Wednesday, July 31, 2019

An "Aberrant" Path for Sand Dollars

Until now, I would have said, without qualification, that the shells or tests of sand dollars are things of subtle beauty.  In a moment of weakness, I might even be tempted to ascribe some aesthetic purpose to the artistry of the five-pronged stars etched delicately on the obverse of the tests, or to their wonderfully symmetrical disk-like shape.  We are, indeed, attracted to the symmetrical.

But certain fossils recently added to my collection challenge my overall assessment of the charm and beauty of sand dollars.

The echinoderm order Clypeasteroida includes the sand dollar whose rigid test is made up of interlocking plates.  (Of interest, this order’s name comes from the Latin clypeus = shield or medallion, and aster = star.)  The sand dollar, whose calcium carbonate plates readily fossilize, is a relatively recent arrival on the world scene.  The University of California Museum of Paleontology notes that echinoids (which include sea urchins, among other taxa) first appear in the late Ordovician (458 to 444 million years ago), but that the sand dollar shows up only in the Paleocene (66 to 56 million years ago). (Introduction of the Echinoidea, UMCP.)

At a recent gem and mineral show, I came upon a small container sporting the label: “fossil sand dollars.”  At $1 apiece, these small fossils were hard to resist, so I purchased a few.  As seen in the example below, these fossils are quite striking with a shape certainly unlike that of any sand dollar I’ve seen.  (Clearly, my specimens had suffered some breakage.)  Unusual they are, beautiful they are not.

These particular specimens belong to the Rotulidae family whose members exhibit digit-like projections of the test (I refer to them as “digits” below, though they clearly are not fingers).  The Rotulidae first enter the sand dollar family during the Miocene (23 to 5 million years ago).  I have concluded these specimens belong to the genus Heliophora which, according to The Echinoid Directory of the Natural History Museum in London, includes a single species, H. orbicularis.  I am particularly persuaded in my identification by those robust protrusions of the test.  As the NHM puts it for this species, “Posterior of test strongly digitate.”  The NHM also notes that this taxon is first found in the fossil record in the late Pliocene (5 to 3 million years ago) and that it is an extant species found on the west coast of Africa.  So, my specific fossils are, in the scheme of things, remnants of very recent additions to the planet’s fauna.

On my H. orbicularis and on Rotulidae, in general, the digits appear at the posterior of the test.  The dorsal side of the test is somewhat raised (meaning the test is not flat) and shows the star etching, while the ventral or oral side is flat and has two holes in it.  The hole in the center of the oral side is the peristome opening which serves the animal’s mouth, while the second hole, nearer the digits, is the periproct opening which serves the anus.  Though the periproct’s location is variable among sand dollar species, in many it is in the rim of the test.  That’s not the case here, based on my understanding of the Heliophora.

The test’s digits need some explaining.  From my reading to date, I’m not sure there’s a scientific consensus about them.  Two aspects of how sand dollars live (lived) are relevant here.  First, sand dollars are filter feeders, with features (including many spines covering the test; these spines are not preserved in fossils) that act to channel microorganisms such as diatoms into its mouth.  The movement of food occurs not just on the oral side of the test, but also on the dorsal side.  The raised dorsal side with grooves that run down it help to channel food toward the oral side of the organism.

Second, sand dollars live on or in sedimentary beds in moving water.  This introduces consideration of how the shape of the test responds to the hydrodynamic forces of moving water.

I assume that, evolutionarily, there has been an interplay between the demands of securing food and those of "sedimentary" living and coping with moving water.  For the Rotulidae with their test digits, paleontologist Adolf Seilacher’s discussion in Morphodynamics (2015) is relevant.  In this volume, he posited that “lunular notches” in sand dollars – the perforations through the test found in several types of sand dollars; this term includes the digits of the Rotulidae – arose for a constellation of different reasons.  These include aid for digging by allowing sand to move more easily from the oral side to the dorsal side; stability by reducing the lift effect of water moving past the test; and feeding by facilitating the movement of food from the dorsal to the oral side.

In an earlier article, Seilacher noted a possible explanation for the concentration of lunular notches or digits in the posterior edge of the Rotulidae.  Members of this family may increase the effectiveness of these digits for filter feeding by positioning themselves in an upright position in the sediment with the digits exposed to the water currents.  (Constructional Morphology of Sand Dollars, Paleobiology, Volume 5, Number 3, 1979.)

I am struck by how the Rotulidae, those sand dollars with digits, break the connection with the common name for this kind of organism, a name which refers to the test’s similarity in shape to a silver or gold dollar coin.  If the only “sand dollars” we knew were Rotulidae, I doubt they’d have that common name.  Indeed, the H. orbicularis in particular hardly looks like a coin and bears little resemblance to the common names given this group of organisms by other cultures and in other languages.  For instance, H. orbicularis certainly doesn’t bring to mind a cookie, a frequent reference point elsewhere in the world (e.g., sea cookie in New Zealand, galleta de mar in Spanish).  Nor does it evoke a flower (e.g., sea pansy in South Africa).  Yet, the French, it would seem, might have no problem with the unusual shapes of the Rotulidae because, in French, a sand dollar is called (I think) oursin plat, meaning flat sea urchin, a common name which sacrifices metaphor for broadly accurate, though boring, descriptors.

In the end, I fear the H. orbicularis and the other Rotulidae leave me cold.  Their digits make them interesting but also render them much less attractive.  Perhaps the most interesting aspect of the digits is they appear in the Rotulidae which enter the fossil record somewhat later than sand dollars in general and are limited to just three genera.  This, it would appear, is a new evolutionary path for sand dollars, but, at least so far, one that remains very much a minor, restricted “experiment” among the Clypeasteroida.  May it remain so.

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