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Reacquisition of the lower temporal bar in sexually dimorphic fossil lizards provides a rare case of convergent evolution.

Simões TR, Funston GF, Vafaeian B, Nydam RL, Doschak MR, Caldwell MW - Sci Rep (2016)

Bottom Line: An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release.The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages.This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada.

ABSTRACT
Temporal fenestration has long been considered a key character to understand relationships amongst reptiles. In particular, the absence of the lower temporal bar (LTB) is considered one of the defining features of squamates (lizards and snakes). In a re-assessment of the borioteiioid lizard Polyglyphanodon sternbergi (Cretaceous, North America), we detected a heretofore unrecognized ontogenetic series, sexual dimorphism (a rare instance for Mesozoic reptiles), and a complete LTB, a feature only recently recognized for another borioteiioid, Tianyusaurus zhengi (Cretaceous, China). A new phylogenetic analysis (with updates on a quarter of the scorings for P. sternbergi) indicates not only that the LTB was reacquired in squamates, but it happened independently at least twice. An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release. The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages. This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

No MeSH data available.


Related in: MedlinePlus

Results of the FEA of the skull of Iguana iguana, testing how distinct models for a LTB would affect mechanical stress during hard biting.Results are displayed as contours representing combined-axes von Mises stress. (a) model A, lizard with quadratojugal ligament; (b) model B, lizard with jugomandibular ligament; (c) model C, lizard with the addition of a complete LTB, sutured to the quadrate; (d) model D, lizard with the addition of a complete LTB, connected to the quadrate by a short quadratojugal ligament. Red arrows indicate the points in model “A” with increase of stress compared to model “B” (the ones with the least amount of increased stress regions); in model “C”, red and black arrows represent areas of increased and reduces stress, respectively, compared to model “A”. Model “D” had a very similar distribution of stress to model “A”.
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f5: Results of the FEA of the skull of Iguana iguana, testing how distinct models for a LTB would affect mechanical stress during hard biting.Results are displayed as contours representing combined-axes von Mises stress. (a) model A, lizard with quadratojugal ligament; (b) model B, lizard with jugomandibular ligament; (c) model C, lizard with the addition of a complete LTB, sutured to the quadrate; (d) model D, lizard with the addition of a complete LTB, connected to the quadrate by a short quadratojugal ligament. Red arrows indicate the points in model “A” with increase of stress compared to model “B” (the ones with the least amount of increased stress regions); in model “C”, red and black arrows represent areas of increased and reduces stress, respectively, compared to model “A”. Model “D” had a very similar distribution of stress to model “A”.

Mentions: Among the four models, model B was the best suited for forceful biting. Different regions of the skull showed reduced stress compared with the other models (Fig. 5), apart from a minor increased stress in the nasal process of the maxilla. Model A had comparable areas of the skull with increased stress, such as: the ventral margin of the orbit, parts of the upper temporal bar, the ventral side of the basisphenoid, and especially on the posterior crest of the quadrate, as well as the quadrate process of the pterygoid. The pressure maps also indicate greater compression in the upper temporal and postorbital bars, and greater compression and tension in the pterygoids for model A when compared to B. Von Mises stress values were particularly higher on the quadrate, and especially on the pterygoid (43.2% higher—Supplementary Table S4). Our results thus suggest that the jugomandibular ligament represents a derived condition in squamates, emerging in forms with robust akinetic skulls, built for strenuous biting, as previously hypothesized28.


Reacquisition of the lower temporal bar in sexually dimorphic fossil lizards provides a rare case of convergent evolution.

Simões TR, Funston GF, Vafaeian B, Nydam RL, Doschak MR, Caldwell MW - Sci Rep (2016)

Results of the FEA of the skull of Iguana iguana, testing how distinct models for a LTB would affect mechanical stress during hard biting.Results are displayed as contours representing combined-axes von Mises stress. (a) model A, lizard with quadratojugal ligament; (b) model B, lizard with jugomandibular ligament; (c) model C, lizard with the addition of a complete LTB, sutured to the quadrate; (d) model D, lizard with the addition of a complete LTB, connected to the quadrate by a short quadratojugal ligament. Red arrows indicate the points in model “A” with increase of stress compared to model “B” (the ones with the least amount of increased stress regions); in model “C”, red and black arrows represent areas of increased and reduces stress, respectively, compared to model “A”. Model “D” had a very similar distribution of stress to model “A”.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4829860&req=5

f5: Results of the FEA of the skull of Iguana iguana, testing how distinct models for a LTB would affect mechanical stress during hard biting.Results are displayed as contours representing combined-axes von Mises stress. (a) model A, lizard with quadratojugal ligament; (b) model B, lizard with jugomandibular ligament; (c) model C, lizard with the addition of a complete LTB, sutured to the quadrate; (d) model D, lizard with the addition of a complete LTB, connected to the quadrate by a short quadratojugal ligament. Red arrows indicate the points in model “A” with increase of stress compared to model “B” (the ones with the least amount of increased stress regions); in model “C”, red and black arrows represent areas of increased and reduces stress, respectively, compared to model “A”. Model “D” had a very similar distribution of stress to model “A”.
Mentions: Among the four models, model B was the best suited for forceful biting. Different regions of the skull showed reduced stress compared with the other models (Fig. 5), apart from a minor increased stress in the nasal process of the maxilla. Model A had comparable areas of the skull with increased stress, such as: the ventral margin of the orbit, parts of the upper temporal bar, the ventral side of the basisphenoid, and especially on the posterior crest of the quadrate, as well as the quadrate process of the pterygoid. The pressure maps also indicate greater compression in the upper temporal and postorbital bars, and greater compression and tension in the pterygoids for model A when compared to B. Von Mises stress values were particularly higher on the quadrate, and especially on the pterygoid (43.2% higher—Supplementary Table S4). Our results thus suggest that the jugomandibular ligament represents a derived condition in squamates, emerging in forms with robust akinetic skulls, built for strenuous biting, as previously hypothesized28.

Bottom Line: An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release.The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages.This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada.

ABSTRACT
Temporal fenestration has long been considered a key character to understand relationships amongst reptiles. In particular, the absence of the lower temporal bar (LTB) is considered one of the defining features of squamates (lizards and snakes). In a re-assessment of the borioteiioid lizard Polyglyphanodon sternbergi (Cretaceous, North America), we detected a heretofore unrecognized ontogenetic series, sexual dimorphism (a rare instance for Mesozoic reptiles), and a complete LTB, a feature only recently recognized for another borioteiioid, Tianyusaurus zhengi (Cretaceous, China). A new phylogenetic analysis (with updates on a quarter of the scorings for P. sternbergi) indicates not only that the LTB was reacquired in squamates, but it happened independently at least twice. An analysis of the functional significance of the LTB using proxies indicates that, unlike for T. zhengi, this structure had no apparent functional advantage in P. sternbergi, and it is better explained as the result of structural constraint release. The observed canalization against a LTB in squamates was broken at some point in the evolution of borioteiioids, whereas never re-occuring in other squamate lineages. This case of convergent evolution involves a mix of both adaptationist and structuralist causes, which is unusual for both living and extinct vertebrates.

No MeSH data available.


Related in: MedlinePlus