Fossil Friday: Turtles All the Way Down
Today’s featured fossil is a most beautiful specimen of the turtle Eurysternum wagleri, the largest turtle species from the Upper Jurassic Solnhofen limestones (Bavaria, Germany). It is 150 million years old and nearly a half meter long. Eurysternum is considered a stem group turtle of the extinct marine taxon Thalassochelydia, which is close to the crown group of turtles and tortoises. I photographed this wonderful fossil in 2015 in Mörnsheim on the occasion of the presentation of a two-volume monograph on the fossils of the Solnhofen Limestone (Arratia et al. 2015), to which I had contributed several chapters.
Problems with Pappochelys
Contrary to the gradualistic expectations of Darwin’s theory, the distinct body plan of turtles appeared abruptly in the Late Triassic with forms like Proganochelys and Proterochersis. The most famous alleged intermediate link is Pappochelys, from the Middle Triassic of Germany, but this status is highly questionable. That is because of three fatal problems with the phylogenetic analysis by Schoch & Suess (2015, 2018), who first described Pappochelys and boldly placed it in the stem group of turtles:
First, this study also placed the Middle Permian “reptile” Eunotosaurus in the stem group of turtles, even though it is generally considered to belong to an unrelated primitive group of sauropsids called Parareptilia (Ruta et al. 2011). As highlighted by Lee (2013), a turtle-relationship of Eunotosaurus arguably would place turtles firmly in the anapsid Parareptilia, which would of course conflict with other evidence for a diapsid relationship of turtles and a diapsid alleged stem-turtle like Pappochelys.
Second, this study placed turtles in a clade with lepidosaurs (lizards and snakes), even though the general consensus in modern vertebrate phylogeny considers turtles as members of the archosaur clade (together with crocs, dinos, and birds). This archosaur relationship is supported by strong anatomical characters (see Mickoleit 2004) as well as strong molecular evidence from multiple phylogenomic studies (Chiari et al. 2012; Crawford et al. 2012; Fong et al. 2012; Shaffer et al. 2013; Wang et al. 2013; Field et al. 2014; Green et al. 2014).
Third, more recent and more comprehensive phylogenetic analysis (Lichtig & Lucas 2021) indeed confirms that neither Eunotosaurus nor Pappochelys is a close relative of turtles at all.
Similar to Pappochelys is Eorhynchochelys, described by Li et al. (2018) from the Late Triassic of China. This alleged stem turtle suffers from comparable problems. One problem is again controversial resolutions in the phylogenetic reconstruction, which has the Permian Claudiosaurus and Acerosodontosaurus as basal stem turtles, while Gardner & Vranken (2020) clearly rejected such a relationship.
Furthermore, Eorrhynchochelys introduces even more incongruence to the character similarities, because it lacked a shell but had a toothless beaked mouth, while other stem turtles have a shell but lack a beaked mouth. At least one of these similarities with modern turtles must have been independently acquired as a convergence (Field Museum 2018). The authors of the study convey this insight with typical Darwinist double-talk, commenting that, “The discovery of this new form reveals a complex early history of turtles.” “Complex history” is the politically correct code word for anything that contradicts simple Darwinian expectations.
Another Evolutionary Icon
Another favorite icon of evolution from the Late Triassic of China is Odontochelys, described by the same authors (Li et al. 2008). It has a ventral shell (plastron) but lacks the dorsal shell (carapax). This was proposed as ancestral state for turtles, prior to the evolution of a closed shell. However, even mainstream evolutionists (e.g., Reiz & Head 2008) have suggested that this does not represent a primitive state at all, but rather a secondary condition in adaptation to a fully marine life, analogous to the reduced carapax of the living leatherback turtle Dermochelys.
Ultimately, the alleged evidence for the stepwise evolution of the turtle body plan evaporates under closer scrutiny and its abrupt appearance remains a fact. This is why eminent vertebrate paleontologist Olivier Rieppel even suggested a saltational origin with a “turtles as hopeful monsters” hypothesis (Rieppel 2001, 2017). He concluded that “Early ontogenetic deviation may cause patterns of morphological change that are not compatible with scenarios of gradualistic, stepwise transformation.” I tend to agree, but this of course raises the question of where the new information to bring about this new body plan suddenly came from. Neo-Darwinism has no answer, while intelligent design theory has no problem in explaining the phenomenon and identifying an adequate cause.
- Arratia G, Schultze HP, Tischlinger H & Viohl G (eds) 2015: Solnhofen – Ein Fenster in die Jurazeit. 2 vols. Verlag Dr. Friedrich Pfeil, Munich (Germany), 620 pp. https://pfeil-verlag.de/publikationen/solnhofen-ein-fenster-in-die-jurazeit/
- Chiari Y, Cahais V, Galtier N & Delsuc F 2012. Phylogenomic analyses support the position of turtles as the sister group of birds and crocodiles (Archosauria). BMC Biology 10(65), 1–14. DOI: https://doi.org/10.1186/1741-7007-10-65.
- Crawford NG, Faircloth BC, McCormack JE, Brumfield D, Winker K & Glenn TC 2012. More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs. Biology Letters 8(5), 783–786. DOI: https://doi.org/10.1098/rsbl.2012.0331.
- Field DJ, Gauthier JA, King BL, Pisani D, Lyson TR & Peterson KJ 2014. Toward consilience in reptile phylogeny: miRNAs support an archosaur, not lepidosaur, affinity for turtles. Evolution & Development 16(4), 189–196. DOI:https://doi.org/10.1111/ede.12081.
- Field Museum 2018. Fossil turtle didn’t have a shell yet, but had the first toothless turtle beak. Science Daily August 22, 2018. https://www.sciencedaily.com/releases/2018/08/180822131011.htm
- Fong JJ, Brown JM, Fujita MK & Boussau B 2012. A phylogenomic approach to the vertebrate phylogeny supports a turtle-archosaur affinity and a possible paraphyletic Lissamphibia. PLoS One 7(11):e48990, 1–14. DOI: https://doi.org/10.1371/journal.pone.0048990.
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- Li C, Fraser NC, Rieppel O, Wu X-C 2018. A Triassic stem turtle with an edentulous beak. Nature 560, 476–479 (plus SI). DOI: https://doi.org/10.1038/s41586-018-0419-1.
- Lichtig AJ & Lucas S 2021. Chinlechelys from the Upper Triassic of New Mexico, USA, and the origin of turtles. Palaeontologia Electronica 24(1):a13, 2–49. DOI: https://doi.org/10.26879/886.
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- Rieppel O 2017. Turtles as Hopeful Monsters. Indiana University Press, Bloomington (IN), 216 pp.https://iupress.org/9780253024756/turtles-as-hopeful-monsters/
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- Wang Z, Pascual-Anaya J, Zadissa A et al. 2013. The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan. Nature Genetics 45(6), 701–706. DOI:https://doi.org/10.1038/ng.2615.