Variabilidad morfológica de la tortuga cretácica Pleurosternon bullockii en las VIII Jornadas de Salas de los Infantes

Varios de los trabajos presentados por integrantes del Grupo de Biología Evolutiva de la UNED en las pasadas VIII Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno trataron sobre el registro europeo de tortugas mesozoicas. Uno de ellos fue la comunicación sobre el estudio de la variabilidad morfológica presente en la tortuga Pleurosternon bullockii, procedente del Berriasiense (Cretácico inferior) de Gran Bretaña.

El análisis cualitativo y cuantitativo de su variabilidad morfológica han permitido establecer que caracteres son más variables y en función de qué factor/es (ontogenia, polimorfismos, dimorfismo sexual y anomalías), proporcionando una mejor comprensión de la variabilidad intraespecífica del caparazón de este taxón y una primera caracterización detallada.

El resumen es el siguiente:

Turtles are a very diverse group of reptiles, which shows a high degree of intraspecific morphological variability. Studies on this variability, and, therefore, those based on the comparisons between different species considering the variability of each of them, are still very scarce for the extinct species, since they are generally represented by a very limited number of specimens. Such studies are even more complex when morphometric characterizations try to be performed, due to this require a considerable amount of well-preserved individuals. In most cases, a relatively high number of well-preserved specimens is not available, especially for the Mesozoic taxa.

Pleurosternidae is a relatively diverse clade of North American and European Mesozoic turtles, known from the Late Jurassic to the end of the Early Cretaceous. Phylogenetic relationships among some members of this clade still arises controversy, due to the lack of information on several representatives. Furthermore, a relatively great range of intraspecific variability has been identified in this family. This caused problems in the interpretation of several characters, and the characterization of some taxa, as is the case of the type species of this clade. It is the European turtle Pleurosternon bullockii, especially represented in the British record, by abundant specimens from the Berriasian (Lower Cretaceous) levels of the Purbeck Limestone Group, found in several localities. In fact, a valid diagnosis for this species considering the current knowledge about this clade is not available, and numerous erroneous interpretations of the anatomy of its shell continue to be present in the literature. Thus, some of the encodings of this form in the matrices of morphological data are affected by these issues.

Knowledge about the European pleurosternids has been remarkably improved during the last years, especially by the description of new forms (e.g. Selenemys lusitanica, Riodevemys inumbragigas, Toremys cassiopeia), but also by the detailed study of poorly- known taxa (e.g. Dorsetochelys typocardium). Considering this remarkable increase in the knowledge of this clade, the so far unpublished detailed revision of the type species (i.e. Pleurosternon bullockii), started several years ago, could be is of great interest. Thus, the main results of this study are presented here.

To achieve with the proposed objective, more than fifty well-preserved shells and partial shells of Pleurosternon bullockii, from United Kingdom, have been analysed. This collection includes the specimens previously cited in the literature, many of them of high historical value, as well as other unpublished or poorly known. The qualitative and quantitative analysis of its morphological variability allow us to establish which characters are more variable, each of them being justified as caused by one or more specific factors (ontogeny, sexual dimorphism, individual variability and anomalies). Therefore, a better understanding of the intraspecific variability of the shell of this species and, for the first time, its detailed characterization, are provided.

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Más información:

• Referencia: Guerrero, A., Pérez-García, A. 2019. Analysis of the morphological variability of the Lower Cretaceous pleurosternid turtle Pleurosternon bullockii. Libro de resúmenes de las VIII Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno: 67-68.
• Imágenes: Selección de algunos ejemplares de Pleurosternon bullockii (arriba). Andrea Guerrero durante la presentación (centro). Diapositiva mostrando la diversidad de Pleurosternidae en Europa (abajo).