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Skeletal Morphogenesis of Microbrachis and Hyloplesion (Tetrapoda: Lepospondyli), and Implications for the Developmental Patterns of Extinct, Early Tetrapods.

Olori JC - PLoS ONE (2015)

Bottom Line: However, early and rapid ossification of the postcranial skeleton, including a well-developed pubis and ossified epipodials, suggests that neither taxon metamorphosed nor were they neotenic in the sense of branchiosaurids and salamanders.Overall patterns of postcranial ossification may indicate postaxial dominance in limb and digit formation, but also more developmental variation in early tetrapods than has been appreciated.The phylogenetic position and developmental patterns of M. pelikani and H. longicostatum are congruent with the hypothesis that early tetrapods lacked metamorphosis ancestrally and that stem-amniotes exhibited derived features of development, such as rapid and complete ossification of the skeleton, potentially prior to the evolution of the amniotic egg.

View Article: PubMed Central - PubMed

Affiliation: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, United States of America.

ABSTRACT
The ontogeny of extant amphibians often is used as a model for that of extinct early tetrapods, despite evidence for a spectrum of developmental modes in temnospondyls and a paucity of ontogenetic data for lepospondyls. I describe the skeletal morphogenesis of the extinct lepospondyls Microbrachis pelikani and Hyloplesion longicostatum using the largest samples examined for either taxon. Nearly all known specimens were re-examined, allowing for substantial anatomical revisions that affect the scoring of characters commonly used in phylogenetic analyses of early tetrapods. The palate of H. longicostatum is re-interpreted and suggested to be more similar to that of M. pelikani, especially in the nature of the contact between the pterygoids. Both taxa possess lateral lines, and M. pelikani additionally exhibits branchial plates. However, early and rapid ossification of the postcranial skeleton, including a well-developed pubis and ossified epipodials, suggests that neither taxon metamorphosed nor were they neotenic in the sense of branchiosaurids and salamanders. Morphogenetic patterns in the foot suggest that digit 5 was developmentally delayed and the final digit to ossify in M. pelikani and H. longicostatum. Overall patterns of postcranial ossification may indicate postaxial dominance in limb and digit formation, but also more developmental variation in early tetrapods than has been appreciated. The phylogenetic position and developmental patterns of M. pelikani and H. longicostatum are congruent with the hypothesis that early tetrapods lacked metamorphosis ancestrally and that stem-amniotes exhibited derived features of development, such as rapid and complete ossification of the skeleton, potentially prior to the evolution of the amniotic egg.

No MeSH data available.


Related in: MedlinePlus

Dermal scales of M. pelikani.Individual scales (A-D) and in rows (E). Note heavy ridge in B and D. Scale bars = 1mm.
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pone.0128333.g001: Dermal scales of M. pelikani.Individual scales (A-D) and in rows (E). Note heavy ridge in B and D. Scale bars = 1mm.

Mentions: ‘Dermal scales’ of ‘microsaurian’ lepospondyls frequently were described in the literature as oat- or rod-shaped elements with a radiating pattern of ornamentation [1,47]. The scales are composed of bone (Fig 1A) and thus ‘dermal scales’ or ‘dermal squamation’ are common terminology for similar features in temnospondyls and other early tetrapods (e.g., [51,52]). Both dorsal and ventral scales were noted previously, and can be distinguished morphologically [15,46]. I also observed two types of dermal ossifications, although association with the ventral or dorsal surfaces of the skeletons was not always clear. In one type of scale (usually dorsal), the overall shape is roughly circular and the ornamentation is composed of relatively short, semi-parallel striae. Additionally, where the longitudinal striae converge at the edge of the scale that is overlapped by imbricating scales, there generally is a net-like pattern of ornamentation formed by shorter, perpendicular striae (Fig 1B). In small specimens with tiny scales that type of ornamentation can resemble a starburst with few, nonparallel, radiating striae and less cross-hatching at the point of convergence. The second type of scale (usually ventral) is more elongate and oval in shape. The striae are accordingly elongate, generally parallel to one another, and there is no cross-hatch pattern where the striae converge (Fig 1C). Both types of scales are unmodified during ontogeny although they do increase in size along with the rest of the body (i.e., larger animals have proportionately larger scales), and the ornamentation becomes stronger. However, the largest individuals of M. pelikani also developed a heavy ridge that curves along the edge of the scale (Fig 1B and 1D) where it may have attached or embedded into the dermis, if not completely embedded. The ridge is thick, broad, and smooth and is not the same as the ‘ridges’ described by Milner [15], which are thick lines produced along the edges of the scales where they overlap when preserved in place. When more than one row of scales is preserved in situ, the ‘ridges’ [15] formed by the overlap make the scales appear square (Fig 1E). When isolated, however, it is apparent that the scales are either round or oval. The heaviest ridges usually are found on the scales from the ventral surface of the body.


Skeletal Morphogenesis of Microbrachis and Hyloplesion (Tetrapoda: Lepospondyli), and Implications for the Developmental Patterns of Extinct, Early Tetrapods.

Olori JC - PLoS ONE (2015)

Dermal scales of M. pelikani.Individual scales (A-D) and in rows (E). Note heavy ridge in B and D. Scale bars = 1mm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4470922&req=5

pone.0128333.g001: Dermal scales of M. pelikani.Individual scales (A-D) and in rows (E). Note heavy ridge in B and D. Scale bars = 1mm.
Mentions: ‘Dermal scales’ of ‘microsaurian’ lepospondyls frequently were described in the literature as oat- or rod-shaped elements with a radiating pattern of ornamentation [1,47]. The scales are composed of bone (Fig 1A) and thus ‘dermal scales’ or ‘dermal squamation’ are common terminology for similar features in temnospondyls and other early tetrapods (e.g., [51,52]). Both dorsal and ventral scales were noted previously, and can be distinguished morphologically [15,46]. I also observed two types of dermal ossifications, although association with the ventral or dorsal surfaces of the skeletons was not always clear. In one type of scale (usually dorsal), the overall shape is roughly circular and the ornamentation is composed of relatively short, semi-parallel striae. Additionally, where the longitudinal striae converge at the edge of the scale that is overlapped by imbricating scales, there generally is a net-like pattern of ornamentation formed by shorter, perpendicular striae (Fig 1B). In small specimens with tiny scales that type of ornamentation can resemble a starburst with few, nonparallel, radiating striae and less cross-hatching at the point of convergence. The second type of scale (usually ventral) is more elongate and oval in shape. The striae are accordingly elongate, generally parallel to one another, and there is no cross-hatch pattern where the striae converge (Fig 1C). Both types of scales are unmodified during ontogeny although they do increase in size along with the rest of the body (i.e., larger animals have proportionately larger scales), and the ornamentation becomes stronger. However, the largest individuals of M. pelikani also developed a heavy ridge that curves along the edge of the scale (Fig 1B and 1D) where it may have attached or embedded into the dermis, if not completely embedded. The ridge is thick, broad, and smooth and is not the same as the ‘ridges’ described by Milner [15], which are thick lines produced along the edges of the scales where they overlap when preserved in place. When more than one row of scales is preserved in situ, the ‘ridges’ [15] formed by the overlap make the scales appear square (Fig 1E). When isolated, however, it is apparent that the scales are either round or oval. The heaviest ridges usually are found on the scales from the ventral surface of the body.

Bottom Line: However, early and rapid ossification of the postcranial skeleton, including a well-developed pubis and ossified epipodials, suggests that neither taxon metamorphosed nor were they neotenic in the sense of branchiosaurids and salamanders.Overall patterns of postcranial ossification may indicate postaxial dominance in limb and digit formation, but also more developmental variation in early tetrapods than has been appreciated.The phylogenetic position and developmental patterns of M. pelikani and H. longicostatum are congruent with the hypothesis that early tetrapods lacked metamorphosis ancestrally and that stem-amniotes exhibited derived features of development, such as rapid and complete ossification of the skeleton, potentially prior to the evolution of the amniotic egg.

View Article: PubMed Central - PubMed

Affiliation: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, United States of America.

ABSTRACT
The ontogeny of extant amphibians often is used as a model for that of extinct early tetrapods, despite evidence for a spectrum of developmental modes in temnospondyls and a paucity of ontogenetic data for lepospondyls. I describe the skeletal morphogenesis of the extinct lepospondyls Microbrachis pelikani and Hyloplesion longicostatum using the largest samples examined for either taxon. Nearly all known specimens were re-examined, allowing for substantial anatomical revisions that affect the scoring of characters commonly used in phylogenetic analyses of early tetrapods. The palate of H. longicostatum is re-interpreted and suggested to be more similar to that of M. pelikani, especially in the nature of the contact between the pterygoids. Both taxa possess lateral lines, and M. pelikani additionally exhibits branchial plates. However, early and rapid ossification of the postcranial skeleton, including a well-developed pubis and ossified epipodials, suggests that neither taxon metamorphosed nor were they neotenic in the sense of branchiosaurids and salamanders. Morphogenetic patterns in the foot suggest that digit 5 was developmentally delayed and the final digit to ossify in M. pelikani and H. longicostatum. Overall patterns of postcranial ossification may indicate postaxial dominance in limb and digit formation, but also more developmental variation in early tetrapods than has been appreciated. The phylogenetic position and developmental patterns of M. pelikani and H. longicostatum are congruent with the hypothesis that early tetrapods lacked metamorphosis ancestrally and that stem-amniotes exhibited derived features of development, such as rapid and complete ossification of the skeleton, potentially prior to the evolution of the amniotic egg.

No MeSH data available.


Related in: MedlinePlus