The sclerotic ring of squamates: an evo-devo-eco perspective

The sclerotic ring consists of several bones that form in the sclera of many reptiles. This element has not been well studied in squamates, a diverse order of reptiles with a rich fossil record but debated phylogeny. Squamates inhabit many environments, display a range of behaviours, and have evolved several different body plans. Most importantly, many species have secondarily lost their sclerotic rings. This research investigates the presence of sclerotic rings in squamates and traces the lineage of these bones across evolutionary time. We compiled a database on the presence/absence of the sclerotic ring in extinct and extant squamates and investigated the evolutionary history of the sclerotic ring and how its presence/absence and morphology is correlated with environment and behaviour within this clade. Of the 400 extant species examined (59 families, 214 genera), 69% have a sclerotic ring. Those species that do not are within Serpentes, Amphisbaenia, and Dibamidae. We find that three independent losses of the sclerotic ring in squamates are supported when considering both evolutionary and developmental evidence. We also show that squamate species that lack, or have a reduced, sclerotic ring, are fossorial and headfirst burrowers. Our dataset is the largest squamate dataset with measurements of sclerotic rings, and supports previous findings that size of the ring is related to both environment occupied and behaviour. Specifically, scotopic species tend to have both larger inner and outer sclerotic ring apertures, resulting in a narrower ring of bone than those found in photopic species. Non-fossorial species also have a larger sclerotic ring than fossorial species. This research expands our knowledge of these fascinating bones; with further phylogenetic analyses scleral ossicles could become an extremely useful character trait for inferring the behaviour of fossil squamates.