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Friday, March 4th, 2022
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| 2:02a | [Paleontology • 2022] Yorkicystis haefneri • Cambrian Edrioasteroid reveals New Mechanism for Secondary Reduction of the Skeleton in Echinoderms
Abstract Echinoderms are characterized by a distinctive high-magnesium calcite endoskeleton as adults, but elements of this have been drastically reduced in some groups. Herein, we describe a new pentaradial echinoderm, Yorkicystis haefneri n. gen. n. sp., which provides, to our knowledge, the oldest evidence of secondary non-mineralization of the echinoderm skeleton. This material was collected from the Cambrian Kinzers Formation in York (Pennsylvania, USA) and is dated as ca 510 Ma. Detailed morphological observations demonstrate that the ambulacra (i.e. axial region) are composed of flooring and cover plates, but the rest of the body (i.e. extraxial region) is preserved as a dark film and lacks any evidence of skeletal plating. Moreover, X-ray fluorescence analysis reveals that the axial region is elevated in iron. Based on our morphological and chemical data and on taphonomic comparisons with other fossils from the Kinzers Formation, we infer that the axial region was originally calcified, while the extraxial region was non-mineralized. Phylogenetic analyses recover Yorkicystis as an edrioasteroid, indicating that this partial absence of skeleton resulted from a secondary reduction. We hypothesize that skeletal reduction resulted from lack of expression of the skeletogenic gene regulatory network in the extraxial body wall during development. Secondary reduction of the skeleton in Yorkicystis might have allowed for greater flexibility of the body wall. Keywords: Cambrian, skeleton, development, echinoderms, evolution  Yorkicystis haefneri n. gen. n. sp. Samuel Zamora, Imran A. Rahman, Colin D. Sumrall, Adam P. Gibson and Jeffrey R. Thompson. 2022. Cambrian Edrioasteroid reveals New Mechanism for Secondary Reduction of the Skeleton in Echinoderms. Proc. R. Soc. B. 289; 20212733. DOI: 10.1098/rspb.2021.2733 | ||
| 2:48p | [Paleontology • 2022] Bashanosaurus primitivus • New Stegosaurs (Ornithischia, Stegosauria) from the Middle Jurassic Lower Member of the Shaximiao Formation of Chongqing, China
ABSTRACT Stegosaurs are a major clade of ornithischian dinosaurs, yet because of their fragmentary fossil record, their interrelationships and early evolution are poorly understood. Here, we describe a new stegosaur, Bashanosaurus primitivus, gen. et sp. nov., and some other indeterminate stegosaur materials. We provide new U-Pb detrital zircon ages for horizons bounding the holotypic quarry in the Lower Member of the Shaximiao Formation, Yunyang, Chongqing Municipality, China, which indicate a Middle Jurassic (Bajocian) age. Bashanosaurus represents the earliest record of stegosaurs in Asia and one of the earliest records of this clade in the world. The dorsal vertebrae, scapula, coracoid, femur, and plates of Bashanosaurus primitivus possess several unique characters among Stegosauria, including the elevation of the parapophyses of dorsal vertebrae on stalks at the base of the transverse processes, a flared distal end of the scapula, and a small acromial process. Among armored dinosaurs (thyreophorans), these features are reminiscent of the basally branching taxon Scelidosaurus, indicating that Bashanosaurus possesses a unique mosaic of plesiomorphic thyreophoran and derived stegosaur features. Phylogenetic analysis shows that Bashanosaurus primitivus is the earliest-diverging stegosaur, along with Chungkingosaurus, from the Upper Member of the Shaximiao Formation, consistent with the early age of the taxon.  Bashanosaurus primitivus, gen. et sp. nov. Dai Hui, Li Ning, Susannah C. R. Maidment, Wei Guangbiao, Zhou Yuxuan, Hu Xufeng, Ma Qingyu, Wang Xunqian, Hu Haiqian and Peng Guangzhao. 2022. New Stegosaurs from the Middle Jurassic Lower Member of the Shaximiao Formation of Chongqing, China. Journal of Vertebrate Paleontology. e1995737. DOI: 10.1080/02724634.2021.1995737 |
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