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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

Heterochrony related to body size and level of skeletal ossification.Line leading to A represents ancestral condition. Line leading to N represents neoteny in the sense of a deceleration of rate of ossification, which produces a derived condition with a lesser degree of skeletal ossification. Line leading to D1 represents an earlier onset of ossification, producing a derived condition in which smaller body size is coupled with the same degree of ossification as ancestral condition. Line leading to D2 represents an acceleration in the rate of ossification combined with earlier onset, which produces a derived condition of smaller body size coupled with a higher degree of skeletal ossification.
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pone.0128333.g038: Heterochrony related to body size and level of skeletal ossification.Line leading to A represents ancestral condition. Line leading to N represents neoteny in the sense of a deceleration of rate of ossification, which produces a derived condition with a lesser degree of skeletal ossification. Line leading to D1 represents an earlier onset of ossification, producing a derived condition in which smaller body size is coupled with the same degree of ossification as ancestral condition. Line leading to D2 represents an acceleration in the rate of ossification combined with earlier onset, which produces a derived condition of smaller body size coupled with a higher degree of skeletal ossification.

Mentions: In contrast to the pattern in branchiosaurids, the mosaic of developmental features in M. pelikani and H. longicostatum may actually point to peramorphosis of skeletal growth in the latter two taxa. If the ancestral tetrapod condition was to grow large while either remaining in the water or gradually completing a transition to land, then neoteny in the sense of a deceleration of somatic development [29,32] would lead to a derived condition with a lesser degree of skeletal ossification than the ancestral condition (Fig 38). In order to produce a derived condition in which smaller body size is coupled with a degree of skeletal ossification similar to the ancestral condition, at a minimum there must be an earlier onset of ossification (i.e., pre-displacement). An additional acceleration in the rate of ossification is necessary to produce a derived condition exhibiting both smaller body size and a higher degree of skeletal ossification, as in lepospondyls (Fig 38).


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

Olori JC - PLoS ONE (2015)

Heterochrony related to body size and level of skeletal ossification.Line leading to A represents ancestral condition. Line leading to N represents neoteny in the sense of a deceleration of rate of ossification, which produces a derived condition with a lesser degree of skeletal ossification. Line leading to D1 represents an earlier onset of ossification, producing a derived condition in which smaller body size is coupled with the same degree of ossification as ancestral condition. Line leading to D2 represents an acceleration in the rate of ossification combined with earlier onset, which produces a derived condition of smaller body size coupled with a higher degree of skeletal ossification.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g038: Heterochrony related to body size and level of skeletal ossification.Line leading to A represents ancestral condition. Line leading to N represents neoteny in the sense of a deceleration of rate of ossification, which produces a derived condition with a lesser degree of skeletal ossification. Line leading to D1 represents an earlier onset of ossification, producing a derived condition in which smaller body size is coupled with the same degree of ossification as ancestral condition. Line leading to D2 represents an acceleration in the rate of ossification combined with earlier onset, which produces a derived condition of smaller body size coupled with a higher degree of skeletal ossification.
Mentions: In contrast to the pattern in branchiosaurids, the mosaic of developmental features in M. pelikani and H. longicostatum may actually point to peramorphosis of skeletal growth in the latter two taxa. If the ancestral tetrapod condition was to grow large while either remaining in the water or gradually completing a transition to land, then neoteny in the sense of a deceleration of somatic development [29,32] would lead to a derived condition with a lesser degree of skeletal ossification than the ancestral condition (Fig 38). In order to produce a derived condition in which smaller body size is coupled with a degree of skeletal ossification similar to the ancestral condition, at a minimum there must be an earlier onset of ossification (i.e., pre-displacement). An additional acceleration in the rate of ossification is necessary to produce a derived condition exhibiting both smaller body size and a higher degree of skeletal ossification, as in lepospondyls (Fig 38).

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