The Mudeaters

Test everything; retain what is good.

1 thessalonians 12:21

The worst problem in the search for the oldest animal fossils is mistaken identity. Sedimentary rocks are replete with irregular structures and small scale disturbances or interruptions of the horizontal bedding or layering. Some of these disturbances are caused by organisms, but many are not. If these disturbances are formed at the same time that the sediment was deposited, they are called primary sedimentary structures. Trace fossils are primary sedimentary structures formed by the burrowing and feeding activities of animals that lived on or in the soft sediment (or, occasionally, hardened sediment).

Usually a well-preserved and well-formed trace fossil is unquestionably biologic in origin, and all paleontologists would agree that the trace was formed by an animal. Yet it can be difficult to define precisely what it is about a trace fossil that makes it convincingly biogenic (formed by life). It is like the statement by an American judge regarding his identification of pornography; he couldn't define precisely what pornography was, but he knew it when he saw it. Regularity of structure, a shape suggesting motion through mud by an animal, and the implausibility of forming a structure solely by physical processes are all criteria used to establish the biologic nature of trace fossils. As you might expect, there are many cases where the true identity of a trace-fossillike structure is not clear.

A sedimentary structure that resembles, but is in fact not, a trace fossil (or a body fossil, for that matter) is called a pseudofossil. Pseudofossils have plagued the study of Precambrian paleontology because many inorganic sediment disturbances look deceptively like fossils. While out in the field, earth scientists always hope their labors will be rewarded by a crucial discovery (such as a find of the oldest animal fossil) and by the professional acclaim that would attend such a discovery. Paleontologists have occasionally made the mistake that A. H. Knoll (1986) calls "highgrading" on the outcrop, passing over many ordinary, inorganic sedimentary structures in Precambrian rocks but collecting and publishing accounts of those that look most like fossils.

Figure 3.1 is a histogram showing the frequency of formally published goofs (cases of mistaken identity) in the search for the oldest animal fossils. As the histogram shows, there are three peaks in the data, one during the interval 1860-1870, a second during 1890-1900 and a third in the interval 1960 — 1970. The first peak includes Eozoon and other Precambrian "fossils" described by Dawson. The second peak is a result of C. D. Walcott's error-prone work in the identification of Precambrian animal fossils. We can only wonder why Wal-cott was so consistently wrong whenever he dealt with Precambrian animals. Perhaps he was striving to fulfill the predictions of Darwin's gradualistic views. Walcott expected to find Precambrian animal fossils, a bias which may account for many of his misidentifica-tions.

The third peak in figure 3.1 is a result of burgeoning research in Precambrian geology beginning in the early 1960s, plus a spate of unfettered enthusiasm for attempts to find the most ancient animal fossil. Seilacher recalls (personal communication, 1987) a presentation given in the early 1960s at a formal geologic meeting. The speaker claimed to have found the oldest evidence for fossil metazoa, and showed the audience photographs of the most convincing specimens. After the presentation, several scientists in the audience expressed grave reservations about the putative biologic origin of these structures. A vote was immediately taken to see how many people in the audience were convinced of the biologic nature of the specimens. The vote was overwhelmingly in favor of the author's conclusions. Seilacher, who was one of the skeptics (and later proved correct), was so dismayed by this incident that it was twenty years before he again worked seriously with Precambrian animal fossils.

As seen in figure 3.1, the number of mistakenly described Precam-brian pseudofossils has fallen off dramatically since the peak in the early 1960s. The drop-off in the numbers of cases of mistaken identity can be attributed, perhaps in large part, to the unflagging skepticism of Preston Cloud, a geologist at the University of California at Santa Barbara. Cloud published several articles (1968, 1973) attacking the uncritical approach taken by many of his colleagues toward the identification of Precambrian animals, particularly those older than the beds containing Ediacaran fossils.

I (M.A.S.M.) have added my own blunder to those tabulated in figure 3.1. As a graduate student in 1982, while doing field work in the Clemente Formation of the Cerros de la Cienega of northern Sonora, Mexico, I came across some markings in the rock that looked suspiciously biogenic (figure 3.2). The rock, a red-colored sandy silt-

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