Abstract
The uppermost Jurassic to lowermost Cretaceous stem turtle Pleurosternon bullockii is the pleurosternid (Testudinata, Paracryptodira) known by the largest number of specimens worldwide, composing the largest European collection of Lower Cretaceous complete and partial shells for a turtle taxon. The availability of numerous specimens as well as their generally good preservation allowed for recent detailed characterization of the shell of this species, including states that are variable at the intraspecific level (individual variability, sexual dimorphism, and ontogenetic development). However, extreme cases of morphological variation corresponding to anomalies have not been addressed in detail, neither for P. bullockii nor for any other member of Paracryptodira. In this context, the study of several shell anomalies in P. bullockii is carried out here. Fourteen specimens showing anomalies are recognized and examined here to determine the frequency and distribution of these shell anatomical deviations. All these anomalies are described and figured. The morphogenetic cause of each of them is discussed. As a consequence, a relatively broad spectrum of anomalies is reported for P. bullockii. None of the anomalies seem to present negative consequences for vital activities of the specimens since none compromised the main functions of the shell.
Highlights
Turtles are a highly specialized group of reptiles, characterized by the presence of a shell composed by osseous plates, generally covered by keratinous scutes
Turtles show relatively high individual variability regarding the frequency of abnormal variants of the shell elements, generally corresponding to anatomical deviations originating during the embryonic development [3,4,6,7]
The large availability of specimens as well as their good preservation has recently allowed for the characterization of the intraspecific variation in the species
Summary
Turtles are a highly specialized group of reptiles, characterized by the presence of a shell composed by osseous plates, generally covered by keratinous scutes This mosaic of shell elements is subjected to a relatively high phylogenetic stability, but a significant degree of intraspecific variability can be recognized [1,2,3,4,5]. The information and, knowledge about this type of variation in extinct taxa are relatively scarce In this context, a detailed study of numerous shell anomalies identified for the European extinct turtle Pleurosternon bullockii (Owen, 1842) [25] is carried out here. The presence of additional shell elements corresponds to one of the most frequent abnormal variants identified here for Pleurosternon bullockii (Figures 1A–D and 2C–E), which is recognized for five different shell components (i.e., neural plates, costal plates, epiplastral plates, vertebral scutes, and gular-extragular scutes)
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