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Observed compactness in ribs. Cross sections are not to scale. Phylogeny from Muizon et al. [19]. Compactness values are given below the taxon name. The age of the fossils must be read at the tip of the branches. The broken branch marks a break of the stratigraphic scale. Mean value for terrestrial mammals is from Buffrénil et al. [11]. Daggers denote extinct taxa.
