These groups still retained their formal Linnaean taxonomic ranks. Some of them are paraphyletic in that, although every organism in the group is linked to a common ancestor by an unbroken chain of intermediate ancestors within the group, some other descendants of that ancestor lie outside the group. The evolution and distribution of the various taxa through time is commonly shown as a "spindle diagram" (often called a "Romerogram" after the American palaeontologist Alfred Romer) where various spindles branch off from each other, with each spindle representing a taxon. The width of the spindles are meant to imply the abundance (often number of families) plotted against time.