Date of Award

August 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Linda C. Ivany

Keywords

Bivalve, Evolution, Growth rate, Longevity

Subject Categories

Physical Sciences and Mathematics

Abstract

The factors involved in promoting long life are extremely intriguing from a human perspective. In part by confronting our own mortality, we have a desire to understand why some organisms live for centuries and others only a matter of days or weeks. What are the factors involved in promoting long life? Not only are questions of lifespan significant from a human perspective, but they are also important from a paleontological one. Most studies of evolution in the fossil record examine changes in the size and the shape of organisms through time. Size and shape are in part a function of life history parameters like lifespan and growth rate, but so far little work has been done on either in the fossil record. The shells of bivavled mollusks may provide an avenue to do just that. Bivalves, much like trees, record their size at each year of life in their shells. In other words, bivalve shells record not only lifespan, but also growth rate. Being abundant both on the surface of the planet today, and in the geologic record, bivalves provide a vessel by which we can explore the factors that influence lifespan from two different perspectives.

Mean body size in marine animals has increased more than 100 fold since the Cambrian. Associated with this increase in body size is thought to be an overall shift in the metabolic rates of organisms as well. Both factors bring attention to the key life history parameters of lifespan and growth rate. Variation in these parameters is not well understood among modern taxa, much less in deep time. Therefore, in Chapter 1, I present a global database of modern bivalve lifespans and growth rates in order to understand if latitudinal patterns exist in life history parameters in today’s oceans. The database consists of over 1,000 entries from 297 species compiled by latitude. The data indicate that 1) lifespan increases, and growth rate decreases, with latitude, both across the group as a whole and within well-sampled species, 2) growth rate, and hence metabolic rate, correlates inversely with lifespan, and 3) opposing trends in lifespan and growth combined with high variance obviate any demonstrable pattern in body size with latitude. These observations suggest that the proposed increase in metabolic activity and demonstrated increase in body size of organisms over the Phanerozoic, should be accompanied by a concomitant shift towards faster growth and/or shorter lifespan in marine bivalves.

Clear latitudinal patterns in both lifespan and growth rate documented in Chapter 1 suggest a role for some environmental factors in promoting lifespan. From a physiological perspective, cool temperatures and low/seasonal food availability are thought to promote long lifespan. However, on the planet today, these two factors covary with latitude so separating their influence using modern organisms is difficult. Fortunately, Earth’s fossil record offers a chance to tease apart these factors. In Chapter 2, I turn to fossils from the Cretaceous and Eocene of Seymour Island, Antarctica. During these times, Antarctica was situated in almost the same latitude as today, but temperatures resembled those of modern day mid-latitude environments (e.g, North Carolina). In this unique setting, I found several co-occurring, unrelated, slow growing, long-lived species of bivalve. Cool temperatures cannot explain these impressive longevities. However, the high latitude setting would have resulted in extended periods of no sunlight and suggests that caloric restriction may be the driving factor in extreme longevity.

Chapters 1 and 2 suggest that growth rate could be the factor through which long-life is selected in the evolution of extreme longevity. Studies of growth rates of bivalves living at similar latitudes, under similar environmental conditions through long spans of geologic time, could help shed light on this question. However, determining growth rates and lifespans of bivalves requires cross-sectioning individuals to reveal internal growth bands. Such methods are time intensive and destructive sampling is often not permitted by museum curators. An alternative method could be to determine age by simply measuring the size of individuals without cross-sectioning, but the nature of growth in bivalves (long-lived in particular) is such that a few millimeters of growth could equate to several decades and introduces a large degree of error. In Chapter 3 then, I explore probabilistic methods for determining age from size in order to constrain population growth parameters without cutting large numbers of individuals. From a small original sample size, I use the relationship between parameters of the von Bertalanffy growth equation to constrain the theoretical age/size distributions of a population of modern Spisula solidissima. From these distributions, age can be assigned to an individual of any given size by drawing at random from the corresponding age/size distribution. This method works extremely well in reconstructing population growth parameters in a modern bivalve and should be applicable to the fossil record as well.

With the three chapters presented here, the foundation has been laid to study life history parameters in the fossil record. Currently, life history data are missing from studies of body size and energetics of organisms through time. The growth rate parameter k of the von Bertalanffy growth equation is the variable that will add a new dimension to our understanding of these fundamental patterns in the history of life on Earth. The conclusions from Chapter 1 predict that the temporal trend in body size is driven by an increase in k through time. The methods in Chapter 3 allow for study of k values through time in the fossil record. Chapter 2 examines lifespans and growth rates of fossil bivalves and provides significant revelations into the factors that influence extreme longevity. Though bivalves have been on the planet for over 500 million years, only a handful of studies have examined their life histories in the fossil record. Besides those presented, other fascinating areas where life history information is needed include studies of survival at mass extinction events and the transitions associated with the Mesozoic Marine Revolution. Incorporation of life history data into paleontological studies can and will provide fascinating insights in the evolution of life on Earth.

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