A New Burgess Shale Polychaete and the Origin of the Annelid Head Revisited

New fossil of Gaoloufangchaeta advances the origin of Errantia (Annelida) to the early Cambrian

Molecular clock estimates suggest the origin of Annelida dates back to the Ediacaran period, which is in discordance with the first appearance of this taxon in the early Cambrian, as evidenced by the fossil records of stem-group and basally branching crown-group annelids. Using new material from the early Cambrian Guanshan biota (Cambrian Series 2, Stage 4), we re-interpret Gaoloufangchaeta bifurcus Zhao, Li & Selden, 2023, as the earliest known errantian annelid. Gaoloufangchaeta has a prominent anterior end bearing three pairs of putatively sensory appendages and a pair of anterior eyes; a muscular eversible pharynx with papillae is identified. The presence of enlarged parapodia with acicula-like structures and long capillary chaetae suggests a pelagic lifestyle for this taxon. Our phylogenetic analyses recover Gaoloufangchaeta within the Phyllodocida (Pleistoannelida, Errantia), extending the origin of Errantia back to the early Cambrian. Our data are in line with the hypothesis that Annelida diverged before the Cambrian and indicate both morphological and ecological diversification of annelids in the early Cambrian.

A mid-Cambrian tunicate and the deep origin of the ascidiacean body plan

Tunicates are an evolutionarily significant subphylum of marine chordates, with their phylogenetic position as the sister-group to Vertebrata making them key to unraveling our own deep time origin. Tunicates greatly vary with regards to morphology, ecology, and life cycle, but little is known about the early evolution of the group, e.g. whether their last common ancestor lived freely in the water column or attached to the seafloor. Additionally, tunicates have a poor fossil record, which includes only one taxon with preserved soft-tissues. Here we describe Megasiphon thylakos nov., a 500-million-year-old tunicate from the Marjum Formation of Utah, which features a barrel-shaped body with two long siphons and prominent longitudinal muscles. The ascidiacean-like body of this new species suggests two alternative hypotheses for early tunicate evolution. The most likely scenario posits M. thylakos belongs to stem-group Tunicata, suggesting that a biphasic life cycle, with a planktonic larva and a sessile epibenthic adult, is ancestral for this entire subphylum. Alternatively, a position within the crown-group indicates that the divergence between appendicularians and all other tunicates occurred 50 million years earlier than currently estimated based on molecular clocks. Ultimately, M. thylakos demonstrates that fundamental components of the modern tunicate body plan were already established shortly after the Cambrian Explosion.

Annelids from the Cambrian (Wuliuan Stage, Miaolingian) Spence Shale Lagerstätte of northern Utah, USA

The Spence Shale Member of the Langston Formation in northern Utah and southern Idaho preserves generally non-biomineralized fossil assemblages referred to as the Spence Shale Lagerstätte. The biota of this Lagerstätte is dominated by panarthropods, both biomineralized and soft-bodied examples, but also preserves diverse infaunal organisms, including species of scalidophorans, echinoderms, lobopodians, stalked filter feeders, and various problematic taxa. To date, however, only a single annelid fossil, originally assigned to Canadia sp., has been described from the Spence Shale. This lone specimen and another recently collected specimen were analyzed in this study using scanning electron microscopy and energy dispersive X-ray spectrometry. The previous occurrence is reassigned to Burgessochaeta cf. B. setigera Walcott, 1911. The new fossil, however, is identified as a novel polychaete taxon, Shaihuludia shurikeni gen. et sp. nov., characterized by the presence of fused, bladed chaetae and a wide body. The occurrence of Burgessochaeta is the first outside the Burgess Shale and its vicinity, whereas Shaihuludia shurikeni gen. et sp. nov. adds to the diversity of annelids in the middle Cambrian and highlights the diversity of the Spence Shale Lagerstätte.

First record of growth patterns in a Cambrian annelid

Early annelid evolution is mostly known from 13 described species from Cambrian Burgess Shale-type Lagerstätten. We introduce a new exceptionally well-preserved polychaete, Ursactis comosa gen. et sp. nov., from the Burgess Shale (Wuliuan Stage). This small species (3-15 mm) is the most abundant Cambrian polychaete known to date. Most specimens come from Tokumm Creek, a new Burgess Shale locality in northern Kootenay National Park, British Columbia, Canada. Ursactis has a pair of large palps, thin peristomial neurochaetae and biramous parapodia bearing similarly sized capillary neurochaetae and notochaetae, except for segments six to nine, which also have longer notochaetae. The number of segments in this polychaete range between 8 and 10 with larger individuals having 10 segments. This number of segments in Ursactis is remarkably small compared with other polychaetes, including modern forms. Specimens with 10 segments show significant size variations, and the length of each segment increases with the body length, indicating that body growth was primarily achieved by increasing the size of existing segments rather than adding new ones. This contrasts with most modern polychaetes, which typically have a larger number of segments through additions of segments throughout life. The inferred growth pattern in Ursactis suggests that annelids had evolved control over segment addition by the mid-Cambrian.

Symbiosis in the Cambrian: enteropneust tubes from the Burgess Shale co-inhabited by commensal polychaetes

The in situ preservation of animal behaviour in the fossil record is exceedingly rare, but can lead to unique macroecological and macroevolutionary insights, especially regarding early representatives of major animal clades. We describe a new complex ecological relationship from the middle Cambrian Burgess Shale (Raymond Quarry, Canada). More than 30 organic tubes were recorded with multiple enteropneust and polychaete worms preserved within them. Based on the tubicolous nature of fossil enteropneusts, we suggest that they were the tube builders while the co-preserved polychaetes were commensals. These findings mark, to our knowledge, the first record of commensalism within Annelida and Hemichordata in the entire fossil record. The finding of multiple enteropneusts sharing common tubes suggests that either the tubes represent reproductive structures built by larger adults, and the enteropneusts commonly preserved within are juveniles, or these enteropneusts were living as a pseudo-colony without obligate attachment to each other, and the tube was built collaboratively. While neither hypothesis can be ruled out, gregarious behaviour was clearly an early trait of both hemichordates and annelids. Further, commensal symbioses in the Cambrian may be more common than currently recognized.

The rise and early evolution of animals: where do we stand from a trace-fossil perspective?

The trace-fossil record provides a wealth of information to track the rise and early evolution of animals. It comprises the activity of both hard- and soft-bodied organisms, is continuous through the Ediacaran (635-539 Ma)- Cambrian (539-485 Ma) transition, yields insights into animal behaviour and their role as ecosystem engineers, and allows for a more refined characterization of palaeoenvironmental context. In order to unravel macroevolutionary signals from the trace-fossil record, a variety of approaches is available, including not only estimation of degree of bioturbation, but also analysis of ichnodiversity and ichnodisparity trajectories, and evaluation of the occupation of infaunal ecospace and styles of ecosystem engineering. Analysis of the trace-fossil record demonstrates the presence of motile benthic bilaterians in the Ediacaran, mostly feeding from biofilms. Although Ediacaran trace fossils are simple and emplaced at or immediately below the sediment surface, an increase in ichnofossil complexity, predation pressure, sediment disturbance and penetration depth is apparent during the terminal Ediacaran. Regardless of this increase, a dramatic rise in trace fossil diversity and disparity took place during the earliest Cambrian, underscoring that the novelty of the Fortunian (539-529 Ma) cannot be underestimated. The Fortunian still shows the persistence of an Ediacaran-style matground ecology, but is fundamentally characterized by the appearance of new trace-fossil architectural plans reflecting novel ways of interacting with the substrate. The appearance of Phanerozoic-style benthic ecosystems attests to an increased length and connectivity of the food web and improved efficiency in organic carbon transfer and nutrient recycling. A profound reorganization of the infaunal ecospace is recorded in both high-energy sand-dominated nearshore areas and low-energy mud-dominated offshore environments, during the early Cambrian, starting approximately during Cambrian Age 2 (529-521 Ma), but continuing during the rest of the early Cambrian. A model comprising four evolutionary phases is proposed to synthetize information from the Ediacaran-Cambrian trace-fossil record. The use of a rich ichnological toolbox; critical, systematic and comprehensive evaluation of the Ediacaran-Cambrian trace-fossil record; and high-resolution integration of the ichnological dataset and sedimentological information show that the advent of biogenic mixing was an important factor in fully marine environments at the dawn of the Phanerozoic.

A Cambrian crown annelid reconciles phylogenomics and the fossil record

The phylum of annelids is one of the most disparate animal phyla and encompasses ambush predators, suspension feeders and terrestrial earthworms¹. The early evolution of annelids remains obscure or controversial^2,3, partly owing to discordance between molecular phylogenies and fossils^2,4. Annelid fossils from the Cambrian period have morphologies that indicate epibenthic lifestyles, whereas phylogenomics recovers sessile, infaunal and tubicolous taxa as an early diverging grade⁵. Magelonidae and Oweniidae (Palaeoannelida¹) are the sister group of all other annelids but contrast with Cambrian taxa in both lifestyle and gross morphology^2,6. Here we describe a new fossil polychaete (bristle worm) from the early Cambrian Canglangpu formation⁷ that we name Dannychaeta tucolus, which is preserved within delicate, dwelling tubes that were originally organic. The head has a well-defined spade-shaped prostomium with elongated ventrolateral palps. The body has a wide, stout thorax and elongated abdomen with biramous parapodia with parapodial lamellae. This character combination is shared with extant Magelonidae, and phylogenetic analyses recover Dannychaeta within Palaeoannelida. To our knowledge, Dannychaeta is the oldest polychaete that unambiguously belongs to crown annelids, providing a constraint on the tempo of annelid evolution and revealing unrecognized ecological and morphological diversity in ancient annelids.

Biogenic Iron Preserves Structures during Fossilization: A Hypothesis: Iron from Decaying Tissues May Stabilize Their Morphology in the Fossil Record

It is hypothesized that iron from biological tissues, liberated during decay, may have played a role in inhibiting loss of anatomical information during fossilization of extinct organisms. Most tissues in the animal kingdom contain iron in different forms. A widely distributed iron-bearing molecule is ferritin, a globular protein that contains iron crystallites in the form of ferrihydrite minerals. Iron concentrations in ferritin are high and ferrihydrites are extremely reactive. When ancient animals are decaying on the sea floor under anoxic environmental conditions, ferrihydrites may initialize the selective replication of some tissues in pyrite FeS₂ . This model explains why some labile tissues are preserved, while other more resistant structures decay and are absent in many fossils. A major implication of this hypothesis is that structures described as brains in Cambrian arthropods are not fossilization artifacts, but are instead a source of information on anatomical evolution at the dawn of complex animal life.

Canadia spinosa and the early evolution of the annelid nervous system

Annelid worms are a disparate, primitively segmented clade of bilaterians that first appear during the early Cambrian Period. Reconstructing their early evolution is complicated by the extreme morphological diversity in early diverging lineages, rapid diversification, and sparse fossil record. Canadia spinosa, a Burgess Shale fossil polychaete, is redescribed as having palps with feeding grooves, a dorsal median antenna and biramous parapodia associated with the head and flanking a ventral mouth. Carbonaceously preserved features are identified as a terminal brain, circumoral connectives, a midventral ganglionated nerve cord and prominent parapodial nerves. Phylogenetic analysis recovers neuroanatomically simple extant taxa as the sister group of other annelids, but the phylogenetic position of Canadia suggests that the annelid ancestor was reasonably complex neuroanatomically and that reduction of the nervous system occurred several times independently in the subsequent 500 million years of annelid evolution.

Sclerite-bearing annelids from the lower Cambrian of South China

Cambrian annelids are strikingly diverse and reveal important details of annelid character acquisition. Their contribution, however, to a wider understanding of the evolution of the trochozoans (encompassing the annelids as well as such groups as the brachiopods and molluscs) remains limited. Thus the early annelids had been linked to a variety of cataphract Cambrian metazoans, notably Wiwaxia and the halkieriids, but recent work assigns such fossils to stem-group molluscs. Here we report two new annelids from the Lower Cambrian Chengjiang Lagerstätte, South China. Ipoliknus avitus n. gen., n. sp. is biramous with neurochaetae and notochaetae, but significantly also bears dorsal spinose sclerites and dorso-lateral dentate sclerites. Adelochaeta sinensis n. gen., n. sp. is unique amongst Cambrian polychaetes in possessing the rod-like supports of the parapodia known as aciculae. This supports phylogenetic placement of Adelochaeta as sister to some more derived aciculate Palaeozoic taxa, but in contrast Ipoliknus is recovered as the most basal of the stem-group annelids. Sclerites and chaetae of I. avitus are interpreted respectively as the remnants and derivatives of a once more extensive cataphract covering that was a characteristic of more primitive trochozoans. The two sets of chaetae (noto- and neurochaetae) and two sets of sclerites (spinose and dentate) suggest that in a pre-annelid an earlier and more complete scleritome may have consisted of four zones of sclerites. Other cataphract taxa from the Lower Palaeozoic show a variety of scleritome configurations but establishing direct links with such basal annelids as Ipoliknus at present must remain conjectural.