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The integument of Cambrian chancelloriids

100%

Bengtson S., Hou X.

Acta Palaeontologica Polonica

2001

tom 46

nr 1

Details of the body surface of the chancelloriid Allonnia from the Lower Cambrian Chengjiang biota in southwestern China elucidate the nature of these enigmatic organisms. Rhombically arranged elements, about 30 x 60 ym, are interpreted as representing imbricating platelets, the distal ends of which projected as spinules from the body surface. Comparisons with other chancelloriids suggest that the flexible integument was continuous with the aragonitic sclerites that sit on the surface like cactus spines, and that both were formed by an epidermal epithelium secreting a continuous exo- and endocuticle. In the sclerites, the exocuticle was mineralized; the unmineralized endocuticle and cellular extensions from the epithelium filled the interior of the sclerites. In the flexible integument the epithelium was overlain by endocuticle and unmineralized exocuticle. This structure of soft integument and sclerites is at variance with proposals of poriferan or ascidian affinity of chancelloriids but in accord with a coeloscleritophoran model.

Skeletonized microfossils from the Lower-Middle Cambrian transition of the Cantabrian Mountains, Northern Spain

100%

Clausen S., Alvaro J. J.

Acta Palaeontologica Polonica

2006

tom 51

nr 2

Two different assemblages of skeletonized microfossils are recorded in bioclastic shoals that cross the Lower–Middle Cambrian boundary in the Esla nappe, Cantabrian Mountains. The uppermost Lower Cambrian sedimentary rocks represent a ramp with ooid−bioclastic shoals that allowed development of protected archaeocyathan−microbial reefs. The shoals yield abundant debris of tube−shelled microfossils, such as hyoliths and hyolithelminths (Torellella), and trilobites. The overlying erosive unconformity marks the disappearance of archaeocyaths and the Iberian Lower–Middle Cambrian boundary. A different assemblage occurs in the overlying glauconitic limestone associated with development of widespread low−relief bioclastic shoals. Their lowermost part is rich in hyoliths, hexactinellid, and heteractinid sponge spicules (Eiffelia), chancelloriid sclerites (at least six form species of Allonnia, Archiasterella, and Chancelloria), cambroclaves (Parazhijinites), probable eoconchariids (Cantabria labyrinthica gen. et sp. nov.), sclerites of uncertain affinity (Holoplicatella margarita gen. et sp. nov.), echinoderm ossicles and trilobites. Although both bioclastic shoal complexes represent similar high−energy conditions, the unconformity at the Lower–Middle Cambrian boundary marks a drastic replacement of microfossil assemblages. This change may represent a real community replacement from hyolithelminth−phosphatic tubular shells to CES (chancelloriid−echinoderm−sponge) meadows. This replacement coincides with the immigration event based on trilobites previously reported across the boundary, although the partial information available from originally carbonate skeletons is also affected by taphonomic bias.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

1 Alvaro J. J.

1 Bengtson S.

1 Clausen S.

1 Hou X.

1 2006

1 2001

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The integument of Cambrian chancelloriids

Skeletonized microfossils from the Lower-Middle Cambrian transition of the Cantabrian Mountains, Northern Spain