Limits...
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 tibia of M. pelikani.A. Stage 1, NHMW1898-2400; proximal toward upper left. B. Stage 2, NHMW1898_X_33; proximal toward upper left. C. Stage 3, St.193, proximal toward top. D. Stage 4, MB.Am.840; proximal toward top right. Proximal end of tibia partially crushed under femur; arrows denote base of proximal end. Intf, intermedial facet; Fem, femur; Fib, fibula; Tib, tibia. Scale bars = 1mm.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4470922&req=5

pone.0128333.g025: Ontogenetic changes in the tibia of M. pelikani.A. Stage 1, NHMW1898-2400; proximal toward upper left. B. Stage 2, NHMW1898_X_33; proximal toward upper left. C. Stage 3, St.193, proximal toward top. D. Stage 4, MB.Am.840; proximal toward top right. Proximal end of tibia partially crushed under femur; arrows denote base of proximal end. Intf, intermedial facet; Fem, femur; Fib, fibula; Tib, tibia. Scale bars = 1mm.

Mentions: When initially ossified, the tibia of M. pelikani is a featureless column of bone (Fig 25A). Neither end of the tibia is greatly expanded at this stage, but the proximal end is barely wider than the distal end. During the next stage of morphogenesis, the proximal end is expanded much more than the distal end, leading to a size disparity that is maintained throughout ontogeny (Fig 25B). In contrast to a prior report by Carroll and Gaskill [1], the distal end is expanded relative to the tibial shaft, although the proximal end begins expansion before the distal end. Later in ontogeny, as a result of asymmetric narrowing of the tibial shaft, the tibia curves medially (Fig 25C). At about the same time, the head of the tibia slants medially. Finally, the distal end of the tibia slants medially to form the intermedial facet (Fig 25D). However, in many specimens with an ossified intermedium, the distal end of the tibia remains flat, although in a few individuals in which the tarsals are not developed yet, the facet is already present. Two specimens exhibit a tibial crest, but developmentally more advanced individuals seem to lack that structure. The ends of the tibia never become rounded and a distinct tibiale facet was not observed in any individual.


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 tibia of M. pelikani.A. Stage 1, NHMW1898-2400; proximal toward upper left. B. Stage 2, NHMW1898_X_33; proximal toward upper left. C. Stage 3, St.193, proximal toward top. D. Stage 4, MB.Am.840; proximal toward top right. Proximal end of tibia partially crushed under femur; arrows denote base of proximal end. Intf, intermedial facet; Fem, femur; Fib, fibula; Tib, tibia. Scale bars = 1mm.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g025: Ontogenetic changes in the tibia of M. pelikani.A. Stage 1, NHMW1898-2400; proximal toward upper left. B. Stage 2, NHMW1898_X_33; proximal toward upper left. C. Stage 3, St.193, proximal toward top. D. Stage 4, MB.Am.840; proximal toward top right. Proximal end of tibia partially crushed under femur; arrows denote base of proximal end. Intf, intermedial facet; Fem, femur; Fib, fibula; Tib, tibia. Scale bars = 1mm.
Mentions: When initially ossified, the tibia of M. pelikani is a featureless column of bone (Fig 25A). Neither end of the tibia is greatly expanded at this stage, but the proximal end is barely wider than the distal end. During the next stage of morphogenesis, the proximal end is expanded much more than the distal end, leading to a size disparity that is maintained throughout ontogeny (Fig 25B). In contrast to a prior report by Carroll and Gaskill [1], the distal end is expanded relative to the tibial shaft, although the proximal end begins expansion before the distal end. Later in ontogeny, as a result of asymmetric narrowing of the tibial shaft, the tibia curves medially (Fig 25C). At about the same time, the head of the tibia slants medially. Finally, the distal end of the tibia slants medially to form the intermedial facet (Fig 25D). However, in many specimens with an ossified intermedium, the distal end of the tibia remains flat, although in a few individuals in which the tarsals are not developed yet, the facet is already present. Two specimens exhibit a tibial crest, but developmentally more advanced individuals seem to lack that structure. The ends of the tibia never become rounded and a distinct tibiale facet was not observed in any individual.

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