Animal Origins And The Fossil Record

The fossil record is our primary window into the history of life. It provides many kinds of information that cannot be inferred from living animals. Fossils give us pictures of extinct forms that may be ancestors of modern animals, provide minimal estimates of the time of origin or divergence of particular groups, reveal episodes of extinctions and radiations, and, in favorable circumstances, offer detailed accounts of the evolution of important structures.

The search for the origins of modern animals begins with an assessment of the Cambrian fossil record. It has been known since before Darwin's time that animal diversity increased dramatically during this period, which spans an age from roughly 545 to 490 million years ago (Ma). Molluscs, arthropods, annelids, chordates, echinoderms, and representatives of most other modern phyla make their first appearance in Cambrian fossil deposits (Fig. 1.1). The emergence of large, complex animal forms and their radiation over a 10 to 25 million year interval in the Early-Middle Cambrian is often referred to as the "Cambrian Explosion."

The appearance of these animals in the Cambrian fossil record gives us only a minimum estimate of their time of origin. The crucial question about the Cambrian Explosion is whether it marks the origin of animals or the origin of modern phyla. Did most phyla first arise in this short period, or did they predate their preservation in the Cambrian fossil record? Although the Precambrian animal fossil record is relatively scarce, several kinds of fossil evidence indicate that the origins of most modern phyla predate the Cambrian. First, the fossil record of some modern groups clearly begins before this period. For example, body fossils of both cnidarians and sponges predate the Cambrian (Fig. 1.2). Both of these groups are diploblastic animals, composed of two tissue layers. The cnidarians have a radically symmetrical body design that distinguishes them from sponges and from a much larger number of modern phyla that are triploblastic—that is, composed of three tissue layers—and have bilaterally symmetrical body designs (the Bilateria). Second, Precambrian deposits contain evidence in the form of trace fossils, the record of the meanderings and burrowings of animals in sediments, which indicate the existence of some bilaterian forms (Figs 1.2 & 1.3d) well before the Cambrian Explosion. A third piece of potential evidence for earlier animal origins is the Ediacaran fauna (575-544 Ma), named for the Australian locale in which they were first discovered.

The biological interpretation of Ediacaran fossils and their relationships, if any, to modern animals remains controversial. Several distinct body plans have been identified, including radially symmetrical types and a number of frond-like and tube-like forms (Fig. 1.3). None of these bear any clear-cut similarity to modern animals, so they have been difficult to place on the tree of animal evolution. Some of the Ediacaran fossils could represent diploblastic forms related to cnidarians or sponges. Others could be primitive bilaterians that possess some, but not all, features of modern bilaterians.

The difficulties in placing Ediacarans in the scheme of animal evolution have led to the proposal that they represent an extinct experiment in multicellular life. On the other hand, perhaps their lack of resemblance to modern groups is exactly what should be expected

Figure 1.1

Cambrian animal fossils

Figure 1.1

Cambrian animal fossils

Representatives of many modern phyla are found in Cambrian deposits and are made up of repeating units. (a) Aysheaia pedunculata, an onychophoran; (b) Burgessochaeta setigera, a polychaete annelid; (c) Pikaia gracilens, a chordate; (d) Olenoides serratus, a trilobitomorph arthropod; (e) Waptia fieldensis, a crustacean-type arthropod.

Source: Photographs from Briggs DEG, Erwin DH, Collier FJ. Fossils of the Burgess shale. Washington, DC: Smithsonian Institution Press, 1994; reprinted by permission from the Smithsonian Institution Press.

of primitive animals. It is possible that the Ediacaran fauna include both extinct types of diploblastic animals and primitive ancestors of modern bilaterians. The fossil record indicates that some Ediacaran forms persisted into the Cambrian, but then died out as bilaterians, sponges, cnidarians, and ctenophores flourished.

Given the uncertainty of the relationship of the Ediacarans to modern phyla and the paucity of body fossils prior to the Cambrian, it is difficult to pinpoint the origins of modern animals based on the fossil evidence. Consequently, biologists have turned to other methods to try to identify when major animal groups diverged. Using the evolution of protein and ribosomal RNA sequences between species to calibrate molecular clocks, estimates of the time of divergence of most animal phyla have been made that range from approximately 650 Ma to more than 1000 Ma. While these estimates remain controversial, even the most conservative estimate suggests a period of more than 100 million years before the beginning of the Cambrian in which most bilaterian phyla had arisen but led a paleontologically cryptic existence.

Figure 1.2

The early fossil record of animals

The appearance of various animal phyla in the fossil record are indicated, relative to the Cambrian and Proterozoic periods. The ages of fossils from particular localities are shown in red at the bottom. Note that the cnidarian and poriferan records clearly predate the Cambrian. Other phyla first appear in the Cambrian, although early members may exist that predate the Cambrian by a considerable period. L, Late; M, Middle; B/T, Botomian plus Toyonian; T/A, Tommotian plus Atdabanian; N-D, Nemakit-Daldynian. Source: Adapted from Knoll AH, Carroll SB. Science 1999;284:2129-2137.

Figure 1.2

The early fossil record of animals

The appearance of various animal phyla in the fossil record are indicated, relative to the Cambrian and Proterozoic periods. The ages of fossils from particular localities are shown in red at the bottom. Note that the cnidarian and poriferan records clearly predate the Cambrian. Other phyla first appear in the Cambrian, although early members may exist that predate the Cambrian by a considerable period. L, Late; M, Middle; B/T, Botomian plus Toyonian; T/A, Tommotian plus Atdabanian; N-D, Nemakit-Daldynian. Source: Adapted from Knoll AH, Carroll SB. Science 1999;284:2129-2137.

It is widely believed that primitive bilaterians may have been very small and their size limited by atmospheric and oceanic oxygen levels. This fact would help to explain their slim fossil record before the Cambrian (Fig. 1.2). In the last few years, evidence has also been gathered that suggests a possible mass extinction at the boundary between the Proterozoic and Cambrian. Whatever the cause of such an event, it may have hastened the extinction of Ediacaran forms and opened up the ecological opportunity for bilaterians to radiate. Environmental and ecological changes may have removed constraints on bilaterians, permitting the evolution of larger animals. In addition, competitive interactions among bilaterians may have

Figure 1.3

Pre-Cambrian animal fossils and traces

(a) Ediacaria, a radially symmetrical form from deposits in Australia. (b) Calcified fossils in limestone from Namibia. (c) Pteridinium, a frond-like ediacaran fossil form built of repeating units. (d) Trace fossils made in sediments by bilaterian animals. Source: Knoll AH, Carroll SB. Science 1999;284:2129-2137.

Figure 1.3

Pre-Cambrian animal fossils and traces

(a) Ediacaria, a radially symmetrical form from deposits in Australia. (b) Calcified fossils in limestone from Namibia. (c) Pteridinium, a frond-like ediacaran fossil form built of repeating units. (d) Trace fossils made in sediments by bilaterian animals. Source: Knoll AH, Carroll SB. Science 1999;284:2129-2137.

facilitated the evolution of skeletonized taxa, more sophisticated predatory and defense behaviors, and the variety of anatomical innovations that unfolded in the Cambrian.

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