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1. The original boneheads: histologic analysis of the pachyostotic skull roof in Permian burnetiamorphs (Therapsida: Biarmosuchia)

   https://doi.org/10.1111/joa.12987  

   Kulik, Zoe T. ; Sidor, Christian A. ( May 2019 , Journal of Anatomy)

   Thickened, pachyostotic skulls are best known in pachycephalosaur dinosaurs, but evolved convergently in Permian burnetiamorphs as well as in some other stem‐mammal groups and Triassic archosauromorphs. Until now, only pachycephalosaur domes have been histologically sampled to reveal patterns of bone tissue microstructure and growth. Using computed tomography and osteohistology, we serially thin‐sectioned one of the smallest burnetiamorph skull caps ever recovered (estimated skull length = 10 cm), as well as an individual nearly twice as large, and here report the first cranial histological data from this clade. We recognize four highly vascularized histological zones visible in coronal thin‐sections, only one of which shares morphological similarities with the tripartite zonation previously reported in pachycephalosaur domes. Zone A forms the endocranial region of the skull cap and records disorganized primary osteons in a fibrolamellar complex. Zone B preserves a border of compact, avascular layers of parallel‐fibered bone surrounding an interior of partially remodeled vascular canals. Interestingly, the outline of Zone B resembles the shape of an incipient skull roof. Zone C forms the thickest portion of the skull cap and is composed of fast‐growing woven bone with minimal osteonal development. The superficial Zone D has a matrix of predominantly woven bone with narrower primary vascular canals than in deeper regions of the skull caps. Unlike in pachycephalosaurs, where primary vascular porosity is thought to decrease through ontogeny, both burnetiamorph skull caps preserve a thick Zone C of highly vascularized tissue. Additionally, the remnants of sutures are visible as radial struts that taper superficially, leaving no trace on the surface of the skull. Even in the smallest individual, the sutures are closed ectocranially, which is unusual, given that some large, presumably adult pachycephalosaur domes preserve open sutural gaps. Although pachycephalosaur and burnetiamorph skull domes are superficially similar, histological analysis reveals differences in their vascularity and construction that imply multiple evolutionary pathways to form an elaborate pachyostotic dome.

    

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2. Permo-Triassic tetrapods and their climate implications

   https://doi.org/10.1016/j.gloplacha.2021.103618  

   Liu, Jun ; Angielczyk, Kenneth D. ; Abdala, Fernando ( August 2021 , Global and planetary change)

   The narrow active temperature ranges of ectothermic tetrapods can be used as proxies for reconstructing paleoclimates. Here we deduce the climatic preferences of major Permo-Triassic tetrapod groups based on their known geographic distributions, the critical thermal limits of living tetrapods, and paleoclimate information from other sources. The resulting preferred temperature sequence of amniotes places most Triassic archosauromorphs at the high end of the spectrum, with preferred temperatures over 32 ◦ C in some cases, followed by captorhinids, pareiasaurs, procolophonids, cynognathian cynodonts, dicynodonts (excluding Lystrosaurus), Proterosuchus fergusi, and finally Lystrosaurus at the lowest preferred temperature. The poleward distribution of Permian Lystrosaurus marks the border of cool temperate climates, whereas Triassic Lystrosaurus delineates the border of the arid zone. Most temnospondyls indicate the availability of perennial water sources. Captorhinids and pareiasaurs preferred dry climates, whereas dicynodonts preferred wetter conditions. Based on current evidence, central Pangea transitioned from an arid zone to a tropical zone during the late Olenekian. 

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3. Cranial synchondroses of primates at birth

   https://doi.org/10.1002/ar.24521  

   Smith, Timothy D. ; Reynolds, Rebecca L. ; Mano, Nanami ; Wood, Brody J. ; Oladipupo, Lanre ; Hughes, Gabriel K. ; Corbin, Hayley M. ; Taylor, Jane ; Ufelle, Alexander ; Burrows, Anne M. ; et al ( October 2020 , The Anatomical Record)

   Cranial synchondroses are cartilaginous joints between basicranial bones or between basicranial bones and septal cartilage, and have been implicated as having a potential active role in determining craniofacial form. However, few studies have examined them histologically. Using histological and immunohistochemical methods, we examined all basicranial joints in serial sagittal sections of newborn heads from nine genera of primates (five anthropoids, four strepsirrhines). Each synchondrosis was examined for characteristics of active growth centers, including a zonal distribution of proliferating and hypertrophic chondrocytes, as well as corresponding changes in matrix characteristics (i.e., density and organization of Type II collagen). Results reveal three midline and three bilateral synchondroses possess attributes of active growth centers in all species (sphenooccipital, intrasphenoidal, presphenoseptal). One midline synchondrosis (ethmoseptal) and one bilateral synchondrosis (alibasisphenoidal synchondrosis [ABS]) are active growth centers in some but not all newborn primates. ABS is oriented more anteriorly in monkeys compared to lemurs and bushbabies. The sphenoethmoidal synchondrosis (SES) varies at birth: in monkeys, it is a suture‐like joint (i.e., fibrous tissue between the two bones); however, in strepsirrhines, the jugum sphenoidale is ossified while the mesethmoid remains cartilaginous. No species possesses an SES that has the organization of a growth plate. Overall, our findings demonstrate that only four midline synchondroses have the potential to actively affect basicranial angularity and facial orientation during the perinatal timeframe, while the SES of anthropoids essentially transitions toward a “suture‐like” function, permitting passive growth postnatally. Loss of cartilaginous continuity at SES and reorientation of ABS distinguish monkeys from strepsirrhines.

    

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4. Underground Down Under: Skull anatomy of the southern blind snake Anilios australis Gray, 1845 (Typhlopidae: Serpentes: Squamata)

   https://doi.org/10.1002/ar.24696  

   Laver, Rebecca J. ; Daza, Juan D. ; Ellis, Ryan J. ; Stanley, Edward L. ; Bauer, Aaron M. ( June 2021 , The Anatomical Record)

   The cranial anatomy of blindsnakes has been markedly understudied, with the small size and relative rarity of encountering these subterranean reptiles being significant limiting factors. In this article, we re‐visit the skull anatomy of the Australian southern blind snakeAnilios australisGray, 1845 using microCT data, and produce the first complete atlas for the cranial anatomy of a representative of this speciose typhlopid genus. The skull is formed by 18 paired and four unpaired elements. We here produce a bone‐by‐bone description of each element as well as an inner ear endocast for each of two specimens differing in skull size. This approach has revealed the presence of a highly perforated dorsal plate on the septomaxilla―a structure convergent with the cribriform plate of the mammalian ethmoid bone―and a feature previously unknown for typhlopid snakes. This detailed anatomical study will facilitate ongoing taxonomic and systematic studies in the genusAniliosas well as provide comparative data for future studies on blindsnake anatomy more broadly.

    

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5. Developmental origins of the crocodylian skull table and platyrostral face

   https://doi.org/10.1002/ar.24802  

   Morris, Zachary S. ; Vliet, Kent A. ; Abzhanov, Arhat ; Pierce, Stephanie E. ( October 2021 , The Anatomical Record)

   The dorsoventrally flattened skull typifies extant Crocodylia perhaps more than any other anatomical feature and is generally considered an adaptation for semi‐aquatic feeding. Although the evolutionary origins of caniofacial flattening have been extensively studied, the developmental origins have yet to be explored. To understand how the skull table and platyrostral snout develop, we quantified embryonic development and post‐hatching growth (ontogeny) of the crocodylian skull in lateral view using geometric morphometrics. Our dataset (n = 103) includes all but one extant genus and all of the major ecomorphs, including the extremely slender‐snoutedGavialisandTomistoma. Our analysis reveals that the embryonic development of the flattened skull is remarkably similar across ecomorphs, including the presence of a conserved initial embryonic skull shape, similar to prior analysis of dorsal snout shape. Although differences during posthatching ontogeny are recovered among ecomorphs, embryonic patterns are not distinct, revealing an important shift in developmental rate near hatching. In particular, the flattened skull table is achieved by the end of embryonic development with no changes after hatching. Further, the rotation of skull roof and facial bones during development is critical for the stereotypical flatness of the crocodylian skull. Our results suggest selection on hatchling performance and constraints on embryonic skull shape may have been important in this pattern of developmental conservation. The appearance of aspects of cranial flatness among Jurassic stem crocodylians suggests key aspects of these cranial developmental patterns may have been conserved for over 200 million years.

    

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