Published on August 14th, 2019 | by David Marshall Episode 102: Small Shelly Fossils
Between the weird and wonderful rangeomorphs of the Ediacaran Period and the world-famous palaeocommunities of the Burgess Shale, the ‘Early Cambrian’ is host to a ‘waste basket’ of fossils untied by their small size and shelly construction.
These small shelly fossils (SSFs) aren’t just a single group of animals, but represent several different invertebrate phyla. Further compounding the difficulty of their identification, each SSF, termed a ‘sclerite’, is part of a larger composite skeleton known as a ‘sclerotome’. Whilst some complete sclerotomes have been preserved, many SSFs still represent multiple jigsaws thrown together and the pictures lost.
Piecing the SSFs back together and building a picture of the Earliest Cambrian is Dr Marissa Betts of the University of New England, Australia. Her work on the SSFs have provided a new framework for the regional stratigraphy of Australia and in this interview, we discuss why this was necessary, how she went about it and finally, what we know about the animals themselves.
Dr Marissa Betts collecting samples in the field.
Bedded Cambrian carbonates in the eastern Flinders Ranges. Prof. John Paterson (University of New England, Armidale) on the left, Assoc. Prof. Glenn Brock (Macquarie University, Sydney) to the right, and Marissa at back.
Samples are collected along measured stratigraphic sections. A tape is run perpendicular to the angle of the bedding and limestone samples were collected at 5–10 m intervals. This photo of Glenn Brock was taken in the Bunkers Ranges (central Flinders Ranges). It shows very nicely how well exposed the lower Cambrian is in the Flinders, and how continuous bedding can be. Some of those beds may be traced for several kilometres.
Often the small shelly fossils are not seen in the field and are only discovered after the rock is acid processed in the lab. However, in some cases, such as with these hyoliths, the fossils occur in such dense accumulations it is difficult to miss them.
Another group of large fossils were the “sponge-like” Archaeocyaths. These, with the aid of a suite of binding microbial taxa, formed the first animal-built reefs. These structures ranged from small metre-high mounds to quite large, even barrier-like edifices with considerable relief from the seafloor. These reefs provided lots of new environments and ecological niches for early animals to inhabit.
Archaeocyaths were early “sponge-like” organisms that built (with the aid of a suite of binding microbial taxa) the first animal-built reefs. These structures ranged from small metre-high mounds to quite large, even barrier-like edifices with considerable relief from the seafloor. These reefs provided lots of new environments and ecological niches for early animals to inhabit. Image: Museum Victoria.
Packing the samples into the vehicle. Each stratigraphic section usually ranges from a few hundred metres to a couple of kilometres. Lithology is logged in detail as we go, and at each stratigraphic horizon we take a sample for fossil extraction, stable isotope geochemistry and an oriented sample for making a thin section.
These foil containers hold the insoluble residues left behind after the samples were dissolved with weak acid (acetic). If you look closely you can even see some of the shelly fossils in quite a few samples. These are dried and then picked through under a microscope with a very small paintbrush to extract each of the tiny fossils.
In addition to stratigraphic sections measured in the field, Marissa’s team also took advantage of several cores that had previously been drilled for exploration purposes. This particular interval intersects a limestone with a rich archaeocyath assemblage.
Hyolith and mollusc phosphatic steinkerns (internal moulds). This is a great example of a very common way that calcareous shelled taxa are preserved in the carbonates. Originally, all of these fossils possessed calcium carbonate shells which have since been dissolved away. The only reason they can be found is because they have undergone taphonomic processes which result in the tiny shells being infilled with phosphate. These internal moulds often do not preserve fine details of the original shells which are important for identification. For these reasons, molluscs are only used as “accessory taxa” in the biostratigraphic work.
The small shelly fossils are kept in little green well slides. Each one of the tiny specks you can see in the wells is an individual microfossil. The biostratigraphic work utilised many thousands of shelly fossils from almost 1500 horizons over 21 stratigraphic sections/drill cores.
Looking at some small shelly fossils down the microscope.
Scanning electron microscopy is critical for being able to discern fine details on the small shelly fossils and identify them properly. Many early Cambrian small shelly fossils are “multi-element” taxa which means their suit of armour (sclerotome) was made from many parts (sclerites) which (usually) fell apart after they died. We have a pretty good handle now of which sclerites belonged to which organisms, and many of them have turned out to be reliable biostratigraphic tools. Note the two bradoriids (top right and bottom right) in this figure.
Eccentrotheca helenia is a great example of a “small shelly success story”. Prior to the discovery of the complete scleritome, this taxon was only known from isolated sclerites. The complete animal lived in a tube and was probably a sessile, filter feeding organism related to brachiopods.
This is a single geological section measured and collected in the Flinders Ranges. It shows the stratigraphic ranges of the shelly fossils, the lithology (rock type) and the oxygen and carbon isotopic curves. This section is a particularly long (2 km of basically continuous outcrop) and it intersects all three of the shelly fossil biozones that we were formally define by Marissa’s team (shown in the orange, purple and green blocks). Each of these zones is defined by the occurrences of particular shelly fossils.
The new shelly fossil biozones (left) were integrated into a global chronostratigraphic context by using the palaeogeographic distributions of key fossils in conjunction with the global carbon isotope curve. Radiometric dates from volcanic ash deposits (letters A-D in the top left) also give reliable dates to work with. This work has provided a reliable way to correlate lower Cambrian successions in South Australia with other parts of the world for the first time.