Recently, in my Herpetology class, my professor informed us about a recent revelation in turtle evolution. The turtles’ closest living relatives are in fact, birds and crocodiles (collectively known as the archosaurs to which I’ll refer to this as the archosaur hypothesis). This is quite different from what I learned while growing up mainly that turtles were a weird outgroup of reptiles (see the figure below). This was even commonly accepted in the scientific community. Intrigued that a major shift in the Tree of Life has happened, I listened intently.
Possible phylogenetic relationships in Turtles. Tree A represents the traditional tree with turtles as an outgroup of reptiles. Tree B represents a different take using morphology as an example. Tree C represents the new tree with turtles a sister group to the archosaurs (the “archosaur hypothesis”). Image from Hedges (2012).
The first papers that questioned turtles’ relationship on the Tree of Life were few and far between; some of which were even inconclusive. However, Kumazawa and Nishida (1999), Cao et al. (2000), and Iwabe et al. (2005), were among the first scientists to question if turtles were actually a sister group to the archosaurs. They had found strong evidence that this was so. But, all three of these were either led or helped by Dr. Kumazawa meaning only a few scientists supported this hypothesis. This changed in 2011 when we got a deluge of papers dealing with turtle relationships. Shen et al. (2011), Crawford et al. (2012), Chiari et al. (2012), Fong et al. (2012), and Lu et al. (2013) all concluded that turtles were actually a sister group to archosaurs (this partially excludes Fong et al. (2012) who thought that turtles were either a sister group to lizards or to the archosaurs).
Again and again and again, we see continual support for the archosaur hypothesis from various different research groups analyzing various different genetic components. I thought this was cool, but my professor by the end of his lesson seemed more annoyed then gratified by all this research. Yes, alright, we get it, turtles are a sister group to archosaurs, move on.
But I had an appreciation for these scientists and their experiments. This is science at its best. We have rigorous testing of a novel hypothesis that has shaken traditional knowledge. And through this rigorous testing, again and again, the hypothesis stands strong.
This may seem odd to praise repeated testing, but lately, science, particularly pharmaceutical science, has been faltering lately. If you have the time, you should definitely read the article here which highlights how repeated testing has taken a backseat in modern science. Why is that? Well, as the article states, it could be that the scientists want to make a name for themselves by testing new things, or journals may have no interest in repeated testing, or it’s just too costly. Any of these could be a factor. This lack of critical, unbiased repeated testing harms science as certain information we could perceive to be correct could actually be false due to human error.
This was why I was pleased to hear these multiple experiments done on turtle phylogeny. It showed me that scientists are still questioning not just science but each other. What’s more, the turtle phylogeny question has still not been accepted by everyone. Morphologists, who study the structure of organisms, disagree with the archosaur hypothesis based on turtle anatomy and the fossil record. However, even they have seemed to accept this hypothesis as the amount of research papers refuting the archosaur hypothesis has gone down considerably since the mid-2000s. What’s more, I even found a morphological paper, Bhart-Anjan et al. (2009), which supported the archosaur hypothesis.
As such, in the space of about two decades, we see a great example of how our perception of science can change from one standpoint to another. First, we have a generally accepted hypothesis (turtles are an outgroup of reptiles). This was then challenged by a few scientists (turtles are actually relatives of archosaurs). Finally, it ballooned to many scientists coming forth and supporting this new hypothesis through repeated testing. To me, I find this awesome. And that is why, my friend, turtle science, is good science.
Bhart-Anjan S., and Bever G. S., 2009. An Archosaur-like Laterosphenoid in Early Turtles (Reptilia: Pantestudines). Breviora 518:1-11.
Cao Y., Sorenson M. D., Kumazawa Y., Mindell D. P., Hasegawa M., 2000. Phylogenetic position of turtles among amniotes: evidence from mitochondrial and nuclear genes. Genes 259:139-148.
Chiari Y., Cahais V., Galtier N., and Delsuc F. 2012. Phylogenomic analyses support the position of turtles as the sister group of birds and crocodiles (Archosauria). BMC biology 10:65.
Crawford N. G., Faircloth B. C., McCormack J. E., Brumfield R. T., Winker K., Glenn T. C., 2012. More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs. Biology Letters 8:783-786.
Fong J. J., Brown J. M., Fujita M. K., and Boussau B. 2012. A phylogenomic approach to vertebrate phylogeny supports a turtle-archosaur affinity and a possible paraphyletic Lissamphibia. PloS one 7:e48990.
Hedges S. B., 2012. Amniote phylogeny and the position of turtles. BMC Biology 10:64.
Iwabe N., Hara Y., Kumazawa Y., Shibamoto K., Saito Y., Miyata T., and Katoh K., 2005. Sister Group Relationship of Turtles to the Bird-Crocodilian Clade Revealed by Nuclear DNA–Coded Proteins. Molecular Biology and Evolution 22:810-813.
Kumazawa Y., and Nishida M., 1999. Complete Mitochondrial DNA Sequences of the Green Turtle and Blue-Tailed Mole Skink: Statistical Evidence for Archosaurian Affinity of Turtles. Molecular Biology and Evolution 16:784-792.
Lu B., Yang W., Dai Q., and Fu J. 2013. Using Genes as Characters and a Parsimony Analysis to Explore the Phylogenetic Position of Turtles. PloS one 8:e79348.
Shen X. X., Liang D., Wen J. Z., and Zhang, P 2011. Multiple genome alignments facilitate development of NPCL markers: a case study of tetrapod phylogeny focusing on the position of turtles. Molecular biology and evolution 28:3237-3252.
The Unapologetic Nerd 