Hierarchical Microstructure of Tooth Enameloid in Two Lamniform Shark Species, Carcharias taurus and Isurus oxyrinchus.

Jana Wilmers,Miranda Waldron,Swantje Bargmann

doi:10.3390/nano11040969

Jana Wilmers, Miranda Waldron + Show 1 more

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https://doi.org/10.3390/nano11040969

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Abstract

Shark tooth enameloid is a hard tissue made up of nanoscale fluorapatite crystallites arranged in a unique hierarchical pattern. This microstructural design results in a macroscopic material that is stiff, strong, and tough, despite consisting almost completely of brittle mineral. In this contribution, we characterize and compare the enameloid microstructure of two modern lamniform sharks, Isurus oxyrinchus (shortfin mako shark) and Carcharias taurus (spotted ragged-tooth shark), based on scanning electron microscopy images. The hierarchical microstructure of shark enameloid is discussed in comparison with amniote enamel. Striking similarities in the microstructures of the two hard tissues are found. Identical structural motifs have developed on different levels of the hierarchy in response to similar biomechanical requirements in enameloid and enamel. Analyzing these structural patterns allows the identification of general microstructural design principles and their biomechanical function, thus paving the way for the design of bioinspired composite materials with superior properties such as high strength combined with high fracture resistance.

Highlights

The most pronounced difference between teeth of the two species is the presence of small tricuspids in C. taurus not found in I. oxyrinchus
The lowest level of the hierarchy, Level 0, is the individual nanoscale crystallite. Hard tissues such as shark enameloid are biological composites of nanoscale mineral crystals arranged in intricate hierarchical patterns interspersed with only minor amounts of remnant protein
Fluorapatite nanocrystallites (Level 0) are arranged in bundles (Level II) that themselves are arranged in a layered pattern over the enameloid cover (Level IV)

Summary

Introduction

The teeth are arranged in multiple rows behind each other in the shark’s jaws (Figure 1) and exhibit a variety of shapes and sizes among different species, ranging from flattened domes over needles to triangular cutting tools with sharp, serrated edges. Even between closely related species, a wide variety of tooth shapes can be found [1] and may be used to identify species [2]. These morphological variations have been attributed to differences in feeding behavior and, mechanical loads [3,4,5]. Optimal functionality is further guaranteed by regular shedding and replacement of the teeth. For spotted ragged-tooth sharks (Carcharias taurus), an average tooth loss rate of 1.06 teeth per day was identified [6], which means an individual spotted ragged-tooth shark will shed over 13,500 teeth in a lifetime

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