This chapter tackles the evolutionary history of Theria and its two clades, Marsupalia and Eutheria. Therians are viviparous amniotes which give birth to young after a period of internal gestation. Although most therians are terrestrial, there are also therians with burrowing, aquatic, and flying forms. Thus, the evolution of these different ways of life led to corresponding diversity in anatomy, body size, and ecology, as well as many striking cases of convergent evolution. The chapter considers the diversity of Metatheria and Eutheria in relation to the key elements of therian reproductive biology and specializations for feeding and locomotion. It notes how mammals front extinction risk at the hands of humans, which includes habitat loss and the introduction of alien species.
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Therians
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Movement, Locomotion, and Migration
This chapter looks into the reasons for the movement, locomotion, and migration of individual organisms across the planet. Complex free-living organisms may respond to changes in their immediate surroundings by actively determining where to position themselves and for how long. Some organisms preferred other locations for breeding and feeding, and many undertake movements from one location to another on a seasonal basis for either reasons of protection, production, or resource availability. The chapter examines the concept of plant dispersal as part of the reproductive process. Meanwhile, animal movement depends on the active transfer of metabolically generated kinetic energy to the surface.
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Upper Motor Neuron Control of the Brainstem and Spinal Cord
Edited by George J. Augustine, Jennifer Groh, Scott Huettel, Anthony-Samuel LaMantia, Leonard E. White, and Dale Purves,
This chapter evaluates how the axons of upper motor neurons arise from cell bodies in higher centers and descend to influence the local circuits in the brainstem and spinal cord. The sources of these upper motor neuron pathways include several brainstem centers and multiple cortical areas in the frontal lobe. The motor control centers in the brainstem are important in postural control, orientation toward sensory stimuli, locomotion, and orofacial behavior, with each center having a distinct influence. The mesencephalic locomotor region initiates locomotion. Two other centers, the vestibular nuclear complex and the reticular formation, make widespread contributions to the maintenance of body posture and position. The reticular formation also contributes to a variety of somatic and visceral motor circuits that govern the expression of autonomic and stereotyped somatic motor behavior. Also in the brainstem, the superior colliculus contains upper motor neurons that initiate orienting movements of the head and eyes.
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Body Support and Locomotion
This chapter considers the diversity of locomotor modes among amphibians and reptiles from a functional perspective, emphasizing the mechanics of locomotion and examining the underlying similarities observed among diverse taxa that have converged on similar locomotor patterns. The 17,000-plus extant species of amphibians and reptiles occupy habitats ranging from the open oceans to the crowns of rainforest trees and the depths of desert soils. Such diverse habitats require different modes of locomotion. Feet that gain purchase on a tree limb are very different from those required to propel their owner through a pond. Furthermore, several alternative solutions to the same locomotor challenge may exist, depending on the evolutionary starting points of the lineages involved. The chapter considers lever systems, terrestrial locomotion with limbs, jumping, terrestrial limbless locomotion, aquatic locomotion, burrowing, climbing, and aerial locomotion.
