Osteoderms of the California Legless Lizard Anniella (Squamata: Anguidae) and Their Relevance for Considerations of Miniaturization

Anniella is a clade within Anguidae including small-bodied serpentiform lizards, previously hypothesized to be miniaturized relative to other parts of Anguidae. Ancestrally, Anguidae displays an extensive armor of dermal ossifications, but the presence or absence of osteoderms in Anniella was a source of some confusion in the past. We confirm that Anniella possesses osteoderms in the dermis deep to nearly every epidermal body scale, as well as remnant ossifications deep to the head scales. The body osteoderms display three basic components: an anterior plate bearing apparent growth ridges, a usually constricted neck portion with a distinct microstructure, and a branching portion divided into a variable number of processes. Under high magnification, collagen bundles similar to those on other lizard osteoderms are visible. The osteoderms of Anniella are reduced in their ossification relative to those of other Anguidae; they are thinner, occupy only the middle regions of the dermal scale component, and their posterior portions appear only as a series of fingerlike processes. Reduction in the osteoderms of Anniella constrasts with reinforced solidity of many of the skull elements, supporting the idea that modularity was involved in the evolution of miniaturization in this clade.