Evolution In The Shadow Of Dinosaurs

The evolutionary fortune of mammals and the premam-malian synapsids was quite different from those of other land vertebrates during the Mesozoic. Although phyleti-cally diverse, Mesozoic mammals appear to have been relatively rare, even by comparison with other small, non-dinosaurian vertebrates. During the Permian and most of the Triassic, premammalian synapsids thrived in terrestrial vertebrate faunas worldwide. By the Late Triassic, however, premammalian cynodonts were eclipsed by the rapidly rising dinosaurs. Throughout the rest of Mesozoic, nonavian dinosaurs completely dominated over mammals.

Mesozoic mammals were small to extremely small animals. The Late Triassic and Early Jurassic mammals are, on average, much smaller than the premammalian cyn-

odonts. The cynodont-mammal transition involved a great decrease in body mass, which apparently took place in the Late Triassic. The smallest known Mesozoic mammal was Hadrocodium, which is estimated to have had a body mass of about 2 g (Luo, Crompton, and Sun, 2001). The largest of well-known Mesozoic mammals attained the size of a fox (e.g., Repenomamus, see Li et al., 2000) or a cat (e.g., the largest Sinoconodon specimens, Luo, Crompton, and Sun, 2001); isolated teeth suggest the presence of several kinds of mammals that were substantially larger (Clemens et al., 2003). The vast majority of early mammals, however, were shrew to mouse sized, a pattern that persisted through the entire 155-Ma history of mammals in the Mesozoic.

Dental structure and body size suggest that most Mesozoic mammals were insectivorous; some larger animals, such as Sinoconodon and gobiconodontid adults, were probably carnivorous, incorporating significant amounts of vertebrate prey (or carrion) in their diets; still others, such as multituberculates, were probably herbivorous to omnivorous.

By analogy with modern placental insectivores, Crompton et al. (1978) hypothesized that small, insectivorous early mammals achieved homeothermy (although not necessarily high resting metabolic rates). Homeothermy would have allowed the earliest mammals to be active and forage nocturnally, in places and at times that would have

figure 1.2. Changing interpretations of Mesozoic mammal phylogeny and the timing of their patterns of diversifications. A, An historical (now abandoned) view of the polyphyletic origin of the Class Mammalia via several independent lineages from premammalian cynodont ancestry, as advocated by Simpson (e.g., 1928a, 1959) and accepted by others in the 1940s-1960s (e.g., Olson 1959). In this view of mammalian evolution, there were four independent origins of mammals (in the white bands) of monotremes, multitubercu-lates, triconodonts (= eutriconodontans), and extant therians. Stratigraphic ranges of mammal lineages here reflect the available fossil record up to the 1940s. B, Single origin of Mammalia but a fundamental dichotomy within mammals (e.g., Hopson and Crompton, 1969; Crompton and Jenkins, 1979). This was a prevalent view from the mid-1960s to the 1980s that was generally accepted by many fossil mammal workers. While the monophyletic origin of Mammalia is supported by current evidence, the early division of the "proto-therians" versus therians (two white bands) within the Mammalia has been abandoned. Stratigraphic ranges of major lineages here reflect the available fossil record up to the 1980s. The current view of interrelationships of Mesozoic mammals is shown in figure 1.1. Source: modified from Cifelli (2001).

figure 1.2. Changing interpretations of Mesozoic mammal phylogeny and the timing of their patterns of diversifications. A, An historical (now abandoned) view of the polyphyletic origin of the Class Mammalia via several independent lineages from premammalian cynodont ancestry, as advocated by Simpson (e.g., 1928a, 1959) and accepted by others in the 1940s-1960s (e.g., Olson 1959). In this view of mammalian evolution, there were four independent origins of mammals (in the white bands) of monotremes, multitubercu-lates, triconodonts (= eutriconodontans), and extant therians. Stratigraphic ranges of mammal lineages here reflect the available fossil record up to the 1940s. B, Single origin of Mammalia but a fundamental dichotomy within mammals (e.g., Hopson and Crompton, 1969; Crompton and Jenkins, 1979). This was a prevalent view from the mid-1960s to the 1980s that was generally accepted by many fossil mammal workers. While the monophyletic origin of Mammalia is supported by current evidence, the early division of the "proto-therians" versus therians (two white bands) within the Mammalia has been abandoned. Stratigraphic ranges of major lineages here reflect the available fossil record up to the 1980s. The current view of interrelationships of Mesozoic mammals is shown in figure 1.1. Source: modified from Cifelli (2001).

been largely impossible for poikilothermic vertebrates. Coupled with their more sensitive hearing and olfaction, larger brains for better sensory and motor coordination, and feeding adaptations for insectivory, homeothermy enabled early mammals to successfully invade the nocturnal niches of terrestrial ecosystems of the Mesozoic. Early mammals survived and diversified in the nocturnal niches that had not been previously exploited by other kinds of small, insectivorous vertebrates. The fact that more than half of all modern mammals, and the vast majority of all small mammals, live nocturnal lives (Ryszkiewicz, 1989) is suggestive of a mammalian ancestral niche from their long sojourn with Mesozoic nights.

The evolutionary causes by which mammals remained predominantly small throughout the rest of the Mesozoic are still not well understood. Several speculative scenarios have been developed to account for this phenomenon, including avoidance of predation by large nonmammalian vertebrates, as well as competitive ecological exclusion by small nondinosaur vertebrates and juvenile dinosaurs (critically reviewed by Lillegraven, 1979).

Crompton (1968) suggested that whereas cynodonts acquired hair as insulation, they did not develop the cooling mechanisms characteristic of most modern mammals (but not monotremes). Faced with the increasing temperatures of the Triassic, they evolved smaller sizes to avoid overheating. Bakker speculated (1971: 655): "Small body size and high surface-area-volume ratios would have eased the difficulties of overheating. Nocturnality would also reduce the difficulties—the mammals could be active in ambient temperatures usually at least a few degrees below the body temperature during the Mesozoic nights and could avoid exposure to direct solar radiation."

The earliest known eutherians were very small, ranging from 5 g for Batodon tenuis, to 7 to 8 g for Prokennalestes and Montanalestes (Wood and Clemens, 2001), and to about 20 g for Eomaia scansoria (Ji et al., 2002). The majority of stem metatherians of Cretaceous age were in a similar size range (chapter 12), with the exception of Didelphodon (Clemens, 1966). Lillegraven et al. (1987) noted that small body mass played an important role in the origin of the earliest eutherians and marsupials during the Early Cretaceous in relation to metabolic and reproductive constraints of these mammals. In fact, it is a general pattern that the earliest members of the mammalian clade tended to be smaller than their later and more derived phylogenetic relatives, as documented by a theoretical study of Cenozoic mammals (Alroy, 1998) and a case study of Late Cretaceous mammals of Wyoming (Lillegraven and Eberle, 1999).

It was only after the dinosaur extinction 65 Ma ago that mammals took off in a great evolutionary radiation, oc cupying diverse ecological niches, with many feeding adaptations and a great range of body sizes. Whereas all but two of the well-known Mesozoic mammals are estimated to be much less than 500 g, by the end of Paleocene the body mass of many mammals had reached hundreds of kilograms and by the Eocene the mass of some mammals could be measured in tons. In a great adaptive radiation that gave rise to most of the modern mammalian orders, Eocene mammals invaded all niches accessible for vertebrates, with bats capable of powered flight, der-mopterans capable of gliding, whales and sirenians adapted to the aquatic environment, and so on.

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