Before the formation of the Central American Isthmus, there was a Central American Peninsula. Here we show that southern Central America existed as a peninsula as early as 19 Ma, based on new lithostratigraphic, biostratigraphic and strontium chemostratigraphic analyses of the formations exposed along the Gaillard Cut of the Panama Canal. Land mammals found in the Miocene Cucaracha Formation have similar body sizes to conspecific taxa in North America, indicating that there existed a terrestrial connection with North America that allowed gene flow between populations during this time. How long did this peninsula last? The answer hinges on the outcome of a stratigraphic dispute: To wit, is the terrestrial Cucaracha Formation older or younger than the marine La Boca Formation? Previous stratigraphic studies of the Panama Canal Basin have suggested that the Cucaracha Formation lies stratigraphically between the shallow-marine Culebra Formation and the shallow-to-upper-bathyal La Boca Formation, the latter containing the Emperador Limestone. If the La Boca Formation is younger than the Cucaracha Formation, as many think, then the peninsula was short-lived (1-2 m.y.), having been submerged in part by the transgression represented by the overlying La Boca Formation. On the other hand, our data support the view that the La Boca Formation is older than the Cucaracha Formation. Strontium dating shows that the La Boca Formation is older (23.07 to 20.62 Ma) than both the Culebra (19.83-19.12 Ma) and Cucaracha (Hemingfordian to Barstovian North American Land Mammal Ages; 19-14 Ma) formations. The Emperador Limestone is also older (21.24-20.99 Ma) than the Culebra and Cucaracha formations. What has been called the "La Boca Formation" (with the Emperador Limestone), is re-interpreted here as being the lower part of the Culebra Formation. Our new data sets demonstrate that the main axis of the volcanic arc in southern Central America more than likely existed as a peninsula connected to northern Central America and North America for much of the Miocene, which has profound implications for our understanding of the tectonic, climatic, oceanographic and biogeographic history related to the formation of the Isthmus of Panama.
Two beds containing large numbers of turritellid gastropods (Family Turritellidae) occurring in the densely fossiliferous Upper Pliocene Pinecrest Sand of Florida formed as a result of upwelling and consequent high biological productivity, together with some degree of physically mediated time averaging. Analyses of size-frequency distribution and shell surface condition, combined with isotopic data on chronological age of individual shells, water temperature and upwelling intensity, suggest that both beds formed relatively quickly, probably in less than 100–200 years. The upper bed, occurring within Petuch's (1982) unit 2 (2.5-2.0 Ma) and containing abundant Turritella apicalis Heilprin, appears to have formed largely as a result of upwelling; the lower bed, occurring in upper unit 6/7 (3.5-2.5 Ma) and containing abundant Turritella gladeensis Mansfield, appears to have formed over a longer period, as a result of upwelling, increased time-averaging, and perhaps cooler overall water temperatures. This study highlights the potential to isolate and examine separately some of the biological and physical factors affecting shell bed formation, and especially to address the role of biological productivity in this process. □Pinecrest Beds, Pliocene, upwelling, turritellid gastropods, taphonomy.
Introduction: TNF is a cytokine that regulates growth, migration, differentiation and apoptosis in normal and diseased intestinal epithelium through complex signaling mechanisms. MMP-9 belongs to a family of endopeptidases involved in tissue injury, repair, and tumorigenesis. MMP-9 is elevated in patients with inflammatory bowel disease (IBD), and its inhibition in murine models of colitis ameliorates disease severity. Since sustained MAP kinase activation is necessary for epithelial cell growth inhibition by TNF and MMP-9 expression, we investigated TNF regulation of MMP-9 expression and the involved TNF receptors in intestinal epithelium. Methods: Young adult mouse colon (YAMC), TNFR1−/ −, or TNFR2−/ − mouse colon epithelial (MCE) cells were treated with TNF (100 ng/ml) or EGF (10 ng/ml) for various times (1–24 h). Cellular lysates, total RNA and conditioned media were analyzed for MMP-9 expression by Western blot analysis, 2-D Difference Gel Electrophoresis and MALDI-TOF, or gene microarray and gelatin zymography, respectively. MMP-9 was studied by immunodetection in mouse colon after TNF injection (104 U/mouse) or in 2 models of IBD with increased endogenous TNF production, IL 10−/ − colitis and mice receiving water with dextran sulfate sodium (5% wt/vol) for 7 days. All were compared to appropriate controls. Results: TNF increased MMP-9 expression 6-fold in YAMC cells at 4 hours by gene microarray and proteomic screening assays. Furthermore, conditioned media derived from TNF-treated cells showed a 20-fold increase in both the active and pro-forms of MMP-9 and a decrease in MMP-2 expression by gelatin zymography. By comparison, EGF enhanced MMP-2 by 3–5-fold but had no effect on MMP-9 expression. TNF increased MMP-9 expression in TNFR2−/ −MCE but not TNFR1−/ −MCE cells, indicating TNFR1 is required for this effect. Treatment of mice with exogenous TNF increased MMP-9 expression in the epithelial and surrounding stromal cells at the base of colonic glands. Induced colitis, either in IL 10−/ − deficiency or exogenous dextran sulfate sodium administration, increased MMP-9 expression similarly. MMP-9 was increased in IL 10−/ − mice with colitis more than 20-fold compared to background control mice. Conclusion: In the mouse colon, MMP-9 expression is increased by TNF or colitis through TNFR1 in epithelial and surrounding stromal cells. Increased expression of MMP-9 through TNFR1 may contribute to epithelial damage, defective epithelial remodeling and tumorigenesis in IBD.
It is shown that the variational method of dealing with the integral equations of scattering problems is equivalent to solving the integral equation directly by Galerkin's method and using the standard formula for the amplitude of the scattered wave. The second method also satisfies the reciprocity theorem. It is therefore suggested that the reciprocity theorem be used as the basis of approximation without the introduction of variational formulas. The error involved in using an approximate solution is discussed and it is shown that only a special set of approximations can lead to accuracy at low frequencies. Some ways in which bounds for the error may be obtained in special problems are also given.
The Neogene stratigraphic record of Florida reveals many richly fossiliferous (both invertebrate and vertebrate), shallow-marine deposits whose ages are poorly constrained, despite over a century of paleontological investigation. Chronostratigraphic analyses are frequently hampered by a general absence of age-diagnostic fossils (micro- or nannofossils in particular), laterally discontinuous sedimentary units, and an overall lack of natural exposure. Consequently, temporal correlations among many of Florida's Neogene units as well as with the global geomagnetic polarity timescale (GPTS) remain tenuous. Strontium isotope ([sup 87]Sr/[sup 86]Sr) geochronology offers considerable potential as an independent method for correlating these deposits with one another as well as with the GPTS. Portions of the Miocene and Pliocene as well as most of the Pleistocene were characterized by rapid increases in the global [sup 87]Sr/[sup 86]Sr of seawater. Marine carbonates formed within these intervals are therefore particularly amenable to investigation by Sr isotope techniques. Ratios ([sup 87]Sr/[sup 86]Sr) from mollusk shells collected throughout Florida within stratigraphic context ranged from 0.70785 which roughly correspond to the interval 30 Ma (late Oligocene) to 1 Ma (late Pleistocene). The data suggest that many accepted correlations based on lithologic and/or faunal similarities within Florida's Miocene and Plio-Pleistocene strata are improbable. While additional samplingmore » is in order, improved correlation between vertebrate and invertebrate biochronologies has already resulted from the stratigraphic based upon this approach.« less
Abstract We have known since Trueman's classic work of 1922 that the Lower Jurassic Gryphaea of Britain exhibit phyletic size increase and heterochronic change in shape. Since Hallam's revisionary work in the 1960s, we have recognized that pronounced and generalized juvenilization of form accompanied this increasing size. This extensive literature provides invertebrate paleontology's most famous example of a biometrically documented, continuous anagenetic trend within a discrete lineage. But Gryphaea has also provoked great frustration because a key datum, required for a full solution, had been theoretically recognized but practically unavailable. We could identify the evolution of shape as paedomorphic, but could not specify the mode of heterochrony for this paedomorphic result because we could not standardize samples by common age or developmental stage. In this paper, we provide sclerochronological data on sizes and shapes at specified ages marked by annual growth bands in two Jurassic sequences of Gryphaea : the classic Lower Jurassic series showing phyletic size increase with paedomorphosis, and an independent Middle–Upper Jurassic series illustrating neither size increase nor heterochrony. We prove that size increase in the classic series occurs entirely by faster growth (larger descendant sizes at the same ages as ancestors), and not by extended age (for descendants lived no longer than ancestors). The well-marked paedomorphosis of form probably arose as a correlated consequence of growing larger by extending and maintaining rapid juvenile growth rates—thus marking the heterochronic mode as a case of neoteny. The independent upper sequence, acting as a different replicate in a natural experiment, shows neither size increase nor heterochrony but does exhibit (in contrast with the classic sequence) evolution toward greater longevity. Hallam's flow tank experiments indicated a strong adaptive advantage in shell stability for both larger size and paedomorphic form. Neotenous development provides an evolutionary pathway to the simultaneous acquisition of both favored traits—thus showing that “constraints” due to “correlations of growth” (Darwin's own phrase for the phenomenon) may be positive in promoting joint evolutionary advantages, and not only neutral (in carrying spandrels along with primary adaptations), or negative (by imposing inadaptive “baggage” upon trends in form through developmental correlation with selected traits).
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