Browsing by Author "Kowalewski, M"

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    Live, dead, and fossil mollusks in Florida freshwater springs and spring-fed rivers: Taphonomic pathways and the formation of multisourced, time-averaged death assemblages
    (Cambridge University Press, 2020-07-20) Kusernik, KM; Means, GH; Portell, RW; Brenner, M; Hua, Q; Kannai, A; Means, R; Monroe, MA; Kowalewski, M
    Taphonomic processes are informative about the magnitude and timing of paleoecological changes but remain poorly understood with respect to freshwater invertebrates in spring-fed rivers and streams. We compared taphonomic alteration among freshwater gastropods in live, dead (surficial shell accumulations), and fossil (late Pleistocene–early Holocene in situ sediments) assemblages from two Florida spring-fed systems, the Wakulla and Silver/Ocklawaha Rivers. We assessed taphonomy of two gastropod species: the native Elimia floridensis (n = 2504) and introduced Melanoides tuberculata (n = 168). We quantified seven taphonomic attributes (aperture condition, color, fragmentation, abrasion, juvenile spire condition, dissolution, and exterior luster) and combined those attributes into a total taphonomic score (TT). Fossil E. floridensis specimens exhibited the greatest degradation (highest TT scores), whereas live specimens of both species were least degraded. Specimens of E. floridensis from death assemblages were less altered than fossil specimens of the same species. Within death assemblages, specimens of M. tuberculata were significantly less altered than specimens of E. floridensis, but highly degraded specimens dominated in both species. Radiocarbon dates on fossils clustered between 9792 and 7087 cal BP, whereas death assemblage ages ranged from 10,692 to 1173 cal BP. Possible explanations for the observed taphonomic patterns include: (1) rapid taphonomic shell alteration, (2) prolonged near-surface exposure to moderate alteration rates, and/or (3) introduction of reworked fossil shells into surficial assemblages. Combined radiocarbon dates and taphonomic analyses suggest that all these processes may have played a role in death assemblage formation. In these fluvial settings, shell accumulations develop as a complex mixture of specimens derived from multiple sources and characterized by multimillennial time-averaging. These findings suggest that, when available, fossil assemblages may be more appropriate than death assemblages for assessing preindustrial faunal associations and recent anthropogenic changes in freshwater ecosystems. Copyright © 2020 The Paleontological Society
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    One fossil record, multiple time resolutions: disparate time-averaging of echinoids and mollusks on a Holocene carbonate platform
    (Geological Society of America, 2017-11-11) Kowalewski, M; Casebolt, S; Hua, Q; Whitacre, KE; Kaufman, DS; Kosnik, MA
    Our quantitative understanding of the temporal resolution of the fossil record is largely based on numerical dating of Holocene bivalves. However, for paleontologically important taxa with less-robust skeletons, no quantitative estimate of time-averaging exists. Here, we compare time-averaging in sympatric accumulations of the echinoid Leodia sexiesperforata and the bivalve Tucetona pectinata on a shallow carbonate platform of San Salvador Island, Bahamas. Using graphite-target and carbonate-target accelerator mass spectrometry (AMS) radiocarbon methods, we dated 30 echinoid tests and 30 mollusk valves collected from surficial sediments at a single site. All echinoid tests yielded post-bomb (taking into account radiocarbon from nuclear weapons testing) ages, and the estimated time-averaging (interquartile age range) is at most 2 yr. In contrast, sympatric mollusk valves spanned 4000 yr with an estimated time-averaging of 1830 yr. This three-orders-of-magnitude difference in the extent of time-averaging quantifies the taphonomic expectation that echinoid tests should degrade more rapidly and experience less time-averaging when compared to more durable mollusk shells. When preserved, fossil echinoids are likely to indicate a more finely resolved fossil record than skeletally robust organisms such as mollusks, and may provide a fundamentally distinct class of paleontological data, potentially adequate for investigating biological and physical processes that operate at subdecadal time scales. Immensely disparate time-averaging of sympatric mollusks and echinoids indicates that, at broader phylogenetic scales, differences in intrinsic skeletal durability may be the main determinant of the temporal resolution of fossil assemblages. Copyright © 2017 Geological Society of America
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    Radiocarbon-calibrated amino acid racemization ages from Holocene sand dollars (Peronella peronii)
    (Elsevier, 2017-04) Kosnik, MA; Hua, Q; Kaufman, DS; Kowalewski, M; Whitacre, KE
    Amino acid racemization (AAR) is widely used as a cost-effective method to date molluscs in time-averaging and taphonomic studies, but it has not been attempted for echinoderms despite their paleobiological importance and distinct biomineralization. Here we demonstrate the applicability of AAR geochronology for dating Holocene Peronella peronii (Echinodermata: Echinoidea) collected from Sydney Harbour (Australia). Using standard HPLC methods we determined the extent of AAR in 74 Peronella tests and performed replicate analyses on 23 tests. Replicate analyses from the outer edge of 23 tests spanning the observed range of D/L values yielded median coefficients of variation <4% for Asp, Glu, and Phe D/L values, which is comparable to the analytical precision. Correlations between THAA D/L values across 178 independently prepared subsamples of 74 individuals are also very high (Spearman ρ ≥ 0.95) for these three amino acids. The ages of 20 individuals spanning the observed range of D/L values were determined using 14C analyses, and Bayesian model averaging was used to determine the best AAR age model. Only three models fit to Glu D/L contributed to the final averaged age model. Modeled ages ranged from 14 to 5496 years, and the median 95% confidence interval for the 54 AAR ages was ±29% of the modelled age. In comparison, the median 2σ confidence interval for nine graphite target 14C ages was ±8% of the median age estimate and the median 2σ confidence interval for 20 carbonate target 14C ages was ±26% of the median age estimate. Overall Peronella yield high-quality D/L values and appear to be a good target for AAR geochronology. ©2016 Elsevier B.V