The Oldest Complex Animals On The Planet

Charnia masoni, the first recorded Ediacaran

In 1957, a schoolboy by the name of Roger Mason discovered something remarkable: a fossil in the ancient rocks in Charnwood Forest, Leicestershire. Previous palaeontological theory stated that the oldest fossils hailed from the base of the Cambrian period some 542 million years ago. Mason´s fossil, however, was over 570 million years old, older than all proposed dates made by some of the most respected scientists of the day and indeed a hundred years of palaeontological dogma.

Across the world, fossils were found in ancient Precambrian rocks with strange swirls and discs, fronds and bubbles. Collectively they are now known as the Ediacara biota and they constitute the oldest animals on the planet. Ediacarans are all strange but some have a resemblance to or at least possess features found in modern day animals.

The oldest known are 730 million years old and come from Namibia. Named Otavia antiquus, they are sponges and the oldest undisputed animals on the planet. However they are very simple: little more than gooey bags of cells held together by collagen molecules. Animals such as humans or giraffes are far more complex. These earlier creatures display bilateral symmetry, where both halves of the organism mirror each other.

The oldest possible Bilaterian, Vermanimalcula, is around 600 million years old, but it is disputed as to whether it is an animal, something more primitive or not a fossil at all. Yet, palaeontologists excavating in the Tacuari region, Uruguay, have found indisputable evidence of the oldest complex, bilateral animals. In 2007, a team, led by geologist Ernesto Pecoits from the University of Alberta, Canada, were examining a mix of sedimentary and igneous rocks in the Tacuari Region, when they came across a tiny track way.

The ancient fossil trackways

No more than two centimetres in length, the tracks could easily have been missed, but in the tortured, twisted mass of sandstone and solidified lava, their regularity stood out. Simple animals are usually unable to move. Bilaterians, however, have a distinct front and back which allows for movement in a particular direction which produces a trackway in the process. By making careful measurements of the structure, Pecoits and his team deduced that the maker was 4 to 7 millimetres long and 1 to 2 millimetres wide and moved just under the thin layer of organic matter on the seabed.

Tiny marks along the edge of the track suggested that the owner had legs of sorts.´When you observe (these tracks) you can see like a furrow and on the side you can see ridges and that's because the organism was pushing the sediment out, and sometimes you see the organism goes up to breathe oxygen and then goes down again,´ Pecoits said. The real excitement came when the tracks were dated. As the sandstone was difficult to date, due to the pounding it had recieved in the continental vice, the team used small crystals within the rock to obtain an upper limit of 600 million years old.

For the lower bound, they used the igneous rocks. As the material was a granitic intrusion into the sedimentary formation, it must have been younger than the sandstone. The final number was 585 million years old. The tracks therefore fall somewhere in between the two, making them the oldest undisputed evidence of Bilaterians, and therefore, complex animals. In my opinion, tracks are some of the most enigmatic fossils that there are. While their strands add another element to creatures extinct for many millions of years, making the armies of organisms, once committed to an evolutionary void, almost tangible.