2003 was just too early for this taxon to be properly nested.
Sinodelphys (Luo et al., 2003) was considered the oldest known metathere (= marsupial) and was compared with Didelphis, the extant Virginia opossum. Here in the large reptile tree (LRT, 1250 taxa, subset Fig. 1) Sinodelphys nests between Chaoyangodens and Brasilitherium + Kuehneotherium among the prototheres, basal egg-laying mammals. Sinodelphys may have been mistakenly nested because Chaoyangodens and Brasilitherium are newer taxa. Several of the other taxa are also more recently published.
The Luo et al. study nests Sinodelphys
just inside the Metatheria, very close to the Eutheria/Metatheria split. Among taxa both analyses have in common, very few have matching sister taxa. Many are not even in the same large clade (Eutheria/Metatheria/Prototheria). This may be due to an over reliance on dental traits in the Luo et all. study and an under reliance of dental traits in the LRT, which employs a wider gamut of taxa (vs. taxon exclusion in the Luo et al. study).
Sinodelphys has a dorsal naris with short ascending processes on the premaxilla, not a terminal naris opening anteriorly. This trait alone nests Sinodelphys with the egg-laying mammals. Even so, a long list of traits support that nesting. Perhaps if Sinodelphys were described today, after so many other prototheres have been reported, it would have been identified as one of them.
With an inch-long skull
this is a tiny Early Cretaceous egg-layer, ancestral to today’s platypus and echidna.
The fingers on both hands are jumbled up (Fig. 3).
If Luo et al. are correct in their manus reconstruction, the only change I would make is to flip it left to right. Note their digit 5 is missing the proximal phalanx (Fig. 5). That is more likely the thumb because then digits 3 and 4 are the longest, as in sister taxa in the LRT.
Luo Z-X, Ji Q, Wible JR and Yuan C-X 2003. An Early Cretaceous tribosphenic mammal and metatherian evolution. Science 302:1934–1939.