Fossils of The Burgess Shale - Faunal Composition

Faunal Composition

As of 2008 only two in-depth studies of the mix of fossils in any part of the Burgess Shale had been published, by Simon Conway Morris in 1986 and by Caron and Jackson in 2008. Caron and Jackson commented that Conway Morris had to rely on a set of specimens that may not have been representative, since their excavators discarded specimens they found uninteresting; and for which the exact level in the rock sequence had not been recorded, making chronological analyses impossible. Both studies noted that the set of species in Walcott's Phyllopod Bed (Conway Morris, 1986) and its expanded version the Greater Phyllopod Bed (Caron and Jackson, 2008) was different from those found in other parts of the Burgess Shale; and Conway Morris commented that faunas at most other Burgess Shale sites resembled those of the Raymond Quarry, which is above and therefore more recent than the Greater Phyllopod Bed (abbreviated "GPB").

Conway Morris found that the shelly fossils in Walcott's Phyllopod Bed were about as abundant as in other shelly fossil deposits, but accounted for only 14% of the Phyllopod Bed fossils. Assuming that, as in modern marine ecosystems, about 70% of the species that lived in the Early Cambrian seas are unsuitable for fossilization, he estimated that the shelly fossils probably represent about 2% of the animals that were alive at the time. Since these shelly fossils are found in other parts of North America and, in many cases, over a much wider range, the Burgess Shale fossils, including the soft-bodied ones, probably show how much diversity could be expected at other sites if Burgess Shale type preservation were found there.

Caron and Jackson found that about 25% of the 172 known species were abundant and widespread throughout the time range of the GPB, while the majority of species were rare, and occurred in a small area for a short period of time. In most layers the five most abundant species accounted for 50% to 75% of individual animals. The species that had wide ranges in time and space may have been generalists, while the rest were specialists in particular types of environment. Alternatively some wide-ranging species may have been opportunists that were quick to recolonize the area after each burial event. The 6 species that appeared in all layers were very probably generalists.

In each burial event layer the commonest species generally has several times as many individuals as the second most common, and accounts for 15% to 30% of individual fossil animals. The more common a species is in one layer, the greater the number of other layers it appears in. These "recurrent" species account for 88% of the individual specimens, but only 27% of the number of species. This suggests that the majority of species were in existence for much shorter periods than the "recurrent" ones. Species that cover shorter periods of time occur mainly in the higher, younger layers. The GPB shows an overall trend of increasing diversity as time progresses.

In almost all layers arthropods are the most abundant and diverse group of fossils in the GPB, followed by sponges. 69.2% of GPB individuals and 63.9% of species lived on the surface of the sea bed; within this group, mobile deposit feeders that extracted food particles from the sediment accounted for 38.2% of the total number of individuals and 16.8% of the total species; the smallest sub-group was mobile hunters and scavengers; and the rest were sessile suspension feeders. Animals that lived in the sediment made up 12.7% of the species and 7.4% of the individuals; the largest sub-group was mobile mobile hunters and scavengers. Bottom-dwelling animals capable of swimming comprised 12.7% of species and 7.4% of individuals. Organisms that spent their whole life swimming were very rare, accounting for only 1.5% of individuals and 8.3% of species.

These patterns – a few common species and many rare ones; the dominance of arthropods and sponges; and the percentage frequencies of different life-styles – seem to apply to all of the Burgess Shale. However the identity of the dominant species differs between sites. For example Marrella splendens is often credited as the commonest animal in the Burgess Shale, because of its abundance among the specimens collected by Walcott, but is only the third-most abundant organism in the Greater Phyllopod Bed, and very rare at other localities.

The overall community and ecology is very similar to that of other Cambrian localities, suggesting a global pool of species that repopulated localities after calamitous burial events occurred.

Caron and Jackson used computer software to simulate the numbers of species that would be found if smaller numbers of specimens were included, and found that the number of species "discovered" kept increasing as the number of specimens increased, rather than reaching a plateau. This suggests that Burgess Shale probably still contains as-yet undiscovered species, although probably very rare ones. Some recently discovered species, known in 2008 only by nicknames like "woolly bear" and "Siamese lantern" are familiar to the collecting teams, but have yet to be formally described and named. The team also nicknamed another discovery as "Creeposaurus", and in 2010 this animal was described and formally named Herpetogaster.