Fossil Shell Preserves Signs of a Prehistoric Tug-of-War

Back in the early days of paleontology, when the meaning and origin of fossils was still in doubt, some naturalists believed that the shells, shark teeth, and other petrified curiosities were attempts by the rock to imitate life. Fossils were not true vestiges of history, it was believed, but instead the product of some "plastic virtue" suffused throughout the non-living Creation. As naturalists began to study fossils more closely, however, they realized that the ancient shells showed signs of growth just like their counterparts along the seashore. Fossils were not crude imitations of life. Instead they were the traces of long-deceased organisms which had been transformed, and while it is easy to take this fact for granted today there are quite a few stunning specimens which beautifully reaffirm that paleontology is the study of ancient life.

As explained by paleontologists James Sprinkle and Jeri Rodgers in the Journal of Paleontology, between about 300-315 million years ago what is now Brown County in north-central Texas was covered by a shallow bay or tidal channel. Bivalve shells, bits of trilobite, shark teeth, crinoids, and other such fossils have been found here, including a large number of brachiopods. Though they might superficially look like just another kind of mollusc, brachiopods belonged to an entirely different phylum, one that flourished during the past but has been reduced to just a handful of species today. You can tell them apart from mollusc shells because brachiopods had upper and lower shells which were hinged at the back (giving some the appearance of an oil-lamp, hence their common name "lamp shells"). Given the abundance of these fossils it is easy to simply collect them and store them away, but a closer look at one specimen in particular documented a years-long struggle between two organisms.

Encrusted on the lower shell of the brachiopod Composita were two organisms: a colony of tiny marine invertebrates called bryozoans and a single individual of a kind of echinoderm called an edrioasteroid, with the bryozoans creating an almost complete ring around the starfish-like echinoderm. Both were trying to eek out a living on the brachiopod shell, and neither was giving up a millimeter of space.

From what Sprinkle and Rodgers were able to tell from the fossil, the sequence of events probably went as follows. The brachiopod which the shell represents grew for a number of years, but eventually died. At this point the two valves of the shell may have become separated, providing prime real-estate for the encrusting invertebrates which lived in the shallow Carboniferous bay. The echinoderm was the first to latch on, gradually growing as it sat on the shell, but sometime later a bryozoan larvae also took up residence there. Since it could reproduce asexually, it soon began creating a colony of other bryozoans which eventually came into contact with the echinoderm.

Once the different invertebrates made first contact, the organisms began to struggle with each other for shell space. The bryozoans continued to grow over the available parts of the shell, surrounding the echinoderm, but they could not overtake it. The echinoderm, on the other hand (or tube foot, if you like), held back the bryozoan colony but could not continue to grow outward. Just how the echnioderm held its ground is unknown, perhaps it had some kind of physical or chemical defense to keep the bryozoans from building up too high along the margins of its living space, but regardless of the weapons used in the struggle Sprinkle and Rodgers describe how the conflict was probably brought to an end:

The stalemate in the edrioasteroid/bryozoan war for living space continued for a while longer as the bryozoan almost completely surrounded the edrioasteroid. By then, both competing organisms had grown to sexual maturity and had probably reproduced. Finally a major storm or flood event buried and killed both organisms, breaking off the bryozoan's projections at this or a later time.

This tug-of-war between marine invertebrates would only have been detectable with the passage of time. Like the growth of the brachiopod itself, it occurred at a rate that would have been nearly imperceptible to us, but it was a dire struggle nonetheless.

Sprinkle, J., & Rodgers, J. (2010). Competition between a Pennsylvanian (Late Carboniferous) Edrioasteroid and a Bryozoan for Living Space on a Brachiopod Journal of Paleontology, 84 (2), 356-359 DOI: 10.1666/09-089R.1