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a Sketch map of the People's Republic of China, showing the position of collecting localities in Guizhou Province. b Simplified geographic sketch map of the two fossil localities. c Partial Cambrian
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The ontogenetic series of the eodiscoid trilobite Pagetia vinusta is first documented, from the crack-out specimens including numerous articulated individuals and many disarticulated sclerites from the Cambrian (Wuliuan) Kaili Formation, Guizhou, southwestern China. Better descriptions of morphological changes during growth are presented, especiall...
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... material was collected from two sections through the Cambrian (Wuliuan) Kaili Formation, in Danzhai and Jianhe counties, Guizhou, South China (Fig. 1). Of the numerous crack-out exuviae preserved in grey-green shale, 126 articulated specimens, including five protaspides, eight meraspides and 113 holaspides, are investigated in this ...
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Citations
... This represents 84% of known agnostid genera, with the remaining 16% excluded because all available images of these genera were of specimens that had undergone compression and/or deformation, or no published photos exist for that genus. The images were sourced from and screened against all formal publications related to agnostids, e.g., [1,2,[53][54][55]. Several illustrations of specimens from published papers were also included in our dataset. ...
... Hexagonal box plots and scatter density plots were created using Python v. 3.11 [63] and were used to show the density of morphospace occupation through time based on the PCA. lications related to agnostids, e.g., [1,2,[53][54][55]. Several illustrations of specimens from published papers were also included in our dataset. ...
The temporal range of eodiscids and agnostoid arthropods overlaps with several early Paleozoic geological events of evolutionary significance. However, the responses of agnostids to these events and how the perturbations associated with them (both abiotic and/or biotic) may have impacted agnostids remain uncertain. To address this uncertainty, we employ geometric morphometrics to reconstruct morphospace occupation for agnostids, thereby elucidating their evolutionary response to geological events during the early Paleozoic. The results indicate that maximum morphospace occupation was reached by Cambrian Series 2 and then declined soon thereafter. Subsequent reductions in agnostid morphospace occupation coincide not only with significant abiotic changes and associated extinction events, such as the Botoman–Toyonian Extinctions (BTEs), the Redlichiid–Olenellid Extinction Carbon Isotope Excursion (ROECE), the Drumian Carbon Isotope Excursion (DICE), and the Steptoean Positive Carbon Isotope Excursion event (SPICE), but also with major evolutionary episodes, such as the Great Ordovician Biodiversification Event (GOBE). These repeated and periodic declines in agnostid morphological diversity following Cambrian Series 2 suggest that the extinction of agnostids reflects the culmination of an episodic reduction in morphological occupancy for agnostids rather than a singular, sudden event. Accordingly, it cannot be tied to a single cause, either abiotic or biotic.
... Carefully investigated ontogenetic sequences, in particular those in which protaspides and meraspides are preserved, have great potential to play a pivotal role in understanding their development. Such studies remain limited, with ontogenetic series documented for only approximately 12 species, including Tsunyidiscidae [Tsunyidiscus acutus (Dai & Zhang, 2011), Tsunyidiscus longquanensis (Zhang & Clarkson, 1993, Tsunyidiscus yanjiazhaiensis (Dai, Zhang & Peng, 2016)], Hebediscidae [Hebediscina longispinus (Zhang & Clarkson, 2012)], Calodiscidae [Neocobboldia chinlinica (Zhang, 1989), Calodiscus lobatus (Cederström et al., 2009), Sinodiscus changyangensis (Dai & Zhang, 2013a)], and Eodiscidae [Pagetia ocellata (Jell, 1970;Shergold, 1991), Pagetia prolata (Jell, 1975), Pagetia vinusta (Cui et al., 2019), Pagetides qianensis (Zhang & Clarkson, 2012), and Pagetia resseri (Hu, 1971)]. ...
... Multiple growth stages have been identified in only a few species (e.g. Cederström et al., 2009;Zhang & Clarkson, 2012;Cui et al., 2019), due to their poorly segmented morphology and an insufficient sample size, limiting our understanding of morphogenesis and trunk segmentation at the transition from late protaspid to early meraspid period in different families of the Eodiscina. In P. ocellata (Jell, 1970;Shergold, 1991) and P. vinusta (Cui et al., 2019), for instance, the protaspides were subdivided into two growth stages based on the appearance of a shallow furrow, differentiating the exoskeleton into protocranidium and protopygidium. ...
... Cederström et al., 2009;Zhang & Clarkson, 2012;Cui et al., 2019), due to their poorly segmented morphology and an insufficient sample size, limiting our understanding of morphogenesis and trunk segmentation at the transition from late protaspid to early meraspid period in different families of the Eodiscina. In P. ocellata (Jell, 1970;Shergold, 1991) and P. vinusta (Cui et al., 2019), for instance, the protaspides were subdivided into two growth stages based on the appearance of a shallow furrow, differentiating the exoskeleton into protocranidium and protopygidium. However, segment addition into the protopygidium with respect to the dorsal exoskeleton has not been found widely across the Eodiscina, and consequently, instars with different numbers of protopygidial segments and size ranges are still unclear or unidentified. ...
A comprehensive review of a full developmental sequence of eodiscinid trilobites reported in recent decades from Cambrian Series 2 and 3 strata is presented. These mostly articulated specimens exhibit detailed morphologies with preservation of even delicate structures at different growth stages, such as the bacculae, axial pores, spine pores, pygidial marginal spines and line coaptative structures. Their trunk segmentation schedules displayed a consistent developmental mode in segment generation and liberation, that is tagmosis and somitogenesis occurred heterochronously after each moulting event, providing clues regarding the potential developmental strategy in isopygous and even macropygous tri-lobites. The fact that the rate of segmentation obviously exceeds that of articulation results in a seemingly prolonged process of the formation of thoracic segments, which might explain why eodiscinid trilobites have a fixed and limited number of thoracic segments. In addition, the relationship between enrollment mechanism and trunk segmentation during eodis-cinid ontogeny confirms this highly unusual growth pattern among the Trilobita, revealing why these early-diverging tri-lobites controlled the rate of segment increase and release during their life cycles, and is thus of interest with regard to the evolution of arthropod body patterning.
... Complete or nearly complete ontogenetic sequences based on analyses of fully articulated, well-preserved specimens from different trilobite lineages and geological ages have been increasingly well studied. Most are considered as classic developmental models representing their respective clades, such as: the eodiscoids Calodiscus (Cederström et al., 2009), Sinodiscus (Dai & Zhang, 2013a), Tsunyidiscus (Dai & Zhang, 2011;Zhang & Clarkson, 2012;Dai, Zhang & Peng, 2016) and Pagetia (Cui et al., 2019); the redlichiids Eoredlichia (Dai & Zhang, 2013b), Zhangshania and Estaingia (Holmes, Paterson & García-Bellido, 2021b); the orytocephalids Balangia (McNamara, Yu & Zhou, 2006), Changaspis (Du et al., 2019), Duodingia (Hou et al., 2015), Duyunaspis (McNamara et al., 2006;Lei, 2016;Dai et al., 2017), Oryctocarella (Du et al., 2020;Dai et al., 2021a,b) and Oryctocephalus (Esteve, Zhao & Peng, 2017); the olenids Parabolina (Clarkson, Taylor & Ahlberg, 1997), Ctenopyge (Clarkson, Ahlgren & Taylor, 2003) and Peltura (Bird & Clarkson, 2003); and the aulacopleurids Elrathia (Hopkins, 2021) and Aulacopleura (Hughes & Chapman, 1995;Hughes, Chapman & Adrain, 1999;Hughes et al., 2017). These developmental sequences, with reliable ontogenetic information throughout the whole life cycle, have enabled insights into how trunk segmentation with respect to the exoskeleton was controlled and regulated, and are critical for improving our understanding of the systematic and phylogenetic relationships within this fossil arthropod group. ...
... (1) The trunk development of E. intermediata and Z. typica is of the protarthrous type, since the onset of the holaspid phase significantly preceded the onset of the epimorphic phase (Fig. 6A, C). From our current understanding, the protarthrous developmental pattern seems widespread not only in the earliest clades, such as Redlichiidae (Dai & Zhang, 2013b), Gigantopygidae , Calodiscidae (Cederström et al., 2009; Biological Reviews (2022) Dai & Zhang, 2013a), Tsunyidiscidae (Dai & Zhang, 2011;Zhang & Clarkson, 2012;Dai et al., 2016), Eodiscidae (Cui et al., 2019), and Oryctocephalidae (Hou et al., 2015;Dai et al., 2017), but also in other later clades such as Shumardiidae (Stubblefield, 1926), Dolichometopidae (Robison, 1967), Aulacopleuridae (Hughes & Chapman, 1995;Hughes et al., 1999Hughes et al., , 2017, and Dalmanitidae (Drage, Laibl & Budil, 2018). It appears that termination of thoracic segment articulation preceded termination of trunk segment expression in various groups from different ages, and thus, this developmental pattern is not exclusive to those Cambrian taxa. ...
... Presumably, trilobites of large size or with many thoracic (or trunk) segments moulted a greater number of times and had a longer lifespan than those of small size or with fewer thoracic segments. For example, according to Jell (1975) and Cui et al. (2019), Pagetia had only two thoracic segments and at least nine instars (two protaspides, four meraspides and three holaspides) and thus must have moulted at least eight times during its life cycle. By a similar reasoning, most other eodiscoids, such as Calodiscus (Cederström et al., 2009), Sinodiscus (Dai & Zhang, 2013a), Tsunyidiscus (Dai & Zhang, 2011;Zhang & Clarkson, 2012;Dai et al., 2016), had three thoracic segments and exhibited more than nine instars and thus must have moulted more than nine times during their life cycles. ...
为了纪念卢衍豪（1913-2000）对我国三叶虫系统分类学和生物地层学研究等方面的卓越贡献，以及他最早研究了莱德利基虫的个体发育序列，我们将莱德利基三叶虫独特的发育模式命名为“卢氏发育模式（Lu’s ontogenesis）”。
The developmental mode of four redlichiid trilobites is summarized, based upon exceptionally well-preserved, articulated specimens from Cambrian Series 2 (stages 3 and 4) strata in southwestern China and South Australia. These relatively complete developmental sequences indicate a balanced rate in segment increase and addition to the thorax during the meraspid phase, which might explain why most redlichiids possess micropygous body patterning during ontogeny. In addition, an analysis of the size distribution, developmental strategy, and distribution of specimen numbers at different growth stages reveals a distinct developmental strategy during the redlichiid life cycle. A relatively short pre-holaspid and a prolonged holaspid phase in these redlichiid taxa offers insight into the developmental control and life strategy in these primitive arthropods.
... However, there is dispute on stratigraphic position of the Pagetia in these regions (Parcha, 2005(Parcha, , 2006Hughes, 2006). Eodiscoid trilobites are globally widely known from Cambrian Series 2 to Wuliuan (Miaolingian) (Jell, 1997;Zhang, 1989;Zhang and Clarkson, 1990;Shergold, 1991;Dai and Zhang, 2013;Dai et al., 2016;Zhang & Yuan, 1981;Yuan et al., 2016;Cui et al., 2019). In the Himalayan Cambrian sections, the eodiscoid Pagetia is well known from the Spiti and Kashmir regions (Hayden, 1904;Reed, 1910;Shah et al., 1995;Sahni & Sudan, 1996;Jell & Hughes, 1997;Singh et al., 2016aSingh et al., , 2017aSingh et al., , 2017bYin et al., 2018). ...
Abundant, though moderately well-preserved, specimens of Pagetia sp. are recorded along with the ptychopariid Xingrenaspis dardapurensis from a new stratigraphic level which lies above the Oryctocephalus salteri biozone in the Spiti region (Himalaya). This occurrence of Pagetia in a higher stratigraphic level (higher than the Oryctocephalus salteri biozone) from the Spiti region helps in understanding the distribution of this taxon in the Cambrian of the Kashmir and Spiti regions of the Himalaya. A Pagetia-Xingrenaspis association from the Kashmir region is already known, the present discovery of a similar association in the Spiti region enables the Wuliuan (Miaolingian) biostratigraphic correlation between the Kashmir and the Spiti regions. The record of the taxon Pagetia from a higher stratigraphic level in the Spiti region contradicts the previous assumption that the Pagetia bearing level in Kashmir is equivalent to the Pagetia-Oryctocephalus indicus (O. indicus biozone, Hayden horizon 2) in the lowest part of the Wuliuan in the Spiti region.
The ontogenetic sequence of an early Cambrian redlichiid trilobite Bathynotus kueichouensis is presented on the basis of numerous articulated specimens from Guizhou, China. The body regionalization of B. kueichouensis witnessed a wide variation in the morphology of trunk segments between different thoracic regions, referred to as the ‘thoracic region 1-4’. A greater size variation of these long pleural spines is recognized from the earlier meraspid degrees in a gradual appearance or disappearance until the holaspid period. The thoroughly inverse variation of these pleural spines in size and form is seen for the first time during the process of trilobite segment formation, which offers an opportunity to trace the evolutionary path of trilobite body structures, associated with their functional morphology and behavior. The ontogenetic strategy of B. kueichouensis is broadly similar to those of other redlichioid representatives where the full ontogenetic sequence exhibited a balanced rate in segment expression and liberation. Such a developmental mode may be the reason why most redlichiids possess a micropygous body patterning during ontogeny and provides insights into the controls of early arthropod developmental segmentation during evolutionary process.
