Function For This Form

Fossil in hand, the question nearly always arises:  why is this specimen shaped the way it is?  The initial presumption is that form reflects function which is not always true.  As this post explores, there are times when a function follows form.

Exploring the interplay of form and function helps turn fossil collecting from an impulsive effort simply to amass neat specimens into an intellectual activity meriting the label “amateur paleontology.”

The gift from a friend of some fossil mollusc shells found along the St. Marys River, Maryland, prompted research to explore why the two bivalve shells shown below sport such robust, overarching concentric ribs.  These shells, collected in the St. Marys Formation, Windmill Point Member, are from the extinct species Lirophora alveata which, according to Fossilworks, lived between 11.608 and 7.246 million years ago (Late Miocene).

In 1903, paleontologist William Healey Dall described L. alveata succinctly:

This fine species is readily recognized by its high, trigonal form, few high, even, recurved concentric ribs, and absence of radial sculpture.  (Contributions to the Tertiary Fauna of Florida, Transactions of the Wagner Free Institute of Science, Volume III, Part VI, October, 1903, p. 1298.)

I had never considered the function of concentric ribs on mollusc shells.  Based on a quick scan of a modern guide to shells, it would appear that concentric ribs aren’t uncommon among bivalve species.  Evolutionary biologist Geerat Vermeij, in his always rewarding A Natural History of Shells (1993), considers the role of shell ornamentation, such as ribs, or the lack thereof, in the burrowing process for infaunal molluscs (those that live in bottom sediment).  Burrowing molluscs face an important challenge:  keep from being exposed as the water’s currents move sediment away.  There are two responses to this challenge.  The first is to burrow quickly and deeply.  For that, smooth shells, those lacking ornamentation, do well.  The second is for the mollusc to burrow shallowly and try to stabilize the surrounding mud and sand, that is, keep such material from being scoured away.  Vermeij notes that certain kinds of shell shapes evolved to accomplish that stabilizing, including “strong concentric ribs” in infaunal clam families, including the Veneridae to which the Lirophora belong.

I would think that the concentric ornamentation might offer more impediment to burrowing than radiating ornamentation.  As the mollusc’s “foot” works at sediment to bury its shell, radiating structures might contribute to the burrowing process, allowing the shell to cut through the mud and sand, and moving the material away.  Concentric ornamentation could be rather counterproductive for that.  Do bivalves with radiating ribs bury themselves deeper than those with concentric ribs?

From this, one might conclude that stability in sediment is the reason this Miocene bivalve featured those distinctive concentric ribs.  Well, it’s probably not the only reason.

Two paleontologists, Adiël A. Klompmaker and Patricia H. Kelley, in an interesting paper titled Shell Ornamentation as a Likely Exaptation:  Evidence from Predatory Drilling on Cenozoic Bivalves (Paleobiology, Volume 41, Number 1, 2014) take us a level deeper into this.  The authors observe that ornamentation on shells, such as ribs, might serve a number of functions, such as:

maintaining a stable life position in the sediment, burrowing, shell strengthening, directing inhalant and exhalant currents, and protecting against predators. (p. 187)

Their research focus is “the degree to which ornamentation is effective against drilling predation” on “several bivalve species with varying strengths of smooth-topped concentric ribs.”  (p. 188)

Ah, another possible function for this form.

One of the species they studied is the extant Lirophora latilirata which, as the picture below shows, is certainly related to the extinct L. alveata.

(This picture is reproduced under the Creative Commons Attribution-NonCommercial ShareAlike 4.0 International license.  This picture was posted by cyric to iNaturalist and can be found here.)

The take-away from their paper is that concentric ribs, such as those featured on Lirophora species (both extinct and extant), defend these bivalves from predation by boring gastropods.  Such ribs appear to be an efficient and economical way for these animals to strengthen their shell without investing the resources required to thicken the entire shell.  Not only are dense concentric ribs difficult to drill through, they minimize the amount of exposed, unstrengthened shell that predators might reach and drill into.

Based on an examination of the two L. alveata specimens shown above, I would go further for this species and offer the following hypothesis.  The outer edges of the curved ribs of L. alveata are not as thick as the bulk of the structures arising from the shell, perhaps reflecting a further economy in the investment of resources to generate ribs.  These thinner overhangs might be sufficient to serve a defensive function, helping to keep snails from gaining access to the shell surfaces between the ribs.  Further, though predators might drill through them with less effort, these predators would have to make a further effort to actually reach the shell surface below.  This hypothesis probably applies as well to the extant L. latilirata pictured immediately above.

There is more to extract from this study relevant to this exploration of concentric ribs.  The authors contend that there is evidence that concentric ribs are a defensive “exaptation,” not an “adaptation.”  Klompmaker  and Kelley distinguish the two terms:

adaptations are features built by natural selection for their current role, whereas exaptation refers to characters that evolved for other usages or for no particular function and were co-opted later for their current role.  (p. 188)

The authors believe that the timing of the emergence of concentric ornamentation in bivalves (first appearing in the Paleozoic and proliferating in the Mesozoic), which preceded the appearance of significant drilling by predatory gastropods, suggests that the defensive role of those ribs came after their initial appearance.  According to this line of argument, because the ribs did not evolve in response to drilling, the defensive function they now serve is an exaptation.  If true, that means that such ribs were first generated by natural selection to serve a different function (stability for shallow burrowing?) or for no discernable function (not every feature of an organism is necessarily serving a current or even a past function).

In the end, it’s always satisfying to find a plausible explanation for the form of a fossil or some feature on it. In this instance, there’s the added bonus that, although the form was driven by one function, another function apparently followed.  In other words, build it (for some function or none) and a function might come.