Among turtles, cases of “gigantism” occur mostly in pleurodiran Pelomedusoides and cryptodirans, but are infrequent
among pleurodiran chelids, which are mostly small-medium sized turtles. Yaminuechelys spp. are extinct South American
long-necked chelids (from the Late Cretaceous–early Paleocene of Patagonia, Argentina) with caparaces almost three
times larger than their extant sister taxon, Hydromedusa tectifera. Since evolutionary changes in size can be analyzed
based on growth dynamics, we studied growth strategies from an osteohistological point of view. We sampled both
extinct (Yaminuechelys maior) and extant (H. tectifera) species, in order to test hypotheses related to the mechanisms
involved in the macroevolution of size within this clade. For this purpose, thin sections of long bone (humerus and
femur) shafts of specimens of different ontogenetic stages for these species were prepared. The osteohistological study
reveals a similar growth dynamic in both taxa, with a poorly vascularized cortex dominated by parallel- fibered bone
and interrupted by lines of arrested growth (LAGs). The huge body size of Y. maior appears to be a consequence of the
prolongation of the growth phase, suggesting that it had a longer lifespan than H. tectifera, allowing to reach greater
sizes. In this way, and assuming that there is no displacement at the beginning of development (e.g., a delay in the earliest
stages of growth) in H. tectifera, the acquisition of a large size in Yaminuechelys would be explained by hypomorphosis
of the former or hypermorphosis of the latter, depending on the reconstruction of the ancestral condition of this clade.
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