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The Origin and Evolution of Mammals - Moodle

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(a)<br />

1<br />

(b)<br />

(d)<br />

me<br />

(c)<br />

me.d<br />

3<br />

styl styl<br />

pa<br />

pr.d<br />

pa.d<br />

pa<br />

2<br />

2<br />

In fact they tend to be even simpler because <strong>of</strong> the<br />

relatively weak development <strong>of</strong> the cingulum,<br />

presumably a secondary reduction. Some have<br />

teeth in which the angle between the main cusps<br />

is obtuse, such as Tinodon from the Jurassic <strong>of</strong> North<br />

America (Fig. 5.14(a)), <strong>and</strong> the Early Cretaceous<br />

Gobiotherodon from Mongolia. Given that this pattern<br />

<strong>of</strong> cusps is similar to that <strong>of</strong> Kuehneotherium it is<br />

presumably an ancestral feature <strong>and</strong> the obtuseangled<br />

‘symmetrodontans’ are almost certainly<br />

paraphyletic (Luo et al. 2002).<br />

1<br />

1<br />

3<br />

3<br />

me<br />

2<br />

(e)<br />

(f)<br />

(g)<br />

1<br />

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7�?<br />

5�<br />

prd<br />

phyd<br />

1 4�<br />

3�<br />

7�<br />

THE MESOZOIC MAMMALS 163<br />

4�<br />

6�<br />

ppr<br />

Kuehneotherium Shuotherium<br />

2<br />

2<br />

pme<br />

past<br />

ANTERIOR<br />

1<br />

pa<br />

3<br />

pal<br />

5<br />

pr<br />

6<br />

2<br />

4<br />

me<br />

Others, the spalacotheriids, are the acute-angled<br />

‘symmetrodontans’, in which the angle between the<br />

three main cusps is less than 90 o . This group shares<br />

certain features with the remainder <strong>of</strong> the<br />

Holotheria, indicating a relationship. <strong>The</strong> most<br />

important is the exaggeration <strong>of</strong> the postvallumprevallid<br />

shearing action, between the back <strong>of</strong> the<br />

upper molar <strong>and</strong> the front <strong>of</strong> the lower, which is the<br />

functional reason for the development <strong>of</strong> the acute<br />

angulation <strong>of</strong> the cusps. Spalacotherium, which<br />

occurs in Late Jurassic to Early Cretaceous deposits<br />

BUCCAL<br />

st<br />

pr.d<br />

1<br />

2 3<br />

pa.d<br />

me.d<br />

6<br />

n<br />

hy.d<br />

4<br />

hyl.d<br />

6<br />

ec.d<br />

Figure 5.13 Kuehneotherium praecursoris (a) Lower jaw in medial, lateral, <strong>and</strong> dorsal views. (b) Medial <strong>and</strong> lateral views <strong>of</strong> upper molar.<br />

(c) Medial <strong>and</strong> lateral views <strong>of</strong> lower molar. (d) Occlusal diagram <strong>of</strong> two upper <strong>and</strong> one lower molars. Length <strong>of</strong> jaw approx. 1.4 cm (Kemp 1982,<br />

from Kermack et al. 1968). (e) Shuotherium dongi lower jaw in dorsal <strong>and</strong> medial views. Length <strong>of</strong> jaw as preserved approx. 1.2 cm (Chow <strong>and</strong><br />

Rich 1982). (f) Upper molar <strong>of</strong> Shuotherium shilongi in anterior <strong>and</strong> occlusal views (Wang et al. 1998). (g) Postulated occlusal relationships<br />

between upper <strong>and</strong> lower molars <strong>of</strong> Shuotherium (left) compared to a tribosphenidan (right) (Wang et al. 1998).

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