The Origin and Evolution of Mammals - Moodle

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postcranial skeleton, and Graybeal et al (1989) on the inner ear structure, the osteology of Morganucodon is extremely well studied (Fig. 5.3(a)–(c)). It was a small animal with a skull 2–3 cm in length and a presacral body length of about 10 cm. In general appearance it would have looked like a shrew or mouse. The skull has all the superficial appearance of a small, modern mammal characterised by the absence of a postorbital bar, slender zygomatic arch, and expanded braincase consisting largely of the parietal bone above. The sidewall of the brain case is formed by a prominent anterior lamina of the periotic behind that is pierced for the V 2 and V 3 branches of the trigeminal nerve, and an epipterygoid, or alisphenoid in front. As in the non-mammalian cynodonts, the latter bone is a broadened, vertical pillar from the base of the skull right up to the roofing bones. The lower jaw is also characteristically mammalian in form, with fully developed coronoid, angular, and articular processes of the dentary. However, unlike most subsequent fossil and modern mammals, there is a double jaw articulation (Fig. 5.3(b)). The fully formed mammalian jaw joint between a dentary condyle and a squamosal glenoid fossa is visible laterally. Medially to, and supported by the dentary and squamosal bones respectively, lie the reduced but still easily recognisable articular and quadrate hinge bones of the reptiles. Indeed, the postdentary bones are altogether cynodont-like, though reduced in size, and they lie in a medial trough of the dentary. Crompton and Luo (1993) have demonstrated the presence of a slender reflected lamina closely attaching to the hind edge of the angular process in a specimen of the Chinese species Morganucodon oehleri (Fig. 4.9(h)). Unfortunately it is not possible to be sure whether or not or a retroarticular process of the articular was also present, as predicted by Allin’s theory of the evolution of the middle ear. Certainly there was a large promontorium on the ventral side of the ear region of the skull, housing the enlarged cochlea and implying that Morganucodon had a significantly enhanced hearing system (Luo et al. 1995). The dentition (Fig. 5.3(b)) is what would be expected in an animal living primarily on insects and other small, easily dealt with invertebrates. The cusps of all the teeth tend to be sharp, and the molariform teeth are primarily adapted for a cutting or THE MESOZOIC MAMMALS 143 shearing action rather than crushing and grinding. There are usually four incisors in both the upper and the lower jaw, followed by modest canines. All of them have wear facets indicating that the upper and lower front teeth actively worked against each other in the course of food collection. There are five upper and four lower premolars, each with a large, laterally flattened and sharp main cusp, a smaller posterior accessory cusp behind it, and practically no trace of a cingulum. The three molariform teeth in both upper and lower jaw together formed a shearing system designed to shred up food, as first analysed by Crompton and Jenkins (1968). Upper and lower molars have the same basic structure of a large, laterally compressed, and sharp main cusp. Two posterior accessory cusps and a single anterior accessory cusp are also present, and lie along a line parallel to the jaw. The upper molars possess a cingulum of fine cuspules that completely surrounds the crown; the lower molars differ in that their cingulum is restricted to the inner, lingual edge of the crown, and the individual cuspules are larger but less numerous. The wear facets on the teeth show that accurate, extensive occlusion occurred between the opposing molar teeth. (Fig. 5.3(c)). The main cusp of a lower molar worked against the gap between the main cusp and the anterior accessory cusp of the corresponding upper molar. At the same time, the main cusp of the upper tooth worked against the equivalent space between the main cusp and the posterior accessory cusp of the lower. As a consequence of the wearing effect of tooth-to-tooth contact, flat facets with sharp free edges developed on the opposing teeth that come to behave like the opposing blades of a pair of scissors. Any resilient food caught between them is liable to be sliced, or sheared into two. As was described earlier, the essence of the mammalian design of postcanine teeth relates to the direction of movement of the lower molars during occlusion. As one of them shears against its opposing upper tooth it is forced to move very slightly inwards as well as upwards, while retaining contact. To achieve this, the lower jaw as a whole must follow a triangular pathway as viewed from behind: upwards and inwards during occlusion, downwards as the jaws opened, and laterally ready for the next bite. Only the molar teeth on one
144 THE ORIGIN AND EVOLUTION OF MAMMALS I 1 (b) Morganucodon C PM 4 M1 M 4 c.pr ang.pr (d) (e) refl. lam (a) cond cond Megazostrodon Dinnetherium art (c) upper Figure 5.3 Morganucodontan skull and dentition. (a) Skull of Morganucodon watsoni in dorsal, ventral, lateral, and posterior views. Skull length approx. 2.6 cm (Kermack et al. 1982). (b) lower jaw in lateral and medial views (Kermack et al. 1973). (c) Occlusal relationships between one upper and two lower molars in side and occlusal views. (d) Molariform dentition of Megazostrodon rudneri in occlusal and side views (Crompton 1974). (e) Dinnetherium lower jaw in lateral view. Length of jaw approx. 5 cm (Crompton and Luo 1993) and Dinnetherium dentition, occlusal view of uppers (top) and external view of uppers and lowers. Shaded areas are wear facets (Crompton and Luo 1993). ang.pr, angular process; art, articular; cond, dentary condyle; c.pr, coronoid process; refl.lam, reflected lamina of the angular. lower
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The Origin and Evolution of Mammals
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The Origin and Evolution of Mammals
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Preface This book arose from twin a
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For Mal /gosia with love and thanks
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Contents 1 Introduction The definit
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CHAPTER 1 Introduction There are ab
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transversely occluding molar teeth
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the Mesozoic mammals, is to do with
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a hierarchy of committees under the
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it means that a 200 Ma igneous rock
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een a more fundamental impact than
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Table 2.3 Classification of marsupi
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(a) (b) (c) humerus radius aquatic
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(a) Westlothiana Limnoscelis PO Wes
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the ankle bones to form an astragal
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(b) (c) (a) Casea rutena Casea broi
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(Fig. 3.2(f)), is actually an ophia
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the first one is enlarged, often to
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Even at their initial appearance in
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(a) Nikkasaurus (b) (d) Reiszia (e)
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Nikkasauridae The family Nikkasauri
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has figured large in discussions of
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(c) Figure 3.9 (continued). Syodon
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a mixture of progressively more spe
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animal with interdigitating, but no
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(e) (a) Anomocephalus Ulemica (b) S
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mandibulae musculature. This, as in
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To date the postcranial skeleton of
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ecognised four subgroups, based mai
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the presence of a deep, longitudina
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collected from the Lystrosaurus Ass
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(a) (d) (c) (b) Leontocephalus V PA
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(a) (c) Lycosuchus Oliveria (d) EVO
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separate families. Lycosuchidae (Fi
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consist of a large labial cusp and
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Procynosuchus (d) Procynosuchus (a)
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They constitute the monophyletic gr
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Although there is no mammal-like co
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Cynognathidae Cynognathus (Fig. 3.2
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(e) (f) Figure 3.22 (continued). Ma
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(c) (d) (a) (b) Kayentatherium EVOL
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Pachygenelus (e) (a) (c) Therioherp
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palatal, and orbitosphenoid bones,
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Wible and Hopson 1993). The interor
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published a cladogram based on rece
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310-320 Ma. By this time, the great
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are forms such as Casea itself, var
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lakes and swamps in the low-lying l
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urrowing and subsisting on a diet o
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Eothyris Haptodus sphenacodontine B
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z (a) (c) a.pt.m. M B PO P P SQ P M
Page 104 and 105: (b) a.pt.m. temp.m. a.pt.m. temp.m.
Page 106 and 107: the dentary bone above the level of
Page 108 and 109: the therocephalian grade was not on
Page 110 and 111: A balance was also achieved in the
Page 112 and 113: Locomotion Ancestral amniote grade
Page 114 and 115: screw-shaped: the front part faces
Page 116 and 117: For the recovery phase, the relativ
Page 118 and 119: SC PRC p.i.f.i tr.min (c) dp.cr ect
Page 120 and 121: the therocephalian pelvis and hindl
Page 122 and 123: spc s.sp s.sp SC PRC (d) delt T AST
Page 124 and 125: no significant novelties to what ha
Page 126 and 127: (a) (c) (e) (g) D D ex. au.m C tym
Page 128 and 129: (a) (c) (d) PMX (f) V n.turb mx.tur
Page 130 and 131: ol.b (a) (b) cer.hem gorgonopsian t
Page 132 and 133: (a) (b) (i) (ii) (iii) (iv) (v) a g
Page 134 and 135: cold stress, the part that usually
Page 136 and 137: conducted the experiment of wrappin
Page 138 and 139: mammals themselves that the enlarge
Page 140 and 141: elevation of maximum aerobic activi
Page 142 and 143: a mammal. The difficulty with all s
Page 144 and 145: collection, ingestion, and assimila
Page 146 and 147: were edaphosaurid and caseid pelyco
Page 148 and 149: CHAPTER 5 The Mesozoic mammals The
Page 150 and 151: (a) (d) AL condylar foramina are se
Page 152 and 153: evidence to relate the haramiyidans
Page 156 and 157: side of the head can be in action a
Page 158 and 159: the lower molars is relatively high
Page 160 and 161: morganucodontan teeth. However, des
Page 162 and 163: adjacent lower molars. A small exte
Page 164 and 165: (a) (b) (d) P4 P4 Plagiaulax P4 Pau
Page 166 and 167: American Morrison Formation genus C
Page 168 and 169: a self-sharpening property. As the
Page 170 and 171: near parasagittal gait (Fig. 5.11(e
Page 172 and 173: pass through the birth canal. Relat
Page 174 and 175: (a) 1 (b) (d) me (c) me.d 3 styl st
Page 176 and 177: (a) (c) Henkelotherium Crusafontia
Page 178 and 179: pubis is excluded from the acetabul
Page 180 and 181: Cretaceous, (Aptian or Albian) of M
Page 182 and 183: eautifully preserved placentals fro
Page 184 and 185: subsequent specimens revealed that
Page 186 and 187: Minimal divergence of tribosphenida
Page 188 and 189: (c) (a) M 1 M 1 M 2 Steropodon M 2
Page 190 and 191: (b) (a) (c) pa.d pr.d me.d Ambondro
Page 192 and 193: crown-group therians) is accepted b
Page 194 and 195: form of the tooth, must reflect sub
Page 196 and 197: of feasibility that a taxon of endo
Page 198 and 199: mammals, by creating environmental
Page 200 and 201: the 11 species of marsupial, of whi
Page 202 and 203: (d) (a) pa A Dasyuromorphia B C pr
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earing two huge claws on digits thr
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that contains the independent ances
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(b) (d) (a) Glasbius Alphadon (c) D
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elieved to be Late Cretaceous in ag
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(Fig. 6.5(c)). The dentition exhibi
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(d) Figure 6.6 (continued). Thylaco
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(a) (c) Caroloameghina and Procarol
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(a) (e) (d) Proargyrolagus Groeberi
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(a) (b) Djarthia Thylacotinga (c) (
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LIVING AND FOSSIL MARSUPIALS 211 M
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Extinct Notoryctemorphia At present
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(f) (g) Wakaleo Diprotodon optatum
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fossil mammals, which might help cl
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30 40 50 60 Figure 6.13 Southern Go
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the Diprotodontia also included gen
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1. Placentalia 2. Edentata 3. Epith
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effectively absent. However, increa
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Asioryctes (a) (b) (d) Cimolestes p
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and Prokennalestes, although it doe
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(a) Leptictidium Palaeoryctidans we
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(a) Purgatorius (c) are part of the
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(a) (d) (e) (g) Protoungulatum Meso
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(a) (f) (e) Hyopsodus Phenacodus (d
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Titanoides (pantodont) (a) (c) (b)
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(a) (b) (d) Trogosus (tillodont) Es
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Mioclaenus Paulacoutoia (didolodont
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their presence. The five orders may
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Xenungulata. Only two Late Palaeoce
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(a) (b) (d) (c) Oxyaena Patriofelis
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continuously growing, and form a gr
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navicular facet, 19 or more thoraci
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(a) (c) Phosphatherium Numidotheriu
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coexist with other characters that
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The salient feature of Widanelfasia
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1 (a) 1. Perissodactyla 2. Titanoth
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(a) (b) BUNODONTIA or SUIFORMES SUI
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time, which suggests that it was on
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condition. This particular combinat
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etween Primates, Dermoptera (colugo
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have postcranial adaptations for ar
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undoubtedly related at a supraordin
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indisputably to occur prior to the
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The Late Cretaceous mammal record i
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interordinal divergences in the Lat
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diversity on Earth (Janis 1993). Fu
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effect of the surrounding sea. Trop
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was high. It lasted from 5 to 3 Ma
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and thus remain in their preferred
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agreement about exactly when within
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References Abdala, F. Redescription
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Ax, P. 1987. The phylogenetic syste
Page 306 and 307:
Bramble, D. M. 1978. Origin of the
Page 308 and 309:
Colbert, E.H. 1948. The mammal-like
Page 310 and 311:
Duvall, D. 1986. A new question of
Page 312 and 313:
Gow, C.E. 1980. The dentitions of t
Page 314 and 315:
Ivakhnenko, M.F. 1990. The late Pal
Page 316 and 317:
Kemp, T.S. 1988a. A note on the Mes
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cladogenesis in dasyurid marsupials
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(eds) Evolution of Tertiary mammals
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McKenna, M.C. and Bell, S.K. 1997.
Page 324 and 325:
(ed) The phylogeny and classificati
Page 326 and 327:
Ray, S. 2000.Endothiodont dicynodon
Page 328 and 329:
Rubidge, B.S., Kitching, J.W., and
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Simpson, G.G. 1970. The Argyrolagid
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Thewissen, J.G.M. 1990. Evolution o
Page 334 and 335:
Novacek, M.J., and McKenna, M.C. (e
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Index Note: Page numbers in italics
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Equoidea 262 Equus 262 Erethizon 28
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Overkill hypothesis 289, 290 Oxyaen
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Tulerpeton 14, 18 Tylopoda 264 Ukha
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