How old are the oldest macrofossils?

When did complex animal life first appear? Ted Nield discovers how we think we know, from high precision isotope chronology of Ediacaran fossils

Geoscientist Online 10 September 2008

Scientists at the NERC Isotope Geosciences Laboratory and the British Geological Survey (BGS) are using the most advanced dating techniques available to produce exciting new age constraints on some of the first macroscopic multicellular life forms - the Ediacaran biota from Charnwood near Leicester UK, the BA was told today by Dr Stephen Nobe (NERC), speaking at the Society-sponsored session on isotopes and Earth science.

Initial data, which still await final confirmation, provide tantalising clues that Ivesheadia lobata may be one of the oldest macroscopic multicellular organisms recognised so far on Earth.

Charnwood has historically been very important in bringing the significance of late Precambrian organisms to light; but even now, new fossil horizons are being discovered in Charnwood by scientists at BGS, Noble told the BA. Isotopes of uranium and lead are then being used to produce precise dates on these rocks - and this technique is used in turn to help calibrate the geological timescale.

Noble said: “We are particularly interested in answering a fundamental question in Earth Science, namely "when did the first complex life forms evolve on Earth?", an event that we know happened before the Cambrian "explosion" during the recently created “Ediacaran Period”. High precision U-Pb ages show that fossil remains of one of the most globally widespread of the Ediacarans, the frond-like Charnia, which looks like but is unrelated to a modern sea pen, lived around 562 million years (Ma) ago at Charnwood. This correlates extremely well with the same type of organisms found at Mistaken Point in Newfoundland, Canada, demonstrating the widespread nature of this biota.”

“Perhaps more significantly, there are even older and as yet poorly understood fossils at Charnwood (Ivesheadia lobata) whose age we are determining at present. These older fossils are preserved together with Charnia (picture) and its relatives at Mistaken Point in rocks that are around 575 to 560 Ma old. However, in Charnwood Ivesheadia lobata is found in rocks that are thought to be older than 575 Ma. Their greater antiquity, inferred from geological evidence, is supported by our initial data - which indicate that the Ivesheadia lobata fossils at Charnwood could be as old as ~614 Ma. They also suggest that this species persisted through a significant portion of the Ediacaran period - around 40 million years.”

The Ediacaran period lasted from 635 to 542 Ma, and fossils from this period are only found in ~ 30 locations on Earth, including Charnwood in the British Midlands. The Ediacaran was a crucial period in Earth history, starting at the end of the last of the "Snowball Earth" global glacial events and ending with the widespread appearance of biomineralised animals at the base of the Cambrian.

During the Ediacaran, the Earth’s oceans became progressively oxygenated, evolutionary rates increased, and life evolved from single-celled forms to the first macroscopic (1-10 cm) and megascopic (up to 1 m) complex organisms. These pioneering multicellular organisms were largely non-skeletal and soft bodied. This contrasts starkly with the single-celled life that existed during the vast amount of geological time prior to the Ediacaran, and also with the Cambrian times when the appearance of animals with hard parts facilitated a richer fossil record due to better preservation.

The generally held view is that Ediacaran animals (or animal-like organisms) included sponges and cnidarians (e.g. jellyfish, corals), early bilateral animals (precursors to molluscs and annelids), and some others forms that died out without leaving recognisable more modern relatives. A vital group of these Ediacaran macroscopic animals are the now-extinct rangeomorphs. The rangeomorphs were a group of spindle, fan and frond-like soft bodied organisms. Many of these forms were tethered to the sea floor by a holdfast that when preserved in isolation from the rest of the organism looks like a ring.

Noble told the BA: “The first rangeomorphs and their holdfasts were discovered in Ediacara Australia by Reg Sprigg in 1946; but their significance was not immediately realised. Around 10 years later schoolgirl Tina Negus found Charnia in Charnwood but her attempt to bring the discovery to the attention of those who could appreciate it was frustrated. About one year after Tina's discovery, on 19 April 1957, three schoolboys climbing in a disused quarry found a large Charnia specimen. One of the boys was Roger Mason, and with the help of his father, was able to contact Trevor Ford (later a professor of geology at the University of Leicester). This then led to further visits to the quarry, removal of the type specimen to a museum, and the publication of the find in a journal. Martin Glaessner in Adelade realised the link between the Australian and Charnwood fossils and published his findings in Nature in 1958.

“Since then Precambrian fossils have been recognised in other locations worldwide and research has moved on to further discoveries of different types of Ediacaran animals, paleoecological analysis, and refinements to the age constraints. For example, work by Guy Narbonne and co-workers at Queen's University in Canada has shown that these probable suspension-feeders formed the oldest deep-water communities, the Avalon assemblage, as far back as 575 Ma and that they died out by the start of the Cambrian. In addition, Narbonne has shown that these rangeomorphs were colonial organisms that constructed the colonies in a fractal, modular way. Further geochronological investigation of these animal communities is a crucial part of current research, connecting using absolute time the rise of these organisms and the large scale processes occurring in the Earth system as a whole. This information forms a fundamental platform for further research seeking to understand the crucial steps that took place to allow the Earth to become a haven for advanced life forms.”

But determining the age of these organisms is by no means straightforward. Says noble: “We take crucial advantage of the fact that many of them are preserved by virtue of burial in volcanic ash. Fortunately, some of the ash layers contain zircon, a mineral that has relatively high concentrations of uranium but is able to exclude virtually all lead when it crystallises. It is also an incredibly robust mineral and so it is an ideal atomic clock. It doesn't allow things to leak out or invade into the crystal to any great extent over time, and almost all of the lead in a zircon turns out being the result of in situ decay of the uranium.

“If one extracts the uranium and lead from the zircons then it is possible to measure their isotopes and determine the age of the mineral grain. In principal this is sounds easy but in practice it is complicated, involving chemical separation and purification followed by time-consuming and exacting analysis. In addition, one must be certain that the zircons being dated are actually from the time of the volcanic eruption that buried the organisms.

“Despite these challenges, the technique has now evolved to such a degree that accurate ages with uncertainties of around +/- 0.1% of the age can be produced. To consistently produce ages this precise it is critical to have an extremely clean laboratory where the total lead and uranium contamination of a sample is vanishingly small, measured in femto- or pico-grams (as low as 300-500 femtograms for Pb, less for U). We also need a high performance mass spectrometer that can measure individual ions with high accuracy and precision. This is because only single zircon crystals around 50 to 200 microns long, or domains within these crystals, are being analysed. The quantity of uranium and lead extracted from these amounts of zircon are extremely small, the total amount of lead being a 10 to 20 picograms, around the same as what was in just a few cubic centimetres of room air in the days of leaded petrol.

“The fact that the Precambrian rocks at Charnwood are dominated by the products of volcanism that makes dating the fossils possible. The Charnwood rocks are part of a volcaniclastic succession. The chemical and physical composition of the rocks is similar to the andesitic rocks and related sediments found in modern island arc systems overlying subduction zones. Some of the Charnwood rocks are intrusive, some are pyroclastic flows resulting from explosive volcanism which at times was extremely violent, while others represent remobilised volcanic debris deposited in deep water. The volcanoes that were active in the Charnwood area were largely submerged, which allowed fragmental material to be erupted from, or slumped off, the volcanoes and deposited as sedimentary layers on the surrounding ocean floor.

“The erupted rocks range from very coarse-grained pyroclastic breccias to fine-grained volcanic ash layers. Many of these volcano-sedimentary rocks are turbidites, deep water sediments that become progressively finer-grained upwards. Examination of these turbidites under the microscope shows that they are composed of angular fragments of fine-grained volcanic rock, volcanic glass shards, and broken mineral grains. The angular nature indicates that the material was not subjected to rounding through long distance transport or multiple stages of sedimentary reworking.

A very similar volcanic and depositional environment occurs on and just offshore of the Caribbean island of Montserrat, and it was watching the volcanism and sedimentation in action there that helped develop better geological models for Charnwood. It is on the topmost, very fine grained parts of the turbidite beds that the Charnwood Ediacarans lived and were buried and preserved, often being buried by a thin layer of volcanic ash. It is the zircons in some of these ash layers that allow us to date the Charnia fossils with a degree of confidence. However, dating the old fossils, Ivesheadia lobata, is proving more difficult because we don't have good ash layers to work with and so we have to approach the geochronology differently.

In this case we have sampled turbidite layers above, at and below the fossil horizon and are dating the volcanic zircons to bracket the age of the fossil horizon. The limited range of composition of the constituents of the turbidite, and the same zircon morphologies of zircons inside small volcanic rock fragments and zircons from the finer-grained horizons support the idea that the volcanic debris was of very limited range in age, and that the youngest zircons provide a maximum age for the fossils. The fact that the turbidites are almost certainly derived from the slumping of debris from active volcanoes during earthquakes means that the zircons are effectively dating the fossils in a manner close to that of the ash layer zircons.

“We are sure that there is still much more Charnwood can tell us about the timing of the rise of animal life on Earth, and what these early forms looked like, and we are planning to do even more geochronology work there" Noble told the BA.

Noble’s presentation included the first robust U-Pb geochronology data to be presented for these important Precambrian fossil localities in the UK - one of the most important records of early deep water soft-bodied animal communities anywhere on Earth.

How old are the oldest macrofossils?

When did complex animal life first appear? Ted Nield discovers how we think we know, from high precision isotope chronology of Ediacaran fossils

Geoscientist Online 10 September 2008

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