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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

Branchial plates of M. pelikani, NHMW1898_X_29.Arrows point to plates. Ventral view, anterior up, medial to the left. Note the triangular processes projecting laterally from the ventral-most branchial plate. Scale bar = 1mm.
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pone.0128333.g005: Branchial plates of M. pelikani, NHMW1898_X_29.Arrows point to plates. Ventral view, anterior up, medial to the left. Note the triangular processes projecting laterally from the ventral-most branchial plate. Scale bar = 1mm.

Mentions: A major point of historical contention concerning Microbrachis is whether or not the taxon possessed external gills. I studied nearly all known specimens of M. pelikani and examined the structures previously identified as branchial plates [1]. The structures are present in specimens spanning nearly the total size range observed for the taxon, suggesting that they persisted throughout ontogeny, although overall their preservation is relatively rare [16]. My interpretation is that the bony structures probably are branchial plates, although the presence of small, round or octagonal scales in the same location in other microsaurs like Pantylus and Saxonerpeton ([1]; pers. obs.), as well as the disorganized arrangement of the plates in Microbrachis, cast some doubt. The best evidence for the identification of the structures as branchial plates comes from NHMW1898_X_29 (Naturhistorisches Museum, Vienna, Austria), an individual that exhibits such good preservation that under high magnification three triangular-shaped structures, perhaps denticles, can be observed projecting along one side of the plates (Fig 5). In that individual and other specimens in which the plates are present, only a shallow impression remains, attesting to the fragility or perhaps low level of ossification of these structures. Impressions are usually round or oval, and in better-preserved specimens there is a raised spot at the center, which produces a donut or ring-shaped element in casts. The structures frequently are associated with the interclavicle, because both are visible mainly when specimens are preserved in ventral view. When present, the branchial plates generally are found close to the edge of the interclavicle, located between it and the anteriormost vertebrae, when the interclavicle has been displaced from the midline.


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

Olori JC - PLoS ONE (2015)

Branchial plates of M. pelikani, NHMW1898_X_29.Arrows point to plates. Ventral view, anterior up, medial to the left. Note the triangular processes projecting laterally from the ventral-most branchial plate. Scale bar = 1mm.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g005: Branchial plates of M. pelikani, NHMW1898_X_29.Arrows point to plates. Ventral view, anterior up, medial to the left. Note the triangular processes projecting laterally from the ventral-most branchial plate. Scale bar = 1mm.
Mentions: A major point of historical contention concerning Microbrachis is whether or not the taxon possessed external gills. I studied nearly all known specimens of M. pelikani and examined the structures previously identified as branchial plates [1]. The structures are present in specimens spanning nearly the total size range observed for the taxon, suggesting that they persisted throughout ontogeny, although overall their preservation is relatively rare [16]. My interpretation is that the bony structures probably are branchial plates, although the presence of small, round or octagonal scales in the same location in other microsaurs like Pantylus and Saxonerpeton ([1]; pers. obs.), as well as the disorganized arrangement of the plates in Microbrachis, cast some doubt. The best evidence for the identification of the structures as branchial plates comes from NHMW1898_X_29 (Naturhistorisches Museum, Vienna, Austria), an individual that exhibits such good preservation that under high magnification three triangular-shaped structures, perhaps denticles, can be observed projecting along one side of the plates (Fig 5). In that individual and other specimens in which the plates are present, only a shallow impression remains, attesting to the fragility or perhaps low level of ossification of these structures. Impressions are usually round or oval, and in better-preserved specimens there is a raised spot at the center, which produces a donut or ring-shaped element in casts. The structures frequently are associated with the interclavicle, because both are visible mainly when specimens are preserved in ventral view. When present, the branchial plates generally are found close to the edge of the interclavicle, located between it and the anteriormost vertebrae, when the interclavicle has been displaced from the midline.

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