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[dinosaur] Mesozoic bird mass and tooth loss + Stereosternum limb development + basal mosasauroid from Japan

Ben Creisler

Some recent non-dino papers:

Free pdf:

ZHOU Ya-Chun, Corwin SULLIVAN, ZHANG Fu-Cheng (2018)
Negligible effect of tooth reduction on body mass in Mesozoic birds.
Vertebrata PalAsiatica (advance online publication)
DOI: 10.19615/j.cnki.1000-3118.180307

Tooth reduction and loss was an important evolutionary process in Mesozoic birds. Analysis of evolutionary trends in the total mass of the dentition, a function of tooth size and tooth number, has the potential to shed light on the evolutionary pattern of tooth reduction and loss, and on the causes of this pattern. Because modern birds lack teeth, however, they cannot provide the basis for a model that would allow estimation of tooth masses in their Mesozoic counterparts. We selected the teeth of crocodilians as analogues of those in Mesozoic birds because the former are the closest living relatives of the latter, and the two groups are similar in tooth morphology, tooth implantation, and tooth replacement pattern. To estimate tooth masses in Mesozoic birds, we formulated four regression equations relating tooth mass to various linear dimensions, which were measured in 31 intact isolated teeth from eight individual crocodiles (Crocodylus siamensis). The results for Mesozoic birds show that dental mass as a proportion of body mass was negligible, at least from the perspective of flight performance, suggesting that selection pressure favoring body mass reduction was probably not the primary driver of tooth reduction or loss. Variations in dental mass among Mesozoic birds may reflect the different foods they ate, and the different types of feeding behavior they displayed.Â


Free paper:

Constanze Bickelmann and Linda A. Tsuji (2018)
A case study of developmental palaeontology in Stereosternum tumidum (Mesosauridae, Parareptilia).
Fossil Record 21: 109-118
doi: https://doi.org/10.5194/fr-21-109-2018

Ontogenetic series of extinct taxa are rare. However, if preserved, fossil embryos and juveniles can provide evidence of developmental plasticity as related to ecological specialization. Here, we describe articulated and isolated juvenile material found in close association with an adult mesosaurid Stereosternum tumidum (MB.R.2089) from Lower Permian sediments in Brazil, housed in the collection of the Museum fÃr Naturkunde Berlin. Stylopodial, zeugopodial, and autopodial elements are not yet completely ossified in the juveniles, as indicated by compression artifacts on the surface of the bone. These correspond to internal ossification processes, which have been demonstrated in other aquatic taxa. Quantitative analysis of measurements in juvenile and adult material reveals differing growth rates between limb elements: hind limb zeugopodia, which are massive and elongate in the adult as needed for propulsion, are already comparatively larger in the juvenile than the humeri, femora, and also the zeugopodia of the forelimb. This pattern differs from that seen in another extinct aquatic reptile, Hovasaurus boulei. Nevertheless, we attribute the accelerated growth rate or earlier onset of ossification to be a potential developmental pathway generating limb element variation in the adult present in 280 million year old mesosaurs, which are known for their fully aquatic lifestyle, in which the hind limbs play a more prominent role than the forelimbs.


Tamaki Sato, Takuya Konishi, Tomohiro Nishimura and Takeru Yoshimura (2018)
A Basal Mosasauroid from the Campanian (Upper Cretaceous) of Hokkaido, Northern Japan.
Paleontological Research 22(2): 156-166
doi: https://doi.org/10.2517/2017PR018

A basal mosasauroid specimen, including a rib and a vertebra from the middle to posterior portion of the trunk, is reported from the lower Campanian Inoceramus (Platyceramus) japonicus Zone in Obira Town, northern Hokkaido, northern Japan. It is the second occurrence of basal mosasauroids sensu lato in Japan after the halisaurine Phosphorosaurus ponpetelegans, but represents a larger individual than the P. ponpetelegans holotype. The Obira specimen predates the early Maastrichtian P. ponpetelegans by about 10 million years, indicating colonization by basal mosasauroids of the northwestern Pacific by at latest the early Campanian age. While the overall morphology of the Obira specimen agrees well with that of a halisaurine vertebra, the presence of well developed zygantra (zygosphenes missing postmortem if present), the inclined condyle, and a distinct precondylar constriction uniquely align the specimen with Pannoniasaurus inexpectatus, a Santonian-aged basal mosasauroid from freshwater deposits in Hungary.