endothermic, or "warm.blooded"; those adapted to cold would behavior. Pristerognathids (left ) weighed about 50 kilograms and have had hairy insulation. The advent of endothermy, competitive. represent a group that included the direct ancestors of mammals. ly superior to the ectothermy ("cold·bloodedness") of typical rep· The reconstructions were made by the author on the basis of fossils tiles, is the basis of author's new classification of land vertebrates. saur (adult weight about 30 kilograms) and others were restored cal similarities between dinosaurs and early birds. Dinosaurs, it ap­ by Michael Raath of the Queen Victoria Museum in Rhodesia and pears, were endother"!ic, and the smaller species required insula. the author on the basis of evidence that some thecodonts, ancestors tion. Feathers would have conserved metabolic heat in cold environ· of the dinosaurs, had insulation and on the basis of close anatomi· ments and reflected the heat of the sun in hot climates such as this. 59 © 1975 SCIENTIFIC AMERICAN, INC <E---MODERN MAMMALS---?> BIRDS o i i GREAT NORTHERN ICE AGE 16 26 25 (50 N 0 Z � W Z 0 W is « a: 50 <.? W a: �----- => DINOSAURS-----?)i � r-_r---+--------------------�--�f_----------r_Tl--_r,_--,_----�,_�--�r_--t_--------ffi � ::;; W 75 � DINOSAUR 12 o COMMUNITIES a: « 20 3: W 00 -' => o 0 � W 100 0 <.? « Z � (fj W « a: W 0 a: 00 -' o « � ::;; ::;; « 125 ::;; 0 0 (5 MINIMUM <.? N « 0 00 00 LATITUDINAL a: 0 W TEMPERATURE « (5 ::;; W -' GRADIENT >- N -' u.. 150 0 DINOSAUR 00 « 0 W COMMUNITIES ::;; 00 ::;; 00 z 0 0 ::J -' (fj 00 � � « Z a: W => 175 ...., Ci « a: <.? W a: => � a: 200 r---+---------------------------------�------�----��._���----------------- � ::;; W MIXED THERAPSID­ � THECODONT COMMUNITIES THECODONT � « 3: W -' 225 o � <.? �-+��-+----------r--���-��--------��--� w i��tr7'",---i-------------------� THERAPSID « � COMMUNITIES W -' a: ------------0 z W 250 o « ALL-ECTOTHERMIC � >­ COMMUNITIES a:-' o(5 W a: � « N W 8 GREAT -' GONDWANA 275 � r-L-+-------------r-��-ff-+���----�----------��--------------------------------� ICE AGE zOO O=> mO a: a: «W o� PREDATOR-PREY SYSTEMS of land vertebrates and the paths groups named and pictured in the illustrations on pages 64 through of descent of successive groups are diagrammed with the preda­ 67_ The relative importance of the live biomass represented by tors (color ) and the prey animals (gray) numbered to refer to the fossils is indicated by the width of the gray and colored pathways. 60 © 1975 SCIENTIFIC AMERICAN, INC high temperatures than at low tempera­ tures, and the endothermic animal's bio­ u B N CLASS chemistry can be finely tuned to operate o CLASS CLASS Z MAMMALIA REPTILIA AVES within a narrow thermal range. w U The adaptation carries a large bioen­ ergetic price, however. The total energy budget per year of a population of endo­ thermic birds or mammals is from 10 to 30 times higher than the energy budget of an ectothermic population of the same u size and adult body weight. The price B N is nonetheless justified. Mammals and o (f) birds have been the dominant large and w { ::;; :! DINOSAURIA medium-sized land vertebrates for 60 (f)er (f)� million years in nearly all habitats. «« --... In view of the advantage of endother­ d8 -... �i3 my the remarkable success of the dino­ I (f) er saurs seems puzzling. The first land­ THERAPSIDA « THECODONTIA vertebrate communities, in the Carbonif­ erous and early Permian periods, were u composed of reptiles and amphibians B N generally considered to be primitive and o W ectothermic. Replacing this first ecto­ ...J « thermic dynasty were the mammal-like "- reptiles (therapsids), which eventually produced the first true mammals near the end of the next period, the Triassic, USUAL CLASSIFICATION of land vertebrates (excluding the Amphihia) is diagrammed about when the dinosaurs were originat­ here in a highly simplified form. The classes are all descended from the original stem rep· ing. One might expect that mammals tiles. Birds (class Aves) were considered descendants of early thecodonts, not of dinosaurs, would have taken over the land-verte­ and endothermy (color) was thought to have appeared gradually, late in the development of mammals and birds. The author proposes a reclassification(see illustration on page brate communities immediately, but 77). they did not. From their appearance in the Triassic until the end of the Creta­ ceous, a span of 140 million years, mam­ energy flow in ectotherms places little those of the mid-Atlantic or East Africa. mals remained small and inconspiCUOUS demand on the bone compartment of the Paleomagnetic data make it possible to while all the ecological roles of large ter­ blood and calcium-phosphate system, reconstruct the ancient positions of the restrial herbivores and carnivores were and so the compact bone of living rep­ continents to within about five degrees monopolized by dinosaurs; mammals did tiles has a characteristic "low activity" of latitude, and sedimentary indicators not begin to radiate and produce large pattern: a low density of blood vessels such as glacial beds and salt deposits species until after the dinosaurs had al­ and few Haversian canals, which are show the severity of the latitudinal tem­ ready become extinct at the end of the the site of rapid calcium-phosphate ex­ perature gradient from the Equator to Cretaceous. One is forced to conclude change. Moreover, in strongly seasonal the poles in past epochs. Given the lati­ that dinosaurs were competitively su­ climates, where drought or winter cold tude and the gradient, one can plot tem­ perior to mammals as large land verte­ forces ectotherms to become dormant, perature zones, and such zones should brates.