Fossils of The Burgess Shale - Significance

Significance

Analysis of the Burgess Shale fossils has been important to the interpretation of the Precambrian and Cambrian fossil records, and thus to scientific understanding of the nature of early evolution. English geologist and palaeontologist William Buckland (1784–1856) realised that a dramatic change in the fossil record occurred around the start of the Cambrian period, 542 million years ago. The earliest Cambrian trilobite fossils are about 530 million years old, but were already both diverse and widespread, suggesting that the group had a long, hidden history. The earliest fossils widely accepted as echinoderms appeared at about the same time Because Darwin's contemporaries had insufficient information to establish relative dates of Cambrian rocks, they had the impression that animals appeared instantaneously. Charles Darwin regarded this sudden appearance of many animal groups with few or no antecedents as a major objection to his theory of evolution, and in 1859 devoted a chapter of The Origin of Species to this problem. He speculated that the phenomenon, now known as the Cambrian explosion, was a product of gaps in the sequence of fossil-bearing rocks and in contemporary knowledge of those rocks.

While some geological evidence was presented to suggest that earlier fossils did exist, for a long time this evidence was widely rejected. Fossils from the Ediacaran period, immediately preceding the Cambrian, were first found in 1868, but scientists at that time assumed there was no Precambrian life and therefore dismissed them as products of physical processes. Between 1883 and 1909 Walcott discovered other Precambrian fossils, which were accepted at the time. However in 1931 Albert Charles Seward dismissed all claims to have found Precambrian fossils. In 1946, Reg Sprigg noticed "jellyfishes" in rocks from Australia's Ediacara Hills. However, while these are now recognized as coming from the Ediacaran period, they were thought at the time to have been formed in the Cambrian. From 1872 onwards small shelly fossils, none more than a few millimeters in size, were found in very Early Cambrian rocks, and later also found in rocks dating to the end of the preceding Ediacaran period, but scientists only started in the 1960s to recognize that these were left by a wide range of animals, some of which are now recognized as molluscs.

Darwin's view – that gaps in the fossil record accounted for the apparently sudden appearance of diverse life forms – still had scientific support over a century later. In the early 1970s Wyatt Durham and Martin Glaessner both argued that the animal kingdom had a long Proterozoic history that was hidden by the lack of fossils. However, Preston Cloud held a different view about the origins of complex life, writing in 1948 and 1968 that the evolution of animals in the Early Cambrian was "explosive". This "explosive" view was supported by the hypothesis of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s—which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.

The fossils of the Burgess Shale were hidden in store rooms until the 1960s. When Whittington and his colleagues first began to publish their Burgess finds in the early 1970s, the fossils became central to the debate about how quickly animals arose, and were interpreted as evidence that all the living bilaterian phyla had appeared in the Early Cambrian, along with many other phyla that had become extinct by the end of the Cambrian.

— = Lines of
descent = Basal node = Crown node = Total group = Crown group = Stem group Crown groups and stem group.

However, at this time, cladistics, which appeared in the 1950s, was starting to change scientists' approach to biological classification. Unlike previous methods, cladistics attempts to consider all the characteristics of an organism, rather than those subjectively chosen as most important. As a result it gives less significance to unique or bizarre characteristics than to those that are shared, since only the latter can demonstrate relationships. Cladistics also emphasises the concept of a monophyletic group, in other words one that consists only of a common ancestor and all its descendants – for example it regards the traditional term "reptile" as useless, since mammals and birds are descendants of different groups of "reptiles", but are not considered "reptiles". The concepts of crown groups and stem groups, first presented in English in 1979, are consequences of this approach. A crown group is a monophyletic group of living organisms, and a stem group is a non-monophyletic set of organisms that do not have all the shared features of the crown group but have enough to distinguish them clearly from close relatives of other crown groups – in very simple terms, they are "evolutionary aunts" of the organisms in the crown group. Phyla are crown groups, and the fact that some of their characteristics are considered defining features is simply a consequence of the fact that their ancestors survived while closely related lineages became extinct.

	lobopods, including Aysheaia and Peripatus armored lobopods, including Hallucigenia and Microdictyon anomalocarid-like taxa, including modern tardigrades as well as extinct animals like Kerygmachela and Opabinia Anomalocaris arthropods, including living groups and extinct forms such as trilobites

Simplified summary of Budd's "broad-scale" cladogram (1996)
of arthropods and their closest relatives

Briggs and Whittington started experimenting with cladistics in 1980 to 1981 and the results, while full of uncertainties, convinced them that cladistics offered reasonable prospects of making sense of the Burgess Shale animals. Other fossil beds discovered since 1980 – some rather small and others rivalling the Burgess Shale – have also produced similar collections of fossils, and show that the types of animals they represent lived in seas all over the world. It appears that most of the major animal lineages had arisen before the time of the Burgess Shale, and before that of the Chengjiang and Sirius Passet lagerstätten about 15 million years earlier, in which very similar fossils are found, and that the Cambrian explosion was complete by then. In the 1990s it was suggested that some Ediacaran fossils from 555 to 542 million years ago, just before the start of the Cambrian, may have been primitive bilaterians, and one, Kimberella, may have been a primitive mollusc. By 1996, with new fossil discoveries filling in some of the gaps in the "family tree", some Burgess Shale "weird wonders" such as Hallucinogenia and Opabinia were seen as stem members of a total group that included arthropods and some other living phyla.