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

Narial region in M. pelikani.Dorsal view of bones in St.200, anterior is up, lateral to the right. Top image includes line drawing interpretation of sutures observed in original photograph (bottom). Fr, frontal; La, lacrimal; Mx, maxilla; N, naris; Na, nasal; Or, orbit; Pmx, premaxilla. Scale bar is in 1 mm increments.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g007: Narial region in M. pelikani.Dorsal view of bones in St.200, anterior is up, lateral to the right. Top image includes line drawing interpretation of sutures observed in original photograph (bottom). Fr, frontal; La, lacrimal; Mx, maxilla; N, naris; Na, nasal; Or, orbit; Pmx, premaxilla. Scale bar is in 1 mm increments.

Mentions: The portion of the skull surrounding the external naris is often the least well preserved area, leading to frequent speculation about the narial margins. Despite having a larger sample size than those used in previous studies [1,9], I did not find evidence of a septomaxilla in M. pelikani. The element may be unossified or perhaps weakly ossified, resulting in poor preservation potential. The only new information that I can provide about the nasal is a more complete description of the contact with the lacrimal. As hinted at in illustrations by Carroll and Gaskill ([1] reference figures 77,78) and Vallin and Laurin ([9] reference figures 2,3), but not discussed by them, the nasal has a short, squared process that projects laterally to meet the anterior end of the lacrimal (Fig 7). That contact contributes to the posterior margin of the external naris and is located anterior to the prefrontal, the latter of which is, thus, excluded from the external naris. As reflected in previously published Thin Plate Splines (TPS; [17]), the anterolateral edge of the nasal that curves posteriorly around the naris to form the lacrimal process becomes relatively shorter in larger individuals, suggesting that the diameter of the naris decreases proportionately as well. The only other change exhibited by the nasal is a transition from angular to rounded edges during growth. A similar change occurs in the frontals, which start out with relatively straight margins in small specimens, but the margins become more sinuous with increased growth (Fig 8).


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

Olori JC - PLoS ONE (2015)

Narial region in M. pelikani.Dorsal view of bones in St.200, anterior is up, lateral to the right. Top image includes line drawing interpretation of sutures observed in original photograph (bottom). Fr, frontal; La, lacrimal; Mx, maxilla; N, naris; Na, nasal; Or, orbit; Pmx, premaxilla. Scale bar is in 1 mm increments.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0128333.g007: Narial region in M. pelikani.Dorsal view of bones in St.200, anterior is up, lateral to the right. Top image includes line drawing interpretation of sutures observed in original photograph (bottom). Fr, frontal; La, lacrimal; Mx, maxilla; N, naris; Na, nasal; Or, orbit; Pmx, premaxilla. Scale bar is in 1 mm increments.
Mentions: The portion of the skull surrounding the external naris is often the least well preserved area, leading to frequent speculation about the narial margins. Despite having a larger sample size than those used in previous studies [1,9], I did not find evidence of a septomaxilla in M. pelikani. The element may be unossified or perhaps weakly ossified, resulting in poor preservation potential. The only new information that I can provide about the nasal is a more complete description of the contact with the lacrimal. As hinted at in illustrations by Carroll and Gaskill ([1] reference figures 77,78) and Vallin and Laurin ([9] reference figures 2,3), but not discussed by them, the nasal has a short, squared process that projects laterally to meet the anterior end of the lacrimal (Fig 7). That contact contributes to the posterior margin of the external naris and is located anterior to the prefrontal, the latter of which is, thus, excluded from the external naris. As reflected in previously published Thin Plate Splines (TPS; [17]), the anterolateral edge of the nasal that curves posteriorly around the naris to form the lacrimal process becomes relatively shorter in larger individuals, suggesting that the diameter of the naris decreases proportionately as well. The only other change exhibited by the nasal is a transition from angular to rounded edges during growth. A similar change occurs in the frontals, which start out with relatively straight margins in small specimens, but the margins become more sinuous with increased growth (Fig 8).

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