Semicircular canals inAnolislizards: ecomorphological convergence and ecomorph affinities of fossil species

Abstract

Anolis lizards are a model system for the study of adaptive radiation and convergent evolution. Greater Antillean anoles have repeatedly evolved six similar forms or ecomorphs: crown-giant, grass-bush, twig, trunk, trunk-crown and trunk-ground. Members of each ecomorph category possess a specific set of morphological, ecological and behavioural characteristics which have been acquired convergently. Here we test whether the semicircular canal system—the organ of balance during movement—is also convergent among ecomorphs, reflecting the shared sensory requirements of their ecological niches. As semicircular canal shape has been shown to reflect different locomotor strategies, we hypothesized that each Anolis ecomorph would have a unique canal morphology. Using three-dimensional semilandmarks and geometric morphometrics, semicircular canal shape was characterized in 41 Anolis species from the Greater Antilles and the relationship between canal shape and ecomorph grouping, phylogenetic history, size, head dimensions, and perch characteristics was assessed. Further, canal morphology of modern species was used to predict the ecomorph affinity of five fossil anoles from the Miocene of the Dominican Republic. Of the covariates tested, our study recovered ecomorph as the single-most important covariate of canal morphology in modern taxa; although phylogenetic history, size, and head dimensions also showed a small, yet significant correlation with shape. Surprisingly, perch characteristics were not found to be significant covariates of canal shape, even though they are important habitat variables. Using posterior probabilities, we found that the fossil anoles have different semicircular canals shapes to modern ecomorph groupings implying extinct anoles may have been interacting with their Miocene environment in different ways to modern Anolis species.

The internal cranial anatomy of Champsosaurus (Choristodera: Champsosauridae): Implications for neurosensory function

“…inferring ecology from the endosseous labyrinth. The following methods for inferring the ecology of Champsosaurus based on the morphology of the semicircular canals were modified from Dickson et al 13 . The morphologies of the semicircular canals of CMN 8920 and CMN 8919 were compared to 59 species (61 specimens) of Lepidosauria and Archosauromorpha, and Youngina as an outgroup taxon (see Supplementary Tables S6-S8 for lists of species and specimen numbers).…”

“…Many studies have landmarked the centerline of the semicircular canals 9,13 ; however, Mennecart and Costeur 37 noted that landmarking the centerline does not take canal thickness into account, and that landmarks should instead be placed on the inner-and outer-most surfaces of the canals. This, unfortunately, cannot be done in this study because some taxa (including Champsosaurus) have some canals that lack a defined inner surface due to confluence with the pars inferior.…”

“…Several studies have also demonstrated a strong phylogenetic signal in the morphology of semicircular canals in many different lineages 13 . This has important implications for Champsosaurus because the phylogenetic position of Choristodera within Neodiapsida is not well understood, where recent phylogenies have placed Choristodera in a polytomy with Archosauromorpha and Lepidosauromorpha 21,22 .…”

Bony labyrinth morphometry reveals hidden diversity in lungless salamanders (Family Plethodontidae): Structural correlates of ecology, development, and vision in the inner ear

“…Further, structures related to sensory perception of environmental stimuli demonstrate a strong form-function relationship and thus are useful for investigating ecomorphological variation (Bock and von Wahlert 1965). Additionally, the size and shape of the semicircular canals are correlated with locomotor behavior in a broad range of vertebrate taxa that perform agile and/or spatially complex behaviors, including arboreal Anolis lizard ecomorphs (Dickson et al 2017), fossorial caecilian amphibians (Maddin and Sherratt 2014), gliding snakes (Boistel et al 2011), andfossorial, aquatic, andarboreal mammals (Lindenlaub et al 1995;Spoor et al 2002;Spoor 2003;Pfaff et al 2015). For example, fossorial (burrowing) mammals (Burda et al 1992;Mason 2004), lizards (Toerien 1963), snakes (Yi and Norell 2015), and amphibians (Maddin and Sherratt 2014) exhibit repeated, parallel evolution of hypertrophic auditory structures believed to contribute to enhanced seismic sensitivity to ground vibrations.…”

“…For example, fossorial (burrowing) mammals (Burda et al 1992;Mason 2004), lizards (Toerien 1963), snakes (Yi and Norell 2015), and amphibians (Maddin and Sherratt 2014) exhibit repeated, parallel evolution of hypertrophic auditory structures believed to contribute to enhanced seismic sensitivity to ground vibrations. Additionally, the size and shape of the semicircular canals are correlated with locomotor behavior in a broad range of vertebrate taxa that perform agile and/or spatially complex behaviors, including arboreal Anolis lizard ecomorphs (Dickson et al 2017), fossorial caecilian amphibians (Maddin and Sherratt 2014), gliding snakes (Boistel et al 2011), andfossorial, aquatic, andarboreal mammals (Lindenlaub et al 1995;Spoor et al 2002;Spoor 2003;Pfaff et al 2015).…”