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A Cretaceous symmetrodont therian with some monotreme-like postcranial features


A new spalacotheriid mammal preserved with a complete postcranium and a partial skull has been discovered from the Yixian Formation1,2,3 of Liaoning, China. Spalacotheroid symmetrodonts4,5,6,7,8,9,10,11 are relatives to modern therians (combined group of marsupials and placentals) and are characterized by many skeletal apomorphies of therians. But unlike the closely related spalacotheroids and living therians, this new mammal revealed some surprisingly convergent features to monotremes in the lumbar vertebrae, pelvis and hindlimb12,13. These peculiar features may have developed as functional convergence to locomotory features of monotremes, or the presence of lumbar ribs in this newly discovered mammal and their absence in its close relatives might be due to evolutionary developmental homoplasy. Analysis including this new taxon suggests that spalacotheroids evolved earlier in Eurasia and then dispersed to North America, in concordance with prevailing geodispersal patterns of several common mammalian groups during the Early Cretaceous period.

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Figure 1: Akidolestes cifellii.
Figure 2: Dentition of Akidolestes cifellii.
Figure 3: Phylogenetic relationships of Akidolestes cifellii.
Figure 4: Comparison of pelvic and hindlimb features of the spalacotheroids Akidolestes and Zhangheotherium , and hindlimb posture of Didelphis and Ornithorhynchus.

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We thank P.-J. Chen for generously providing this fossil for our study; A. R. Tabrum for his skilful preparation of the fossil; H.-C. Zhang, J.-G. Sha, X.-N. Yang and Y.-K. Shi for their help with this research; R. L. Cifelli, Q. Ji, Z. Kielan-Jaworowska, T. Martin and J. R. Wible for sharing ideas on early mammals; K. C. Beard, M. R. Dawson and T. Martin for discussion on mammalian evolutionary biogeography; M. R. Dawson and J. R. Wible for improving the manuscript; and M. A. Klingler for assistance with graphics. Support was provided by the National Natural Science Foundation of China, the National Science Foundation (USA) and the National Geographic Society and Carnegie Museum of Natural History (Z.-X.L.). G.L. acknowledges the Nanjing Institute of Geology and Palaeontology and the Institute's State Key Laboratory of Palaeobiology and Stratigraphy, and funding from the Ministry of Science and Technology of China (‘973’ project funding to C.-S. Wang).

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Correspondence to Zhe-Xi Luo.

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Supplementary information

Supplementary Notes

This file contains additional details and analysis of the morphology of Akidolestes cifellii and additional references. (PDF 811 kb)

Supplementary Table 1

Matrix for relationships of Akidolestes. (PDF 281 kb)

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Li, G., Luo, ZX. A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature 439, 195–200 (2006).

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