Amplectobeluid Radiodont Guanshancaris gen. nov. from the Lower Cambrian (Stage 4) Guanshan Lagerstätte of South China: Biostratigraphic and Paleobiogeographic Implications

Simple Summary “Anomalocaris” kunmingensis is the most common radiodont in the Guanshan biota, which has been recently reassigned to the family Amplectobeluidae. However, its generic assignment is still uncertain. Our new specimens reveal more characteristic of “Anomalocaris” kunmingensis, warranting the erection of a new genus, Guanshancaris gen. nov. Brachiopod shells bearing embayment injury, and broken trilobites closely associated with the frontal appendages, may indicate durophagous habits of Guanshancaris. The spatial and temporal distribution of Amplectobeluidae, documented in various soft-bodied biota, indicates that this group was restricted to low latitude regions from Cambrian Stage 3 to Drumian, and implies an ecological preference for a shallow-water environment. Abstract Radiodonta, an extinct stem-euarthropod group, has been considered as the largest predator of Cambrian marine ecosystems. As one of the radiodont-bearing Konservat-Lagerstätten, the Guanshan biota (South China, Cambrian Stage 4) has yielded a diverse assemblage of soft-bodied and biomineralized taxa that are exclusive to this exceptional deposit. “Anomalocaris” kunmingensis, the most abundant radiodont in the Guanshan biota, was originally assigned to Anomalocaris within the Anomalocarididae. Despite this taxon being formally assigned to the family Amplectobeluidae more recently, its generic assignment remains uncertain. Here, we present new materials of “Anomalocaris” kunmingensis from the Guanshan biota, and reveal that the frontal appendages possess two enlarged endites; all endites bear one posterior auxiliary spine and up to four anterior auxiliary spines; three robust dorsal spines and one terminal spine protrude from the distal part. These new observations, allied with anatomical features illustrated by previous studies, allow us to assign this taxon to a new genus, Guanshancaris gen. nov. Brachiopod shell bearing embayed injury and incomplete trilobites, associated with frontal appendages in our specimens, to some extent confirm Guanshancaris as a possible durophagous predator. The distribution of amplectobeluids demonstrates that this group is restricted to Cambrian Stage 3 to Drumian, and occurs across South China and Laurentia within the tropics/subtropics belt. Moreover, the amount and abundance of amplectobeluids evidently decreases after the Early–Middle Cambrian boundary, which indicates its possible preference for shallow water, referring to its paleoenvironmental distribution and may be influenced by geochemical, tectonic, and climatic variation.


Introduction
Radiodonts, a large soft-bodied stem group of euarthropods [1,2] with a cosmopolitan distribution from Cambrian to Ordovician, have been viewed as giant apex predators [3,4]   All specimens were gathered from yellowish-to-greenish mudstones interbedded by thin-bedded siltstone and sandstone [33,43,46]. Some were further prepared with fine needles under high magnification using stereomicroscopes. Fossils were photographed with a Canon EOS 5D Mark II digital camera with a Canon EF-S 60 mm macro lens (general view) and an EF-S 80 mm micro lens (detailed view), and was controlled using the EOS Utility 3.2 program for remote shooting. Images were processed in Digital Photo Professional 4. Camera lucida drawing were made using a Zeiss Discovery V12 microscope and prepared with CorelDRAW Graphics Suite 2022.
The descriptive terminology of the frontal appendage mainly follows that of Lerosey-Aubril and Pates [23] and Guo et al. [47]. Radiodont frontal appendages can be separated into two major regions for amplectobeluids, anomalocaridids and tamisiocaridids, namely, peduncle and distal articulated region. The term 'peduncle' [48] (also called a 'base' sensu [49]) is used to describe proximal podomeres of frontal appendages, which have more weakly defined articulations and bear no endite or reduced endite; the 'distal articulated region' (defined by Cong et al. [25]; equivalent to 'claw' sensu [49]) refers to the distal part of appendages, where endites project from the ventral side of podomeres and may bear sophisticated auxiliary spines. An angle on dorsal margin can normally be identified as the boundary between the peduncle and the distal articulated region. The term 'endite' is employed to describe the ventral spinose or setose projection on podomeres, and the term 'auxiliary spines' refers to spines projecting from the endite.
Diagnosis. Amplectobeluid with frontal appendages consisting of 15 podomeres, including 2 peduncular podomeres and 13 distal articulated podomeres; podomeres tall rectangle in shape; endites on the proximal-most podomere in the distal articulated region and distal-most peduncular podomere enlarged, with the latter being slightly smaller than the former; proximal-most endite in the distal articulated region possesses four anterior auxiliary spines, increasing in length towards the tip of the endite; proximal endites distal to the proximal-most one bear up to three anterior auxiliary spines; distal endites bear paired auxiliary spines; all endites bear one posterior auxiliary spine projecting from the middle position; three thickened dorsal spines protrude from the distal-most three podomeres and one terminal spine on the distal-most podomere.
Remarks. Guanshancaris kunmingensis was originally assigned to Anomalocaris based on the number of podomeres and alternation in the length of the endites on odd/even numbered podomeres (Figure 1 in [7]), which are shared with typical Anomalocaris species (Figures 1-7 in [52]). This taxon was later generally retrieved as a basal member of Amplectobeluidae in several phylogenetic studies (e.g., [16,17,23,44,53]), and thus was most recently formally assigned to the Amplectobeluidae [10]. However, this Guanshan taxon was still simply treated as either Anomalocaris kunmingensis or "Anomalocaris" kunmingensis, lacking a formal generic name [10]. In this contribution, we established a new genus, Guanshancaris gen. nov., for this taxon within the family Amplectobeluidae.
Guanshancaris can be confidently identified as Amplectobeluidae, as presently defined, by the diagnostic features of the frontal appendage, including an elongated rectangle shape of the podomeres, the largest enlarged proximal-most endite in the distal articulated region, the endite on podomere five is larger than podomere three (counting from posterior to anterior end) in the distal articulated region, as well as the thickened dorsal spines and terminal spine in the distal region. A recently reported amplectobeluid frontal appendage from the Parker Formation resembles Guanshancaris in many regards, such as 13 podomeres in the distal articulated region; two enlarged endites, respectively, on the distal-most peduncular podomere and proximal-most podomere in the distal articulated region; and three anterior auxiliary spines protruding from the proximal-most podomere at an oblique angle in the distal articulated region [54] (p. 778). Hence, here we consider that the appendage from the Parker Formation may represent the species of Guanshancairs outside the Guanshan biota.
In the context of Amplectobeluidae, Guanshancaris uniquely possesses relatively large peduncular endites, as well as larger and stouter endites distal to the enlarged proximalmost one in the distal articulated region ( Figure 2; Table 1). The condition that the endites of Guanshancaris bears auxiliary spines differentiates it from Amplectobelua, in which the spiniform endites are generally devoid of auxiliary spines ( Figure 15 in [55] and Figure 4 in [24]). Guanshancaris also differs from Amplectobelua by the length of the proximal-most enlarged endite in the distal articulated region less than one-third of the length of the appendage, whereas the exceptionally long endite is one-third to nearly half as long as the length of the appendage in Amplectobelua [24,55]. The enlarged endite in Guanshancaris is not inclined to the distal end as much as Amplectobelua symbrachiata from the Chengjiang biota ( Figure 4 in [24]) and Amplectobelua stephenensis from the Burgess Shale (    Description. The frontal appendage of Guanshancaris kunmingensis has an elongated shape tapering towards the distal end. Lengths of the complete appendages range from 25.1 mm to 81.9 mm (measuring along dorsal margin). The most complete appendage specimen consists of 15 podomeres, containing 2 podomeres in the peduncle (p), which attach to 13 podomeres in the distal articulated region (po) at an angle of approximately 160 • along the dorsal margin ( Figures 3B,D and 4H,J). The configuration of the proximal podomeres is elongated rectangular, and has a gradual transition to trapezium towards the distal end, with some tapering in width ventrally ( Figures 3A-D and 4A,B). The height/length ratio of the podomeres is approximately ∼2.0. Endites join to the ventral surface at an oblique angle of equal or less than 90 • , of which the length alternates on the distal-most peduncular podomere (p1) and podomeres in the distal articulated region (po1-12) (longer ones on odd podomeres). Although most of the endites are not completely preserved, the length of the endites is approximately equal or exceeds the height of the attached podomeres in light of the width of the preserved base of the endites. The length of the endites on p1 and po1-12 diminishes towards the distal end, with the exception of the endite on po5, which is longer than the endite on po3 (Figures 3A-D and 5A,B). Two specimens of our material are smaller than other specimens, and the articulations between podomeres are not observable, which represent possible juvenile individuals ( Figure 5).  The proximal-most peduncular podomere is weakly sclerotized (p2 in Figures 3A-D and 4A,B), which may bear an endite inclining towards the distal end (pe2 in Figure  4H,J,K). The endites of p1 and po1 are larger and stouter than other endites, and the latter is thicker ( Figures 3B,D and 4A,B,H,J,K). The endite on p1 is not preserved completely in our specimens, which can be seen in one posterior auxiliary spine in WLQ GLF 07 ( Figure  4K) and one possible anterior auxiliary spine in the holotype ( Figure 3B  The proximal-most peduncular podomere is weakly sclerotized (p2 in Figure 3A-D and Figure 4A,B), which may bear an endite inclining towards the distal end (pe2 in Figure 4H,J,K). The endites of p1 and po1 are larger and stouter than other endites, and the latter is thicker ( Figures 3B,D and 4A,B,H,J,K). The endite on p1 is not preserved completely in our specimens, which can be seen in one posterior auxiliary spine in WLQ GLF 07 ( Figure 4K) and one possible anterior auxiliary spine in the holotype ( Figure 3B,D). The endite on po1 is flanked by one large posterior auxiliary spine from nearly midpoint of the endite, and up to four anterior auxiliary spines, which increase in size towards the end of the endite ( Figures 3G, 4K and 5A,B). Endites on proximal podomeres distal to po1 bear one posterior, and up to three anterior, auxiliary spines (Figures 3A-D and 5A-D). One pair of auxiliary spines projects from the endites on distal podomeres ( Figure 5A-D). All the auxiliary spines form an acute angle with the endites (Figures 3 and 4). Three  An oval and an elongated carapace are associated with the frontal appendage in G WLQ 04 ( Figure 5A,B), which may represent the central head shield and lateral P-elem the fracture embayment in this shell is 0.96 mm in width ( Figure 6D,E). Two sets of adjoining drape-like arcs can be seen in the embayment, which is wrapped around by a Ushaped relief (white arrows in Figure 6D,E). Although the co-occurrence of frontal appendages with trilobites and brachiopod shells does not indicate their predator-prey relationship directly, the existence of the brachiopod with the embayment injury, associated with the frontal appendage of Guanshancaris, increases the possibility of their predatory relationship.  Remarks. Guanshancaris kunmingensis, which was originally reported by Wang et al. [7], is the most common radiodont taxa in the Guanshan biota. In their study, p1 has a tiny endite and one or two pairs of auxiliary spines present on the endites of po4-14 in the frontal appendage of G. kunmingensis [7]. In our specimens, p1 has an enlarged endite, and An oval and an elongated carapace are associated with the frontal appendage in GLF WLQ 04 ( Figure 5A,B), which may represent the central head shield and lateral P-element of Guanshancaris kunmingensis; the oval head shield measures 12.3 mm from the overlapping edge to the incomplete edge at its furthest point, and 9.6 mm at its widest point; the lateral P-element is 13.4 mm in length and 4 mm in width separately. The P-element is composed of two parts, which overlap each other. The oval head shield is not entire, and is partly overlapped by the P-element on one side. Both of these carapaces do not show a reticulate pattern. Noticeably, the shells of brachiopods, the cephalic and thoracic regions of trilobites, as well as unidentified fragments, are located immediately adjacent to the frontal appendages in most of our specimens ( Figures 3A-D, 4A,B,G-J and 6A,B). In GLF WLQ 09, one brachiopod shell has a concave embayed fracture in its top-left section, which clamps between the two dorsal spines of a frontal appendage ( Figure 6A-C). The estimated maximum shell length, as well as width, is 6.0 mm and 4.2 mm, respectively; the fracture embayment in this shell is 0.96 mm in width ( Figure 6D,E). Two sets of adjoining drape-like arcs can be seen in the embayment, which is wrapped around by a U-shaped relief (white arrows in Figure 6D,E). Although the co-occurrence of frontal appendages with trilobites and brachiopod shells does not indicate their predator-prey relationship directly, the existence of the brachiopod with the embayment injury, associated with the frontal appendage of Guanshancaris, increases the possibility of their predatory relationship.
Remarks. Guanshancaris kunmingensis, which was originally reported by Wang et al. [7], is the most common radiodont taxa in the Guanshan biota. In their study, p1 has a tiny endite and one or two pairs of auxiliary spines present on the endites of po4-14 in the frontal appendage of G. kunmingensis [7]. In our specimens, p1 has an enlarged endite, and the endites on podomeres in the distal articulated region are flanked by one posterior auxiliary spine, and up to four anterior auxiliary spines ( Figures 3A-D and 5A-D). Furthermore, the enlarged endite on the distal-most peduncular podomere can also be seen in Wang et al. [7], which does not receive their attention, but has been mentioned in Jiao et al. [10] (p. 259). An oral cone with tetraradial configuration in Guanshan is described by Zeng et al. [42] from the Gaoloufang section. Due to the great abundance of frontal appendages of G. kunmingensis in this section, they deduce that this tetraradial oral cone may be G. kunmingensis. Recently, a pair of frontal appendages of G. kunmingensis, associated with a tetraradial oral cone bearing small and large plates, was reported from the Lihuazhuang section of the Guanshan biota (Figures 2-4 in [10]), which confirms the presence of a tetraradial oral cone in G. kunmingensis.
Laminacaris chimera, a species with combination of characters shared by hurdiids and other radiodont families, was reported in the Chengjiang biota by Guo et al. [47]. Frontal appendages of L. chimera have 15 podomeres and 2 large endites on p1 and po1, which show similarity with G. kunmingensis (Figures 2A,B and 4C-F; Table 1). However, the podomeres have a higher height/length ratio (approximatively~2.0) in G. kunmingensis than L. chimera. In L. chimera, each podomere bears a single endite [47], whereas the endites are paired on the podomeres of G. kunmingensis. The length of the endites on podomeres 4-14 is shorter than the height of the attached podomeres (Figures 1 and 2A-D in [47]) in L. chimera, whereas in G. kunmingensis, the endites are approximately equal or longer than the podomeres to which they attach ( Figures 3A-D and 5A-D). The shape of the endites on p1 and po2 in L. chimera is straight blade-like, and resembles the morphology of the endites of hurdiids ( Figure 4F), which is not observable in G. kunmingensis. The endite on po2 bears small auxiliary spines between the larger auxiliary spines, and all the auxiliary spines arranged perpendicular to the endites in L. chimera ( Figures 1H and 2E in [47]). In G. kunmingensis, the morphology of the endites on po2 distinctively differs from L. chimera by one large posterior auxiliary spine, and up to four anterior auxiliary spines protruding from the endite with a certain obliquity, which gradually become larger towards the end of the endite (Figures 3G, 4K and 5A,B).

Guanshancaris as a Possible Durophagous Predator
As a member of Amplectobeluidae, Guanshancaris tends to use a variety of feeding structures, such as frontal appendages and oral cone, to consume biomineralised prey, which is represented by trilobites and brachiopods [57,58]. That is, morphofunctional information in Guanshancaris may identify its durophagous eating strategy. The endites apparatus of the frontal appendages in Guanshancaris is distinct from other amplectobeluids: two large proximal endites of Guanshancaris are not as stout and large as Amplectobelua and Lyrarapax, however, they may make it more flexible for Guanshancaris to grasp prey; the endites in the distal articulated region are longer and stouter than other amplectobeluids, which is beneficial for smashing shells powerfully. These functional morphologies may complement the inability of the oral cone to produce certain injuries to trilobite exoskeletons or brachiopods shell [1,3,13]. It is noteworthy that similar frontal appendage structure in mature large individuals with complete auxiliary spines also exists in some small ones ( Figure 5A-D), possibly reflecting raptorial feeding habits in juveniles. Specimens of the Burgess Shale arthropod Sidneyia inexpectans, with fragmented juvenile trilobites within the gut tract, show that some gnathobase-bearing arthropod taxa may consume exoskeletons [59]. Although it is not discovered in Guanshan yet, in Amplectobeluidae, the gnathobase-like structure used to manipulate and masticate food items is a diagnostic trait [22], so it may reveal the generality of durophagous feeding in this group. Moreover, the oral cone that potentially belongs to Guanshancaris possesses many scale-like nodes on the surface (Figure 2 in [20]), which implies that this feeding structure is likely to be more sclerotized than previously thought. Therefore, Guanshancaris, the only representative of amplectobeluids in the Guanshan biota, is most likely to be a durophagous animal.
The presumed durophagy of Guanshancaris is also, to some extent, supported by the evidence of the durophagous shell-breaking on a brachiopod shell ( Figure 6). Embayment is a kind of scar shape produced by Cambrian durophagous predators on brachiopods ( Figure 6C-E; [60,61]). Adjoining drape-like arcs (white arrows in Figure 6D,E) may show self-repairability of the brachiopod after the sublethal predation, which was previously reported in the Guanshan biota ( Figure 2 in [60]). In our specimen, the brachiopod shell bearing repaired damages, stuck in the middle of two dorsal spines of Guanshancairs, may imply a predator-prey relationship between them ( Figure 6). Among all known animals in the Guanshan biota, such injury on brachiopod shells is most likely caused by radiodonts in the act of predation [57,58,[60][61][62]. Compared to other Guanshan radiodonts, the morphology of Guanshancaris appendages, such as the enlarged proximal endites and relatively stout dorsal spines, is best suited to grasping and attacking hard prey. Therefore, although this co-occurrence of frontal appendage and injured brachiopod shell reflects the predatorprey relationship with little possibility between the two individuals, it is likely that this relationship does exist between the two species. With a great abundance in Guanshan, trilobites may serve as another food source for Guanshancaris. A number of trilobite-dominated aggregates in Emu Bay Shale [14], and elliptical aggregates containing exoskeletal remains of bivalved arthropods, hyoliths, and trilobites in Maotianshan Shale [58], are interpreted as coprolites, which may be excreted by radiodonts based on the size of the coprolites. Nevertheless, more direct evidence of the durophagy of Guanshancaris is awaiting the further discovery of gut content and coprolites of this animal.
In summary, the durophagy of specific radiodont taxa has been discussed for a long time [14,57,58,61,62]. Here, we add new evidence of Guanshancaris on the basis of two aspects: the functional morphology of the frontal appendages and a brachiopod shell with preingestive breakage associated with frontal appendage. The weight of the evidence reviewed above leads us to assume that Guanshancaris, the most common radiodont in Guanshan, may be a durophagous predator that is likely able to attack brachiopods and trilobites using its frontal appendages.

Paleoenvironmental Distribution
Amplectobeluids documented in various Cambrian Konservat-Lagerstätten provide further evidence to suggest that this group has an ecological preference for the shallowwater environment (Table 2). Ramskoeldia, Lyrarapax, and Amplectobelua symbrachiata are nearly restricted to the Chengjiang biota, which deposits a shallow-water setting of the epeiric platform, with fossils either autochthonous or parautochthonous [63,64]. Amplectobelua symbrachiata also occurred in the Niutitang Formation of Guizhou, South China, which deposited in offshore shelf-basin facies of the Yangtze Platform [65]. A possible Ramskoeldia consimilis found outside of South China occurred in the Latham Shale, which is generally considered as having been deposited in a proximal shelf above the fair-weather wave [22]. A recently reported amplectobeluid of indeterminable species from the Fandian biota in South China was deposited in a similar setting with the Chengjiang biota [66]. Guanshancaris is preserved in the Guanshan biota, deposited in an offshore transition between the fair-weather wave base and the storm wave base [33,46]. Amplectobelua is a widely distributed species of amplectobeluids, which occurs from the outer shelf adjacent to a carbonate ramp (Wheeler Formation, UT, USA, [67]; Burgess Shale, British Columbia, Canada, [3]; Kinzer Formation, PA, USA, [8]) to a lower shoreface setting (Chengjiang biota, Yunnan, South China, [64]; Niutitang Formation, Guizhou, South China, [65,68]). Although the Kinzer Formation, where an affinis species of Amplectobelua symbrachiata occurred [8], was deposited in a low-energy deep environment with intermittent, pulsed sedimentation, the sediment of this deposit may be transformed from further shallow inboard or elsewhere [30]. A few amplectobeluid taxa appear to be restricted to deep-water environments, as exemplified by Amplectobelua stephenensis from the Miaolingian Burgess Shale (Canada) and Wheeler Formation (USA) of Laurentia [3,69], and putative Guanshancaris from the Parker Quarry Lagerstätte (Franklin Basin succession of Parker Formation, northwest Vermont, USA, [56,70]). Considering both the diversity and abundance of amplectobeluids, however, it appears to be extremely rare in these three deposits ( Table 2). The relationship between the changes of amplectobeluid species abundance and the depositional environment of assorted deposits may imply that this group is more suitable for shallow-water environments. With adequate sunlight and oxygen, the shallow-water setting was a preferable place for prey such as trilobites to live, which may provide a sufficient food for amplectobeluids.

Spatio-Temporal Distribution of Amplectobeluidae
During the Early and the Middle Cambrian (Series 2 and Miaolingian), four genera of the family Amplectobeluidae have been reported from distinct paleocontinents. The biogeographical and temporal distribution of Amplectobeluidae is summarized in Figure 7.
Amplectobeluids appear to decline in both diversity and abundance in Stage 4, but have a wider distribution longitudinally, occurring across Laurentia and South China (Figure 7). Guanshancaris kunmingensis from the Guanshan biota (Palaeolenus and Megapalaeolen Zone; [7,10]) constitutes the only representative of the family Amplectobeluidae from South China during this period. The amplectobeluids in Laurentia are represented by Amplectobelua aff. symbrachiata from the Kinzers Formation (Bonnia-Olenellus Zone) of Pennsylvania [8], a potential Guanshancaris species from the Parker Quarry Lagerstätte (Bolbolenellus euryparia Zone or overlying Nephrolenellus multinodus Zone) of Vermont [54,69], and Ramskoeldia consimilis? from the Latham Shale (Olenellus Zone, Bristolia Subzone) of California, USA [22,72].
During the middle Cambrian (Miaolingian Series), records of amplectobeluid continued to be sparse and apparently limited to Laurentia, including Amplectobelua stephenensis from the Burgess Shale of Canada (Latest Glossopleura Zone to early-middle Bathyuriscus-Elrathina Zone; [12]), and Amplectobelua cf. A. stephenensis from the Wheeler Formation (Drumian Stage) of Utah, USA (Bolaspidella trilobite Zone, Ptychagnostus atavus agnostoid Zone; [69]), which represents the youngest fossil record of amplectobeluids. Ultimately, this group virtually disappeared from the upper Miaolingian rock record ( Figure 7A).
The apparent decline of amplectobeluids after Cambrian Stage 4 may be related to the tectonic, climate, and geochemical changes at the Early-Middle Cambrian boundary (EMC), which is also the reason which caused the mass extinction event of archaeocyathids and redlichiid/olenellid trilobites (ROECE) [73], such as volcanically [74][75][76] or eustatically [76][77][78] associated redox, carbon negative excursions [73], and the oligotrophic environment caused by aggravated N loss, as well as enhanced P input [78]. Hurdiids thrived from the Wuliuan onwards [79], which may indicate their interspecific competitive relationship with amplectobeluids of the same ecological niche.
According to the pattern of the paleogeographic distribution of amplectobeluids, this group occurred in South China and Laurentia from Cambrian Stage 3 to Drumian, restricted to the subtropical to tropical belt ( Figure 7). Thus, in contrast to hurdiids [79], amplectobeluids have the preference for warm water, as seen in anomalocaridids and tamisiocaridids [49,80], which may be controlled by changes in sea temperatures and climate zones.

Conclusions
New material of "Anomalocaris" kunmingensis uncovers new characteristics of this species: the most proximal enlarged endite in the distal articulated region and the distalmost secondary-enlarged peduncular endite; elongated endites in the distal articulated region; several distally-swelled anterior auxiliary spines and one posterior auxiliary spine protruding from endites with an acute angle. These characteristics prompt a reassignment of "Anomalocaris" kunmingensis to a new genus, Guanshancaris gen. nov., and support its amplectobeluids affinities. Moreover, the co-occurrence of the frontal appendages of Guanshancaris kunmingensis comb. nov. and fragments of trilobites and brachiopods with embayment injury may indicate the durophagous eating habit of Guanshancaris kunmingensis, and enhance the possibility of amplectobeluids as a durophagous predator, as well as adding to the understanding of its feeding ecology. In addition to the improved morphological information provided by the new specimens, the distribution date of Amplectobeluidae suggests that both the variety and quantity of this group reached its maximum in the Early Cambrian and apparently decreased in the Middle Cambrian, which may be caused by geochemical and eustatic changes added by volcanic activity. From the spatio-temporal and paleoenvironmental distribution of amplectobeluids, this group prefers shallow water in tropical/subtropical regions.