Morphological disparity and evolutionary patterns of Cambrian hyoliths

Hyolitha represent one of the major constituents of the Cambrian Evolutionary Fauna, first appearing in the Terreneuvian and rapidly diversifying soon after. Recent work has both enriched the hyolith fossil record and expanded our understanding of their biology, but studies documenting the evolutionary trajectory of Cambrian hyoliths remain scarce. Here we present the first study of changes in morphological disparity in Cambrian hyoliths over time with the aim of characterizing the evolutionary trajectory of hyoliths throughout their primary period of diversification. Our results show that hyoliths occupied distinct regions of morphospace at different times during the Cambrian, with an expansion in morphospace occupation associated with the increase in hyolith diversity in the early Cambrian. Both the Sinsk Event and multiple abiotic factors led to a decline in hyolith diversity in the Miaolingian, and morphological disparity also contracts in association with this reduction in diversity.

T H E Cambrian represents both the time of first appearance and a period of major divergence for most Metazoan phyla (Conway Morris 2003;Budd 2013;Smith & Harper 2013;Wood et al. 2019;Zhang & Shu 2021).One such phylum is the Hyolitha, which first emerges along with the earliest small shelly fossils (SSFs) near the base of the Cambrian and then rapidly diversifies and increases in abundance soon thereafter.The high abundance of hyoliths during the Cambrian makes them one of the more important components of the Cambrian evolutionary fauna (Sepkoski 1981), with hyoliths frequently recovered from early and middle Cambrian deposits.The majority of recent hyolith studies have focused on their occurrence (Pan et al. 2019;Peel et al. 2020;Liu et al. 2021aLiu et al. , 2021bLiu et al. , 2022b)), biological affinities (Moysiuk et al. 2017;Sun et al. 2018a;Li et al. 2019;Liu et al. 2020) and palaeoecology (Kimmig & Pratt 2018;Sun et al. 2018bSun et al. , 2019;;Liu et al. 2021a), which have all served to expand understanding of both hyolith biodiversity and distribution.Despite their known importance, studies of hyolith evolution are scarce, with little comparative work or broad-scale analysis of the evolutionary trajectory of hyoliths during the Cambrian.Previous studies have sometimes implied that distinct hyolith assemblages are present during different intervals of the Cambrian (Qian 1999) but hyoliths, for the most part, have generally been treated as a fixed element of the Cambrian Evolutionary Fauna (Sepkoski 1981;Zhuravlev & Wood 1996;Li et al. 2007;Maloof et al. 2010).Detailed discussion of hyolith evolutionary trajectories or the factors that may have driven any changes in hyolith morphology or diversity through the Cambrian are largely absent.To address this issue, we examine the global-scale fossil record of Cambrian hyoliths to document hyolith morphospace occupation, with the aim of quantifying changes in hyolith morphology over time.Differences in morphological disparity over time are then considered in the light of changes in taxonomic diversity to best understand the evolutionary pattern for hyoliths throughout the Cambrian.

MATERIAL AND METHOD
Published data from the Paleobiology Database (https:// paleobiodb.org) and recent sources (Appendix S1) were collated to construct a database of Cambrian hyolith genera, including their respective stratigraphic ranges and palaeocontinental distributions.In total, 115 genera of Cambrian hyoliths are included in our dataset (Appendix S1), covering all valid genera known across the four epochs of the Cambrian and incorporating specimens preserved in both Konservat-Lagerst€ atten and carbonate residues.The geographic distribution of Cambrian hyolith genera encompasses 10 palaeocontinents: South China, North China, Australia, Antarctica, West Gondwana, Siberia, Laurentia, Baltica, Avalonia, and Mongolia.The assignment of taxa to palaeocontinents is based on existing Cambrian palaeocontinental reconstructions (Torsvik & Cocks 2013;Zhao et al. 2018).A spindle diagram of hyolith genus-level diversity and diversity curves for both Cambrian hyolith clades (Orthothecida and Hyolithida) were constructed using PAST 4.12b (Hammer et al. 2001).
For all 115 Cambrian hyolith genera, we compiled a morphological dataset with a total of 20 morphological characters, with all chosen characters considered pertinent to hyolith classification.Examples of traits include conch shape, skeleton conch and opercula structures considered important for hyolith taxonomy (clavicles, cardinal processes, ligula and helens), and types of ornamentation (Appendix S1).It should be noted that, for approximately 50% of the genera in our study, the characteristics of the operculum remain unknown, largely because the operculum is often not preserved (Marek & Yochelson 1976;Malinky & Berg-Madsen 1999).For these taxa, all characters associated with the operculum were listed as unknown (coded as a ? in our analyses).To assess the potential impact of this large number of unknown character states, we also performed all analyses on a dataset in which character traits associated with the operculum were excluded (Fig. S1) to assess for any differences.Character states for each taxon were coded based on descriptions and figures of hyoliths in the existing literature.To compare changes in morphospace for hyolith assemblages across the epochs of the Cambrian, non-metric multidimensional scaling (NMDS) in PAST 4.12b (Hammer et al. 2001) was used, based on the Euclidean distance metric (chosen for its low-stress value; Takane et al. 1977) and with results for each Cambrian epoch visualized as convex hulls.Taxa were assigned to epochs based on the interval in the Cambrian in which the taxon first appeared.Given that only a single taxon (as newcomer) was assigned to the Furongian based on this method, this interval was omitted from the NMDS analysis.Covariation and changes in the morphology of hyoliths (including conch, opercula and key structures) through time were tested using non-parametric multivariate analysis of variance (PERMANOVA), using 9999 permutations carried out across all axes of variation (Collyer et al. 2015) (see Table S5 for the full list).
Analyses of disparity were carried out using the R package dispRity (v1.7.0;Guillerme 2018).Both sum of ranges (SOR) and sum of variances (SOV) were used to summarize different aspects of disparity, calculated using Euclidean distance on the two NMDS axes using R v4. 2.3 (https://cran.r-project.org/bin/windows/base/old/4.2.3/R-4.2.3-win.exe).SOR captures the total amount of morphospace occupied and thus reflects patterns of overall expansion and contraction.SOV measures the dispersion of taxa around the centroid and reflects the density of taxa in the occupied region of morphospace (Ciampaglio et al. 2001). Both Hopkins (2022) and Esteve & Su arez (2023) have previously noted that values for both SOR and SOV are sensitive to small sample size.Because the number of Furongian hyolith taxa is much less than in the other epochs we sampled (see our later discussion of the 'Furongian Gap'), all disparity calculations are based upon resampling with replacement (1000 replicates).
Analysis of multivariate homogeneity of groups of variances (Anderson 2006) was performed in PAST 4.12b (Hammer et al. 2001), using the Tukey, Mann-Whitney and Kruskal-Wallis tests, to test for significant differences between the SOR, SOV and median of centroids values for each hyolith assemblage.
All illustrated fossil specimens are deposited in the Early Life Institute and Department of Geology (ELI; http://geology.nwu.edu.cn/).

Diversity and distribution of Cambrian hyoliths
Global diversity curves for hyoliths through the four epochs and 10 stages of the Cambrian are shown in Figure 1.Hyolitha genus richness at epoch level increases sharply from the Terreneuvian and reaches a peak in Series 2, followed by a steep decline in the Miaolingian and low richness in the Furongian (Fig. 1A; Table S1).However, the diversity curves through the Cambrian for the two hyolith clades (hyolithids and orthothecids) differ from each other and also from the total diversity curve for all hyoliths (Fig. 1B; Table S2).The diversity of orthothecids decreases in Stage 3, whereas hyolithids decrease in diversity during the Wuliuan (Fig. 1B). Figure 2A shows the distribution of Cambrian hyoliths across the 10 palaeogeographical regions.Many regions such as South China, Laurentia, Australia and west Gondwana provide a near complete record of fossiliferous formations throughout the Cambrian (Table S3).North China, Mongolia and Antarctica are poorly sampled with lower levels of diversity compared with better sampled regions, and the distribution of hyoliths is patchy in some intervals due to this (Fig. 2A).Looking at all of the palaeocontinents collectively (Fig. 2B-D), it is clear that hyoliths are much more common in Series 2, where their diversity is highest (Fig. 2A, C), whereas hyoliths taxa are depauperate during the Furongian (Fig. 2E).The pattern of diversity change across most regions is similar, with a steep decline in the total number of taxa after Series 2, except for Baltica and west Gondwana, where hyolith assemblages did not decline until after the Miaolingian (Fig. 2A, C).There are very few genera reported from the Furongian (Fig. 2A, E; Table S4), and only a single new genus, Kygmaeoceras (Yochelson et al. 1969), first emerges in the Furongian.Furongian hyolith genera are sporadically preserved in Laurentia, Baltica and west Gondwana (Fig. 2E).

The emergence of hyoliths in the Terreneuvian
The hyoliths first appeared in the Fortunian (Fig. 3), and these early taxa have been the focus of a great deal of research (Qian 1978;Maloof et al. 2010;Kouchinsky et al. 2012;Guo et al. 2014;Yang et al. 2014;Zhang et al. 2015Zhang et al. , 2016;;Yang & Steiner 2021).Faunal assemblages in the Fortunian have been referred to as belonging to 'tube world' because assemblages at this time are dominated by various tubular taxa (Budd & Jackson 2016).Hyoliths are one of the major components of the Terreneuvian tube world, with few morphological features.For example, Conotheca and Ladatheca were two of the most widely distributed and common hyolith genera in the early Cambrian.Both taxa consist of a long, straight conch with no dorsoventral differentiation (round or oval cross-section), a sharp apex and a low dispersive angle.Opercula (Fig. 4A-D) are usually oval shaped, lacking clavicles and cardinal processes, with a lid-shaped inner surface and flat outer surface.This lack of features is typical of Fortunian hyoliths, and the relatively 'simple' tubular morphology has resulted in some taxonomic confusion and difficulty in differentiating these early hyoliths from other Terreneuvian fossils.For instance, Anabarites and Cambrotubulus also have a tubular or conical shape (Kouchinsky et al. 2009) and are common constituents of latest Ediacaran and earliest Cambrian faunal assemblages (Qian 1999;Li et al. 2007).The identification of these particular taxa can be difficult due to how they are often preserved, predominantly as internal moulds lacking distinct morphological characters.Because early orthothecids also lack distinguishing features, they can be challenging to differentiate from these other tube-like fossils (Kouchinsky et al. 2009), especially in cases in which hyolith specimens are lacking the apical region, as for Conotheca (Steiner et al. 2004, figs 3-17).For this reason, it is likely that many problematic fossils from the Terreneuvian, such as Salanytheca, Tiksitheca and Kugdatheca, have previously been aligned with hyoliths, until later studies redesignated them as synonyms of Anabarites (Missarzhevsky 1982;Signor et al. 1987, fig. 5.2).This high level of morphological similarity between the earliest hyoliths and other tubular taxa has somewhat obscured our understanding of when hyoliths first emerged.
Based on reports of Terreneuvian hyoliths, we suggest that the first orthothecids appeared across several palaeocontinents at approximately the same time (Fig. 3), in the middle of the Fortunian (Maloof et al. 2010;Kouchinsky et al. 2012;Guo et al. 2014;Yang et al. 2014;Betts et al. 2018).On the Siberian Platform, the earliest forms of hyoliths are Ladatheca (Bokova 1985;Missarzhevsky 1989), Loculitheca, Spinulitheca (Bokova 1985) and Circotheca (Meshkova 1974), which occur from the Purella Biozone of the Nemakit-Daldyn Formation (Fortunian in age; Fig. 3).Turcutheca is the first orthothecid genus reported from the Mount Terrible Formation in Australia, a formation that has been proposed to represent the top of the Fortunian to the base of the Cambrian Stage 2 (Jago et al. 2002, fig. 3;Jacquet et al. 2017;Betts et al. 2018).Brasier & Singh (1987, fig. 6.6-6.15) reported hyoliths from the Chert-Phosphorite Member (base of the Lower Tal Formation) in India (West Gondwana palaeocontinent), corresponding to the Anabarites trisulcatus and Purella Assemblage zones of Siberia (Hughes et al. 2005).However, given that only fragments of tubes are figured (Brasier & Singh 1987, fig. 6.6-6.15), it is difficult to accurately assess these specimens and they require revision.Conotheca subcurvata was reported as the earliest hyolith from West Gondwana, recovered from the Heraultia Limestone (Stage 2) in France (Devaere et al. 2014).Fortunian hyoliths in Avalonia and Laurentia are problematic (in terms of taxonomic assignation), a consequence of poor preservation and the typically featureless internal moulds that comprise the specimens.Ladatheca from Newfoundland and Nevada has been the focus of numerous studies (Grabau 1900;Hollingsworth 1999) and was initially considered an orthothecid, however, the taxon has since been revised as problematic or possibly a polychaete (Landing et al. 1988(Landing et al. , 1989;;Landing 1993;Landing et al. 2023).South China yields the earliest definitive hyoliths in the Fortunian, first appearing in the Anabarites trisulcatus-Protohertzina anabarica Zone from the Zhongyicun Member of the Zhujiaqing Formation and the Yanjiahe Formation (Qian 1978(Qian , 1989;;Luo et al. 1982;Guo et al. 2014;Yang et al. 2014) (Fig. 3).Conotheca, Neogloborilus and Lophotheca are common hyolith taxa from Fortunian deposits of South China (Qian 1978) (Fig. 3).Early specimens of 'Circotheca' from the Meishucunian/Zhujiaqing Formation (Fortunian) are now considered to be misidentified specimens of Conotheca (Guo et al. 2014)  Most Terreneuvian hyolith records represent orthothecids, with only rare publications reporting definitive hyolithid specimens.The first appearance of hyolithids is still under debate, largely because the key characters that differentiate the group, such as helens, are rarely preserved in Terreneuvian deposits.Despite these uncertainties, it has generally been presumed that hyolithids appeared after the first orthothecids (Skovsted et al. 2020;Liu et al. 2022a).Orłowski & Waksmundzki (1986) reported that the earliest hyolithid with helens appeared in the Czarna Formation from Poland, which correlates to the Platysolenites Zone.However, Stachacz (2012) showed that the Czarna Formation was younger than previously thought and suggested that it was instead equivalent to Cambrian Stage 3. Kouchinsky et al. (2017) reported that the earliest appearance of hyolithids preserved with potential retractile helens was probably in the upper Tommotian (Stage 2) in southeastern Siberia (Fig. 3), with Aimitus and Burithes considered the earliest hyolithids (Sysoev 1966;Rozanov et al. 1969;Meshkova et al. 1976;Val'kov 1987).Aimitus however, lacks key hyolithid characters (helens, lateral sinus, rooflets on the operculum etc.) (Sysoev 1966) and the stratigraphic age of Burithes is probably younger than first thought.Because any revision of these taxa remains pending, both taxa have been included in our analysis here.The earliest hyolithids of South China are Adyshevitheca and Microcornus from the Xihaoping Member of the Dengying Formation (Qian & Zhang 1983).The Xihaoping Formation was initially regarded to correlate with the pre-trilobitic Meishucunian Stage (Shiyantou Formation, Stage 2) in eastern Yunnan (Duan 1983(Duan , 1987).However, it has since been revised as corresponding with the Aldanian-Lenan Stage rather than the Meishucunian Stage, due to the presence of key hyolith and brachiopod taxa (Qian & Zhang 1983).Recently, the Xihaoping Formation was considered to correlate with the early Chiungchussuan and the lower part of the Yu'anshan Member (Parabadiella trilobite Zone, Cambrian Age 3) (Zhang et al. 2021a).Although some stratigraphic uncertainties exist, it is likely that the first hyolithid probably appeared towards the top of the Cambrian Stage 2 or the base of Stage 3 (Fig. 3) and typically coincides with the first appearance of trilobites (Rozanov et al. 1969;Val'kov 1987).

The diversification of hyoliths in Series 2
Cambrian Series 2 is not only the time of peak diversity for hyoliths, but also the time when there are marked changes in conch and operculum morphology.Hyoliths developed a distinct ventral and dorsal conch, features exemplified in taxa such as Triplicatella (Skovsted et al. 2014;Liu et al. 2020) and Paratriplicatella (Pan et al. 2019).Some conchs also preserve crests and deep carinas that taper towards the dorsum, together with a short ligula (Liu et al. 2020).This morphology affects the outline of the operculum, resulting in a variety of shapes, including subtriangular, triangular and hemispherical cup-shaped opercula.Conchs of selected Series 2 hyoliths also start to become curved and some groups even have isotropically coiled conchs.For instance, the isotropically coiled Protowenella, initially regarded as a monoplacophorid, helcionelloid or gastropod, has recently been identified instead as a hyolith, due to the presence of a bilaterally symmetrical operculum (Peel 2021).Michniakia represents another taxon with an isostrophical conch (Kouchinsky et al. 2022), and some hyoliths such as Neogloborilus applanatus Qian & Zhang, 1983 (Fig. 5A-J) and Cupitheca mira He in Qian 1977 (Fig. 6A-G) have strong dorsally curved conchs rather than the typical straight conical conch.Additionally, many hyolith conchs preserved obvious lateral margins that converge ventrally, and form a deep groove on the apex (Fig. 6L, T), rather than the flat surface seen in older forms.Some hyolith steinkerns preserve a sharply pointed or tubular distal region towards a pointed conch apex (Fig. 6P, U), but for crack-out hyolith material the apical part consists of flattened triangular areas on the sides of a linear tube (Liu et al. 2020(Liu et al. , 2022b)).The opercula of Cambrian Series 2 hyoliths are increasingly complex compared with their earlier counterparts in the Terreneuvian.This includes a more distinct cardinal and conical shield, and the development of clavicles and cardinal processes (Figs 5O, P, 6I-K).The recently erected family Paramicrocornidae first appeared in Series 2, and has been suggested as being representative of an intermediate evolutionary stage between the Orthothecida and the Hyolithida (Liu et al. 2022a(Liu et al. , 2022b)).It contains the species Paramicrocornus ventricosus (Fig. 5K-W) and Paramicrocornus zhenbaensis (Fig. 6I-K, M-P), taxa that both possess many hyolithid features, such as a complex platyclaviculate operculum with a prominent cardinal process, but lack helens, a characteristic feature of hyolithids (Zhang et al. 2018;Pan et al. 2019;Skovsted et al. 2020;Liu et al. 2022aLiu et al. , 2022b)).Moreover, typical hyolithid possessing the helens in Series 2 are well preserved and frequently described with detailed systematic palaeontology, such as Microcornus parvulus, the 'Linevitus' malongensis from the Guanshan Biota (Liu et al. 2021a), and Haplophrentis reesei from South China (Sun et al. 2017).

Hyoliths during the Miaolingian and Furongian
Overall hyolith morphology remains generally stable throughout the Miaolingian, but with some distinct morphological differences when compared with taxa from Series 2. These differences include changes in the surface ornament of both the conch and opercula, the development of a furrow on the conch and an increased number of clavicles on the interior of the opercula (Fig. 7).Surface ornament on the conch does change over time, from smooth to transverse growth lines in the early Cambrian, to striated or longitudinal ornament in the mid-Cambrian (Contitheca chrosniaki; Marek et al. 1997), to reticulate surface ornament in younger specimens (Malinky et al. 2009, fig.4E-F).The ventral longitudinal furrows became deeper on some genera, for example Gracilitheca destombesi, Contitheca chrosniaki (Marek et al. 1997), Holmitheca, Foerstetheca dubecensis and Shandongolithes thakal (Kruse 2002).The operculum also changes, with an increase in the number of clavicles in younger strata (Marek 1967), and the ornamentation begins to display a radially ribbed surface in addition to the concentric ornament on the outer surface, such as in Slapylites (S. signatulus from the Czech Republic (Valent et al. 2017), S. after from Morocco (Marek et al. 1997, figs 3.3, 4, 7, 10, 16-20;7.6, 8)).

Morphospace occupation of Cambrian hyoliths
Results for NMDS based on the Euclidean distance matrix indicate that hyoliths in the Terreneuvian, Series 2 occupied different areas of morphospace compared with Miaolingian hyoliths (Fig. 8A).PERMANOVA indicates that there are significant differences in morphospace occupation for hyoliths between the three epochs (F = 4.492, p = 0.0001), however the three hyolith assemblages from the three different epochs do still overlap in morphospace (Fig. 8A).The pairwise comparison of Terreneuvian hyoliths with both Series 2 and Miaolingian hyoliths is significant (p < 0.05, see Table S5 for details of pairwise comparisons).Series 2 hyoliths occupy a larger area of morphospace than hyoliths from the Terreneuvian and Miaolingian.This indicates that the variance in morphospace increases from the Terreneuvian to Series 2, but is subsequently reduced during the Miaolingian.
An increase in disparity during Series 2 (Fig. 8A) coincides with the expansion of morphospace occupation (Fig. 8B).The SOV (Fig. 8C; Table S6) and the median distance from centroid (MOC) (Fig. 8D) change little throughout the Cambrian, but the SOR (Table S7) shows a distinct change (Fig. 8B).SOR thus increases from the Terreneuvian to Series 2 and then decreases in the Miaolingian.The increase in SOR from the Terreneuvian to Series 2 is smaller than the fall in SOR from Series 2 to the Miaolingian (Fig. 8B).The MOC (Table S8) indicates how tightly genera are clustered around the 'centroid' and measures the clustering of taxa around a 'central point' (Fig. 8D).The increased mean and median of Series 2 morphospace centroids from the Terreneuvian indicates that the genera morphologies of Series 2 are less clustered around the centroid.The small decline in MOC is thus likely to reflect that genera during the Miaolingian are slightly more clustered around the centroid.All three values SOR, SOV and MOC for both Terreneuvian and Miaolingian hyoliths are smaller than the SOR, SOV and MOC of Series 2 hyoliths (Fig. 8B-D).This indicates that hyoliths expand their morphological disparity from the Terreneuvian through to Series 2, but then gradually decrease from Series 2 to Miaolingian.
Results for NMDS, SOR, SOV and MOC analyses using the 'without operculum traits' dataset are similar to those obtained with the total dataset in which operculum characters are included (Fig. S1).
decline for the Hyolitha during the Cambrian (Figs 1, 2, 8).These results show that the Cambrian Series 2 was when the largest increase in both diversity (Fig. 1) and geographic range occurred for hyoliths (Fig. 2), while during the Miaolingian there is a marked decline in both diversity and geographic range.Based on the data and results presented here, we suggest that the evolutionary trajectory of hyoliths can be subdivided into three evolutionary phases: 1. Origination (Terreneuvian): first appearance of hyoliths, with initial assemblage dominated by orthothecids; 2. Radiation (Series 2): a phase of rapid radiation and ecological expansion (this is the peak of hyolith diversity with an abundance of both orthothecids and hyolithids and a diverse range of morphologies); and 3. A survival period (Miaolingian to Furongian): a phase of decline in which only a few rare hyolith genera still exist, coinciding with a decrease in morphological disparity (genera extant at this time almost all first appeared during Series 2 and are all hyolithids).
Regional diversity patterns (Fig. 2C) confirm that the major diversification of hyoliths during Series 2 coincides F I G .7 .Skeletal morphological change (operculum and conch) of hyoliths through the Cambrian.The opercula of young genera were relatively complex, including a differential cardinal shield and conical shield, with increased clavicles.The morphology of the conch shifted from conical to pyramidal with a distinct venter and dorsum, and developed a furrow with varying depth on the venter.The surface ornament of the conch evolved from weak to strong.
L I U E T A L .: THE EVOLUTION OF CAMBRIAN HYOLITHS 1 1 with a global rise in biodiversity, a period of time that is generally referred to as the Cambrian Radiation (Erwin 1991;Smith & Harper 2013;Wood et al. 2019;Zhang & Shu 2021).The decline in hyolith diversity in the Miaolingian (Fig. 1A) also mirrors global biodiversity at the time, with a large reduction in biodiversity in both the Botoman and Toyonian stages (c.513-508 Ma).This biodiversity drop has been linked to the Sinsk Event, an extinction event most probably caused by extensive shallow marine anoxia (Zhuravlev & Wood 1996, 2018, 2020).The Sinsk Event is characterized by the rapid decline of shallow-water benthic faunas, with perhaps the most notable change being the extinction of the archaeocyaths (Zhuravlev & Wood 1996).Although hyoliths have been reported from a wide variety of water depths (Conway Morris 1986;Kruse et al. 1995), they are typically thought to live in shallow, normal marine environments and are much more prevalent in fine-grained sediments (Berg-Madsen & Malinsky 1999).It is thus reasonable that this expansion of anoxic waters into continental margins and epicontinental seas also greatly affected hyolith diversity.However, in our study we find that orthothecids and hyolithids manifest different diversity patterns during this interval of global change (Fig. 1B).Orthothecids decrease in diversity during Stage 4 (Fig. 1B) but hyolithid diversity increases during Stage 4, before declining in the subsequent Miaolingian period.This differentiated response suggests that hyolithids were less affected by the Sinsk Event while orthothecids were.It is difficult, however, to explain the reason for this difference, largely because detailed understanding of hyolith ecology and physiology is currently lacking.It is possible that the difference in skeleton configurations and life mode between the two groups could explain the contrasting diversity profiles.Moysiuk et al. 2017;Skovsted et al. 2020;Liu et al. 2021a).Orthothecids, in contrast, with retractable planar opercula, complex digestive tracts, and strong tuft-like arrangement of the tentacles, are considered deposit feeders (Runnegar et al. 1975;Horn y 1998;Malinky 2003;Valent et al. 2011;Liu et al. 2020Liu et al. , 2021b)).It is possible that filter feeders were better able to take advantage of newly available ecological niches following the Sinsk Event.Echinoderms and lingulate brachiopods, both filter feeders, change little in diversity across the Sinsk Event (Zhuravlev & Wood 1996, fig. 3;Nardin & Lefebvre 2010;Zamora et al. 2017), which may suggest that filter feeders had a relative advantage at this time.This could possible explain why the diversity of hyolithids did not decline in the Sinsk Event while that of orthothecids did decline.
The possible relative advantage that hyolithids had over orthothecids in low-oxygen conditions after the Sinsk Event, may be directly related to specific aspects of their morphology, such as the presence of helens and long ligula, which meant that they were better equipped to either survive or exploit times of environmental stress.Orthothecid morphology does, however, change in multiple ways during the Cambrian, suggesting possible adaptation to changes in the environment at the sedimentwater interface.Changes in the cross-section of the orthothecid conch, which transforms from round to subtriangular (i.e.into a differentiated ventral and dorsal conch) then to a kidney shape with deeper ventral longitudinal furrows (Fig. 7) have been suggested to be an adaptation for dealing with a decrease in firm substrate (Malinky 2009), with changes in the substrate a result of the Cambrian substrate revolution (Bottjer et al. 2000).Orthothecid genera that first appear in the Miaolingian, such as Contitheca and Gracilitheca, have deeper furrows than their predecessors, which may have also been a response to an increase in the prevalence of softer sediments.The furrows would have raised the aperture of the conch above the sediment-water interface, the function of which may have been to avoid the fouling of the interior by increasingly mobile sediments.A shift in surface ornamentation from transverse lines to longitudinal ribs is proposed to not only strengthen the conch but also streamline the flow of water over and around the conch to increase stability on the sea floor (Malinky 2009).There is also an increase in the number of clavicles on the opercula, the significance of which remains an open question (Marek 1967), but which may have served to reinforce the hinge between the operculum and conch and increase stability in strong currents (Fig. 7).Given that we find little difference in our results when traits associated with the operculum are removed from our analyses (Fig. 8, Fig.S1) it does not seem that morphological changes to the opercula were particularly important in the evolutionary history of orthothecids or even hyoliths in general.This may be because the cross-section of the conch somewhat matches the outline and shape of the opercula (Malinky & Berg-Madsen 1999, text-fig. 3), meaning that changes in the operculum may be correlated with or reflect changes in conch morphology.However, opercula are relatively rare compared with conches in the fossil record, which limits the possibility to fully assess the relative import of opercula disparity over time or determine which structures or ornamentation may have been present on the inner surface of the operculum.What is clear is that evolutionary disparity and patterns of change for Cambrian hyoliths are mainly reflected in morphological characters of the conch.
Hyolithids did not continue to diversify following the Sinsk Event, instead they experienced a sudden decline in diversity during the Wuliuan.Abiotic factors may explain this decrease, including global sealevel rise (Haq & Schutter 2008) and higher sea surface temperatures.The Wuliuan represents a dynamic period in Earth history, as the world changed from cold to greenhouse conditions (Emanuel 2006;Zhuravlev & Wood 2008;Erwin 2009).Seawater chemistry was also changing, fluctuating between an aragonitic and calcitic sea (Stanley 2008;Zhuravlev & Wood 2008;Kouchinsky et al. 2012;Wood & Zhuravlev 2012;Li et al. 2021).The fragility and collapse of middle-late Cambrian ecosystems are likely to have been due to all of these environmental changes, which primarily occurred at the boundary between Stage 4 and the Wuliuan and affected a number of clades, including hyolithids and trilobites (Zhang et al. 2021b).
After a decline in diversity during Series 2 and the Miaolingian, surviving hyolith genera in the Furongian were predominantly endemic, restricted to the high-latitude Mediterranean Province (mainly from Laurentia, Baltica and west Gondwana) (Fig. 2E).It is difficult to be certain whether the low number of hyolith genera recorded from the Furongian is a true signal or whether it is reflective of preservation bias, therefore any results for this time should be viewed with scepticism.That said, nearly all metazoan groups during the Furongian decrease in diversity, an interval that has been referred to as the 'Furongian Gap', a period of low diversity sandwiched between the Cambrian Radiation and the Great Ordovician Biodiversification Event.In many regions, however, the Furongian is poorly represented and not intensively sampled, suggesting that the low diversity recorded during this interval may be the result of sampling biases (Harper et al. 2019).Recent work has suggested that there may actually be no significant biodiversity gap and that marked fluctuations in marine life during the Furongian are a reflection of the volatility of environmental conditions at the time (Deng et al. 2023).Regardless of whether low hyolith numbers during the Furongian are a sampling artefact or whether they represent a real signal of low diversity, hyoliths survived through the late Cambrian and then increased in diversity during the Ordovician.This Ordovician radiation may reflect a return to conditions suitable for their proliferation (Malinky et al. 2004;Valent 2010) (Fig. 1), but any recovery should be considered regional, given that it was confined to European peri-Gondwana and African Gondwana in the Ordovician (Malinky et al. 2004;Valent 2010).Despite this increase in diversity, hyoliths would never again be the dominant benthic group that diversified around the world in the early-mid Cambrian.

CONCLUSION
This study represents the first attempt to quantitatively analyse the genus-level diversity and morphological disparity of Cambrian hyoliths.Biodiversity curves and morphospace analysis of Hyolitha from the Terreneuvian to the Furongian suggest that the evolutionary trajectory of Cambrian hyoliths can be divided into three phases.The initial phase involves the emergence of orthotheciddominated hyolith assemblages in equatorial regions during the Terreneuvian.In the second phase, during Series 2, the hyoliths rapidly radiated, with the group reaching a peak in both morphological disparity and diversity during this time.Although both hyolithids and orthothecids were common in Series 2, orthothecids begin to decline in diversity during the Sinsk Event.Several more complex morphological features first appear during Series 2, including the development of a differentiated ventral and dorsal pyramidal conch with a furrow, long ligula and helens, and an increase in operculum complexity.Possibly because of multiple abiotic changes, hyolith diversity and morphological disparity decreases in the Miaolingian, with only a single new genus first appearing in the Furongian.This is designated herein as the survival stage.The survivors were endemic and heavily restricted to the Mediterranean Province, and their diversity remained low until their recovery during the Ordovician.

F
I G . 2 .Gamma diversity (c-diversity) of Cambrian hyoliths.A, bar chart showing the number of hyoliths genera per Cambrian epoch from the 10 defined biogeographical regions.B-E, palaeogeographical distribution of Cambrian hyoliths from different regions with their genus-level diversity during the four Series of the Cambrian.Base map from Torsvik & Cocks 2013; Zhao et al. 2018.Abbreviations: AN, Antarctica; AU, Australia; AV, Avalonia; BA, Baltica; LA, Laurentia; MO, Mongolia; NC, North China; SC, South China; SI, Siberia; WG, west Gondwana.

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I G . 8 .A, the morphospace of Cambrian hyoliths from different Cambrian epochs based on the Euclidean distance matrix, using the non-metric multidimensional scaling (NMDS) method.The sum of ranges (B), sum of variances (C) and median of centroids (D) show the changes in disparity of hyolith genera from the Terreneuvian to the Miaolingian (95% confidence intervals plotted).