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The population structure of Late Pliocene and Early Pleistocene Neptunea angulata, Gastropoda and an investigation into bias in the fossil record and museum collections.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. (Paleobiology)
2016 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesisAlternative title
En undersökning av populationsstrukturen hos Neptunea angulata under sen Pliocen och tidig Pleistocen samt partiskhet inom fossilfynd och museisamlingar. (Swedish)
Abstract [en]

The fossil population structure of the gastropod Neptunea angulata from the late Pliocene and Early Pleistocene was investigated in this project in order to contribute to a wider study on the influence of predation on populations and the evolutionary history of organisms. Over time, predator-prey relationships can drive evolution in a way similar to the Red Queen Hypothesis. However, before the effects of predation can be understood one must determine how the population dynamics functioned without the influence of predation. There are a number of problems that arise when determining population dynamics for fossil assemblages. These are usually caused by missing fossil data and the uncertain nature of their absence. Bias is rife within fossils at various stages from post-portem processes to when they are present in museum collections. One way to estimate these biases is to investigate the population structures of the fossils both directly from the fossil record and from museum collections.

The variation in oxygen istopes found in N. angulata shells corresponded to yearly cycles which then were counted to determine the age of the specimen at time of death. Measuring the length of the spiral at yearly intervals provided the growth rate for the organism while it was alive. The growth rates were then used to determine the ages of specimens based on their size. The resulting ages were organized into an age distribution graph which was used to determine any museum bias. Bias in the preservation was also investigated by measuring the taphonomic damage of organisms of different size and then determine the distributions of size vs taphonomy.

The ages of three specimens were found to differ even though the organisms had similar whorl lengths. As a result the growth equations differed and so different age distributions were calculated from each growth equation. All the age distributions demonstrated that the museum collections did show some bias against the smaller sized and thus younger specimens. There also appeared to be a size bias towards small N. angulata within the fossil record, with the extremely small individuals missing. The majority of the smallest specimens found in the field collections were not actually N. angulata specimens. A major problem with the results was a lack of data and a small sample size and it is highly recommended that an extensive collection and review of material be undertaken to fully determine the population structure present in the fossil assemblages. Other parts of the study, for example, the growth rates also require larger data sets in order for the confidence of the data to be improved. 

Abstract [sv]

I detta projekt studerades snäckpopulationer (Neptunea angulata) från Pliocen och Pleistocen för att besvara bredare frågeställningar om predation och dess inflytande på populationer och evolution. Innan man kan fastställa effekten av predation så behöver man förstå hur populationen betedde sig utan predationstryck. Flertalet problem uppstår när man studerar fossila populationer: tillgången på data kan vara begränsad och det kan vara svårt att se vad som saknas och varför. Information från fossil förloras från tiden som organismen dör fram till att de återfinns i museisamlingar. Ett sätt att uppskatta informationsförlusten är att studera fossil såväl i fossilbäddar som i museisamlingar.

Genom att använda sig av den observerade cykliska skillnaden i stabila syreisotoper mellan olika tillväxtzoner i skalen av Neptunea angulata var det möjligt att uppskatta åldern på en organism vid en viss längd. Hastigheten med vilken snäckan växte beräknades genom att mäta förändringen i längd mellan olika åldrar. Genom att beräkna hur snabbt en snäcka växte så var det möjligt att använda storleken på fossilerna för att uppskatta dess ålder då den dog. Fossilen organiserades efter ålder för att visa populationsstrukturer. Förluster av fossil efter deposition uppskattades undersöktes genom att bestämma om mindre storleksgrupper var mer skadad än större storleksgrupper .

Den varierade tillväxten hos olika snäckor användes för att beräkna dess åldrar. Det upptäcktes att museisamlingar tenderade att inneha större och äldre individer. Det tycktes också finnas färre små fossil av Neptunea angulata inom opartisk samling. Några av de extrement små individerna saknades helt och majoriteten av de minsta fossilen var inte ens Neptunea angulata. Skador på fossilen var större ju mindre individerna var. Mängden data inverkade negativt på denna studie och därför rekommenderas en omfattande genomgång av de tillgängliga samlingarna för att bättre kunna besvara frågor kring denna population i framtiden.

Place, publisher, year, edition, pages
2016. , 41 p.
Series
Examensarbete vid Institutionen för geovetenskaper, ISSN 1650-6553 ; 364
Keyword [en]
Bias, age distributions, taphonomy, population structure.
Keyword [sv]
Partiskhet, åldersfördelning, tafonomi, populationsstrukturer.
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-296325OAI: oai:DiVA.org:uu-296325DiVA: diva2:937470
Educational program
Master Programme in Earth Science
Presentation
2016-06-01, Geocentrum, Villavägen 16, Uppsala, 13:00 (English)
Supervisors
Examiners
Available from: 2016-06-15 Created: 2016-06-15 Last updated: 2016-06-15Bibliographically approved

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