Morphological variables from life and death assemblages of the marine bivalve Eucallista purpurata

Name: Morphological variables from life and death assemblages of the marine bivalve Eucallista purpurata
License: https://creativecommons.org/licenses/by-nc-sa/2.5/ar/
Keywords: MOLLUSKS

Datos de investigación

Morphological variables from life and death assemblages of the marine bivalve Eucallista purpurata

Autores: Cristini, Paula Andrea Icon

; Ballarre, Josefina Icon

Publicador: Consejo Nacional de Investigaciones Científicas y Técnicas

Fecha de depósito: 22/04/2025

Fecha de creación: 16/04/2024

Clasificación temática:

Paleontología

Resumen

Mollusc shells are among the most abundant and well-preserved organisms in sediments worldwide, but their shells are usually subject to fragmentation due to exposure time in the TAZ, which compromises their preservation potential. The shell strength of valves from life and death assemblages from the sandy beaches of Santa Clara (37°50′49″S, 57°30′16″W) and Mar Chiquita (37°44′45″S, 57°25′1.5″W) on the Bonaerense coast, southwest Atlantic Argentina, was analysed to capture the effect of taphonomic processes over time and to determine how intrinsic properties and taphonomic variables influence their shell strength. To achieve this, the mechanical resistance profile of the bivalve Eucallista purpurata was assessed using point load and microindentation tests, and the mineralogical composition and organic matter content of the shell, as well as morphological variables, were measured. We concluded that, as expected, shells from life assemblages were more resistant to compression than those from death assemblages, but surprisingly, right valves were more resistant to compression than left valves in both Mar Chiquita and Santa Clara death assemblages. No differences were observed between broken and unbroken shells.

Métodos

Sampling The life assemblage (LA) was sampled after a tidal arrival at Mar Chiquita, at low tide, in August 2021. Specimens were taken to the laboratory where they were kept in tanks of seawater until sacrificed (within 48 hours of sampling) with menthol to extract the soft parts, leaving only the shells. Menthol is commonly used as an anaesthetic, as is ethanol (Gianelli et al. 2015). Crystals of menthol were placed in seawater (inside the tanks where the individuals were kept) and after a few hours, when the foot was fully extended and insensitive to touch, the soft tissues were removed from the shells (Gunkel and Lewbart 2008; Cristini et al. 2021). The shells were divided into left and right valves (LV and RV respectively) and stored in a refrigerator to prevent organic decay and consequent alteration in shell strength. Two bulk samples of disarticulated E. purpurata were collected from Mar Chiquita and Santa Clara beaches on a single day in October 2021. More than 500 shells of E. purpurata were collected from DAs, and based on their taphonomic condition (no visible evidence of mechanical damage) 378 shells from DAs (178 from Mar Chiquita and 200 from Santa Clara) and 200 shells from LA were used for morphological measurements and mechanical experiments (see explanation below in the sections on morphological and mechanical characterisation, respectively). To avoid taphonomic bias, shells with no discernible evidence of macroscopic mechanical, chemical or biological alteration were used (Figure 2) (Roy et al. 1994; Zuschin and Stanton 2001). The only exception was a few shells from life assemblages that had a small break on the anterior and/or posterior edge, possibly due to a predator such as birds during the tidal arrival (Arrighetti et al. 2005). The remaining shells were used for mineralogical characterisation (see explanation below in the mineralogical characterisation section). No radiocarbon dates have been obtained in the present study. As with the shells from the life assemblages, all the shells were stored in a refrigerator to prevent organic decay and consequent changes in shell strength Morphological characterisation Two hundred shells from LAs and 378 shells from DAs (178 from Mar Chiquita and 200 from Santa Clara) were used for morphological measurements and mechanical experiments. All shells were divided into left and right valves (RV and LV, respectively). Length, cross-sectional width, height and shell thickness were measured using a digital caliper (Wembley CD-150 resolution 0.01 mm/0.0005’) following Kosnik et al. (2006) (Figure 3). Width in cross section was used as an indicator of concavity. In addition, shells were weighed using a digital balance (accuracy 0.0001 g). Scanning electron microscopy (SEM) images were taken using a Carl Zeiss Sigma microscope at Lamarx Laboratory (Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba) to examine the crystal configuration of the shells. Mechanical characterisation Shell strength mechanical assays. Shells from LAs (n = 200) and DAs (n = 378) were subjected to compression load measurements using an Instron Emic 23–50 Universal Testing System. The system consists of a load frame for specimen mounting and compressive load application, and a control console for calibration, test setup, load – displacement measurements, and test operation control. The equipment was fitted with a 10,000 N load cell, providing an accuracy level of 1/1000 of the load at a crosshead speed of 1 mm/min. Shells were positioned at maximum stability (maximum supported area), with the shell opening facing downwards (West and Cohen 1996), between plates and loaded to failure (Figure 4). For each shell, load versus displacement data were plotted, and the maximum load (Pmax) was measured in Newtons (N, unit of force). Shell strength was determined by dividing the force measured in Newtons (maximum applied load) by the resistant area of the shell (m2), which is defined as the fracture stress and expressed in Pascals (Cristini et al. 2021). The projected area of the cubic shape of the species and the position of the shells in the mechanical tests were used to estimate the resistant area of the shell. The projected area was calculated from an ellipse (A = length/2 × height/2 × π) (Powell and Stanton 1985; Cristini et al. 2021).

Palabras clave: MOLLUSKS, PRESERVATION, MARINE COAST, ACTUOPALEONTOLOGY, ARGENTINA

Alcance geográfico

.

Identificador del recurso

URI: http://hdl.handle.net/11336/259183

Colecciones

Datos de Investigación(IIMYC)
Datos de Investigación de INSTITUTO DE INVESTIGACIONES MARINAS Y COSTERAS

Datos de Investigación(INTEMA)
Datos de Investigación de INST.DE INV.EN CIENCIA Y TECNOL.MATERIALES (I)

Citación

Cristini, Paula Andrea; Ballarre, Josefina; (2025): Morphological variables from life and death assemblages of the marine bivalve Eucallista purpurata. Consejo Nacional de Investigaciones Científicas y Técnicas. (dataset). http://hdl.handle.net/11336/259183

Condiciones de uso

Las buenas prácticas científicas esperan que se otorgue el crédito adecuado mediante una citación. Utilice un formato de citación y aplique estas normas de reutilización.

Excepto donde se diga explícitamente, este item se publica bajo la siguiente descripción: Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Unported (CC BY-NC-SA 2.5)