Single Celled Giant – Responsible for Ancient Trace Fossils?
One of the most momentous events in the history of life on our planet took place around 545 million years ago, at a time when the Earth looked very different than today. For a start it was spinning faster on its axis, meaning that our 24-hour orbit was something unimaginable back then, days would have lasted something like 20 hours. The atmosphere would have been completely alien to us, in fact had a human being been able to travel back in time to the Cambrian, they would have needed to bring breathing apparatus as the air with its lack of oxygen would have been quite toxic to us.
The continents with which we are so familiar today, would have been unrecognisable, in the western hemisphere; the fragmented parts of a super continent were being pulled apart by an ocean ridge, in the east, another, larger remnant of this huge landmass stretched almost pole to pole. Most of the United Kingdom lay off the coast of Gondwana, the name given to the area of land in the eastern hemisphere, although rock that is found in Scotland today was actually thousands of miles away in the western hemisphere. The geological period we now know as the Cambrian was first defined in 1835 by an English professor of geology – Adam Sedgewick. He mapped rock strata in Wales and showed that it was formed after older Precambrian rocks but before the rocks believed to have been formed in the Silurian period.
As our understanding of geology has improved, these Welsh rocks are not used anymore to delineate the start of the Cambrian, strata in Newfoundland are regarded as indicative and distinctive enough to be seen as forming the boundary between the Precambrian and the Cambrian. The starting point of the Cambrian is marked by the appearance of trace and body fossils, indicating diverse animal life. Trace fossils are preserved evidence of activity, for example a fossil cast of a worm burrow or a trail left in soft mud on the ocean floor that has become preserved.
During the Cambrian, there was a rapid acceleration in evolution and a vast array of new creatures suddenly appear in the fossil record. Animals with hard parts such as shells and external skeletons evolved and this phenomenon has been termed the “Cambrian Explosion”. Prior to the evolution of hard parts that have a higher potential for fossilisation, only a very few types of fossil were known – many of which were trace fossils, such as tracks preserved in mud as a creature crawled over the seabed. The first trilobites appear, amazing arthropods, distant ancestors of spiders, mite, insects and crustaceans. trilobites are one of the “star turns” in the fossil record. These marine creatures evolved into at least nine Orders and survived right through the Palaeozoic, with the very last of their kind becoming extinct at the end of the Permian approximately 250 million years ago.
An Illustration of a Typical Trilobite
Illustration Credit: Everything Dinosaur
To view a model of a trilobite – from Everything Dinosaur’s fossil replica series:-
Model trilobite and other prehistoric anmals: Prehistoric Animal Models.
Up until now, the trace fossils, many of which are preserved trackways, are believed to have been made by complex, soft-bodied organisms, perhaps ancestors of the armoured trilobite. This has been an appropriate assumption as; in a relatively short period of geological time, the Cambrian explosion occurs and a myriad of life forms are recorded as fossils. It had been widely accepted by palaeontologists and ichnologists that trace fossils such as these trackways were made by soft bodied, complex, bio-symmetrical organisms, a fair assumption, since just a few million years later a great diversity of complex life occurred and these animals must have had complex ancestors and life would have been abundant. Being soft-bodied there is little fossil evidence to be found, the fossils were thought to be disproportional to the amount of life forms that had already evolved.
However, new insight into the habits and lifestyle of a very ancient creature has cast doubt on this assumption, it appears that single-celled, very simple organisms are capable of leaving tracks in soft mud and it could be these life forms, not more complex ones that left the Early Cambrian and Precambrian tracks in mud that became trace fossils.
New research into a little known, distant relative of microscopic amoebas called Gromia sphaerica has proved that these little organisms travel across the sea floor and as they do, they leave trackways behind. These tracks are very similar to the fossilised tracks found in Precambrian rocks. It has been suggested that the Precambrian trace fossils may not be evidence of complex organisms but the preserved trails of an ancient type of Gromia.
If this is the case, then the Cambrian explosion becomes even more of a mystery, as what evidence we have of complex organisms in Precambrian strata may be reduced if some of these tracks are attributed to types of single-celled organism.
G. sphaerica was believed to be sessile and of a epifaunal habit (living on the sea floor, either on soft sediment or attached to rocks, but not moving). Observations from the bottom of the Arabian Sea indicated this. However, a group of researchers have located a colony of the coast of the Bahamas and these creatures are vagrants (move over the sea floor), leaving trails up to half-a-metre in length.
Gromia sphaerica is gigantic for a single-cell eukaryote (cell enclosed by a membrane), some specimens have been measured with diameters in excess of 30 mm, about the size of a large marble. These creatures were recorded wandering across the seabed by Mikhail Matz of the University of Texas at Austin and a group of fellow researchers
A distant relative of amoebas, these strange organisms have left trails in the Caribbean seabed that resemble the trace fossils found in Precambrian rocks.
The red arrow in the picture is pointing to a trail left by a Gromia sphaerica. The round lump is the organism itself, covered in a light coating of sediment. The trail consists of two small ridges on the outside, and one thin bump running down the middle. This pattern is consistent with certain trace fossils. Scientists are confident that prior to the Cambrian, life forms were exploiting the surface layers of marine sediments, now this new evidence may indicate that multi-celled, bio-symmetrical organisms were not as abundant as the fossil record indicates, if some of these trace fossils can be attributed to the ancestors of Gromia sphaerica.
Commenting on the research, Matz stated:
“If these guys were alive 600 million years ago, and their traces got fossilised, a palaeontologist who had never seen this thing would not have a shade of doubt attributing this kind of trace to the activity of a big, multicellular, bilaterally symmetrical animal”.
“This is a very important discovery,” Shuhai Xiao of Virginia Polytechnic Institute added. “The fact that protists can make traces has important implications for how we interpret many trace fossils.”
This research could have dramatic implications for the Cambrian Explosion, if single-celled organisms were making some of the trackways, complex animals were not as abundant as previously thought. To quote the researcher Matz, every type of animal phyla around today “suddenly burst out of a magic box. It wasn’t a gradual development”.
In addition, genetic analysis of the water-filled cells of G. sphaerica reveal tantalising evidence that it could be one of the oldest type of organism on the planet – a living fossil.
“There’s a 1.8 billion year old fossil from the Stirling Formation in Australia that looks just like one of their traces, and with a discoidal body impression similar to these guys [G. sphaerica]“. Matz commented.
“We have not proved anything, but we might be looking at the ultimate living macroscopic fossil”.
The Stirling Formation in south-western Australia is famous for its fossils of Precambrian life, many types of soft-bodied animal are preserved in what was a shallow, tidal, sandy sea bottom. Sharing this ancient world, a part of the geological time scale called the Cryptozoic (time of invisible life due to the paucity of the fossil record), were jellyfish.
Jellyfish are another example of a living fossil, this type of animal may have swam in prehistoric seas, as much as one billion years ago.
To read a related article on jellyfish and life before the Cambrian explosion:
Everything Dinosaur supplies a model of a Medusa type jellyfish, part of a set of fossil animal replicas that include belemnites and ammonites.