Abstract
Squamate reptiles are considered to exhibit indeterminate growth. Nevertheless, current literature disputes the available definitions of this growth type, presents new theoretical models, and questions its universality in cold-blooded vertebrates. We have followed up on our previous research employing micro-CT to explore growth plate cartilage (GPC) in the epiphysis of long bones, which is responsible for longitudinal skeletal growth by the endochondral ossification process. We focused on numerous and highly diversified group of the Iguania clade comprising Acrodonta (agamas and chameleons) and Pleurodonta (“iguanas”). We recorded the absence of GPC in most of the examined adult Pleurodonta specimens and interpret it as an irreversible arrest of skeletal growth. This finding clearly rejects the universality of indeterminate growth in lizards. On the other hand, we found apparent GPC preservation in most of the adult specimens belonging to Acrodonta. This suggests a preserved ability to continue body growth throughout most of their life. We discuss the uncovered disparity between Acrodonta and Pleurodonta and emphasize the importance of GPC degradation timing.
Highlights
Postnatal skeletal growth connected with the increase in skeletal size is traditionally divided into the determinate and indeterminate type[1]
The presence of the growth plate cartilage was scored in the following www.nature.com/scientificreports cases: (1) epiphysis and diaphysis separated by a wide radio-translucent band corresponding to non-calcified growth cartilage in the area near metaphysis; (2) the trabeculae not extended into the metaphysis; and (3) the suture between the epiphysis and the metaphysis was present (Figs. 2a, SI2, SI3)
The absence of the growth plate cartilage was scored when: (1) the radio-translucent band corresponding to non-calcified growth cartilage was absent, which is a sign of growth plate senescence, degradation or even its complete absence; (2) the suture between the epiphysis and the metaphysis was not present; and (3) the whole proximal part of the femur was occupied by a continuous network of bone trabeculae (Figs. 2b, SI4, SI5)
Summary
Postnatal skeletal growth connected with the increase in skeletal size is traditionally divided into the determinate and indeterminate type[1]. Except for very old senescent individuals, retained GPC nearly throughout their entire life We interpreted this dual pattern of body growth in monitor lizards as an extreme case of heterochrony[22]. We focused on Iguania (sensu33) comprising the subclades of Acrodonta (chameleons and agamas) and Pleurodonta (Iguanidae and related families, hereafter called “iguanas”) Those two crown subclades of lizards (for phylogenetic relationships see Fig. 1) have undergone convergent evolution. The goals of this study were (1) to employ advanced imaging techniques to evaluate the presence/absence of GPC in examined specimens; (2) to score the species according to the pattern of GPC persistence in adults; (3) to test the universality of the presumed indeterminate growth in this clade, and (4) to discuss the putative life-history parameters responsible for the body growth pattern
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