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An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution

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An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution R ESEARCH A RTICLES 21. Despite the absence of key information about sedi- as potential “alluvial fans” and “deltas” (34, 35) and quence of tens to hundreds of repeated layers (or packages ment caliber, stream load, original gradients, and more than 100 additional locations exhibiting similar of layers too fine to resolve in MOC images) of essentially original basin morphometry that would be needed to topographic relations (valleys entering depressions). As identical thickness and outcrop expression. perform a quantitative assessment of the hydrology of October 2003, some 200 MOC images covering 24. M. C. Malin, K. S. Edgett, Science 290, 1927 (2000). of the Holden NEbasin, it is somewhat informative approximately 80 locations had been acquired and in- 25. W. E. Galloway, in Deltas, Models for Exploration, to examine those aspects of the system for which spected. All of these images show features quite differ- M. L. Broussard, Ed. (Houston Geological Society, some reasonable assumptions can be made. The rel- ent from those discussed in this work, generally falling Houston, TX, 1975), pp. 87–98. atively low gradient (0.35°) of the well-exposed me- into two categories. The most prevalent category is one 26. W. Nemec, in Coarse-Grained Deltas, A. Colella, D. B. Prior, ander zone seen in Fig. 2A, and the measurement of in which the floors of the valley and crater are concor- Eds., Intl. Assn. Sedimentol. Spec. Pub. 10, 3 (1990). a typical channel width of 50 m, permits the calcu- dant, showing no discernible expression of deposition 27. T. C. Blair, J. G. McPherson, J. Sediment. Res. A 64, lation of flow velocity using the Manning equation (e.g., MOC images E04-01284, E23-01302, and R02- 450 (1994). with appropriately gravity-modified parameters (33) 00995). In these cases, alluvial deposits may exist but 28. GMT—The Generic Mapping Tools (http://gmt.soest. if one assumes a range of possible flow depths and have been buried by some process that filled the crater, hawaii.edu). Manning roughness coefficients. For values of rough- or may have once existed but have since been com- 29. P. Wessel, W. H. F. Smith, Eos 72, 441 (1991). ness corresponding to 0.04 on Earth (a bed mostly of pletely stripped away. In a relatively small number of 30. R. P. Miller, J. Geol. 45, 432 (1937). fine-grained materials but with some stones), flow cases (the second category), a discernible apron of 31. J. Maizels, Palaeogeogr. Palaeoclimatol. Palaeoecol. depths of a few meters on Mars would flow at a few material is seen at the point where the valley enters the 76, 241 (1990). meters per second, producing discharges of a few crater. Although the aprons have some attributes of 32. Mars Channel Working Group, Geol. Soc. Am. Bull. hundred cubic meters per second. Terrestrial field alluvial fans (they are conical in three-dimensional form, 94, 1035 (1983). experience suggests that this rate is consistent with have longitudinal slopes Ն2° and convex transverse 33. P. Komar, Icarus 37, 156 (1979). the size and configuration of the meanders seen sections, and occur adjacent to high-standing relief), 34. N. A. Cabrol, E. A. Grin, Icarus 149, 291 (2001). (although perhaps on the high end of such an esti- they have three characteristics that distinguish them 35. G. G. Ori, L. Marinangeli, A. Baliva, J. Geophys. Res. mate). Were this discharge to occur today, it would from the fan described in this work: They consist of a 105, 17629 (2000). fill the existing, eroded floor of Holden NECrater to single (rather than multiple) lobe of material, they lack 36. We thank R. A. MacRae for stimulating discussions, and R. the –1300 m level (the level at which both major a radial (or distributary) pattern of conduits, and they M. E. Williams and V. R. Baker for their perceptive and valleys entering the crater lose definition) in roughly display concentric steps in their surface’s descent to the insightful comments and suggestions that were instru- 20 years. Although fraught with uncertainties owing crater floor (e.g., MOC images E02-00508 and R02- mental in refining and focusing this paper. We acknowl- to dependencies on climate, catchment basin size and 00093). The concentric steps are unique to the aprons, edge the contribution to this work made by the MGS/ geometry, and lake volume, the Holden NEvalues fall as the adjacent crater walls do not display such forms MOC and Mars Odyssey/THEMIS operations teams at within a range that includes comparable desert envi- (that is, the steps are not wave-cut terraces). In some Malin Space Science Systems, Arizona State University, the ronment lakes such as the Great Salt Lake in Utah cases, the volume of the apron appears to be equal to Jet Propulsion Laboratory ( JPL), and Lockheed Martin As- and the Sea of Galilee (inflow rates of tens to hun- the volume of the valley (e.g., MOC images E05-02330, tronautics. Supported by JPL contract 959060 and Arizona dreds of cubic meters per second, lake volumes of 109 E09-00340, and E11-00948). These aprons appear to be State University contract 01-081 (under JPL contract to 1011 m3, and filling times of decades). These the result of mass movements rather than fluvial pro- 1228404 and NASA prime contract task 10079). calculations simply show that the relations are inter- cesses, with the concentric steps resulting from succes- nally consistent with similar relations seen on Earth, sive surges of the material as it moved out of the valley 18 August 2003; accepted 28 October 2003 not necessarily that the situations are identical. or, more likely, as the expression of compressive stress Published online 13 November 2003; 22. As part of our study, we targeted 158 locations identi- in the material as it came to rest within the crater. 10.1126/science.1090544 fied by previous investigations [e.g., appendix B in (34)] 23. We use the term “rhythmically layered” to denote a se- Include this information when citing this paper. previously known from the Yixian Formation An Early Cretaceous Tribosphenic [125 million years ago (Ma) (12)] by a long list of apomorphies (13, 14). Numerous den- Mammal and Metatherian tal and skeletal apomorphies also distinguish Sinodelphys from all Cretaceous eutherians (including Eomaia from the Yixian Forma- Evolution tion) (2, 10, 15–18). Sinodelphys is also more Zhe-Xi Luo,1,2* Qiang Ji,2,3 John R. Wible,1 Chong-Xi Yuan4 derived than the stem boreosphenidans (4) outside the therian crown group (metathe- Derived features of a new boreosphenidan mammal from the Lower Cretaceous rians ϩ eutherians) in several dental apo- Yixian Formation of China suggest that it has a closer relationship to met- morphies, but is less advanced than other atherians (including extant marsupials) than to eutherians (including extant metatherians including Deltatheridium (3)in placentals). This fossil dates to 125 million years ago and extends the record dental formula (13, 14). Hairs are preserved of marsupial relatives with skeletal remains by 50 million years. It also has many as carbonized filaments and impressions foot structures known only from climbing and tree-living extant mammals, around the torso of the holotype (Fig. 1). The suggesting that early crown therians exploited diverse niches. New data from pelage appears to have both guard hairs and this fossil support the view that Asia was likely the center for the diversification denser underhairs close to the body surface. of the earliest metatherians and eutherians during the Early Cretaceous. Description and comparison. Sinodel- phys szalayi is more closely related to extant marsupials than to extant placentals and Marsupials are one of the three main lineages includes all extinct mammals that are more stem taxa of boreosphenidans in its many of extant mammals (Monotremata, Marsupia- closely related to extant marsupials than to marsupial-like apomorphies in the skeleton lia, and Placentalia) (1, 2). Extant marsupials, extant placentals (3). Both metatherians and and anterior dentition (Fig. 1). The posterior such as the opossum, kangaroo, and koala, eutherians (including extant placentals) are upper incisors (I3, I4) are mediolaterally are a subgroup of the Metatheria, which also subgroups of the northern tribosphenic mam- compressed with an asymmetrical, lanceolate mal clade or Boreosphenida (2, 4, 5). Here we (nearly diamond) outline in lateral view. This report a new boreosphenidan mammal with feature is characteristic of “didelphid-like” 1Carnegie Museum of Natural History, Pittsburgh, PA close affinities to metatherians, and discuss marsupials and the stem metatherians for 2 15213, USA. Department of Earth Science, Nanjing its implications for the phylogenetic, biogeo- which incisors are known (19–24), but it is University, Nanjing 200017, China. 3Chinese Academy of Geological Sciences, Beijing 100037, China. 4China graphic, and locomotory evolution of the ear- absent in all known Cretaceous eutherians University of Geosciences, Beijing 100083, China. liest eutherians and metatherians. and mammals outside crown Theria (7, 10, *To whom correspondence should be addressed. E- Sinodelphys szalayi (6) gen. et sp. nov. is 25–27). The first upper premolar (P1) is pro- mail: [email protected] distinguishable from all mammals (7–11) cumbent and close to the upper canine, fol- 1934 12 DECEMBER 2003 VOL 302 SCIENCE www.sciencemag.org R ESEARCH A RTICLES lowed by a large diastema behind (Fig. 1C), a The wrist and ankle of Sinodelphys have head with its navicular facet is asymmetrical derived feature of Late Cretaceous metathe- many marsupial-like apomorphies (Figs. 2 with regard to the main axis of the astragalar rians and Cenozoic “didelphid-like” marsupi- and 3). In the manus of Sinodelphys, the neck (Fig. 3), as is typical of Cretaceous als (3, 19–23). Sinodelphys has a mixture of carpals have a hypertrophied hamate (relative metatherians (18).
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