Dana H. Geary

Paleogeographic evolution of the late Miocene Lake Pannon in Central Europe

Abstract The paleogeographic evolution of Lake Pannon within the Pannonian basin is reconstructed with eight maps, ranging from the Middle Miocene to the Early Pliocene. The maps are based on the distribution of selected biozones and specific fossils, and on complementary sedimentological and seismic information. Our reconstruction shows that the history of Lake Pannon can be divided into three distinct intervals: an initial stage with low water level, which resulted in isolation from the sea at about 12 Ma and might have led to temporary fragmentation of the lake; an interval of gradual transgression lasting until ca. 9.5 Ma; and a long late interval of shrinkage and infilling of sediments that persisted into the Early Pliocene. The deep subbasins of the lake formed during the transgressive interval, in more basinward locations than the deep basins of the preceding Sarmatian age. The southern shoreline, running parallel with the Sava and Danube rivers along the northern foot of the Dinarides, changed very little during the lifetime of the lake, while the northern shoreline underwent profound changes.

Patterns of evolutionary tempo and mode in the radiation of Melanopsis (Gastropoda; Melanopsidae)

The Paratethyan basins of eastern Europe and western Asia became isolated from marine influence in the Late Miocene, and were the sites of several remarkable endemic radiations of brackish and freshwater organisms. Here I describe the patterns of tempo and mode before and during the radiation of the gastropod Melanopsis in the Pannonian basin of eastern and central Europe, and I explore the underlying mechanisms of evolutionary change. The most ancient melanopsid species in this area, M. impressa , was present in freshwater areas marginal to the basin well before the radiation. Widely spaced samples of M. impressa indicate that this species underwent a period of stasis lasting at least 7 m.y. The end of stasis corresponded with the extinction of the last of the normal marine fauna in the basin, suggesting that the lack of other fauna and/or reduced salinity in the basin permitted expansion of the melanopsids from the basin margins into the basin proper. Stasis ended with the onset of changes in size, shouldering, and ontogeny, which led eventually to M. fossilis . Change occurred over a 2-m.y. interval; a series of intermediates is present for all three characters. Within-sample correlations provide no evidence that the three characters are constructionally linked; instead they appear to be changing independently. The mode of change in the M. impressa–M. fossilis lineage appears to have been anagenetic. Alterations in the rate and direction of selection (and/or genetic links between characters) are probably required to explain the overall slowness of the change. Most Pannonian basin melanopsid species arose by rapid cladogenesis in the Middle Pannonian Stage. Physical factors in the basin probably influenced the timing of this diversification; contrasting patterns of variation and diversity between two melanopsid clades suggest that intrinsic factors influenced the extent of diversification.

The influence and interaction of temperature, salinity, and upwelling on the stable isotopic profiles of strombid gastropod shells

We constructed stable isotope profiles for three specimens of the gastropod Strombus from localities on the Pacific coast of Panama, the Caribbean coast of Panama, and the eastern coast of Florida. The three shells belong to individuals from two conspecific populations and a closely related species. All are from very shallow water (<10 m deep). Strong contrasts among localities in seasonal temperature, salinity, and upwelling are apparent in the isotope profiles. The Pacific shell profile exhibits pronounced variation in δ 18 O, corresponding to strong seasonal variation in temperature and salinity

The usefulness of bivalve stable isotope profiles as environmental indicators; data from the eastern Pacific Ocean and the southern Caribbean Sea

We constructed stable isotope profiles for 16 Recent venerid bivalve shells from four environmental regions in the eastern tropical Pacific and the southwestern Caribbean. The environments sampled were: 1) the Gulf of Chiriqui, a nonupwelling region in the eastern Pacific; 2) Panama Bay, an upwelling region in the eastern Pacific; 3) the Caribbean non-upwelling region of northern Panama and northwestern Colombia west of Cartegena; and 4) the Caribbean upwelling region of northern Colombia east of Cartagena and the northern Gulf of Venezuela. The four environments are generally well-characterized by the shell profiles. In particular, the 6180 range is greatest in upwelling environments, and, to a lesser extent, is greater in the eastern Pacific than in the Caribbean.

A comparative study of naticid gastropod predation on Varicorbula caloosae and Chione cancellata, Plio-Pleistocene of Florida, U.S.A.

Abstract Naticid gastropod predation on members of the bivalve family Corbulidae has been reported to be unusual in its lack of stereotypy, low rate of success, and lower frequency than predicted by a net energy maximization model. The cause behind these atypical patterns has been attributed to layers of conchiolin within the valve microstructure of corbulids, which are thought to inhibit shell penetration by naticids. The mechanism of shell penetration by drilling gastropods, as well as scattered instances of typical predation rates and patterns on corbulids suggest, however, that factors other than conchiolin layers should be investigated as potential causes of anomalous predation. We examined naticid-prey interactions by comparing naticid predation on Varicorbula caloosae and Chione cancellata from the Plio-Pleistocene of Florida. We tested for borehole site selectivity and prey size selectivity, two typically stereotyped behaviors. We compared the degree of predation success on both species, and compared observed prey-preference rankings with rankings predicted from cost/benefit analyses based on the net energy maximization model of Kitchell et al. (1981). We found that naticid predatory behavior was generally stereotyped in both species. The cases of unstereotyped predatory behavior on V. caloosae may be linked to its small size, rather than the presence of conchiolin layers. We also found that predicted rankings calculated from cost/benefit analyses were generally inconsistent with observed rankings; the size range of C. cancellata predicted to be the most profitable was not preferred. Size and size selectivity, however, are predictable from the net energy maximization model because they are behaviors that maximize net energy gain (Kitchell, 1986). Therefore, our results support the idea of naticids as energy maximizing predators, but we question the assumptions of the Kitchell et al. (1981) model, especially when applied to fossil assemblages.

Ancient Lake Pannon and its endemic molluscan fauna (Central Europe; mio-pliocene)

Lake Pannon existed from approximately 12 to 4 Mya, situated in the Pannonian basin of central-eastern Europe. The birth of Lake Pannon is defined by its loss of contact with adjacent marine areas. After these contacts were closed 12 Mya, they were never re-established. The physical history of Lake Pannon, including its palaeogeography, palaeobathymetry, sedimentology and chronostratigraphy, is currently well understood and provides an excellent context in which to investigate a variety of palaeobiological questions. Lake Pannon harboured a spectacular endemic malacofauna, including over 900 described species and many endemic genera. Among bivalves, the families Cardiidae (> 220 species) and Dreissenidae (> 130 species) predominate. These groups evolved from survivors of the “Sarmatian Sea”, the brackish-marine water body that occupied the Pannonian basin prior to the formation of the lake (see Dumont, this volume). Among gastropods, the prosobranch families Hydrobiidae (> 180 species) and Melanopsidae (> 100 species) predominate. Several pulmonate gastropods evolved remarkable endemics adapted to deep basinal habitats. The fact that Lake Pannons malacofauna had its ancestry in both the relict marine and surrounding freshwater systems contributed to its overall high diversity. In addition, the long lifetime of the lake (approximately 8 My) probably played a role in the establishment of such high endemism. Although a great many of the hundreds of endemic species apparently evolved geologically rapidly (i.e. < 105 years), evolution in several lineages appears to have been anagenetic and geologically gradual, with morphological intermediates spanning 1–2 My or more.

Iterative changes in Lake Pannon Melanopsis reflect a recurrent theme in gastropod morphological evolution

Abstract Iterative evolutionary changes are of special interest because they imply that the recurring morphological changes had a cause that also repeated itself and might therefore be possible to uncover. We describe a set of iterative morphological changes in melanopsid gastropods from the ancient, long-lived Lake Pannon. First in the Pannonian Age, and again in the Pontian Age approximately three million years later, a smooth-shelled ancestor gave rise to a shouldered descendant. In both cases, the morphological change was probably coincident with a shift from habitats just outside the lake (e.g., rivers and streams) to habitats within the lake itself. Many other convergent examples exist in which a smooth-shelled river dweller is closely related to a shouldered and/or ribbed lacustrine snail. The frequency of this type of morphological change suggests that it has an adaptive basis; response to differing predators or hydrodynamic conditions seem the most plausible explanations, but the functional nature of these morphological changes remains unknown.

A gradual morphologic transition during a rapid speciation event in marginellid gastropods (Neogene: Dominican Republic)

We document a speciation event between two species of Prunum (Marginellidae: Gastropoda) in Pliocene strata of the northern Dominican Republic. The ancestral species, P. coniforme, is widely distributed in the Mio-Pliocene of the Dominican Republic and Jamaica, and has a range of at least 11 m.y. The descendant species, P. christineladdae, is endemic to the northern Dominican Republic. The ancestral species persists after its descendant arises. The transition between species is marked by strati- graphic and morphologic intermediates, and occurs during an interval estimated to be between 73,000 and 275,000 years (representing 0.6-2.5 percent of the duration of the ancestral species). Although the transition takes more than a geologic instant, the overall pattern of morphologic change is best described by the model of punctuated equilibrium. The change between species apparently involved a habitat shift into deeper water, and occurred during accelerated deepening. All of the Neogene Dominican species of Prunum appear to have had depth-defined ranges.

An unusual pattern of divergence between two fossil gastropods; ecophenotypy, dimorphism, or hybridization?

-The temporal dimension of fossil sequences provides a critical component to the study of intraspecific dynamics and species formation. Here I report on the branching and subsequent morphological evolution of two gastropods (Melanopsis fossilis and M. vindobonensis) from the Late Miocene of the Pannonian basin in eastern and central Europe. Although morphological divergence between species is rapid, intermediates between the two species co-occur with typical individuals for approximately 1 m.y. and then disappear. The long-term persistence of intermediates followed by their ultimate disappearance is a pattern that, to my knowledge, has not been previously observed. Distinguishing genetic from ecophenotypic influences on shell form in freshwater prosobranchs is difficult. Nevertheless, consideration of the temporal, geographic, lithologic, and paleoecologic patterns of this sequence suggests that the morphologic differences between M. fossilis and M. vindobonensis had some genetic basis. Whether these forms were initially morphs of a single species or two species with some hybridization between them is impossible to determine. In either case, the morphological changes that resulted in M. vindobonensis were rapid, but the attainment of complete isolation between M. fossilis and M. vindobonensis apparently did not occur until approximately 1 m.y. later. Dana H. Geary. Department of Geology and Geophysics, 1215 W. Dayton Street, University of Wisconsin, Madison, Wisconsin 53706 Accepted: June 11, 1991

The paradox of gradualism: phyletic evolution in two lineages of lymnocardiid bivalves (Lake Pannon, central Europe)

Abstract Patterns preserved in the fossil record are of the highest importance in addressing questions about long-term evolutionary processes, yet both the description of pattern and its translation into process can be difficult. With respect to gradual phyletic change, we know that randomly generated sequences may exhibit characteristics of a “trend”; apparent patterns, therefore, must be interpreted with caution. Furthermore, even when the claim of a gradual trend can be statistically justified, interpretation of the underlying mechanisms may be challenging. Given that we can observe populations changing rapidly over tens or hundreds of years, it is now more difficult to explain instances of geologically gradual (as opposed to punctuated) change. Here we describe morphologic change in two bivalve lineages from the late Miocene Lake Pannon. We evaluate change according to the model-based methods of Hunt. Both lineages exhibit size increases and shape changes over an interval of nearly 4 million years. Size and two shape variables in the conjungens lineage are best fit by a model of directional evolution; remaining shape variables mostly conform to unbiased random walks. Body-size evolution in the diprosopum lineage is also significantly directional but all shape variables are best fit by the unbiased random walk model; the small number of sampling intervals available for this lineage (n  =  6) makes determination of the actual pattern more difficult. Model-fitting results indicate that the parallel trajectories of increasing log shell height over time in the two lineages can be accounted for by an underlying trend shared by both lineages, suggesting that the size increases may be a shared response to the same cause. The pace of phenotypic change, measured as Lynchs Δ, is slower than the neutral expectation for all size and shape traits. Our examples illustrate well the paradox of gradualism; the sequences exhibit significant directional morphological evolution, but rates of change as measured over the long-term are apparently too slow for directional selection or even drift to be the cause. Viewing long-term phenotypic evolution in terms of populations tracking peaks on adaptive landscapes is useful in this context. Such a view allows for intervals of directional selection (during times of peak movement—resulting in the overall trends we can detect) interspersed with intervals of stasis (during times of peak stability—resulting in overall changes that appear to proceed more slowly than the neutral expectation). The paradox of gradualism thus reduces to (1) peak movements and their drivers, which are not restricted in rate as are population-genetic drivers, and (2) the maintenance of stasis, on which no consensus exists. We can identify no environmental parameter in the central European Neogene that exhibits consistent change across the interval of gradual morphologic change. It may be that in Lake Pannon the long-term persistence of generally ameliorating conditions (plentiful resources and habitat space, few predators or competitors) resulted in geologically slow but consistent peak shifts, which in turn facilitated size increase and shape change in these lineages.