Wonderful Life (Part III)
Mar. 18th, 2005 05:03 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
prof_pangaeaExcerpt from:
Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould
III. The First Fauna of the Cambrian Explosion
Our hypothetical advocate of the cone and the ladder might be willing to give ground on the these first two incidents from the dim mists of time, but he might then be tempted to dig his entrenchment across the Cambrian boundary. Surely, once the great explosion occurs, and traditional fossils with hard parts enter the record, then the outlines must be set, and life must move upward and outward in predictable channels.
Not so. As noted in chapter II, the initial shelly fauna, called the Tommotian to honor a famous Russian locality, contains far more mysteries than precursors. Some modern groups make an undoubted first appearance in the Tommotian, but more of these fossils may represent anatomies beyond the current range. The story is becoming familiar -- a maximum of potential pathways at the beginning, followed by decimation to set the modern pattern.
The most characteristic and abundant of all Tommotian creatures, the archaeocyathids, represent a long-standing problem in classification. The familiar litany plays again. These first reef-forming creatures of the fossil record are simple in form, usually cone-shaped, with double walls -- cup within cup. In the traditional spirit of the shoehorn, they have been shunted from one modern group to another during more than a century of paleontological speculation. Corals and sponges have been their usual putative homes. But the more we learn about archaeocyathids, the stranger they appear, and most paleontologists now place them in a separate phylum destined to disappear before the Cambrian had run its course.
Even more impressive is the extensive disparity just now being recognized among organisms of the "small shelly fauna." Tommotian rocks house an enormous variety of tiny fossils (usually one to five millimeters in length) that cannot be allied with an modern group (Bengston, 1977; Bengston and Fletcher, 1983). We can arrange these fossils by outward appearance, as tubes, spines, cones, and plates, but we do not know their zoological affinities. Perhaps they are merely bits and pieces of an era of early, still imperfect skeletonization; perhaps the covered familiar organisms that later developed the more elaborate shells of their conventional fossils signatures. But perhaps -- and this interpretation has recently been gaining favor among aficionados of the small shelly fauna -- most of the Tommotian oddballs represent unique anatomies that arose early and disappeared quickly, For example, Rozanov, the leading Russian expert on this fauna, concludes his recent review by writing:
Early Cambrian rocks contain numerous remains of very peculiar organisms, both animals and plants, most of which are unknown after the Cambrian. I tend to think that numerous high-level taxa developed in the early Cambrian and rapidly became extinct (1986, p. 95).
Once again, we have a Christmas tree rather than a cone. Once again, the unpredictability of evolutionary pathways asserts itself against our hope for the inevitability of consciousness. The Tommotian contained many modern groups, but also a large range of alternative possibilities. Rewind the tape into the early Cambrian, and perhaps this time our modern reefs are built by archaeocyathids, not corals. Perhaps no Bikini, no Waikiki; perhaps, also, no people to sip rum swizzles and snorkle amidst great undersea gardens.
Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould
III. The First Fauna of the Cambrian Explosion
Our hypothetical advocate of the cone and the ladder might be willing to give ground on the these first two incidents from the dim mists of time, but he might then be tempted to dig his entrenchment across the Cambrian boundary. Surely, once the great explosion occurs, and traditional fossils with hard parts enter the record, then the outlines must be set, and life must move upward and outward in predictable channels.
Not so. As noted in chapter II, the initial shelly fauna, called the Tommotian to honor a famous Russian locality, contains far more mysteries than precursors. Some modern groups make an undoubted first appearance in the Tommotian, but more of these fossils may represent anatomies beyond the current range. The story is becoming familiar -- a maximum of potential pathways at the beginning, followed by decimation to set the modern pattern.
The most characteristic and abundant of all Tommotian creatures, the archaeocyathids, represent a long-standing problem in classification. The familiar litany plays again. These first reef-forming creatures of the fossil record are simple in form, usually cone-shaped, with double walls -- cup within cup. In the traditional spirit of the shoehorn, they have been shunted from one modern group to another during more than a century of paleontological speculation. Corals and sponges have been their usual putative homes. But the more we learn about archaeocyathids, the stranger they appear, and most paleontologists now place them in a separate phylum destined to disappear before the Cambrian had run its course.
Even more impressive is the extensive disparity just now being recognized among organisms of the "small shelly fauna." Tommotian rocks house an enormous variety of tiny fossils (usually one to five millimeters in length) that cannot be allied with an modern group (Bengston, 1977; Bengston and Fletcher, 1983). We can arrange these fossils by outward appearance, as tubes, spines, cones, and plates, but we do not know their zoological affinities. Perhaps they are merely bits and pieces of an era of early, still imperfect skeletonization; perhaps the covered familiar organisms that later developed the more elaborate shells of their conventional fossils signatures. But perhaps -- and this interpretation has recently been gaining favor among aficionados of the small shelly fauna -- most of the Tommotian oddballs represent unique anatomies that arose early and disappeared quickly, For example, Rozanov, the leading Russian expert on this fauna, concludes his recent review by writing:
Early Cambrian rocks contain numerous remains of very peculiar organisms, both animals and plants, most of which are unknown after the Cambrian. I tend to think that numerous high-level taxa developed in the early Cambrian and rapidly became extinct (1986, p. 95).
Once again, we have a Christmas tree rather than a cone. Once again, the unpredictability of evolutionary pathways asserts itself against our hope for the inevitability of consciousness. The Tommotian contained many modern groups, but also a large range of alternative possibilities. Rewind the tape into the early Cambrian, and perhaps this time our modern reefs are built by archaeocyathids, not corals. Perhaps no Bikini, no Waikiki; perhaps, also, no people to sip rum swizzles and snorkle amidst great undersea gardens.
