This paper presents a method for constraining the age of a clade with the ages of the earliest fossil specimens in that clade's outgroups. Given a sufficiently deep, robust, well-resolved, and stratigraphically consistent cladogram, this method can yield useful age constraints even in the absence of specific information about the fossil preservation and recovery rates of individual taxa. The algorithm is applied to simulated data sets to demonstrate that this method can yield robust constraints of clade ages if there are sufficient fossil outgroups available and if there is a finite chance that additional outgroups may be discovered in the future. Finally, the technique is applied to actual fossil data to explore the origin of modern placental mammals. Using data from recently published cladograms, this method indicates that if all Mesozoic eutherians are regarded as outgroups of Placentalia, then the last common ancestor of modern placental mammals and their Cenozoic allies lived between 65 and 88–98 million years ago, depending on the assumed cladogram and the number of outgroups included in the analysis.

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