Osteological Variation among Extreme Morphological Forms in the Mexican Salamander Genus Chiropterotriton (Amphibia: Plethodontidae): Morphological Evolution And Homoplasy

Osteological variation is recorded among and within four of the most distinctive species of the Mexican salamander genus Chiropterotriton. Analysis of the data is consistent with the monophyletic status of the genus and documents previously unrecorded intraspecific and interspecific variation. Most of the recorded variation involves qualitative and quantitative proportional differences, but four fixed differences constitute autapomorphic states that affirm and diagnose some species (C. dimidiatus, C. magnipes). Osteological variation in 15 characters is analyzed with respect to predictions generated from four hypotheses: 1) phylogeny, 2) adaptation to specific habitats (the four species include cave-dwelling, terrestrial, and arboreal forms), 3) size-free shape, and 4) size. High levels of intraspecific variation suggest that the characters studied are not subject to rigid functional constraints in salamanders, regardless of size. The pattern predicted by the hypothesis based on size differences seen among these four Chiropterotriton species matches most closely the observed pattern of relative skull robustness. Since size change and heterochrony are often associated in plethodontid evolution, it is likely that changes in developmental timing play a role in the morphological transitions among these morphologically diverse taxa. Webbed feet, miniaturization, body shape, and an unusual tarsal arrangement are morphologies exhibited in species of Chiropterotrition that are shown to be homoplastic with other clades of tropical plethodontids. Although extensive homoplasy in salamanders might be seen as a roadblock to unraveling phylogenetic hypotheses, the homologous developmental systems that appear to underlie such homoplasy may reveal common and consistent evolutionary processes at work

Osteological Variation among Extreme Morphological Forms in the Mexican Salamander Genus Chiropterotriton (Amphibia: Plethodontidae): Morphological Evolution And Homoplasy.

Osteological variation is recorded among and within four of the most distinctive species of the Mexican salamander genus Chiropterotriton. Analysis of the data is consistent with the monophyletic status of the genus and documents previously unrecorded intraspecific and interspecific variation. Most of the recorded variation involves qualitative and quantitative proportional differences, but four fixed differences constitute autapomorphic states that affirm and diagnose some species (C. dimidiatus, C. magnipes). Osteological variation in 15 characters is analyzed with respect to predictions generated from four hypotheses: 1) phylogeny, 2) adaptation to specific habitats (the four species include cave-dwelling, terrestrial, and arboreal forms), 3) size-free shape, and 4) size. High levels of intraspecific variation suggest that the characters studied are not subject to rigid functional constraints in salamanders, regardless of size. The pattern predicted by the hypothesis based on size differences seen among these four Chiropterotriton species matches most closely the observed pattern of relative skull robustness. Since size change and heterochrony are often associated in plethodontid evolution, it is likely that changes in developmental timing play a role in the morphological transitions among these morphologically diverse taxa. Webbed feet, miniaturization, body shape, and an unusual tarsal arrangement are morphologies exhibited in species of Chiropterotrition that are shown to be homoplastic with other clades of tropical plethodontids. Although extensive homoplasy in salamanders might be seen as a roadblock to unraveling phylogenetic hypotheses, the homologous developmental systems that appear to underlie such homoplasy may reveal common and consistent evolutionary processes at work

Tibial spur development.

<p>Five states of tibial spur development in <i>Chiropterotriton</i>: (a) absent, tibia smooth; (b) absent, tibial ridge; (c) present, detached; (d) present, attached; (e) present, attached with foramen. Osteological character 16.</p

<i>Pseudoeurycea leprosa</i> foot.

<p>Drawing of left, rear foot of <i>Pseudoeurycea leprosa</i> (MVZ 147092), showing skeletal elements and the extent of interdigital webbing. Dorsal view; stippled areas represent cartilage. Elements as labeled in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127248#pone.0127248.g005" target="_blank">Fig 5</a>.</p

<i>Chiropterotriton priscus</i> feet.

<p>Drawings of front, left (a) and rear, left (b) feet of <i>Chiropterotriton priscus</i> (MVZ 192794), showing skeletal elements and the extent of interdigital webbing. Dorsal view; stippled areas represent cartilage. Elements as labeled in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127248#pone.0127248.g005" target="_blank">Fig 5</a>.</p

<i>Pseudoeurycea leprosa</i> skull.

<p>Drawings of dorsal (a) and ventral (b) aspects of the skull of <i>Pseudoeurycea leprosa</i> (MVZ 147092). Stippled areas represent cartilage. Elements as labeled in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127248#pone.0127248.g001" target="_blank">Fig 1</a>.</p

Frequency (%) of osteological character variation in the anterior skull.

<p>Frequencies calculated by considering right and left sides separately. Ten specimens of each species were examined.</p><p>Frequency (%) of osteological character variation in the anterior skull.</p

<i>Chiropterotrition priscus</i> body shape.

<p>Photograph showing the strikingly similar external appearance of (a) <i>Chiropterotrition priscus</i> (MVZ 138885) and certain members of the genus <i>Pseudoeurycea</i> [(b) <i>P</i>. <i>leprosa</i> (MVZ 132796)].</p

<i>Chiropterotriton dimidiatus</i> skull.

<p>Drawings of dorsal (a) and ventral (b) aspects of the skull of <i>Chiropterotriton dimidiatus</i> (MVZ 103967). Stippled areas represent cartilage. Elements as labeled in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127248#pone.0127248.g001" target="_blank">Fig 1</a>.</p