Prior to the dramatic appearance of the Cambrian hard-bodied fauna, the only multicellular fossils to be found belong to the Ediacaran assemblage of soft-bodied marine organisms. These are commonly described using the word 'enigmatic' as they are extinct and we have to speculate as to what they really were. In the past, an evolutionary scenario has prevailed regarding the Ediacaran biota: the Avalon assemblage is the oldest, with relatively low numbers of genera. The White Sea and Nama assemblages come later, with increased taxonomic diversity. The main doubts about this diversification story relates to the inferred palaeoenvironments. According to one researcher, "these three biotas represent an environmental-ecological gradient involving little evolutionary change or bio-geographic provinciality."
The new research puts the emphasis on morphology. The methodology involved recording the presence or absence of 50 morphological characters for 272 occurrences of Ediacaran species. The authors explored ways of comparing the morphological diversity, with some unexpected results.
Surprisingly, however, as shown by Shen and colleagues, these earliest Ediacara life forms already occupied a full morphological range of body plans that would ever be realized through the entire history of Ediacara organisms. "In other words, major types of Ediacara organisms appeared at the dawn of their history, during the Avalon Explosion," Dong said. "Subsequently, Ediacara organisms diversified in White Sea time and then declined in Nama time. But, despite this notable waxing and waning in the number of species, the morphological range of the Avalon organisms were never exceeded through the subsequent history of Ediacara."
The left panel shows that number of genera (black line) and how they are dissimilar from each other (gray line) in the three Ediacara assemblages. The right panel shows morphospace occupation of the three Ediacara assemblages. Although there are fewer genera in Avalon than latter assemblages, Avalon fossils occupy the full range of Ediacara morphospace, indicating an explosive evolutionary pattern (the Avalon explosion) similar to the Cambrian explosion of animals that occurred about 542 million years ago. (Source)
The authors conclude that here is evidence for an explosion of Ediacaran life that parallels the Cambrian Explosion.
"The rapid increase of morphospace at the beginning of Ediacara evolution parallels the disparity patterns of the Cambrian explosion: a rapid evolution of body plans followed by taxonomic diversification within the limits of a predefined morphospace. [snip] Regardless of the veracity of these causative explanations, the marked parallels between the Cambrian and Avalon explosions suggest that the decoupling of taxonomic and morphological evolution is not unique to the Cambrian explosion and that the Avalon explosion represents an independent, failed experiment with an evolutionary pattern similar to that of the Cambrian explosion."
Needless to say, these trends are not Darwinian. This is acknowledged by one of the authors in a press release:
"The explosive evolutionary pattern was a concern to Charles Darwin, because he expected that evolution happens at a slow and constant pace," said Shuhai Xiao, associate professor of geobiology at Virginia Tech. "Darwin's perception could be represented by an inverted cone with ever expanding morphological range, but the fossil record of the Cambrian Explosion and since is better represented by a cylinder with a morphological radiation at the base and morphological constraint afterwards."
Furthermore, Darwin's branching model does not fit the post-Cambrian fossil record. Recent posts have noted this in the case of the mammals, beetles and flowering plants.
Wherever we look, we find evidences of abrupt appearance. Gradualism in the fossil record relates only to the "equilibria" phases of lineages. Darwin was right to be concerned - surely this is enough to falsify his theory!
The Avalon Explosion: Evolution of Ediacara Morphospace
Bing Shen, Lin Dong, Shuhai Xiao, Michal Kowalewski
Science 319, 4 January 2008: 81-84.
Ediacara fossils [575 to 542 million years ago (Ma)] represent Earth's oldest known complex macroscopic life forms, but their morphological history is poorly understood. A comprehensive quantitative analysis of these fossils indicates that the oldest Ediacara assemblage - the Avalon assemblage (575 to 565 Ma) - already encompassed the full range of Ediacara morphospace. A comparable morphospace range was occupied by the subsequent White Sea (560 to 550 Ma) and Nama (550 to 542 Ma) assemblages, although it was populated differently. In contrast, taxonomic richness increased in the White Sea assemblage and declined in the Nama assemblage. These diversity changes, occurring while morphospace range remained relatively constant, led to inverse shifts in morphological variance. The Avalon morphospace expansion mirrors the Cambrian explosion, and both events may reflect similar underlying mechanisms.
2 explosive evolutionary events shaped early history of multicellular life, EurekAlert, 3 January 2008.
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