Exceptionally Preserved Cambrian Trilobite Digestive System Revealed in 3D by Synchrotron-Radiation X-Ray Tomographic Microscopy

The Cambrian ‘Orsten’ fauna comprises exceptionally preserved and phosphatised microscopic arthropods. The external morphology of these fossils is well known, but their internal soft-tissue anatomy has remained virtually unknown. Here, we report the first non-biomineralised tissues from a juvenile polymerid trilobite, represented by digestive structures, glands, and connective strands harboured in a hypostome from the Swedish ‘Orsten’ fauna. Synchrotron-radiation X-ray tomographic microscopy enabled three-dimensional internal recordings at sub-micrometre resolution. The specimen provides the first unambiguous evidence for a J-shaped anterior gut and the presence of a crop with a constricted alimentary tract in the Trilobita. Moreover, the gut is Y-shaped in cross section, probably due to a collapsed lumen of that shape, another feature which has not previously been observed in trilobites. The combination of anatomical features suggests that the trilobite hypostome is functionally analogous to the labrum of euarthropods and that it was a sophisticated element closely integrated with the digestive system. This study also briefly addresses the preservational bias of the ‘Orsten’ fauna, particularly the near-absence of polymerid trilobites, and the taphonomy of the soft-tissue-harbouring hypostome

Controls on gut phosphatisation : the trilobites from the Weeks Formation Lagerstätte (Cambrian; Utah)

Despite being internal organs, digestive structures are frequently preserved in Cambrian Lagerstätten. However, the reasons for their fossilisation and their biological implications remain to be thoroughly explored. This is particularly true with arthropods--typically the most diverse fossilised organisms in Cambrian ecosystems--where digestive structures represent an as-yet underexploited alternative to appendage morphology for inferences on their biology. Here we describe the phosphatised digestive structures of three trilobite species from the Cambrian Weeks Formation Lagerstätte (Utah). Their exquisite, three-dimensional preservation reveals unique details on trilobite internal anatomy, such as the position of the mouth and the absence of a differentiated crop. In addition, the presence of paired pygidial organs of an unknown function is reported for the first time. This exceptional material enables exploration of the relationships between gut phosphatisation and the biology of organisms. Indeed, soft-tissue preservation is unusual in these fossils as it is restricted to the digestive structures, which indicates that the gut played a central role in its own phosphatisation. We hypothesize that the gut provided a microenvironment where special conditions could develop and harboured a source of phosphorus. The fact that gut phosphatization has almost exclusively been observed in arthropods could be explained by their uncommon ability to store ions (including phosphorous) in their digestive tissues. However, in some specimens from the Weeks Formation, the phosphatisation extends to the entire digestive system, suggesting that trilobites might have had some biological particularities not observed in modern arthropods. We speculate that one of them might have been an increased capacity for ion storage in the gut tissues, related to the moulting of their heavily-mineralised carapace

Digestive and appendicular soft-parts, with behavioural implications, in a large Ordovician trilobite from the Fezouata Lagerstätte, Morocco

Trilobites were one of the most successful groups of marine arthropods during the Palaeozoic era, yet their soft-part anatomy is only known from a few exceptionally-preserved specimens found in a handful of localities from the Cambrian to the Devonian. This is because, even if the sclerotized appendages were not destroyed during early taphonomic stages, they are often overprinted by the three-dimensional, mineralised exoskeleton. Inferences about the ventral anatomy and behavioural activities of trilobites can also be derived from the ichnological record, which suggests that most Cruziana and Rusophycus trace fossils were possibly produced by the actions of trilobites. Three specimens of the asaphid trilobite Megistaspis (Ekeraspis) hammondi, have been discovered in the Lower Ordovician Fezouata Konservat-Lagerstätte of southern Morocco, preserving appendages and digestive tract. The digestive structures include a crop with digestive caeca, while the appendages display exopodal setae and slight heteropody (cephalic endopods larger and more spinose than thoracic and pygidial ones). The combination of these digestive structures and the heteropody has never been described together among trilobites, and the latter could assist in the understanding of the production of certain comb-like traces of the Cruziana rugosa group, which are extraordinarily abundant on the shallow marine shelves around Gondwana.This work has been supported by the Spanish Ministry of Economy and Competitiveness, project number CGL2012- 39471/BTE.Peer reviewe

A xandarellid artiopodan from Morocco – a middle Cambrian link between soft-bodied euarthropod communities in North Africa and South China

NB. A corrigendum [correction] for this article was published online on 09 May 2017; this has been attached to this article as an additional file. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ © The Author(s) 2017. The attached file is the published version of the article

The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group

Background: Thylacocephala is a group of enigmatic extinct arthropods. Here we provide a full description of the oldest unequivocal thylacocephalan, a new genus and species Thylacares brandonensis, which is present in the Silurian Waukesha fauna from Wisconsin, USA. We also present details of younger, Jurassic specimens, from the Solnhofen lithographic limestones, which are crucial to our interpretation of the systematic position of Thylacocephala. In the past, Thylacocephala has been interpreted as a crustacean ingroup and as closely related to various groups such as cirripeds, decapods or remipeds. Results: The Waukesha thylacocephalan, Thylacares brandonensis n. gen. n. sp., bears compound eyes and raptorial appendages that are relatively small compared to those of other representatives of the group. As in other thylacocephalans the large bivalved shield encloses much of the entire body. The shield lacks a marked optical notch. The eyes, which project just beyond the shield margin, appear to be stalked. Head appendages, which may represent antennulae, antennae and mandibles, appear to be present. The trunk is comprised of up to 22 segments. New details observed on thylacocephalans from the Jurassic Solnhofen lithographic limestones include antennulae and antennae of Mayrocaris bucculata, and endites on the raptorial appendages and an elongate last trunk appendage in Clausocaris lithographica. Preserved features of the internal morphology in C. lithographica include the muscles of the raptorial appendage and trunk. Conclusions: Our results indicate that some `typical' thylacocephalan characters are unique to the group; these autapomorphies contribute to the difficulty of determining thylacocephalan affinities. While the new features reported here are consistent with a eucrustacean affinity, most previous hypotheses for the position of Thylacocephala within Eucrustacea (as Stomatopoda, Thecostraca or Decapoda) are shown to be unlikely. A sister group relationship to Remipedia appears compatible with the observed features of Thylacocephala but more fossil evidence is required to test this assertion. The raptorial appendages of Thylacocephala most likely projected 45 degrees abaxially instead of directly forward as previously reconstructed. The overall morphology of thylacocephalans supports a predatory mode of life

Functional morphology of giant mole crab larvae: a possible case of defensive enrollment

BACKGROUND: Mole crabs (Hippidae) are morphologically distinct animals within Meiura, the “short-tailed” crustaceans. More precisely, Hippidae is an ingroup of Anomala, the group which includes squat lobsters, hermit crabs, and numerous “false” crabs. Within Meiura, Anomala is the sister group to Brachyura, which includes all true crabs. Most meiuran crustaceans develop through two specific larval phases. The first, pelagic one is the zoea phase, which is followed by the transitory megalopa phase (only one stage). Zoea larvae are rather small, usually having a total size of only a few millimeters. Zoea larvae of some hippidan species grow significantly larger, up to 15 mm in size, making them the largest known zoea larvae of all anomalan, and probably all meiuran, crustaceans. It has been suggested that such giant larvae may be adapted to a specific defensive strategy; i.e., enrollment. However, to date such giant larvae represent a rarity. METHODS: Eight specimens of large-sized hippidan larvae from museum collections were photographed with a Canon Rebel T3i digital camera under cross-polarized light. Additionally, one of the specimens was documented with a Keyence BZ-9000 fluorescence microscope. The specimen was subsequently dissected to document all appendages in detail. UV light (377 nm) was used for illumination, consistent with the specimen’s autofluorescence capacities. For high-resolution images, composite imaging was applied. RESULTS: All specimens differ in important aspects from all other known hippidan zoea larvae, and thus probably represent either previously unreported larvae or stages of known species, or larvae of unknown species. The sixth pleon segment articulates off the telson, a condition not previously reported in hippidan zoea larvae, but only for the next larva phase (megalopa). The larvae described here thus most likely represent the ultimate pelagic larval stages, or rare cases of ‘early megalopae’. The morphological features indicate that giant hippidan larvae perform defensive enrollment. CONCLUSIONS: Our investigation indicates a larger morphological diversity of hippidan larvae than was known previously. Moreover, their assumed functional morphology, similar to the condition in certain stomatopod larvae, indicates a not yet directly observable behavior by these larvae, namely defensive enrollment. In a wider context, we are only just beginning to understand the ecological roles of many crustacean larvae. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s40851-016-0052-5) contains supplementary material, which is available to authorized users

Major issues in the origins of ray‐finned fish ( A ctinopterygii) biodiversity

Ray‐finned fishes ( A ctinopterygii) dominate modern aquatic ecosystems and are represented by over 32000 extant species. The vast majority of living actinopterygians are teleosts; their success is often attributed to a genome duplication event or morphological novelties. The remainder are ‘living fossils’ belonging to a few depauperate lineages with long‐retained ecomorphologies: P olypteriformes (bichirs), H olostei (bowfin and gar) and C hondrostei (paddlefish and sturgeon). Despite over a century of systematic work, the circumstances surrounding the origins of these clades, as well as their basic interrelationships and diagnoses, have been largely mired in uncertainty. Here, I review the systematics and characteristics of these major ray‐finned fish clades, and the early fossil record of A ctinopterygii, in order to gauge the sources of doubt. Recent relaxed molecular clock studies have pushed the origins of actinopterygian crown clades to the mid‐late P alaeozoic [ S ilurian– C arboniferous; 420 to 298 million years ago ( M a)], despite a diagnostic body fossil record extending only to the later M esozoic (251 to 66 M a). This disjunct, recently termed the ‘ T eleost G ap’ (although it affects all crown lineages), is based partly on calibrations from potential P alaeozoic stem‐taxa and thus has been attributed to poor fossil sampling. Actinopterygian fossils of appropriate ages are usually abundant and well preserved, yet long‐term neglect of this record in both taxonomic and systematic studies has exacerbated the gaps and obscured potential synapomorphies. At the moment, it is possible that later P alaeozoic‐age teleost, holostean, chondrostean and/or polypteriform crown taxa sit unrecognized in museum drawers. However, it is equally likely that the ‘ T eleost G ap’ is an artifact of incorrect attributions to extant lineages, overwriting both a post‐ P alaeozoic crown actinopterygian radiation and the ecomorphological diversity of stem‐taxa.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109271/1/brv12086.pd