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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

Postparietals of M. pelikani.Rough sculpture displaying ‘knitting’ across the midline suture of the contralateral postparietals of St.201 (Narodini Museum, (now National Museum Prague), Prague, Czech Republic). Scale bar = 1mm.
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pone.0128333.g003: Postparietals of M. pelikani.Rough sculpture displaying ‘knitting’ across the midline suture of the contralateral postparietals of St.201 (Narodini Museum, (now National Museum Prague), Prague, Czech Republic). Scale bar = 1mm.

Mentions: As partially described by Carroll and Gaskill [1], dermal sculpture is modified during growth. In the smallest specimen with a preserved skull (skull length = 7mm) the parietal, tabular, squamosal, and dentary are marked with deep, widely spaced, and straight grooves that radiate from the ossification centers. Except for the frontal, which exhibits faint striae, all other dermal skull elements have not yet begun to develop ornamentation. This lag is characteristic of a pattern of sculpture elaboration found in many individuals of M. pelikani in which the parietal, tabular, squamosal, post-parietal, post-orbital, and articular exhibit the most prominent dermal ornamentation, followed by the frontal, jugal, and postfrontal, which appear moderately sculptured in comparison. The development of the sculpture on the nasal, lacrimal, prefrontal, postfrontal, maxilla, and premaxilla is delayed further and these elements remain relatively smooth in earlier growth stages. During the next stage of sculpture development (skull length ~10–13mm) weak crenulations form at the ossification centers of the most strongly ornamented elements and the once straight grooves show more sinuous traces. During later stages of growth (skull length ~14–17mm), these grooves are more densely spaced and tend to bifurcate into multiple pathways. Additionally, many grooves anastomose, resulting in the appearance of alternating grooves and ridges frequently described in the literature (e.g., [1,47]). In specimens of still larger size (skull length ~18mm+) the crenulations at the ossification centers become so rough that they appear fragmented into many small islands of raised ornamentation. Moreover, the radiating grooves and ridges deepen and thicken while converging to become continuous with those radiating from neighboring dermal elements (Fig 3). Further development of the sculpture produces more densely packed crenulations and ridges that resemble a net-like pattern observed only in the largest individuals (skull length ~25mm+).


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

Olori JC - PLoS ONE (2015)

Postparietals of M. pelikani.Rough sculpture displaying ‘knitting’ across the midline suture of the contralateral postparietals of St.201 (Narodini Museum, (now National Museum Prague), Prague, Czech Republic). Scale bar = 1mm.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g003: Postparietals of M. pelikani.Rough sculpture displaying ‘knitting’ across the midline suture of the contralateral postparietals of St.201 (Narodini Museum, (now National Museum Prague), Prague, Czech Republic). Scale bar = 1mm.
Mentions: As partially described by Carroll and Gaskill [1], dermal sculpture is modified during growth. In the smallest specimen with a preserved skull (skull length = 7mm) the parietal, tabular, squamosal, and dentary are marked with deep, widely spaced, and straight grooves that radiate from the ossification centers. Except for the frontal, which exhibits faint striae, all other dermal skull elements have not yet begun to develop ornamentation. This lag is characteristic of a pattern of sculpture elaboration found in many individuals of M. pelikani in which the parietal, tabular, squamosal, post-parietal, post-orbital, and articular exhibit the most prominent dermal ornamentation, followed by the frontal, jugal, and postfrontal, which appear moderately sculptured in comparison. The development of the sculpture on the nasal, lacrimal, prefrontal, postfrontal, maxilla, and premaxilla is delayed further and these elements remain relatively smooth in earlier growth stages. During the next stage of sculpture development (skull length ~10–13mm) weak crenulations form at the ossification centers of the most strongly ornamented elements and the once straight grooves show more sinuous traces. During later stages of growth (skull length ~14–17mm), these grooves are more densely spaced and tend to bifurcate into multiple pathways. Additionally, many grooves anastomose, resulting in the appearance of alternating grooves and ridges frequently described in the literature (e.g., [1,47]). In specimens of still larger size (skull length ~18mm+) the crenulations at the ossification centers become so rough that they appear fragmented into many small islands of raised ornamentation. Moreover, the radiating grooves and ridges deepen and thicken while converging to become continuous with those radiating from neighboring dermal elements (Fig 3). Further development of the sculpture produces more densely packed crenulations and ridges that resemble a net-like pattern observed only in the largest individuals (skull length ~25mm+).

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