Shell variability in the stem turtlesProterochersisspp.

Abstract

BackgroundTurtle shells tend to exhibit frequent and substantial variability, both in bone and scute layout. Aside from secondary changes, caused by diseases, parasites, and trauma, this variability appears to be inherent and result from stochastic or externally induced flaws of developmental programs. It is, thus, expected to be present in fossil turtle species at least as prominently, as in modern populations. Descriptions of variability and ontogeny are, however, rare for fossil turtles, mainly due to rarity, incompleteness, damage, and post-mortem deformation of their remains. This paper is an attempt at description and interpretation of external shell variability in representatives of the oldest true turtles, Proterochersis robusta and Proterochersis porebensis (Proterochersidae, the sister group to all other known testudinatans) from the Late Triassic (Norian) of Germany and Poland.MethodsAll the available shell remains of Proterochersis robusta (13 specimens) and Proterochersis porebensis (275 specimens) were studied morphologically in order to identify any ontogenetic changes, intraspecific variability, sexual dimorphism, and shell abnormalities. To test the inferred sexual dimorphism, shape analyses were performed for two regions (gular and anal) of the plastron.ResultsProterochersis spp. exhibits large shell variability, and at least some of the observed changes seem to be correlated with ontogeny (growth of gulars, extragulars, caudals, and marginals, disappearance of middorsal keel on the carapace). Several specimens show abnormal layout of scute sulci, several others unusual morphologies of vertebral scute areas, one has an additional pair of plastral scutes, and one extraordinarily pronounced, likely pathological, growth rings on the carapace. Both species are represented in a wide spectrum of sizes, from hatchlings to old, mature individuals. The largest fragmentary specimens of Proterochersis porebensis allow estimation of its maximal carapace length at approximately 80 cm, while Proterochersis robusta appears to have reached lower maximal sizes.DiscussionThis is the second contribution describing variability among numerous specimens of Triassic turtles, and the first to show evidence of unambiguous shell abnormalities. Presented data supplement the sparse knowledge of shell scute development in the earliest turtles and suggest that at least some aspects of the developmental programs governing scute development were already similar in the Late Triassic to these of modern forms.

Ontogenetic development of the European basal aquatic turtle <i>Pleurosternon bullockii</i> (Paracryptodira, Pleurosternidae)

“…Thus, these midsagittal ridges progressively decrease as ontogeny progresses, being absent in the mature stage. In fact, minor modifications in the carapace ornamentation pattern comparable with those recognized here for P. bullockii have been recognized as resulting from the ontogenetic variation in several species of extinct and extant turtles (Hay, 1908;Zangerl, 1953;de Broin, 1977;Somers et al, 2000;Danilov and Parham, 2008;Jansen and Klein, 2014;Cadena, 2015;Szczygielski et al, 2018).…”

“…As previously observed among representatives of other turtle groups (e.g. Sinemydidae, Proterochersidae or Baenidae; Shao et al, 2017;Smith et al, 2017;Szczygielski et al, 2018;Li et al, 2019), in shell elements that are ontogenetically dependent, the magnitude of shape change, as well as the growth rate, varies at the specific level. Therefore, in general, the ontogenetic development of P. bullockii is not shared with that known for any other pleurosternid.…”

“…These specimens have been compared with the characters of the shell that are ontogenetically variable in juveniles of other extinct (e.g. Brinkman et al, 2013;Jansen and Klein, 2014;Szczygielski et al, 2018;Li et al, 2019) and extant turtles (e.g. Somers et al, 2000;Pritchard, 2008;Fish and Stayton, 2014;Pate and Salmon, 2017).…”

New insights on the anatomy and ontogeny of the largest extinct freshwater turtles

“…Among turtles, ontogenetic changes in fossils have been documented in Mesozoic and Cenozoic taxa, including modifications of the anterior scutes of the carapace in Proterochesis spp. ( Szczygielski et al., 2018 ); progressive skull stretching and enlargement of posterior and lateral emargination in the Late Cretaceous Bauremys elegans from Brazil ( Mariani and Romano 2017 ); and reduction of keels in the neural series of the Eocene Neochelys franzeni from Germany ( Cadena 2015 ). None of these taxa represent giant turtles (the largest-bodied species in their ecological category or clade, following Vermeij, 2016 ).…”

Analysis of the Sexual Variability in the Shell of the Spanish Upper Cretaceous Turtle Dortoka Vasconica (Pan-Pleurodira, Dortokidae)

“…Turtles exhibit a wide assemblage of sexually dimorphic characters that can manifest through sexual shape dimorphism (Kuchling, 1997;Pritchard, 2008;Bonnet et al, 2010) or sexual size dimorphism (the most studied form of dimorphism between the sexes; Leuteritz & Gantz, 2013;Regis & Meik, 2017). For the extinct forms, only a few of these sexually dimorphic characters are available (Bonnet et al, 2010;Cadena et al, 2013;Limaverde et al, 2020;Szczygielski et al, 2018), which considerably complicates the evaluation of their sexual dimorphism. In this study, the quantitative approach demonstrated an evident sexual shape dimorphism in the morphology of the anal notch of Dortoka vasconica, the two sexual morphotypes previously observed by Pérez- being characterized.…”