The key to taking the measure of biodiversity lies in a downward adjustment of scale. The smaller the organism, the broader the frontier and the deeper the unmapped terrain.
~ Edward O. Wilson, Naturalist, 1994, p. 363-364.
“Whatever it is, it’s very, very little.”
~ line spoken by one of three jowly and mustachioed men crowded around a microscope in a cartoon by James StevensonSpecies diversity is a crucial gauge of the health of an ecosystem. Botanist and ecologist Ruth Patrick, working with diatoms in river systems, showed that the health of those water environments can be measured by the diversity of the living communities they host (an indicator known as the Patrick Principle). (See, Patrick Principle(s), a post on this blog.) Comparing species diversity over time, particularly when ancient environments are involved, is important for understanding the effects of environmental change (most obviously something like, say, the end-Permian mass extinction). But it’s a profound challenge. How does one know whether the floral or faunal diversity for any time period and any location has been accurately measured? It’s particularly a problem given how limited our knowledge is of the smallest organisms living today, much less those tiny entities that lived in the past. This issue came to the fore recently for me as I sifted through a sample of matrix from a Pliocene formation.
A year ago, while on vacation at Bradenton Beach, Florida, a friend came upon massive grey mounds of shelly sand stretching across one end of a parking lot next to a local beach. He was not the first to recognize that this material, dug from a nearby quarry, was a treasure trove of Pliocene Epoch fossil mollusk shells. Climbing these piles, he began collecting fossil shells with purpose, knowing there was a death sentence hanging over this scene. Yes, all too soon, this beach parking lot had been repaved with this three-million-year-old mixture of sand and fossils.
The fossil shells he saved from destruction are breathtaking in their diversity and their state of preservation. Here are just a very few of the specimens he rescued.
- Cancellaria conradiana (a Nutmeg Shell)
- Terebraspira sp. (a Tulip Shell, though, for the life of me, I cannot figure out its species)
- Fasciolaria apicina (a Tulip Shell)
- Strombus floridanus (a Conch).
Knowing of my peculiar interest in microfossils (specifically, the shells from foraminifera and ostracodes), my friend collected the matrix he extracted from his finds and passed it on to me. His container of choice for this offering was a coffee can whose delightfully rusting exterior belied the beauty of the myriad fossils hidden inside. After working on this rich mixture for a bit, I find that I’m spending surprisingly little time collecting and identifying my usual suspects. Instead, I have been diverted (in both senses of the word – redirected and entertained) by the many, many micromollusks that leaven this matrix.
A definition is in order. Though somewhat widely used, the term micromollusk (or micromollusc in all of the English-speaking world outside of the U.S., which also uses mollusc instead of mollusk) is new to me and apparently not some codified category of mollusk. In most instances, this term appears to be applied to any mollusk whose adult form does not exceed 5 mm in size. The 2007 symposium titled Micromolluscs: Methodological Challenges – Exciting Results used the definition of “molluscs no larger than 5 mm.” Authors in this symposium also employed the label microgastropod to describe any adult gastropod up to 5 mm in size. This definition is highly arbitrary and I find no particular reason for the 5 mm threshold dividing micro and macro. Yet, given its usage elsewhere, it’s what I will employ in this post. In fact, it’s certainly more useful for analysis than my preferred definition of a microfossil and, by extension, a micromollusk or a microgastropod: “a specimen whose study requires magnification.”
Given that the matrix in the coffee can had, in effect, been screened already (component pieces had to be small enough to fit inside larger fossil shells), any specimens that came to me had to be relatively small, most likely falling below the 5 mm threshold. I did some further screening, using a sieve with 1 mm square mesh which allows me to focus on still smaller specimens. But, as is obvious from the several specimens pictured below, some that exceed 1 mm in a dimension are able to slip through.
Yes, there are foraminifera and ostracodes here in fairly large numbers, but what is striking is the abundance of micromollusks, principally microgastropods. It’s a phenomenon I have not encountered to such an extent with any other matrix I’ve explored for my usual tiny quarries.
The purported source of the material probably explains why I am awash in these minute specimens. The piles on the Bradenton Beach parking lot likely came from the quarries of SMR Aggregates, Inc., which mines material for use in the construction of roads, construction of buildings’ foundations, landscaping, and making of concrete. If this is the actual source of this material, then its geological origins are the prized Pinecrest Beds of the Tamiami Formation.
The Pinecrest Beds are considered to be among “some of the most species rich and densely packed fossil horizons known in the world.” (Portell, SMR Aggregates, p. 7.) I describe the Pinecrest Beds as being from the Upper Pliocene Epoch (this epoch ended about 2.58 million-years-ago) based on an analysis by paleontologist Lauck Ward (Diagnostic Mollusks From the APAC Pit, Sarasota, Florida, Plio-Pleistocene Stratigraphy and Paleontology of Southern Florida, Special Publication #36, Florida Geological Survey, 1992, Figure 1.) In contrast, Portell, et al. posited that these beds straddled the Pliocene/Pleistocene boundary (SMR Aggregates, p. 5). There is agreement that much of the Pinecrest was deposited in a relatively shallow (perhaps less than 100 feet deep), tropical to temperate marine environment. Obviously, it was a very fecund environment with apparently high levels of species diversity.
Here is one slide with a selection of microgastropods from this material. There are 61 specimens pictured here. (There’s certainly something to be said about microfossils – storage is not the problem it is with every other fossil I collect.)
And here are three examples of these microgastropods:
For better or worse (possibly for the worse), the identification of this first specimen is based principally on Pliocene Molluscs From the Yorktown and Chowan River Formations in Virginia, written by geologist Lyle D. Campbell (Virginia Division of Mineral Resources, Publication 127, 1993, p. 103-104, image 520 on Plate 42). [The discussion of this species has been edited after I initially uploaded this post.] In fact, based on Campbell, I originally identified this as Turbonilla (Chemnitzia) beta, a species he asserted was new. I don’t see the name being used later, and probably only the genus name (without the subgenus designation) is accurate. I should note that Campbell did extensive work on mollusks, amassing a large and scientifically important collection (see below). But, this particular volume of his has been taken to task. Paleontologists Lauck W. Ward and Norman L. Gilinksy criticized it severely for myriad faults, ranging from “inadequate descriptions” to the “lumping together of clearly different taxa.” And, from my perspective (an amateur looking for help in identifications), one of their most damning comments was that “the poor quality of the original specimen photographs makes clear identifications difficult.” (Biostratigraphic Analysis of the Chowan River Formation (Upper Pliocene) and Adjoining Units, the Moore House Member of the Yorktown Formation (Upper Pliocene) and the James City Formation (Lower Pleistocene), Virginia Museum of Natural History, Memoir Number 3, Part A, 1993, p. 8).
For this second shell, I find several morphologically similar specimens identified as Teinostoma in Campbell (p. 60-61, and specimens 266 through 274 on Plate 27). Also, several similar specimens identified as in the Teinostoma genus are pictured in Micromollusks of the Lower Pinecrest Beds, Upper Tamiami Formation, Sarasota County, Florida which appears on the Jaxshells (Jacksonville Shells) website. The diversity of species within this genus thwarts me at present.
I am fairly confident about the final one, Ringicula guppyi, based on several sources, including the Florida Museum of Natural History, gastropod section of its Invertebrate Paleontology Image Galleries. The photograph of the specimen with UF Catalog Number of 137290 is quite persuasive.
My (belated) discovery of micromollusks, and of microgastropods in particular, has reinforced my appreciation of the epigraph from biologist E.O. Wilson that begins this post. As Wilson went on to observe, “Most of Earth’s largest species – mammals, birds, and trees – have been seen and documented.” (Naturalist, p. 364.) Size matters when it comes to our natural impulse to collect, identify, and categorize, if only because the big stuff is what’s easiest to find. It’s the little that gets overlooked and ignored.
Seashells are an excellent case in point. When malacologist Philippe Bouchet and his colleagues undertook to make a complete inventory of the diversity of extant mollusk species at a site in New Caledonia, they felt compelled to explain why such an effort was necessary. They noted that, despite the fact that mollusks are regularly included in analyses of marine biodiversity, and that the long standing interest in seashells by malacologist and collectors has produced a great deal of information about species diversity, there is a serious gap.
[M]uch of the literature focuses on macromolluscs (‘seashells’) that can be collected by picking in the field, and tends to ignore or grossly underestimate the smaller species. This is probably because micromolluscs require specific collecting/sorting attention and have a reputation to present formidable taxonomic difficulties.
(Philippe Bouchet et al., Assessing the Magnitude of Species Richness in Tropical Marine Environments: Exceptionally High Numbers of Molluscs at a New Caledonia Site, Biological Journal of the Linnean Society, Volume 75, 2002, p. 422.)Bouchet et al. showed how important a component the smallest mollusk species is of a fauna. They found that, of the 2,581 mollusk species found at their site, fully a third (33.59%) were 4.1 mm in size or smaller (no 5 mm threshold used here). It strikes me that, if work on living mollusks, such as that by Bouchet and colleagues, still needs to be done, the data on micromollusks in the fossil record must present much greater challenges to any consideration of changes in biodiversity reaching back into deep time.
The micromollusks of the Pliocene Pinecrest Beds have played a central role in analyses of changes in marine biodiversity in the western Atlantic coastal plain. Before the early 1990s, it was generally understood that molluscan biodiversity in the western Atlantic tropical and subtropical environments had reached a peak across the Miocene and Pliocene Epoch, and then suffered serious losses, leading to a presently impoverished molluscan fauna. In essence, there are fewer species living today than there were in the Pliocene.
In 1993, paleontologist Warren D. Allmon and colleagues challenged that assessment with an article in Science titled Diversity of Atlantic Coastal Plain Mollusks Since the Pliocene (Vol. 260, June 11, 1993). For their reading of diversity in the Pliocene, they relied largely on gastropod fossils from the Pinecrest Beds. They identified 460 species of gastropod larger than 5 mm and another 40 smaller than that from that geological location. But, recognizing there is a size bias in the fossil record, they turned to data on all extant shallow water gastropods in Florida which showed that approximately 22% of those extant gastropods were 5 mm or smaller. They assumed that this ratio was also applicable in the Pliocene and, so, estimated that there was a total of approximately 590 gastropod species living in the Pliocene in the area where the Pinecrest Beds were laid down. Thus, of this total, an estimated 130 were microgastropods.
Armed with these estimates and data on the diversity of extant gastropods, Allmon et al. concluded that there had not been a loss of molluscan species diversity over this time period. Although they found that fully 70% of the gastropod species in the Pinecrest are presently extinct, this loss has been made up by the appearance of many new species – rates of extinction seemed to have been balanced by rates of origination. They acknowledged that more species might well turn up with further work in the Pinecrest, but suggested that any increase in Pliocene numbers would be balanced by the identification of new extant species. The report on Pinecrest micromollusks posted on the Jaxshells (Jacksonville Shells) website (cited above) suggests that discovery of new micromollusk species continues apace, as does identification of new extant species (thought it's agnostic about whether they are in balance).
Micromollusks, microgastropods aren’t just curiosities. They are integral parts of their environment, past and present, and, as such, have much to tell us. So much so that, in 2015, the National Science Foundation awarded a grant to the University of Florida so that two collections of the Florida Museum of Natural History could be saved and permanently housed. One of them, a collection of mollusks collected by Lyle Campbell, is described as “an exceptional, nationally unique inventory of fossil micro-mollusks.” Under this grant, these collections will become research and teaching tools. Products from this grant include two identification guides and 1,500 images – all to be made available online. Frankly, I cannot wait.
And, if research applications aren’t sufficient motivation to get excited about micromollusks, I also embrace the sentiment attributed to Immanuel Kant:
Look closely. The beautiful may be small.