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The Oldest Animal Fossils?

By Hans-Dieter Sues

Charles Darwin noted that the oldest fossils known in his day already represented quite complex life forms such as trilobites, an immensely diverse group of extinct marine arthropods most closely related to horseshoe crabs, spiders, and their relatives. We now date these remains as middle Early Cambrian in age.

Because Darwin assumed evolutionary change to be gradual, he reasoned that the roots of animal life must be sought far back in time. However, there was no fossil record to support his contention. Darwin even hoped that rocks at the bottom of the ocean might yield clues, but we now know that no ocean floor older than about 200 million years survives.

Since Darwin’s day paleontologists have searched far and wide for the earliest traces of animal life. This effort has led to the recovery of a diversity of fossils from sedimentary rocks of the Ediacaran, deposited just prior to the Cambrian Period, which began 542 million years ago. However, recent studies have shown that many of the Ediacaran organisms are not closely related to any of the major lineages of animals known from later periods, and some may not even be animals at all. (See my blog “Weird Wonders at the Dawn of Animal Life” posted on April 14, 2010).

A number of biologists have attempted to determine the first appearance of the major lineages of animals by estimating rates of evolutionary change in DNA and various other molecules and thus developing “molecular clocks.” These studies have pushed the origins of these lineages far back in time, long before any undisputed record of animal fossils. One problem with this approach is that different molecules evolved at different rates and thus the molecular clocks are not in synch.

In a recent article in Nature Geoscience a team of American researchers led by Adam Maloof (Princeton University) has reported on intriguing small fossils from limestone of the Trezona Formation in the Flinders Ranges of South Australia that they interpret as an early sponge-like organism. Although the Trezona Formation has not been dated radiometrically rocks of the overlying Nuccaleena Formation have yielded an age of 635.2 million years.

The new fossils can attain a size of several millimeters and appear on the rock surface in a great variety of shapes, ranging from circular to anvil- and V-shaped in outline.

Oldest_Animal_Fossil_FIG1.jpg

Princeton geoscientist Adam Maloof holds a chunk of limestone from South Australia that may contain the oldest animal fossils discovered to date. The fossils, visible as red shapes, suggest the existence of sponge-like animals well over 635 million years ago.

Courtesy and copyright of Dr. Adam Maloof (Department of Geosciences, Princeton University).

In order to interpret the structure of these organisms, Maloof and his colleagues cut 50-micrometer-thick slices of fossil-bearing limestone. Each slice was then imaged digitally, and the resulting images were stacked into three-dimensional representations using new software. This process established that the variety of shapes seen on the rock surface represented different sections of irregularly shaped bodies.

The new fossils reveal a surprisingly complex internal organization, with chambers connected by canals that have diameters of up to one millimeter. They certainly resemble sponges. However, the body walls of most modern and extinct sponges contain microscopic needles (spicules) that are composed of either calcium carbonate or silica and strengthen the walls. The absence of such spicules in the fossils from the Trezona Formation puts into question their interpretation as sponges (although the researchers noted the presence of tiny blobs of silica in a few specimens, which may represent altered spicules). Or does it?

A major challenge in the search for early animal life is that the stem-forms of present-day animals probably lacked many, indeed most, of the characteristic attributes of these groups. In other words, we may be unable to recognize such forms as related to their descendants. The absence of spicules in the new Australian fossils could suggest that either sponges had not yet evolved these features or that the needles dissolved as the limestone enclosing the fossils underwent chemical changes due to geological processes.

The preserved details of internal structure in the fossils from the Trezona Formation do suggest a relationship to sponges. The geological age of the fossils is consistent with estimates for the first appearance of sponges based on molecular clocks as well as with geochemical records of molecules (“biomarkers”) specific to this group of animals.

Oldest_Animal-Fossil_FIG2.jpg

The Princeton researchers, in conjunction with experts at Situ Studio (Brooklyn, New York), used a serial-grinding and imaging process to analyze hundreds of slices through a single fossil embedded in limestone. Special computer techniques were then used to create a digital reconstruction of the specimen, generating a three-dimensional model of the ancient creature.

Courtesy and copyright of Dr. Adam Maloof (Department of Geosciences, Princeton University).

The earliest known undisputed body fossils of sponges come from the Ediacaran of South Australia and are some 90 million years younger than the fossils from the Trezona Formation. During the succeeding Cambrian Period, sponges and various sponge-like organisms diversified rapidly, along with most major groups of present-day animals during what is commonly referred to as the “Cambrian Explosion.”

The search for the earliest animals continues to be an exciting area of research. While our understanding of the fossil record has made tremendous advances since Darwin’s time a great deal remains to be learned about the origin and early evolution of multicellular life on our planet.

Hans-Dieter-Sues.jpgHans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time.

A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.

Blog entries by Hans-Dieter Sues >>

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Comments

  1. Antonio Melhado
    Birigui SP- Brasil
    April 21, 9:22 am

    Os poríferos são animais importantes para a biologia evolucionista, por se tratar de um estágio da vida antiquissima e pouco conhecida, que é a passagem do protozoa para o metazoa, ou seja do unicelular para os pluricelulares.

  2. me
    canada
    March 18, 2012, 4:06 pm

    perfect for my earth science homework thanks it took forever to find an article on determing earths past through fossil evidence

  3. steven robertson
    mariposa, ca
    July 3, 2011, 11:45 pm

    crazy… really you’d think life was much, much older than that. virus’ and bacteria can be so much simpler than one could think and cravings for things like meatd can be so strong and intense.