Article: Population analysis and orientation studies of graptoloids from the Middle Ordovician Utica Shale, Quebec
Three large populations of graptoloids from the Middle Ordovician Utica Shale of Quebec contain Orthograptus quadrimucronatus micracanthus and Amplexograptus praetypicalis. Detailed orientation studies show that the two species reached the bedding plane at different times and were probably present in the water mass as monospecific shoals. Some size ranges of each population are orientated, suggesting that current sorting occurred. Few siculae are present, either because of current winnowing or because of geographical separation of growth stages in life. Length-frequency graphs of complete specimens suggest that both species grew throughout life. Survivorship analysis indicates that some populations died from constant environmental stress while others lived long enough for increasing length (or age) to become a handicap. The origin of synrhabdosomes is considered to be taphonomic.