Early Origins of Turtles

This project integrates molecular, morphological, and developmental data with the fossil record to determine whom turtles are related to and the evolutionary origin of their body plan

Turtles have one of the most bizarre body plans of any animal alive today, with a boney shell that encases both the pelvic and shoulder girdles. The turtle shell is made up of over 50 individual bones. The evolutionary origin—or how the shell forms, when it forms, and the initial impetus for its initial transformation—has fascinated scientists for centuries. Additionally, partly as a result of their unique body plan, whom turtles are related to has remained one of the more vexing phylogenetic problems in the 21st century. These interrelated questions formed the corpus of my graduate and postgraduate work, and I continue to pursue these interesting questions. This broadly collaborative project integrates developmental, soft-tissue anatomy, and high-resolution computed tomography data of both extant and extinct animals with rapidly emerging molecular data, all within a phylogenetic context, to address these interesting questions. Key collaborators include Bruce Rubidge (University of the Witwatersrand), Gabriel Bever (Johns Hopkins Medical Institute), Emma Schachner (University of Louisiana), and Torsten Scheyer (University of Zurich).

Much of my graduate and postdoctoral work focused on the phylogenetic origin of turtles and the evolutionary origin of their unique body plan. Prior to my work, very little was known about the evolutionary history of some of the bones that make up the complex turtle shell (Lyson and Gilbert 2009; Lyson et al. 2013), when the initial transformations of the shell occurred (Lyson et al. 2010; Lyson et al. 2013b), how the turtle got its scapula inside its ribcage (Lyson and Joyce 2012), and the order in which various components of the shell were assembled (Lyson et al. 2013b).

Since joining the Museum, I have continued my work on these interesting questions and have developed collaborations with additional scientists throughout South Africa, the United States, and Europe. Specifically, we have published a number of papers where we integrated molecular, soft-tissue anatomy (including histology), and developmental data with data from the fossil record to determine how and when the transformations involved in the unique abdominal muscle respiratory system took place (Lyson et al. 2014). My collaborators and I used CT scans to analyze the skull of the oldest fossil turtle and created one of the largest amniote morphological matrices to determine the phylogenetic origin of turtles and the evolutionary origin of their skull (Bever et al. 2015). More recently, we have combined osteological, histology, and finite element analyses to propose a burrowing ecology for the earliest fossil turtles, and we have hypothesized that this burrowing ecology was the impetus for the broadening of the ribs and the earliest origin of the shell (Lyson et al. 2016).

To request PDFs please email Tyler Lyson.

Lyson, T. R. & Bever, G. S. 2020. Evolutionary Origin of the Turtle Body Plan. Annual Review of Ecology, Evolution, and Systematics 51, 143-166. DOI: 10.1146/annurev-ecolsys-110218-024746

Bever, G. S., T. R. Lyson, D. J. Field, and B.-A. S. Bhullar. 2016. The amniote temporal roof and the diapsid origin of the turtle skull. Zoology 119(6):471–473. DOI: 10.1016/j.zool.2016.04.005.

Summary
Two taxa, Eunotosaurus africanus and Pappochelys rosinae, were recently and independently described as long-anticipated stem turtles whose diapsid skulls would cement the evolutionary link between turtles and other modern reptile lineages. Detailed CT analysis of the stratigraphically older and phylogenetically stemward of the two, Eunotosaurus, provides empirical insight into changing developmental trajectories that may have produced the anapsid cranial form of modern turtles and sets the stage for more comprehensive studies of early amniote cranial evolution.

Lyson, T. R., B. S. Rubidge, T. M. Scheyer, K. de Queiroz, E. R. Schachner, R. M. H. Smith, J. Botha-Brink, and G. S. Bever. 2016. Fossorial origin of the turtle shell. Current Biology 26(14):1887–1894. DOI: 10.1016/j.cub.2016.05.020.

Summary
The origin of the turtle shell is a major evolutionary transition whose initial function was unknown. Lyson et al. present a strongly supported idea that a burrowing ecology and adaptations related to digging favored the initial transformations on the road to the modern turtle shell. Only later was the shell co-opted for protection.

Bever, G. S., T. R. Lyson, D. J. Field, and B.-A. S. Bhullar. 2015. Evolutionary origin of the turtle skull. Nature 525:239–242. DOI: 10.1038/nature14900.

Summary
Here we use high-resolution computed tomography and a novel character/taxon matrix to study the skull of Eunotosaurus africanus, a 260-million-year-old fossil reptile from the Karoo Basin of South Africa, whose distinctive post-cranial skeleton shares many unique features with the shelled body plan of turtles.

Field, D. J., J. A. Gauthier, B. L. King, D. Pisani, T. R. Lyson, and K. J. Peterson. 2014. Toward consilience in reptile phylogeny: miRNAs support an archosaur, not lepidosaur, affinity for turtles. Evolution and Development 16(4):189–196. DOI: 10.1111/ede.12081.

Summary
Previously, a study using microRNAs (miRNAs) placed turtles inside diapsids, but as sister to lepidosaurs (lizards and Sphenodon) rather than archosaurs. Here, we test this hypothesis with an expanded miRNA presence/absence dataset and employ more rigorous criteria for miRNA annotation. 

Lyson, T. R., E. R. Schachner, J. Botha-Brink, T. M. Scheyer, M. Lambertz, G. S. Bever, B. Rubidge, and K. de Queiroz. 2014. Origin of the unique lung ventilatory apparatus of turtles. Nature Communications 5:5211. DOI: 10.1038/ncomms6211.

Summary
We show through broadly comparative anatomical and histological analyses that an early member of the turtle stem lineage has several turtle-specific ventilation characters: rigid ribcage, inferred loss of intercostal muscles, and osteological correlates of the primary expiratory muscle. Our results suggest that the ventilation mechanism of turtles evolved through a division of labor between the ribs and muscles of the trunk in which the abdominal muscles took on the primary ventilatory function. 

Joyce, W. G., Schoch, and T. R. Lyson, 2013. The girdles of the oldest fossil turtle, Proterochersis robusta, and the age of the turtle crown. BMC Evolutionary Biology 13:article number 266. DOI: 10.1186-1471-2148-13-266.

Summary
Proterochersis robusta from the Late Triassic (Middle Norian) of Germany is the oldest known fossil turtle (i.e., amniote with a fully formed turtle shell), but little is known about its anatomy. A newly prepared, historic specimen provides novel insights into the morphology of the girdles and vertebral column of this taxon and the opportunity to reassess its phylogenetic position.

Joyce, W. G., I. Werneburg, and T. R. Lyson. 2013. The hooked element in the pes of turtles (Testudines): a global approach to exploring primary and secondary homology. Journal of Anatomy 223(5):421–441. DOI: 10.1111/joa.12103.

Summary
The hooked element in the pes of turtles was historically identified by most paleontologists and embryologists as a modified fifth metatarsal and often used as evidence to unite turtles with other reptiles with a hooked element. Some recent embryological studies, however, revealed that this element might represent an enlarged fifth distal tarsal. We herein provide extensive new myological and developmental observations on the hooked element of turtles and reevaluate its primary and secondary homology using all available lines of evidence.

Lyson, T. R., B.-A. S. Bhullar, G. S. Bever, W. G. Joyce, K. de Queiroz, A. Abzhanov, and J. A. Gauthier. 2013. Homology of the enigmatic nuchal bone reveals novel reorganization of the shoulder girdle in the evolution of the turtle shell. Evolution and Development 15(5):317–325. DOI: 10.1111/ede.12041.

Summary animation

The animation is based on the work by Tyler Lyson, PhD. The animation shows how the bone at the front and center of the turtle shell is derived from paired shoulder girdle bones (cleithra, blue). In addition, three of the bones found in the plastron (belly shell) are also derived from shoulder girdle bones (clavicles, pink; and interclavicle, green). The animation takes you through known fossil taxa that help bridge the morphological gap separating the highly modified shoulder girdle found in turtles from the more generalized shoulder girdle.

Lyson, T. R., G. S. Bever, T. M. Scheyer, A. Y. Hsiang, and J. A. Gauthier. 2013. Evolutionary origin of the turtle shell. Current Biology 23(12):1113–1119. DOI: 10.1016/j.cub.2013.05.003.

Summary animations

The animation shows how various fossils, particularly Eunotosaurus and Odontochelys, bridge the morphological gap between a generalized animal body plan to the highly modified body plan found in living turtles.

Lyson, T. R., E. A. Sperling, J. A. Gauthier, A. M. Heimburg, and K. J. Peterson. 2012. microRNAs support a Testudines-Lepidosaur clade. Biology Letters 8:104–107. DOI: 10.1098/rsbl.2011.0477.

Lyson, T. R., and W. G. Joyce. 2012. Evolution of the turtle bauplan: the topological relationship of the scapula relative to the ribcage. Biology Letters 8(6):1028–1031. DOI: 10.1098/rsbl.2012.0462.

Lyson, T. R., G. S. Bever, B.-A. S. Bhullar, W. G. Joyce, and J. A. Gauthier. 2010. Transitional fossils and the origin of turtles. Biology Letters 6(6):830–833. DOI: 10.1098/rsbl.2010.0371.

Lyson, T. R., and S. Gilbert. 2009. Turtles all the way down: loggerheads at the root of the chelonian tree. Evolution and Development 11(2):133–135. DOI: 10.1111/j.1525-142X.2009.00325.x.