Sponge-like Fossils may be Evidence of Earliest Animals Known to Science
A team of researchers writing in the scientific journal “Nature Geoscience” claim that an analysis of limestone strata (stromatolites formed by bacteria) from the Flinders range in South Australia, may have produced evidence of the first complex animals known. Analysis of these strange “body” fossils undertaken by Geoscientists based at Princeton University (New Jersey, United States), suggests the presence of primitive sponge-like organisms dating from approximately 650-640 million years ago. These fossils pre-date the presence of other complex animal fossils in the fossil record by about seventy million years.
Is there Evidence of Ancient Sponges preserved in these Limestone Formations?
Picture Credit: A. Maloof/Princeton University
The picture shows two stromatolite formations, the pen on the right of the picture provides a scale. Stromatolites are layered, mound-like structures, sometimes reaching more than a metre tall. These structures are built up over many years in shallow and warm tropical waters by sheets of bacteria growing over the seabed and trapping sediment. Evidence for stromatolites have been found in rocks dating from 3 billion years of age.
The research team, led by Professor Adam Maloof and assisted by Geoscientist Catherine Rose suggest that the strange, bizarre calcite based forms preserved in the limestone are the fossilised remains of sponges. The shapes, which vary from ovals, anvils, wishbones and rings some of which measure more than one centimetre long represent body fossils according to the Princeton University researchers.
Broadly there are two main types of fossils. Firstly, there are body fossils, these preserve something of the bodily remains of animals or plants. For example, the cast exoskeleton of a lobster or trilobite, fossil bones, teeth, plant remains and such like. Secondly, there are trace fossils, these preserve evidence of the activity of animals such as tracks, insects bore holes in wood, burrows in sediment and footprints. Technically, there are other sorts of fossils such as what is referred to as “chemical fossils” evidence preserved in rocks of the chemicals that we know can only be produced by life processes. There are also micro-fossils, tiny, microscopic body and trace fossils such as pollen grains and other fossil material that is only able to be viewed using microscopes or other technology.
The American based researchers suspected that the peculiar shapes entombed in the limestone might be parts of the bodies of ancient organisms, the use of sophisticated 3-D imaging techniques affirm their theory.
The theory is controversial, as if the scientists are correct, then they have discovered fossils of creatures that existed in the Cryogenian period. This geological period is estimated to have lasted from about 850 million years ago to approximately 635 million years ago, although the exact duration of this particular part of the geological time-scale is disputed, it is thought that the Earth went through dramatic climate changes resulting in a global Ice Age, when the entire planet become covered in ice, leading to the concept of a “snowball Earth”. To have discovered the remains of relatively sophisticated creatures that have survived beyond the Cryogenian into the Ediacaran period would be a remarkable discovery and enhance the reputation of the Flinders Formation as a major location for Proterozoic fossil finds.
Professor Maloof commented:
“No one was expecting that we would find animals that lived before the ice age [global freezing event – “snowball Earth”] and since animals probably did not evolve twice, we are suddenly confronted with the question of how a relative of these reef-dwelling animals survived the “snowball Earth”.”
A spokesperson from the National Science Foundation, which funded the study, stated that the find is at least seventy million years older than any other evidence of animal body forms in the known fossil record.
Professor Maloof added:
“People have certainly proposed complex organisms, like eukaryotic algae [organisms with cells that contain a nucleus] or protists, and have even proposed animals in the form of trace fossils (preserved tracks) prior to the sponges that we report. But I think we could confidently say that our sponges are the first somewhat convincing body fossils of an animal before the Ediacaran period.”
The Ediacaran hills, part of the Flinders Range in South Australia have already provided fossil evidence of some amazing soft bodied organisms. In 1946, geologist Reginald Sprigg was searching for signs of mineral deposits when he discovered the remains of several bizarre Precambrian animals. Creatures such as Tribrachidium, Yelovichnus and Spriggina (which is named in honour of him). These animals comprise a range of organisms classified as Ediacaran fauna and are so unusual and strange that most cannot be associated with known Phylum. For example, Spriggina could be a primitive, basal trilobite or possibly a segmented worm.
Due to the fact that the animal fossils were made of the same mineral material as the limestone rock (calcite), the scientists were unable to remove them from the matrix. Instead, the researchers collaborated with Situ Studio, a specialist imaging company to produce digital models of the strange structures.
By carefully slicing off thin layers of rock, each about 50 microns in diameter and photographing it after each layer was removed, the team managed to create a 3-D model of the animals. The model revealed a series of irregularly shaped bodies up to one centimetre in size (marble sized) with an internal network of interconnected millimetre-wide channels. The presence of the channels and their structure are the interesting elements to the palaeontologists. Such structures are found in sponges, it is these channels that have water drawn into them that permit these organisms to feed. Could these microscopic channels be evidence of Precambrian sponge-like lifeforms?
Is this Evidence of a 640 million year old Sponge-like Animal?
Picture Credit: A. Maloof/Situ Studio
The picture shows one of the 50 micron wide rock slices, one of 500 thin layers of rock examined. The area of interest, a potential fossil is outlined in blue.
The researchers suggest that at least three of the specimens had a short, tube-shaped pedicle at the base. This may have been used to attach the creature to a solid surface.
A Digital Image Showing the Microscopic Network of Channels
Picture Credit: A Maloof/Situ Studio
The oldest fossil sponges previously found lived around 520 million years ago. However, chemical traces from materials of cell membranes (steranes) of sponges were discovered in 2009 in sedimentary rocks more than 635 million years old.
However, the theory put forward by the Princeton team is disputed. In a dismissive statement; Dr. Jim Gehling, a Senior Research Scientist from the South Australian Museum said that he saw no convincing evidence to support the interpretation that the “coco-pop breakfast-cereal-like forms” were ancient sponges.
He went on to add:
“They may just as easily be mineralised bacterial cells or some other sort of single-celled microbes. Sponges should have first appeared in the Cryogenian Period about 650 million years ago – judging by times of branching in family trees in the Animal Kingdom, based on molecular clocks and the discovery of sponge biomarkers (steranes) preserved in rocks of this age.”
However, it is difficult to interpret the fossil evidence as scientists have no real data on what the earliest types of sponge-like organisms actually looked like.
Dr. Gehling stated:
“The problem is that we have no idea what the very earliest sponges may have looked like. This means that the discovery of any weird shape in rocks of this age may lead to claims of the ‘oldest sponge-grade fossils’.”
Our knowledge of Proterozoic lifeforms has increased dramatically over the last thirty years or so. New insights into existing fossil specimens have occurred thanks in the main to the use of sophisticated new examination techniques.