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Reading between the lines: these lines may represent the fossilized skeleton of a primitive marine animal.

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Dear PaleoScene,

Please let me know what this fossil is. I found it in Wiarton, Ontario, while working in the quarry.

Thank you,

ANDRICUS PETERS

Dear Andricus,

From your photos, my first impression is that this is a weathered example of an enigmatic fossil called Sphenothallus. This type of fossil usually has a long and narrow but flexible, gently tapering tube with a cup-like attachment disc, or holdfast, at the pointed end, which once anchored the organism to the sea floor, or to a hard surface such as a shell or another Sphenothallus.

The genus was named in 1847 by James Hall for a set of fossils from New York. Derived from the Greek words sphen, meaning "wedge-shaped," and thallos, "young shoot," the name reflects Hall's belief that the fossils represented land plants. Many organisms that do not move about (called sessile organisms) both fossil and living--sea lilies, for example-- were thought by early naturalists to be plants and were named accordingly.

Since then, Sphenothallus has been recognized as the remains of a marine animal. One palaeontologist named similar fossils Serpulites, which simply means "like Serpula," a known modern marine worm that builds itself a tube to live in. Serpulites is now thought to be just another form of Sphenothallus. Most recently, Sphenothallus has been assigned to the large group of animals called Cnidaria, which includes corals, jellyfish, hydra, and a host of other primitive organisms, most of which live in the sea. There it joins a group of other tubelike fossils, some of which lived at the same time and in the same environments.

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In Sphenothallus, the broader end of the tube was open, and evidence suggests that it may have been inhabited by a polyp (similar to a modern hydra) that could have used its tentacles to grab small swimming or planktonic prey as it passed by. Often, all that remains of the fragile tube, or theca, are two narrow longitudinal thickenings, so the fossil may appear to be just two almost parallel, threadlike lines in the rock, tapering to a point at one end. The space between the lines may hold dark traces of the original thecal material; the space may be slightly raised if the tube had filled with mud after the animal died; or it may be collapsed if the tube was empty when it was buried. Occasionally, branching forms occur, and multiple individuals are often found clustered together.

The host rocks in which Sphenothallus is preserved suggest that it thrived in conditions that would be considered harsh for many other marine organisms--soft fine mud; varying salinity; and, often, low oxygen. These conditions combine to preserve delicate features that might otherwise have been destroyed by predators or scavengers on the sea floor. The rock in the Wiarton area has been interpreted as fine mud deposited in poorly oxygenated shallow lagoons that once existed there, perfect for preserving these features. Sphenothallus may have been equally happy in normal marine conditions, where traces of holdfasts have also been observed.

This fossil is widely distributed, and looks pretty much the same throughout its range. It has been found in rocks ranging in age from Lower Cambrian (515 million years) to Late Carboniferous (320 million years), and geographically across four continents: in China, South Korea, the Czech Republic, Bolivia, British Columbia, Ontario, Quebec, and many parts of the United States. It is not uncommon to uncover it among the thinly bedded banded horizons of the Eramosa Formation (from the Silurian period, 425 mil lion years old) in quarries around Wiarton, although it may be overlooked in favour of more rare fossils that "look like something."

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But there are other, more prosaic ways to interpret your fossil. If it encompasses not just the thin curved lines, but also the general amorphous shapes surrounding them, we could be looking at a broader sheet-like strip, such as a piece of an algal or bacterial mat, similar to pond slime, that had torn off from the sediment surface and twisted over on itself. In this case, the curved lines could represent the edge of the crumpled sheet. Since the dark bands in the Eramosa sediments were caused by cyclical or seasonal accumulations of slime, this explanation also fits with the location.

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Either way, though your fossil may represent a lowly life form, it was one that quietly outlasted many grander things and was preserved to see the light of day again.

Janet Waddington is assistant curator of Paleontology in the ROM's Department of Natural History.
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Title Annotation:PALEOSCENE
Author:Waddington, Janet
Publication:ROM Magazine
Geographic Code:1CANA
Date:Sep 22, 2008
Words:760
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