Enameloid Microstructure Research Articles

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

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.

New chondrichthyans characterised by cladodont-like tooth morphologies from the Early Cretaceous of Austria, with remarks on the microstructural diversity of enameloid

ABSTRACT Cladodontomorphii represents an archaic clade of chondrichthyan fishes characterised by distinct tooth morphologies referred to as the cladodont type. This group of cartilaginous fishes first occurred during the early Palaeozoic Era as revealed from the fossil record and were long thought to have gone extinct at the Permian-Triassic mass extinction event. However, a recently reported chondrichthyan tooth assemblage from the Early Cretaceous of France suggests that cladodontomorphs might have survived the catastrophic events at the Permian-Triassic boundary, probably by occupying deep-sea refuge environments. Here, we describe two new chondrichthyan genera based on isolated teeth recovered from Valanginian (Early Cretaceous) deep-water deposits of Austria, including a total of three new species tentatively assigned to the cladodontomorph families Falcatidae (Cretacladoides ogiveformis gen. et sp. nov. and C. noricum sp. nov.) and Ctenacanthidae (Natarapax trivortex gen. et sp. nov.). In addition, an enameloid microstructure analysis had led to the identification of a distinct multilayered enameloid including a parallel-, tangled-, and radial-bundled enameloid, whose phylogenetic distribution within Chondrichthyes is here discussed in detail, leading to the conclusion that the herein described cladodont-like taxa, together with those reported from the French Early Cretaceous might be closely related to ancient Palaeozoic taxa. The ZooBank electronic publication LSID is: urn:lsid:zoobank.org:pub:C06FD718-F54F-4C57-A45E-8C0D7CC8EB83

Dental Structure of the Late Cretaceous (Maastrichtian) Guitarfish (Neoselachii: Batoidea)Myledaphus pustulosusfrom the Hell Creek Formation of Garfield County, Montana

In this study, we examine surface collected teeth of Myledaphus pustulosus from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of Garfield County, Montana. Macroscopic analysis of teeth shows that 54% of the teeth have pronounced attritional wear due to a durophagous diet. A higher percentage of unworn teeth than worn teeth (64% vs 27%) have acid damage due to predation or scavenging. Ultrastructural analysis of whole and sectioned teeth show that the enameloid of the teeth is 100–125 µm thick and is composed of a single crystallite enameloid in which apatite crystallites show no preferred orientation. Orthodentine in the tooth crown is composed of dense material with numerous canals for odontoblast processes which extend nearly to the tooth surface. Dentinal tubules surround odontoblasts most of the way through the enameloid. The enameloid microstructure of M. pustulosus is consistent with the pattern seen in other batoids.

On the enameloid microstructure of Archaeobatidae (Neoselachii, Chondrichthyes)

In this study, we present new data regarding the enameloid microstructure of the oldest batoid family, Archaeobatidae. First, all the teeth were etched superficially with HCl 10% for 5 s and photographed in the SEM. Afterwards, the same teeth were embedded in Canadian Balsam, polished and etched again with HCl 10% in order to reveal the enameloid microstructure before being photographed a second time. The enameloid layer of Archaeobatidae consists on a superficial single crystallite enameloid (SCE) with a parallel bundled enameloid (PBE) in all the taxa studied, but only in Toarcibatis and Cristabatis there exists a tangled bundled enameloid (TBE) under it. The structural complexity and diversity found in Archaeobatidae are comparable to that recently described in others fossil batoids. Our data suggest a general trend to “simplification” in batoid enameloid up till the homogeneous single crystallite enameloid that is present in the majority of current batoids; which contrasts with the increasing structural complexity present in selachimorphs.

New insights into the enameloid microstructure of batoid fishes (Chondrichthyes)

Chondrichthyan teeth are capped with a hypermineralized tissue known as enameloid. Its microstructure displays a hierarchical organization that has increased in structural complexity from a homogenous single-crystallite enameloid (SCE) in early Chondricthyans to the complex multilayered enameloid found in modern sharks (consisting of bundles of crystallites arranged in intriguing patterns). Recent analyses of the enameloid microstructure in batoid fishes, focused on Myliobatiformes and fossil taxa, point to the presence of a bundled (or fibred) multilayered enameloid, a condition proposed as plesiomorphic for Batoidea. In this work, we provide further enameloid analysis for a selection of taxa covering the phylogeny of batoids. Our SEM analysis shows a superficial layer of SCE, where individualized crystallites are clearly discernable, capping the teeth in most of the species studied. A bundled double-layered enameloid was found only in a Rhinoidei, Rhina ancylostoma Bloch & Schneider, 1801. We conclude that the most widespread condition among extant batoids is a monolayer SCE lacking microstructural differentiation, probably plesiomorphic at least for crown batoidea. We suggest that the complex bundled enameloid present in other batoids is a convergent character that has appeared repeatedly during the evolution of batoids, probably as a mechanical adaptation towards moderate durophagous diets.

Chondrichthyan tooth enameloid: past, present, and future

Enameloid is a hard mineralized tissue covering chondrichthyan and actinopterygian teeth. Over the past 40 years, it has been extensively studied in various extinct and extant sharks, leading to the broad use of microstructural characters to differentiate between hybodont and neoselachian teeth. However, the chondrichthyan taxic diversity is disproportionately high compared to the number of taxa explored for enameloid microstructure, and the generalization of these few observations to the whole group is problematic. Indeed, many other groups, in particular modern rays and skates, have been completely overlooked, and almost nothing is known about their tooth histology. Furthermore, the recent discovery of typical neoselachian character in cladodontomorph sharks teeth clearly indicates that we have had an over-simplified perception of the chondrichthyan enameloid distribution, which put into question the previously proposed evolutive scenarios dealing whith this tissue. We propose a brief historical overview of the study and understanding of chondrichthyan enameloid diversity and briefly discuss preparation issues encountered when dealing with the study of chondrichthyan hypermineralized tissues. Then, the variation of enameloid microstructures encountered in ctenacanthiforms, hybodonts, selachimorphs, and batomorphs is explored, summarized, and discussed. Although the full extent of the diversity and variability of the enameloid microstructure in many of these groups and others remains to be fully determined, we are able to show that most possess a much more complex enameloid microstructure than expected, and propose a revised and more fitting chondrichthyan enameloid terminology, based on the recognition of two main units: an external Single Crystallite Enameloid (SCE) and an internal Bundled Crystallite Enameloid (BCE). Our study reveals new insights in the understanding of character distribution among batomorphs and sets a framework for tackling global chondrichthyan tooth enameloid evolution. © 2015 The Linnean Society of London

Simplification of the enameloid microstructure of large stingrays (Chondrichthyes: Myliobatiformes): a functional approach

The microstructure of neoselachian teeth is characterized by a triple-layered enameloid, despite its purported absence in batoids since their appearance during the early Jurassic. This feature is thought to have been secondarily lost, as an adaptation to durophagy in batoids. The monophyletic Myliobatiformes are an ecologically diverse clade of batoids that includes very specialized taxa such as the filter-feeding mobulids and the durophagous myliobatids. Their diversity and recent evolutionary history (Palaeogene) make them an ideal model for assessing adaptive constraints that influence enameloid microstructure. This first study focusing only on batoid enameloid shows a very reduced single-layered enameloid in the derived filter feeders. However, most other taxa show a double-layered enameloid, which is probably plesiomorphic for batoids. It is concluded that the triple-layered enameloid is not a synapomorphy of the neoselachii as a whole, but of Selachimorpha only, and that diet specialization in derived myliobatoids indeeds impact their enameloid microctructure, which is confirmed by their fossil diversity. © 2013 The Linnean Society of London

Enameloid microstructure of some Cretaceous Hexanchiformes and Synechodontiformes (Chondrichthyes, Neoselachii): New structures and systematic implications

Modern neoselachian sharks may be separated from more basal relatives by the presence of tooth enameloid comprising three layers. Although enameloid microstructure studies were mostly used in the aim of differentiating supposed basal neoselachians from hybodonts, differences in the enameloid organization among neoselachians have been recognized suggesting the potential for use of enameloid microstructure as a phylogenetic tool within the neoselachian sharks. The enameloid microstructure of five taxa of neoselachian sharks belonging to two orders, the Hexanchiformes and Synechodontiformes, has been studied. The Hexanchiformes are a monophyletic order with extant representatives, whereas the extinct Synechodontiformes have been considered as monophyletic, paraphyletic, or polyphyletic by different authors. This study has revealed numerous new enameloid microstructures such as amalgamated crystallites in the internal units [parallel-bundled enameloid (PBE) and tangled-bundled enameloid], cavities at the shiny-layered enameloid (SLE)/PBE boundary, radial furrows in the PBE, as well as different original organizations at the base of the crown and in the PBE at the level of the cutting edges. Pachyhexanchus pockrandti (Hexanchiforme) and Welcommia bodeuri have the most of the features in common and they share more characters with those of Paraorthacodus sp. and Sphenodus sp. than with Synechodus sp.

Vertebrate microremains from the Early Cretaceous of southern Tunisia

Microremains of various sharks, actinopterygians and crocodiles have been recovered from two sites in the Douiret Formation and three sites in the Aïn el Guettar Formation in southern Tunisia. The presence of an actinistian is also suggested based on histological study of hemisegments of lepidotrichia. Convergence in dental enameloid microstructure between neoselachian sharks and actinopterygians sharing a tearing dentition is also documented. The vertebrate assemblage of the Douiret Formation suggests a pre-Aptian age for this formation and the presence of Bernissartia in the Aïn el Guettar Formation confirms faunal exchange between Africa and Europa during the Early Cretaceous.

Enameloid microstructure of the serrated cutting edges in certain fossil carcharhiniform and lamniform sharks

The triple-layered enameloid organization of neoselachian teeth has proven to be a reliable systematic character of the group. This study uses scanning electron microscopy to investigate the orientation of the parallel enameloid bundles in the area of the serrated cutting edges in certain fossil elasmobranchs. The examined teeth come from two Upper Cretaceous Squalicorax species and the Upper Miocene carcharhiniforms Galeocerdo sp., Carcharhinus sp., and Hemipristis serra. The parallel bundles are revealed by surface etching, which removes the superficial shiny-layered enameloid. In the teeth of Squalicorax, the bundles around the cutting edge bend once, before they reach the serrations. The studied carcharhiniform species show a more complicated pattern with a change of parallel bundle course inside the serrations. H. serra teeth do not display the first bending of the bundles, whereas it was present in the other two carcharhiniforms. The course of the crystalline bundles in both Squalicorax species is not affected by the presence of the serrations, regardless of the twofold difference in tooth size between them. In the carcharhiniform species, the bended bundles occur within the primary and secondary serrations and are always associated with them. This feature might have functional significance by strengthening the cutting edge or could simply develop as a consequence of the enameloid mineralization around the individual serrae.

Pseudodalatias henarejensis nov. sp. a new pseudodalatiid (Elasmobranchii) from the Middle Triassic of Spain

ABSTRACT Pseudodalatiids, a chondrichthyan family of uncertain phylogenetic affinities, have been hitherto exclusively known from the tooth-based species Pseudodalatias barnstonensis (Sykes, 1971), which has a stratigraphic range restricted to the Upper Triassic of Europe. Pseudodalatias presents a characteristic dentition which allows it to hold and cut its prey, showing a neoselachian design, but lacking the triple-layered enameloid microstructure of neoselachian teeth. The discovery of Pseudodalatias henarejensis nov. sp. in the Ladinian of Spain extends the stratigraphical range and the palaeogeographical distribution of this family. This new species also demonstrates that a cutting-clutching dentition evolved progressively in the family Pseudodalatidiidae. Pseudodalatiids are likely to represent stem-batoids or stem-neoselachians rather than aberrant hybodonts.

Enameloid microstructure in the oldest known chondrichthyan teeth

AbstractPrevious studies on tooth enameloid microstructure in several chondrichthyan taxa spanning the phylogeny of the group provided support for the homology of chondrichthyan tooth enameloid. This hypothesis requires that a single crystallite enameloid (SCE) monolayer must be present in the teeth of the most primitive chondrichthyan. However, the dental microstructure of the earliest sharks has yet to be investigated. We have studied the tooth enameloid microstructure of the two oldest tooth‐bearing shark species currently known, Leonodus carlsi Mader (1986) and Celtiberina maderi Wang (1993), from the lowermost Lochkovian (Lower Devonian) of Spain. Our study demonstrates the presence of a SCE monolayer in the teeth of both species. These results show that a superficial cap of SCE is present in the oldest shark teeth known, which suggest its presence in the most basal chondrichthyans.

The homology and phylogeny of chondrichthyan tooth enameloid

A systematic SEM survey of tooth microstructure in (primarily) fossil taxa spanning chondrichthyan phylogeny demonstrates the presence of a superficial cap of single crystallite enameloid (SCE) on the teeth of several basal elasmobranchs, as well as on the tooth plates of Helodus (a basal holocephalan). This suggests that the epithelial-mesenchymal interactions required for the development of enameloid during odontogenesis are plesiomorphic in chondrichthyans, and most likely in toothed gnathostomes, and provides phylogenetic support for the homology of chondrichthyan and actinopterygian enameloid. Along the neoselachian stem, we see a crownward progression, possibly modulated by heterochrony, from a monolayer of SCE lacking microstructural differentiation to the complex triple-layered tooth enameloid fabric of neoselachians. Finally, the occurrence of fully-differentiated neoselachian enameloid microstructure (including compression-resistant tangle fibered enameloid and bending-resistant parallel fibered enameloid) in Chlamydoselachus anguineus, a basal Squalean with teeth that are functionally "cladodont," is evidence that triple-layered enameloid microstructure was a preadaption to the cutting and gouging function of many neoselachian teeth, and thus may have played an integral role in the Mesozoic radiation of the neoselachian crown group.

Microstructure of pharyngeal tooth enameloid in the parrotfish Scarus rivulatus (Pisces: Scaridae)

The microstructure of parrotfish pharyngeal teeth was examined using scanning electron microscopy to infer possible mechanical properties of the dentition with respect to their function. Parrotfish tooth enameloid is formed from fluorapatite crystals grouped into bundles. In the upper and lower pharyngeal jaw, the majority of the crystal bundles are orientated either perpendicularly or vertically to the enameloid surface. The only exception is in the trailing apical enameloid in which the majority of bundles are orientated perpendicularly or horizontally to the trailing surface. A distinct transition occurs through the middle of the apex between the leading and trailing enameloid in teeth of the lower pharyngeal jaw. This transition appears less distinct in the teeth of the upper pharyngeal jaw. Enameloid microstructure indicates that shear forces predominate at the apex of the teeth. In the remainder of the enameloid, the microstructure indicates that wear is predominant, and the shear forces are of less importance.