Above: Cross-section of the fossil bivalve Cucullaea raea from Seymour Island, Antarctica. Arrows point to yearly growth increments. Only a portion of the shell is shown here as this individual lived for over 100 years! Photo credit: Lars Beierlein.
My research interests in the field of paleontology lie in understanding how the so-called ‘pagent of life’ has unfolded throughout Earth’s history. To do this, I examine variations in lifespan and growth rates of modern and fossil bivalves to answer large-scale macroevolutionary questions.
What controls lifespan? The longest-lived, non-colonial animal on the planet is the ocean quahog clam, Arctica islandica. The oldest specimen known to science is over 5 centuries old! At least a dozen other modern bivalves are known to reach lifespans in excess of 100 years. However, almost nothing is known about the fossil record of bivalve lifespans and the factors that may be influencing the evolution of such interesting life histories.
Lifespans and growth rates of modern bivalves - Though the fossil record of bivalve longevity is poorly understood, lifespans of modern bivalves, especially those serving as fishery targets, have been well studied. With the assistance of several undergraduate students, the paleontology group at Syracuse University compiled a global database of over 1000 records of lifespan and growth rate data for modern bivalves (Moss et al., 2016). We documented a latitudinal pattern of increasing lifespan and decreasing growth rate with latitude. We also found that growth rate (von Bertalanffy k) and hence metabolic rate, correlates with lifespan. Our findings have significant implications for large-scale macroevolutionary trends in the fossil record. For example, across the Phanerozoic there is a well-documented increase in the mean body size of animals, and a presumed concomitant increase in metabolic rates. If metabolic rates have indeed increased, then there should be a shift towards faster growth and/or shorter lifespans in marine bivalves over their evolutionary history.
Fig 2. Moss et al. (2016)
Temperature versus food – The pattern described above suggests that something about the environment influences lifespan. Two possibilities are cold temperatures and seasonal food availability. Separating the influence of these on lifespan in modern settings is difficult as they co-vary with latitude. Fortunately, Earth’s history offers non-analog settings that present opportunities to deconvolve these two factors. Moss et al., (2017) examined lifespans of fossil bivalves from the Eocene and Cretaceous of Seymour Island, Antarctica. During these periods in Earth’s history, temperatures on Seymour Island resembled those of modern day North Carolina. Despite the warm temperatures, multiple unrelated taxa in this high paleolatitude setting exhibit unusually long life and slow growth. Thus, the seasonal food availability, driven by the extreme polar light regime, may be more important in long life than cold temperatures.
Fig 2. Moss et al. (2017). Annual growth increments in three species of Seymour Island bivalves. A, polished thick section under reflected light (Lahillia larseni, Cretaceous); B, polished thin section under polarized light (Cucullaea raea, Eocene); C, thick section stained with Mutvei's solution (Retrotapes antarcticus, Eocene).