Archive for October, 2013

Wooster’s Fossil of the Week: A cheilostome bryozoan and serpulid worm bryolith from the Recent of Massachusetts

October 13th, 2013

Cheilostome Serpulid Muffin TopA bryolith is a mobile, unattached mass of bryozoans. Cheilostome bryozoans are especially good at forming bryoliths because of their hardy skeletons and relatively rapid rates of growth. The above specimen is a bryolith collected by my good friend Al Curran in March 2008 from Duck Creek in Cape Cod Bay near Wellfleet, Massachusetts. It is a modern specimen, so not actually a fossil, but I present it here because these objects have a good fossil record. The bottom view of the bryolith is below.
Cheilostome Serpulid Muffin ReverseThe tubes twisting about in this mass are those of polychaete serpulids. These are filter-feeding “tubeworms” common on marine shells, hardgrounds and rocks since the Triassic. We’ve met them many times in this blog. They are frustrating to identify from the tube alone because the soft anatomy (especially the genitalia, if you can imagine them) are needed to sort out most taxa. They tend to live on the undersides and cryptic spaces of hard substrates, which you can see when comparing the top and bottom of the above specimen.
Cheilostome Serpulid Muffin closerWith this closer look (above) we can see the fabric of the bryozoan skeleton (the zoarium). Individual zooecia (the skeletal tubes of the living zooids) are coming into focus. It appears from the intergrown nature of the serpulid tubes and bryozoan that these two groups were living together at the same time.
Cheilostome Serpulid Muffin closer yetIn this even closer view we see a serpulid tube embedded in a matrix of cheilostome zooecia. The apertures of the zooecia are now visible, and a bit of the frontal walls.
Cheilostome Serpulid Muffin closestThis is the closest I could get with our camera equipment. The frontal walls and apertures of the zooecia are easily seen. In life each aperture would have had a little door (an operculum). The frontal walls are a beautiful lattice-work of calcite.

I hesitate to suggest an identification for this cheilostome bryozoan because one of the world’s experts, my English good friend Paul Taylor, reads this blog. Nevertheless, I think these are of the widespread genus Schizoporella. Paul will correct me quickly if I’m wrong!

References:

Kidwell, S.M. and Gyllenhall, E.D. 1998. Symbiosis, competition, and physical disturbance in the growth histories of Pliocene cheilostome bryoliths. Lethaia 31: 221-239.

Klicpera, A., Taylor, P.D. and Westphal, H. 2013. Bryoliths constructed by bryozoans in symbiotic associations with hermit crabs in a tropical heterozoan carbonate system, Golfe d’Arguin, Mauritania. Marine Biodiversity: http://dx.doi.org/10.1007/s12526-013-0173-4 .

Wooster’s Fossil of the Week: A gastropod/coral/hermit crab combination from the Pliocene of Florida

October 6th, 2013

Septastrea marylandica_585These two shells show a lovely symbiosis between shallow marine hermit crabs and encrusting scleractinian corals. I was first introduced to the concept of “pagurized” shells by my friends Paul Taylor and Sally Walker. They showed me the many ways by which shells that were carried around by hermit crabs display particular evidence of this specific use, from characteristic wear patterns to patterns of encrustation and boring. Further, there are some situations, such as that shown above, where encrusters and hermit crabs have developed a mutually beneficial relationship that may have even been depended upon by the crabs.

What we have here are gastropod (snail) shells that have been completely encrusted by the scleractinian coral Septastrea marylandica (Conrad, 1841). These are found in great abundance in the Pliocene Pinecrest Sand (foraminiferal zone N20) near Fruitville, Sarasota County, Florida. What is most cool is that the corals have completely encrusted these spiraling snail shells and more. If you look carefully at the aperture of the specimen on the left you see the lower surface of the coral with no snail shell. The coral had encrusted the whole shell and continued to grow from the original aperture outward, elongating the twisting tube farther than the snail ever grew. Why (and how) did it do this?

The answer is that the shells were occupied by hermit crabs. The corals extended the aperture of the shell with the crab shuffling about in the opening. The crabs gained the advantage of a shell that essentially grew along with them, meaning they did not have to make the dangerous switch to a larger shell as often. The corals gained by being carried about into diverse microenvironments, extending their feeding possibilities. Nice arrangement, and elegant fossils to show it.
Septastrea closeSeptastrea marylandica (Conrad, 1841) is a scleractinian coral. We’ve seen this order before on this blog, but usually as a recrystallized version of the original aragonitic shell. In these specimens the aragonite is still preserved in excellent detail. Each of the individual “cups” (corallites) above contained a single coral polyp in life. The radiating vertical walls are called septa and are related to the original soft parts of the polyps. The polyps extended tentacles from these corallites into the surrounding seawater. The tentacles were lined (as they are today) with stinging cells called nematocysts for subduing very small items of prey, such as larvae or tiny arthropods. Corals thus represent an ecological group of sessile benthic epifaunal predators. Sessile means stationary, benthic means on the seafloor, and epifaunal means on the surface of the seafloor (that is, not in the substrate itself). Curiously, then, these corals that encrusted shells with hermit crabs in them became in a sense vagrant rather than benthic because they were moved about on the seafloor. You don’t hear about vagrant benthic corals very often!

References:

Allmon, W.D. 1993. Age, environment and mode of deposition of the densely fossiliferous Pinecrest Sand (Pliocene of Florida): Implications for the role of biological productivity in shell bed formation. Palaios 8: 183-201.

Darrell, J.G. and Taylor, P.D. 1989. Scleractinian symbionts of hermit crabs in the Pliocene of Florida. Memoir of the Association of Australasian Palaeontologists 8:115–123.

Laidre, M.E. 2012. Niche construction drives social dependence in hermit crabs. Current Biology 22: R861–R862.

Petuch, E J. 1986. The Pliocene reefs of Miami: Their geomorphological significance in the evolution of the Atlantic coastal ridge, southeastern Florida, USA. Journal of Coastal Research 2: 391-408.

Taylor, P.D. and Schindler, K.S. 2004. A new Eocene species of the hermit-crab symbiont Hippoporidra (Bryozoa) from the Ocala Limestone of Florida. Journal of Paleontology 78: 790-794.

Vermeij , G.J. 2012. Evolution: Remodelling hermit shellters. Current Biology 22: R882-R884. [Really. The title is spelled exactly this way.]

Walker, S.E. 1992. Criteria for recognizing marine hermit crabs in the fossil record using gastropod shells. Journal of Paleontology 66: 535-558.

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