Tooth Loss in Turtles

An online paper by Davit-Béal, Tucker and Sire (2009) examined tooth loss in several tetrapod clades. They mentioned tooth loss in “toads in Lissamphibia, turtles and birds in Sauropsida, and baleen whales, pangolins, anteaters, sloths, armadillos and aardvark in Mammalia.” There were other extinct forms that also lost all* their teeth, including pterosaurs, certain poposaurs and other dinosaurs (ornithomimosaurs). I want to focus on turtles today.

Origins Questioned
Davit-Béal et al. (2009) wrote: The origin of turtles from ancestral sauropsids is still unclear and largely debated. Molecular data are partially congruent with morphological characters supporting diapsid rather than anapsid turtle relationships [Rieppel & deBraga, 1996deBraga & Rieppel, 1997; see Laurin & Reisz, 1995 and Lee, 1997 for Parareptilia (anapsid) turtle relationships]. However, the molecular data conflict with palaeontological data as to where exactly turtles fit within diapsids (Rieppel, 1999). Phylogenetic studies either place turtles close to the lepidosaumorphs (tuatara, snakes and lizards) (e.g.Hill, 2005) or close to the archosauromorphs (crocodiles and birds) (e.g. Hedges & Poling, 1999Iwabe et al. 2005). The turtle ancestor diverged from the other diapsids between 285 and 270 Ma (McGeoch & Gatherer, 2005) but its origin remains a mystery. The most ancient and well-known turtle is Proganochelys quenstedti (late Triassic, 220 Ma).”

 Toothlessness in turtles according to Davit-Béal et al. (2009).

Figure 1. Clicking will NOT enlarge image. Toothlessness in turtles according to Davit-Béal et al. (2009). Click to enlarge. Green lines are turtles with palatal teeth. Red lines are turtles without any teeth.

Written Just Before Odontochelys
Timing is everything and the Davit-Béal paper was likely “in press” when the Odontochelys (225 mya) paper was published (Li et al. 2008). Odontochelys is the only known primitive turtle with teeth on the rims of its jaws. Proganochelys and several other early turtles retained palatal teeth. Here the primitive diadectomorph Stephanospondylus (290 mya) is in the clade of turtles, prior to the development of the carapace and plastron 75 million years before turtles with shells.

From left to right the skulls of Stephanospondylus, Odontochelys and Proganochelys

Figure 2. From left to right the skulls of Stephanospondylus, Odontochelys and Proganochelys demonstrating tooth loss and other skull traits.

Notes from Davit-Béal (2009) 
Davit-Béal et al. (2009) reported, “P. quenstedti was roughly similar to the species that live today, except for, among other characters, the presence of several rows of conical teeth on the vomers, palatines and pterygoids (Fig. 6D), which make it unique among Testudinata as the other ancient turtles lack these teeth (Joyce, 2007). The maxilla, pre-maxilla and dentary are edentulous but the pre-maxillary has tooth vestiges (Kordikova, 2002). Although the common ancestor of all living turtles was aquatic, the earliest turtles clearly lived in a terrestrial environment (Joyce & Gauthier, 2004Scheyer & Sander, 2007).  As suggested for birds, the presence of a keratinized beak that was efficient for food uptake probably relaxed the functional pressure on teeth, which were probably lost through a similar process to that described in birds (see above). In the turtle’s ancestor, as in the bird’s ancestor, the beak minimized the negative consequences of tooth loss. In turtles, teeth were retained on the palate longer than in jaws. Teeth were lost in the vomers and palatines first, then later on the pterygoids.”

Stepping Back in Time and Phylogeny
The present tree indicates that a sister to Orobates and Diadectes preceded Stephanospondylus and Arganaceras was a basal pareiasaur sister. These taxa are widely considered to be herbivores. The 16 teeth in the left premaxilla (4) and maxilla (12) of Orobates were larger anteriorly and the tooth row ended below the mid orbit. The 15 to 21+ teeth in the left premaxilla and maxilla of Diadectes followed these patterns, but were smaller overall. The sisters Oradectes and Silvadectes were not much different.

Stephanospondylus Teeth
Romer (1925) illustrated 27 teeth in the premaxilla (3) and maxilla (24) of Stephanospondylus extending posteriorly beyond the mid point of the orbit. Each tooth had a smaller diameter but a longer length.

Odontochelys Teeth
In the one and only known toothed turtle (Li et al. 2009) 6 teeth were in the premaxilla and 18 in the maxilla. Distinct from Stephanospondylus, the premaxillary teeth were smaller than the maxillary teeth. Instead of four, there were six premaxillary teeth. The maxillary teeth started off small anteriorly, but were larger posteriorly and extended behind the orbit. As in Stephanospondylus the first maxillary tooth was slightly larger than the others.

Summary
Smaller and an increased number of teeth often precede the complete loss of teeth, as seen here in turtles. Herbivores often lose their teeth, as seen here in turtles. Turtle ancestors provide one of the few examples of herbivores that became insectivores and piscivores. Perhaps they were always omnivores. Like birds, early turtles developed a shearing keratinous beak, providing a substitute for teeth. They have no other similarities that I know of and must have developed beaks for distinct reasons.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Baur G 1887. On the phylogenetic arrangement of the Sauropsida: Journal of Morphology, v. 1, n. 1:93-104.
Davit-Béal T, Tucker AS and Sire J-Y 2009.
 Loss of teeth and enamel in tetrapods: fossil record, genetic data and morphological adaptations. Journal of Anatomy 2009 April; 214(4): 477–501. doi:  10.1111/j.1469-7580.2009.01060.x
Gaffney ES 1990. The comparative osteology of the Triassic turtle Proganochelys, Bull. Am. Mus. Nat. Hist. 194: 1–263.
Layson TR, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA 2010. Transitional fossils and the origin of turtles. Biology Letters June 9 2010. doi: 10.1098/rsbl.2010.0371
Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008.
 An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
Romer AS 1925. Permian amphibian and reptilian remains described as Stephanospondylus. Journal of Geololgy 33: 447-463.
Stappenbeck R 1905. Uber Stephanospondylus n. g. und Phanerosaurus H. v. Meyer: Zeitschrift der Deutschen Geologischen Gesellschaft, v. 57, p. 380-437.

wiki/Proganochelys
wiki/Odontochelys

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