Living Large in the Cretaceous Anne Weil
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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149 of this changii,a dinosaur species found in the same both in the original and in the residual issue1,overturn this outdated view.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 2dǁ1, 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 dǁ2 . 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 dǁ2 . 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 116 NATURE | VOL 433 | 13 JANUARY 2005 | www.nature.com/nature © 2005 Nature Publishing Group 13.1 n&v113 MH 7/1/05 5:15 pm Page 117 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 Ȋ-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 Ȋ 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.