Ben Creisler
New tetrapod papers:
Free pdf:
Tetrapod life on land was the result of a lengthy process, the final steps of which resulted in full independence of amniotic tetrapods from the aquatic environment. Developmental strategies, including growth rate and the attainment of sexual maturity, played a major role in this transition. Early amniotes, such as Ophiacodon, tended to reach sexual maturity in a year while most non-amniotic Paleozoic tetrapods (including Devonian tetrapods and temnospondyls) became adult after 3 to 11 years. This ontogenetic transition is accompanied by a drastic change in growth rate and bone microstructure suggesting faster growth dynamics in early amniotes than in Devonian tetrapods and temnospondyls. Was the acquisition of a faster development (earlier sexual maturity and faster growth rate) a drastic evolutionary event or an extended process over geological time? To answer this question, the limb bone histology of two Early Permian (i.e., 270â290 million-year-old) stem-amniote seymouriamorphs, Seymouria sanjuanensis and Discosauriscus austriacus, were investigated. We used three-dimensional bone paleohistology based on propagation phase-contrast synchrotron microtomography. Both seymouriamorphs display relatively fast bone growth and dynamics (even though cyclic in the humerus of D. austriacus). This significantly contrasts with the slow primary bone deposition encountered in the stylopods of temnospondyls and Devonian (i.e., 360 million-year-old) stem tetrapods of similar sizes. On the basis of skeletochronological data, the seymouriamorph D. austriacus retained a long pre-reproductive period as observed in Devonian tetrapods and most temnospondyls. The combination of characteristics (faster growth rate but long pre-reproductive period) suggests that the shift toward an amniotic developmental strategy was an extended process in the evolutionary history of amniotes.
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Free pdf:
Free pdf:
http://www.ivpp.ac.cn/cbw/gjzdwxb/pressonline/202004/P020200415364051586740.pdfChroniosuchians have the earliest representatives in Gansu, China, but the Chinese records were scarcer compared to Russia, especially the Triassic one. In Xinjiang, there was only one specimen reported from the upper part of the Guodikeng Formation. Here three new chroniosuchian specimens are reported from three new stratigraphic horizons: the Quanzijie Formation (middle Permian), the base of the Guodikeng Formation (upper Permian), and the Jiucaiyuan Formation (Lower Triassic). The osteoderm from the Jiucaiyuan Formation represents the first definite Triassic chroniosuchian from China. The new findings increase the chroniosuchian diversity and their time range in China. The bystrowianid chroniosuchian specimens from the Guodikeng and Jiucaiyuan formations demonstrated that this group survived in the end-Permian mass extinction in Xinjiang, China.
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Also may be of interest (not free):
Pedro P. Rizzato, ÂAnna Pospisilova, ÂEric J. Hilton & FlÃvio A. Bockmann (2020)
Ontogeny and homology of cranial bones associated with lateralâline canals of the Senegal Bichir, Polypterus senegalus (Actinopterygii: Cladistii: Polypteriformes), with a discussion on the formation of lateralâline canal bones in fishes.
Journal of Anatomy (advance online publication)
doi:
https://doi.org/10.1111/joa.13202https://onlinelibrary.wiley.com/doi/10.1111/joa.13202
The association between lateralâline canals and skull bones in fishes has been the subject of several studies and raised a series of controversies, particularly with regard to the hypothesized role of lateralâline organs (i.e. neuromasts) in osteogenesis and the consequences for hypotheses of homology of the bones associated with lateralâline canals. Polypteridae, a group of freshwater fishes that occupies a key phylogenetic position as the most basal extant lineage of rayâfinned fishes (Actinopterygii), provides an interesting model for the study of the relationships between lateralâline canals and skull bones. We describe the development of bones associated with lateralâline canals in the Senegal Bichir, Polypterus senegalus, and use these data to reâaddress previous hypotheses of homology of skull bones of polypterids. We demonstrate that the lateralâline canals constitute a separate component of the dermatocranium that may interact with a membranodermal component, thereby forming compound bones in the adult. Differences in the interactions between these components determine the characteristics of the development of each independent bone in the skull of adult P. senegalus. Our results shed light on longâstanding controversies about the identity of skull bones such as the rostral, preopercle, and sphenotic in Polypteridae, and suggest the presence of an ancestral twoâcomponent pattern of formation of bones associated with lateralâline canals in bony fishes. These findings reveal the need to reâaddress previous hypotheses of homology of bones associated with lateralâline canals in different groups of bony fishes, especially fossil taxa.