Abstract
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.
Highlights
The study of fossilized ontogenies is largely confined to postnatal stages with ossified skeletal elements, due to preservational factors
We describe a remarkable specimen containing an adult Stereosternum tumidum next to juvenile material from Lower Permian sediments in Brazil; a specimen long neglected in the fossil collections at the Museum für Naturkunde Berlin
The secondarily fully aquatic parareptilian clade Mesosauridae is renowned for an exceptional fossil record including the preservation of multiple growth stages, tail regeneration, along with the earliest report ofviviparity in amniotes (Rieppel, 1993; Delfino and Sánchez-Villagra, 2010; Piñeiro et al, 2012a)
Summary
The study of fossilized ontogenies is largely confined to postnatal stages with ossified skeletal elements, due to preservational (taphonomic) factors If preserved, they provide valuable insight into the life history of extinct taxa, as well as evolutionary trajectories. Ossification sequences in fossils are available for some “fish” (Cloutier, 2010), temnospondyl and lepospondyl amphibians (Fröbisch, 2008; Fröbisch et al, 2010, 2015), “younginiforms” (sensu Bickelmann et al, 2009) (Currie, 1981; Caldwell, 2002), mosasaurs (Caldwell, 2002), sauropterygians (Hugi and Scheyer, 2012), ichthyosaurs (Caldwell, 1997), sauropsids (Delfino and Sánchez-Villagra, 2010) and mammals (Sánchez-Villagra, 2010) Most of this information is derived from secondarily aquatic taxa, which is the result of a taphonomic bias (Fröbisch et al, 2010). Gaining a comprehensive picture of changes in development and growth in extinct (fossil) taxa in addition to what we know about development in modern animals, as approached in this and similar studies, contributes to our understanding of evolution, developmental trajectories, life history strategies, and more (Fröbisch et al, 2010)
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