Topic 34 Mammals Class Mammalia Mammals class Mammalia represented by more than 6 400 species Includes the largest extant animals blue whale and some of the most intelligent such as elephants cetaceans and primates Basic plan body of mammals four legged terrestrial animals but some mammals are adapted for life at sea in the air in the trees or on two legs Mammals are amniotes that have hair and produce milk Blue whale 33 m Bumblebee bat 30 40 mm Mammals Shared derived traits Mammals differ from reptiles by shared derived traits 1 Mammals have mammary glands which produce milk Origin of mammary glands is unclear Soft tissues are poorly fossilized Hypothesized to have evolved from sweat glands 2 Mammals have hair covering their bodies at some stage of their life cycle Fine cylindrical keratin fibres Insulation protection and sensory perception whiskers Mammals Shared derived traits 3 Three middle ear bones Formed by the relocation of leftover jaw bones to the middle ear Allows the detection of higher frequency sounds than reptiles which have one Mammalian ear Fig 34 39 The evolution of the mammalian ear bones ear bone 4 Mammals are endothermic Mammals can regulate their body temperature independently of the external environment Independent origin from avian endothermy Hair and fat also help with thermoregulation Endothermy together with a four chambered heart supports a high metabolic rate Mammals Shared derived traits 5 Dentary squamosal jaw joint Mammals have a unique jaw joint formed between the dentary lower jawbone that carries the teeth and squamosal small cranial bone bones Articular and quadrate reduced in size and incorporated into the ear In most gnathostomes incl early synapsids the jaw joint consists of the articular small bone back of lower jaw and the quadrate small bone back of upper jaw Fig 25 7 The origin of mammals Mammals Shared derived traits 6 Differentiated teeth reptile dentition between species Mammalian dentition is much more complex and differentiated than Greater diversity of tooth morphology both within an individual and Wolf skull Incisors canines premolars and molars Adapted for different functions such as cutting piercing tearing and Dental diversity reflects the wide range of diets among mammals Mammalian teeth also occlude more closely than those of reptiles Allows food to be ground more finely Finely ground food has a greater surface area permitting more rapid Beaver skull grinding digestion Mammals have two sets of teeth over their lifespan Milk teeth and adult teeth https doi org 10 1038 449413a Mammals Shared derived traits 7 Larger brains Mammals typically have a larger brain than other vertebrates of equivalent size Supports complex behaviours and higher cognitive functions Mammal limbs in line with body Other mammalian characteristics The vertical placement of limbs beneath the body cf lateral placement in most reptiles Better supports the body enabling more efficient movement Limbs modified for running jumping burrowing climbing and swimming The diaphragm contributes to lung ventilation The diaphragm is a sheet of muscle at the base of the rib cage Mammals copulate using an intromittent organ for internal fertilization Most mammals are viviparous giving birth to live young Evolution of mammals from synapsids Permian diversification Fig 25 7 The origin of mammals Synapsids evolved during the Carboniferous period and became the dominant tetrapods during the Permian period evolving into diverse herbivorous and carnivorous forms Early non mammalian synapsids lacked hair had a sprawling gait and laid eggs e g reptile like sailbacks like Dimetrodon The Permian mass extinction drastically reduced synapsid diversity Origin of mammals By the late Triassic some synapsids evolved mammalian traits like hair small size and higher metabolic rates though they still laid eggs Included numerous Therapsida and Cynodont species True mammals emerged during the Jurassic period and diversified into many short lived lineages All non mammalian synapsids are extinct Mammals rapidly diversified after the Cretaceous mass extinction Slide by Neil Harris Dimetrodon Royal Tyrrell Museum WC Dimetrodon WC Cynognathus the largest predatory cynodont of the Triassic Early evolution of mammals By the early Cretaceous three main lineages of mammals had emerged Monotremes Marsupials and Eutherians Coexisting with Cretaceous non avian dinosaurs most early mammals were small bodied insectivores Mammals did not undergo a significant adaptive radiation until after the Cretaceous After the Cretaceous mass extinction 66 mya mammals rapidly diversified evolving into large predators herbivores flying species and aquatic forms Underwent adaptive radiation to exploit a wide range of ecological roles previously dominated by dinosaurs Fig 25 23 Adaptive radiation of mammals Major clades of extant mammals Three main lineages of extant mammals are based on modes of birth and development Two clades are single orders Order Monotremata Monotremes diverged from other mammals 180 mya Order Marsupialia Marsupials diverged from eutherians 140 mya The third clade contains many orders Clade Eutheria placental mammals Fig 34 43 Exploring mammalian diversity Monotremes Monotremes order Monotremata are a small group of egg laying mammals consisting of echidnas and the platypus Only five extant species 1 platypus and 4 echidnas that are found in Australia and New Guinea Monotremes provide milk to young when they hatch Monotremes have no nipples milk is secreted from patches of specialized hairs Monotremes feed on invertebrates and lack teeth as adults Marsupials and Eutherians Shared derived traits Shared derived traits of clade Marsupials Eutherians clade Theria 1 Viviparous development of embryos within the parental body 2 A placenta provides initial nourishment to the embryo The placenta consists of extraembryonic membranes and the lining of the uterus The placenta exchanges nutrients metabolic waste and dissolved gasses between the mother s and embryo s blood 3 Nursing on milk from nipples Nipples are conical outlets from the mammary glands a i r e h T e d a C l Eutherians Marsupials Monotremes Live birth nipples simple placenta yolk sac placenta Lactation hair etc Ancestral mammal Marsupials Marsupials order Marsupialia include opossums kangaroos and koalas 360 extant species all but one in the southern hemisphere The embryo briefly develops within a placenta in the mother s uterus The placenta is