A specimen of Glyptograptus from the Silurian of Dob's Linn is described that possessed a bifurcating virgula, with a biserial stipe along each virgula. The colony lived for some considerable time with this morphology, showing that at least some graptoloids could survive and grow with major modifications to the rhabdosome. The specimen is left under open nomenclature, but if further material is discovered it may be appropriate to name a new species. Growth abnormalities in graptolites are not common, but many examples of repaired damage, rhabdosome malformations, and abnormal growth of thecae, virgellae and virgulae are known. Growth abnormalities can provide important evidence about graptolite ecology, biology and evolution, and so should be described and illustrated whenever possible.