The relationship between generation gland morphology and armour in Dragon Lizards (Smaug): a reassessment of ancestral states for the Cordylidae

in Amphibia-Reptilia
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Abstract

To substantiate the claim of a relationship between generation gland morphology and degree of body armour in cordylid lizards, we studied the nine species in the genus Smaug. We predicted that well armoured species in this clade will have multi-layer generation glands, and lightly armoured species two-layer glands. Gland type was determined using standard histological techniques after sectioning a glandular patch of one adult male per species. A total of 133 specimens were examined for data on tail and occipital spine lengths (which were used as indicators of armour). We found that species with multi-layer generation glands (S. giganteus, S. breyeri, and S. vandami) have relatively long tail and occipital spines, while species with two-layer glands (S. mossambicus, S. regius, S. barbertonensis, S. warreni, and an undescribed species) have relatively short spines. Smaug depressus possesses both multi-layer and two-layer glands, and this variation was linked to regional variation in spine length. An ancestral state reconstruction for the Cordylidae showed that the two-layer state always results from the reduction of layers from a multi-layer precursor, and that reduction always culminates in two-layer glands and not in one-layer glands. This finding suggests that the one-layer state in the Ninurta-Chamaesaura-Pseudocordylus clade is most probably plesiomorphic, and therefore the ancestral state at the Cordylidae and Cordylinae nodes. Given the observed relationship between type of generation gland and body armour, this finding would suggest that the most recent common ancestor of the Cordylidae was lightly armoured.

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References
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Figures
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    Photomicrographs of a) the ventral aspect of the thigh of a cordylid lizard, showing the patch of generation glands and the row of femoral gland pores; and cross-sections through b) a protruding two-layer type generation gland of Smaug depressus, (NMB R10876) showing the two mature generation layers; c) a protruding multiple-layer type generation gland of S. vandami (NMB R8545) showing the multiple mature generation layers; (GG = generation gland; MG = mature generation layer; PG = presumptive generation layer; FG = femoral gland; SG = stratum germinativum).

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    Photographs of eight Smaug species depicting differences in body colour and spinosity.

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    Photographs depicting a) the four lateral tail spines on whorls three and four that were measured for each individual of Smaug investigated (the insert shows how the spine measurement was performed), and b) the two outer occipital spines on each side of the head that were measured for each individual of Smaug investigated. Spine measurements were taken along the long axis of the outer or both of the two occipital scales.

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    Ancestral state reconstruction of generation gland type for the Cordylidae. The phylogenetic tree is based on the maximum-likelihood analyses of Stanley et al. (2011) and Stanley and Bates (2014). Pie charts show the relative likelihood of each generation gland type at the respective nodes.

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    Morphological data for samples of the nine species of Smaug examined for this study. Determination of tail and occipital spine ratios is explained under ‘Materials and methods’.

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    Scatterplots of tail spine ratio (= ln tail spine length)/ln(SVL)) versus occipital spine ratio (= ln occipital spine length)/ln(SVL)) for a) 66 individuals, representing eight of the nine Smaug species, and b) 37 individuals of S. depressus from various localities in the Soutpansberg region. Solid symbols represent species/populations displaying multi-layer generation glands, and open symbols represent species/populations with two-layer generation glands.

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