# The Fenestras Elisabeth Complex (Nova Structura) in the Parietal Bone of Plecotus auritus: Morphology, Topography, and Functional Significance

## TL;DR

This paper describes a new skull structure in brown long-eared bats that may help reduce weight and manage vibrations from echolocation.

## Contribution

The discovery of a novel fenestrated complex in the parietal bone of Plecotus auritus, with potential biomechanical and acoustic functions.

## Key findings

- The Fenestras Elisabeth complex consists of multiple small and larger fenestrations in a species-specific pattern.
- The complex may reduce skull weight while maintaining structural integrity and redistributing mechanical stresses from ear movements.
- Individual variability and lateral asymmetry suggest adaptive plasticity in this region of the skull.

## Abstract

The skulls of mammals are shaped by a balance between mechanical stability, sensory function, and weight reduction. In bats, which rely heavily on echolocation and highly mobile external ears, even small structural modifications of the skull may have important functional consequences. In this study, we describe a previously unrecognized fenestrated complex in the posterolateral region of the parietal bone of the brown long-eared bat (Plecotus auritus), which we name the Fenestras Elisabeth complex. Using micro-computed tomography, we show that this complex consists of multiple small openings arranged in a repeatable, species-specific pattern, together with a paired set of larger fenestrations and a centrally located depression. Although the number and shape of individual openings vary between specimens, the overall topographic organization of the complex is consistent across all examined skulls. We suggest that this fenestrated architecture may contribute to local lightening of the skull while maintaining structural integrity, and may also play a role in redistributing mechanical stresses and microvibrations associated with ear movements during echolocation. Our findings highlight how subtle skeletal modifications can reflect functional and evolutionary adaptations in bats.

Fenestrations of mammalian skull bones are rare and poorly understood, particularly within the parietal bone. In bats—a group characterised by advanced sensory specialisation and echolocation—superficial modifications of the cranial vault may have functional significance, yet their occurrence and organisation remain insufficiently documented. In this study, we describe an exceptional fenestrated complex in the posterolateral parietal bone of Plecotus auritus, comprising structural elements not previously recorded within Vespertilionidae. The aim of the study was to characterise in detail the morphology, topography, and variability of the surface structure termed Fenestras Elisabeth (nova structura), with particular emphasis on its relationship with the auditory region and its potential biomechanical–acoustic significance. The material consisted of ten skulls of P. auritus, examined using micro-CT scanning, 3D reconstruction, and qualitative analysis of fenestrated structures and their topographic relationships. Within the posterolateral parietal region, we identified an extensive and repeatable fenestrated complex comprising numerous fenestrae parietales Elisabeth, paired fenestrula Elisabeth, a central depression (recessus acousticus parietalis), and a bordering fissure (fissura occipitalis mastoidea, nova structura topographica). The complex exhibited a stable spatial organisation despite individual variation in the number and shape of the openings. All fenestrations were confined to the posterolateral zone, and the contact between the fenestrae and the fissura occipitalis mastoidea represented a diagnostic feature. Our analysis suggests that the Fenestras Elisabeth complex may be associated with combined biomechanical and acoustic constraints: (1) a biomechanical role—reducing strain in the parietal lamina during movements of the extremely mobile and elongated pinnae; and (2) an acoustic role—modulating micro-vibrations transmitted to the inner-ear structures. Individual variability and lateral asymmetry of fenestration patterns indicate a high degree of adaptive plasticity in this region of the skull. This study provides the first documentation of a large, structurally organised parietal-bone fenestration complex in Plecotus auritus, expanding current knowledge of bat cranial morphology and suggesting a likely functional significance for these previously unknown bony structures.