Living Large in the Cretaceous Anne Weil

13.1 n&v113 MH 7/1/05 5:15 pm Page 116

news and views

Mammalian palaeobiology Living large in the Cretaceous Anne Weil

Discoveries of large, carnivorous mammals from the Cretaceous 100 YEARS AGO challenge the long-held view that primitive mammals were small and “Average Number of Kinsfolk in Each Degree.” uninteresting. Have palaeontologists been asking the wrong questions? May I ask you to insert yet another brief communication on the above subject, lthough more than two-thirds of has no living descendants. Repenomamus because private correspondence shows mammalian evolution occurred is closely related to Gobiconodon,another that paradoxical opinions are not yet wholly A between about 180 million and mammal discovered in the Lujiatun beds4, dispelled? The clearest way of expressing 65.5 million years ago, many people think and perhaps more distantly to the much statistical problems is the familiar method of that these early mammals were not very smaller Jeholodens that was discovered higher black and white balls, which I will now adopt. exciting. Mesozoic mammals are usually in the Yixian Formation1,5. Plunge both hands into a dark bag partly portrayed as rat-sized, nocturnal prey ani- If R.robustussupped on young dinosaurs, filled with black and white balls, equal in mals, ecologically marginalized and con- did R. giganticus go after the adults? None number, and well mixed. Grasp a handful in strained from evolving diverse body types of the dinosaurs described so far from the the right hand, to represent a family of boys and sizes until the extinction event at the Lujiatun beds2 is very big; most published and girls. Out of this unseen handful extract end of the Cretaceous removed non-avian specimens have skull lengths near or less one ball, still unseen, with the left hand. dinosaurs from the scene. Two fascinating than 10 cm. Repenomamus giganticus was There will be on the average of many similar discoveries of near-complete fossil skeletons, longer and heavier than adults of Sinovenator experiments, as many white as black balls, described by Hu et al. on page 149 of this changii,a dinosaur species found in the same both in the original and in the residual issue1,overturn this outdated view. Neither deposits6,for instance. However, modern- handful, because the extracted ball will be is of a small mammal. One is more than a day mammalian carnivores that weigh less as often white as black. Using my previous metre long. The other appears to have a dis- than 21.5 kg prey mostly on animals of less notation, let the number of balls in the membered juvenile dinosaur in its stomach. than half their weight7.IfR. giganticus original handful be 2d. Consequently, the Both skeletons were found in the Lujiatun behaved like living mammals, it might have number in the residual handful will be 2d1, fossil beds at the base of the Yixian Forma- preyed on dinosaurs weighing less than 7 kg. and the average number in it either of white tion in northeastern China. They are at Indeed, although the new R. robustus speci- —1 or of black balls will be half as many, or d2 . least 128 million years old, dating from the men provides evidence that it ate young It makes no difference to the average result Early Cretaceous period. The diversity and dinosaurs, how much of its diet was com- whether the hitherto unseen ball in the astounding preservation of fossils from the posed of dinosaurs — or even of meat — is left hand proves to be white or black. In Yixian is well established; from feathered open to speculation. Many living mammal- other words, it makes no difference in the dinosaurs to insects, it continues to produce ian carnivores, particularly those under the estimate of the average number of sisters scientific riches2.These latest finds should 21.5-kg threshold, also eat invertebrates and or of brothers whether the individual from trigger another avalanche of questions and plants7, and their diets can vary considerably whom they are reckoned be a boy or a girl; speculation among palaeontologists. with season. Small mammals related to —1 it is in both cases d2 . The reckoning may The dinosaur-eater belongs to a species Repenomamus,such as Jeholodens,have been proceed from one member of each family of large mammal, Repenomamus robustus, reconstructed as insectivores5. taken at random, or from all its members which was described first from a skull3.The Despite the frequently made generaliza- taken in turn. Francis Galton new specimen is more complete — and on its tion that Mesozoic mammals were rat-sized, From Nature 12 January 1905. left side, under its ribs where a mammal’s palaeontologists have known for some time stomach might well have been,lies a fragmen- that this was not the case. Larger mammals 50 YEARS AGO tary and disarticulated skeleton of a young include Kollikodon from the Early Cretaceous The “Proceedings” for 1954 of the Croydon Psittacosaurus, estimated to have been about of Australia8, and Schowalteria9 and Bubo- Natural History and Scientific Society 14 cm long. The devourer of this little dino- dens10 from the Late Cretaceous of North contains interesting articles on deneholes… saur was more than half a metre long, and is America.But exactly how large those animals Deneholes are excavations in underlying estimated1 to have weighed 4–6 kg. were is a mystery, because Schowalteria is chalk reached by vertical shafts through the Repenomamus robustus is a runt,however, known only from the front end of a fragmen- overload… The age of the deneholes seems next to its newly discovered relative, Repeno- tary skull,Kollikodon from a partial lower jaw to be pre-Roman, and they are probably of mamus giganticus.Hu et al.1 provide the with three teeth, and Bubodens from a single the Iron Age. Many explanations have been first description of this Mesozoic mammal. tooth. These mammals were at least as large given as to why they were made; but none Curled on one side, the skeleton looks like as R. robustus, and may have been as large as is satisfactory. Underground granaries or nothing so much as that of a sleeping dog. R. giganticus,but because their remains are stores have been suggested, or pits for Uncurled, R. giganticus would have been so incomplete it is hard to tell. The fossil of obtaining chalk for agriculture; but, if the about 105 cm long, and the authors estimate R. giganticus,however, is nearly complete, latter explanation be the correct one, why that it would have weighed about 12–14 kg. and its height and length are indisputable. have they been so carefully made?… It Both Repenomamus species had proportion- Hypotheses developed to explain the would seem clear… that some connexion ately shorter legs than living mammals, but evolution of mammalian size often focus must exist between these artificial caves their posture may have been similar to that of on dinosaurs. The most frequently repeated and the earth-houses of northern Scotland. living quadrupeds of the same size.They were speculation is that Mesozoic mammals were But unfortunately we do not really know squat, toothy, heavily built animals, in some forced to remain small by a combination of why these latter were made, either. respects reminiscent of the Tasmanian devil heavy predation pressure from dinosaurs and From Nature 15 January 1955. (Sarcophilus) or of the ratel (Mellivora).They the saturation of ecological niches by large belong to an early mammalian lineage that reptiles. Are the mammals from the Lujiatun

news and views

beds large because the dinosaurs are small? Anne Weil is in the Department of Biological purely theoretical grounds, but was soon This question may be premature, as the fossil Anthropology and Anatomy, Duke University, verified experimentally6 and represents one deposits are under active excavation and 08 Biological Sciences Building, Box 90383, of the triumphs of astrophysics. description of the fauna is not complete. Yet Durham, North Carolina 27708-0383, USA. Once carbon is formed, the other ele- the two new specimens of Repenomamus e-mail: [email protected] ments — especially those,such as oxygen and prompt a reversal of the question, if only 1. Hu, Y., Meng, J., Wang, Y. & Li, C. Nature 433, 149–152 (2005). neon,that can be created simply by adding yet in speculation: how might mammals have 2. Zhou, Z., Barrett, P. M. & Hilton, J. Nature 421, 807–814 more -particles — are readily made without influenced dinosaur evolution? It seems likely (2003). effective destruction of 12C.Moreover,under- 3. Li, J., Wang, Y., Wang, Y. & Li, C. Chin. Sci. Bull. 46, 782–785 that small dinosaurs experienced predation (2001). standing the rate at which the triple- process pressure from mammals. Indeed, in describ- 4. Li, C., Wang, Y., Hu, Y. & Meng, J. Chin. Sci. Bull. 48, 1129–1134 proceeds is fundamental to understanding ing the diminutive S. changii,which lies (2003). many mechanisms in astrophysics beyond evolutionarily at the base of a lineage closely 5. Ji, Q., Luo, Z.-X. & Ji, S.-A. Nature 398, 326–330 (1999). the production of elements. It is important 6 6. Xu, X., Norell, M. A., Wang, X.-L., Makovicky, P. J. & Wu, X.-C. related to that of birds, Xu et al. express Nature 415, 780–784 (2002). for the generation of energy inside stars more surprise that, although the avian lineage 7. Carbone, C., Mace, G. M., Roberts, S. C. & Macdonald, D. W. massive than the Sun, and for their appear- 7 continued an evolutionary trend towards Nature 402, 286–288 (1999). ance in the later stages of stellar evolution .It small size, closely related dinosaurian lin- 8. Flannery, T. F., Archer, M., Rich, T. H. & Jones, R. Nature 377, also influences the properties of giant stars, eages became larger again. Maybe these small 418–420 (1995). and is relevant to the formation of the very 9. Fox, R. C. & Naylor, B. G. Neues Jb. Geol. Paläontol.-Abhandlung. dinosaurs got larger — or got off the ground 229, 393–420 (2003). first stars in the Universe. — to avoid the rapacious mammals. ■ 10.Wilson, R. W. Dakoterra 3, 118–132 (1987). Curiously,however,the rate of the triple- process has not been accurately determined over the entire range of temperatures at Astrophysics which it is astrophysically important. Recent calculations of stellar structure and nucleo- synthesis use rates produced by the NACRE The process of carbon creation (Nuclear Astrophysics Compilation of Mounib El Eid Reaction Rates) collaboration8.These data include a mixture of measurements,theoret- In the Universe, the element carbon is created only in stars, in a ical predictions and extrapolations, but are remarkable reaction called the triple- process. Fresh insights into the subject to continual reassessment. reaction now come from the latest experiments carried out on Earth. Working with data from particle-acceler- ator facilities,Fynbo and colleagues1 analysed n the first few moments of the Universe’s ‘resonant reaction’. This occurs when the the inverse process, where 12C decays into existence — the famous ‘three minutes’ — energy of the captured particle matches the two or three -particles through the creation Ino elements heavier than helium were difference between the energy of the nuclear of the unstable isotopes 12N and 12B. They made, with the exception of a tiny amount state and the threshold energy — the mini- used the decay properties of these nuclei to of lithium. So how were the other elements, mum energy required to initiate the reaction. search for or confirm resonant states in the including the carbon that is so important to This prediction meant that the probability of 12C system, which are expected to have ener- life on Earth, created? On page 136 of this a 8Be nucleus capturing another -particle gies in the range of 106 electronvolts (MeV). issue, Fynbo et al.1 present new and exciting was dramatically increased. It was based on They found a broad resonance at one energy measurements of the rate of the level, 11.23 MeV. But they could not nuclear fusion reactions that produce confirm the resonance at 9.1 MeV 12C. The element is mainly synthesized assumed in NACRE’s figures.The main inside stars when they evolve to the difference in the rate occurs in the red-giant and later stages (Fig.1). temperature ranges below 5107 K, The starting point for the relevant where the reaction proceeds much reactions is helium, 4He, the nucleus of more quickly, and above 109 K, where which is known as the -particle. In it is slower. 2,3

1952, Edwin Salpeter suggested that The consequences of this new rate CREDNER & S. KOHLE/OBS. BONN the nuclear fusion process leading to will need to be investigated in detail. T. the synthesis of 12C is a two-step But the higher rate at low temperatures process, with two -particles combin- will affect our understanding of the ing to form a minuscule amount of an evolution of the first generation of unstable form of the element beryllium stars. In such stars, the lack of heavy (8Be). Although the lifetime of 8Be is elements implies that the CNO only about 1016 seconds, the close (carbon–nitrogen–oxygen) cycle can’t proximity of atomic nuclei inside the operate to deliver the energy and to dense matter of a star in principle transform hydrogen into helium, allows the capture of a third -particle until some small amount of carbon is to form 12C. Hence the term ‘triple-’ created. This is only possible through for the presumed process of carbon the triple- reaction, and at higher formation. But the probability of this temperatures (near 108 K) that are in a occurring seemed too low to explain the range where the reaction rate has the abundance of carbon in the Universe. higher value obtained by the new evalu- Fred Hoyle4 and Dunbar et al.5 then ation. The net effect is that, with the recognized a crucial point, in predict- Figure 1 Carbon factories. This three-colour composite image9 new rate, this phase of evolution of ing that the third -particle could be of the constellation Auriga includes several red-giant stars, first-generation stars is expected to be captured through what is called a a primary site of carbon synthesis. shorter. At the high-temperature end,