Estimating Taxonomic Diversity Using Centrum Growth Profiles and Stinger Morphology of 36 Million Year Old Stingrays From North Dakota
Stingrays are a diverse and popular group of vertebrates; however, nothing is known about the relationships between growth biology and climate change. Freshwater stingrays once inhabit the United States and Canada during very warm times in the geologic record. No stingray material has been recorded from the northern part of the United States for the last 33 million years. The Earth’s climate cooled from 50 to 33 million year ago when many warm adapted organisms were relegated to warmer, southern latitudes in North America. Today, freshwater stingrays only inhabit subtropical and tropical environments. We are interested in the freshwater stingrays that lived just prior to the climatic cooling that changed the area of what is now North Dakota 33 million years ago. Our goal was to estimate alpha diversity of stingrays from a fluvial fossil deposit that existed 34 million years ago. We predicted the fossil stingrays from North Dakota to grow very slow compared to their modern counterparts. Fossil elements of stingrays are represented by their individual vertebral centra, stingers, and teeth. No complete specimens have been identified. We examined 36 isolated vertebral centra and estimated the number of growth cessation marks on each centrum. Centrum radial distance (mm) was measured from the notochord foramen to each annulus and plotted. We could not find any published data on age and growth of extant freshwater stingrays for comparison. The growth profiles were compared to marine Dasyatis pastinaca, Common Stingray, which are found in Northeastern Atlantic Ocean. There is complete overlap in the von Bertalanffy growth curves and parameters of the two datasets with no evidence for slow growth rates from ages 1-7 years old. Stingers were described morphologically and stinger thickness and median ridge thickness (mm) was measured for each specimen. Chronological ages for individual specimens ranged from 0 to 8 years old with two significantly distinct growth profiles; small and large profiles. Three stinger morphotypes were recognized. Measurements of stinger median ridge thickness indicated there were two small morphotypes and one significantly larger morphotype. Our data suggest there were three taxa of stingrays that lived in the river channels of North Dakota 36 million years ago. Two taxa were small and one taxon was somewhat larger. Our next goal is to determine whether there are three distinct morphologies in the vertebral centra. There is little evidence of old individuals in the fossil dataset suggesting two hypotheses; 1) older individuals did not exist in the population, and 2) older individuals lived in another habitat (habitat partitioning). The use of fossils stands to provide great insight into the effects of climate change on the age and growth biology of fishes. Our research indicates that diversity of freshwater stingrays was higher than expected. Climatic cooling may have caused regional extinctions of freshwater stingrays because freshwater stingrays are found in tropical and subtropical areas today.
Centrum Growth Profiles, Stinger Morphology, Stingrays, North Dakota, Biology, Ecology and Evolutionary Biology
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