. Bulletin of the British Museum (Natural History), Geology. BARTONIAN MAMMALS OF HAMPSHIRE BASIN 311 2>1 .1- .8- & T. h. preecei ^H/9 = ° **3 = • T. h. hmlvticug ^}/2~ ^ M, = A M2 =4 Mj =A A =2 ~Y T T ~Y ~Y T 20 Text-figure 35 Scatter diagrams of length (1) against width (w) in M1/2 and M3 of Treposciurus helveticus helveticus Schmidt-Kittler from Eclepens B and T. helveticus preecei subsp. nov. from Creechbarrow. Measurements in millimetres. Lines join teeth of one individual. nearer the hypocone. Some specimens have smooth enamel and no metalophule I at all and there ar
. Bulletin of the British Museum (Natural History), Geology. BARTONIAN MAMMALS OF HAMPSHIRE BASIN 311 2>1 .1- .8- & T. h. preecei ^H/9 = ° **3 = • T. h. hmlvticug ^}/2~ ^ M, = A M2 =4 Mj =A A =2 ~Y T T ~Y ~Y T 20 Text-figure 35 Scatter diagrams of length (1) against width (w) in M1/2 and M3 of Treposciurus helveticus helveticus Schmidt-Kittler from Eclepens B and T. helveticus preecei subsp. nov. from Creechbarrow. Measurements in millimetres. Lines join teeth of one individual. nearer the hypocone. Some specimens have smooth enamel and no metalophule I at all and there are complex intermediates between most of these types. Fig. 7 shows a doubled meta- conule 2, whilst Fig. 9 shows a single, huge, undivided metaconule such as occurs in the late Lutetian Protadelomys cartieri from Egerkingen y, Switzerland. One very curious M1/2 (M35514) is relatively elongated, has no metalophule I, a very shallow sinus and a strong lingual cingulum completely spanning the sinus and rising distally to about a third the crown height of the hypocone. There is less variation in the lower molars. No M1/2s show features typical of T. mutabilis and the lower molars of T. intermedius have no special distinguishing characters. M35524 is peculiar in having a double mesoconid. An M3 (M37348) deserves special mention (PI. 17, fig. 6); unlike the typical shape shown in PI. 17, fig. 4, its outline is a parallelogram, the acute angles being the mesiolingual and distobuccal corners. Phylogenetic relationships. From the polymorphic pattern described above it is possible to formulate a phylogenetic model for the three Treposciurus species. This involves a cladogenesis with T. helveticus as the common ancestor of T. mutabilis and T. intermedius. The two Ludian species would have developed by selection of different morphs already present in the Bartonian species. The two daughter species are thus more specialized than the ancestral species. This speciation event seems to have o
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