Two species of squirrel are currently found in the UK, the native red, which has been reduced to a fraction of its former range, and the grey, an American species introduced in the 19th century. The larger grey both outcompetes the red in many habitats and carries a virus fatal to reds.
The skull of the grey squirrel is distinctly elongated in shape compared to the more globular skull of the red. The widest point of the red squirrel skull is near the centre of the zygomatic arch, whereas in the grey it is located on the temporal bone at the rear of the arch. In these specimens, the vestigial premolar is missing, but the socket can be seen at the front of the tooth row.