I recently visited the Calvert Marine Museum (CMM) in Solomons, Maryland, and the word that I’ve been contemplating since that visit is balance, in various of its manifestations.
At a fundamental level, natural history museums (and, I guess, all museums) have to strike a balance between, and among, many valid, though often competing, elements. Among these are the depth of its coverage and its breadth, the complexity of information conveyed and its accessibility to visitors, and the popularity of topics covered and their importance. Challenging to say the least.
The CMM navigates nicely through the various shoals upon which a natural history museum might founder. Typically with an appropriate level of detail, its exhibits capture the natural history and beauty, the complex maritime history, and some of the present troubles of the Chesapeake Bay, this continent’s largest estuary. An attentive visitor will come away with a new, or, perhaps, renewed, appreciation of many aspects of ancient and contemporary life in, on, and along the waters of the Chesapeake. Not much more that one might ask for from this museum.
To my delight, balance is itself one of the central themes that the CMM is conveying. More precisely, balance in the natural world. This is perhaps clearest in the contents and labels of the varied aquaria showing marine life in the Bay. These explicitly present the issue of the balance between predator and prey. Significantly, the museum makes it clear that we have to face the fact that many of the environmental balances that may have marked the Bay from ancient times have been disrupted by various invasive species. None more destructive than . . .
Of course, I would argue that a museum dedicated to the Chesapeake Bay has to give a prominent place to fossils collected from the Calvert Cliffs whose Miocene sedimentary formations stretch from Chesapeake Beach in the north to Drum Point to the south, where the Patuxent River and Bay meet. In this regard, the CMM does not disappoint. The paleontology exhibits offer a richness of fossil shark teeth, the fossils most sought after from the cliffs. A few appear below.
Yet, how refreshing is the central line of thinking about Calvert Cliffs fossils at the CMM (at least, as I reconstruct it). I suspect it goes something like this – “Sure, we have to offer lots of shark teeth because that’s what the casual visitor and the typical fossil collector who might come through will want to see. Nevertheless, that’s a skewed view of what lived here in the Miocene. Above all, we’ve got to balance those teeth with lots and lots of mollusc shells.” Amen, says this fossil shell collector. Here are a few.
I was engrossed by the array of fossil shells. The species diversity is staggering in its richness, reflecting, I assume, the fecundity of the Salisbury Embayment, the arm of the Atlantic that covered this area during the Miocene.
When I saw all of the various kinds of gastropods (some are shown in the first two pictures above), particularly my favorite oyster drills (subject of a previous post), my first reaction was to wonder whether these years in the Miocene might have been bad times for their bivalve prey (some are shown in the bottom three pictures). But then again, when I consider the various bivalves, it’s not hard to notice how many of them sport thick shells, and, if not that, then a large size. Paleontologists Gregory Dietl and Patricia H. Kelley have written, ". . . size and thickness of the shell are two general defenses in molluscs that are effective against shell-crushing crabs or fish, as well as shell-drilling snails, not to mention boring parasites." ( The Fossil Record of Predator-Prey Arms Races: Coevolution and Escalation Hypotheses, appearing in The Fossil Record of Predation, edited by Michal Kowaleswki and Patricia H. Kelley, The Paleontological Society Papers, Volume 8, October 2002, p. 364.)
And therein lies a story that I wish the CMM had sought to tell, at least a bit, with these fossils. Yes, it's hinted at by the way the gastropods are grouped in an upper level of the display where they sit above the bivalve specimens. I'll concede it's possible that perhaps the museum did explicitly address this somewhere for the fossil molluscs and I just missed it. If that's the case, I apologize, but, if I missed it, I would assume so would most visitors. Regardless, more might have been done with it.
For me, the story of how the balance of power between predator and prey is struck and how it changes holds an endless fascination, particularly when it comes to gastropods and their bivalve prey. My collection of fossil shells offers clear evidence of this struggle in the partial and complete holes drilled into many bivalve shells (and some gastropod shells). Indeed, examined closely, some of the specimens on display at the CMM show evidence of drilling.
As I prepared this post, I explored some of the literature on the evolutionary importance and consequences of predatory-prey interactions. I initially thought of devoting the post to that topic, but, frankly, while I have found the debate on this topic to be deeply interesting, my own understanding of it has become less and less clear. I've concluded that I don’t know enough to do any kind of justice to the thought-provoking debate between the two primary hypotheses – coevolution and escalation – whose distinctions I find subtle and elusive. For an excellent discussion of these hypotheses, I recommend the piece cited above by Dietl and Kelley. To give a flavor of the differences between these two hypotheses (and reveal my ignorance), I would note that, in the predator-prey configuration, coevolution involves an evolutionary response by each of the interacting predator and prey species – one responding to the other, and vice versa. In contrast (I think), the escalation hypothesis posits that the driving force for evolutionary change is a species’ enemies. As a consequence, under this hypothesis, predator-prey interactions are more likely to promote an evolutionary response by the prey, while evolutionary change by the predator will often be a response to pressures from the predator’s own enemies, not to any change in its prey.
As an aside, I would note that some of Kelley’s work has involved analyses of the evidence of interactions between certain Miocene gastropod and bivalve taxa using fossils from formations in these very Calvert Cliffs. She found evidence that increased shell thickness accompanied anti-predation success by bivalve species, but no concomitant prey-related evolution by the gastropod species she studied. Rather, what changes she noted in the predators appeared to be more likely responses to their enemies. (See, for example, Coevolutionary Patterns of Naticid Gastropods of the Chesapeake Group: An Example of Coevolution?, Journal of Paleontology, Volume 66, 1992.)
No, I’m not asking that this predator-prey story in all of its subtlety be told in the CMM, but I wanted more than the hint I got.
So, because the museum doesn’t deserve a post that ends on a somewhat negative note, I have to say that I was greatly impressed by a display that greets visitors to the paleontology exhibits. Titled There’s More Than One Way To Make A Fossil, it succinctly identifies the various ways that organic remains become fossils. Certainly not technical in its explanations, its virtue is its conciseness and in the message it conveys – its title says it all. It displays fossils reflecting unaltered preservation, permineralization, recrystallization, replacement, carbonization, compression, as well as preservation by being captured in tar or amber, and through the creation of molds, casts, and traces. Forgive the poor photograph which appears below, it doesn’t do the display justice.