Bryozoans from the Early Cambrian Cambrian Xiannüdong Formation of Shaanxi Province, China.

Molecular clock studies have suggested that the Phylum Bryozoa, or Moss Animals, first appeared in the Early Cambrian, which is consistent with the appearance of nearly all other Animal phyla at this time. However, for a long time the earliest known fossil Bryozoans came from the Early Ordovician, at which point six of the eight known orders of Bryozoans appear abruptly. Several putative Cambrian Bryozoan fossils, such as Pywackia, Archaeotrypa, and Marcusodictyon, were described, but none of these was universally accepted as a Bryozoan. In 2021 a more plausible Brozoan, Protomelission gatehousei, was described from the Early Cambrian of Australia and South China. In this it was possible to identify several Brozoan traits, including monomorphic zooid capsules, modular construction, organic composition, and a simple linear budding growth geometry, leading to the conclusion that this was probably a stem-group Bryozoan.

Protomelission gatehousei from the Cambrian Wirrealpa Limestone, South Australia. (a)–(g) Holotype, SADME 10470. (a) Front side of the colony showing the seven series of zooids. Top box corners indicate the area shown in (f); bottom box corners show the broken-off part in (c). (b) The top broken part of (a). (c) The lower broken part of (a). (d) Oblique lateral view of the bilaminate colony. (e) Enlarged view of (d) showing the staggered budding pattern and the curved basal walls of the two back-to-back layers (arrows and tailed arrows) in the bifoliate colony. (f) Quincuncial arrangement of sub-hexagonal zooids with broken frontal walls. (g) Lateral view of uncovered zooids; note the minute spoon-shaped structure (arrow) at the proximal end of basal wall extending backwards underneath the distal part of the parent zooid. (h), (i) SADME 10470-2. (h) Lateral view of a broken colony, showing the largely broken frontal walls (tailed arrows) and basal walls of opposite layer (arrows). (i) Enlarged view of three adjacent zooids. Note the dome shape of the distal part of frontal wall (tailed arrows), and almost circular orifice of zooid. Abbreviations: B, basal wall; F, frontal wall. Zhang et al. (2021).

However, while Protomelission gatehousei shows enough Bryozoan-like features that most palaeontologists have accepted it to be at least a stem group Bryozoan, the specimens used to describe the species lacked the definitive Bryozoan soft-tissue anatomy and diagnostic skeletal microstructure which would be necessary for complete conformation, leaving the identity of these fossils open to challenge.

In a paper published in the journal Nature on 3 June 2026, Baopeng Song (宋宝鹏) and Zhifei Zhang (张志飞) of the Department of Geology at Northwest University, Luke Strotz, also of the Department of Geology at Northwest University and also of the Department of Earth Sciences at Utrecht University, Timothy Topper, again of the Department of Geology at Northwest University, and of the Department of Palaeobiology at the Swedish Museum of Natural History, Andrej Ernst of the Institut für Geologie at Universität Hamburg, Zhiliang Zhang of the Department of Geology at Northwest University, and the Institut für Geologie at Universität Hamburg, Mei Luo (罗梅), again of the Department of Geology at Northwest University, Lars Holmer, again of the Department of Geology at Northwest University, and of the Department of Earth Sciences at Uppsala University, Yue Liang (梁悦), Yazhou Hu (胡亚洲), Caibin Zhang (张彩彬), and Yanlong Chen (陈延龙), all of the Department of Geology at Northwest University, and Glenn Brock, once again of the Department of Geology at Northwest University, and of the School of Natural Sciences at Macquarie University, describe new specimens of Protomelission gatehousei from the Early Cambrian Xiannüdong Formation of southern Shaanxi Province, China, as well as a second new species of Bryozoan from the same formation.

Notably, these fossils preserve soft-tissue features in exceptional fidelity, including internal moulds of membranous sacs in the zooid chambers, which allow the unequivocal placement of these taxa within the Phylum Bryozoa. The presence of two separate Bryozoan taxa within these Early Cambrian deposits pushes the origin of the group still earlier, confirming that this group appeared during the Cambrian explosion.

Specimen of Protomelission gatehousei from the Xiannüdong Formation in which the membranous sacs are preserved (ELI DYCX 8-001). (a) Front side of the colony. The outlined area is magnified in (h). (b) Back side of the colony. The outlined area is magnified in (j). (c) Lateral view of the bifoliate colony. (d) Oblique lateral view of the bifoliate colony showing the hollow arched mesotheca (arrow). (e) Partial enlargement of (c) showing the staggered budding pattern. (f), (g) X-ray tomographic microscopy images showing the longitudinal section of the colony and the orifice of autozooids (arrowheads) (f, oblique lateral view; (g) lateral view). (h) Quincuncial arrangement of sub-hexagonal membranous sacs with elliptical orifice. Note the 10-μm gap present between adjacent membranous sacs, indicating the loss of skeletal walls during the taphonomic processes. The outlined area is the membranous sac magnified in (i). (i) Enlarged view of a membranous sac showing the orifice (asterisk), circular fibres (arrow) and longitudinal fibres (arrowhead). These features suggest muscle preservation in the membranous sac. (j) Enlarged view of a zooid. Note that the aperture is coated with secondary phosphate. (k) Enlarged view of a zooid. Note that the secondary phosphate coating of the aperture is partially stripped away. (l), (m) Enlarged view of the membranous sac showing the longitudinal fibres in (l) arrowhead, and circular fibres in (m), arrow. These features suggest muscle preservation in the membranous sac. Scale bars, 500 μm (a)–(d), 50 μm (e), (i)–(k), 200 μm (f), 150 μm (g), 100 μm (h) and 30 μm (l), (m). Song et al. (2026).

These new specimens show Protomelission gatehousei as forming upright colonies with two curved lamellar sheets of zooids back-to-back, with the largest colonies being 1-2 mm in width and about 3 mm high, tapering towards their tip. Each of these lamellae has six-to-eight rows of zooids, with budding originating from a planar mesotheca.

Soft-tissue preservation of Protomelission gatehousei. (a)–(e) ELI DYCX 8-005. (a) Front side of the colony, box corners indicate the area shown in (d). (b) The back side of the colony. (c) Lateral view of the bifoliate colony. (d), (e) Enlarged view of elongated hexagonal zooids. Note the longitudinally neatly arranged cylindrical structures on the both sides of the ridge-like orifice, which are possible secondary coatings of protective shields. (f) Protective shields developed in an extant Cheilostome Bryozoan, Valdemunitella sp. photographed by Dennis Gordon (Wellington). Song et al. (2026).

The new species described is named Dayingomelission hexaclitia, where 'Dayingomelission' means 'honeycomb from Daying' and 'hexaclitia' means 'six slopes' in reference to the sloped, hexagonal apertures of its autozooids. Colonies of Dayingomelission hexaclitia form a sheet-like grown covering the substrate. This sheet is interpreted as having spread by linear branching, with a single row of zooids diverging to form two new rows. Each autozooid is hexagonal and box-like, between 200 µm to 400 µm in diameter, and separated from its neighbours by a double-walled structure. All vertical walls show this double-walled structure, while the basal wall is planar, sometimes showing a slight curvature. 

Specimens of Dayingomelission hexaclitia from the Xiannüdong Formation showing the colony and cystids. (a), (b) ELI ZJBX 10-001 (holotype). (a) Oblique view of the front side of a unilaminate colony form clearly showing the regular hexagonal, compactly arranged, honeycomb-shaped cystids. The outlined area is shown in (b). (b) Hexagonal cystid with vertical wall and ring septa clearly evident (arrow). (c)–(e) ELI ZJBX 10-002. (c) Front side of a unilaminate colony form. The bottom outlined area shows the cystids magnified in (d); whereas the top outline shows the cystids magnified in (e). (d) Enlarged view of adjacent cystids. Note the hexagonal vertical wall (arrow) and the basal exterior wall of cystids (arrowheads). (e) Row bifurcation showing change in zooid width along rows. (f)–(i) ELI ZJBX 3-001. (f) Front side of a unilaminate colony form with styles indenting the zooidal chambers. (g) Oblique view showing hexagonal cystids with styles. (h) Oblique view of colony surface. Note that the styles arise in the endozone and extend through most of exozone. (i) Enlarged view of the vertical wall with planar spherulitic fabric. Scale bars, 500 μm (a), (c), 80 μm (b), 100 μm (d), 200 μm (e), 300 μm (f), (g), 100 μm (h) and 25 μm (i). Song et al. (2026).

Both species have hexagonal zooids with a box-shaped profile and a non-porous phosphatized or silicified skeleton. These are more-or-less uniform in size, and angled at 30-75° to the median lamina or basal exterior wall. They have preserved phosphatized internal structures interpreted as membranous sacks, the outer end of which comprises an elliptical orifice surrounded by an undulating fold. These are made up of densely packed circular and longitudinal fibres interpreted as annular and longitudinal muscles. Longitudinally aligned cylindrical structures, possibly representing protective shields or a broad operculum are present in some specimens. In others sac is attached to the cystid wall in the inner part of the zooid cell.

Membranous sacs preserved in situ in the autozooid cystids of Protomelission gatehousei and Dayingomelission hexaclitia and colonial growth reconstruction of Protomelission gatehousei . (a), (b) Protomelission gatehousei  ELI DYCX 8-016. (a) Front side of a bifoliate colony showing the eight series of zooids. The outlined area is magnified in (b). (b) Enlarged view of a zooid. Note that the membranous sac (arrow) is preserved in the cystid (arrowhead). (c)–(g) Dayingomelission hexaclitia ELI DYCX 8-004. (c) Front side of a unilaminate colony, with ten series of zooids, all with membranous sacs and cystids. The outlined area is magnified in (g). (d) Back side of the colony showing the membranous sacs of the zooids and the gap between the sacs. The outlined area is magnified in (e). (e) Enlarged view showing capsule￾like membranes and gaps. (f) X-ray tomographic microscopy image showing the longitudinal section of zooids with membranous sacs and cystids. (g) Enlarged view highlighting that the membranous sacs (arrow) are captured in the cystids (arrowhead), and the membranous sacs are in contact with the cystids 20 μm from the apertures (ligamentous attachment, asterisks). (h) Three-dimensional reconstruction of a Bryozoan zooid with protruding lophophore. (i) Longitudinal section of reconstructed Bryozoan zooid. Greyish white, cystid; translucent white, membranous sac and tentacles; pink, polypide excluding tentacles. (j) Reconstruction of Protomelission gatehousei , front surface view. Scale bars, 500 μm (a), (c), (d), 40 μm (b), 200 μm (e), (f) and 100 μm (g). Song et al. (2026).

Both Protomelission gatehousei and Dayingomelission hexaclitia show most of the key features associated with Palaeozoic Bryozoans, including  aspects of their colony morphology, their skeletal architecture,  the presence of soft-tissue structures such as membranous sacs, as well as annular and longitudinal musculature. Notably they contain a number of features associated with the Class Stenolaemata, including styles and  a free-walled colony organisation, which would make both species crown-group Brozoans. This makes it more likely that they were biomineralized in life, although it is impossible to determine the initial composition of their skeletons. Brozoans are known to have undergone a number of independent biomineralization events, with a molecular clock analysis indicating that the first of these was likely to have happened in the Early Cambrian. These results also imply that the common ancestor of the organic￾walled Gymnolaemata and the mineralized Stenolaemata probably originated in the early Cambrian (Terreneuvian) or even perhaps in the Ediacaran Period.

Phylogenetic relationships of Bryozoans. A 50% majority-rule consensus phylogenetic tree inferred using morphological characters and Bayesian analysis based on a matrix of 22 taxa and 50 characters. Node values are Bayesian posterior probability support values. Coloured areas indicate the three taxonomic classes that comprise the Bryozoa along with outgroups, with Protomelission and Dayingomelission belonging to Stenolaemata. Song et al. (2026).

The presence of two species of Bryozoan in the Early Cambrian Xiannüdong Formation of Shaanxi Province, as well as one of these species being present in the lower Wirrealpa Limestone of South Australia makes it likely that Bryazoans had already diversified and become widespread in the Early Cambrian. This lends support to the idea that the tentative mineralised Bryomorphs from the Lower Cambrian of Nevada recently described by Pruss et al. (2022) are also Bryozoans, and that Moss Animals were therefore widespread in shallow Cambrian seas, particularly Archaeocyath reef-associated carbonate platform settings. 

See also...

Did the earliest Vertebrates have four eyes?

Vertebrates have complex, camera-type eyes which have been a source of interest to evolutionary biologists since the nineteenth century, when this seemed an unusually complex system, which it was difficult to imagine arising through a series of gradual steps. Modern evolutionary biologists are less concerned by this, recognising that even a very simple eye is better than no-eye-at-all, and that therefore a complex eye could arise step-wise from the simplest cluster of light-sensitive cells, but beyond this have been able to give no real explanation of what the eyes of our earliest Chordate ancestors looked like. 

Camera eyes comprise a comprise of a spherical lens, a retina, an iris, and a set of muscles exterior to the main eye structure, which can be used to alter the shape of the lens, enabling it to focus an image on the hemispherical retina, which are detected by the optic nerve, and transmitted to the brain.

Almost all modern Vertebrates have two lateral camera eyes, although some groups have lost these, and, curiously, some Lizards have a third such eye on the top or back of their heads, which is derived from the pineal complex of the brain.

Eyes in Vertebrate fossils are often identified by the preservation of the pigmentation from the retinal epithelium, which is rich in melanin, as dark stains, and/or by impressions left by the hard lens. The oldest purportative fossil Vertebrate eyes are seen in Metasprigginna walcotti, a probable Chordate from the Burgess Shale of Canada, dated to about 505 million years before the present. In these fossils a hemispherical shape has been interpreted as the retina, and an associated circular area as the lens. The earliest known example of melanostomes (the cells which contain the pigment melanin) being preserved in the eye of a Vertebrate is the Devonian Jawless Fish Euphanerops longaevus, from the Escuminac Formation of Canada, which has lateral eyes with abundant such cells, inferring the presence of a retina.

No non-Vertebrate Chordates possess a camera eye. The Lancets, or Amphioxi, have four clusters of photoreceptor cells, but are not thought to be able to produce an image (unsurprising since they also lack a brain). Salps, which are planktonic Tunicates, have a multiple stage life cycle, with an colonial adult phase which reproduces sexually, and a solitary adult phase which reproduces asexually. The larval form of the colonial Salp has three pigment cup eyes, while the larval form of the solitary stage has a single eye. During the embryonic development of Vertebrates, the paired eyes arise from a section of the anterior neural plate which also gives rise to the pineal organ, leading some biologists to speculate that these three organs are analogous to the three eyes of the Salp larvae. 

In a paper published in the journal Nature on 21 January 2026, Xiangtong Lei of the Center for Vertebrate Evolutionary Biology and Institute of Palaeontology at Yunnan University, Sihang Zhang, also of the Center for Vertebrate Evolutionary Biology, and of the State Key Laboratory for Vegetation Structure, Functions and Construction at Yunnan University, Peiyun Cong, also of the Center for Vertebrate Evolutionary Biology, and State Key Laboratory for Vegetation Structure, Functions and Construction at Yunnan University, as well as the Oxford University Museum of Natural History, Jakob Vinther of the Palaeobiology Research Group and School of Biological Sciences at the University of Bristol, Sarah Gabbott of the Centre for Palaeobiology & Biosphere Evolution at the University of Leicester, Fan Wei again of the Center for Vertebrate Evolutionary Biology, and State Key Laboratory for Vegetation Structure, Functions and Construction at Yunnan University, and Xing Xu, once again of the Center for Vertebrate Evolutionary Biology and Institute of Palaeontology at Yunnan University, and of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, identify organs which they believe are eyes in two species of Myllokunmingids (Early Chordate Animals which may be ancestral Vertebrates) from the approximately 518-million-year-old Chengjiang Biota of Yunnan Province.

Lei et al. consider Myllokunmingids such as Haikouichthys ercaicunensis and Myllokunmingia fengjiaoa to be the earliest known Vertebrates. For their study they examined six specimens of Haikouichthys ercaicunensis and four slabs which each contained multiple specimens of an as yet unnamed new Myllokunmingid. In both species they found that the head region typically has four black spots, two larger spots being placed laterally on the head, and two smaller spots facing forward. Previous studies have identified the larger of these spots as eyes, while the forward-pointing spots have been identified as nasal sacs. 

General morphology of the lateral eyes and pineal complex with their preserved melanosomes in two species of Myllokunmingidae from the Chengjiang biota. (a)-(b) Haikouichthys ercaicunensis (YNGIP-90281) with its enlarged eye region (b). (c) Carbon (red) and iron (green) element mapping of the same region in (b), arrows denote the position of figured melanosomes in (g) and (h). (d) General morphology of the unnamed Myllokunmingid  (YNGIP 90291-b,). (e) Enlarged eye region of the unnamed Myllokunmingid (YNGIP 90292-a), illustrating lateral eyes (circles in dotted line) and pineal/parapineal organs (arrows). (f) Carbon (red) and iron (green) element mapping of the same region in (e), arrows denote the position of figured melanosomes in (i), (j). (g)-(h) Melanosomes in the eyes (g) and pineal complex (h) of Haikouichthys ercaicunensis. (i)-(j) Melanosomes in the eyes (i) and pineal complex (j) of the unnamed Myllokunmingid. Scale bars are 2 mm (a); 1 mm (d); 200 μm (b), (c), (e), and (f); 500 mm (g)-(j). Lei et al. (2026).

Energy dispersive X-ray, Raman spectroscopy, and X-ray photoelectron spectroscopic analysis of the lateral eyes and the forward facing spots are enriched in organic carbon. Examination under a scanning electron microscope revealed that these organic patches are made up of oblong or cylindrical microbodies, which measure 200-1200 nm in length, and 200-900 nm in width. Most of these microbodies appear deformed or fused together, and they are associated with pyrite minerals and a clay matrix.

In the lateral eyes of Haikouichthys ercaicunensis these microbodies are consistently oval in shape, ranging from 250 to 900 nm in length and from 200 to 800 nm in width. Element mapping suggests that these objects are carbonaceous structures with a small central hole. In the unnamed Myllokunmingid, there are two morphotypes of microstructures present, the first similar to those seen in Haikouichthys ercaicunensis, and the second being cylindrical in shape and between 400 and 1200 nm in length and between 200 nm and 550 nm in width. These structures also have a central hole. Transverse sections of the melanosomes of some living Vertebrates have also shown such a central hole.

Lei et al. next investigated the molecular composition of the microstructures using Time-of-Flight Secondary Ion Mass Spectrometry. This revealed that in both species the microstructures contained the pigments eumelanin and phaeomelanin, both of which are found in living Vertebrates, confirming that these structures are in fact melanosomes. 

The melanosomes in the lateral eyes of Haikouichthys ercaicunensis appear to be largely distributed on the horizontal axis, while those of the unnamed Myllokunmingid are spread along a diagonal axis, with the two types of melanosomes present having different distributions and pigment contents; the cylindrical cells have a higher eumelanin content (which would have made them browner in colour) while the ovoid cells have a higher phaeomelanin content (which would have made them oranger in colour). 

In living Vertebrates, melanosomes are found in the iris, choroid and retinal pigment epithelium, but layers of ovoid and cylindrical melanosomes are found only in the retinal pigment epithelium. The observed structures in the eyes of the unnamed Myllokunmingid are consistent with a retinal pigment epithelium with a similar structure. However, in the six specimens of Haikouichthys ercaicunensis examined only ovoid melanosomes could be observed. However, rather than interpreting this as a more primitive state, Lei et al. note that in the Lamprey Mayomyzon pieckoensis and the Cartilaginous Fish Bandringa rayi from the Carboniferous Mazon Creek Fauna of Illinois, a preponderance of ovoid melanosomes have also been observed in eye structures, and that relatively few living Vertebrates have have been investigated to determine what forms of melanosomes are present in their retinas.

In both Chengjiang Myllokunmingids, the central spots are smaller than the lateral spots, about 160-240 µm in diameter in Haikouichthys, and about 90-120 µm in diameter in the unnamed Myllokunmingid. These were also found to be carbonaceous in composition, and to contain microbodies which appeared to be melanosomes; in each species these were consistent with the bodies found in the larger lateral eyes, with only oval melanosomes in Haikouichthys and both cylindrical and oval forms in the unnamed Myllokunmingid. Based upon this, Lei et al. conclude that these medial organs are also preserved retinas.

Carbonaceous preservation of Myllokunmingids eyes and median dark s pots (a-h). (a)-(b) Haikouichthys ercaicunensis (YNGIP-90285) showing lateral eyes (grey) and pineal eyes (green) with lens (blue). (c) Carbon element map of Haikouichthys ercaicunensis (YNGIP-90285) head. (d)-(e) Haikouichthys ercaicunensis (YNGIP-90296) showing lateral eyes (grey) and pineal eyes (green) with lens (blue). (f) Carbon element map of Haikouichthys ercaicunensis  (YNGIP-90296), arrows indicating left pineal eye. (g)-(i) Eyes of Haikouichthys ercaicunensis showing lens (arrows). (g) YNGIP-90283. (h) YNGIP-90284. (i)  YNGIP-90289. (j), (m) lens in Elonichthys peltigerus (ROM56794). (k), (n) Lens in Platysomus circularis (PF7333). (l), (o) Lens in Bandringa rayi (ROM56789). Scale bars are 200 μm (a)-(f); 50 μm (g)-(i); and 500 mm (m)-(o). Lei et al. (2026).

As well as melanosomes within their retinas, both species show preserved lenses, which are ovoid in structure, and about one fifth of the size of the associated retinas. These structures are preserved as impressions with some relief, suggesting that they represent an original structure which was somewhat decay resistant. This placement, size, and composition is consistent with the interpretation of these structures as eye lenses, which are harder tissue than other components of the eyes, and have been found in other Vertebrate fossils, including the Middle Cambrian vertebrate Metaspriginna walcotti.

The similarity of the lateral eyes of the two Myllokunmingid species from the Chengjiang Fauna to those found in later Vertebrate fossils is taken by Lei et al. to indicate that camera eyes had appeared by the Early Cambrian. The combination of a large retinal pigment epithelium and smaller lens is consistent with a fluid-filled retinal sphere with an iris opening within which the lens is suspended, as seen in living Vertebrates. Such eyes would almost certainly have been capable of image formation, although the quality of such images is impossible to know. 

The median, forward-facing spots on Myllokunmingids have previously been interpreted as nasal sacs, or possibly pineal organs. The former explanation seems unlikely, as nasal sacs otherwise appear to have been quite a late development, not found in many later stem Vertebrates, and probably first evolving in Galeaspids (probable stem Gnathostomes) between 435 and 370 million years ago. Lei et al. report the discovery of melanosome-bearing tissues and lenses in these spots, which are again inconsistent with an interpretation as nasal sacs. They instead interpret them as paired pineal organs functioning as a second pair of camera eyes.

Lei et al. also note that the Middle Cambrian stem Vertebrate Metaspriginna walcotti also has a pair of dark spots between the lateral eyes, preserved as carbonaceous films, and that these also appear to have associated spherical objects, which may also have been lenses, suggesting that this species may also have had a second pair of median eyes.

In Lampreys, the pineal organ is photosensitive, helping the Animal to respond to changes in light levels within the environment. In Mammals, the pineal organ is entirely internal, but it is associated with aligning the neuroendocrine system with the day/night cycle. In Lizards, the pineal organ is also associated with the neuroendocrine system, but in some species retains a photoreceptive capacity. It has therefore previously been suggested that the pineal organ may have developed from some sort of precursor eye, something that has entered popular culture as the 'third-eye' theory. Lei et al. suggest that the pineal organ may have begun as a pair of photosensitive organs acting as additional camera eyes. 

The presence of complex visual systems in the earliest Vertebrates suggests that this sense was of key importance to the success of the group from very early in its history. Both the photoreceptive cells of Vertebrates and the cells of the retinal ganglion arise from nurosensory cell precursors also present in Tunicates. A theoretical model has previously been developed in which the camera eye developed via two rounds of whole-genome duplication, the first allowing for a divergence between the photoreceptor cells and the optical ganglion cells, the second between the pineal complex and the lateral eyes. The apparent presence of a second pair of camera eyes associated with the pineal complex in Early Cambrian Myllokunmingids may represent a transitional stage, in which the genes associated with the development of the eyes have been duplicated, but only just started to evolve towards the modern pineal complex.

Evolutionary scheme of visual system in early Vertebrates. (a) Thalia (Tunicata). (b) Haikouichthys. (c) Euphanerops. (d) Generalised Lamprey. (e) Sacabambaspis. (f) Shuyu. (g) Aphyocharax. Coloured regions show positions of key sensory organs: blue, eyes; red, pineal. Light grey lines represent body outlines. Coloured bars represent the suggested acquisition of key characters. Abbreviations: br, brain; p, pineal; pp, parapineal; TG, total group. Cyclostome represents the Petromyzontidae and Myxinoidea total groups and Gilpichthys, which was recovered in a polytomy with those two groups. Cyclostome and Gnathostome total groups in this topology recovered in a polytomy with Metaspriggina and (Haikouichthys + Myllokunmingia). Lei et al. (2026).

Euphanerops longaevus, an anaspid-like fossil from the Devonian Escuminac Formation of Canada, which has been suggested as a stem-Agnathan (jawless Fish) also has paired median dark patches which have been shown to be carbonaceous films with structures identical to the melanosomes of its lateral eyes. Living Lampreys have a pineal eye and a smaller parapineal eye, both of which have functioning retinas (but not lenses) and are used to detect changes in light conditions. The stem Gnathostome (jawed Fish) Sacabambaspis has two pineal openings, which Lei et al. suggest are analageous to the pineal and parapineal eyes of Lampreys. Later stem Gnathostomes, such as the Galeaspids, only have a single such opening, suggesting a progressive loss of this system. Crown Gnathostomes have lost this opening completely, but some have a preserved pineal window, with an area of thin, semitransparent skull overlaying a pigmented area associated with the pineal complex. Thus an image-forming pineal complex was slowly replaced with a light sensitive organ regulating the production of the hormone melatonin, which regulates sleep patterns. Most crown Vertebrates possess both pineal and parapineal organs, sugesing that this complex was originally paired.

During the Cambrian Explosion, early Animals went through a phase of remarkable morphological innovation, with each new development changing the ecological environment in which all Animals lived, particularly as predation became more common. It has been suggested that higher levels of ultraviolet radiation in shallow waters during the Cambrian may have made the rapid evolution of vision more important, although it is likely that the evolution of predator-prey relationships would have been sufficient to drive this. The appearance of large (for the Cambrian) predators such as Radiodonts, gilled Lobopods, and stem Chaetognaths, all of which developed complex visual systems, would have made it important for smaller, non-predatory Animals such as Myllokunmingids to develop equivalent systems to evade predation and survive. 

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Toledodiscus valverdi: A new species of Eodiscide Trilobite from the Early Cambrian of Toledo Province, Spain.

The Eodiscides were a group of Agnostid Trilobites, which appeared in the late Early Cambrian and persisted till the end of the Middle Cambrian. They were small in size, with only two or three thoracic segments. Within the Eodiscina, the Weymouthiidae were a group of blind (eyeless) Trilobites lacking free cheeks, known only from the latest Early Cambrian and earliest Middle Cambrian.

In a paper published in the journal Historical Biology on 14 February 2025, Luis Collantes of the Yunnan Key Laboratory for Palaeobiology and MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment at Yunnan University, and Sofia Pereira of the Centro de Geociências at the Universidade de Coimbra, describe a new species of Weymouthiid Eodiscide Trilobite from the Upper Marian (Cambrian 'Stage 4') of Toledo Province, Spain.

The new species is described from eight isolated cephala, three isolated pygidia and six complete specimens, preserved as internal and external moulds, collected from a site within the 'metasedimentary unit’ of the Schist-Greywacke Complex of the Schist-Greywacke Complex, about 25 km to the southwest of the city of Toledo. It is given the name Toledodiscus valverdi:, where 'Toledodiscus' refers to the location where the specimens were discovered, and 'valverdi' honours the late agronomist and palaeontologist Ildefonso Recio Valverde for his contributions to the study of the palaeontology of Toledo Province and the conservation and dissemination of the palaeontological heritage of the Toledo Mountains.

Toledodiscus valverdi, Soleras Formation, upper Marianian, Totanés, Toledo, Spain, all in dorsal view. (a) MPT–01306, holotype. (b) MPT–01312. (c) MPT–01313. (d) MPT–01309. (e) MPZ 2021/337. (f) MPT–01307. (g) MPT–01319. (h) MPT–01308. (i) MPT–01318. (j) MPT–01310. (k) MPT–01315. Scale bars are 2 mm. Collantes & Periera (2025).

Toledodiscus valverdi resembles members of the genus Serrodiscus, and in particular Serrodiscus bellimarginatus, with some specimens initially being identified as belonging to that species. However, unlike any member of the genus Serrodiscus, Toledodiscus valverdi has ocular ridges a structure usually absent in all Weymouthiids. Weymouthiids, like other Eodiscides, lacked eyes, and therefore have no obvious use for ocular ridges; some other Eodiscides do retain these structures, which are assumed to be a 'primitive' trait within the group, inherrited from a sighted ancestor. This could be interpreted as a sign that Toledodiscus is a more primitive genus than Serrodiscus, and possibly ancestral to it. However, Toledodiscus valverdi also has a well-defined occipital structure, something considered an advanced trait in Weymouthiids, and absent in all known members of the genus Serrodiscus.

Idealized schematic illustration of the representative dorsal characters of Toledodiscus valverdi. Noah Schamuells in Collantes & Periera (2025).

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Timorebestia koprii: A giant stem-group Chaetognath from the Early Cambrian Sirius Passet Lagerstätte of Peary Land, North Greenland.

The Cambrian Explosion was marked by a sudden radiation of numerous Animal taxa, and the simultaneous expansion into numerous ecological roles. Exactly why this started remains unclear, but the evolution of predatory behaviour and the subsequent arms race between predators and prey is considered to be a significant part of the subsequent rapid evolutionary divergence. The 'explosion' is now recognised to have had three distinct steps; the appearance of Worm-like organisms in the terminal Ediacaran, the appearance of hard parts in the earliest Cambrian, and an rapid evolutionary diversification which produced the majority of modern phyla in Cambrian Stage 3.

Chaetognaths, or Arrow Worms, may have been the first group of Bilaterian Animals to specialise in hunting within the water column, with the Paraconadonts of the Earliest Cambrian now recognised as the grasping spines of early Chaetognaths. Amiskwia sagittiformis, a Burgess Shale fossil first described by Charles Doolittle Walcott in 1911, shows a strong similarity to modern Chaetognaths, possessing a roughly tubular body with paired lateral and tail fins. Despite this, it was for a long time thought unlikely to be a Chaetognath, as no specimen had ever been found with grasping spines, considered to be a key feature of the group. However, recent studies of these fossils have shown that they have an internal jaw apparatus, similar to that seen in Gnathiferans, a group which phylogenetic studies have suggested are closely related to the Chaetognaths, with the two forming a single clade, the Chaetognathifera. This combination of Chaetognath-like paired lateral and tail fins with an Gnathiferan-like internal jaw apparatus, raises interesting questions about the origin of these groups; where the two traits present in the ancestors of all Chaetognathiferans? Or was Amiskwia sagittiformis a member of one lineage that had had convergently gained a trait associated with the other? Understanding the answer to this question could have implications for our understanding of not just the origins of the four modern phyla, the Chaetognatha, Gnathostomulida, Micrognathozoa, and Rotifera.

In a paper published in the journal Science Advances on 3 January 2023, a team of scientists led by Tae-Yoon Park of the Division of Earth Sciences at the Korea Polar Research Institute, and the University of Science and Technology, describe a new, giant, stem-group Chaetognath from the Early Cambrian Sirius Passet Lagerstätte of Peary Land, North Greenland, and discuss its implications for the origin of the group.

The new species is named Timorebestia koprii, where 'Timorebestia' means 'fear-inducing beast' (it also appears to be a reference to Robert Burn's line 'timorous beastie', from the poem 'To a Mouse', though this is not stated), and 'koprii' derives from KOPRI, the acronym for the Korea Polar Research Institute. The species is descibed from 13 specimens collected from several horizons at the main Sirius Passet Lagerstätte locality; specimens were found over 12 m of exposure, although most were from within a particular fossiliferous interval of between 5 m and 7 m.

Holotype (MGUH 34286) of Timorebestia koprii. (A) to (C) Entire specimen. (D) and (E) Jaw apparatus in the anterior region of trunk. (A) Wavelength-dispersive x-ray spectrometry map of carbon on the specimen surface. (B) Polynomial texture mapping visualization using specular enhancement, illuminated from top left. (C) Interpretative drawing. (D) Carbon map of jaw apparatus indicating some indistinct enrichment of carbon within it. (E) Polynomial texture mapping image illuminated from top left of jaw apparatus. (F) Interpretative drawing of jaw apparatus based on tracing of multiple illumination angles. Abbreviations: Bp, basal plate; Lb, lateral bars; Jw, jaw; Ja, jaw apparatus; G, gut; Tm, transverse muscles; Fr, fin rays; Ps, posterior structure; Lm, longitudinal muscles. Park et al. (2024).

Timorebestia koprii is a wide bodied 'Amiskwiiform' with lateral fins running along the majority of the length of its trunk and a well-developed rounded caudal fin; these fins are rayed, with no division between the trunk and caudal fins. It's fore-end has a distinct tapering 'head' with paired antennae. An internal jaw apparatus is preserved as a pair of blunt anterior elements, and a single anterior plate. longitudinal bands of discrete muscles can be seen in the trunk, as well as additional outer circular or transverse muscles, although these are much more sparse. A digestive tract can be seen running from the head to just in front of the caudal fin.

Additional specimens of Timorebestia koprii (A) MGUH 34287, the largest preserved individual imaged with high dynamic range based on multiple images taken with different incident illumination angles while submerged in water. (B) Interpretative drawing. (C) MGUH 34288, another very large individual preserving less detail in low angle illumination. (D) High dynamic range image. (E) Interpretative drawing. (F) MGUH 34289, laterally preserved specimen. (G) Interpretative drawing. Abbreviations: An, antennae; Cr, caudal region; Hd, head; Mu, muscles; Mgc, mineralized gut contents; G, gut; Tr, trunk. Park et al. (2024).

Timorebestia koprii shows considerable variation in size, with the smallest being about 22 mm in length, while the largest is an incomplete specimen with a preserved bodylength of 206 mm, plus 92 mm antennae, giving a total preserved length of 298 mm, and an estimated original length in excess of over 300 mm. This is remarkable, as most extant Chaetognaths are only a few millimetres in length, while the largest living species, Pseudosagitta gazellae, reaches about 10 cm in length, similar to the largest described fossil species to date, Capinatator praetermissus, from the Burgess Shale of British Columbia.

Digital 3D model of Timorebestia koprii. Reconstruction showing internal and external anatomy (red, musculature; blue, ventral ganglion; black, jaw apparatus; green, gut). (A) Lateral view. (B) Dorsal view. (C) Ventral view. (D) Ventral view excluding musculature. Park et al. (2024).

Timorebestia koprii shares a general bodyplan with Amiskwia sagittiformis, which is not seen in any extant group of Gnathiferans. Since Amiskwia sagittiformis was the first discovered organism with this bodyplan, Park et al. refer to these Animals as 'Amiskwiiforms'. They also note the presence of another, as yet unnamed stem-Chaetognath in the Sirius Passet Lagerstätte, which does have external grasping spines, and is interpreted as being closer to the crown group than either Timorebestia koprii or Amiskwia sagittiformis. Thus it is assumed that an internal jaw apparatus is the ancestral state in Chaetognaths and Gnathiferans. The accessory pair of transverse elements within these internal jaws resembles the uncus elements of Rotifers (which are closely related to Chaetognaths and Gnathiferans). The external transverse or circular muscles seen in Amiskwiiforms are absent in modern Chaetognaths, however, Rotifers and Mictognanozoans have strong circular muscles and Gnathostomulids have numerous, but much thinner circular muscles, similar to those seen in Timorebestia koprii, suggesting that this might be the ancestral trait in Chaetognathiferans. Both Timorebestia koprii and the unnamed Sirius Passet species show phosphatized structures which appear to be a ventral ganglion with lateral neuron somata, something which strongly suggests they are more closely related to Chaetognaths than to other groups. A phylogenetic analysis carried out by Park et al. suggests that Timorebestia koprii and Amiskwia sagittiformis are the earliest (known) branching taxa on the Chaetognath branch of the Chaetognathiferan tree.

Ventral ganglion comparisons and phylogenetic relationships. (A) and (B) Timorebestia koprii MGUH 34290 and an interpretative drawing highlighting the presence of a paired set of bilobed structures (arrowed), mineralized by phosphate interpreted as lateral neuron somata of a ventral ganglion. (C) and (D) A small undescribed Chaetognath from Sirius Passet, MGUH 34299, preserving the ventral ganglion (arrowed) as paired phosphatized structures. (E) to (G) Confocal laser scanning microscope images of the extant Chaetognath Sagitta sp. (E) Histochemical labelling of nuclei (blue) and α-tubulin (green). (F) Same view as in (E), with only immunolocalization nuclei (blue). (G) Magnified view of the ventral nerve centre and the lateral neuron somata (arrowed) enriched in nuclei (blue). (H) Summary of phylogenetic analysis placing Timorebestia koprii on the Chaetognath stem. Schematic reconstructions at the tips indicate relative association of the jaw apparatus, pedal ganglion in Rotifers, and ventral ganglion. Park et al. (2024).

The large size of Timorebestia koprii is surprising; the largest known specimen of Amiskwia sagittiformis is only about 35 mm long, while the previous largest known fossil Chaetognath, Capinatator praetermissus from the Burgess Shale reaches only about 100 mm, comparable to the size of the largest living species, Pseudosagitta gazellae. The large size of Timorebestia koprii makes it one of the largest Early Cambrian pelagic species, which in combination with its complex swimming apparatus and long antennae, would probably have made it a top predator in its environment, an interpretation which is supported by the presence of the Bivalved Arthropod Isoxys volucris within the digestive tract of many specimens.

Reconstruction of Timorebestia koprii in the pelagic ecosystem preserved in Sirius Passet. Other taxa shown in the foreground are Kiisortoqia, Siricaris, Kerygmachela, Pauloterminus, Kleptothule, and Isoxys. Further in the background are two Radiodonts: Tamisiocaris and an Amplectobeluid. Robert Nicholls in Park et al. (2024).

This is surprising, as Chaetognaths are close to the bottom of the food chain in modern oceans, feeding on tiny zooplankton and, in the case of the largest species, very small Fish. However, they are one of (if not the) earliest pelagic predatory groups to appear, with Paraconadonts such as Protohertzina appearing in the oldest Cambrian deposits, so it is perhaps unsurprising that they were for a time the top predators in the Cambrian oceans. The first Panarthropods are thought to have colonised the water column around the transition from Cambrian Stage 2 to Cambrian Stage 3, or roughly 525 to 522 million years ago. The Sirius Passet Lagerstätte is thought to be between 523 and 518 million years old, making it a window into a time when one ecological realm was replacing another, including the replacement of Chaetognaths as top pelagic predators by the emerging Panarthropods.

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Three new species of Artiopodan Euarthropod from the Cambrian Chengjiang Biota of Yunnan Province, China.

The Artiopodans, which comprise Trilobites plus a diverse range of Trilobite-like Arthropods, form an important part of all Palaeozoic marine communities, distinguished by a series of trunk appendages in similar morphology. Many biologists consider that the Artiopodans are still extant today, although they are divided as to whether the Chelicerates (Sea Spiders, Horseshoe Crabs, and Arachnids) or Mandibulates (Crustaceans, Insects, Millipedes and Centipedes) are living Artiopodans. Fossil Artiopodans are divided into two groups, the non-mineralised Trilobitomorphs and the mineralised Vicissicaudates.

The first Artiopodan fossil from the Chengjiang Biota of Yunnan Province was described in 1985, since when this Konservat-Lagerstätte has been recognised as having the highest known diversity of non-Trilobite Artiopodans of any known Cambrian deposit, and in particular the site from which most information about the appendages of these Arthropods has been gained.

In a paper published in the journal Acta Palaeontologica Polonica on 8 September 2023, Zhu Yuyan, Zeng Han, Lui Yao, and Zhao Fangchen of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, describe three new species of Artiopodans from the Chengjiang Biota.

All three new species come from the Kuangshan section in Malong District, which is about 95 km to the northeast of Chengjiang, the type locality for the Chengjiang Biota. Here a yellowish mudstone considered to be part of the Maotianshan Shale Member of the Yu’anshan Formation has an intermixture of siltstone beds, and is interpreted to have been reworked since its original deposition.

The first new species described is named Zhugeia acuticaudata, where 'Zhugeia' refers to Zhuge Mountain where the Kuangshan section is located, which in turn is named after the ancient Chinese statesman Zhuge Liang, who reputedly once camped there, and 'acuticaudata' means 'spiney tail'. It is placed in the Trilobitomorph order Xandarellida, which is defined by having a semicircular cephalic shield (head-plate) with stalk lateral eyes that extend posteriorly to cover multiple thoracic tergites (plates covering back segments) and a pygidium (tail) with a broad median spine.

Artiopodan Euarthropod Zhugeia acuticaudata from the Cambrian Stage 3 Chengjiang fauna at Malong, Yunnan, China; holotype NIGP 200049. (A) Whole specimen, under high-angle light (A₁), inverted red channel image (A₂), interpretative drawing (A₃). (B) Right eye. (C) Left eye. (D) Left genal spine. (E) Thoracic tergites under low-angle light (E₁), under high-angle light (E₂). (F) Sixth to ninth thoracic tergites under low-angle light (F₁), under high-angle light (F₂). (G) Median pygidial spine under low-angle light (G₁), under high-angle light (G₂). (H) Lateral pygidial spine under low-angle light (H₁), fluorescence photograph under 532 nm green laser (H₂). Abbreviations: br, bradoriid; ce, cephalon; ct1–ct2, thoracic tergite number covered by cephalon; e, eye; gs, genal spine; ls, lateral pygidial spine; ms, median pygidial spine; py, pygidium; t1–t9, thoracic tergites. Zhu et al. (2023).

Zhugeia acuticaudata has a semi-elliptical cephalon with a pair of genal spines (spines emerging from the 'cheek'-section of the cephalon, and a pair of ovoid lateral eyes near anterior margin of the cephalon. The posterior of the cephalon covers the two forewardmost thoracic tergites. There are nine partially overlapping tergites with pleural spines (spines emerging from the outer edges of the tergites). The pygidium is small with two small lateral spines and an elongated needle-like median spine.

The second new species is named Tonglaiia bispinosa, where 'Tonglaiia' derives from 'Tonglai', an ancient name for Malong County, and 'bispinosa' means 'double-spined'. The phylogenetic affinities of this species are unclear.

Artiopodan Euarthropod Tonglaiia bispinosa from the Cambrian Stage 3 Chengjiang fauna at Malong, Yunnan, China; holotype NIGP 200050. (A) Whole specimen under high-angle light (A₁), under low-angle light (A₂), interpretative drawing (A₃). B. Left eye. (C) Left cephalic spine. (D) Right cephalic spine. (E) Right appendage. (F) Thoracic tergites (F₁) with spines (F₂). (G) Pygidium. Abbreviations: ap, appendage; ce, cephalon; cs, cephalic spine; e, eye; ml, marginal line around cephalon; pa, pleural angle; py, pygidium; sp, marginal spine; t1–t7, thoracic tergites; ts, terminal spine. Zhu et al. (2023).

Tonglaiia bispinosa has an elliptical cephalon with a pair of marginal cephalic spines and two oval eyes situated near its anterior margin. There are seven partially overlapping tergites with posterolateral marginal spines. The pygidium is small with a pair of triangular spines at its posterior end.

The third new species described is placed in the Vicissicaudate genus Sidneyia, which was first described by Charles Doolittle Walcott in 1911, with a single species Sidneyia  inexpectans, based upon specimens from the Burgess Shale, and given the specific name 'malongensis', meaning 'from Malong'. This is the second species of Sidneyia from Yunnan, with Sidneyia minor having been described from the Early Cambrian Xiaoshiba Biota.  Specimens have also been assigned to Sidneyia cf inexpectans  from the Mantou Formation of North China.

Artiopodan Euarthropod Sidneyia malongensis from the Cambrian Stage 3 Chengjiang fauna at Malong, Yunnan, China; paratype NIGP 200052. (A) Whole specimen under high-angle light (A₁), under low-angle light (A₂), interpretative drawing (A₃). (B) Thoracic tergites under high-angle light (B₁), under low-angle light (B₂). (C) Abdomen. Abbreviations: as1–as2, abdominal segments; ce, cephalon; e, eye; t1–78, thoracic tergites; tf, tail fluke. Zhu et al. (2023).

Sidneyia malongensis is described from two specimens. It has an oval-shaped exoskeleton composed of cephalon, thorax, and abdomen, with a body length of about 31 mm, and a maximum width is about 21 mm in the holotype and 15 mm in the paratype. The cephalon is semi-elliptical, measuring 6 mm long and 17 mm wide in holotype. The anterior margin of the cephalon is rounded, and posterior margin is straight. Lateral eyes are located at the genal angles of the cephalon. Eyes are also associated with notches on cephalon, although eye stalks are unclear. The presence of numerous wrinkles near the anterior margin indicates the convexity of the cephalon. The thorax consists of eight imbricated tergites of approximately equal length, measuring 16 mm long in the holotype and 12 mm long in the paratype. The overlapping area between adjacent thoracic tergites accounts for about one third of the sagittal length of each tergite. The posterior margin of the first three thoracic tergites is almost straight. The thorax is widest at the third or fourth thoracic tergite, measuring 8 mm wide in holotype, after which it gradually narrows and curves posteriorly. The pleural angle of each tergite is nearly equal along the thorax. No marginal spines are found on thoracic tergites. The abdomen consists of two overlapping cylindrical segments of subequal dimensions, measuring 4 mm long and 4 mm wide in holotype, as well as a tail fluke. The abdominal segments are narrower than the last thoracic tergite. The overlapping area between adjacent abdominal segments accounts for about one-fifth to one-fourth of sagittal length of each segment, with an average length of 1 mm. The tail fluke consists of a central triangular telson and a pair of flanked flaps, and is approximately as long as the abdominal segments.

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