Introduction Amniotes appear in Paleozoic era Pennsylvanian of

Introduction Amniotes appear in Paleozoic era (Pennsylvanian of Carboniferous) Sauropsida—gave rise to turtles, lizards, snakes, dinosaurs, and birds Synapsida—gave rise to mammals

FIGURE 01: Phylogeny of Tetrapods Adapted from Gauthier, J, A. G. Kluge and T. Rowe. , Cladistics, 4 (1988): 105– 209; adapted from Benton, M. J. Vertebrate Palaeontology, Second edition. Chapman and Hall, 1997; and adapted from Carroll, R. L. Vertebrate Palaeontology and Evolution. W. H. Freem

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Synapsida • Dominated terrestrial faunas during Permian and early Triassic • Many synapsid groups went extinct during Permian extinction event • Therapsida survived – Gave rise to cynodonts – Later, otherapsids dwindle • Mammals arose from cynodont ancestor in late Triassic

Synapsid Characters • Single temporal fenestra on skull • Jaw muscles move to outer braincase FIGURE 02: Diagrammatic views of skulls of amniotes showing some of the arrangements of temporal openings

Synapsid phylogeny Phylogeny adapted from Wible, J. R. , et al. , American Museum Novitates 3149 (1995) 1– 19; adapted from Rowe, T. , J. of Vertebrate Palaeontology, 8 (1998): 241– 264; adapted from Rowe, T. Mammal Phylogeny: Mesozoic Differentiation, Multituberculates, Monotre FIGURE 03: Phylogeny and representative skulls of synapsids

Synapsid Characters • Maxilla contacts quadratojugal bone • Caniniform maxillary teeth • Narrow neural arches on trunk vertebrae

Synapsids • Permian synapsids include a paraphyletic group called Pelycosaurs FIGURE 05: Reconstructed skeletons of primitive and derived synapsids, showing changes in the postcranial skeleton. Modified from Romer, A. S. Vertebrate Paleontology. University of Chicago Press, 1966.

Adapted from Jenkins, FA Jr. , Journal of Zoology 165 (1971): 303– 315; Adapted from Jenkins, F. A. Jr. , Evolution 24 (1970): 230 -252 Adapted from Jenkins, F. A. Jr. Primate Locomotion. Academic Press, 1974.

Therapsids • Appear in middle Permian • Diverse assembalage • Include early cynodonts

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Therapsid Characters • Enlarged temporal opening • Sagittal crest and zygomatic arches • Upper canines enlarged • More upright limb posture • Deep acetabulum

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Therapsid Characters • Feet shortened • External auditory meatus within squamosal • Jaw joint in line with occiput • Anterior coronoid bone absent

Cynodont therapsids • Masseteric fossa on dentary bone • Two occipital condyles • Zygomatic arches flare laterally • Angular bone reduced • Teeth absent from pterygoid bone

Cynodont therapsids • Incisors spatulate • Accessory cusps on postcanine teeth • Partial secondary palate • Ribs reduced on lumbar vertebrae • Calcaneal heel present on foot

Early Mammals FIGURE 10: Reconstruction of Eozostrodon, a Triassic mammal of the family Morganucodontidae Adapted from Crompton, A. W. & F. A. Jenkins, Jr. , Biological Reviews 43 (1968): 427.

Stem Mammals • Dentary-squamosal jaw articulation • Tabular bone absent in skull • Medial wall of orbit enclosed • Double-rooted cheek teeth • Cheek teeth include premolars and molars

FIGURE 09: Stages in the development of the lower jaw and ear region in a young opossum Monodelphis (Didelphidae) Adapted from Ghiselin, M. T. , and Pinna, G. New Perspectives on the History of Life. California Academy of Sciences, 1994.

FIGURE 08: Selected major stages in the evolution of the mammalian jaw joint and ear region Adapted from Ghiselin, M. T. , and Pinna, G. New Perspectives on the History of Life. California Academy of Sciences, 1994.

Stem Mammals • • Precise occlusion of cheek teeth Diphyodont dentition Mandibular symphysis reduced Derived features of soft tissues – Mammary glands with teats – Viviparity – Separate anal and urogenetal openings – Digastricus muscle lowers jaw

Theria • Tribosphenic molars • Supraspinous fossa on scapula • Cochlea spiraled

Important Transformations • Evolution of a new squamosal-dentary jaw joint • Articular, quadrate, and angular bones of lower jaw detach and become part of ear apparatus (malleus, incus, and tympanic ring, respectively)

Other Transformations Evolution of: • More complex cusps on molars • Secondary palate – Facilitated suckling • Parasagittal movement of limbs (more mammalian limb posture) • Endothermy

Early Mammals • Small bodied (e. g. 20– 30 grams) • Premolars and molars in cheek teeth • Large brain size • Probably insectivorous diet • Probably nocturnal and arboreal • Lactation and suckling likely

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Mesozoic Multituberculates • Herbivorous or omnivorous • Appear in late Jurassic – Fossil record spans 100 million years • Similar in appearance to rodents – 2 or 3 incisors above and 2 on bottom – Diastema – Massive, blade-like lower premolars

Multituberculates FIGURE 16: Ptilodus (order Multituberculata) skull Modified from Romer, A. S. Vertebrate Paleontology. University of Chicago Press, 1966.

Multituberculates • Some retained cervical ribs • Unusual chewing mechanics Multituberculates began to decline in late Paleocene—probably due to competition with condylarths, early primates, and rodents

Cretaceous Mammals • Metatherians and eutherians evolved from a clade of Mesozoic mammals • Angular process on dentary • Tribosphenic molars • Lower molars have talonid in some forms

Cretaceous Mammals FIGURE 17: Molar of Aegialodon, an early Cretaceous tribosphenidan (family Aegialodontidae) Parts c and d modified from Romer, A. S. Vertebrate Paleontology. University of Chicago Press, 1966.

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Cretaceous Mammals • Much biotic interchange between continents in Jurassic—sets the stage • Flowering plants become dominant flora • Insects radiate with flowering plants in Cretaceous • Dinosaur fauna begins to decline • Mammal faunas begin to radiate

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