Medium bluish gray laminated to thin-bedded dolostone; massive ridge former (near top of this unit) tan dolostone, silty; very fine-grained sandstone beds, granule conglomerates
Vermiforma • 49 Siltstone and quartzite
Light gray dolostone; indistinctly laminated; medium-bedded; scattered carbonate chips; white chert (top of this unit = top of El Arpa Formation?)
40 meters Siltstone, reddish, fissile, saddle former
70 meters Sandy dolomite
25 meters Siltstone and sandstone
20 meters Tan brownish carbonate; white chert
Light-colored dolostone; nearly barren of plants; forms a prominent bald spot
Laminated medium to dark gray dolostone; white chert (replacement) brown top of unit
Very fine-grained sandstone and siltstone; weathers reddish Granule conglomerate Cross-bedded sandstone
Interbedded reddish shale and brownish sandstone and sandy dolostone
Oolite conglomerates MM-82—78 seems conglomeratic at base; laminated and not oolitic toward top
14 meters Rainstorm member —strange fossils in red silty sandstone
Reference here is made to the Rainstorm Member of the Johnnie Formation, a correlative sedimentary unit in the United States.
MM-82—79 (here I made sketches). Volcanoes; burrowing surface of bed; burrows viewed perpendicular to bedding; frond?; this shape could not be made by scratching
measured Pitiquito (quartzite)
measured Gamuza (Formation); chert MM-82-80
When I picked up sample MM-82-79 and handed it to Jack Stewart, he said something like, "I've seen a lot of problematic sedimentary structures in this rock unit, but this one really looks like a trace fossil." Needless to say, I was quite pleased with my find for the day. However, the work of interpretation was just beginning.
The rock, a reddish-colored sandy siltstone, contained four elongate lobe-shaped objects, each with concentric U-shaped ridges at the end of the lobe (figures 3.2 and 3.3). The U-shaped ridges looked a lot like the concentric layers of sediment that form in many types of animal-built burrows.
When I returned to Santa Barbara I showed the objects to Preston Cloud. Cloud, who by 1982 had acquired a ferocious reputation as the premier debunker of Precambrian "fossils," agreed that they could be biologic. Encouraged by this, I published a photograph of these structures in the journal Geology, describing them as "probable metazoan traces."5'6
After publication I returned to Sonora and attempted to find more specimens of convincing trace fossils in the Clemente Formation, the rock unit that had produced these specimens, but without success. This was not an idle quest; I had my professors and fellow graduate and undergraduate students, both Mexican and American, engaged in searching and splitting Clemente Formation siltstones and sandstones. We simply struck out.
I showed the 1982 specimens again to Cloud. He still felt that they could be biologic, although he was intrigued by the fact that all four of the lobes were oriented in approximately the same direction. This made him suspicious, but he wasn't sure why.
In 1984, while I was completing my doctoral dissertation, Australian paleontologist Malcolm R. Walter visited Santa Barbara. I showed him the enigmatic structures, and he immediately came up with a way to form these lobes without invoking animal activity. Walter had seen flow
Figure 3.3: Enlargement of two of the specimens in figure 3.2. Scale bar = 1 cm.
structures resembling tiny lava flows coming off of the flanks of volcanoes, forming in association with sediment fluid-escape cones called sand volcanoes.
Sand volcanoes can form when water-saturated sediment is exposed to air and then disturbed by compactional forces or jostled by earthquakes. When this occurs, the sediment settles and forces water to move upward. The water sometimes follows a cylindrical conduit roughly resembling the vent of an igneous volcano. Sediment entrained in the water stream is deposited where the dewatering flow meets the air and can be deposited in a broad sediment cone or sand volcano. These sand volcanoes are often only a few centimeters in diameter, much smaller than their igneous counterparts. At the center of the sand mound is a small collapse pit, which looks like the vent, or caldera, in an igneous volcano.
When small sand volcanoes are preserved in ancient sediments, they are called pit-and-mound structures. Sometimes a particularly fluid slurry is ejected from the sand volcano vent. This slurry can flow down and beyond the flanks of the sand volcano, forming a lobe of fluidized sediment that can settle to create a sedimentary structure. This structure looks very much like a trace fossil with characteristic backfilled layers.
This was the interpretation I settled on for my dissertation text. Dianna McMenamin and I also opted for it in our 1990 book The Emergence of Animals: The Cambrian Breakthrough. Bruce Runnegar intimated that he was particularly convinced by our arguments in Emergence that this was a pseudofossil. Alas, as is often the case when dealing with a small number of enigmatic specimens, these arguments proved wrong.
Before proceeding further, I would like to mention the dangers of reversing oneself in science. I have had to reverse my earlier interpretation of this specimen not once, but twice in print. Not only is this painfully embarrassing, but it can do damage to one's professional reputation, a most valuable thing for a scientist.
As Peter Medawar once put it, science is the "art of the soluble," and the whole point of science is to come up with the correct solutions. There is a verifiable objectivity to the process of science unmatched in nonsci-entific fields. When the reward structures in science are functioning well, the greatest rewards go to the scientists who make the most important discoveries, the scientists who make the most correct interpretations, and those who solve the most important and difficult problems correctly. As a subscriber to this notion of science, I strive to get things right in my scientific work. Sometimes, in the face of new information, this may mean changing my stance on some issue. All too often, this process is skewed by political factors or by reluctance to change one's mind.
If one trusts one's own judgment, and then one feels compelled to change one's mind repeatedly about an issue, this may be an indication that the issue at hand is one of considerable importance (if not, better stop trusting one's own judgment). But intellectual caution is not always what is needed. As Daniel Dennett once said, we "often learn more from bold mistakes than from cautious equivocation." In my opinion, as a paleontologist one is not doing one's job unless one can occasionally be shown to be wrong. This is actually a healthy sign, for it indicates that the work is proceeding well within the realm of testable hypothesis and verifiable (or falsifiable) prediction.
My discovery in 1995 of Ediacaran fossils below the Clemente oolite of the Clemente Formation (the oldest convincing Ediacaran fossils known; see chapter 9) led me to reevaluate my interpretation of the structures in figures 3.2 and 3.3. I remember now standing in the room in the Biogeology Clean Lab that held Preston Cloud's enormous reprint collection and asking him whether my specimens were Vermiforma, a fossil he had described a few years before from the North Carolina slate belt.7 He was noncommittal but reiterated that Vermiforma was a body fossil.
I recently returned to Cloud, Wright, and Glover's 1976 paper on Vermiforma. The fossils were found in Proterozoic strata of North Carolina. As part of this article, Cloud had published the description of a new species, Vermiforma antiqua. The journal American Scientist is a very unusual place to publish a description of a new species. American Scientist articles are generally summaries of already published research advances, not the original research itself. Cloud perhaps felt that such an unusual fossil merited an unusual publication venue.
As per Cloud's original description, Vermiforma is represented by seven individual specimens and fragments of several other specimens on a bedding plane surface (figure 3.4). The fossils occur in a laminated, greenish, volcaniclastic sediment. Cloud, Wright, and Glover inferred the deposi-tional environment of the fossils to be "rather deep water," although no conclusive paleobathymetric indicators were associated with the fossils. They inferred from zircon Pb-U isotope dates an age of "close to 620 m.y." for the fossils, but due to structural complexities of the rocks in which they occur, their age is not closely constrained. The age of the fossils falls somewhere between 555 and 680 million years.8
Cloud interpreted Vermiforma antiqua as the body fossil of an elongate metazoan with possible annelid worm affinities. Cloud's figure 3 (repro-
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