9.1: The Mesozoic Era
(Continued from the previous page)
In geology texts, the Mesozoic Era, which began some 250 million years ago (mya), is usually called the "Age of Reptiles." It is divided into three periods, the Triassic, Jurassic, and Cretaceous (see Table 9.1). In the typical description, encountered in texts on geology, paleontology, and evolutionary biology, it is "the era during which reptiles dominated life on earth until their extinction at the end of the Cretaceous Period, 65 million years ago." The idea that reptiles "ruled" the Mesozoic dates back to Cuvier, who in the early nineteenth century asserted there had once been a time when reptiles were dominant.¹ It also comes from the popular misconception that dinosaurs were all large. Such was not the case—while some dinosaurs weighed 80 tons, others were as small as chickens (Lambert 1991: 115; Norman 1985: 38). It comes, also, in part, from the common belief that the category "dinosaur" contains more different kinds of animals than it actually does. Only two orders of Mesozoic reptiles are classed as dinosaurs:
Other ancient creatures, traditionally categorized as reptiles and popularly included with dinosaurs, were not dinosaurs at all: pterosaurs (flying, batlike creatures, formerly known as pterodactyls), mosasaurs (whalelike marine predators), and plesiosaurs (includes two general categories, one a long-necked, paddle-flippered type, the other seallike). Moreover, many extant reptile forms (e.g., crocodilians, turtles, lizards) date back at least as far as the earliest dinosaurs and have been widespread ever since. Many other types of animals, far too numerous even to mention, were also the dinosaurs' contemporaries.
Another tendency contributing to the belief that the Mesozoic Era was "ruled by reptiles" is the inclination to believe that animals, known only from fossils, had the full suite of characters defining modern reptiles. For instance, most people think of pterosaurs as egg-laying, cold-blooded, flying reptiles with naked, scaly skin. In fact, however, in some cases, where the texture of a pelt has been preserved in fossils, pterosaurs can be seen to have been fur-bearing animals (Broili 1927, 1938, 1939; Goldfuss 1831; Lambert 1991: 22; Seeley 1870; Sharov 1971; Wellnhofer 1991). According to Wellnhofer (1991: 164), this
direct proof of a hairlike body covering seems to have confirmed the warm-bloodedness of pterosaurs once and for all, as only mammals, i.e., warm-blooded creatures, have hair today.
Various researchers have pointed out that, besides fur, the elevated metabolism characteristic of flying animals, and the fact (known from fossils) that pterosaurs could live in a chilly habitat, make it highly likely that pterosaurs were warm-blooded (Wellnhofer 1991). There is no fossil evidence that pterosaurs laid eggs (ibid). Studies of their bones have shown that pterosaurs grew rapidly, as do mammals and birds, not slowly like reptiles (ibid). Warm-blooded young usually require parental care to prevent death by exposure. So pterosaurs, which were abundant during the Mesozoic, may not have even been reptiles, let alone "ruling" reptiles.
No single person has seen all the fossils found over the years—or even most of them—but fossils are nevertheless widely assumed to substantiate the basic tenets of evolutionary biology. The stories told about fossils are familiar to most biologists, but the fossils themselves are not. The reason for this discrepancy is straightforward. It is much easier to read and assimilate a story about the fossil data than to evaluate the evidence itself. Part of the story told about fossils is the claim that mammalian fossils from the Mesozoic are rare. Romer (1966: 197) emphasizes this point:
Mammals presumably came into existence toward the end of the Triassic [~251-206 mya], but we know extremely little about their history during almost the entire span of the Mesozoic [~251-65 mya]. The oldest known mammals appear in the "Rhaetic" beds at the Triassic-Jurassic boundary [~206 mya]. In the Jurassic [~206-144 mya], almost all known mammalian remains come from two English localities ... and one small bone pocket at Como Bluff, Wyoming. In the early Cretaceous [~130 mya], ... our knowledge is confined to fragments from the Trinity sands of Texas and the Wealden of Southern England. Further, almost all the earlier Mesozoic remains consist of isolated teeth, or, at the most, jaws; prior to the Upper Cretaceous [~75 mya] we have not one satisfactory skeleton and very little skull material. Even in the Upper Cretaceous, mammal remains are rare, mostly fragmentary, and are found only in a few areas in North America and Mongolia. In consequence we are still in the dark about much of the history of Mesozoic mammals. We know little about the dental anatomy of most of the forms which have been found; and the sparseness of the record suggests that groups which have escaped detection may well have existed.
During the forty years since Romer wrote these words, the fossils of more Mesozoic mammals have been found, but they are still considered rare. This rarity has been explained in various ways. Thus, it is said that in the early days of fossil hunting, when the great museum collections were being assembled, the emphasis was largely on dinosaurs, and that mammals were supposedly neglected. Being warm-blooded, mammals are also thought to have been more common inland, at higher elevations—conditions under which fossils are rarely preserved because erosion tends to destroy them.
One fact, however, is particularly salient. Mesozoic terrestrial fossils of all kinds are extremely sparse (Romer 1966). Even in the Paleocene, the first epoch of the Cenozoic Era ("Age of Mammals"), such deposits are well known only in western North America (Romer 1966: 336). During the preceding, vast period of time that geologists call the Mesozoic Era, which lasted some 180 million years, terrestrial deposits are sparse indeed. Our knowledge of Mesozoic land animals has been obtained almost exclusively from three regions of the world (southern England, Mongolia, and western North America). Moreover, the fossils from these three sites cover a very small percentage of the time interval in question. As Clemens, et al. (1979: 8) note,
It is emphasized that for the present, negative evidence has little value for Mesozoic mammals. That is, the absence of a group of mammals at a particular time and place [in the fossil record] generally cannot be taken as an indication that it did not in fact occur then and there. The only areas in which negative evidence may be given some, but not conclusive weight [i.e., because the samples are not so inadequate as elsewhere] are those for the Rhaeto-Liassic (Late Triassic or Early Jurassic) of southwestern Britain, the Late Jurassic of southern England and western United States, the Late Cretaceous of Mongolia and the Late Cretaceous of the Rocky Mountain and High Plains of North America.
Outside southern England, Mongolia, and western North America, animals unknown today may well have been abundant throughout the Mesozoic and Paleocene. During this entire time, continental Europe, Africa, Asia (except Mongolia), Australia, South America, Antarctica, and eastern North America are largely a blank, either very poorly known, or not known at all. Given such huge gaps in the data, claims of the rareness of early mammals largely lack empirical verification.
But a review of the literature prompts the suspicion that the most significant reason for the reported preponderance of reptile remains in Mesozoic fossils lies not so much in the actual rarity of mammalian fossils, but rather in a strong tendency of paleontologists to classify fossils as "reptile" if from the Mesozoic, and as "mammal" if they date from a later time. For instance, in referring to an early collector, a modern paleontologist wrote the following: "He [i.e., the collector] knew that the teeth were from the 'secondary' (Mesozoic) and therefore really ought to be reptilian" (Norman 1985: 10). This bias seems to be especially pronounced when it comes to large animals.
The reasons for this bias are subtle, complex, and sheltered by tradition. One obvious explanation, however, is stereotypic thinking. Consider the distinction between fossil mammals and fossil reptiles, already alluded to in the case of pterosaurs. When we hear that an animal is a "mammal," a number of traits spring to mind. Many of these features (warm-bloodedness, four-chambered heart, mammary glands, parental care, diaphragm, raised external ear, viviparity, hair, etc.) are not, or are only very rarely, preserved in fossils. "As a result," notes dinosaur expert David Norman, "the first true mammals are recognised as fossils almost by weight of opinion, rather than anything more scientific" (Norman 1985: 10).
Even in the case of living animals, some of these traits are not valid criteria for distinguishing reptiles from mammals. Crocodiles have hearts with four chambers and care for their young. Even various kinds of frogs exhibit parental care (Campbell 1987: 650). Certain mammals are cold-blooded, including manatees, hyraxes, sloths, tenrecs, naked mole-rats, platypuses, and some bats (Nowak 1991, p.1289; Rosas 1994; Walker 1983: 118, 454, 857).
Scales are usually considered diagnostic of reptiles, but these features are seen also in certain mammals (Nowak 1991). In some, scales are limited to certain regions of the body, for example, scaly-tailed squirrels (Anomaluridae), which have tails that are partially scaled, beavers (Castor), which have tails that are entirely scaled, pichiciegos (Chlamyphorus), Asiatic shrew-moles (Uropsilus), echimyids (Echimyidae), and some porcupines (Atherurus, Trichys). In others, most of the body is covered. Examples include the Javan rhinoceros, armadillos, and pangolins. On the other hand, pterosaurs, traditionally pictured as naked and scaly, are now known to have been furry. The modern leatherback turtle (Dermochelys), classed as a reptile, is warm-blooded, lacks a bony shell, and has leathery, scaleless skin.
Many reptiles, including about half of the extant snakes and lizards, give live birth. But it is usually unknown whether a given extinct animal was viviparous or laid eggs. Moreover, in those exceptional cases where the mode of birth can be determined for a fossil form, the findings are sometimes contrary to stereotype. Ichthyosaurs are traditionally classified as reptiles, but fossils show they gave live birth to their young in the sea as dolphins do.
Nevertheless, when a paleontologist finds in a fossil two or three bony features that usually distinguish modern mammals from modern reptiles, the conclusion is often that all the other features stereotypically associated with a mammal were also present in the corresponding extinct animal.
The same is true of artists' renderings of extinct animals. Let an artist draw a gray or brown animal with fur and ears that stand up from the head and the viewer will have the strongest tendency to think of the pictured animal as a mammal, with all the traits characteristic of mammals. Let her base an illustration on the same set of bones and draw a green animal with naked skin and no ears, and the viewer will think "reptile." It is important always to keep in mind that drawings purporting to represent the outward appearance of Mesozoic animals often reflect the theoretical biases of the artist and are frequently based on scant fossil evidence. Although pictured as complete and intact, the animals illustrated are in many, perhaps even most, cases actually known only from a fragment of a jawbone or a few teeth.
The ensuing pages, which make up the remainder of Section 9, examine each of the major categories of modern mammals (and even some of the minor ones). For each type of modern mammal (e.g., rodents, carnivores, armadillos, whales, etc.), we will see that a similar category of Mesozoic precursors is known from the fossil record. These precursors have been largely ignored, perhaps because their existence does not fit with the accepted claim of orthodox theory that the many different types of extant mammals are derived from a single "generalized" Cretaceous form. The existence of such precursors does fit, however, with stabilization theory. If new forms of life typically arise through stabilization processes, we are not looking for a common ancestor of mammals, or a common ancestor of whales. Instead, our expectation is that ancestral similarity sets would produce descendant similarity sets of a comparable nature. Thus, various early rodentlike forms would give rise to various more modern rodent forms. The accepted view is not that new forms might be expected to arise from precursor forms similar to themselves (e.g., marine mammals from earlier marine mammals), though this idea may seem straightforward and plausible to the uninitiated reader. For example, widely accepted theory claims that whales are descended from a tiny, shrewlike animal, and that the whole transformation required only 10 or 20 million years. NEXT PAGE >>
1. In 1808 Cuvier identified as a giant marine reptile a fossil from a mine near Maastricht in the Netherlands. He named it Mosasaurus. He also identified as a large flying reptile a fossil found in Bavaria, which he named pterodactyl (Pterodactyle Cuvier, 1809). These finds led him to propose there had been a time when reptiles had been predominant (Rudwick 1997). His diagnoses of these animals as reptiles are called into question in subsequent pages of this section.
Most shared on Macroevolution.net:
9.1: The Mesozoic Era - © 2008 Macroevolution.net