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Fossil Record A palaeontological open-access journal of the Museum für Naturkunde

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Foss. Rec., 21, 109-118, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
03 Apr 2018
A case study of developmental palaeontology in Stereosternum tumidum (Mesosauridae, Parareptilia)
Constanze Bickelmann1 and Linda A. Tsuji2 1Museum für Naturkunde, Leibniz-Insitut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany
22477 Folkway Drive, Mississauga, ON L5L2J7, Canada
Abstract. Ontogenetic series of extinct taxa are rare. However, if preserved, fossil embryos and juveniles can provide evidence of developmental plasticity as related to ecological specialization. Here, we describe articulated and isolated juvenile material found in close association with an adult mesosaurid Stereosternum tumidum (MB.R.2089) from Lower Permian sediments in Brazil, housed in the collection of the Museum für Naturkunde Berlin. Stylopodial, zeugopodial, and autopodial elements are not yet completely ossified in the juveniles, as indicated by compression artifacts on the surface of the bone. These correspond to internal ossification processes, which have been demonstrated in other aquatic taxa. Quantitative analysis of measurements in juvenile and adult material reveals differing growth rates between limb elements: hind limb zeugopodia, which are massive and elongate in the adult as needed for propulsion, are already comparatively larger in the juvenile than the humeri, femora, and also the zeugopodia of the forelimb. This pattern differs from that seen in another extinct aquatic reptile, Hovasaurus boulei. Nevertheless, we attribute the accelerated growth rate or earlier onset of ossification to be a potential developmental pathway generating limb element variation in the adult present in 280 million year old mesosaurs, which are known for their fully aquatic lifestyle, in which the hind limbs play a more prominent role than the forelimbs.
Citation: Bickelmann, C. and Tsuji, L. A.: A case study of developmental palaeontology in Stereosternum tumidum (Mesosauridae, Parareptilia), Foss. Rec., 21, 109-118,, 2018.
Publications Copernicus
Short summary
Mesosaurs were reptiles that lived around 280 mya. They were the first land reptile group to return to the water. Study of a fossil from the Museum für Naturkunde, Berlin reveals a previously unobserved baby skeleton near the adult. This small skeleton can tell us about how these creatures grew; the lower segment of the hind limb is already proportionally larger than the upper hind limb and the lower and upper forelimbs. This may be because young mesosaurs needed to swim right from birth.
Mesosaurs were reptiles that lived around 280 mya. They were the first land reptile group to...