Patterns of growth in giant extinct reptiles through a microanatomical perspective.

MARIA EUGENIA PEREYRA; PAULA BONA; IGNACIO A. CERDA; BÁRBARA DESÁNTOLO; MARCOS JANNELLO; LEONARDO S. FILIPPI

Simposio; 5th International Symposium on Palaeohistology?, 2019, Julio de 2019, Cape Town, South Africa.; 2019

Several extant reptile lineages (e.g., turtles, crocodylians) have reached giant sizes throughout their evolutionary history. In this way, extinct taxa closely related to extant chelids (Hydromedusa) and alligatorids (Caiman) have developed sizes that exceeded at least three times than their relative current forms. We perform an osteohistological study to analyze topics related to growth rate and evolutionary mechanisms involved in the large sized forms of these two taxa. For this propose, thin sections of the shaft of fossil log bones material (humerus and femur) of Caiman sp. (MACN-5687), Yaminuechelys maior and Yaminuechelys aff. maior (MLP-14-9-23-1 and MAU-Pv-PR-455) were analyzed. Caiman sp. material is an almost complete right humerus (~18cm length) which belongs to a huge specimen that comes from The Upper Miocene of North East Argentina (Entre Ríos province). Fossil materials of Chelidae correspond to a humerus of Yaminuechelys maior (~18cm length) and a femur of Yaminuechelys aff. maior (~13cm length) form the Lower Paleocene of Patagonia (Argentina). Yaminuechelys is an extinct genus recovered as the sister taxa of the extant genus Hydromedusa in almost all the recent phylogenetic analysis. Thin sections of extant material, a juvenile and an adult of Caiman latirrostris (humerus length of 5.7cm and 10.8 cm respectively) and Hydromedusa tectifera (humerus length of 1.8 cm and 4.8 cm respectively and femur length of 2.1 cm and 4.9 cm respectively), were also analyzed. In the case of the extant turtle parallel-fibered bone is the main tissue (with the fibers arranged concentrically) with low vascularization in juvenile and practically avascular in adult. In the middle cortex of Yaminuechelys aff. maior. highly vascularized tissue was observed, with the collagen fibers arranged in different orientations; however, Yaminuechelys maior has low vascularization and collagen fibers are arranged concentrically. On the other hand, in juvenile and adult Caiman latirostris the matrix is parallel-fibered bone type (with concentric collagen fibers). Only in adults, cyclical growth marks are identified. In Caiman sp., most of the matrix is the same asthe observed in adult form of Caiman latirostris; however, two distinct regions were also observed in the inner and outer cortex. In these regions, the collagen fibers are perpendicular with respect to the axis of the bone. These results suggest that growth rates were not significantly different between extant taxa and their correspondent fossil representatives. Therefore, large body sizes in fossil forms appear to be achieved by a prolongation of the growth phase during their lives. Additionally, Yaminuechelys aff. maior reveals a higher growthrate during the early ontogeny than the deduced for Hydromedusa. This particular feature possibly plays a very important role for the large body size achievement of this taxon. Although, the growth rate of Caiman sp. appears to be relatively constant, the accelerated early phase of development reported in Yaminuechelys cannot be ruled out in this archosaur. Rapid growth pulses recorded in Caiman sp. can also improve the reaching of large sizes.