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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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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Bushizheia yangi: A new species of Euarthropod from the Early Cambrian Chengjiang Lagerstätte.

Over 250 species have been recorded from the Cambrian Series 2, Stage 3 Chengjiang biota of Yunnan Province, China, with Arthropods representing circa 33% of these species. These include taxa with soft anatomy preservation that are instrumental to elucidating early Euarthropod evolution. Among these, Arthropods with a grasping appendage, the so-called great or frontal appendages, are iconic animals of the Chengjiang biota. 

In a paper published in the journal Palaeontologica Electonica, Robert O’Flynn and Denis Audo of the Yunnan Key Laboratory for Palaeobiology and International Joint Laboratory for Palaeobiology and Palaeoenvironment at Yunnan University, Mark Williams of the School of Geography, Geology and the Environment at the University of Leicester and the International Joint Laboratory for Palaeobiology and Palaeoenvironment at Yunnan University, Dayou Zhai, also of the Yunnan Key Laboratory for Palaeobiology and International Joint Laboratory for Palaeobiology and Palaeoenvironment at Yunnan University, and Hong Chen, again of the Yunnan Key Laboratory for Palaeobiology at Yunnan University, and Yu Liu, once again of the Yunnan Key Laboratory for Palaeobiology and International Joint Laboratory for Palaeobiology and Palaeoenvironment at Yunnan University, describe a new species of Euarthrapod from the Chengjiang Biota.

The new species is known only from a single specimen with a well-preserved dorsal exoskeleton that is divided into head, thorax, and pygidium, but is most notable for its long frontal appendages that resemble those of Radiodonts and the Euarthropod Kiisortoqia soperi. O'Flynn et al. discuss what happened to the ‘great appendage’ with reference to two possibilities for the evolution of the primary antennae or ‘great appendage’: (1) that limb-like ‘great appendages’ evolved to form chelicerae, i.e. innervated from the deutocerebrum; or (2) that the labrum represents the transformed frontal ‘great appendage’, i.e. innervated from the protocerebrum.

 

Bushizheia yangi (YKLP 11421). (A) Photograph of dorsal view; (B) composite line drawing of dorsal view. Both scale bars are 10 mm. Abbreviations: app?ex, exopod of a post-frontal head limb appendage; cw, compression wrinkle; en, endopod; gr, groove; hl, frontal head limbs; hs, head shield; ps; pygidial segment; pyg, pygidium; tg1-8, tergites 1 through 8; ts, tailspine. O'Flynn et al. (2020).

The new species is described from a single specimen recovered from the Yu’anshan Member pf the Chiungchussu Formation at Jiucun Town in Chengjiang County. The Yu’anshan Member is informally divided stratigraphically into four lithologically and palaeontologically distinctive parts. The specimen was recovered from the third stratigraphic interval of the Yu’anshan Member from laminated mudstones
of the early Cambrian (Series 2, Stage 3). It is deposited in the Yunnan Key Laboratory for Palaeobiology at Yunnan University, Kunming, as specimen number YKLP 11421.

The specimen was prepared mechanically with a mounted steel needle under a binocular microscope, where only a small amount of matrix was required to be removed by a gentle scraping action. 

Fluorescence microscopy and photography (a Leica DFC7000 T linked to a Leica M205 FA fluorescence stereo microscope) were used to increase the contrast between the specimen (not autofluorescent) and the matrix (weakly autofluorescent). For photographs in natural colour, O'Flynn et al. captured several images with a Digital SLR Camera (a Canon EOS 5DS R and a Nikon D700) fitted with a 100 mm and a 105 mm macro lens, respectively, with extension rings for magnification factors above 1:1. The photographs were mostly captured in cross-polarised light to limit reflection of light and increase contrast. O'Flynn et al. also captured photographs with cropped light (light coming from a low vertical angle, highlighting structures at the surface) from various angles (default: from the top left) to reveal the micro relief of the fossil. The specimen was also investigated with Computed Tomography using a ZEISS Xradia 520 Versa, but no additional structures were revealed using this method.

Measurements were made on digital photographs using the image processing software ImageJ. Morphological characters measured include: (1) the total length, measured from the anterior margin of the head shield to the posterior termination of the tailspine; (2) the head, measured from the anterior margin to the posterior margin of the head shield; (3) tergites one through eight, measured from the anterior margin to the posterior margin of the tergite; (4) the frontal head limb, measured from the distal termination to the proximal margin of the podomeres; and (5) the pygidium, measured from the posterior margin of the posterior-most tergite to the posterior termination.

The new species is named Bushizheia yangi, where 'Bushizheia' derives from Bǔshízhě, the Mandarin for predator: an allusion to Kiisortoqia, from the Kalaallisut (Greenlandic) word kiisortoq, meaning predator, and 'yangi' honours Zhixin Yang, who recovered and prepared the fossil.

The genus Bushizheia is is ascribed to the total group Euarthropoda, as defined by the presence of fully Arthrodised bodies and limbs, because it has sclerotised trunk tergites, sclerotisation of post-antennal appendages, and does not have any isolated tergites in the cephalic segments.

It features a long, frontal grasping appendage that is morphologically like that of Radiodonts, but in combination with a completely Arthrodised body organisation; this distinguishes Bushizheia from all other Euarthropods, except Kiisortoqia.

Bushizheia and Kiisortoqia appear allied to each other by the possession of frontal ‘great appendage’-like head limbs. However, Bushizheia is sufficiently distinct to warrant erection of a monospecific genus. It differs from Kiisortoqia by: (1) Bushizheia  has a large pygidium with paired short falcate spines and a terminal tailspine; Kiisortoqia has a small tail shield with neither spines nor a tailspine and twice the number of thoracic tergites; (2) trilobation of thoracic tergites effaced; (3) dorsal spine is present on each podomere of the frontal head limb. 

 

Bushizheia yangi (YKLP 11421). (A), (C) Tungsten photography lighting of left frontal head limb and dorsal view of head shield, respectively; (B), (D) fluorescent photography lighting of the same. All scale bars are 2 mm. Abbreviations: cw, compression wrinkles; ds, dorsal spines; hl, frontal head limb. O'Flynn et al. (2020).

Bushizheia yangi is a Euarthropod with a semi-circular head shield lacking dorsal compound eyes, eight thoracic tergites, and a large, semi-circular pygidium. Frontal head limbs circa 33% of body length, strong, composed of at least 12 podomeres, each with paired spines laterally.

Bushizheia yangi is circa 54 mm long. The habitus is sub-elliptical in dorsal aspect, with a length (anterior margin of the head shield to the posterior termination of the tailspine) of circa 250% of the width (maximum width in the anterior half of the body at the third and fourth tergites). The length of the head shield is circa 20% of the total length. The trunk tapers from the fourth tergite backwards to a large pygidium (the tail shield of the exoskeleton covering the fused posterior segments of the body). The pygidium’s margin is spinose.

 

Bushizheia yangi (YKLP 11421). (A) Detail of pygidium; (B) detail of exopod setae. Scale bar 3A is 10 mm. Scale bar is 1 mm. Abbreviations: en, endopod; exs, exopod setae; pyg, pygidium; ts, tailspine. O'Flynn et al. (2020).

The head shield is a simple, semi-circular shield, with smoothly rounded posterolateral margins, and the length is circa 70% of its width.

The trunk consists of eight tergites. The anteriormost three tergites are virtually of equal length; the length is circa 20% of the width. Tergites 3-4 are widest; the width and length of tergites subsequently decrease posteriorly in width and length. Axially, the posterior border of each tergite overhangs the following tergite by circa 17% of its length, but by less abaxially. The first, second, and third tergites are reflexed anteriorly, whereas tergites posterior to that are transverse or reflexed posteriorly. The tergopleurae of the first and second tergites terminate bluntly posterolaterally. The tergopleurae of tergites 3-8 are extended into posterolateral projections.

 

Reconstruction of (A) Bushizheia yangi juxtaposed with (B) Kiisortoqia soperi in dorsal view. Both scale bars are 10 mm. O'Flynn et al. (2020).

The pygidium is a semi-circular shield and its length is circa 70% of its width; the margins bear eight paired short falcate spines, which curve very slightly posteriorly. The pleural regions of the pygidium’s anterior-most segment are effaced, but a transverse furrow is visible axially. The tailspine is sub-triangular and incompletely articulated; it is as wide as long, circa 8% of the total length, and circa 30% of the total length of the pygidium.

The frontal head limbs reach a length of circa 33% of the total length of the body: 12 podomeres extend beyond the head shield, each with lateral endites that bear two inward facing setae, and terminal setae that are outward facing. Podomeres 2-7 are of equal length but decrease in width distally.

All visible post-frontal head limb appendages consist of either an endopod or an exopod. The one (incompletely) preserved endopod comprises four narrow podomeres. The two preserved exopods are paddle-shaped flaps, fringed with setae. Martin Stein in his description of Kiisortoqia soperi, identified four cephalic appendiferous segments (a head incorporating the antennular plus three post-antennular limb-bearing segments), considered to be part of the ground pattern of Euarthropoda. The distal exopod of the anteriormost post-frontal head limb appendage of Bushizheia yangi appears beneath the head shield, and may, based on its position, be the fourth cephalic appendage. The limb immediately posterior of this, adjacent to the first trunk tergite, appears to be the first trunk appendage.

Bushizheia yangi is dorsoventrally compressed and lies parallel to lamination; its well-articulated nature (i.e., tergites preserved attached, exopods preserved in situ). This state of preservation, especially the presence of articulated frontal head limbs, suggests limited post-mortem transportation: in, e.g. in extant Crustaceans, antennae can easily disarticulate. Compression wrinkles in the head shield that parallel its margin, suggest localised, weak sclerotisation. It is, therefore, presumed that the head shield was a simple, convex shield, sclerotised but without substantial biomineralisation.

As a result of the state of preservation, not all podomeres of the frontal appendages are accessible for a straightforward description. The exact number of podomeres cannot be established; some are obscured by sediment. Lateral endites can be confirmed for at least podomeres 2-7, 9, 11, 12, and medial endites bearing two firm setae at their tips can be confirmed for at least 4, 5, 7, 11, and 12. Rotation of podomere 2 evinces widely spaced double rows of endites.

Limbs with exopod setae are visible to the right of the two anterior-most thoracic tergites; the exopod of the posterior-most cephalic limb protrudes from beneath the head shield directly adjacent to the border of the head shield and first tergite, and is significantly rotated posteriorly, as is, albeit to a lesser extent the exopod of the first trunk limb. Only a single posteriorly situated endopod is, due to a degradation or complete absence of the overlying tergite, visible. No additional limbs were revealed by palaeontological techniques. The exact number of podomeres in the frontal appendages cannot be ascertained due to poor preservation of the distal-most ones. The proximal portions of the exopods are unavailable for description, so it cannot be established as to whether they were bipartite and divided into proximal and distal portions as in Leanchoilia illecebrosa.

Bushizheia yangi and Kiisortoqia soperi possess a long, grasping frontal appendage very similar to that of the Radiodonts. However, neither of these two species closely resemble Radiodonts. Could the presence of raptorial appendages in Bushizheia yangi and Kiisortoqia soperi be symplesiomorphic, shared by Bushizheia yangi and Kiisortoqia soperi, but also with Radiodonts linked earlier in the clade, to which Martin Stein alluded? Or are they an interesting case of convergence in organisms that are not closely related (not monophyletic), which independently evolved similar traits as a result of having to adapt to similar life habits?

It is beyond the scope of O'Flynn et al.'s study to attempt to solve the Arthropod head problem. Indeed, the preservation of Bushizheia yangi does not allow assessment of the neurological connection of the raptorial appendage. Hence, O'Flynn et al. only compare them to other appendages based on their morphology. Therefore, O'Flynn et al. explore the significance of the raptorial appendage of Bushizheia yangi and Kiisortoqia soperi in the context of some current interpretations of the homology of cephalic Arthropodan appendages; more precisely, the interpretation of the nature of the ‘great appendage’ of Radiodonts.

Many palaeontologists consider that the Radiodonts, and other ‘great appendage’ Arthropods belong to the lineage leading to Chelicerates, and the ‘great appendage’ is homologous to the chelicera. Following this line of reasoning, the presence of the ‘great appendage’ in Bushizheia yangi and Kiisortoqia soperi would, in this case, most likely be homologous to the ‘great appendage’ of radiodonts. The presence of this ‘great appendage’ would tie Bushizheia yangi and Kiisortoqia soperi, and ‘great appendage’ and ‘short-great appendage’ Arthropods together. More precisely, within this group, Bushizheia yangi and Kiisortoqia soperi and Radiodonts would be one of the sister groups of Chelicerates and ‘short-great appendage’ Arthropods. The problem being to reconcile the apparently primitive scarcity of sclerites displayed by Radiodonts with them being included in Euarthropoda, especially when considering that Bushizheia yangi and Kiisortoqia soperi are fully sclerotised. One solution to this problem would be to consider that Radiodonts subsequently lost part of their tergites, perhaps due to their nektobenthic life habits. The sclerotisation of Bushizheia yangi and Kiisortoqia soperi in this scenario would then represent a primitive trait.

Another, currently more widely held view, is that Radiodonts are not Euarthropods, but belong to a series of sister groups to Euarthropods (so called ‘Stem-Group’), which documents the progressive transition from weakly sclerotised, segmented Animals resembling Lobopodians to fully sclerotised Animals, the Euarthropods. However, the results of two important phylogenies supporting this theory were criticised in a 2011 study by Joachim Haug, Dieter Waloszek, Andreas Maas, Yu Liu, and Carolin Haug, due to apparent discrepancies in the coding of some characters and taxa. The debate on the nature of Radiodonts is not closed yet: several phylogenetic analyses published in the last decade support a ‘stem-group’ affinity for Radiodonts. In this scheme, Bushizheia yangi and Kiisortoqia soperi clearly belong to Euarthropods, and in this scenario the appendage similarity is either a convergence with the ‘great appendage’ of Radiodonts, or, the ‘great appendage’ was retained in early Euarthropods, and lost, or modified beyond recognition in most lineages of Euarthropods.

The convergence hypothesis agrees with the interpretation of Graham Budd, who considered the ‘great appendage’ to represent a pre-antennular appendage that was transformed into the labrum in the group including Euarthropods. If the hypothesis that the ‘great appendage’ was lost or reduced into the labrum is accepted, then the presence of ‘great appendage’-like frontal head limbs require convergence. Such convergence can be easily explained if Bushizheia yangi and Kiisortoqia soperi used their frontal head limbs in a fashion like predatory Radiodonts.

Contrary to this last hypothesis, the composition and morphology of the raptorial appendage of Bushizheia yangi bears a striking resemblance to the plesiomorphic state of the raptorial appendage within Radiodonts (i.e. the predatory, microphagous species). Other, probably distantly related species also have a seemingly similar raptorial appendage, e.g. the Bradoriid Kunmingella douvillei. This could give credence to the hypothesis that the great appendage was not reduced in a group including Euarthropods, but rather was lost or reduced in a group within Euarthropods.

In the case of Bushizheia yangi, since O'Flynn et al. do not have access to the ventral surface of the Animal, they cannot describe the position of the raptorial appendage. This prevents observation of the connection of this appendage to the head, and its exact nature. O'Flynn et al. can only hope that better preserved specimens may help to understand this animal better, and perhaps lead to a better understanding of the Arthropod head.

Despite its overall similarity to Kiisortoqia soperi and the morphological similarity of its appendages with those of the Radiodonts, the posterior of the animal is quite different. Disparity between Bushizheia yangi and Kiisortoqia soperi in this regard may well indicate that these are not closely related organisms. On a similar line of thought, similitude between Bushizheia yangi and Trilobitomorphs due to the presence of a pygidium could have been acquired by convergent evolution, an allusion to the conspicuous resemblance of the appendages of Aglaspidid-like Arthropods with those of the Nektaspid Naraoia spinosa. Alternatively, a plesiomorphic pygidium was retained in various Arthropod groups; such a scenario conflicts with several studies in which the presence of the pygidium is synapomorphic for a clade composed by Xandarellida, Nektaspida, Conciliterga, and Trilobita.

Artiopoda (e.g. Xandarellida, Nektaspida, Conciliterga and Trilobita) are arthropods with filiform antennulae. Bushizheia yangi is ipso facto outside Artiopoda, ergo the presence of the pygidium is not synapomorphic for a clade composed by the aforementioned groups. If the interpretation of a plesiomorphic pygidium is correct, this would indicate that this structure was retained in some Arthropod groups (e.g. Bushizheia yangi and Trilobitomorpha) but lost in many others.

A variety of Arthropods from the Chengjiang biota were most likely predators. The frontal head limbs of Bushizheia yangi are sturdy, predatory appendages, reminiscent of the ‘great appendage’ of Radiodonts. The presence of limbs posterior of these frontal head limbs that bear paddle-shaped exopods suitable for swimming could be indicative of a nektobenthic habit. No eyes could be found in the specimen. If the eyes in Bushizheia yangi were situated under the anterior border of the head shield, as in e.g. Leanchoilia illecebrosa, it is possible that they, together with the other ventral features of that area, were not preserved. The alternative hypothesis is that Bushizheia yangi was blind. Although visual perception in animals plays an important role in prey detection, other senses are used by animals in pinpointing their prey; the absence of eyes does not preclude Bushizheia yangi from being a predator. In the case where visual perception is lacking, Bushizheia yangi may have been an ambush predator, lying in wait, and catching unsuspecting prey with its anteroventral raptorial appendage.

Bushizheia yangi possesses a fully arthrodised body and limbs and consequently rests comfortably within the Euarthropoda. The present data neither confirms nor refutes any of the current interpretations of the homology of cephalic arthropodan appendages and although the phylogenetic position of Bushizheia yangi is difficult to resolve because few ventral structures are visible, the many Euarthropod plesiomorphies possessed by the taxon may indicate a basal position of the species, and it demonstrates the importance of fossils in characterising ground patterns.

The dorsoventrally flattened exoskeleton and paddle-shaped exopods that are suitable for swimming suggest a nektobenthic mode of life and Bushizheia yangi likely used its ‘great appendage’-like frontal head limbs for predation.

See also...

Online courses in Palaeontology. 

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