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

Abstract

Abstract. Several British specimens of relatively complete and partial shells of small pleurosternid turtles, found in the Purbeck Limestone Group (Berriasian, Lower Cretaceous), are analysed in detail here. Despite having been found more than a century ago, most of them remained unpublished until now. Due to the scarce knowledge available to date about the small individuals of Pleurosternidae from the British record, their taxonomic status was doubtful. Thus, some authors proposed their attribution to a new but not defined taxon of small size, whereas others suggested that they could represent juvenile individuals of Pleurosternon bullockii, the adults of which were also found in the Purbeck Limestone Group but at different levels. Knowledge about the shell of the adult individuals of Pleurosternon bullockii has notably increased recently, abundant information on its intraspecific variability being currently available. In this context, a detailed study of the small specimens through qualitative and quantitative approaches is performed here. The results are evidence a significant range of shape variability because of ontogenetic development, as well as other types of intraspecific variation such as inter-individual variation. As a consequence, their attribution to juvenile individuals of Pleurosternon bullockii is justified, providing new data on the ontogenetic development of a basal form.

An alternative interpretation of small-bodied turtles from the “Middle Purbeck” of England as a new species of compsemydid turtle

“…The best-preserved small-bodied turtles from Beckles' Pit were recently figured by Guerrero and Pérez-García (2021b). Although we here disagree with their interpretation of this material representing juveniles of Pleurosternon bullockii, we fully agree with their anatomical observations.…”

“…In our estimation, the analyses of Guerrero and Pérez-García (2021b) are insufficient to suggest conspecificity of the material at hand. Instead, the small-bodied turtles can readily be diagnosed as a new species of extinct turtle using a series of discrete characters otherwise used for this purpose.…”

“…2). In the rare instance where referred material provides important insights, we refer the reader to the figures of Guerrero and Pérez-García (2021b).…”

“…More than a century later, Lapparent de Broin and Murelaga (1999) listed a number of characteristics that distinguish the small-bodied material from Beckles' collection from large-bodied Pleurosternon bullockii and suggested that the small-bodied form might represent a new species of compsemydid turtle, but they refrained from naming it, likely because this was not the focus of their study. Guerrero and Pérez-García (2021b) recently figured and described most of the small-sized turtles available from the Beckles' collections. Similar to Lydekker (1889a) and Lapparent de Broin and Murelaga (1999), Guerrero and Pérez-García (2021b) highlighted numerous differences between these small-sized turtles and Pleurosternon bullockii, but sided with Lydekker (1889a) by interpreting the differences as ontogenetic.…”

“…Guerrero and Pérez-García (2021b) recently figured and described most of the small-sized turtles available from the Beckles' collections. Similar to Lydekker (1889a) and Lapparent de Broin and Murelaga (1999), Guerrero and Pérez-García (2021b) highlighted numerous differences between these small-sized turtles and Pleurosternon bullockii, but sided with Lydekker (1889a) by interpreting the differences as ontogenetic.…”