Consider all the taxa (gold bars) that extend to near the K/T extinction (65 mya). Those are Oviraptorids, Troodontids, Dromaeosaurids, Alvarezsaurids, Euenantiornithes, Hesperornithiforrms and Ichthyornis.
Each of those is a candidate for a euphyletic (ancestral) relationship with corresponding modern bird taxa.
For example the taxa labelled "modern birds" (which should read "Some modern birds") can represent the taxa of modern birds that developed from Ichthyornis.
The other modern birds that developed from the other taxa (eg from Euenantiornithes etc) are simply not shown on this chart.
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