Turtle Neck Evolution

By: Taylor Auletto, Nick Botelho, Jeremy Breen, and Rebecca Johnson (Stonehill College, BIO323: Evolution, Spring 2019)

The neck is a seriously underappreciated part of the body of most organisms. Think about it: what if you didn’t have a neck? Or, what if you were incapable of moving your neck? Weird, huh? The proper development and functioning of a neck are essential to survival for many species. This is especially true for turtles. Turtles have the unique ability to retract their neck into their shell when necessary; without this ability, they would be highly susceptible to predators and other such dangers. As such, researchers Christine Böhmer and Ingmar Werneburg wanted to investigate the way the cervical vertebral (CV) column appears across multiple extinct and living turtle taxa for the first time. The cervical vertebral column is essentially composed of segments of the skeleton that make up the neck. Turtles have 8 (referred to as CV1-CV8) of these segments. What’s more, these segments have been conserved through evolution for millions of years. What we already know, though, is that there are specializations in the CV column that are different (i.e., specialized) in different groups of turtles, depending on the method by which they retract their necks. These differences are known to be due to differences in how Hox genes have been modified through evolution. Hox genes are genes that are involved in the body plan of all organisms. Böhmer and Werneburg state that their work ultimately aims to understand how evolution has worked to create the differences in CV columns that we can see today.

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The way in which Böhmer and Werneburg conducted their research was relatively simple. In fact, they were able to gather all their data and results from strictly morphological analyses! In other words, all their presented information comes from visual methods rather than experiments. The authors analyzed a total of 77 cervical vertebrae from 3 turtle groups (stem turtles, Pleurodira, and Cryptodira) and eight other groups of animals. Specifically, a 3D landmark-based geometric morphometric analysis was utilized where the vertebrae of interest were scanned and imported into the software Landmark. We’ll spare some of the details here, as they are a bit confusing to explain and we do not wish to distract from the important points here. Also used for setting up the analyses was software PAST, Relative Warps (RW) Analysis, and a Principal Component Analysis (PCA). When combined, these methods will reveal the relationships between the vertebrae of interest! What’s more, they were able to visualize patterns and details that were otherwise impossible to tell. This would lead to new understandings of the evolution of turtle necks!

The authors were able to come to the general conclusion that turtles display a huge amount of differences in the way their CV appears. These differences occur in several different ways. For example, one of these ways is the shape and height of neural spines. The researchers also made comparisons of Hox gene expression between turtles and other animals, including mice, lizards, crocodiles, archosaurs, and chickens. Some notable conclusions of these comparisons included a prediction of the number of Hox genes in the vertebral column of turtles and differences in expression of certain Hox genes between turtles and all other vertebrates. The authors also make note of different patterns in the CV column between turtle species with different methods of neck retraction, and a potential lead into the development of the fixed eighth CV during the late Triassic period if more research is conducted. It is also important to mention that many, if not all, of the conclusions as explored by these scientists would be better supported if genetic analyses were combined with these morphological techniques. This is true because many studies that focus only on the appearance of something are highly subjective. In other words, there could be major bias and/or missing pieces of information that could lead to different conclusions. Overall, this article described some seriously interesting science and we cannot wait to see the next development on this topic!

Citation: Böhmer, C., and I. Werneburg. 2017. Deep time perspective on turtle neck evolution: chasing the Hox code by vertebral morphology. Scientific Reports 7: 8939.

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