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Heterochronic characters coded for Xiphosura encompassing aspects of thoracetron and telson morphology, showing paedomorphic (−1), neutral (0), and peramorphic (+1) conditions. A character unavailable for coding in a species is considered missing data (?) and does not contribute to the species score.
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The occupation of new environments by evolutionary lineages is frequently associated with morphological changes. This covariation of ecotype and phenotype is expected due to the process of natural selection, whereby environmental pressures lead to the proliferation of morphological variants that are a better fit for the prevailing abiotic condition...
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Norway spruce has a wide natural distribution range, harboring substantial physiological and genetic variation. There are three altitudinal ecotypes described in this species. Each ecotype has been shaped by natural selection and retains morphological and physiological characteristics. Foliar spectral reflectance is readily used in evaluating the p...
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... The results emerging from our study are consistent with the findings of Lamsdell 126 , who hypothesised, by studying the body trait variations of horseshoe crabs in response to aquatic-terrestrial transitions, a strong role of heterochronic changes in shaping their morphological evolution in presence of ratchet-like mechanisms. In particular, heterochrony was suggested to act on phenotypes by producing sudden peramorphoclines and paedomorphoclines and altering the pre-existing allometric trajectories 126,127 . Coherently with these findings, our results suggest that species with the most extreme morphologies (i.e., sabertoothed cats, in our case) evolve such features by deeply altering common patterns of evolutionary integration, like the braincase/face covariation that underpins CREA pattern 128,129 . ...
Among evolutionary trends shaping phenotypic diversity over macroevolutionary scales, CREA (CRaniofacial Evolutionary Allometry) describes a tendency, among closely related species, for the smaller-sized of the group to have proportionally shorter rostra and larger braincases. Here, we used a phylogenetically broad cranial dataset, 3D geometric morphometrics, and phylogenetic comparative methods to assess the validity and strength of CREA in extinct and living felids. To test for the influence of biomechanical constraints, we quantified the impact of relative canine height on cranial shape evolution. Our results provided support to CREA at the family level. Yet, whereas felines support the rule, big cats, like Pantherinae and Machairodontinae, conform weakly if not at all with CREA predictions. Our findings suggest that Machairodontinae constitute one of the first well-supported exceptions to this biological rule currently known, probably in response to the biomechanical demands and developmental changes linked with their peculiar rostral adaptations. Our results suggest that the acquisition of extreme features concerning biomechanics, evo-devo constraints, and/or ecology is likely to be associated with peculiar patterns of morphological evolution, determining potential exceptions to common biological rules, for instance, by inducing variations in common patterns of evolutionary integration due to heterochronic changes under ratchet-like evolution.
... This traditional narrative has been overturned in recent years, with extinct horseshoe crabs shown to have greater ecological and morphological diversity than modern forms (Lamsdell, 2016) and a number of xiphosuran clades exhibiting marked shifts in morphology linked to heterochronic shifts in development as they occupy non-marine environments (Lamsdell, 2021a, b). Despite these morphological and ecological changes, horseshoe crabs are thought to have maintained a consistent post-embryonic developmental trajectory (Lamsdell, 2021a;Bicknell et al. 2022) and exhibit a neuroanatomy conserved at least since the Carboniferous (Bicknell et al., 2021b), making xiphosurans an important group for studying the patterns and drivers of mosaic evolution (Hopkins & Lidgard, 2012;Hunt et al. 2015). ...
... The ontogeny of Lunataspis borealis reveals conflicting adherence to the generalized ontogenetic trajectories identified across Xiphosura (Lamsdell, 2021a), with different tagma (Lamsdell, 2013) exhibiting different trends, suggesting some degree of developmental modularity. The changes observed within the thoracetron, specifically the transition from a semicircular to a more angular outline and the apparent reduction of visible tergite margins and axial nodes, fit the previously identified trends. ...
... Interestingly, a decrease in both the length of genal carapace extensions and lateral eye size is observed in eurypterid ontogeny (Lamsdell & Selden, 2013). As the generalized developmental trajectory for Xiphosura is recognized in both Belinurina and Limulina (Lamsdell, 2021a;Bicknell et al. 2022) it is possible that Lunataspis, which resolves as the basalmost xiphosuran outside of the Belinurina and Limulina clades, is exhibiting a mixture of ancestral and derived ontogenetic trajectories and that the highly conserved developmental trajectory of Xiphosura developed somewhere within its stem lineage. ...
... This traditional narrative has been overturned in recent years, with extinct horseshoe crabs shown to have greater ecological and morphological diversity than modern forms (Lamsdell, 2016) and a number of xiphosuran clades exhibiting marked shifts in morphology linked to heterochronic shifts in development as they occupy non-marine environments (Lamsdell, 2021a, b). Despite these morphological and ecological changes, horseshoe crabs are thought to have maintained a consistent post-embryonic developmental trajectory (Lamsdell, 2021a;Bicknell et al. 2022) and exhibit a neuroanatomy conserved at least since the Carboniferous (Bicknell et al., 2021b), making xiphosurans an important group for studying the patterns and drivers of mosaic evolution (Hopkins & Lidgard, 2012;Hunt et al. 2015). ...
... The ontogeny of Lunataspis borealis reveals conflicting adherence to the generalized ontogenetic trajectories identified across Xiphosura (Lamsdell, 2021a), with different tagma (Lamsdell, 2013) exhibiting different trends, suggesting some degree of developmental modularity. The changes observed within the thoracetron, specifically the transition from a semicircular to a more angular outline and the apparent reduction of visible tergite margins and axial nodes, fit the previously identified trends. ...
... Interestingly, a decrease in both the length of genal carapace extensions and lateral eye size is observed in eurypterid ontogeny (Lamsdell & Selden, 2013). As the generalized developmental trajectory for Xiphosura is recognized in both Belinurina and Limulina (Lamsdell, 2021a;Bicknell et al. 2022) it is possible that Lunataspis, which resolves as the basalmost xiphosuran outside of the Belinurina and Limulina clades, is exhibiting a mixture of ancestral and derived ontogenetic trajectories and that the highly conserved developmental trajectory of Xiphosura developed somewhere within its stem lineage. ...
Horseshoe crabs as a group are renowned for their morphological conservatism punctuated by marked shifts in morphology associated with the occupation of non-marine environments and have been suggested to exhibit a consistent developmental trajectory throughout their evolutionary history. Here, we report a new species of horseshoe crab from the Ordovician (Late Sandbian) of Kingston, Ontario, Canada, from juvenile and adult material. This new species provides critical insight into the ontogeny and morphology of the earliest horseshoe crabs, indicating that at least some Palaeozoic forms had freely articulating tergites anterior to the fused thoracetron and an opisthosoma comprising 13 segments.
... For discussion of the incorrect substitution of the term paedomorphosis with "neoteny" see McNamara (1986) and Martynov et al. (2020: 8-9). Although paedomorphosis is a very important mode of evolution (Gould, 1977;Korshunova et al., 2018;Lamsdell, 2020), it does not imply that ancestral developmental patterns are readily disappear during evolutionary modifications. Confirmed examples of paedomorphosis in nudibranch molluscs show that before a distinct paedomorphic organization was formed, for example in the nudibranch families Corambidae or Okadaiidae, a significant amount of gradual modifications of an ancestral organizations (= ancestral ontogenetic cycles) had occurred . ...
Various evaluations of the last common bilaterian ancestor ( lcba ) currently suggest that it resembled either a microscopic, non-segmented motile adult; or, on the contrary, a complex segmented adult motile urbilaterian. These fundamental inconsistencies remain largely unexplained. A majority of multidisciplinary data regarding sedentary adult ancestral bilaterian organization is overlooked. The sedentary-pelagic model is supported now by a number of novel developmental, paleontological and molecular phylogenetic data: (1) data in support of sedentary sponges, in the adult stage, as sister to all other Metazoa; (2) a similarity of molecular developmental pathways in both adults and larvae across sedentary sponges, cnidarians, and bilaterians; (3) a cnidarian-bilaterian relationship, including a unique sharing of a bona fide Hox-gene cluster, of which the evolutionary appearance does not connect directly to a bilaterian motile organization; (4) the presence of sedentary and tube-dwelling representatives of the main bilaterian clades in the early Cambrian; (5) an absence of definite taxonomic attribution of Ediacaran taxa reconstructed as motile to any true bilaterian phyla; (6) a similarity of tube morphology (and the clear presence of a protoconch-like apical structure of the Ediacaran sedentary Cloudinidae) among shells of the early Cambrian, and later true bilaterians, such as semi-sedentary hyoliths and motile molluscs; (7) recent data that provide growing evidence for a complex urbilaterian, despite a continuous molecular phylogenetic controversy. The present review compares the main existing models and reconciles the sedentary model of an urbilaterian and the model of a larva-like lcba with a unified sedentary(adult)-pelagic(larva) model of the lcba .
... Some multispecies trends are reported in the fossil record as successive directional shifts toward increasingly paedomorphic or peramorphic states (paedomorphoclines and peramorphoclines, respectively, reviewed by Jablonski, 2020;Lamsdell, 2021;McNamara, 2012). Such patterns are intriguing as potential cases of heightened evolvability in particular directions, but most require formal phylogenetic analysis and more detailed morphometrics to confirm the stepwise, anagenetic dynamics portrayed in such studies. ...
Evolvability is best addressed from a multi-level, macroevolutionary perspective through a comparative approach that tests for among-clade differences in phenotypic diversification in response to an opportunity, such as encountered after a mass extinction, entering a new adaptive zone, or entering a new geographic area. Analyzing the dynamics of clades under similar environmental conditions can (partially) factor out shared external drivers to recognize intrinsic differences in evolvability, aiming for a macroevolutionary analog of a common-garden experiment. Analyses will be most powerful when integrating neontological and paleontological data: determining differences among extant populations that can be hypothesized to generate large-scale, long-term contrasts in evolvability among clades; or observing large-scale differences among clade histories that can by hypothesized to reflect contrasts in genetics and development observed directly in extant populations. However, many comparative analyses can be informative on their own, as explored in this overview. Differences in clade-level evolvability can be visualized in diversity-disparity plots, which can quantify positive and negative departures of phenotypic productivity from stochastic expectations scaled to taxonomic diversification. Factors that evidently can promote evolvability include modularity—when selection aligns with modular structure or with morphological integration patterns; pronounced ontogenetic changes in morphology, as in allometry or multiphase life cycles; genome size; and a variety of evolutionary novelties, which can also be evaluated using macroevolutionary lags between the acquisition of a trait and phenotypic diversification, and dead-clade-walking patterns that may signal a loss of evolvability when extrinsic factors can be excluded. High speciation rates may indirectly foster phenotypic evolvability, and vice versa. Mechanisms are controversial, but clade evolvability may be higher in the Cambrian, and possibly early in the history of clades at other times; in the tropics; and, for marine organisms, in shallow-water disturbed habitats.
... Disparate xiphosurid forms record a complex evolutionary history of diversification, possibly through habitation of marginal environments and show evidence for differential heterochronic patterns compared to extant horseshoe crab species (Sekiguchi et al., 1982;Shuster Jr & Sekiguchi, 2003;Lamsdell, 2016Lamsdell, , 2021Haug & Rötzer, 2018a). For instance, some belinurids resemble juvenile forms of modern xiphosurids with elongate tail spines, expressed thoracetronic segmentation, hypertrophied ophthalmic spines, and absence or reduced genal spines (Racheboeuf et al., 2002;Haug et al., 2012;Haug & Rötzer, 2018b;Haug & Haug, 2020;Lamsdell, 2021;Lustri et al., 2021). ...
... Disparate xiphosurid forms record a complex evolutionary history of diversification, possibly through habitation of marginal environments and show evidence for differential heterochronic patterns compared to extant horseshoe crab species (Sekiguchi et al., 1982;Shuster Jr & Sekiguchi, 2003;Lamsdell, 2016Lamsdell, , 2021Haug & Rötzer, 2018a). For instance, some belinurids resemble juvenile forms of modern xiphosurids with elongate tail spines, expressed thoracetronic segmentation, hypertrophied ophthalmic spines, and absence or reduced genal spines (Racheboeuf et al., 2002;Haug et al., 2012;Haug & Rötzer, 2018b;Haug & Haug, 2020;Lamsdell, 2021;Lustri et al., 2021). Conversely, austrolimulids, the other primarily marginal-marine group, show splayed and overdeveloped genal spines and often reduced thoracetronic sections (Pickett, 1984;Lamsdell, 2016;Lerner et al., 2017;Bicknell, 2019;Bicknell et al., 2021b). ...
Records of evolutionary stasis over time are central to uncovering large-scale evolutionary modes, whether by long-term gradual change or via enduring stability punctuated by rapid shifts. The key to this discussion is to identify and examine groups with long fossil records that, ideally, extend to the present day. One group often regarded as the quintessential example of stasis is Xiphosurida, the horseshoe crabs. However, when, how and, particularly, why stasis arose in xiphosurids remain fundamental, but complex, questions. Here, we explore the protracted history of fossil and living xiphosurids and demonstrate two levels of evolutionary stability: developmental stasis since at least the Pennsylvanian and shape stasis since the Late Jurassic. Furthermore, shape and diversity are punctuated by two high-disparity episodes during the Carboniferous and Triassic-transitions that coincide with forays into habitation of marginal environments. In an exception to these general patterns, body size increased gradually over this period and, thus, cannot be described under the same, often-touted, static models of evolution. Therefore, we demonstrate that evolutionary stasis can be modular and fixed within the same group at different periods and in different biological traits, while other traits experience altogether different evolutionary modes. This mosaic in the tempo and mode of evolution is not unique to Xiphosurida but likely reflects variable mechanisms acting on biological traits, for example transitions in life modes, niche occupation and major evolutionary radiations.
... Through mapping 258 ecological affinity of species onto a phylogeny and subsequent ancestral state reconstruction, the 259 phylogeny can be used to better estimate when ecological shifts occurred. When combined with 260 phylogeny-based analyses of heterochronic trends (Lamsdell, 2021), phylogenetic paleoecology 261 can reveal whether heterochronic shifts predate changes in environmental occupation, or whether 262 a change in ecological affinity results in a subsequent concerted heterochronic trend. One group of organisms that have emerged as an excellent case study into the relationship 266 between heterochronically-mediated morphological change and shifts in ecologic occupation are 267 the Xiphosura, or horseshoe crabs (Lamsdell, 2016;. ...
... In 275 both cases the radiation within non-marine environments was associated with exploration of 276 novel regions of morphospace (Lamsdell, 2016) resulting from adaptive refinement (type 3 277 novelty) and subsequent innovation. Detailed knowledge of the ontogeny of extant 278 permits the underlying mechanism (or mode) of evolution in these cases to be assessed through a 283 method called heterochronic weighting (Lamsdell, 2021). 284 Under heterochronic weighting, aspects of an organism's morphology are analyzed to 285 determine peramorphic, paedomorphic, and neutral expressions with peramorphic conditions 286 being assigned a positive (+1) score, paedomorphic conditions being assigned a negative (-1) 287 score, and neutral conditions being assigned a score of 0. The overall heterochronic weighting 288 14 for a given species is determined by the average of the scores for the number of characters that 289 can be coded for that species, accounting for patterns of mosaic evolution that are prevalent 290 among morphological traits (Hopkins and Lidgard, 2012;Hunt et al., 2015) and the fact that it is 291 individual traits, not species themselves, that are peramorphic or paedomorphic (Gerber and analyses would perform node-based calculations including inferred ancestral states and 304 constantly reevaluate the peramorphic or paedomorphic condition for a given trait at each node 305 of the phylogeny. ...
... However, due to coarse or uneven phylogenetic sampling (or a need to rely on 306 lower-ranked trait criteria for the majority of species within an analysis), most analyses are likely 307 to have to rely on tip-based calculations that result in a less precisebut equally accurate -308 identification of heterochronic trends within lineages. 309 Initial study of ecological and heterochronic trends within Xiphosura identified 310 heterochronic weightings significantly distinct from what would be expected from a random 311 15 distribution in three of the four major xiphosurid clades (Lamsdell, 2021). Two of these clades, 312 Belinurina and Austrolimulidae, transition and radiate into non-marine environments, and their 313 clade heterochronic weightings express values that sat far outside the randomized distribution 314 and did not occur within the randomized weights, with an extreme preponderance to peramorphy 315 among austrolimulids and paedomorphy among belinurines. ...
Much of the focus of palaeobiological studies in the last century can be summarized as seeking to understand how evolutionary lineages occupy new regions of morphological, ecological, and geographic space, or are excluded from those spaces. A desire to understand the processes that lead to morphological change unites a variety of biological disciplines focusing on topics ranging from studies of organism genomics to broad scale macroevolutionary analyses. There is increasing recognition that a hierarchical approach, incorporating both intrinsic genealogical processes and external ecological factors, is necessary to understand the mechanisms behind the drivers of phenotypic change. One of the most important issues that remains to be resolved regards the generation and fixation of morphological changes within evolutionary lineages, including whether the evolution of novel morphologies facilitates expansion to previously unoccupied environments (a developmental push mechanism) or whether a shift in ecological occupation results in subsequent morphologic change (an ecological pull mechanism). The geological record affords a unique perspective on morphological change, preserving both evidence of environmental change through shifts in sedimentology and the changing morphology of evolutionary lineages; as such, palaeontology provides a long-term view of the relationship between ecological and morphological shifts. This review focuses on the ways that phylogenetic palaeoecology, which utilizes phylogenetic frameworks in concert with palaeoecological data, can be leveraged to explore these questions. It begins by reviewing the literature on novelty and innovation – the origination of new morphologies and their proliferation within ecosystems – within a hierarchical framework and the role of heterochrony as the primary mechanism by which phenotypic change occurs before exploring evidence for developmental push and ecological pull as competing drivers of morphological shifts. Drivers of morphological shifts are examined through analysis of heterochronic trends in horseshoe crab evolution and comparison with case studies on angiosperm plants, giant ground sloths, and megatooth sharks.
... However, there has been considerable historical confusion over the oldest available fossil xiphosuran genus name, which has been recognized alternately as Belinurus König (with a publication date of either 1820 or 1851) or the synonymous Bellinurus Pictet, 1846. Most recent treatments (e.g., Selden and Siveter, 1987;Anderson and Selden, 1997;Lamsdell, 2016Lamsdell, , 2021Bicknell and Pates, 2020) have favored Bellinurus Pictet, 1846 as the available name; however, Haug and Haug (2020) recently argued that Belinurus König, 1820 is valid and has priority, a position then followed by Lamsdell (2020), prompting a reinvestigation of the taxonomic history of the genus. Upon review, it is clear that neither of the previously recognized authorities for Belinurus are accurate and that the two candidate type species for each genus are, in fact, synonyms. ...
... The next treatment of the group, by Selden and Siveter (1987), followed Morris in recognizing Bellinurus Pictet as having priority but considered B. bellulus König, c. 1851 the valid type species. Subsequent workers all recognized Bellinurus Pictet, 1846 as the appropriate taxon name and authority (Schultka, 1994;Anderson and Selden, 1997;Lamsdell, 2016Lamsdell, , 2021Bicknell and Pates, 2020). Very few of these treatments considered the issue of the appropriate type species, although Bicknell and Pates (2020) listed both B. bellulus (ascribed to Pictet, 1846) and B. trilobitoides (Buckland) as distinct, valid species. ...
In the first half of the nineteenth century, a marked shift occurred in our understanding and treatment of the chelicerate fossil record, with the differentiation and recognition of entirely extinct genera for the first time. At the heart of this taxonomic revolution were the Eurypterida (sea scorpions) and Xiphosura (horseshoe crabs), although both groups were in fact considered crustaceans until Lankester's (1881) seminal comparative anatomical study of the extant xiphosuran Limulus Müller, 1785 and modern scorpions. The oldest available eurypterid genus is Eurypterus deKay, 1825; the oldest available fossil arachnid genus name is that of the scorpion Cyclophthalmus Corda, 1835. However, there has been considerable historical confusion over the oldest available fossil xiphosuran genus name, which has been recognized alternately as Belinurus König (with a publication date of either 1820 or 1851) or the synonymous Bellinurus Pictet, 1846. Most recent treatments (e.g., Selden and Siveter, 1987; Anderson and Selden, 1997; Anderson et al., 1997; Lamsdell, 2016, 2021; Bicknell and Pates, 2020) have favored Bellinurus Pictet, 1846 as the available name; however, Haug and Haug (2020) recently argued that Belinurus König, 1820 is valid and has priority, a position then followed by Lamsdell (2020), prompting a reinvestigation of the taxonomic history of the genus. Upon review, it is clear that neither of the previously recognized authorities for Belinurus are accurate and that the two candidate type species for each genus are, in fact, synonyms. Given the convoluted and at times almost illogical history of the competing names, along with the most recent controversy as to which has priority, we present a complete history of the treatment of the genus to resolve the issue.
... More recently, expanded phylogenetic analyses have been proposed (Lamsdell, 2016(Lamsdell, , 2020(Lamsdell, , 2021, and the composition of Xiphosura is constrained to solely Lunataspis, Kasibelinuridae and Xiphosurida. As for synziphosurines, formerly suggested as stem Xiphosura, in recent analyses they have been excluded from the xiphosurans, and considered as a polyphyletic group comprising stem euchelicerates and stem dekatriatans (Dekatriata = Chasmataspidida + Eurypterida + Arachnida; Lamsdell, 2016Lamsdell, , 2020Selden, Lamsdell, & Liu, 2015). ...
Houia is a possible annectent form between horseshoe crabs and eurypterids (sea scorpions). However, fossils are rare, previously only known from a single locality. Here, we report a new species of this genus, Houia guangxiensis sp. nov., from the Lower Devonian Cangwu Formation of Guangxi Province, South China. This discovery extends the geographical distribution of Houia from the Yangtze to the Cathaysia Block, supporting the living age of this genus in the Early Devonian (Lochkovian–Emsian). It not only adds to the record of this rare ‘intermediate’ form but also suggests potential relationships of shallow‐water species in the Yangtze and Cathaysia blocks. Houia guangxiensis sp. nov. from the Lower Devonian of the Cathaysia Block, South China.
... Conversely, most Paleozoic and early Mesozoic forms record evolutionary exploration (Bicknell, 2019;Bicknell, Amati & Hernández, 2019;Bicknell et al., 2019a;Bicknell, Naugolnykh & Brougham, 2020;Bicknell et al., in press;Bicknell, Hecker & Heyng , 2021;Bicknell & Pates, 2020). The evolutionary history of these earlier species illustrate morphological plasticity and exploration of different ecological niches (Lamsdell, 2016;Lamsdell, 2020a;Bicknell et al., 2019b). Belinurina-a clade containing Belinuridae-is a particularly diverse group known from the Carboniferous and Permian that successfully colonized freshwater environments. ...
... We follow the systematic taxonomy of Lamsdell (2013), Lamsdell (2016), Lamsdell (2020a), Bicknell, Lustri & Brougham (2019), and Bicknell & Pates (2020 and anatomical terms presented in Selden & Siveter (1987), Haug & Rötzer (2018b), and Selden, Simonetto & Marsiglio (2019). ...
... The group also has an exceptional diversity and disparity, which is unusual when compared to the Late Mesozoic and Cenozoic forms (Bicknell, 2019;Bicknell et al., 2019a;Bicknell et al., in press-a). Several attempts to colonize freshwater environments likely drove the Belinurina to evolve features that contrast the 'typical' xiphosurid morphology, and added to their extreme diversity and disparity (Lamsdell, 2016;Lamsdell, 2020a). Furthermore, freshwater environments can only sustain small populations (Wang et al., 2019) compared to marine conditions and are susceptible to isolating small populations (see DeWoody & Avise, 2000). ...
Xiphosurida is an ingroup of marine Euchelicerata often referred to as “living fossils”. However, this oxymoronic term is inapplicable for Paleozoic and early Mesozoic forms, as during these periods the group experienced notable evolutionary radiations; particularly the diverse late Palaeozoic clade Belinurina. Despite the iconic nature of the group, select species in this clade have been left undescribed in the light of recent geometric morphometric and phylogenetic considerations and methodologies. To this end, we re-describe Prolimulus woodwardi Fritsch, 1899 using new and type specimens to reveal more details on appendage anatomy and possible ecology. Furthermore, we present geometric morphometric and phylogenetic analyses that uncover relationships between P. woodwardi and other belinurids without genal spines. Both approaches highlight that a clade containing Prolimulus Fritsch, 1899, Liomesaspis Raymond, 1944, Alanops Racheboeuf, Vannier & Anderson, 2002 and Stilpnocephalus Selden, Simonetto & Marsiglio, 2019 may exist. While we do not erect a new group to contain these genera, we note that these genera exemplify the extreme limits of the Belinurina radiation and a peak in horseshoe crab diversity and disparity. This evidence also illustrates how changes in heterochronic timing are a key evolutionary phenomenon that can drive radiations among animals.
