Determinate growth is predominant and likely ancestral in squamate reptiles

Body growth is typically thought to be indeterminate in ectothermic vertebrates. Indeed, until recently, this growth pattern was considered to be ubiquitous in ectotherms. Our recent observations of a complete growth plate cartilage (GPC) resorption, a reliable indicator of arrested skeletal growth, in many species of lizards clearly reject the ubiquity of indeterminate growth in reptiles and raise the question about the ancestral state of the growth pattern. Using X-ray micro-computed tomography (µCT), here we examined GPCs of long bones in three basally branching clades of squamate reptiles, namely in Gekkota, Scincoidea, and Lacertoidea. A complete loss of GPC, indicating skeletal growth arrest, was the predominant finding. Using a dataset of 164 species representing all major clades of lizards and the tuataras, we traced the evolution of determinate growth on the phylogenetic tree of Lepidosauria. The reconstruction of character states suggests that determinate growth is ancestral for the squamate reptiles (Squamata) and remains common in the majority of lizard lineages, while extended (potentially indeterminate) adult growth evolved several times within squamates. Although traditionally associated with endotherms, determinate growth is coupled with ectothermy in this lineage. These findings combined with existing literature suggest that determinate growth predominates in both extant and extinct amniotes.

Methods
We analyzed 194 femoral bones from lizards belonging to three clades of Squamata (Gekkota, Scincoidea, and Lacertoidea). We analyzed the growth plate cartilage (GPC) in the proximal part of the femur. We employed X-ray micro-computed tomography (µCT) and micro-radiography (µRTG) for vizualization of GPC. We scored the GPC state in a binary fashion, as either present (1) or absent (0). Ancestral state reconstruction was performed in R using the Hidden State Speciation and Extinction (HiSSE) model implemented in the R package hisse. Additionally, we performed the ancestral states reconstruction using maximum parsimony, as implemented in the R package castor. The reconstructed states were plotted using the R package phytools.