Abstract
The world-famous ‘Orsten’ Konservat-Lagerstätte has yielded detailed information about Cambrian arthropods and their morphology. Internal organs or soft tissues have, however, rarely been reported, an obvious palaeobiological drawback. In this study, we employed synchrotron radiation X-ray tomographic microscopy (SRXTM) to study microscopic ‘Orsten’ arthropods from the Cambrian of Sweden: Skara minuta and two phosphatocopine species, Hesslandona sp. and Hesslandona trituberculata. This exceptionally high-resolution technique reveals internal organs or soft tissues that allow detailed comparison with equivalent structures in extant crustaceans and functional inferences to be made. The S. minuta specimen shows the digestive system and muscles that extend to the extremities. The slanting anterior portion of the head and anterior position of the mouth with a straight oesophagus suggest a primarily brushing and scraping way of feeding. The prominent head appendage muscles indicate muscle strength and good capacity for food manipulation. In the phosphatocopines the bulbous labrum is one of the most prominent morphological structures of the body. All specimens analysed reveal pairs of muscle bundles within the labrum. Based on comparisons with extant crustacean relatives, these muscles would fulfil the function of moving the labrum up and down in order to open the buccal cavity. The results of this pilot study demonstrate that there is still much to be learned about the ‘Orsten’ taxa.
Highlights
The ‘Orsten’ Lagerstatte from Kinnekulle, on the southern border of Lake Vanern, Sweden, contains remarkably wellpreserved minute fossils from bituminous limestones (‘Orsten’) of uppermost mid-Cambrian through Furongian age (e.g. [1,2,3,4,5,6]). The discovery of these remarkable fossils in the mid-1970s has been followed by a sequence of investigations revealing, among other things, morphological details of exceptional interest for understanding the evolution of, and relationships among, early arthropods
Diagenetic phosphatisation can produce a variety of shapes and textures that may be mistaken for fossilised soft tissues [24,25,26]
Taphonomy The fossils of the ‘Orsten’ Konservat-Lagerstatte are preserved by means of phosphate encrustation and impregnation of both external and, as shown in this study, internal organs of animals during early diagenesis, producing pristine three-dimensional preservation of fossils ([4,6] and references therein)
Summary
The ‘Orsten’ Lagerstatte from Kinnekulle, on the southern border of Lake Vanern, Sweden, contains remarkably wellpreserved minute fossils from bituminous limestones (‘Orsten’) of uppermost mid-Cambrian through Furongian (upper Cambrian) age (e.g. [1,2,3,4,5,6]). Approximately one hundred fairly well-preserved, juvenile specimens of the agnostoid Agnostus pisiformis have been recovered in ‘Orsten’type preservation [2], only one specimen interpreted as a polymerid trilobite hypostome with associated soft tissues has hitherto been discovered [12]. This collectively suggests that the arthropod faunas of this age are taphonomically biased and that the dominance of polymerids and agnostoids in the conventional fossil record does not necessarily represent the true, original faunal composition of arthropods
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