Cerion excelsior Gould, 1984

Original Description

"Cerion excelsior, the subfossil smokestack giant, from Great lnagua, Maya­guana, and East Plana Cay is by far the largest of all Cerion yet discovered. It reaches its maximal size not by growing big whorls (though the Mayaguana population exhibits enlarged whorl siz as well, see below) but by growing more whorls than any other Cerion.  The type specimen (Fig. 13), from the southeast corner of Great Inagua (locality 833 of Gould and Woodruff) has 11⅝ post protoconch whorls (I have never before seen a Cerion population with a mean of more than 10¼ whorls), while its sample averages 10.71. The type sample, 833 on Inagua, avenges 47.37 mm in height and 12.53 mm in width (the smaller Inagua sample, number 734 from Conch Shell Point, averages 41.02 in height, 11.55 in width, and grows a mean number of 10.14 whorls; the largest of all, the Mayaguana sample from caves at locality 930, exhibits mean values of 58.10 in height. 14.33 in width, and grows an average of 10.54 whorls).

     As a second consequence of their unusual number of whorls (and for reasons related to Cerion's general rules of growth as explained in this paper), C. excelsior is also the highest spired of all normal to large-sized Cerion. Mean height/width ratio of the adult shell is 3.78 for locality 833, 3.56 for 734, and 4.06 for 930. No other giant Cerion, and no other species of normal size, reaches 3.0 in mean height/width.

     Cerion excelsior has a white shell, rarely with weak mottling; subfossils of good preservation generally retain traces of any color originally present. It develops moderate to strong, evenly spaced ribs, effaced in the middle whorls on some specimens. but always present on initial whorls, and, again, on the final whorl ­a common pattern for ribbing in Cerion.  It begins growth with an apex triangular in cross-section and undergoes, at about the fourth postprotoconch whorl, the usual transition to a parallel-sided shell of mid-ontogeny. The transition is not so sharp as in members of the C. dimidiatum complex, but is more pronounced than the even gradation displayed by such members of the mottled morphotype as C. gubernatorium.  It corresponds with the clearly definable, yet even, transition so characteristic of the ribby morphotype (C. glans on New Providence and Eleuthera, or C. abacoense on Little Bahama Bank). Indeed, for the first 7 or 8 whorls, C. excelsior looks much like such white-shelled, ribby species as C. abacoense.

     The whorls of middle ontogeny may be strict1y parallel-sided, producing the striking smokestack seen in some specimens (Fig. 2), or gently expanding (Fig. 1). The aperture is distinctive. The final whorl becomes rounded, jutting out slightly from the preceding spire. Growth of the generating curve back on the penultimate whorl is less than in most Cerion The aperture itself is unusually large, jutting our far beyond the spire, and with its longest axis inclined at an angle of up to 60º to the coiling axis. The lip of the aperture is wide and strong but only slightly reflected. I suspect that this strong, large, and unusually inclined aperture represents an intensification of the usual changes that occur when Cerion's aperture is deposited. We know that apertural strength is positively corre­lated with whorl number and final size (Gould 1977), and the large, discordant aperture of C. excelsior may be an automatic consequence of its unusually large size.

     Derivatio nominis.  In recognition of its highest spire among all Cerion, ex­celsior, or "ever upward," the motto of my home state and its emblem, the Empire State Building, a smokestack giant if ever there was one.

     Temporal and geographic distribution (Fig. 13) C. excelsior is clearly a discrete taxon, not a recurrent tendency of growth to large size in certain environmental circumstances.  All C. excelsior share a time and region; they are well bounded spatiotemporally. We have found this species on two adjacent islands in the southeastern Bahamas, Great Inagua and Mayaguana.  A  fragment, mislabelled C. piratarum (the modem Mayaguana species) in collections of the Mollusc Deparment, M.C.Z., Harvard University (No. 75045), demonstrates that this species also inhabited the adjacent island of East Plana Cay. All these islands, though situated in proximity, are separated by deep ocean, and transport must have occurred across water barriers.

The various fossil populations also show dear and consistent geographic vari­ation. The Inagua populations are smaller, with 833 larger than 734 (Fig. 13). The three Mayaguana samples are all considerably larger, making C. excelsior on Mayaguana the largest of all Cerion. Shells of the one measurable Mayaguana sample average 72.4 mm in size (height plus width), compared with 59.9 and 52.6 for the two Inagua samples. The largest of all Cerion specimens, the only shell ever collected with a height in excess of 70 mm (Fig. 1), a single specimen from a recent (but lithified) red soil pock within a Pleistocene dune, measures 89.1 mm in size. The measured Mayaguana sample does not exceed the Inagua samples in whorl number, and its larger size therefore results from unusually large whorl sizes as well (the Inagua samples do not grow particularly large whorls).

     Although we have found no living C. excelsior, all specimens come from the most recent of geological deposits and should be labeled subfossils. None have been discovered within eolianites or stratified soils of the Pleistocene dunes. All have been found weathering out of superficial Holocene caliche deposits or within red soil pockets developed during Holocene times in the older dunes. At localicy 734 on Inagua, the freshest specimens show signs of hybridization with the modern taxon C. rubicundum, while on Mayaguana, at locality 936, the youngest specimens also seem to be hybridizing with rhe surviving taxon C. piratarum. Thus, I conclude that C. excelsior survived until quite recently. Some of its heritage may remain by introgressive hybridization into modern species, as so often happens in Cerion.

Hololype.   Department of Invertebrate Paleontology, Museum of Comparative Zoology, Harvard University, No. 29182. Locality 833 of Gould and Woodruff. Holocene of Great lnagua. Height 54.2 mm, width 13.0 mm, 11⅝ whorls."  (Gould, 1984:193-194)
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