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Bird embryos uncover homology and evolution of the dinosaur ankle.

Ossa-Fuentes L, Mpodozis J, Vargas AO - Nat Commun (2015)

Bottom Line: Unexpectedly, we found the ASC derives from the embryonic intermedium, an ancient element of the tetrapod ankle.The fact that the intermedium fails to fuse early with the tibiale and develops an ossification centre is unlike any other amniotes, yet resembles basal, amphibian-grade tetrapods.The ASC originated in early dinosaurs along changes to upright posture and locomotion, revealing an intriguing combination of functional innovation and reversion in its evolution.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Santiago 7800003, Chile.

ABSTRACT
The anklebone (astragalus) of dinosaurs presents a characteristic upward projection, the 'ascending process' (ASC). The ASC is present in modern birds, but develops a separate ossification centre, and projects from the calcaneum in most species. These differences have been argued to make it non-comparable to dinosaurs. We studied ASC development in six different orders of birds using traditional techniques and spin-disc microscopy for whole-mount immunofluorescence. Unexpectedly, we found the ASC derives from the embryonic intermedium, an ancient element of the tetrapod ankle. In some birds it comes in contact with the astragalus, and, in others, with the calcaneum. The fact that the intermedium fails to fuse early with the tibiale and develops an ossification centre is unlike any other amniotes, yet resembles basal, amphibian-grade tetrapods. The ASC originated in early dinosaurs along changes to upright posture and locomotion, revealing an intriguing combination of functional innovation and reversion in its evolution.

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The intermedium is present in birds and it becomes the ascending process.Whole-mount cartilage staining is shown for (a) Chilean tinamou, (b) mallard duck, (c) chicken, (d) rock pigeon, (e) budgerigar and (f) zebra finch. At HH29-HH31 three condensations appear, from lateral to medial: the calcaneum (also known as fibulare), intermedium and tibiale. Following the intermedium into later stages (HH32-35) reveals that it becomes the ascending process. At late stages (HH36), the three proximal cartilages of the ankle fuse. Five embryos per stage were used for Chilean Tinamou, Mallard Duck and Rock Pigeon. Ten embryos per stage were used for Zebrafinch, Budgerigar and Chicken. asc, ascending process cal, calcaneum; dt, distal tarsals; F, fibula; int, intermedium; T, tibia; tib, tibiale. Scale bars, Stages HH29-31, 100 μm; HH34, 200 μm; HH36-38, 300 μm.
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f1: The intermedium is present in birds and it becomes the ascending process.Whole-mount cartilage staining is shown for (a) Chilean tinamou, (b) mallard duck, (c) chicken, (d) rock pigeon, (e) budgerigar and (f) zebra finch. At HH29-HH31 three condensations appear, from lateral to medial: the calcaneum (also known as fibulare), intermedium and tibiale. Following the intermedium into later stages (HH32-35) reveals that it becomes the ascending process. At late stages (HH36), the three proximal cartilages of the ankle fuse. Five embryos per stage were used for Chilean Tinamou, Mallard Duck and Rock Pigeon. Ten embryos per stage were used for Zebrafinch, Budgerigar and Chicken. asc, ascending process cal, calcaneum; dt, distal tarsals; F, fibula; int, intermedium; T, tibia; tib, tibiale. Scale bars, Stages HH29-31, 100 μm; HH34, 200 μm; HH36-38, 300 μm.

Mentions: The cartilaginous development of the bird ankle is shown in Fig. 1, which can be compared with the common pattern for non-avian amniotes in Supplementary Fig. 1. While other studies have failed to document an intermedium in birds, we found that it was present in all six orders examined. It forms around stage HH29, in the usual pattern for amniotes: after the calcaneum and before the tibiale, temporally as well as spatially (from lateral to medial, Fig. 1, HH29-31. Compare with Supplementary Fig. 1a,b). The tibiale has been labelled alternatively as a centrale in other amniotes1112; however, we use the term tibiale following previous embryological studies of birds. Diffuse alcian blue staining is observed between elements, connecting the intermedium and tibiale, which may explain why modern studies only report a large tibiale13. However, distinct centres of cartilage formation for the tibiale and intermedium are evident (Fig. 1), especially on dissection (Supplementary Figs 2–4) and in histological sections (Fig. 2 and Supplementary Figs 5 and 6a). The latter reveals that ‘diffuse cartilage' corresponds to weak and disperse staining in the disorganized mesenchyme between elements, which is also present between the tibiale and calcaneum (Supplementary Fig. 6b).


Bird embryos uncover homology and evolution of the dinosaur ankle.

Ossa-Fuentes L, Mpodozis J, Vargas AO - Nat Commun (2015)

The intermedium is present in birds and it becomes the ascending process.Whole-mount cartilage staining is shown for (a) Chilean tinamou, (b) mallard duck, (c) chicken, (d) rock pigeon, (e) budgerigar and (f) zebra finch. At HH29-HH31 three condensations appear, from lateral to medial: the calcaneum (also known as fibulare), intermedium and tibiale. Following the intermedium into later stages (HH32-35) reveals that it becomes the ascending process. At late stages (HH36), the three proximal cartilages of the ankle fuse. Five embryos per stage were used for Chilean Tinamou, Mallard Duck and Rock Pigeon. Ten embryos per stage were used for Zebrafinch, Budgerigar and Chicken. asc, ascending process cal, calcaneum; dt, distal tarsals; F, fibula; int, intermedium; T, tibia; tib, tibiale. Scale bars, Stages HH29-31, 100 μm; HH34, 200 μm; HH36-38, 300 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4660350&req=5

f1: The intermedium is present in birds and it becomes the ascending process.Whole-mount cartilage staining is shown for (a) Chilean tinamou, (b) mallard duck, (c) chicken, (d) rock pigeon, (e) budgerigar and (f) zebra finch. At HH29-HH31 three condensations appear, from lateral to medial: the calcaneum (also known as fibulare), intermedium and tibiale. Following the intermedium into later stages (HH32-35) reveals that it becomes the ascending process. At late stages (HH36), the three proximal cartilages of the ankle fuse. Five embryos per stage were used for Chilean Tinamou, Mallard Duck and Rock Pigeon. Ten embryos per stage were used for Zebrafinch, Budgerigar and Chicken. asc, ascending process cal, calcaneum; dt, distal tarsals; F, fibula; int, intermedium; T, tibia; tib, tibiale. Scale bars, Stages HH29-31, 100 μm; HH34, 200 μm; HH36-38, 300 μm.
Mentions: The cartilaginous development of the bird ankle is shown in Fig. 1, which can be compared with the common pattern for non-avian amniotes in Supplementary Fig. 1. While other studies have failed to document an intermedium in birds, we found that it was present in all six orders examined. It forms around stage HH29, in the usual pattern for amniotes: after the calcaneum and before the tibiale, temporally as well as spatially (from lateral to medial, Fig. 1, HH29-31. Compare with Supplementary Fig. 1a,b). The tibiale has been labelled alternatively as a centrale in other amniotes1112; however, we use the term tibiale following previous embryological studies of birds. Diffuse alcian blue staining is observed between elements, connecting the intermedium and tibiale, which may explain why modern studies only report a large tibiale13. However, distinct centres of cartilage formation for the tibiale and intermedium are evident (Fig. 1), especially on dissection (Supplementary Figs 2–4) and in histological sections (Fig. 2 and Supplementary Figs 5 and 6a). The latter reveals that ‘diffuse cartilage' corresponds to weak and disperse staining in the disorganized mesenchyme between elements, which is also present between the tibiale and calcaneum (Supplementary Fig. 6b).

Bottom Line: Unexpectedly, we found the ASC derives from the embryonic intermedium, an ancient element of the tetrapod ankle.The fact that the intermedium fails to fuse early with the tibiale and develops an ossification centre is unlike any other amniotes, yet resembles basal, amphibian-grade tetrapods.The ASC originated in early dinosaurs along changes to upright posture and locomotion, revealing an intriguing combination of functional innovation and reversion in its evolution.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Santiago 7800003, Chile.

ABSTRACT
The anklebone (astragalus) of dinosaurs presents a characteristic upward projection, the 'ascending process' (ASC). The ASC is present in modern birds, but develops a separate ossification centre, and projects from the calcaneum in most species. These differences have been argued to make it non-comparable to dinosaurs. We studied ASC development in six different orders of birds using traditional techniques and spin-disc microscopy for whole-mount immunofluorescence. Unexpectedly, we found the ASC derives from the embryonic intermedium, an ancient element of the tetrapod ankle. In some birds it comes in contact with the astragalus, and, in others, with the calcaneum. The fact that the intermedium fails to fuse early with the tibiale and develops an ossification centre is unlike any other amniotes, yet resembles basal, amphibian-grade tetrapods. The ASC originated in early dinosaurs along changes to upright posture and locomotion, revealing an intriguing combination of functional innovation and reversion in its evolution.

Show MeSH
Related in: MedlinePlus