Bahamas
Fossil Cerions again
04-05-2010 19:52
As a sequel to
his previous
paper on
fossil Cerion
from the Bahamas,
Paul Hearty (2010) has published a new paper on
this topic.
His abstract is as follows:
Despite the nearly 600 named species of the land snail Cerion, studies of the geological and paleonto- logical framework of modern species are few. To address this deficiency, the biostratigraphic succession of Cerion was investigated at several areas on Long Island, Bahamas. A chronostratigraphic framework was developed through whole-rock and Cerion land snail aminostratigraphies. About 175 individual Cerion shells from last interglacial and Holocene deposits were age-ranked using stratigraphic position and amino acid racemization (AAR) geochronology. AAR ages were generated using an existing AAR-14C age model for Cerion from the central Bahamas. The age structure of Cerion fossils in sediments was determined with AAR ages, and the magnitude of ‘‘dead carbon’’ anomalies was evaluated using this chronological approach.
Temporal changes in gross shell morphology were examined from four study areas. The last inter-glacial, marine isotope stage/substage (MIS) 5e (Aminozone E) is characterized by generally large shells and in some cases, bimodal sets of very small (a shells) and very large forms (b shells) coexisting in the same stratigraphic levels (primarily soils), which may encompass the transition from between MIS 5e and 5d/c. Similar bimodality of nearly identical a and b shell forms and sizes is observed at other late MIS 5e sites from the furthest reaches of Great Bahama Bank (including Long, Exumas, Eleuthera, and New Providence Islands). The widespread distribution of a and b forms in soils capping MIS 5e marine and eolian deposits implies that there may have been a synchronous, regional morphological convergence on Great Bahama Bank. None of these forms are observed in Holocene deposits of Aminozone A.
The earliest MIS 1 Cerion appear in a oolite deposited 6500 a BP, and are of intermediate size compared to the Pleistocene a and b forms. As MIS 1 progressed, the diversity of shell sizes and shapes increased into modern times. The greater variety of shell forms over the past 1000–2000 a suggests that humans may have played a role in the introduction and redistribution of Cerion across the region. The potential for frequent and widespread human introductions, combined with the propensity of Cerion to hybridise freely may explain the farrago of shell sizes and shapes in the recent snail faunas of Long Island and other Bahama islands.
Reference:
Hearty, P.J., 2010. Chronostratigraphy and morphological changes in Cerion land snail shells over the past 130 ka on Long Island, Bahamas. - Quarternary Geochronology 5: 50-64.
His abstract is as follows:
Despite the nearly 600 named species of the land snail Cerion, studies of the geological and paleonto- logical framework of modern species are few. To address this deficiency, the biostratigraphic succession of Cerion was investigated at several areas on Long Island, Bahamas. A chronostratigraphic framework was developed through whole-rock and Cerion land snail aminostratigraphies. About 175 individual Cerion shells from last interglacial and Holocene deposits were age-ranked using stratigraphic position and amino acid racemization (AAR) geochronology. AAR ages were generated using an existing AAR-14C age model for Cerion from the central Bahamas. The age structure of Cerion fossils in sediments was determined with AAR ages, and the magnitude of ‘‘dead carbon’’ anomalies was evaluated using this chronological approach.
Temporal changes in gross shell morphology were examined from four study areas. The last inter-glacial, marine isotope stage/substage (MIS) 5e (Aminozone E) is characterized by generally large shells and in some cases, bimodal sets of very small (a shells) and very large forms (b shells) coexisting in the same stratigraphic levels (primarily soils), which may encompass the transition from between MIS 5e and 5d/c. Similar bimodality of nearly identical a and b shell forms and sizes is observed at other late MIS 5e sites from the furthest reaches of Great Bahama Bank (including Long, Exumas, Eleuthera, and New Providence Islands). The widespread distribution of a and b forms in soils capping MIS 5e marine and eolian deposits implies that there may have been a synchronous, regional morphological convergence on Great Bahama Bank. None of these forms are observed in Holocene deposits of Aminozone A.
The earliest MIS 1 Cerion appear in a oolite deposited 6500 a BP, and are of intermediate size compared to the Pleistocene a and b forms. As MIS 1 progressed, the diversity of shell sizes and shapes increased into modern times. The greater variety of shell forms over the past 1000–2000 a suggests that humans may have played a role in the introduction and redistribution of Cerion across the region. The potential for frequent and widespread human introductions, combined with the propensity of Cerion to hybridise freely may explain the farrago of shell sizes and shapes in the recent snail faunas of Long Island and other Bahama islands.
Reference:
Hearty, P.J., 2010. Chronostratigraphy and morphological changes in Cerion land snail shells over the past 130 ka on Long Island, Bahamas. - Quarternary Geochronology 5: 50-64.
What fossils can learn
06-11-2008 20:50
In a thorough paper, Hearty & Schellenberg
present the results of their chronostratigraphic work
on Cerions from the Bahamas. With their study object
they follow in the footsteps of Goodfriend and Gould,
to name just a few well known. As such that is no
story for success. I recall a co-student to do
morphometric studies on Cerion uva from the
collections of P. Wagenaar Hummelinck, only to
conclude after measuring thousands of specimens that
the uniformity of the populations on Aruba, Curacao
and Bonaire is most strikingly.
With modern techniques Hearty and Schellenberg were able to resolve stratigraphic subunits with the Holocene and late Pleistocene of fossil and living Cerion from San Salvador Island. Morphometric studies on samples spanning a 140 kyr time period revealed interglacial sequences with a trend to increasing shell size.
Their detailed study is not only a nice contribution from a geological viewpoint, it also shows that a fossil record might be necessary to reach the conclusions they were able to draw. Their time-succession shows a continuous and directional morphological change on island-wide scale. The period of greatest rate of change coincides with the onset of interglacials, which hints to the availability of a richer diet, resulting in faster growing, more globose shells. One of their working hypothesis for future studies is that ecophenotypy as an evolutionary mechanism may explain the temporal changes in shell shape.
A second study on fossils, this time from the Mediterranean, is the paper by Altaba on Balearena from Mallorca. There are some similarities: it also considers an island fauna, and also applies morphometrics. Yet there is an interesting difference in methodology. Altaba uses geometric morphometrics applying landmarks and a principal component analysis. I find his analysis more appealing.
For the analysis he uses the program PAST, which is probably better known amoung paleontologists than amoung taxonomists working on recent specimens. I found it an interesting freeware program, albeit working only under Windows. The cross-platform alternative is MorphoJ and the acquiring of landmarks can be done use ImageJ, either from photos or drawings. As usual, my preference is platform independent software.
References:
Altaba, C.R., 2007. A new genus and species of Enidae (Gastropoda: Pulmonata) from the Quartenary of the Balearctic Islands (Western Mediterranean). - Zootaxa 1595: 43-52.
Hearty, P.J. & Schellenberg, S.A., 2008. Integrated Late Quartenary chronostratigraphy for San Salvador Island, Bahamas: patterns and trends of morphological change in the land snail Cerion. Palaeo3 267: 41-58.
With modern techniques Hearty and Schellenberg were able to resolve stratigraphic subunits with the Holocene and late Pleistocene of fossil and living Cerion from San Salvador Island. Morphometric studies on samples spanning a 140 kyr time period revealed interglacial sequences with a trend to increasing shell size.
Their detailed study is not only a nice contribution from a geological viewpoint, it also shows that a fossil record might be necessary to reach the conclusions they were able to draw. Their time-succession shows a continuous and directional morphological change on island-wide scale. The period of greatest rate of change coincides with the onset of interglacials, which hints to the availability of a richer diet, resulting in faster growing, more globose shells. One of their working hypothesis for future studies is that ecophenotypy as an evolutionary mechanism may explain the temporal changes in shell shape.
A second study on fossils, this time from the Mediterranean, is the paper by Altaba on Balearena from Mallorca. There are some similarities: it also considers an island fauna, and also applies morphometrics. Yet there is an interesting difference in methodology. Altaba uses geometric morphometrics applying landmarks and a principal component analysis. I find his analysis more appealing.
For the analysis he uses the program PAST, which is probably better known amoung paleontologists than amoung taxonomists working on recent specimens. I found it an interesting freeware program, albeit working only under Windows. The cross-platform alternative is MorphoJ and the acquiring of landmarks can be done use ImageJ, either from photos or drawings. As usual, my preference is platform independent software.
References:
Altaba, C.R., 2007. A new genus and species of Enidae (Gastropoda: Pulmonata) from the Quartenary of the Balearctic Islands (Western Mediterranean). - Zootaxa 1595: 43-52.
Hearty, P.J. & Schellenberg, S.A., 2008. Integrated Late Quartenary chronostratigraphy for San Salvador Island, Bahamas: patterns and trends of morphological change in the land snail Cerion. Palaeo3 267: 41-58.