Key to skulls of British
Rodents and Lagomorphs

British rodents include squirrels, mice, rats and voles. Lagomorphs are rabbits and hares.

Rodent and lagomorph skulls are easy to recognise by their twolarge incisors, top and bottom, separated from the cheekteeth by a large diastema. Theskulls are fragile, however, and it is unusual to find complete examples.

Question onegives you three differences between rodent and lagomorph skulls. Even if the teeth are missing,you should be able to see the sockets of the "tiny extra incisors".

1.a) Tiny "extra" incisor behind each large one in upper jaw,
Palate reduced to a bridge between the first 3 cheek teeth,
Sides of
rostrum open lattice.
b) Only 2 incisors in each jaw, solid palate, solid sides to rostrum. (Rodent) 2
The coypu, which was exterminated in Britain around 1989, was muchlarger than any other British rodent.
2.a) a) Overall length greater than 100mm. COYPU
b) b) Overall length less than 100mm 3
3.a) a) Postorbital processes drawn out to a sharp point. SQUIRREL
b) b) Postorbital processes absent, or reduced to a smooth bump. 4
a) Postorbital process of squirrel
4.a) a) Overall length greater than 35mm. 5
b) b) Overall length less than 35mm. 6
5.a) - Temporal ridges unite to form a crest between eye sockets,
- Cheek teeth
b) Temporal ridges widely separated, or are difficult to detect, teeth with roots. (Rat) 6
6.a) - Temporal ridges almost straight and parallel along the upper sides of the braincase. COMMON
b) - Temporal ridges slight or absent, curved along upper sides of braincase if present. SHIP RAT
6.a) Four cheek teeth. DORMOUSE
b) Three cheek teeth 7
To continue further in this key you will, ideally, need a powerful handlens or alow-power microscope to view teeth and their roots, and a micrometer to measure to fractionsof a millimetre. You may, however, just be able to get by with good eyesight and a ruler !

To count tooth roots, pull out the tooth and count the socket holes in the skull.

7.a) First upper molar with three or more roots. (Mouse) 8
b) First upper molar rootless, or with one or two roots. (Vole) 9
8.a) - First upper molar with 5 roots,
- Upper cheek tooth row 2.8mm or less.
b) - First upper molar with 3 roots,
- Upper cheek tooth row 2.9 - 3.4mm.
c) - First upper molar with 4 roots,
- Upper cheek tooth row 3.7mm or more.
9.a) Cheek teeth rootless .FIELD VOLE
b) Cheek teeth develope two roots with age. In very young animals this may be difficult to see. BANK VOLE
Sub-species of the european vole Microtus arvalis are found on Orkney and Guernsey. They are the only voles on those islands.

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