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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

Ontogenetic changes in the humerus of H. longicostatum.Stage 5 not pictured. A. Stage 1, CGH3; proximal toward top right. B. Stage 2. CGH247; proximal toward top left. C. Stage 3, CGH3028; proximal toward top left. D. Stage 4, NHMW1898_X_23; proximal toward top left. Dpc, deltopectoral crest; Entf, entepicondylar foramen; Scs, subcoracoscapularis attachment point. Scale bars = 1mm.
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pone.0128333.g032: Ontogenetic changes in the humerus of H. longicostatum.Stage 5 not pictured. A. Stage 1, CGH3; proximal toward top right. B. Stage 2. CGH247; proximal toward top left. C. Stage 3, CGH3028; proximal toward top left. D. Stage 4, NHMW1898_X_23; proximal toward top left. Dpc, deltopectoral crest; Entf, entepicondylar foramen; Scs, subcoracoscapularis attachment point. Scale bars = 1mm.

Mentions: In the smallest-known specimen of H. longicostatum, the humerus is a featureless column of bone with flat proximal and distal surfaces (Fig 32A). During the next stage of morphogenesis, the ends of the humerus become convex and can be distinguished from the shaft (Fig 32B). With further growth the proximal and distal ends of the humerus become highly domed relative to the condition in M. pelikani (Fig 32C). At the same level of morphogenesis a small, distally located deltopectoral crest is present. Next, a small attachment point for the subcoracoscapularis develops (Fig 32D). Finally, the distal condyles are differentiated, including the formation of a shallow fossa or groove between them. I did not observe any clear torsion in the humeri of the specimens that I examined, but many are broken or poorly preserved. One specimen (NHMW1898_X_23; Fig 32D) has an entepicondylar foramen, which is oval and elongate, as in M. pelikani. Presence of that foramen was unreported in prior studies of H. longicostatum.


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

Olori JC - PLoS ONE (2015)

Ontogenetic changes in the humerus of H. longicostatum.Stage 5 not pictured. A. Stage 1, CGH3; proximal toward top right. B. Stage 2. CGH247; proximal toward top left. C. Stage 3, CGH3028; proximal toward top left. D. Stage 4, NHMW1898_X_23; proximal toward top left. Dpc, deltopectoral crest; Entf, entepicondylar foramen; Scs, subcoracoscapularis attachment point. Scale bars = 1mm.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g032: Ontogenetic changes in the humerus of H. longicostatum.Stage 5 not pictured. A. Stage 1, CGH3; proximal toward top right. B. Stage 2. CGH247; proximal toward top left. C. Stage 3, CGH3028; proximal toward top left. D. Stage 4, NHMW1898_X_23; proximal toward top left. Dpc, deltopectoral crest; Entf, entepicondylar foramen; Scs, subcoracoscapularis attachment point. Scale bars = 1mm.
Mentions: In the smallest-known specimen of H. longicostatum, the humerus is a featureless column of bone with flat proximal and distal surfaces (Fig 32A). During the next stage of morphogenesis, the ends of the humerus become convex and can be distinguished from the shaft (Fig 32B). With further growth the proximal and distal ends of the humerus become highly domed relative to the condition in M. pelikani (Fig 32C). At the same level of morphogenesis a small, distally located deltopectoral crest is present. Next, a small attachment point for the subcoracoscapularis develops (Fig 32D). Finally, the distal condyles are differentiated, including the formation of a shallow fossa or groove between them. I did not observe any clear torsion in the humeri of the specimens that I examined, but many are broken or poorly preserved. One specimen (NHMW1898_X_23; Fig 32D) has an entepicondylar foramen, which is oval and elongate, as in M. pelikani. Presence of that foramen was unreported in prior studies of H. longicostatum.

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