Tardigrades constitute one of the most important group in the challenging Antarctic terrestrial ecosystem. Living in various habitats, tardigrades play major roles as consumers and decomposers in the trophic networks of Antarctic terrestrial and freshwater environments; yet we still know little about their biodiversity. The Eutardigrada is a species rich class, for which the eggshell morphology is one of the key morphological characters. Tardigrade egg morphology shows a diverse appearance, and it is known that, despite rare, intraspecific variation is caused by seasonality, epigenetics, and external environmental conditions. Here we reportDactylobiotus ovimutanssp. nov. from King George Island, Antarctica. Interestingly, we observed a range of eggshell morphologies from the new species, although the population was cultured under controlled laboratory condition. Thus, seasonality, environmental conditions, and food source are eliminated, leaving an epigenetic factor as a main cause for variability in this case.

Phylum Tardigrada (water bears), well known for their cryptobiosis, includes small invertebrates with four paired limbs and is divided into two classes: Eutardigrada and Heterotardigrada. The evolutionary origin of Tardigrada is known to lie within the lobopodians, which are extinct soft-bodied worms with lobopodous limbs mostly discovered at sites of exceptionally well-preserved fossils. Contrary to their closest relatives, onychophorans and euarthropods, the origin of morphological characters of tardigrades remains unclear, and detailed comparison with the lobopodians has not been well explored. Here, we present detailed morphological comparison between tardigrades and Cambrian lobopodians, with a phylogenetic analysis encompassing most of the lobopodians and three panarthropod phyla. The results indicate that the ancestral tardigrades likely had a Cambrian lobopodian–like morphology and shared most recent ancestry with the luolishaniids. Internal relationships within Tardigrada indicate that the ancestral tardigrade had a vermiform body shape without segmental plates, but possessed cuticular structures surrounding the mouth opening, and lobopodous legs terminating with claws, but without digits. This finding is in contrast to the long-standing stygarctid-like ancestor hypothesis. The highly compact and miniaturized body plan of tardigrades evolved after the tardigrade lineage diverged from an ancient shared ancestor with the luolishaniids.

Chaetognaths, with their characteristic grasping spines, are the oldest known pelagic predators, found in the lowest Cambrian (Terreneuvian). Here, we describe a large stem chaetognath, Timorebestia koprii gen. et sp. nov., from the lower Cambrian Sirius Passet Lagerstätte, which exhibits lateral and caudal fins, a distinct head region with long antennae and a jaw apparatus similar to Amiskwia sagittiformis . Amiskwia has previously been interpreted as a total-group chaetognathiferan, as either a stem-chaetognath or gnathostomulid. We show that T. koprii shares a ventral ganglion with chaetognaths to the exclusion of other animal groups, firmly placing these fossils on the chaetognath stem. The large size (up to 30 cm) and gut contents in T. koprii suggest that early chaetognaths occupied a higher trophic position in pelagic food chains than today.

The Sesong Formation is a member of the Taebaek Group, Korea, which extends from late Cambrian Series 3 to middle Furongian in age. Recent studies on the trilobites of the Sesong Formation have contributed significantly to the revision of the biostratigraphy. However, trilobites in the lower part of the formation, which may include the “StephanocareZone”, have remained essentially overlooked since the establishment of the biozone, making it difficult to correlate with the equivalent biozones of North China. Here we report trilobite faunas from the lower part of the Sesong Formation in two different sections, the Seokgaejae and the Jikdong sections, which yield two species ofJiulongshaniaamong other species. Species ofJiulongshaniahave been known to occur successively in North China, so are useful for detailed correlation. Specimens ofStephanocare richthofeniare fragmentary and rarely occur in association withJiulongshania regularis, whileJiulongshaniaspecies occur throughout the studied intervals. Accordingly, it is reasonable to extend the previously establishedJiulongshaniaZone of the uppermost part of the underlying Daegi Formation into the lower part of the Sesong Formation. By doing so, theJiulongshaniaZone is correlated with theBlackwelderiaZone of North China with confidence. The lowermost part of the Sesong Formation in the Jikdong section yields a fauna includingJ. regularis, which implies that the boundary between the Daegi and Sesong formations is diachronous within the Taebaeksan Basin. The Daegi/Sesong formation boundary in Korea is comparable to the Zhangxia/Gushan boundary in North China in that it displays an abrupt change from a carbonate-dominant facies to a shale-dominant facies. The correlation employing theJiulongshaniaspecies indicates that the facies shift occurred significantly earlier in Shandong, North China than in the Taebaeksan Basin, Korea.

Although trilobites have provided research subjects for more than two centuries, their head segmentation has remained unresolved. Four glabellar furrows (SO and S1–S3) marking the segmental boundaries are generally present in the cephalic axis, but there are trilobites with one more pair of furrows, the so-called S4, in the cephalic axis, causing confusion in understanding trilobite head segmentation. Recent advances in developmental biology and palaeontology have shed light on the arthropod head problem, and thus, trilobite head segmentation can be reviewed in the light of this knowledge. Based on the information from the anatomy of exceptionally preserved trilobites and artiopodans closely related to trilobites, it is inferred that trilobite head contains five segments: the anteriormost ocular segment potentially associated with the hypostome, the antennal segment and the following three segments with walking legs. When present, the S4 furrows are situated where the eye ridges meet the cephalic axis of trilobites, indicating that the furrows are incised ‘within’ the anteriormost segment in trilobites with an anteriorly enlarged frontal lobe. Trilobites of the Order Redlichiida, the most primitive stock, show variable conditions in the frontal glabellar conditions, while in other more derived groups, the condition is rather constant. The frontal glabellar condition, therefore, could provide a clue to elucidate the unresolved Cambrian trilobite phylogeny and the Cambrian roots of the post-Cambrian trilobites.