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.
