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[dinosaur] Evolutionary origin of mammal jaw yaw + mammal neck length + bird furcula bone histology + Genibatrachus + more

Ben Creisler

Some recent non-dino papers that may be of interest:

Free pdf:

David M. Grossnickle (2017)
The evolutionary origin of jaw yaw in mammals.
Scientific Reports 7, Article number: 45094 (2017)

Theria comprises all but three living mammalian genera and is one of the most ecologically pervasive clades on Earth. Yet, the origin and early history of therians and their close relatives (i.e., cladotherians) remains surprisingly enigmatic. A critical biological function that can be compared among early mammal groups is mastication. Morphometrics and modeling analyses of the jaws of Mesozoic mammals indicate that cladotherians evolved musculoskeletal anatomies that increase mechanical advantage during jaw rotation around a dorsoventrally-oriented axis (i.e., yaw) while decreasing the mechanical advantage of jaw rotation around a mediolaterally-oriented axis (i.e., pitch). These changes parallel molar transformations in early cladotherians that indicate their chewing cycles included significant transverse movement, likely produced via yaw rotation. Thus, I hypothesize that cladotherian molar morphologies and musculoskeletal jaw anatomies evolved concurrently with increased yaw rotation of the jaw during chewing cycles. The increased transverse movement resulting from yaw rotation may have been a crucial evolutionary prerequisite for the functionally versatile tribosphenic molar morphology, which underlies the molars of all therians and is retained by many extant clades.


Patrick Arnold, Eli Amson & Martin S. Fischer (2017)
Differential scaling patterns of vertebrae and the evolution of neck length in mammals.
Evolution (advance online publication)
DOI: 10.1111/evo.13232

Almost all mammals have seven vertebrae in their cervical spines. This consistency represents one of the most prominent examples of morphological stasis in vertebrae evolution. Hence, the requirements associated with evolutionary modifications of neck length have to be met with a fixed number of vertebrae. It has not been clear whether body size influences the overall length of the cervical spine and its inner organization (i.e., if the mammalian neck is subject to allometry). Here, we provide the first large scale analysis of the scaling patterns of the cervical spine and its constituting cervical vertebrae.

Our findings reveal that the opposite allometric scaling of C1 and C2-C7 accommodate the increase of neck bending moment with body size. The internal organization of the neck skeleton exhibits surprisingly uniformity in the vast majority of mammals. Deviations from this general pattern only occur under extreme loading regimes associated with particular functional and allometric demands. Our results indicate that the main source of variation in the mammalian neck stems from the disparity of overall cervical spine length. The mammalian neck reveals how evolutionary disparity manifests itself in a structure that is otherwise highly restricted by meristic constraints.


Now in final form...

Free pdf:

Mark E. Samuels, Sophie Regnault & John R. Hutchinson​ (2017)
Evolution of the patellar sesamoid bone in mammals. 
PeerJ 5:e3103
DOI:  https://doi.org/10.7717/peerj.3103

The patella is a sesamoid bone located in the major extensor tendon of the knee joint, in the hindlimb of many tetrapods. Although numerous aspects of knee morphology are ancient and conserved among most tetrapods, the evolutionary occurrence of an ossified patella is highly variable. Among extant (crown clade) groups it is found in most birds, most lizards, the monotreme mammals and almost all placental mammals, but it is absent in most marsupial mammals as well as many reptiles. Here, we integrate data from the literature and first-hand studies of fossil and recent skeletal remains to reconstruct the evolution of the mammalian patella. We infer that bony patellae most likely evolved between four and six times in crown group Mammalia: in monotremes, in the extinct multituberculates, in one or more stem-mammal genera outside of therian or eutherian mammals and up to three times in therian mammals. Furthermore, an ossified patella was lost several times in mammals, not including those with absent hindlimbs: once or more in marsupials (with some re-acquisition) and at least once in bats. Our inferences about patellar evolution in mammals are reciprocally informed by the existence of several human genetic conditions in which the patella is either absent or severely reduced. Clearly, development of the patella is under close genomic control, although its responsiveness to its mechanical environment is also important (and perhaps variable among taxa). Where a bony patella is present it plays an important role in hindlimb function, especially in resisting gravity by providing an enhanced lever system for the knee joint. Yet the evolutionary origins, persistence and modifications of a patella in diverse groups with widely varying habits and habitats—from digging to running to aquatic, small or large body sizes, bipeds or quadrupeds—remain complex and perplexing, impeding a conclusive synthesis of form, function, development and genetics across mammalian evolution. This meta-analysis takes an initial step toward such a synthesis by collating available data and elucidating areas of promising future inquiry.


Jessica Mitchell, Lucas J. Legendre, Christine Lefèvre & Jorge Cubo (2017)

Bone histological correlates of soaring and high-frequency flapping flight in the furculae of birds.

Zoology (advance online publication)

doi: http://dx.doi.org/10.1016/j.zool.2017.03.004





Furculae of soaring birds have increased Haversian bone density.

High-frequency flapping flight is not different from low-frequency flapping.

Depression forces and size increase in soaring birds may explain Haversian bone increase



The furcula is a specialized bone in birds involved in flight function. Its morphology has been shown to reflect different flight styles from soaring/gliding birds, subaqueous flight to high-frequency flapping flyers. The strain experienced by furculae can vary depending on flight type. Bone remodeling is a response to damage incurred from different strain magnitudes and types. In this study, we tested whether a bone microstructural feature, namely Haversian bone density, differs in birds with different flight styles, and reassessed previous work using phylogenetic comparative methods that assume an evolutionary model with additional taxa. We show that soaring birds have higher Haversian bone densities than birds with a flapping style of flight. This result is probably linked to the fact that the furculae of soaring birds provide less protraction force and more depression force than furculae of birds showing other kinds of flight. The whole bone area is another explanatory factor, which confirms the fact that size is an important consideration in Haversian bone development. All birds, however, display Haversian bone development in their furculae, and other factors like age could be affecting the response of Haversian bone development.


Taylor B. Wise and C. Tristan Stayton (2017)
Side-necked Versus Hidden-necked: A Comparison of Shell Morphology Between Pleurodiran and Cryptodiran Turtles.
Herpetologica 73(1):18-29. 2017 
doi: http://dx.doi.org/10.1655/HERPETOLOGICA-D-15-00038

This study explores the potential morphological differences between pleurodiran and cryptodiran turtle shells. Pleurodires (Suborder Pleurodira), as suggested by their name, retract their heads into the shell by bending the neck to the side; this is different from cryptodires (Suborder Cryptodira), which withdraw their necks along a sagittal plane. Pleurodires also have evolved a unique pelvic morphology typified by two columnar pelvic elements sutured to both the carapace and plastron; in contrast, the pelvic girdle in cryptodires is loosely connected to the shell. These differences might put very different selective pressures on shell shape in these animals: The different head retraction strategies in the two groups could lead pleurodires to evolve wider and flatter anterior apertures while the differences in pelvic structure might permit the evolution of flatter carapaces (particularly in the posterior carapace) and narrower bridges as compared to cryptodires. We used 3D landmark data to characterize shell shape and used phylomorphospace plots to examine patterns of diversification within pleurodires. We also used analyses of variance to examine morphological differences between the shell shapes of cryptodire and pleurodire turtles. We found significant phylogenetic signal in pleurodire shell shape. Clustering of pleurodires in shape space primarily followed phylogeny, with all major clades occupying distinct regions of shell shape space. Pleurodires overlapped most strongly in shell shape with aquatic cryptodires, both swimmers (including sea turtles) and bottom-walkers (such as chelydrids). Future studies could seek additional support for the hypotheses, through mechanical tests of shell performance, that the mechanism of neck retraction or the presence of columnar pelvic girdles can influence the functional performance of turtle shells.


Free pdf:

Genibatrachus baoshanensis  

Ke-Qin Gao and Jianye Chen (2017)
A New Crown-Group Frog (Amphibia: Anura) from the Early Cretaceous of Northeastern Inner Mongolia, China.
American Museum Novitates Number 3876 :1-39. 2017 
doi: http://dx.doi.org/10.1206/3876.1

Based on 12 well-preserved skeletons of postmetamorphic individuals, a new crown-group frog taxon is named and described from the Lower Cretaceous Guanghua (upper part of Longjiang) Formation (stratigraphic equivalent of the world-famed Yixian Formation) exposed in Dayangshu Basin, Hulunbuir, in the far northeast of Inner Mongolia, China. The new taxon, Genibatrachus baoshanensis, documents another early Cretaceous anuran having reduction of the presacral vertebrae to eight in number, similar to several frog taxa of roughly the same age from Spain and Brazil. The new frog also displays several features that are ontogenetically and phylogenetically informative, including ontogenetic fusion of the palatine to the sphenethmoid, and ontogenetic fusion of ribs to the diapophyses of the posterior trunk vertebrae. In addition, the new discovery extends the geographic range of early Cretaceous frogs of the Jehol Biota northward to near the 50th parallel north in East Asia.