The Basics:
Aspidella is an Ediacaran disk-shaped fossil. It was most likely the imprint of the holdfast of a stalked organism, such as an erniettomorph or rangeomorph. The Ediacaran Period spans 94 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 541 Mya.
A General Overview of Fossils
(Correlating to the Aspidella Fossils found at Hewitts Cove)
What Are Fossils?
Fossils are accidental by-products of life preserved in the geological record. Some are recent additions to the record, some are incredibly ancient. They help us understand the history of life on Earth and the history of the Earth itself. What is a fossil? A good general answer is: “The remains, impression or trace of any organism that lived in past ages” Fossils are formed in all sorts of curious ways by the processes of nature. Life comes in a wide variety of forms and so therefore do fossils. Since each life-form can be fossilized in several different ways the variety found in fossils is amazing.
Why/How Fossils Form
Why fossils form at all is simply a matter of chance. The right material in the right place at the right time will produce a fossil. Most organisms never make it as fossils but the lucky ones end up partly or wholly preserved, awaiting discovery by someone curious like you. Things end up as fossils due to natural processes and are preserved in all sorts of interesting and sometimes unexpected ways.
Trace fossils
Plant and animal activity often modifies the environment. These modifications together with plant and animal products leave traces behind in the sedimentary material they live on or in. Footprints and other tracks: Animals with any kind of feet can leave footprints behind in soft, moist sediment or volcanic ash. If the sediment or ash is firm enough when a new layer washes or falls over it the impressions will be preserved and become trace fossils. The original layer hosts the footprint indentation and the overlying layer carries a cast of the indentation. Both versions of the footprint are trace fossils.
Specific Information on The Ediacaran Aspidella
Introduction
Ediacaran fossils have baffled scientists since their original discovery in 1868. As the oldest unambiguously multicellular organisms with tissues, Ediacaran organisms (or simply Ediacarans) represent a key stage in the evolution of life, and understanding their fossil record is essential to understanding the status of life prior to the great radiation of forms seen in the Early Cambrian. However, the bizarre shapes and structures of many Ediacarans have made placing them in phylogenetic context with modern taxa and determining their ecological roles difficult.
The Ediacaran Aspidella
Aspidella are mysterious organisms, even by Ediacaran standards. Specimens consist of a fossil structure that superficially resembles a small crater. The name ‘Aspidella’ was first used by Elkanah Billings in 1872 to describe ring-shaped fossils from Newfoundland. The nature of Aspidella is the subject of much discussion and debate. The general consensus is that they were relatively sessile organisms, similar in appearance, but with perhaps no evolutionary relation, to modern members of the order Pennatulacea (the sea pens). They, and many other Ediacaran taxa, are believed to have each had a soft stalk with a frond-like appendage at one end and a stiff base at the other. This base anchored the Aspidella to the sea floor.
The Possibility of Tectonic Streaching
Because the shape and orientation of Hewitt’s Cove’s Aspidella fossils, it has been speculated that these Aspidella were shallow-water dwellers that evolved a streamlined shape and aligned themselves with currents or light sources for photosynthesis. It is also speculated that they formed below the storm wave-base, where it is unlikely strong currents were present. Measurements of the orientation of the Aspidella raises the possibility of another explanation for both the unusual shape of the Aspidella and their orientation: tectonic deformation. Ancient marine fossils are commonly known to become stretched and distorted in a particular direction by the stress/strain of tectonic events. A tectonic event could have formed the fossils into the unusual shape and orientation found at Hewitt's cove.
Evidence for Moving
All examined thin sections of laminated rock showed no signs of lamination disruption . This suggests Aspidella had a stationary lifestyle, or were only mobile above the sediment-water interface. The lack of lamination disruption above the Aspidella fossils indicates that the organisms anchored themselves to the surface of the sea floor or rested upon it. Had they buried their holdfasts any distance below the top layer of sediment, lamination disturbance above the Aspidella fossils would be expected. The lack of horizontal disturbances around the Aspidella indicates that the organisms were also incapable of, or at least seldom engaged in, lateral movement. Finally, the lack of any disturbance around all observed Aspidella implies that the organisms were buried without a struggle. The Aspidella were either too immobile to fight through the sediment load or were already dead at the time of burial.
Logging Requirements:
Notes:
1. Please do not post answers in logs. Email answers and then log the EarthCache.
2. You may find it initially difficult to find the fasthold impressions. Locate GZ using the picture above and look fairly low.
3. You will need to measure. You may want to bring a rule.
Use Caution: Rocks and seaweed are slippery when wet!
Questions:
1. Measure the width and height of one of the holdfast imprints. Submit the answers with the rest of the answers.
2. Measure the height above ground level of the holdfast imprint you found (use the average height if you found a grouping) and submit answer.
3. Are the holdfast imprints consistent in shape or are they divergent?
4. What shape are the holdfast Imprints? Are they circular, oval, etc.
5. Are the holdfast imprints consistent in size with only small variations and or are they divergent is size with large variations?
6. Here is the tough question! Write a Paragraph answering the following questions using what you learned from your observations and the above write-up. Do you ascertain that the holdfasts are impressions from the same organism or different organisms? Were the organisms aquatic or land based? How were the fasthold imprints formed? Feel free to add anything else you found Interesting!
References:
Information for this EarthCache was was complied from the following sources:
A Field and Laboratory Study of the Ediacaran Fossils of Hewitt's Cove
Wikipedia: Aspidella
Thanks:
Thanks to Jacob Benner, Senior Lecturer Earth and Ocean Sciences, Tufts University; Dr. Kenneth G. Galli, Lab Manager / Lecturer Earth & Environmental Sciences, Boston College: and Stephen Mabee, State Geologist, Massachusetts Geological Survey for their valuable assistance in researching both fossils in general and local fossil sites. Thanks to Loni Fournier, Hingham Conservation Commission for granting permission to place the EarthCache in the Bouve Conservation Area.
*Congradualions to sarahbrian77 for FTF!*
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