Abstract
The early Cambrian Guanshan biota of eastern Yunnan, China, contains exceptionally preserved animals and algae. Most diverse and abundant are the arthropods, of which there are at least 11 species of trilobites represented by numerous specimens. Many trilobite specimens show soft-body preservation via iron oxide pseudomorphs of pyrite replacement. Here we describe digestive structures from two species of trilobite, Palaeolenus lantenoisi and Redlichia mansuyi. Multiple specimens of both species contain the preserved remains of an expanded stomach region (a “crop”) under the glabella, a structure which has not been observed in trilobites this old, despite numerous examples of trilobite gut traces from other Cambrian Lagerstätten. In addition, at least one specimen of Palaeolenus lantenoisi shows the preservation of an unusual combination of digestive structures: a crop and paired digestive glands along the alimentary tract. This combination of digestive structures has also never been observed in trilobites this old, and is rare in general, with prior evidence of it from one juvenile trilobite specimen from the late Cambrian Orsten fauna of Sweden and possibly one adult trilobite specimen from the Early Ordovician Fezouata Lagerstätte. The variation in the fidelity of preservation of digestive structures within and across different Lagerstätten may be due to variation in the type, quality, and point of digestion of food among specimens in addition to differences in mode of preservation. The presence and combination of these digestive features in the Guanshan trilobites contradicts current models of how the trilobite digestive system was structured and evolved over time. Most notably, the crop is not a derived structure as previously proposed, although it is possible that the relative size of the crop increased over the evolutionary history of the clade.
Highlights
The trilobite fossil record is rich in terms of diversity and abundance, most species are known only from certain heavily mineralized exoskeletal sclerites
The taphonomy of the Guanshan biota is most similar to that of the Chengjiang biota [46, 47]. Both the Chengjiang and Guanshan biotas are unusual among BurgessShale type fossils for having limited pyritization of non-mineralized features [38], pyrite framboidal crystals and spherical aggregates have typically been replaced with iron oxide pseudomorphs, especially in weathered specimens [46]
Specimens have articulated librigena and sometimes the hypostome is evident. Both of these features indicate that such specimens are body fossils rather than exuvia
Summary
The trilobite fossil record is rich in terms of diversity and abundance, most species are known only from certain heavily mineralized exoskeletal sclerites. The specimen was imaged using synchrotron-radiation X-ray tomography, and as a result, the authors were able to discern delicate features, such as a J-shaped esophagus indicating that the mouth was ventrally and posteriorly directed They located a relatively large expanded region which they interpreted as the crop, and several pairs of lobe-like extensions on the alimentary canal which they interpreted as digestive glands. Some doubt has been raised over the latter interpretation, primarily because the lobes project ventrally rather than latero-dorsally, as in adult trilobites [23] Even if they are digestive glands, the fact that the specimen represents a very early growth stage (early meraspid) has kept open the possibility that the crop decreased in relative size over ontogeny, in which case the two-type model based on adult morphologies would still be valid [7, 20, 23]. Soft-bodied fossils range from the Palaeolenus biozone to the Megapalaeolenus biozone of the regional Canglangpuan Stage of
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