What Is A Mammal

Living mammals are easily recognized by a suite of characteristics that distinguish them from all other vertebrates. Most obvious are an external covering of hair (except in certain highly specialized types) and nourishment of the young by milk produced in the mother's mammary glands. The heart has four chambers, allowing separation of blood flow to the lungs (for reoxygenation) from circulation to the rest of the body. There is a muscular diaphragm, related to increased oxygen consumption. Mammals are endothermic and, consequently, generally have higher metabolic rates and higher activity levels than are found in other vertebrates except birds. Vision, hearing, and olfaction tend to be highly developed, and the brain (especially the cerebrum) is relatively larger and more complex than in other vertebrates. Most of these features, however, are rarely (or never) preserved in fossils.

Fortunately, many skeletal features diagnostic of extant mammals are often preserved in fossils. These include a single lower jaw bone, the dentary; a dentary-squamosal articulation between the lower jaw and the skull; three middle ear ossicles; diphyodonty (two sets of teeth with sequential replacement in all except some primitive Mesozoic forms, but molars not replaced); heterodont dentition, typically with complex molar crowns and multiple molar roots, and associated with precise occlusion; a secondary bony palate; a single bony nasal opening; paired occipital condyles; five regionally differentiated sections of the vertebral column, the first two vertebrae at the cranial end modified to allow rotation; ribs usually limited to the thoracic region; modification of the shoulder girdle (including further reduction of the coracoid); reorganization of the pelvic girdle (elongation of the ilium and separation

Cynodont Ear Bones

Fig. 3.1. Evolution of mammalian characters: transformation of jaw joint and origin of middle-ear ossicles. (A) The cynodont Thrinaxodon, in which the quadrate and articular functioned both as the jaw joint and part of the hearing apparatus; (B) the extant marsupial Didelphis, in which the jaw joint is between the dentary and squamosal, and the auditory ossicles (modified from the quadrate and articular of cynodonts; enlarged in inset) are located behind the jaw joint; (C) lower jaw of fetal mammal, showing the developmental similarity to the phylogenetic origin of jaw and auditory features; (D) reconstruction of Morgan-ucodon (compare with A); (E) occlusal relationships in Morganucodon; (F) medial view of the lower jaw of Morganucodon, showing relationship of dentary-squamosal jaw joint to postdentary bones and auditory ossicles. (From Crompton and Jenkins, 1979.)

of its gluteal and iliac surfaces, reorientation of the ischium and pubis posterior to the acetabulum); and separate centers of ossification for the shaft (diaphysis) and ends (epiphyses) of long bones, which result in better-defined joints and determinate growth (Figs. 3.1, 3.2; see also Figs. 2.1, 2.4). The skull and postcranial skeleton of mammals generally comprise fewer elements than in nonmammalian tetrapods, as a result of both fusion and loss of bones.

Although there is little difficulty in distinguishing mammals from other vertebrates in present-day faunas, it has long been recognized that the distinction breaks down when one considers the fossil record. The transition between mammalian forerunners (cynodont therapsids, discussed in the next section) and the earliest mammals now includes many known intermediate stages (discussed in the next section and in Chapter 4) that document the mosaic evolution of "mammalian" traits. Consequently, how to recognize the first mammal has become controversial: which character(s) should be considered most important for recognizing a mammal? Even if a node-based definition of Mammalia is applied, practical identification of mammals (or any other taxon) in the fossil record is ultimately based on anatomical characters. The acquisition of a well-developed dentary-squamosal joint as the only jaw articulation has traditionally been considered to be the most important indication that the mammalian boundary has been crossed, but even here transitional forms are known that possess this articulation in combination with a joint between the articular and the

Marsupial Pelvic Girdle

Fig. 3.2. Evolution of mammalian characters. (A-C) Shoulder girdles and (D-H) pelvic girdles of cynodonts and primitive mammals (not to scale): (A, D) cynodont; (B, F) Morganucodon; (C, G) echidna (extant monotreme); (E) Oligokyphus (tritylodont); (H) Tupaia (tree shrew, an extant placental). (From Jenkins and Parrington, 1976.)

Fig. 3.2. Evolution of mammalian characters. (A-C) Shoulder girdles and (D-H) pelvic girdles of cynodonts and primitive mammals (not to scale): (A, D) cynodont; (B, F) Morganucodon; (C, G) echidna (extant monotreme); (E) Oligokyphus (tritylodont); (H) Tupaia (tree shrew, an extant placental). (From Jenkins and Parrington, 1976.)

quadrate or between the surangular and the squamosal. Other important mammalian synapomorphies include postcanine teeth with two or more roots, diphyodont rather than continuous alternate tooth replacement, a petrosal promon-torium (the bony swelling enclosing the cochlea and forming the medial wall of the middle-ear cavity), and a bony floor of the cavum epiptericum (the fossa in the braincase that houses the sensory ganglion of the trigeminal nerve, cranial nerve V; Miao, 1991; Kielan-Jaworowska, 1992; No-vacek, 1993; Luo et al., 2002).

As the fossil record of the therapsid-mammal transition improves, it has also become apparent that many "mammalian" characters arose independently multiple times by convergence, making recognition of Mammalia even more problematic. (For example, critical features that may have evolved more than once among mammals include the three ossicles of the middle ear and the dentary-squamosal joint.) Rowe (1988) therefore proposed that a distinction be made between the definition of the group, based on ancestry and taxonomic content, and its diagnosis, based on morphology

Adopting the "crown-group" concept (that higher taxa should be restricted to descendants of the most recent com mon ancestor of two or more extant lineages), Rowe defined Mammalia as all taxa stemming from the last common ancestor of monotremes and therian mammals. Although this approach may seem to provide a neat solution to the ambiguity of what constitutes a mammal, such a restrictive definition excludes many fossil groups long accepted as mammals on anatomical grounds. At the same time, it necessitates the creation of several new higher taxa (Mammaliamorpha, Mammaliaformes) to encompass successive outgroups to Mammalia. Perhaps most objectionable is the volatile composition of Mammalia that results from the instability of the position of monotremes (e.g., Lucas, 1992). A crown-group definition of Mammalia is no more biologically real (or less arbitrary) than any other definition, and in this case conflicts with widely held morphological definitions. In agreement with most students of Mesozoic mammals, a more inclusive stem-based definition of Mammalia is employed in this volume (see also Luo et al., 2002; Kielan-Jaworowska et al., 2004; Kemp, 2005), essentially equivalent to Rowe's Mam-maliformes. Thus Mammalia as used here includes all taxa more closely related to monotremes and therians than to tritheledonts or tritylodonts (see Fig. 4.2).

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  • Lana
    Where is the root of the ear?
    9 years ago

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