Cambrian Artiopoda Reveals a Constraint in Euarthropod Brain Evolution

Abstract

Fossilized traces of neuropils, nerves and ganglia have demonstrated that cerebral organization in Cambrian arthropods conforms to a ground pattern defining one of today’s two existing euarthropod clades, Mandibulata and Chelicerata^1–8. Artiopoda - a third clade including trilobites and soft-bodied relatives - persisted until the late Carboniferous^9,10, but its cerebral organization has remained unknown. Here we identify and reconstruct fossilized neural traces of the artiopodan Xandarella spectaculum¹⁰, which reveal an expanded prosocerebrum associated with paired ocelli, a truncated protocerebrum supplied by substantial lateral eyes, and salient deutocerebral antennular lobes. This arrangement predicts reliance on chemosensory-guided foraging, with visual processing largely limited to dorsal orientational cues and simple local motion signals. The artiopodan brain thus reveals clade-specific modifications of homologous domains of the euarthropod cerebral ground pattern^4,6–8 established in the early Cambrian.

Since the first identification of fossilized brains in Cambrian euarthropods^1–3, intermittent discoveries of further examples have demonstrated unexpected genealogical correspondences between cerebral arrangements in Cambrian stem euarthropods and phylogenetically related modern taxa^4–8,11. Neuroanatomical diversification of morphogenetically fixed cerebral domains, recognized in extant euarthropods and indicated in fossilized species, demonstrates that already in the lower Mid-Cambrian the distinctive cerebral arrangements of radiodontan, mandibulate and chelicerate ground patterns^4–8,11 correspond to, respectively, cerebral arrangements typifying modern Onychophora, Pancrustacea, and Chelicerata¹¹. Such graphic correspondences support the view that it is the distinctive traits of the unsegmented cerebrum rather than exoskeletal attributes that resolve fossils as belonging to specific lineages and branches of the arthropod tree of life⁸.

In contrast to Pancrustacea and Chelicerata, there is a third major clade of Cambrian euarthropods that did not reach the Holocene. This is Artiopoda, erected by Hou and Bergstrom in 1997 and diagnosed by them^9,10 as “lamellipedian arthropods with trilobite-like appearance, broad tergum, and usually a complete set of fairly undifferentiated post-antennal (sic) appendages.” Artiopoda included Trilobita¹², whose biomineralized (calcified) exoskeletons impeded fossilization of soft tissues. This condition contrasts with that of numerous trilobite-like artiopod taxa which possessed exoskeletons that were more amenable to occasional preservation of soft tissue during fossilization¹³. According to the overall similarity of artiopodan shapes and often appendicular uniformity, remains of neural tissue in one taxon might be generally representative of the clade as a whole. Here we describe neural traces in two non-biomineralized artiopodans. One, Xandarella spectaculum^9,10 provides well-defined traces of a tripartite cerebrum equipped with prosocerebral ocelli and protocerebral lateral eyes. The other is a hitherto undescribed artiopodan that provides clearly defined segmental ganglia.

The initial 1991 denomination of X. spectaculum⁹ and its subsequent 1997 description in the context of the Chengjiang euarthropod fauna leave little to be added here¹⁰. Only two modifications pertaining to the head shield will require consideration. As described in earlier reports^9,10, the overall dorsal view of X. spectaculum shows a teardrop-shaped body comprising three approximately equal parts (Figure 1). A head shield comprising two coalesced tergites covers a discrete supraesophageal cerebrum which is described here. The head shield also covers the hypostome and five pairs of segmental appendages. Seven narrow tergites, each associated with a pair of appendages, comprise the mid-trunk¹⁰. This then tapers as four deeper abdominal tergites, together overlying 25 pairs of appendages⁹. Here we focus exclusively on the head shield and its underlying fossilized neural components.

Figure 1. Dorsal view of Xandarella spectaculum (CN 115286).

Dorsal view of X. spectaculum in which the prominent lateral eyes of the protocerebrum (ce2-eye) are revealed as oval swellings at the caudal edge of the first tergite of the head shield. The two lateral eyes are equidistant from a medial area that is flanked by two small oval visual units interpreted as ocelli (arrowheads). Scale bar: 1 cm

The two tergites comprising the head shield (Fig. 1) do not obviously overlap but appear fused, the back margin of the larger one curving obliquely forwards to cross the midline. Its passage both sides of the midline is interrupted about half-way across by an oval recess within which resides a substantial eye. The eye is arranged in the tergite such that it may be obscured dorsally. This is suggested by a raised crumpled patch where the tergite appears to cover the eye (left inset, Supplementary Figure S1), although the right side suggests the eye is hooded possibly affording a lateral view. Complicating whether the eye is truly obscured dorsally is that in most fossils of Xandarella the eyes are visible from above, as in Figures 1 and 2, suggesting that if there is a covering then its taphonomic fragility artifactually reveals the eye. The eye’s receptive layer is thus likely directed downwards, forwards and laterally, viewing close to the substrate on which the animal walked. The eyes are not the only visual sensors, however. Flanking the midline and bordering the back margin of the anterior tergite is a pair of mirror-symmetric lenticular reliefs, indicating paired ocelli (Fig. 1, Supplementary Figure S2). These are likely homologues of what have been termed the paired “frontal organs”^14,15 (also lately ‘medial eyes’) situated on the anterior sclerite of the artiopodan Helmetia expansa¹⁵. Their presence in Xandarella and rostral placement at the anterior tergite determines them as indicators of the prosocerebral domain ce1. The pair of ‘frontal organs’ is also visible in Fig. 57A, G of the 1997 description of Kuamaia lata¹³. In naraoiid artiopodans¹³, visible bulges anterior to the hypostome suggest cup-like accommodations of double or triple ocelli viewing upwards, as would those identified in the xandarellid Sinoburius^10,16 (see Fig. 77 C, D, in ref 10). The location of these ocelli, just in front of the back edge of the anterior tergite, suggests that the anterior tergite of the Xandarella head shield may be an enormously expanded homologue of the discrete anterior sclerite (aka rostral plate) of Helmetiidae¹⁵. In X. spectaculum, the paired ocelli overlie what is here interpreted as a photoreceptive surface that directly caps the lobes of the brain’s prosocerebrum (Fig. 2b).

Figure 2. The part and counterpart of Xandarella spectaculum specimen YKLP 11356.

a. Part (rotated 180 degrees around its anterior-posterior axis to facilitate comparison with the counterpart), showing the left eye, isolated here, but visible in situ in the counterpart, and cerebrum (bracketed and enlarged in panel b). b. Individually separating colors as adjustable gray levels resolves details of cerebrum. The arrow indicates the left ocellus. c. The counterpart shows the left eye within its socket and the empty socket of the right eye, as well as the cerebrum (bracketed and enlarged in d). d. Fossilized cerebrum with furrowed outlines of the protopod and basis (asterisks), the first three of five true segmental appendages beneath the head shield. The arrow indicates the right ocellus; rings indicate the trajectory of left antennular nerve. e. The passage of the optic nerve is indicated by the passage of the cryptic eyestalk beneath the overlying head shield; arrowheads indicate the eyestalk trajectory. Arrow indicates the ocellus. f, g. In panel f, the left eye shown in Fig. 1 is uncovered and shows little evidence for a compound eye; however, color substitution (panel g) of the densest part of the left eye of YKLP11356 suggests a palisade of repeated units along the eye’s upper margin. Scale bar a, c = 5 mm; b, d, e = 2mm; f, g = 1mm

Only one of the 12 known specimens of X. spectaculum reveals fossilized neural tissue. This is shown in the part and counterpart of specimen YKLP 11356¹⁷ (Fig. 2a-d), indicating cleavage slightly tilted around the anterior-posterior axis. In the fossil’s counterpart the eye is shown nestled within a depression in the head shield surrounded by remnants of the cuticle that covered the eye (Fig. S1). At the more dorsal level of the part, the eye socket is incomplete but the attachment of the eye to the eyestalk can still be made out. Figure 2e shows the channel in the head shield accommodating the optic nerve. The channel is the eyestalk, which leads from the eye socket centrally to the cerebrum. Although the xandarellid eye has been suggested as comprising some hundred facets⁹, this is still uncertain (Fig. 2f). Color substitution of the assumed receptive layer of the eye (Fig. 2g) indicates clumped arrangements of what might have been photoreceptors as well as some marginal columnar-like elements the widths of which would not exceed 50 microns. Similarly, large lateral eyes of the related artiopod Cindarella¹⁸ are suggestive of arrayed lenslets (see Figs. 1d, 8b in ref 18) which could in principle endow acute vision, a function already evolved in Radiodonta, predatory stem group arthropods equipped with large multi-facetted eyes¹⁹.

Both the part and counterpart of YKLP 11356 support a bilaterally symmetric organization of cerebral traces, which are resolved as dark profiles against an intensely dense brown matrix (Fig. 2a, d). Here these profiles have been digitally clarified by adjusting gray tones for each color range of the CMYK palette, thereby adjusting contrast of the spectral range as shown in Figures 2b, d. Serial images focusing on micron-intervals capture all the profiles in both part and counterpart (Fig. 3a; Supplementary Figure S2). These were used to reconstruct the bilaterally symmetric cerebrum, detailed in Figure 3c. The brain is clearly organized as three discrete levels of elaboration along its anterior-posterior axis. Most anteriorly, a bridge of fibers links two apical lobes each side of thin nerves extending to the underlying protocerebrum. The two apical lobes are associated with dense deposits that correspond to the afore-mentioned lenticular reliefs of the head shield that are here interpreted as ocelli (Fig. 2b, d). The optic lobes, supplied by the optic nerve, denote the protocerebral domain ce2, as do sparse traces bordering the optic lobes and a pair of robust recurrent columnar lobes extending in front of the prosocerebrum. Further caudally, and mainly resolved in the counterpart, a less dense cluster of traces together represent substantial lobes at the level of the origin of the paired antennular nerves. These nerves arise from the paired post-ocular chemosensory appendages thus defining the identity of these lobes as the deutocerebral domain ce3¹¹.

Figure 3. Artiopodan cerebrum and ventral ganglia.

a. Serial photographs of specimen YKLP 11356 shown mirror-symmetric (see Suppl. Fig. S2). The panels show stepped focal images of fossil tracts and neuropil at three levels of the part (p) and at two levels of the counterpart (cp). b. Reconstruction obtained from overlapping silhouette-type infill of each image (see Suppl. Fig. S2). c. The neural traces resolve a pair of lobes connected by a bridge overlying the central connections. That the paired ocelli abut these lobes identifies them as the prosocerebrum. Although the optic nerves leading from the lateral eyes are obscured by the eyestalk, their passage beneath the first tergite of the head shield can be extrapolated to their root at their small terminal lobes in protocerebrum which are dwarfed by the paired antennular lobes at the level of the deutocerebrum (ce3). The protocerebrum also provides paired recurrent lobes that partially overlap ce3. The caudal extension of the brain appears damaged, but its initial trajectory suggests that the ensuing ventral cord splits into two circumesophageal tracts. d. The tracts are here assumed to be contiguous with segmental ganglia of the type resolved in the artiopodan shown in Figure 4. Abbreviations: ce1-Proso, prosocerebral domain ce1; ce1-eye, one of the paired prosocerebral ocelli; ce2-Proto, protocerebral domain; ce2 op. lo., protocerebral optic lobe; Op. N, optic nerve; ce2-rec, recurrent protocerebral lobes; ce3-Deu, antennular lobe; Ant. N., antennular nerve (on left in images p, panel a); e. f, esophageal foramen; Cir, initial trajectories of the circumesophageal nerve leading to segmental ganglia.

Specimen YKLP 11356 provides reconstruction only of the cerebrum and part of its caudal extension towards the ventral nerve cord, which splits into two halves that rejoin after extending around the gut. The reconstruction of a complete central nervous system shown in Figure 3d includes ganglia from another artiopod specimen (YKLP 11141) shown in Figure 4. These are here deployed in the reconstruction of the artiopodan central nervous system (Fig. 3d). This specimen provides well-defined isomorphic segmental ganglia. YKLP 11141 also shows a partially preserved head shield bordered by a laterally positioned eye comprising what may suggest aligned ommatidia as suggested for the xandarellid Cindarella¹⁸.

Figure 4. Artiopodan segmental ganglia of YKLP 11141.

a. Ventral view of X. showing discrete segmental ganglia overlying the gut. The head shield is partially preserved. A lateral eye (inset a’) flanks the head shield. The gut is indicated by segmentally arranged gut diverticuli and glands. Inset a’. Enlargement of ultraviolet (UV)-illuminated eye with serial units (bracketed) of what may be lenses. Inset b’. UV illumination reveals segmentally paired digestive glands (arrows) and caeca beneath a ladder-like arrangement of ganglia and their connectives. b. White light illumination suggests gut sediments have fallen out (arrowed) exposing the gut’s approximate width. c. Combined white light and UV resolve the gut and the ventral chain of ganglia. d. Tracing of ganglia from boxed area in c. Their diagrammatic representation has been ‘married’ to the cerebral nervous system of Xandarella shown in Fig. 3c. Scale: a = 5mm, a’=1mm; b = 1mm, b’ = 2mm.

Cerebral organization of Xandarella implies behavioral constraints.

Recognizing in fossils the morphogenetic divisions that define the cerebral organization of extant euarthropods relies on sensory characters that denote each of the cerebrum’s asegmental divisions¹¹. The most anterior of these is domain ce1, also called the prosocerebrum, identified by two sensory attributes, the labrum and rostral eyes, which include the nauplius eyes of crustaceans²⁰ and the ocelli, their homologues in hexapods. The second domain, ce2 (protocerebrum), is denoted by the lateral compound eyes of Mandibulata and Xiphosura. Domain ce3 (deutocerebrum) is indicated by its post-ocular appendages: paired antennules of Mandibulata and chelicerae of Chelicerata.

Developmental genetic studies of the insect cerebrum²¹ and that of amphipod crustaceans²² demonstrate that specific arrangements of computational neuropils further characterize each domain and that the volume and elaboration of such circuits reflect their prominence in mediating behaviors^23,24. The prosocerebrum (ce1) incorporates the central complex¹¹, whereas the optic neuropils serving the compound eyes and the mushroom bodies are constrained to the protocerebrum (ce2), the volume of which also reflects the importance of its neuropils in learning and memory and multisensory integration^11,25. Hypertrophy of the antennular lobes, which define the deutocerebrum (ce3), likewise reflects their importance in chemosensory-driven behaviors^26,27.

The arrangement of specific sensory surfaces – ocelli, lateral eyes, antennules – associated with each of the paired lobes of the xandarellid cerebrum confirms its ground pattern organization as three domains typifying the asegmental euarthropod brain (Fig. 5a, b). However, despite their invariable order, the ce1-ce3 domains in Xandarella (Fig. 5e) show significant differences from their representation and homologous domain architecture in Mandibulata (Fig. 5c, d). Although the prosocerebral neuropils of X. spectaculum serve a rostral visual system indicated by the paired ovoid reliefs either side of the midline (Supplementary Fig. S3) these are unlikely to have required such voluminous neuropils. The prosocerebrum’s two rostral lobes are linked heterolaterally by a bridge of connections suggesting the most rostral neuropil – the prosocerebral bridge – of the arthropod central complex¹¹ (Figs. 2b, 3b). The lateral lobes of the prosocerebrum reveal trace neuropils that coincide in shape and location with the ovoid ocelli (Fig. 2b, d; panels p in Fig. 3a). Compared with the prominent fossilized protocerebral (ce2) traces in mandibulates such as Fuxianhuia²⁸ and Jianfengia⁸ that serve their compound eyes, the ce2 domain of X. spectaculum appears greatly reduced: just a single pair of terminal neuropils supplied from out-of-proportion large lateral eyes abut a slender midline neuropil (Fig. 2b, d); there is no indication of seriate optic neuropils as in mandibulates supplying a robust mid-brain^8,11. In Xandarella, which might be expected to have a substantial optic nerve linking the large lateral eyes to the protocerebrum, a barely resolvable fragment suggests unremarkable slender nerves most of which are hidden in their cryptic eyestalks embedded in the head shield (Fig. 2e; Suppl. Fig. S2). In contrast to the protocerebral lobes, the counterpart of YKLP 11356 (cp in Fig. 3a) reveals that its caudal cerebral domain (ce3), defined by the paired antennular lobes provides by far the largest sensory neuropil (Fig. 3a); its relative dimensions exceed the antennular lobes of extant olfaction-dependent insects¹¹ (Fig. 5d). Compared with those of mandibulates and chelicerates (Fig. 5c-f), the artiopodan cerebral ground pattern thus reveals a clade-specific elaboration of homologous domains, demonstrating that euarthropod brain evolution was tightly constrained within a conserved domain architecture that was already established in the early Cambrian.

Figure 5. Alignment of extant mandibulate, artiopod and Cambrian ‘great appendage’ euarthropod brains.

(a) In living euarthropods, the combinatorial expression of homologous genes (for source references see Strausfeld et al.⁴¹) defines three cerebral domains (b): these are the prosocerebrum (ce1); the protocerebrum (ce2); and the deutocerebrum (ce3). Specified by the gene collier,⁴² T1 is the first true segment of the body. Notably, the cerebral traits defining each lineage are independent of Hox-code determination of trunk segmental tagma such as T2-T4, which contribute gnathal appendages in extant mandibulates. (c-f) Schematic showing the morphogenetic divisions of the cerebra of two extant mandibulates (c, d) aligned with the corresponding neuroanatomical domains of the artiopodan Xandarella (e) and the Cambrian leanchoiliid chelicerate (f). As in Cambrian chelicerates the linear brains of extant mandibulates are characterized by their sensory systems and computational centers (summarized in g). The artiopodan cerebrum is unusual in that its protocerebrum is exceptionally small compared to its large prosocerebrum, suggesting that in artiopodans navigational computation was the primary function of the combined visual systems. The paired antennules and large folded deutocerebra of Artiopoda suggest a major reliance on chemosensory information.

The expanded dimensions of the xandarellid prosocerebral domain ce1 are telling. In modern mandibulates the ce1 neuropils are dominated by the prosocerebral bridge and underlying central complex^11,29 the elaboration of which across different species reflects their role in multimodal integration in foraging and navigation³⁰. Today’s isopods, which are benthic foragers, possess elaborate central complexes where an archlike bridge maps a series of axons bundles into the underlying central body. Here in Xandarella the neural traces are so well preserved as to indicate a similar arrangement of an arch attached to the next level by slender connections (Fig. 3a, c). As remarked above, Xandarella’s prominent ce1 domain contrasts with its less endowed protocerebrum, which in extant pancrustaceans would be voluminous not only because of its sequential visual neuropils but also because of other sensory circuits and the paired mushroom bodies¹¹ responsible for computing place memory, amongst other associations^32,33. Here the ce2 protocerebrum of Xandarella suggests a simple optic lobe serving each lateral eye with very little room for any other attributes other than a shallow Y-shaped extension of a pair recurrent lobe overlapping ce1 (Fig. 2a, c); these suggest paired but exiguous mushroom bodies comprising a few dozen parallel fibers as seen in Collembola³⁴.

It is the large, seemingly folded, olfactory/chemosensory lobes of Xandarella that denote an expansive deutocerebrum likely devoted to odorant discrimination and thus playing a major role in behavior. The elongated antennules of Xandarella would have been active in assessing the benthic substrate for chemical and tactile cues. However, foraging can also require information about space. In extant pancrustaceans paired ocelli and lateral eyes together can mediate navigation. If the xandarellid ocellar cuticle was transparent in life, as it is in extant insects ³⁵, then even at low light intensities changes in the direction or polarization of illumination could function as compass-like references informing the ce1’s central complex about rotational change of direction³⁶ integrated with information from the eyes about visual flow³⁷. Totally darkened ocelli would signal inversion, triggering righting movements. These parameters would be integrated with mechano- and chemosensory information obtained by the antennules, the lobes of which in extant mandibulates connect to more rostral neuropils of the protocerebrum¹¹. Integrated multimodal information would mediate path integration and thus a return to a starting location³⁸ within a circumscribed chemical ecology suited to that species. Evidence from soft tissue remnants in various artiopodans³⁹ showing gut contents is suggestive of benthic scavengers. Yet the overall shapes of artiopodans and their simple sensory–motor organization were poorly equipped to survive local disruption; Xandarella went extinct before the Ordovician. The exception was Trilobita¹², whose calcified exoskeleton likely delayed their final demise until the extreme geo-ecological disruptions at the end of the Permian⁴⁰.

METHODS

Material Provenance.

Descriptions herein refer to specimens retrieved from the Cambrian (Series 2, Stage 3) Eoredlichia-Wutingaspis trilobite biozone, Yu’anshan Member, Chiungchussu Formation, Haikou. The specimens used here are curated at the Yunnan Key Laboratory for Palaeobiology (YKLP), Institute of Paleontology, Yunnan University, Yunnan, Kunming, China.

Photomicroscopy.

For light microscopy of fossil material, digital images were taken using a Nikon D3X attached to a Leica M205C photomicroscope (Leica Microsystems; Wetzlar, Germany). Images were transferred to Adobe Photoshop CS5 (Adobe Systems; San Jose, CA) and processed using the Photoshop camera raw filter plug-in to adjust sharpness, luminance, texture, and clarity. Unless otherwise stated, colors were untouched and are those typical of Chengjiang fossils. For reconstructing neural traces serial images focusing on micron-intervals capture all the relevant profiles in both part and counterpart of specimen YKLP 11356 as shown in Fig. 3a and in supplemental Figure S2. These were used to reconstruct the bilaterally symmetric cerebrum, detailed in Figure 3b. For ultraviolet illumination (Figure 4a’,b’,c), fossils were photographed using a Leica MZ10 F stereomicroscope with appropriate filter blocks to evoke intense green fluorescence. Flexible fiberglass light guides were used to combine UV fluorescence and white-light illumination.

Isolating neural traces in specimen YKLP 11367.

Cerebral traces initially resolved as dark profiles against an intensely dense brown background were digitally clarified by converting color images to monochrome, using the Adobe Photoshop Black & White function for luminosity adjustments of each color of the CMYK palette.

Interpretive drawings and tracings.

Drawings were made using Adobe Illustrator for which layered tracings and reconstruction were generated using photographic images.

Data Availability.

Specimens used or referred to here were curated by X. H. at the Yunnan Key Laboratory for Palaeobiology (YKLP), Institute of Paleontology, Yunnan University, Yunnan, Kunming, China. Data reported in this study are included in the article and the supplementary material. Any additional information regarding the data reported in this article is available from N.J.S. upon reasonable request.