Bacon, Brought Home

Natural History, June, 1999 by Stephen Jay Gould

This essay is not structured as a mystery yarn, so I spoil nothing, while (I hope) enhancing the intended intellectual theme by providing the true nature of hysteroliths up front. Hysteroliths are the internal molds of certain brachiopod shells (just as bucardites, discussed above, are internal molds of certain clamshells). Brachiopods are not closely related to clams, but they also grow shells made of two convex valves that open on a hinge located at one end of the shell and close by bringing the two valves together all along their edges. Therefore, if you make an internal mold by pouring plaster of paris into the closed shell, the resulting object will look roughly like a flattened sphere, with the degree of flattening specified by the convexity of the shell. Highly convex shells can produce nearly spherical molds (as in the fat clamshells that make bucardites). Shells of lower convexity--including most brachiopods and all the groups that make hysteroliths--yield more flattened molds.

Since molds are negative impressions of surrounding shapes, the suggestive parts of hysteroliths record features on the interior of a brachiopod shell in reverse. The slit that suggested a vulva and gave hysteroliths their name marks the negative impression of a raised and narrow linear ridge--called the median septum--that runs right down the middle of many brachiopod shell interiors, effectively dividing the valve in half. (For a clarifying analogy, think of the ridge as a knife and the slit as a cut.) The less pronounced "male" features on the other side of some hysteroliths record, in positive relief, a cylindrical groove on the shell interior that houses part of the feeding skeleton (detached from the shell itself and rarely fossilized) in some groups of brachiopods.

By the mid-eighteenth century, paleontologists had reached a correct consensus. They knew that hysteroliths were internal molds of brachiopods, and they had also identified the particular kinds of brachiopods that left such impressions on their molds. They also recognized, of course, that the admittedly striking similarity with human genitalia recorded a sheer, if curious, accident, with no causal meaning or connection whatsoever.

We therefore obtain, in the story of hysteroliths, a clean, clear, and lovely example of science operating admirably by following the canonical definition of its very being and distinctiveness--a procedure dedicated to the sweetest of all goals: the construction of an accurate piece of natural knowledge. This odyssey through two centuries and several interesting stages progresses from the puzzled agnosticism of Agricola's first mention in 1546 to Linnaeus's unchallenged conclusion of 1753.

I certainly do not deny the broad outline of this story. Agricola and Gesner possessed few clues for choosing from a wide range of alternatives--from the correct answer that eventually prevailed, to an inorganic origin by plastic forces circulating through rocks, to generation by various ancient animals as a meaningful symbol that might even cure or alleviate human ailments of the genital organs. The correct answer may not have fulfilled all human hopes and uses, but hysteroliths really are brachiopod molds, and science supplied the tools for proper resolution.