24 research outputs found

    A comparative study of eggshells of Gekkota with morphological, chemical compositional and crystallographic approaches and its evolutionary implications

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    <div><p>The Gekkota is an important clade in the evolution of calcified eggshells in that some of its families lay rigid eggshells like archosaurs. However, the fundamental differences and similarities between the mechanism of rigid eggshell formation of the Gekkota and Archosauria have not been investigated thoroughly due to the lack of knowledge of gekkotan eggshells. Here, we report for the first time a comprehensive analysis of morphological, chemical compositional, and crystallographic features of rigid and soft gekkotan eggshells. Exhaustive morphological description provided common characters for gekkotan eggshells, as well as unique features of each species. We found that elemental distribution of rigid gekkotan eggshells is different from that of avian eggshells, especially in the case of Mg and P. In addition, the crystallographic features (size, shape, and alignment of calcite grains) of gekkotan eggshells are completely different from those of archosaur eggshells. The result of this study suggests that soft gekkotan eggshells are morphologically more similar to tuatara eggshells rather than soft eggshells of derived squamates. The chemical compositional analysis suggests that the eggshell may act as a mineral reservoir for P and F as well as Ca. More importantly, all chemical compositions and crystallographic features imply that the gekkotan eggshell formation may begin at the outer surface and growing down to the inner surface, which is opposite to the direction of the archosaur eggshell formation. This character would be crucial for identifying fossil gekkotan eggs, which are poorly known in paleontology. All these lines of evidence support that soft gekkotan and tuatara eggshells share the primitive characters of all lepidosaurid eggshells. Finally, gekkotan and archosaur rigid eggshells represent a typical example of convergent evolution in the lineage of the Sauropsida.</p></div

    Elemental mapping images of gekkotan eggshells.

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    <p>The panels in the same row are on the same scale. A colored bar on A1 shows the intensity of signal with red stands for higher concentration. Outside of eggshell is up. (A) <i>Gekko gecko</i>. (B) <i>Paroedura pictus</i>. (C) <i>Paroedura stumpfii</i>. (D) <i>Phelsuma grandis</i>. (E) <i>Correlophus ciliatus</i>. (F) <i>Rhacodactylus leachianus</i>. (G) <i>Eublepharis macularius</i>.</p

    Phylogenetic relationship of the Gekkota and related major clades in the Sauropsida modified from [3,9,10].

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    <p>The blue branches represent soft-shelled egg-layers while red branches rigid-shelled ones. The gekkotan genera used in this study are in parentheses.</p

    Thin section images of gekkotan eggshells.

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    <p>A1–G1 were taken under plane-polarized light and A2–G2 cross-polarized light. White bars on the left side of the figures represent the boundary between the layers mentioned in the text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s001" target="_blank">S1 Text</a>. Outside of eggshell is up, consistent with <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.g003" target="_blank">Fig 3</a>. (A) <i>Gekko gecko</i>. White arrows point to the inner thick faint and outer thin dark bands, respectively. Black arrows mark the pore-like structures. (B) <i>Paroedura pictus</i>. White arrows point to the polygonal extinction patterns. Black arrows mark the location of ridge-like ornamentations. (C) <i>Paroedura stumpfii</i>. A white arrow in C1 marks a calcite concretion extending toward the interior of the eggshell. White arrows in C2 mark the triangular extinction pattern. (D) <i>Phelsuma grandis</i>. White arrows in D1 and D2 point a dark band and calcite granules, respectively. (E) <i>Correlophus ciliatus</i>. Extinction patterns are polygonal to triangular in shape (white arrows in E2). (F) <i>Rhacodactylus leachianus</i>. Extinction patterns are longitudinally thick, compared to those of <i>Correlophus ciliatus</i> eggshell (white arrows). The pore-like structure is marked by a black arrow. (G) <i>Eublepharis macularius</i>. Extinction pattern is not prominent, compared to other soft gekkotan eggshells but similar in shape (white arrows).</p

    A hypothetical cladogram of rigid eggshells evolution in sauropsids.

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    <p>The red branches represent the rigid calcified eggshells, the blue represent the “tuatara-styled” eggshells, and the black represents the derived squamate eggshell. The archosaur eggshells are characterized by the nucleation sites at their inner margin. The tuatara and soft gekkotan eggshells are morphologically similar to each other and characterized by the “stem-like” and “cap-like” structure in the inner and outer regions, respectively. The derived squamate eggshells have a shallow calcareous layer on the surface without calcite “stem-like” structure into the shell membrane. The eggshell microstructure of the Dibamidae is unknown.</p

    Inverse pole figure (IPF) maps and lower hemisphere pole figures of gekkotan eggshells.

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    <p>Each row of lower hemisphere pole figures in right columns is corresponding to the area bounded by the white bars on the IPF map. The hexagonal columns in the IPF map show the direction of c-axis orientation. The big columns represent the main direction and small columns represent the subordinate direction (i.e., correspond to a strong signal and weak signal marked by white arrows in the pole figures, respectively). Note that a- and b-axes were not considered in the construction of hexagonal columns. The numbers above the color scale represent the number of data points used in pole figure construction. The numbers on the side of the color scale are the intensity of the signal. Outside of eggshell is up. Dashed lines indicate the boundary layer (E–G). (A) <i>Gekko gecko</i>. Note that lower hemisphere pole figures were constructed only for the area to the left of a dashed line in order to avoid any disturbance caused by ornamentation and pore-like structure in the right area. (B) <i>Paroedura pictus</i>. (C) <i>Paroedura stumpfii</i>. (D) <i>Phelsuma grandis</i>. (E) <i>Correlophus ciliatus</i>. (F) <i>Rhacodactylus leachianus</i>. (G) <i>Eublepharis macularius</i>. (H) An arrow in hexagonal column points to the direction of c-axis (upper left). An IPF legend shows the relationship between the color and c-axis orientation (lower left). A cylinder on the right shows c-axis orientation schematically. Red-colored parts of the IPF map show the region where c-axis of calcite crystal is aligned perpendicular to the eggshell surface. In contrast, blue- and green-colored parts are equivalent to the calcite crystals that have horizontally aligned c-axis (i.e., parallel to the eggshell surface).</p

    Line profile analyses of gekkotan eggshells.

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    <p>Vertical yellow bars represent the locations where analyses were conducted. Horizontal red bars on the graph are auxiliary lines that mark the same position in the eggshell. The boundaries of the eggshell are marked by black bars. The units are in weight percent. Outside of eggshell is up. (A1–A2) <i>Gekko gecko</i>. A1 shows the elemental profile of the main eggshell, while A2 represents the result of ornamentation area. Note the correlation between the level of Mg and S (white arrows). A black arrow points a small Ca peak caused by a very thin layer consisting of polygonal structure of the covering layer. (B) <i>Paroedura pictus</i>. Note the correlation between the level of Mg and S (white arrows). (C) <i>Paroedura stumpfii</i>. A drop of Ca level reflects the empty spaces in the porous layer rather than low concentration of Ca in the porous layer (a white arrow). (D) <i>Phelsuma grandis</i>. Note the correlation between the level of Mg and S in the main eggshell (white arrows). The level of P gradually decreases from the inner surface of the columnar layer, but it increases near the outer surface of the eggshell. (E) <i>Correlophus ciliatus</i>. The hatched rectangle marks the crack. The level of Mg, P, and Ca begins to change at the same position (i.e., the boundary between the shell membrane and calcareous layer). (F) <i>Rhacodactylus leachianus</i>. Note the level of Mg and P changes more abruptly at the boundary between the shell membrane and calcareous layer compared to <i>Correlophus ciliatus</i> eggshell. (G) <i>Eublepharis macularius</i>. Note that <i>Eublepharis macularius</i> eggshell has a relatively short calcareous layer.</p

    Schematic views of the seven different gekkotan and representative derived squamate eggshells.

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    <p>Most of the terminology followed [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.ref002" target="_blank">2</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.ref015" target="_blank">15</a>] and otherwise new terms are proposed in this study. The light gray areas in <i>Gekko gecko</i> (A), <i>Paroedura stumpfii</i> (C), and <i>Phelsuma grandis</i> (D) eggshells show the location and shape of “dark bands”. The dark gray areas in rigid gecko eggshells (A–D) signify the shape of extinction pattern under polarized light. The jargon “calcareous layer” is used in soft gekkotan eggshell to refer to stem-like and cap-like structures, collectively. The figures are not drawn to scale. In <i>Gekko gecko</i> eggshell (A), a pore-like and bulbous structures are associated with the ornamentation. The eggshell of <i>Paroedura stumpfii</i> has a peculiar porous layer (C). The mixed layer of <i>Rhacodactylus leachianus</i> eggshell (F) can be further differentiated into two sub-layers. The protein fibers in <i>Eublepharis macularius</i> eggshell (G) are undulated unlike other soft eggshells (E, F). Note that soft eggshells of derived squamates do not have stem-like structure that extends to the shell membrane (H).</p

    Secondary electron images of gekkotan eggshells.

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    <p>Each column is outer, radial, and inner views, respectively. White and black bars on the side of a central column represent the boundary between the layers mentioned in the text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s001" target="_blank">S1 Text</a>. Outside of eggshell is up. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s004" target="_blank">S2</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s010" target="_blank">S8</a> Figs for details. (A) <i>Gekko gecko</i>. Ribbon-like structures in A3 are remnants of the shell membrane. (B) <i>Paroedura pictus</i>. White arrows in B1 point pore-like structures. Note columnar structure (white arrows) and ridge-like ornamentations (black arrows) in B2. The inner surface is characterized by a lot of pits. (C) <i>Paroedura stumpfii</i>. White arrows in C1 mark the fragments of sawdust so that they should be neglected. Note the absence of columnar structure in the plain layer and the presence of ornamentation (a white arrow) in C2. Protruding calcite concretions are distributed in C3 (white arrows). (D) <i>Phelsuma grandis</i>. Note that the boundary between the blocky and columnar layers in D2 is very similar to the inner surface of other rigid gekkotan eggshells where needle-like structure exists (a white arrow). (E) <i>Correlophus ciliatus</i>. The calcareous layer is weakly columnar in shape and morphologically different from the very thin calcareous layer of derived squamate eggshells. A white arrow in E3 marks the margin of the boundary layer. (F) <i>Rhacodactylus leachianus</i>. The gradual boundary between the capsule-like and flattened granules is marked in F1 (a white arrow) in the outer surface. Note the convex mounds in the boundary layer (white arrows) in F3. (G) <i>Eublepharis macularius</i>. Note that protein fibers near the calcareous layer have wave-like undulation in G2 (white arrows). The outline of eggshell is similar to those of scincid lizard eggshells [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.ref039" target="_blank">39</a>].</p

    Backscattered electron (BSE) images of gekkotan eggshells.

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    <p>White bars on the side represent the certain boundary mentioned in the text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s001" target="_blank">S1 Text</a>. Outside of eggshell is up. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s011" target="_blank">S9</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199496#pone.0199496.s017" target="_blank">S15</a> Figs for details. (A) <i>Gekko gecko</i>. White arrows point to the circular structures with a central hole, whereas black arrows mark the vesicles. Note the enigmatic bulbous structure in ornamentation (a white arrowhead) covered with a very thin layer consisting of polygonal structure in the outermost part of the eggshell (a black arrowhead). (B) <i>Paroedura pictus</i>. Note the circular structures with a central hole (white arrows) and abundant vesicles (black arrows). The ridge-like ornamentation is marked by a black arrowhead. (C) <i>Paroedura stumpfii</i>. Circular structures with a central hole (white arrows) are distributed in the eggshell except the inner portion of the plain layer. Vesicles are marked by black arrows. Note the highly irregular pores in the porous layer and a pore-like orifice (a black arrowhead). (D) <i>Phelsuma grandis</i>. Abundant circular structures (white arrows) and a few vesicles are present (black arrows). Note needle-like structures between the blocky and columnar layers (two lower white arrowheads). Spherical granular ornamentations are connected to the main eggshell (two upper white arrowheads) covered with a very thin covering layer (a black arrowhead). (E) <i>Correlophus ciliatus</i>. The boundary layer is marked by a dashed line. Note that calcareous layer occupies one-half of the eggshell although they coexist with protein fibers (fibers are represented by black dots and lines; white arrows). Pore-like and small chamber-like structures are pointed by black and white arrowheads, respectively. (F) <i>Rhacodactylus leachianus</i>. The boundary layer of the eggshell is marked by a dashed line. It has deeper mixed layer with a more compact outer portion than <i>Correlophus ciliatus</i> eggshell. The protein fibers are marked by white arrows. Note pore-like structure (a black arrowhead). (G) <i>Eublepharis macularius</i>. Columnar structure is clearly observable with pore-like structures between columns (black arrowheads). The stem-like structure of each columns consists of calcite and protein fibers (white arrows).</p
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