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Only rarely can we look at a fossil and directly determine mode of birth. We can, however, with a high degree of certainty assign many fossils to particular taxonomic categories. Moreover, studies of plate tectonics have generated detailed information that has now been synthesized into a fairly exact picture of the relative positions of the continents in past geologic ages. When past continental contacts are considered in conjunction with fossil data, clear conclusions can be reached concerning the time of origin of placental mammals. For example, consider the primates, members of the mammalian order to which we ourselves belong. The very earliest known primate fossils date back no further than the late Cretaceous (approx. 70 mya). But the absence of earlier primate fossils does not mean primates themselves were absent at an earlier date. For all terrestrial organisms the fossil record is next to nonexistent in the early and middle Cretaceous, a period of about sixty million years (beginning at about 144 mya). Moreover, because their tropical forest habitat is not conducive to the formation of fossils, primates are rarely preserved even in much more recent strata. Primates therefore could easily have existed long before the date corresponding to the earliest known primate fossils.
Those fossils that actually are known strongly suggest primates—and therefore placental mammals—are much older than is generally believed. The first fact to consider in this connection is that monkey (cercopithecoid) fossils are known from the mid-Tertiary both of Africa and South America. At that time, South America had long been isolated from Africa by an ocean barrier, since about 105 mya. (SEE PLATE TECTONICS ANIMATION.) Primates are not aquatic animals. With few exceptions (Nasalis, Haplalemur, Homo), they are, in fact, notorious for their complete inability to swim. Even a narrow moat will efficiently contain these animals in zoos. How, then, did monkeys reach both of these continents? It seems utterly implausible to suppose they came into being on one of the two landmasses and then swam across open ocean to colonize the other. Apparently, the data can be explained only by assuming that monkeys arose prior to the separation of South America and Africa, which would imply that the origin of placental mammals occurred sometime before 105 mya.
|Ring-tailed Lemurs: The geographic distribution of fossil lemurs is inconsistent with the usual story told about the origin of placental mammals.|
Monkeys are not the only primates with this sort of paleogeographic distribution — Lemurs are also known from the early Miocene of Africa and the Eocene of South America. Since monkeys, lemurs, and all other primates are placental mammals, these facts imply that the origin of placental mammals occurred before 105 mya. The same inference concerning the antiquity of the origin of placental mammals can be drawn on the basis of the distribution of fossil ungulates and elephants. The fact that stegosaurid/pangolins are known from Jurassic strata is also consistent with this conclusion. Some writers have suggested monkeys and lemurs “rafted” across the Atlantic Ocean on dead trees and debris. But this is implausible. Drifting at random, without provisions on an open ocean, a primate would face almost inevitable death. Death by thirst or drowning would be the two most likely dooms, but starvation and fatal exposure to the elements would also be high on the list of probable fates. Other writers speak of “waif dispersal,” but this term seems to be little more than an erudite way of saying “they got there somehow.”
But primates may have arisen much earlier. Fossil lemurs are common in the early Tertiary deposits of North America and Eurasia. Together these two modern continents composed the proto-continent “Laurasia." The earliest known specimen is a lemur molar from the late Cretaceous (or early Paleocene) of Montana. Extant lemurs are limited to Madagascar, an island that, according to Rabinowitz et al. (1983) has been isolated by an ocean barrier since at least 150 mya, that is, the late Jurassic. About latest time that a lemur could have walked from Madagascar to Laurasia was in the early Cretaceous — at about 170 mya. These facts suggest that primates predate 170 mya and, consequently, that origin of placental mammals occurred in (or prior to) the early Jurassic — a time when it is generally agreed that the latest synapsids were still in existence.
Similar conclusions can be reached by considering the paleogeographic distribution of other placental mammals. For instance, until recently it had been thought the lesser anteater (Tamandua), which is a placental, evolved in South America subsequent to the isolation of that continent. But a well-preserved specimen of Tamandua, recently discovered in the Messel oil shales of Germany, dates to the middle Eocene. Again, the most recent time prior to the Eocene that an anteater could have walked from Germany to South America was before the formation of the Tethys Sea (~170 mya), which long separated the northern and southern continents. Tamanduas, also, must predate 170 mya. The age and paleogeographic distribution of stegosaurid/pangolins and ankylosaur/armadillos suggest these creatures, also, came into existence prior to the middle Jurassic. This conclusion again suggests placental mammals date back to the time of the synapsids. But if placentals actually are as ancient as the synapsids, then the story cannot be correct that orthodox theory tells about mammals evolving from a generalized Cretaceous precursor. Moreover, as the next section shows, if we conclude that the origin of placental mammals occurred far earlier (i.e., at least 100 million years earlier) than generally supposed, it becomes possible to explain things that orthodox theory can’t.
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