Way back in the summer of 2008, my good friend Paul Taylor (the Natural History Museum, London), John Sime (Senior Independent Study student at the time) and I explored the Pierre Shale (Upper Cretaceous) of the Black Hills region in South Dakota, Wyoming and Montana. We collected numerous conchs and internal molds (steinkerns) of the abundant ammonite Baculites. Some of the internal molds had beautiful networks of connected tear-drops molded in the sediment that had filled the empty shells on the Cretaceous seafloor (image above). These represent skinny bryozoan colonies that lived attached to the inside walls of the shells. These bryozoans were soft-bodied with no hard parts and so were preserved as molds within molds, a kind of taphonomic complexity we always enjoy.
In 2012, Paul and I published a description of these bryozoans, describing them as a new genus and species of ctenostome bryozoan: Pierrella larsoni (see Wilson and Taylor, 2012, and this Wooster Fossil of the Week entry). Above are photographs of ammonite steinkerns showing the undersides P. larsoni (figure 28.1 of Wilson and Taylor, 2012). The specimen above on the left is from Red Bird, Wyoming, and the one on the right is from the Heart Tail Ranch in South Dakota. The scale bars are 10 and 5 mm respectively.
One of the most interesting feature of Pierrella is a remarkable “pleated collar” at the zooid aperture. In the image above, we’re looking at a mold of the collar, which was, like everything else in this bryozoan, soft tissue. From Figure 28.4 of Wilson and Taylor (2012); scale bar = 50 µm.
Now the cool “living fossil” part. Several years later, Russian scientists examined metalliferous seafloor nodules in the deep-sea Clarion-Clipperton Fracture Zone of the eastern Pacific Ocean. This area is receiving considerable attention for deep-sea mining, so teams of ecologists and biologists are surveying the biodiversity in this zone to assess the damage that could be done to this dark ecosystem around 5000 meters deep. Turns out there is a considerable bryozoan fauna down there.
And guess who is a part of this deep-sea community? Pierrella! In 2018, Grischenko et al. published a paper describing Pierrella plicata, a modern species of a genus described from the Late Cretaceous. Just last week, Schwaha et al. (2021) described the anatomy of this taxon. Above images are from their figure 2: (b) colonies attached to the surface of an arenaceous foraminiferan, showing dispersed zooids and thin proximal cystid appendage; (c) a single zooid; (d) the apertural folds — our Pierrella pleated collar! (Abbreviations: ap – aperture, pca/cd – proximal cystid appendage/cd, z – zooid.)
So our little Cretaceous Pierrella was recognized in the Modern by its fancy apertural collar, which was preserved in an unusual way we still don’t fully understand. The wonders of paleontology.
The modern Pierrella is an example of a “living fossil“, meaning that it very closely resembles an ancient fossil form. Paleontologists are not thrilled with this category, so I keep it in quotation marks. A popular misconception is that a “living fossil” has “not evolved” during some extended interval (about 75 million years in our case here). However, evolution proceeds along all sorts of pathways that are not always possible to see with a fossil-to-living comparison. Internal organs may have changed dramatically and we wouldn’t know. There is also the possibility of evolutionary convergence. Certainly the environmental differences are extraordinary for Pierrella: the Cretaceous version was found in shell interiors in relatively shallow waters whereas the modern form is about 5000 meters deep. Still, the similarities between fossil and modern Pierrella are intriguing.
You know who really likes “living fossils”? Creationists! They have the mistaken belief that these organisms show that no evolution took place between fossil and modern taxa. The fossil to them is just an example of the modern a few thousand years older. Check out the “living fossil” pages for Answers in Genesis and the Institute for Creation Research. I would love to see them add Pierrella to their benighted lists so I could call them out on it.
References:
Grischenko, A.V., Gordon, D.P., and Melnik, V.P. 2018. Bryozoa (Cyclostomata and Ctenostomata) from polymetallic nodules in the Russian exploration area, Clarion-Clipperton Fracture Zone, eastern Pacific Ocean-taxon novelty and implications of mining. Zootaxa, 4484(1), 1-91.
Schwaha, T., Grischenko, A.V., and Melnik, V.P. 2021. Morphology of ctenostome bryozoans: 4. Pierrella plicata. Journal of Morphology. doi: 10.1002/jmor.21344
Wilson, M.A. and Taylor, P.D. 2012. Palaeoecology, preservation and taxonomy of encrusting ctenostome bryozoans inhabiting ammonite body chambers in the Late Cretaceous Pierre Shale of Wyoming and South Dakota, USA. In: Ernst, A., Schäfer, P. and Scholz, J. (eds.) Bryozoan Studies 2010; Lecture Notes in Earth Sciences 143: 399-412.
