Leonardo Maiorino, Andrew A. Farke, Tassos Kotsakis and Paolo Piras (2017)
Macroevolutionary patterns in cranial and lower jaw shape of ceratopsian dinosaurs (Dinosauria, Ornithischia): phylogeny, morphological integration, and evolutionary rates.
Evolutionary Ecology Research 18: 123-167
Organisms: Ceratopsians were herbivorous, beaked dinosaurs, ranging from 1 m to 9 m in body length, usually four-footed, and with a bony frill that extended backwards from the cranium over the nape of the neck. Known from Asia, Europe, and North America, they appeared in the Late Jurassic and persisted until the end of the Late Cretaceous.
Questions: Which evolutionary processes drive the phenotypic evolution of skulls and lower jaws within Ceratopsia? What is the degree of morphological integration between the skull and lower jaw, and between the snout and frill among clades? Finally, are there any morphological evolution rate shifts across the ceratopsian phylogeny?
Data: Photographs from 121 ceratopsian skulls and 122 lower jaws in lateral view, both from original photos and published pictures. Fifty-five ceratopsian species are represented in the sample.
Methods: We investigated cranial and lower jaw shape changes across ceratopsians applying two-dimensional geometric morphometrics. We also investigated the morphological variation of the snout and the frill. Using phylogenetic generalized least squares regression, we estimated the degree of phylogenetic signal in size and shape data, as well as in the shape–size relationship. We performed phenotypic evolutionary rate analysis on shape data to describe phenotypic shifts across the phylogeny. Using a rarefied version of Escoufier’s RV coefficient, we tested morphological integration between skulls and lower jaws, and between snouts and frills. Finally, we explored the potential link between cranial and frill shape evolution in ceratopsians and the radiation of angiosperms using a linear regression model.
Results: Skull, snout, and frill shapes differ among clades (with the exception of leptoceratopsids and protoceratopsids). Lower jaws show distinct morphologies among groups. Size and shape changes are phylogenetically structured. The frill drives the morphological variation of the skull, co-varying much more with the lower jaw than with the snout. The frill appears to evolve to co-vary better with the lower jaw in the more morphologically derived clades than in basal ones. A significant linear relationship does exist between cranial shape and angiosperm occurrences, suggesting the hypothesis that the frill evolved in response to changes in dietary compositions associated with the turnover between gymnosperms and angiosperms during the Cretaceous. Significant negative shifts in evolutionary rates characterize skull, snout, frill, and lower jaw shapes, corresponding to nodes where psittacosaurids diverge from other taxa. In contrast, a significant positive shift in skull and snout shape rate of evolution characterizes the clade Ceratopsoidea.
Conclusion: The frill is the main driving force in the overall cranial shape within Ceratopsia and evolved secondarily to better co-vary with the lower jaw to produce a more efficient masticatory apparatus. The changes in frill shape are correlated with the angiosperm diversification that occurred in the Cretaceous and thus correlated with changes in diet. Ceratopsians exhibit a slowdown in the phenotypic evolutionary rate in the Early Cretaceous and an acceleration of the phenotypic rate in the Late Cretaceous.
Ji Qiang, Wang Xuri, Ji Yannan, et al. (2016)
Feathers or scales?
Journal of Geology 40(4):535-544
This paper briefly introduces the structures, morphotypes, developmental processes, origin and early evolution of feathers, and discusses the characters of "single longate filamentous feathers". It is considered that those elongate filamentous structures are highly modified scales, rather than feathers. As we know, feathers are homologous with scales of reptiles, and they are derived from the scales of dinosaurs. However, feathers can be distinguished from scales biologically by the differentiation of follicle collar and the development of barb ridges, and morphologically by the development of calamus and barbs. In addition, this paper also discusses a psittacosaurid dinosaur with elongate filamentous structures housed by Senckenbergische Naturforschende Gesellschaft, Frankfurt of Germany and a basal neornithischian dinosaur (*Kulindadromeus zabaikalicus*) from the Jurassic of Siberia with small scales and feather-like structures. It is inferred that both of them do develop highly modified scales, rather than feathers. Maybe feather-like structures can coexist with scales and potentially be widespread among the entire dinosaur clade, but they are not feathers. It is an unwarranted imagination that feathers may have been present in the earliest dinosaurs.