The answer, of course, is quite simple, twice as long as half its length. Unfortunately, this answer is usually not applicable when conducting scientifically rigorous operations. A few of our student employees ran into this type of problem recently. Defining what “half” of something is isn’t always a straightforward procedure.
The “something” in question was the snail genus Turritella. Turritella is a very common type of marine gastropod in Cenozoic deposits. Our students, Darrah Jorgensen and Tait Earney are tasked with tabulating the relative abundance of various taxa of marine organisms in the samples they are working with. In order to minimize bias with regard to this type of tabulation, we usually use some specific criteria for “counting” individuals. If you are working with vertebrate remains, an “individual” is represented by two opposing limb elements, one left, and one right. Working with invertebrates, particularly Turritella, makes this task a bit more daunting.
Turritella shells are conical spirals that are often broken. If they are broken, the aperture, or anterior opening, is usually missing; only the posterior end is present. Individuals are generally “counted” in tabulations if more than half a shell is represented in the sample. However, how do you recognize half of a conical spiral, especially when Turritella specimens occur in various sizes?
Today’s entry by Darrah Jorgensen discusses this problem.
“While discussing the job with Tait, I have noticed some things we may be doing differently. We have own system for processing their samples. We were trained together by Dr. Belanger and Dr. Anderson. However, we work independently. This allows us to work at our own pace and we often create our own methods within the parameters of our training.
The other day Tait and I were discussing how we determined which Turritella shells to pull out. We were told to pull the specimen if more than half the shell was present. This would avoid counting specimens twice by inadvertently counting both the anterior and posterior portions. What we were hung up on was the subjective nature of this. What I define as half may be different from Tait’s definition, which may be different from that of the graduate students. This causes a lot of different sizes of “halves”. He and I were discussing how to standardize this definition. In the end, we didn’t find a useful way to do this, but we did determine that different definitions could cause a variation in specimen count.
I think we are all a little worried about screwing up this project; I know I am. From what I am told, that fear never really goes away. This project is pretty different from the lab and field prep I have done in the past. I guess that is just part of the fun.”
Sometimes, it’s the incredibly mundane questions that have a marked impact on research results…