Strange Aeons

When he penned the above quotation, Lovecraft was apparently not thinking of fossils, but rather of ancient gods that are destined to wreak havoc on the Earth following their awakening when "the stars are right". However, fossils (although indisputably dead) lie protected throughout the countless millennia of geological time, only to emerge for a kind of scientific resurrection when discovered and reconstructed by eager palaeontologists. Although they do not wreak havoc with anything except our evolutionary theories (unless and until Jurassic Park becomes a reality), many ancient animals project a sense of awe and mystery worthy of Lovecraftian deities - just think of large predators like Dimetrodon and Tyrannosaurus, or better yet the enormous pterodactyl Quetzalcoatlus. Be that as it may, this page is devoted to the immensity of geological time and the nature of Earth's environments during the "strange aeons" that we study - the Permian and Carboniferous Periods. Geological Time - H.P. Lovecraft, The Nameless City

One of the most astonishing discoveries in the history of geology was the fact that the age of the Earth is measured not in thousands or millions of years but in billions - approximately 4.5 of them. During this vast span of time, the global climate has undergone dramatic changes, the continents have drifted, spun, and collided across the planet's surface, and life has diversified and spread into every available environment. When discussing the evolution of the animals we study, we often find ourselves casually mentioning intervals of geological time that span tens or even hundreds of millions of years, but even a one million year period seems almost incomprehensibly long when measured against the familiar context of human lives and human history. December 31, 2001, will mark the end of the second millennium of the Gregorian calendar - only 998 more millennia to go before we get to the year 1 million A.D.! The idea of Darwinian evolution becomes much easier to swallow once one appreciates the enormous length of time over which mutation and natural selection have acted to bring about evolutionary change.

Geological time is divided into eras, which are in turn divided into periods; other levels of this hierarchy exist, but are less frequently used. The eras and periods have individual names, which refer both to a span of time and to the rocks deposited within that time. Permian rocks, for instance, were laid down during the Permian Period, between approximately 300 and 250 million years ago. Triassic rocks, being slightly younger, are normally found lying on top of Permian ones unless erosion has removed part of the record. The boundaries between time units often reflect major changes in ancient ecosystems, so that a collection of fossils made in Permian rocks just below the Permian/Triassic boundary may look very different from a collection made in Triassic rocks just above the boundary. The major units of geological time (with boundary ages in millions of years), and a sampling of important events in the evolution of life, are given below:

Most of our work concentrates on just two geological periods, the Carboniferous and the Permian. It was during this time that the ancestors of modern tetrapods - four-limbed animals such as mammals, birds, reptiles, and amphibians - began to diversify on a large scale. Like historians using ancient documents to comprehend the roots of modern events, we find that the fossil record of this diversification offers insights into the ecological and evolutionary processes that shaped vertebrate life as it exists today.

The Carboniferous World

Most of our knowledge of Carboniferous ecosystems comes from deposits in North America and Europe, which formed in proximity to the equator - recall that the continents were then in very different positions from those they occupy today, as they began to coalesce into a single enormous land mass called Pangaea. The Carboniferous Period takes its name from the abundance in these deposits of extensive coal layers, which formed through the accumulation and decay of countless trees and plants in what were apparently warm, humid swamps. This vegetation was extremely exotic by modern standards, consisting of ferns, relatives of the modern horsetail (often of enormous size), and tree-like lycopods. A unique fossil locality at Joggins, Nova Scotia (see illustration) has yielded small tetrapods, including the oldest known amniotes, that were apparently trapped inside hollow lycopod stumps. When the stumps filled with sediment, the animals were entombed and preserved.

The Joggins locality, and many other sites, reveal a diverse fauna of amphibians and in some cases small early reptiles (or, more properly, "parareptiles"). Although amphibians first emerged in the Devonian Period, these early forms were few in number and retained numerous fish-like features; during the Carboniferous, many more species appeared and took on a wide variety of shapes and sizes. Relatively primitive tetrapods, such as the temnospondyls (see our Bestiary page) gave rise to the small, often more specialised lepospondyl amphibians, and also - perhaps via the lepospondyls - to the first reptiles. The pelycosaurs, ultimate ancestors of mammals and a highly important group during the Early Permian, were already quite diverse by the end of the Carboniferous and clearly distinct from all other reptiles. However, herbivorous vertebrates were scarce, and Carboniferous vertebrate communities seem to have been ultimately dependent on insects and aquatic animals as sources of food.

The Permian World

The vertebrates of the early part of the Permian are known mainly from the southwestern United States; the quarry pictured here is in New Mexico. Although some interpretations suggest that the climate was becoming increasingly arid and inhospitable in this region, moist environments persisted at least in the vicinity of the inland sea that separated the New Mexican deposits from their counterparts in Texas. The fauna generally shows a basic continuity with that of the preceding Carboniferous Period, as pelycosaurs and temnospondylous and lepospondylous amphibians continued to predominate. However, recent work (some of it conducted in our lab) has hinted at the existence of a second, partially isolated ecological community, inhabiting upland regions and consisting primarily of small terrestrial animals. This stands in contrast to the situation at most Early Permian localities, where many large and/or semi-aquatic labyrinthodonts and pelycosaurs occur and often are found together with fossil fishes.

During the later part of the Permian, the most significant deposits occur in southern Africa and, to a lesser extent, in Russia. Like the other southern continents, Africa was heavily glaciated during the Carboniferous and Early Permian, but subsequently a temperate environment characterised by lush vegetation and wide river floodplains became established as the ice sheets retreated. It supported a fauna that differed dramatically from anything seen during the Early Permian, as amphibians had declined somewhat in numbers and ecological importance and the pelycosaurs had all but disappeared. The dominant tetrapods in this brave new Late Permian world were the therapsids, descendants of the pelycosaurs but separated from them by a conspicuous evolutionary gap that is bridged only by a single poorly known North American animal called Tetraceratops. The therapsids were both abundant and diverse, and included for the first time a large proportion of herbivores. The most important of these were stocky, beaked, and often tusked animals called dicynodonts; although some were small burrowers, others were larger and must have lumbered over the floodplains like reptilian cattle. As the continents continued to converge, many groups of animals were widespread, occurring in both Russian and South African deposits and often (much more sparsely) in other regions as well. In the plant kingdom, the well known "Glossopteris flora" occurred throughout the southern hemisphere, and provided powerful ammunition for early proponents of continental drift (since it could hardly have attained this distribution unless the continents had been very close together).

Conventional wisdom has it that the Permian Period ended apocalyptically, with what may have been the greatest mass extinction recorded in the entire history of life on Earth. However, there is no evidence for an extraterrestrial impact of the kind that annihilated the dinosaurs some 185 million years later, and it is possible that the so-called "mass extinction" was actually an ecological decline that occurred over a significant span of time rather than in a geological instant. The clearest evidence for an extinction comes from marine invertebrates, but many land vertebrates certainly disappeared around the end of the Permian as well. During the following Triassic Period, the surviving therapsids were increasingly displaced by another group - the archosaurs, which included the crocodilians, pterosaurs and dinosaurs that dominated the Mesozoic Era.