Fossil Record 21(1): 109-118, doi: 10.5194/fr-21-109-2018
A case study of developmental palaeontology in Stereosternum tumidum (Mesosauridae, Parareptilia)
expand article infoConstanze Bickelmann, Linda A. Tsuji§
‡ Museum für Naturkunde, Leibniz-Insitut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany§ 2477 Folkway Drive, Mississauga, ON L5L2J7, Canada
Open Access

Ontogenetic series of extinct taxa are rare. However, if preserved, fossilembryos and juveniles can provide evidence of developmental plasticity asrelated to ecological specialization. Here, we describe articulated andisolated juvenile material found in close association with an adultmesosaurid Stereosternum tumidum (MB.R.2089) from Lower Permiansediments in Brazil, housed in the collection of the Museum fürNaturkunde Berlin. Stylopodial, zeugopodial, and autopodial elements are notyet completely ossified in the juveniles, as indicated by compressionartifacts on the surface of the bone. These correspond to internalossification processes, which have been demonstrated in other aquatic taxa.Quantitative analysis of measurements in juvenile and adult material revealsdiffering growth rates between limb elements: hind limb zeugopodia, which aremassive and elongate in the adult as needed for propulsion, are alreadycomparatively larger in the juvenile than the humeri, femora, and also thezeugopodia of the forelimb. This pattern differs from that seen in anotherextinct aquatic reptile, Hovasaurus boulei. Nevertheless, weattribute the accelerated growth rate or earlier onset of ossification to bea potential developmental pathway generating limb element variation in theadult present in 280 million year old mesosaurs, which are known for theirfully aquatic lifestyle, in which the hind limbs play a more prominent rolethan the forelimbs.