Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida

Abstract

Horseshoe crabs are an iconic group of extant chelicerates, with a stunning fossil record that extends to at least the Lower Ordovician (~480 million years ago). As such, the group has retained significant biological and palaeontological interest. The sporadic nature of descriptive and systematic research into fossil horseshoe crabs over the last two centuries has spread information on the group across more than 200 texts dating from the early nineteenth century to the present day. We present the most comprehensive pictorial atlas of horseshoe crabs to date to pool these important data together. This review highlights taxa such as Bellinurus lacoei and Limulus priscus that have never been documented with photography. Furthermore, key morphological features of the true horseshoe crab (Xiphosurida) families—Austrolimulidae, Belinuridae, Limulidae, Paleolimulidae, and Rolfeiidae—are described. The evolutionary history of horseshoe crabs is reviewed and the current issues facing any possible biogeographic work are presented. Four major future directions that should be adopted by horseshoe crab researchers are outlined. We conclude that this review provides the basis for innovative geographic and geometric morphometric studies needed to uncover facets of horseshoe crab evolution.

Full text

REVIEW
published: 09 July 2020
doi: 10.3389/feart.2020.00098
Frontiers in Earth Science | www.frontiersin.org 1July 2020 | Volume 8 | Article 98
Edited by:
Paul Antony Selden,
University of Kansas, United States
Reviewed by:
James Lamsdell,
West Virginia University, United States
Jason Dunlop,
Museum für Naturkunde Leibniz
Institut für Evolutions und
Biodiversitätsforschung, Germany
*Correspondence:
Russell D. C. Bicknell
rdcbicknell@gmail.com
Specialty section:
This article was submitted to
Paleontology,
a section of the journal
Frontiers in Earth Science
Received: 22 January 2019
Accepted: 20 March 2020
Published: 09 July 2020
Citation:
Bicknell RDC and Pates S (2020)
Pictorial Atlas of Fossil and Extant
Horseshoe Crabs, With Focus on
Xiphosurida. Front. Earth Sci. 8:98.
doi: 10.3389/feart.2020.00098
Pictorial Atlas of Fossil and Extant
Horseshoe Crabs, With Focus on
Xiphosurida
Russell D. C. Bicknell 1
*and Stephen Pates 2
1Palaeoscience Research Centre, School of Environmental and Rural Science, University of New England, Armidale, NSW,
Australia, 2Museum of Comparative Zoology, Department of Organismic and Evolutionary Biology, Harvard University,
Cambridge, MA, United States
Horseshoe crabs are an iconic group of extant chelicerates, with a stunning fossil
record that extends to at least the Lower Ordovician (∼480 million years ago). As
such, the group has retained significant biological and palaeontological interest. The
sporadic nature of descriptive and systematic research into fossil horseshoe crabs
over the last two centuries has spread information on the group across more than
200 texts dating from the early nineteenth century to the present day. We present the
most comprehensive pictorial atlas of horseshoe crabs to date to pool these important
data together. This review highlights taxa such as Bellinurus lacoei and Limulus priscus
that have never been documented with photography. Furthermore, key morphological
features of the true horseshoe crab (Xiphosurida) families—Austrolimulidae, Belinuridae,
Limulidae, Paleolimulidae, and Rolfeiidae—are described. The evolutionary history of
horseshoe crabs is reviewed and the current issues facing any possible biogeographic
work are presented. Four major future directions that should be adopted by horseshoe
crab researchers are outlined. We conclude that this review provides the basis for
innovative geographic and geometric morphometric studies needed to uncover facets
of horseshoe crab evolution.
Keywords: Xiphosura, Xiphosurida, synziphosurines, horseshoe crab, pictorial atlas, evolution
INTRODUCTION
Chelicerates, a group that includes arachnids (spiders, scorpions), eurypterids (sea scorpions),
and Xiphosura (the so-called horseshoe crabs) have a stunning and extensive fossil spanning the
early Palaeozoic to today and an exceptional modern diversity (Dunlop, 2010). Of these taxa,
extant horseshoe crabs have been subject to detailed anatomical (van Der Hoeven, 1838; Owen,
1872; Lankester, 1881; Shuster, 1982; Shultz, 2001; Bicknell et al., 2018b,c,d), biochemical (Kaplan
et al., 1977; Botton and Ropes, 1987), physiological (Sokoloff, 1978), morphological (Lee and
Morton, 2005; Chatterji and Pati, 2014; Jawahir et al., 2017), and population dynamic (Botton,
1984; Brockmann, 1990; Gerhart, 2007) studies over the past two centuries. Furthermore, the
impressive fossil record of this group, and apparent morphological conservatism that allowed
survival of all five big mass extinctions, have driven extensive palaeontological interest in the group
(Babcock et al., 2000; Rudkin and Young, 2009; Sekiguchi and Shuster, 2009; Krzeminski et al.,
2010; Briggs et al., 2012; Dunlop et al., 2012; Lamsdell, 2013; Błazejowski, 2015; Lamsdell and
Mckenzie, 2015; Bicknell et al., 2018b,c, 2019b; Bicknell, 2019;Figure 1). Despite this extensive

Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 1 | The geological and morphological history of horseshoe crabs across the Phanerozoic. Number of named species is presented as well as suggested
palaeoenvironment (Tables 1–7). A major transition to freshwater conditions occurred between the Devonian and Carboniferous. This was concurrent with a decrease
in synziphosurine taxa and an increase in xiphosurids. Limulids had a diversification event in the Triassic and there was a transition back to dominantly marine
conditions in the Jurassic. Dashed lines represent ghost lineages.
research, numerous avenues for further research remain for
horseshoe crabs, and we highlight three here. Firstly, the
evolutionary relationship between synziphosurines (the so-called
“Synziphosura”) and Xiphosura (Lamsdell, 2013, 2016; Legg
et al., 2013; Garwood and Dunlop, 2014). To help clarify
this relationship, Lamsdell (2013) removed synziphosurines
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
from Xiphosura and arrayed them within Prosomapoda and
Planaterga. Secondly, there are a number of specimens that
have been described in open terminology (Haug et al., 2012;
Lamsdell et al., 2020) and despite the recent effort to bring
taxa into recognized families, and genera, and erect new
groups where appropriate (Bicknell, 2019; Bicknell et al.,
2019e; Lamsdell et al., 2020), there remain an array of
individuals that require taxonomic revision. Lastly, some genera
appear to have been extensively over-split (Dunbar, 1923;
Størmer, 1972; Fisher, 1984; Anderson, 1994; Haug et al.,
2012; Kin and Błazejowski, 2014; Haug and Rötzer, 2018b).
We therefore present a pictorial review of horseshoe crabs to
aid current and future researchers in (1) the morphology and
re-evaluation of taxa, (2) the determination of evolutionary
relationships, and (3) the confirmation of species validity
(Waterston, 1985; Selden and Siveter, 1987).
The palaeontological and evolutionary histories, broad
taxonomy of families (Størmer, 1955; Novozhilov, 1991), and
phylogenetic relationships (Lamsdell, 2013, 2016) of horseshoe
crabs has often been reviewed (Bergström, 1975; Selden and
Siveter, 1987; Anderson and Selden, 1997; Anderson and
Shuster, 2003; Rudkin and Young, 2009). However, a document
illustrating all horseshoe crab taxa has not been presented
since Woodward (1866, 1867, 1879),Dix and Pringle (1929,
1930),Eller (1938b), and Raymond (1944). We have therefore
collated images of all species considered horseshoe crabs (see
taxa Dunlop et al., 2019), in a vital step toward understanding
the true diversity and extent of Xiphosura (Lamsdell, 2013). We
also present taxonomic descriptions of the facets that define
members of xiphosurid families and consider of lifestyle and
diversity of each group. We have focused on Xiphosurida
as there are more taxa in this group than stem xiphosurids
and synziphosurines. Nonetheless, synziphosurines and non-
xiphosurid xiphosurans (previously considered Kasibelinuridae)
are also briefly considered. It is vital to note that a thorough
taxonomic revision of all species is beyond the intended
scope of this review—namely the depiction and discussion
of major horseshoe crab groups—but the images and details
here represent the basis for such future work. The ultimate
goal of this work is to depict all taxa in an open-access
environment for future researchers to use as a reference point
to continue research into this somewhat enigmatic group
of chelicerates.
TERMINOLOGY
The following definitions are provided to clarify terminology
used in descriptions. See Figure 2 for a depiction of
these features.
Somite: Fundamental unit or division that construct
arthropod bodies (Lamsdell, 2013; Dunlop and Lamsdell, 2017).
Tergite: Physical expression of somites as discrete plates on
the dorsal exoskeleton (Lamsdell, 2013; Dunlop and Lamsdell,
2017).
Prosoma: Anterior body section consisting of six somites
(Dunlop and Lamsdell, 2017). Prosoma refers to the anterior
A
B
C
FIGURE 2 | Depiction of horseshoe crab features outlining the key
morphological aspects of horseshoe crabs. (A) Reconstruction of
Cyamocephalus loganensis showing main morphological features of
synziphosurines. (B) Reconstruction of Euproops danae, showing main
morphological features of belinurids. (C) Reconstruction of Limulus
polyphemus, showing main morphological features of Limulina. Car, cardiac
lobe; Cep, cephalothorax; Oph, ophthalmic ridge; Ops, opisthosoma; Pro,
prosoma; Tel, telson; Ter, tergite; Thor, thoracetron.
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
section of synziphosurines and xiphosurans (Dunlop, 2010;
Dunlop and Lamsdell, 2017). The prosoma in Xiphosurida is
combined with the two most anterior opisthosomal sections to
produce the cephalothorax (Dunlop, 2010; Dunlop and Lamsdell,
2017).
Cephalothorax: Anterior body section of Xiphosurida.
Combination of two most anterior opisthosomal segments with
prosoma (Dunlop, 2010).
Ophthalmic ridge: Ridge above the lateral compound eye that
extends anteriorly and posteriorly relative to the compound eye
(Størmer, 1955).
Cardiac lobe: Lobe in the center of the
prosoma/cephalothorax that extends into
opisthosoma/thoracetron (Størmer, 1955).
Opisthosoma: Posterior section of the arthropod body,
consisting of up to 13 tergites (Dunlop and Lamsdell, 2017). Used
here for synziphosurines and non-xiphosurid xiphosurans as the
group lack a fused opisthosoma (=thoracetron) (Lamsdell, 2013).
Thoracetron: Posterior section of Xiphosurida that
is a fused solid plate. Shultz (2001) also suggested the
termed tergum for this feature. The section may have
expressed tergites.
Telson: Most posterior section of the xiphosuran exoskeleton,
styliform and highly mobile (Eagles, 1973). Also called
a tailspine.
INSTITUTIONAL ACRONYMS
AM F: Australian Museum, Sydney, NSW, Australia. AMNH:
American Museum of Natural History, New York, USA.
B: Geomuseum der WWU Münster, Germany. BGS.GSE:
British Geological Survey, Keyworth, England, UK. BMSC:
Buffalo Museum, Buffalo, NY, USA. CM: Carnegie Museum
of Natural History, Pittsburgh, Pennsylvania, USA. CCMGE:
Chernyshev Central Research Geological Exploration Museum,
St. Petersburg, Russia. GIN: Geological Institute of the Russian
Academy of Sciences, Moscow, Russia. GIUS: Faculty of Earth
Sciences, Silesian University, Sosnowiec, Czech Republic. GSC:
Geological Survey of Canada, Ottawa, Canada. GZG INV:
Geowissenschaftliches Zentrum der Georg-August-Universität
Geowissenschaftliches Museum, Göttingen, Germany. ISEA:
Museum of the Institute of Systematics and Evolution of
Animals, Polish Academy of Sciences, Warsaw, Poland. L, LL:
Manchester Museum, University of Manchester, Manchester,
England, UK. LPI: Chengdu Geological Center, Chengdu, China.
MAN: Muséum-Aquarium de Nancy, Lorraine, France. MAS Pal:
Museum am Schölerberg, Osnabrück, Germany. MB.A.: Museum
für Naturkunde Leibniz-Insitut, Berlin, Germany. MCZ: Museum
of Comparative Zoology, Harvard University, Cambridge, MA,
USA. MGSB: Museo Geológico del Seminario de Barcelona,
Barcelona, Spain. Specimens ending in MLU, HAU-WIL: Institut
für Geologische Wissenschaften und Geiseltalmuseum Martin
Luther University Halle-Wittenberg, Halle, Saale, Germany.
MM: Manitoba Museum, Winnipeg, Canada. MMF: Geological
Survey of New South Wales, Londonderry, NSW, Australia.
MMO B: Municipal Museum of Ostrava, Ostrava, Czech
Republic. MNHN: Museum National d’Histoire Naturelle
of Paris, Paris, France. MNHP: Národní muzeum, Prague,
Czech Republic. MSNM: Museo Civico di Storia Naturale di
Milano, Milan, Italy. NHMUK PI: Natural History Museum,
London, UK. NME: Geologisch-Paläontologischen Sammlung
des Naturkundemuseums Erfurt, Germany. NMK D: Wolfgang
Munk collection in Naturkundemuseum Kassel, Ottoneum
in Kassel, Germany. NMS: National Museums of Scotland,
Edinburgh, Scotland. NMW: National Museum of Wales,
Cardiff, United Kingdom. NSM: Nova Scotia Museum, Halifax,
NS, Canada. NYSM: New York State Museum, Albany, NY,
USA. OUMNH: Oxford University Museum of Natural History,
Oxford, England, UK. NMV P: Museums Victoria, Carlton,
Victoria, Australia. PIN: Paleontological Museum of Yu A
Orlov, Moscow, Russia. NHM-UIO: Natural History Museum,
University of Oslo, Oslo, Norway. PMSL:Natural History
Museum of Slovenia, Ljubljana, Slovenia. SLK: Leunissen
private collection. SMF: Forschungsinstitut Senckenberg,
Frankfurt am Main, Germany. SMNH: Swedish Museum of
Natural History, Stockholm, Sweden. SMNS: State Museum
of Natural History Stuttgart, Stuttgart, Germany. SNSB-BSPG:
Staatliche Naturwissenschaftliche Sammlungen Bayern –
Bayerische Staatssammlung für Paläontologie und Geologie,
Munich, Germany. SPW: Poschmann private collection.
TMP: The Royal Tyrrell Museum, Drumheller, AB, Canada.
TsNIGR: Chernyshev Central Research Geological Museum,
St. Petersburg, Russia. UCM: University of Colorado Museum
of Natural History, Boulder, CO, USA. UM:Paleontology
Center of University of Montana, MT, USA. UMUT PA:
The University Museum of the University of Tokyo, Tokyo,
Japan. USNM: United States National Museum, Washington,
DC, USA. USTL: Laboratoire de paléontologie de l’université
de Lille-1, Poitiers, France. UTGD: Geology Department,
University of Tasmania, Tasmania, Australia. U.W.: University
of Wisconsin Geology Museum, Madison, WI, USA. W.U.:
Wichita State University, Wichita, KS, USA. YPM IP: Division
of Invertebrate Paleontology in the Yale Peabody Museum,
New Haven, CT, USA. YPM IZ: Division of Invertebrate
Zoology in the Yale Peabody Museum, New Haven, CT, USA.
ZIK: Ukrainian Academy of Sciences, 252.150 Kiev, Ukraine.
ZPAL: Institute of Paleobiology, Polish Academy of Science,
Warsaw, Poland.
DIVISIONS OF HORSESHOE CRABS
Synziphosurines
First appearing in at least the early Ordovician of Morocco,
synziphosurines went extinct in the Mississippian (Tables 1–
4,Figures 3–9) (Anderson and Selden, 1997; Moore et al.,
2005b, 2007; Krzeminski et al., 2010; Van Roy et al., 2010;
Briggs et al., 2012). There are 13 synziphosurine genera and
20 species. Anderella, Borchgrevinkium, Camanchia, Legrandella,
Venustulus, and Weinbergina are currently considered to belong
to the clade Prosomapoda (the group that also contains
Xiphosura, Figures 4,5), while Bembicosoma, Bunaia, Bunodes,
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
TABLE 1 | Horseshoe crabs with currently uncertain suprageneric affinities.
Taxon Family Geological information (where
detailed) and country
Time
period
Environment Citation for figured
specimens
Figured here
Drabovaspis
complexa
Chlupá ˇ
c, 1963
Unspecified Letná Formation, Czech Republic Ordovician Marine Chlupá ˇ
c, 1963, 1965, 1999;
Bergström, 1968; Ortega
Hernández et al., 2010
Figure 3D
Unnamed
synziphosurine
Unspecified Lower Fezouata Formation,
Morocco
Ordovician Marine Van Roy et al., 2010; Martin
et al., 2016
Figure 3C
Unnamed xiphosuran Unspecified Upper Fezouata Formation,
Morocco
Ordovician Marine Van Roy et al., 2010;2015;
Lefebvre et al., 2016
Figure 3E
Dibasterium durgae
Briggs et al., 2012
Unspecified Herefordshire
Konservat-Lagerstätte, England,
UK
Silurian Marine Briggs et al., 2012; Sutton et al.,
2014
Figures 3A,B
Ordered time period and alphabetically by genus.
TABLE 2 | Taxa in Prosomapoda that are potentially related to Xiphosura.
Taxon Family Geological information
(where detailed) and
country
Time period Environment Citation for figured specimens Figured here
Camanchia grovensis
Moore et al., 2011
Unspecified Wenlock Scotch Grove
Formation, Iowa, USA
Silurian Marine Moore et al., 2011 Figure 4F
Venustulus
waukeshaensis
Moore et al. 2005
Unspecified Waukesha
Konservat-Lagerstätte,
Brandon Bridge Formation,
Wisconsin, USA
Silurian Marine (sensu
Wendruff,
2016)
Moore et al., 2005b Figure 4C
Borchgrevinkium
taimyrensis
Novojilov, 1959
Unspecified Sheshenkarinskoy Suite,
Kazakhstan
Devonian Freshwater Novojilov, 1959 Figure 4D
Legrandella lombardii
Eldredge, 1974
Unspecified Icla Formation, Bolivia Devonian Marine Eldredge, 1974; Shuster, 2001;
Shuster and Anderson, 2003; Bicknell
et al., 2019a
Figure 5
Anderella parva
Moore et al. 2007
Unspecified Bear Gulch Limestone,
Montana, USA
Carboniferous Marine Moore et al., 2007 Figures 4B,E
Weinbergina opitzi
Richter and Richter,
1929
Weinberginidae Hunsrück Slate, Germany Devonian Marine Richter and Richter, 1929; Størmer,
1955; Lehmann, 1956; Eldredge,
1974; Stürmer and Bergström, 1981;
Novozhilov, 1991; Shuster, 2001;
Shuster and Anderson, 2003; Jansen
and Türkay, 2010; Rust et al., 2016
Figure 4A
Ordered by family, time period and alphabetically by genus.
Cyamocephalus, Limuloides, Pasternakevia, and Pseudoniscus
have been placed into Planaterga (Figures 6–9;Lamsdell, 2013).
Synziphosurines are characterized by large prosomal shields,
unfused opisthosoma with nine to 11 segmented and expressed
tergites (Størmer, 1934, 1955; Rudkin et al., 2008; Lamsdell,
2013; Selden et al., 2015). In extreme cases, the three most
posterior tergites form a narrow postabdominal (pretelson)
section leading to a styliform telson. Lateral compound
eyes are known from Legrandella lombardii and Pseudoniscus
roosevelti (Eldredge, 1974; Bergström, 1975; Bicknell et al.,
2019a). Furthermore, Pasternakevia podolica (Krzeminski et al.,
2010) and Weinbergina opitzi (Lehmann, 1956; Stürmer and
Bergström, 1981) show evidence for putative ocular features.
The remaining taxa lack such ocular features and were
possibly blind (Bicknell et al., 2019a). Appendages are known
from at least Anderella parva,Venustulus waukeshaensis,
and Weinbergina opitzi (Richter and Richter, 1929; Størmer,
1934; Stürmer and Bergström, 1981; Moore et al., 2005a,b,
2007). Synziphosurines inhabited marine to marginal marine
environments, and the general lack of thick prosomal margin
suggests that the group may not have burrowed, and instead
potentially moved above the substrate (Størmer, 1952; Bergström,
1975; Stürmer and Bergström, 1981; Lamsdell et al., 2013).
Affinities of synziphosurines are actively debated due to the
few useful synapomorphies that have been identified to date
(Anderson et al., 1998), which has resulted in an unnatural
grouping of assorted stem euchelicerates (Krzeminski et al.,
2010; Lamsdell, 2013, 2016; Lamsdell and Mckenzie, 2015;
Selden et al., 2015). To build on the phylogenetic work
presented in Lamsdell (2013), in which Lamsdell highlighted that
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TABLE 3 | Taxa in clade Planaterga, excluding the group Dekatriata, sensu Lamsdell (2013) that traditionally represent synziphosurine groups.
Taxon Family Geological information
(where detailed) and
country
Time period Environment Citation for figured specimens Figured here
Bunodes lunula
Eichwald, 1854
Bunodidae Oesel Group, Saaremaa
Island, Estonia
Silurian Marine Eichwald, 1854; Woodward, 1866, 1867;
Zittel, 1881; Vogdes, 1917; Eldredge, 1974;
Bergström, 1975; Novozhilov, 1991; Bicknell
et al., 2019a
Figure 6
Limuloides horridus
(Woodward, 1872)
Bunodidae Leintwardine Formation,
England, UK
Silurian Marine (sensu
Gladwell,
2018)
Woodward, 1872 Figure 7H
Limuloides limuloides
(Woodward, 1865)
Bunodidae Leintwardine Formation,
England, UK
Silurian Marine (sensu
Gladwell,
2018)
Woodward, 1865, 1866, 1867; Zittel, 1881;
Gaskell, 1908; Vogdes, 1917; Størmer,
1955; Bergström, 1975; Novozhilov, 1991;
Bicknell et al., 2019a
Figures 7A–C
Limuloides salweyi
(Woodward, 1872)
Bunodidae Leintwardine Formation,
England, UK
Silurian Marine (sensu
Gladwell,
2018)
Woodward, 1872 Figure 7D
Limuloides speratus
Woodward, 1872
Bunodidae Leintwardine Formation,
England, UK
Silurian Marine (sensu
Gladwell,
2018)
Woodward, 1872 Figure 7G
Pasternakevia
podolica Selden and
Drygant, 1987
Bunodidae Ustye Suite Series, Russia Silurian Marine Selden and Drygant, 1987; Krzeminski et al.,
2010
Figures 7E,F
Bembicosoma
pomphicus
Laurie, 1899
Unspecified Reservoir Formation,
Scotland, UK
Silurian Marine Laurie, 1899; Anderson and Moore, 2003 Figure 8F
“Bunaia”heintzi
Størmer, 1934a
Unspecified Ringerike Sandstone,
Norway
Silurian Marine Størmer, 1934, 1955; Novozhilov, 1991 Figure 8E
Bunaia woodwardi
Clarke, 1919
Unspecified Vernon Formation, New
York, USA
Silurian Marine Clarke, 1919; Eldredge, 1974; Selden and
Nudds, 2008; Rudkin and Young, 2009
Figures 8B,D
Cyamocephalus
loganensis
Currie, 1927
Unspecified Patrick Burn Formation,
Scotland, UK; Wenlock
Limestone (?), Shropshire,
England, UK
Silurian Marine Currie, 1927; Eldredge and Plotnick, 1974;
Anderson, 1999; Bicknell et al., 2019a
Figure 8A
Pseudoniscus
aculeatus
Nieszkowski, 1859
Unspecified Oesel Group, Saaremaa
Island, Estonia
Silurian Marine Nieszkowski, 1858; Woodward, 1866,
1867; Vogdes, 1917; Eldredge, 1974;
Bergström, 1975
Figure 9B
Pseudoniscus clarkei
Ruedemann, 1916
Unspecified Vernon Formation, New
York, USA
Silurian Marine Ruedemann, 1916; Selden and Nudds,
2008; Bicknell et al., 2019a
Figure 9E
Pseudoniscus falcatus
(Woodward, 1868)
Unspecified Patrick Burn Formation,
Scotland, UK
Silurian Marine Woodward, 1868; Ruedemann, 1916;
Størmer, 1952, 1955; Bergström, 1975;
Novozhilov, 1991; Bicknell et al., 2019a
Figure 9A
Pseudoniscus
roosevelti
Clarke, 1902
Unspecified Vernon Formation, New
York, USA
Silurian Marine Clarke, 1902; Størmer, 1955; Eldredge,
1974; Novozhilov, 1991; Bicknell et al.,
2019a
Figures 9C,D
Indeterminate
synziphosurine
Unspecified Ardenno- Rhenish Massif,
Germany
Devonian Marginal
marine
Poschmann and Franke, 2006 Figure 8C
Ordered by family, time period, and then genus. Synonyms mentioned in Dunlop et al. (2019):Pseudoniscus =Neolimulus.Bunodes =Exapinurus.Limuloides =Hemiaspis. ? denote
uncertain formation assignment.
synziphosurines comprise both possible stem-horseshoe crabs
and stem arachnids, images of all accepted synziphosurines are
presented here (Figures 3–9).
Non-xiphosurid Xiphosura
First appearing in at least the Upper Ordovician of Canada and
potentially the Lower Ordovician of Morocco the group contains
taxa that have been considered stem-xiphosurids (Tables 1,4,
Figures 10–12;Rudkin and Young, 2009). There are eight
genera and 10 species in this group. Two genera—Maldybulakia
and Willwerathia—lack a family and the remaining six genera
are considered stem-xiphosurids (formerly Kasibelinuridae,
although this family was considered unhelpful by Bicknell et al.,
2019c as it is a paraphyletic group). Non-xiphosurid xiphosurans
are defined as chelicerates with a cardiac lobe extending to the
anterior prosomal shield (Lamsdell, 2013). Species of this group
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TABLE 4 | Taxa considered non-xiphosurid Xiphosura and stem xiphosurids.
Taxon Group Geological information
(where detailed) and
country
Time period Environment Citation for figured specimens Figured here
Lunataspis aurora
Rudkin et al., 2008
Stem xiphosurid Churchill River Group,
Canada
Ordovician Marine Rudkin et al., 2008; Rudkin and
Young, 2009; Dunlop, 2010; Young
et al., 2013; Bicknell et al., 2019a
Figure 10B
“Belinurus”
alleghenyensis
Eller, 1938b
Stem xiphosurid Chadakoin Formation, New
York State, USA
Devonian Marginal marine
(sensu Engelder
and Oertel, 1985)
Eller, 1938b; Bicknell et al., 2019c Figure 10C
Elleria morani
(Eller, 1938a)
Stem xiphosurid Venango Formation,
Pennsylvania, USA
Devonian Marginal marine Eller, 1938a; Størmer, 1955;
Babcock et al., 1995
Figure 10D
Kasibelinurus
amicorum
Pickett, 1993
Stem xiphosurid Mandagery Sandstone,
Australia
Devonian Marine Pickett, 1993; Itow et al., 2003;
Bicknell et al., 2019a,c
Figure 11A
“Kasibelinurus" randalli
Beecher, 1902
Stem xiphosurid Chadakoin Formation,
Pennsylvania, USA
Devonian Marginal marine Beecher, 1902; Babcock et al.,
1995; Bicknell et al., 2019c
Figures 11B–D
Pickettia carteri
(Eller, 1940)
Stem xiphosurid Cattaraugus Formation,
Pennsylvania, USA
Devonian Marine (sensu
Wilmarth, 1938)
Eller, 1940; Bicknell et al., 2019c Figure 10A
Maldybulakia angusi
Edgecombe, 1998b
Unspecified Sugarloaf Creek Formation,
NSW, Australia
Devonian Freshwater Edgecombe, 1998a,b Figures 12C,F,G
Maldybulakia malcomi
Edgecombe, 1998b
Unspecified Boyd Volcanic Complex,
NSW, Australia
Devonian Freshwater Edgecombe, 1998a,b Figures 12B,E
Maldybulakia mirabilis
(Tesakov and
Alekseev, 1992)
Unspecified Sheshenkarinskoy Suite,
Kazakhstan
Devonian Freshwater Tesakov and Alekseev, 1992 Figure 12D
Willwerathia laticeps
Størmer, 1936
Unspecified Köppen quarry, Willwerath,
Klerf Formation, Germany
Devonian Marginal marine Størmer, 1936; Anderson et al.,
1998; Poschmann and Franke,
2006
Figure 12A
Taxa order alphabetically by grouping, time period, and then genus. Synonyms mentioned in Dunlop et al. (2019):Maldybulakia =Lophodesmus. Note “Kasibelinuridae” is not used
here as the group is considered paraphyletic (Bicknell et al., 2019b).
can also have ophthalmic ridges, but this is taxon-specific and
may be taphonomically controlled. Select taxa have preserved
eyes: Kasibelinurus amicorum (Pickett, 1993; Dunlop and Selden,
1998)Lunataspis aurora (Rudkin et al., 2008; Rudkin and Young,
2009), and putatively Willwerathia laticeps (Anderson et al.,
1998). Appendages are not known from this group of horseshoe
crabs. Similar to synziphosurines, these taxa are mostly marine.
Select non-xiphosurid xiphosurans, such as Lu. aurora, show
a remarkable morphological similarity to xiphosurids (Rudkin
et al., 2008).
Xiphosurida
True horseshoe crabs are an extant order that first appeared in
the Devonian (Figure 1). Key characteristics of true horseshoe
crabs are a large, keeled, crescentic cephalothorax with anteriorly
located lateral compound eyes, a thoracetron of fused tergites
containing one or two sections, and a styliform telson (Anderson
and Selden, 1997; Rudkin et al., 2008; Briggs et al., 2012;
Lamsdell, 2016). There are 30 genera and at least 82 species in
Xiphosurida that are arrayed across the two suborders Belinurina
and Limulina (Tables 5–7). Belinurina comprises only the family
Belinuridae. Limulina comprises the superfamily Limuloidea,
which includes Austrolimulidae, Limulidae, Paleolimulidae, and
Rolfeiidae, and the genera Bellinuroopsis and Valloisella (sensu
Lamsdell, 2016).
Belinurina
All taxa within this sub-order are members of the family
Belinuridae. The fossil record of Belinuridae spans possibly from
latest Devonian, with the example of Bellinurus kiltorkensis (Eller,
1938b), through to the Carboniferous and the Permian (Figure 1)
and this family has the second largest generic diversity in
Xiphosurida, with seven genera Alanops,Anacontium,Bellinurus,
Euproops, Liomesaspis, Prolimulus, and Xiphosuroides, and 37
named species (Table 5,Figures 13–21). Belinurids have domed
cephalothoraxes with flattened margins, genal spines that are
either flat, posteriorly extending, or vestigial (Størmer, 1955),
and ophthalmic ridges that curve posteriorly from the lateral
compound eyes (Størmer, 1955; Fisher, 1977; Haug et al.,
2012), which sometimes extend into ophthalmic spines (Fisher,
1977). The thoracetron is fused and ranges between round,
trapezoidal, or triangular shapes (Størmer, 1955). Euproops and
Bellinurus species have between five and seven articulated and
expressed thoracetronic tergites with lateral spines (Størmer,
1955; Bergström, 1975; Fisher, 1977; Haug et al., 2012; Lamsdell,
2016). Anacontium,Liomesaspis,Prolimulus, and Xiphosuroides
species have no exposed tergites and no marginal spines
(Størmer, 1955; Shpinev and Vasilenko, 2018). Where known,
the telson is styliform and elongate for all genera (Bergström,
1975). Appendages are known from select belinurids. Chelicerae
and prosomal appendages are known from Euproops danae
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FIGURE 3 | Taxa considered possible horseshoe crabs that currently lack definitive affinities. (A,B) Dibasterium durgae: reconstructed in 3D from the Silurian-aged
Herefordshire Konservat-Lagerstätte, England, UK. OUMNH C.29640, holotype (A) Ventral view. (B) Dorsal view. (C) An unnamed xiphosuran from the lower
Ordovician-aged Upper Fezouata Formation, Morocco. YPM IP 227586. (D) Drabovaspis complexa from the Ordovician-aged Letná Formation, Czech Republic.
MNHP L23577, holotype. This taxon is also considered to have aglaspidid affinities (Dunlop et al., 2019). (E) Two unnamed synziphosurines from the lower
Ordovician-aged Lower Fezouata Formation, Morocco. YPM IP 517856. Photo credit: (A,B) Russell Garwood (also see Briggs et al., 2012); (C) Russell Bicknell; (D)
Javier Ortega Hernández; (E) Jessica Utrup.
(Mazon Creek Konservat-Lagerstätte, Carbondale Formation,
USA; Schultka, 2000; Haug et al., 2012; Haug and Rötzer, 2018b;
Bicknell et al., 2019b) and Alanops magnificus (Montceau-les-
Mines Konservat-Lagerstätte, Great Seams Formation, France;
Racheboeuf et al., 2002; Bicknell et al., 2019b).
Belinurids are an extremely well-studied group of xiphosurids
reflecting the expansive literature on the life mode, ontogeny
and taxonomy of the group (e.g., Fisher, 1977, 1979; Anderson,
1994; Haug et al., 2012; Haug and Rötzer, 2018a,b; Bicknell
et al., 2019d). Belinurids were the most successful horseshoe
crab group in exploiting freshwater conditions (Fisher, 1984;
Lamsdell, 2016). It has been suggested, that select taxa
were likely effective at sub-aerial activity (more so than
extant taxa) as cephalothoracic appendages were arranged
similarly to extant xiphosurids, permitting more on-land
exploration than is observed in extant taxa (Racheboeuf
et al., 2002; Haug and Rötzer, 2018b). Euproops danae
specifically had morphological characteristics that may have
mimicked co-occurring leaves and arachnids (Dunbar, 1923;
Fisher, 1979; Todd, 1991; Filipiak and Krawczynski, 1996),
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FIGURE 4 | Taxa in Prosomapoda that are not within Planaterga or Xiphosura. (A) Weinbergina opitzi from the Devonian-aged Hunsrück Slate Rheinland, Germany.
MB.A.1987. (B,E) Anderella parva from the Carboniferous-aged Bear Gulch Limestone, Montana, USA. (B) CM 54200, holotype. (E) CM 54201, paratype (C)
Venustulus waukeshaensis from the Silurian-aged Waukesha Lagerstätte, Wisconsin, USA. YPM IP 204461. (D) Borchgrevinkium taimyrensis from the Devonian-aged
Sheshenkarinskoy Suite, Kazakhstan. PIN 12711, holotype. (F) Camanchia grovensis from the Silurian-aged Wenlock Scotch Grove Formation, Iowa, USA.
U.W.4018/1a, holotype. Photo credit: (A) Andreas Abele, (B,C,E) Russell Bicknell, (D) Dmitry E. Shcherbakov, (F) Carrie A. Eaton. All converted to gray scale.
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 5 | Legrandella lombardii from the Devonian-aged Icla Formation, Bolivia. (A–C,E,F) AMNH 029273, holotype. (A) Lateral view. (B) Anterior view of prosoma.
(C) Dorsal view of prosoma. (E) Ventral view of prosoma. (F) Lateral view of telson. (D) AMNH 029274, plastoparatype. Dorsal view of prosoma. Photo credit:
Russell Bicknell.
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 6 | Examples of Bunodes lunula from the Silurian-aged Oesel Group, Saaremaa Island, Estonia. (A) NMS G.2001.10.1. (B) YPM IP 212839. (C) NYSM
19113. (D) NYSM 19114. (E) Slab showing two specimens. AMNH 028734. Photo credit: (A) Bill Crighton; (B–E) Russell Bicknell.
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FIGURE 7 | Limuloides and Pasternakevia.(A–C) Limuloides limuloides from the Silurian-aged Leintwardine Formation, England, UK. (A) BGS.GSE 32393. (B)
NHMUK PI. In. 60018. (C) NHMUK PI. In. 48422. (D) Limuloides salweyi from the Silurian-aged Leintwardine Formation, England, UK. NHMUK PI. In. 61510,
holotype. (E,F) Pasternakevia podolica from the Silurian-aged Ustye Suite Series, Russia. (E) ISEA I–F/MP/3/1499/08. (F) ZIK 35611, holotype. (G) Limuloides
speratus from the Silurian-aged Leintwardine Formation. NHMUK PI. I. 1180. (H) Limuloides horridus from the Silurian-aged Leintwardine Formation, England, UK.
NHMUK PI. In. 61509, holotype. Photo credit: (A) David Marshall; (B–D,G,H) Stephen Pates; (E) Bła˙
zej Bła˙
zejowski; (F) Ewa Krzeminska.
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FIGURE 8 | “Synziphosurines” currently lacking a family assignment. (A) Cyamocephalus loganensis from the Silurian-aged Patrick Burn Formation, Scotland, UK.
NHMUK PI. I. 16521, holotype. (B,D) Bunaia woodwardi from the Silurian-aged Vernon Shale, New York, USA. (B) NYSM 9911. (D) NYSM 9910. (C) Indeterminate
(Continued)
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FIGURE 8 | synziphosurine from the Devonian-aged Klerf Formation, Germany. SPW 831-D. (E) “Bunaia” heintzi from the Silurian-aged Ringerike Sandstone, Norway.
NHM-UIO PMOA4361, holotype. (F) Bembicosoma pomphicus from the Silurian-aged Reservoir Formation, Scotland, UK. NMS G.1897.32.146, holotype. Photo
credit: (A) Javier Ortega Hernández; (B,D) Russell Bicknell; (C) Markus Poschmann; (E) Hans Arne Nakrem; (F) Bill Crighton.
FIGURE 9 | Species within Pseudoniscus.(A) Pseudoniscus falcatus from the Silurian-aged Patrick Burn Formation, Scotland, UK. NHMUK PI. In. 44122, holotype.
(B) Pseudoniscus aculeatus from the Silurian-aged Oesel Group, Saaremaa Island, Estonia. AMNH 029281. (C,D) Pseudoniscus roosevelti from the Silurian-aged
Vernon Shale, New York, USA. (C) NMS G.2004.45.5a. (D) NYSM 4762. (E) Pseudoniscus clarkei from the Silurian-aged Vernon Shale, New York, USA. NYSM
E1030. (D,E) were photographed under ethanol. Photo credit: (A) Lucie Goodayle, NHM, London; (B,D,E) Russell Bicknell; (C) Bill Crighton.
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FIGURE 10 | Stem xiphosurids from Canada and the USA. (A) Pickettia carteri from the Devonian-aged Cattaraugus Formation, Pennsylvania, USA. BMSC E 9644,
holotype. (B) Lunataspis aurora from the Ordovician-aged Churchill River Group, Canada. MM I-4000A, holotype. (C) “Belinurus” alleghenyensis from the
Devonian-aged Chadakoin Formation, New York, USA. Cast of CM11065, holotype. (D) Elleria morani from the Devonian-aged Venango Formation, Pennsylvania,
USA. CM11574, holotype. (C,D) were coated with ammonium chloride sublimate. Photo credit: (A) KC Kratt; (B) Permission to reproduce photographs granted by
Graham Young and the Manitoba Museum; (C,D) Russell Bicknell.
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FIGURE 11 | Non-xiphosurid xiphosuran species from Australia and USA. (A) Kasibelinurus amicorum from the Devonian-aged Mandagery Sandstone, Australia. AM
F68969, holotype. (B) “Kasibelinurus” randalli from the Devonian-aged Chadakoin Formation, Pennsylvania, USA. (B) USNM PAL 484524. (C,D) “Kasibelinurus”
randalli from the Devonian-aged Venango Formation, Pennsylvania, USA. (C) YPM IP 09010, holotype. (D) YPM IP 30656, paratype. Photo credit: (A) Josh White;
(B–D) Russell Bicknell.
although this suggestion remains to be thoroughly explored.
The ontogeny of fossil belinurids has been documented using
Euproops sp. from the Osnabrück Formation (Pennsylvanian)
of Germany (Haug et al., 2012), and E. danae from the
Mazon Creek Konservat-Lagerstätte (Pennsylvanian) of the USA
(Haug and Rötzer, 2018b). The apparently large belinurid
diversity almost definitely reflects over-splitting during the
early twentieth century (Anderson, 1997; Lamsdell, 2016)
and grouping Euproopidae with Belinuridae (Dunlop et al.,
2019). A re-evaluation of the family is therefore needed
(Selden and Siveter, 1987) and should build on Anderson
(1994),Haug et al. (2012), and Haug and Rötzer (2018b)
who synonymised Euproops species after determining that
cephalothoracic compression produced variable, supposedly
species-diagnostic features (Haug and Rötzer, 2018b; Shpinev,
2018).
Limulina
This sub-order comprises the superfamily Limuloidea, the
families Paleolimulidae and Rolfeiidae, and the genus
Bellinuroopsis. Limulina has a fossil record ranging from
the Devonian to Recent. The diagnostic feature that separates
Limuloidea from Belinurina is the fusion of the two most
posterior thoracetronic tergites (sensu Lamsdell, 2016).
Paleolimulidae
This family has a fossil record spanning the Carboniferous
to Permian (Table 6). Three genera construct Paleolimulidae:
Moravurus, Paleolimulus, and Xaniopyramis and there are six
species within these three genera (Figure 22). The morphology of
paleolimulids broadly resembles that of modern horseshoe crabs,
but members of this group are smaller than extant taxa (Størmer,
1955; Shuster, 2001). Paleolimulids have a domed cephalothorax,
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FIGURE 12 | Xiphosuran taxa within genera Maldybulakia and Willwerathia.(A) Willwerathia laticeps from the Devonian-aged Klerf Formation, Germany. Cast of
Leunissen collection specimen SLK lb, cast number SPW 1308-D. (B,E) Maldybulakia malcomi from the Devonian-aged Boyd Volcanic Complex, NSW, Australia. AM
F102533, holotype. (B) Dorsal view. (E) Lateral view. (C,F,G) Maldybulakia angusi from the Devonian-aged Sugarloaf Creek Formation, NSW, Australia. (C)
Reconstruction presented in Edgecombe (1998b, Figure 12). (F) AM F102560. (G) AM F102565, cast of holotype. (D) Maldybulakia mirabilis from the Devonian-aged
Sheshenkarinskoy Suite, Kazakhstan. PIN No. 249/1, holotype. (B,E–G) Coated in ammonium chloride sublimate. (B,E–G) Converted to gray scale. Photo credit: (A)
Markus Poschmann; (B,E–G) Patrick Smith; (C) Permission to use reconstruction granted by Gregory Edgecombe, (D) Alexander S. Alekseev.
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
TABLE 5 | Sub-order Belinurina after Dunlop et al. (2019).
Taxon Family Geological information
(where detailed) and
country
Time period Environment Citation for figured specimens Figured here
Bellinurus kiltorkensis
Baily, 1869
Belinuridae Kiltorcan Formation, Republic
of Ireland
Devonian-
Carboniferous
Freshwater
(sensu Bluck,
1967)
Baily, 1870; Cole, 1901; Eller,
1938b
Figure 14F
Alanops magnifica
Racheboeuf et al.,
2002
Belinuridae Montceau-les-Mines
Konservat-Lagerstätte, Great
Seams Formation, France
Carboniferous Freshwater Racheboeuf et al., 2002; Perrier
and Charbonnier, 2014; Bicknell
et al., 2019b
Figures 13A,B
Bellinurus arcuatus
Baily, 1863
Belinuridae Pennine Middle Coal Measures
Formation, England, UK; South
Wales Lower Coal Measures
Formation, Wales, UK,
Carboniferous Freshwater Baily, 1863, 1870; Dix and Pringle,
1929; Eller, 1938b; Parkes and
Sleeman, 1997
Figure 13C
Bellinurus baldwini
Woodward, 1907
Belinuridae Pennine Middle Coal Measures
Formation, England, UK
Carboniferous Freshwater Woodward, 1907; Eller, 1938b;
Novozhilov, 1991
Figure 13E
Bellinurus bellulus
Pictet, 1846
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK; Pennine Middle Coal
Measures Formation,
Lancashire, England, UK
Carboniferous Freshwater Pictet, 1846; Baily, 1863; Baldwin,
1905, 1906; Dix and Pringle, 1929;
Eller, 1938b
Figure 13D
Bellinurus carwayensis
Dix and Pringle, 1929
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK
Carboniferous Freshwater Dix and Pringle, 1929 Figure 13C
Bellinurus concinnus
Dix and Pringle, 1929
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK
Carboniferous Freshwater Dix and Pringle, 1929; Eller, 1938b Figure 14B
Bellinurus grandaevus
Jones and Woodward,
1899
Belinuridae Canso Group, Parrsboro, Nova
Scotia, Canada; Riversdale
Group, Nova Scotia, Canada
Carboniferous Freshwater Jones and Woodward, 1899; Eller,
1938b; Copeland, 1957a
Figure 14D
Bellinurus iswariensis
(Chernyshev, 1928)
Belinuridae Almaznaya Formation; Ukraine;
Mospinskaya Formation,
Ukraine; Smolyaninovskaya (?)
Formation, Russia
Carboniferous Freshwater
(sensu Eros et al.,
2012)
Chernyshev, 1928; Eller, 1938b;
Shpinev, 2018
Figure 14C
Bellinurus koenigianus
Woodward, 1872
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK; Pennine Middle Coal
Measures Formation, England,
UK
Carboniferous Freshwater Woodward, 1872; Dix and Pringle,
1929; Eller, 1938b; Bergström,
1975
Figure 14E
Bellinurus lacoei
Packard, 1885
Belinuridae Mazon Creek
Konservat-Lagerstätte,
Carbondale Formation, Illinois,
USA
Carboniferous Freshwater
(sensu Fisher,
1979)
Packard, 1885 Figure 14A
Bellinurus
longicaudatus
Woodward, 1907
Belinuridae Pennine Middle Coal Measures
Formation, England, UK
Carboniferous Freshwater Woodward, 1907; Eller, 1938b Figure 15C
Bellinurus lunatus
(Martin, 1809)
Belinuridae Pennine Middle Coal Measures
Formation, Rochdale, England,
UK; Upper Silesia Coal Basin,
Czech Republic
Carboniferous Freshwater Martin, 1809; Prantl and P ˇ
ribyl,
1956; Filipiak and Krawczynski,
1996; Krawczynski et al., 1997
Figures 15A,B
Bellinurus
metschetnensis
(Chernyshev, 1928)
Belinuridae Belaya Kalitva Formation,
Ukraine
Carboniferous Freshwater
(sensu Eros et al.,
2012)
Chernyshev, 1928; Eller, 1938b;
Shpinev, 2018
Figure 15D
Bellinurus morgani Dix
and Pringle, 1930
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK
Carboniferous Freshwater Dix and Pringle, 1930; Fisher, 1982 Figure 15E
Bellinurus pustulosus
Dix and Pringle, 1929
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK
Carboniferous Freshwater Dix and Pringle, 1929; Eller, 1938b Figure 16D
(Continued)
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TABLE 5 | Continued
Taxon Family Geological information
(where detailed) and
country
Time period Environment Citation for figured specimens Figured here
Bellinurus reginae
Baily, 1863
Belinuridae Canso Group, Parrsboro, Nova
Scotia, Canada; Karviná
Formation (?), Upper Silesia,
Poland; South Wales Lower
Coal Measures Formation,
Wales, UK
Carboniferous Freshwater Baily, 1863; Woodward, 1867;
Zittel, 1881; Vogdes, 1917;
Copeland, 1957a; Novozhilov,
1991; Parkes and Sleeman, 1997
Figures 16C,E
Belinurus šustai Prantl
and Pˇ
ribyl, 1956
Belinuridae Karviná Formation, Czech
Republic.
Carboniferous Freshwater
(sensu Dopita
and Kumpera,
1993)
Prantl and Pˇ
ribyl, 1956 Figure 17A
Bellinurus stepanowi
Chernyshev, 1928
Belinuridae Almaznaya Formation, Ukraine;
Kamenskaya Formation,
Russia
Carboniferous Freshwater
(sensu Eros et al.,
2012)
Chernyshev, 1928; Eller, 1938b;
Shpinev, 2018
Figure 16B
Bellinurus silesiacus
Roemer, 1883
Belinuridae Upper Silesia Coal Basin,
Poland
Carboniferous Freshwater Roemer, 1883; Eller, 1938b Figure 16A
Bellinurus trechmanni
Woodward 1918
Belinuridae Pennine Upper Coal Measures
Formation, England, UK;
Sprockhövel Formation,
Germany
Carboniferous Freshwater Woodward, 1918; Trechmann and
Woolacott, 1919; Eller, 1938b
Figure 17B
Bellinurus trilobitoides
(Buckland, 1837)
Belinuridae Bickershaw
Konservat-Lagerstätte,
England, UK; Clay Ironstone,
England, UK; ?Pennine Upper
Coal Measures Formation,
England, UK.
Carboniferous Freshwater Buckland, 1837; Prestwich, 1840;
Anderson et al., 1997; Bicknell and
Pates, 2019b
Figure 17D
Bellinurus truemani
Dix and Pringle, 1929
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK; Sprockhövel Formation,
Germany
Carboniferous Freshwater Dix and Pringle, 1929; Eller,
1938b; Schultka, 1994;
Brauckmann, 2005
Figure 17C
Euproops anthrax
(Prestwich, 1840)
Belinuridae Pennant Sandstone Formation,
Wales, UK; South Wales Upper
Coal Measures Formation,
Wales, UK
Carboniferous Freshwater Prestwich, 1840; Størmer, 1955;
Bergström, 1975; Novozhilov,
1991
Figure 18F
Euproops bifidus
Siegfried, 1972
Belinuridae Flöz Dreibänke Formation,
Germany
Carboniferous Freshwater Siegfried, 1972; Brauckmann,
1982, 2005
Figure 18D
Euproops cambrensis
Dix and Pringle, 1929
Belinuridae South Wales Lower Coal
Measures Formation, Wales,
UK
Carboniferous Freshwater Dix and Pringle, 1929 Figure 18C
Euproops danae
(Meek and Worthen,
1865)
Belinuridae Almaznaya Formation; Ukraine;
Beeman Formation, New
Mexico, USA; Donets Black
Coal Basin, Ukaraine;
Farrington Group, England,
UK; Mazon Creek
Konservat-Lagerstätte,
Carbondale Formation, Illinois,
USA; Riversdale Group,
Canada; Smolyaninovskaya
Formation, Russia; Uffington
Shale; West Virginia, USA
Carboniferous Freshwater Meek and Worthen, 1865;
Packard, 1885; Chernyshev, 1928;
Raymond, 1945; Copeland,
1957b; Murphy, 1970; Ambrose
and Romano, 1972; Fisher, 1979;
Anderson, 1994; Babcock and
Merriam, 2000; Shuster, 2001;
Rudkin and Young, 2009; Lucas
et al., 2014; Bicknell et al., 2018d,
2019b,d; Haug and Rötzer,
2018b; Shpinev, 2018; Tashman
et al., 2019; Haug and Haug, 2020
Figure 19
Euproops longispina
Packard, 1885
Belinuridae Allegheny Formation,
Pennsylvania, USA
Carboniferous Freshwater Packard, 1885 Figures 18A,B
Euproops mariae
Crônier and Courville,
2005
Belinuridae Graissessac Shale and Coal,
Graissessac Basin, France
Carboniferous Freshwater Crônier and Courville, 2005 Figure 18E
Euproops meeki Dix
and Pringle, 1929
Belinuridae South Wales Upper Coal
Measures Formation, Wales,
UK
Carboniferous Freshwater Dix and Pringle, 1929 Figure 20D
Euproops orientalis
Kobayashi, 1933
Belinuridae Jido Series, Korea Carboniferous Freshwater Kobayashi, 1933 Figure 20C
(Continued)
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TABLE 5 | Continued
Taxon Family Geological information
(where detailed) and
country
Time period Environment Citation for figured specimens Figured here
Euproops rotundatus
Prestwich, 1840
Belinuridae Coal Measures Westhoughton,
England, UK; Orzesze Beds,
Upper Silesia Coal Basin,
Poland; South Wales Upper
Coal Measures Formation,
Wales, UK; Pennine Middle
Coal Measures Formation,
Lancashire, England, UK
Carboniferous Freshwater Prestwich, 1840; Woodward,
1867; Bölsche, 1879; Baldwin,
1902, 1906; Gaskell, 1908;
Vogdes, 1917; Størmer, 1955;
Filipiak and Krawczynski, 1996;
Krawczynski et al., 1997;
Anderson et al., 1999; Schultka,
2000; Lomax et al., 2016; Haug
and Haug, 2020
Figure 20B
Euproops sp. Belinuridae Bear Gulch Limestone,
Montana, USA; Mazon Creek
Konservat- Lagerstätte,
Carbondale Formation, Illinois,
USA; Piesberg quarry,
Osnabrück Formation
Germany; Windsor Group,
Canada
Carboniferous Freshwater Copeland, 1957b; Schram, 1979;
Brauckmann, 1982; Schultka,
2000; Haug et al., 2012; Bicknell
et al., 2019b; Haug and Haug,
2020
Figure 20A
?Liomesaspis birtwelli
(Woodward, 1872)
Belinuridae Pennine Lower Coal Measures
Formation, England, UK
Carboniferous Freshwater Woodward, 1872:Gaskell, 1908;
Bergström, 1975; Fisher, 1984
Figure 21C
Prolimulus woodwardi
Fritsch 1899
Belinuridae Kladno Formation, Czech
Republic
Carboniferous Freshwater
(sensu Hannibal
and Feldmann,
1981)
Fritsch, 1899; Prantl and Pˇ
ribyl,
1956; Novozhilov, 1991; Štamberg
and Zajíc, 2008
Figures 21D–F
Liomesaspis laevis
Raymond, 1944
Belinuridae Bickershaw
Konservat-Lagerstätte,
England, UK; Meisenheim
Formation, Germany; Mazon
Creek Konservat- Lagerstätte,
Carbondale Formation, Illinois,
USA; Montceau-les-Mines
Konservat-Lagerstätte, Great
Seams Formation, France
Carboniferous-
Permian
Freshwater Raymond, 1944; Størmer, 1955;
Vandenberghe, 1960; Müller,
1962; Novozhilov, 1991; Malz and
Poschmann, 1993; Anderson,
1997; Anderson et al., 1997;
Schindler and Poschmann, 2012
Figures 21A,B
Anacontium brevis
Raymond, 1944
Belinuridae Wellington Formation,
Oklahoma, USA
Permian Freshwater Raymond, 1944 Figure 21H
Anacontium carpenteri
Raymond, 1944
Belinuridae Wellington Formation,
Oklahoma, USA
Permian Freshwater Raymond, 1944 Figure 21G
Liomesaspis
leonardensis (Tasch,
1961)
Belinuridae Wellington Formation, Kansas,
USA
Permian Freshwater Tasch, 1961 Figure 21I
Xiphosuroides
khakassicus Shpinev
and Vasilenko, 2018
?Belinuridae Sarskaya Formation,
Khakassia, Russia
Carboniferous Freshwater Shpinev and Vasilenko, 2018 Figure 20E
Taxa order by time-period and then alphabetically by genus. Synonyms mentioned in Dunlop et al. (2019):Belinuridae =Euproopidae and Liomesaspididae;Bellinurus =Belinurus,
Steropsis and Koenigiella;Euproops =Prestwichia and Prestwichianella;Liomesaspis =Pringlia and Palatinaspis. ? denotes uncertain taxonomic affinities and formation assignment.
ophthalmic ridges that converge anteriorly to lateral compound
eyes and genal spines that extend posteriorly as far as the fourth
thoracic tergite (Lerner et al., 2016). The thoracetron is fused
and has an angular axial section with transverse and longitudinal
thoracetronic ridges occasionally present (Raymond, 1944;
Siveter and Selden, 1987; Novozhilov, 1991), along with
a styliform telson (Pickett, 1984; Seegis, 2014). Moveable
thoracetronic spines are occasionally preserved (Seegis, 2014).
Unique features of select taxa include the additional articulation
between the thoracetron and telson known from Paleolimulus
signatus and the expressed opercular (VIII) tergite producing a
free thoracetronic lobe in Pa. woodae and Xaniopyramis linseyi
(Størmer, 1952; Babcock et al., 2000; Lerner et al., 2016). Rare
specimens preserve soft-parts. Paleolimulus signatus (Insect Hill
Konservat-Lagerstätte, Wellington Formation, USA, Permian)
preserves cephalothoracic and thoracetronic appendages
(Dunbar, 1923; Raymond, 1944; Størmer, 1952; Babcock and
Merriam, 2000; Bicknell et al., 2019b). These appendages are
strikingly similar to modern horseshoe crabs (Størmer, 1955;
Bicknell et al., 2019b). Xaniopyramis linseyi (Upper Limestone
Group, Scotland, Carboniferous) preserves impressions of
cephalothoracic appendage muscles (Siveter and Selden, 1987).
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
TABLE 6 | Taxa in the suborder Limulina.
Taxon Family Geological information
(where detailed) and locality
Time period Environment Citation for figured specimens Figured here
Moravurus rehori
Pˇ
ribyl, 1967
Paleolimulidae Kyjovice Formation, Czech
Republic
Carboniferous Marine (sensu
Bábek et al.,
2004)
Pˇ
ribyl, 1967 Figure 22C
Paleolimulus woodae
Lerner et al., 2016
Paleolimulidae Horton Bluff Formation, Nova
Scotia, Canada
Carboniferous Marine Lerner et al., 2016 Figure 22B
Xaniopyramis linseyi
Siveter and Selden,
1987
Paleolimulidae Upper Limestone Group,
England, UK
Carboniferous Marine Siveter and Selden, 1987 Figure 22A
Paleolimulus signatus
(Beecher, 1904)
Paleolimulidae Barneston Limestone Kansas,
USA; Francis Creek Shale
Member, Illinois, USA; Insect
Hill Konservat-Lagerstätte,
Wellington Formation, Kansas,
USA; Pony Creek Shale
Konservat-Lagerstätte, Wood
Siding Formation, Kansas,
USA
Carboniferous–
Permian
Marine Beecher, 1904; Dunbar, 1923;
Størmer, 1955; Novozhilov, 1991;
Babcock et al., 2000; Shuster,
2001; Shuster and Anderson,
2003; Bicknell et al., 2019b
Figures 22D,F
Paleolimulus
kunguricus
Naugolnykh, 2017
Paleolimulidae Philippovian Formation, Russia Permian Marine Naugolnykh, 2017, 2018 Figure 22G
?Paleolimulus
juresanensis
Chernyshev, 1933
Paleolimulidae Maltchev or Belogor Beds. No
certain formation (T.
Tolmacheva pers. Comms.
2018)
Permian Marine Chernyshev, 1933 Figure 23E
Rolfeia fouldenensis
Waterston, 1985
Rolfeiidae Cementstones Group,
Scotland, UK
Carboniferous Marine Waterston, 1985 Figure 23B
Bellinuroopsis
rossicus Chernyshev,
1933
Unspecified Lebedjan Formation, Russia Devonian Marine Chernyshev, 1933; Eller, 1938b;
Størmer, 1955; Novozhilov, 1991
Figure 23A
The taxa are order by family, time-period and then alphabetically by genus and species. Synonyms mentioned in Dunlop et al. (2019):Paleolimulidae =Moravurdiae.Bellinuroopsis =
Neobelinuropsis.Paleolimulus =Prestwichia. ? denotes uncertain taxonomic affinities.
Paleolimulid species were mostly marine taxa and their
morphologies, similar to extant horseshoe crabs, reflect this life
mode. They may have therefore variably explored swimming and
burrowing life modes, with these ecological inferences related
to the presence of movable thoracic spines (Siveter and Selden,
1987). Paleolimulus woodae lacked thoracetronic movable spines
and may have been capable of swimming, while Xaniopyramis
linseyi, adorned with large thoracetronic spines, would have likely
burrowed (Siveter and Selden, 1987; Lerner et al., 2016). The
diversity of Paleolimulidae has previously been overstated and
Paleolimulus is now considered a paraphyletic group (Lamsdell,
2016; Lerner et al., 2017; Bicknell, 2019). Many paleolimulid
forms are now considered to be austrolimulids (discussed below),
so continued research into these taxa is needed to uncover the
true disparity of forms within this family and diversity of both
austrolimulids and paleolimulids (Bicknell, 2019).
Rolfeiidae
This monospecific family consists of Rolfeia fouldenensis and
is known from the Carboniferous-aged Cementstones Group,
Scotland (Table 6,Figure 23). The cephalothorax is domed,
exhibiting small genal spines, and a thick cephalothoracic
margin. The species has a cardiac lobe narrows anteriorly
and ophthalmic ridges that cross the lateral compound
eyes, converging at the cardiac lobe (Waterston, 1985). The
thoracetron is fused with visible tergal divisions and the
opercular tergite is fully expressed. Large fixed and small
moveable thoracetronic spines are known from R. fouldenensis
(Waterston, 1985; Selden and Siveter, 1987; Lamsdell, 2016) and
the telson is styliform. Lamsdell (2016) suggested that transverse
cephalothoracic ridge nodes were characteristic of the family;
however, as the holotype considered here lack these features, this
feature may be treated tentatively. Presently, no appendages are
known from this group (Waterston, 1985).
Rolfeia fouldenensis is the only species exhibiting large fixed
thoracetronic spines extending laterally, coupled with smaller
moveable thoracetronic spines (Clarkson, 1985). These spines
likely provided the thoracetron with more surface area to
prevent individuals from sinking into the substrate (Anderson,
1994) when they were not suspended in water (Siveter and
Selden, 1987). Originally thought to be a possible paleolimulid
due to tergal expression on the thoracetron (Waterston, 1985),
the unique characters of both moveable and overdeveloped
fixed spines, coupled with an expressed opercular tergite, were
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
TABLE 7 | Fossil taxa in superfamily Limuloidea.
Taxon Family Geological information
(where detailed) and locality
Time period Environment Citation for figured specimens Figured here
?Paleolimulus
longispinus Schram,
1979
Austrolimulidae Bear Gulch Limestone,
Montana, USA
Carboniferous Marginal Marine Schram, 1979; Hagadorn, 2002;
Haug et al., 2012
Figures 25B,C
?Paleolimulus jakovlevi
Glushenko and Ivanov,
1961
Austrolimulidae Araukaritovaya Formation,
Ukraine
Permian Marine Glushenko and Ivanov, 1961 Figure 26E
Panduralimulus
babcocki Allen and
Feldmann, 2005
Austrolimulidae Maybelle Limestone, Texas,
USA
Permian Marginal marine Allen and Feldmann, 2005 Figures 25A,F
Tasmaniolimulus
patersoni Bicknell,
2019
Austrolimulidae Jackey Shale, Tasmania,
Australia
Permian Freshwater Ewington et al., 1989; Itow et al.,
2003; Bicknell, 2019
Figure 24B
Austrolimulus fletcheri
Riek, 1955
Austrolimulidae Beacon Hill Shale, NSW,
Australia
Triassic Freshwater Riek, 1955; Novozhilov, 1991; Itow
et al., 2003; Rudkin and Young,
2009; Bicknell and Pates, 2019b;
Bicknell et al., 2019e
Figure 24A
Dubbolimulus peetae
Pickett, 1984
Austrolimulidae Ballimore Formation, NSW,
Australia
Triassic Freshwater Pickett, 1984; Itow et al., 2003 Figure 24C
?Paleolimulus
fuchsbergensis
Hauschke and Wilde,
1987
Austrolimulidae Exter Formation, Germany Triassic Freshwater Hauschke and Wilde, 1987;
Hauschke, 2014
Figure 26D
Psammolimulus
gottingensis Lange,
1923
Austrolimulidae Solling Formation, Germany Triassic Freshwater Lange, 1922; Meischner, 1962;
Novozhilov, 1991; Kustatscher
et al., 2014; Bicknell and Pates,
2019b; Bicknell et al., 2019b
Figure 26A
Vaderlimulus tricki
Lerner et al., 2017
Austrolimulidae Thaynes Group, Idaho, USA Triassic Marginal marine Lerner et al., 2017 Figure 25E
Casterolimulus kletti
Holland et al., 1975
Austrolimulidae Fox Hills Formation, North
Dakota, USA
Cretaceous Freshwater Holland et al., 1975 Figure 25D
Albalimulus bottoni
Bicknell and Pates,
2019b
?Limulidae Ballagan Formation, Scotland,
UK
Carboniferous Marine Bicknell and Pates, 2019b Figures 27A,B
Limulitella bronnii
Schimper, 1853
Limulidae Grés á Voltzia Formation,
France
Triassic Freshwater Schimper, 1853; Pfannenstiel,
1928; Wincierz, 1960; Novozhilov,
1991; Gall and Grauvogel-Stamm,
1999; Röhling and Heunisch, 2010
Figure 28A
Limulitella henkeli
von Fritsch, 1906
Limulidae Jena Formation, Germany Triassic Marine (sensu
Bła˙
zejowski et al.,
2017)
von Fritsch, 1906; Hauschke and
Mertmann, 2015
Figure 28B
?Limulitella sp. Limulidae Bernburg Fordmation,
Germany
Triassic Marine to
freshwater
Hauschke et al.,
2005
Hauschke and Wilde, 2000 Figure 30A
Limulitella sp. Limulidae Sakamena Group, Madagascar Triassic Marine Hauschke et al., 2004 Figure 29E
Limulitella sp. Limulidae Lower Wellenkalk Member,
Muschelkalk, Netherlands
Triassic Marine Zuber et al., 2017 Figure 28C
?Limulitella sp. Limulidae Buntsandstein, Germany Triassic Marine Hauschke and Wilde, 2008 Figures 29C,D
?Limulitella sp. Limulidae Lower Muschelkalk,
Netherlands
Triassic Marine Hauschke et al., 2009;
Klompmaker, 2019
Figure 28D
Limulitella tejraensis
Bła˙
zejowski et al.,
2017
Limulidae Ouled Chebbi Formation,
Tunisia
Triassic Freshwater Bła ˙
zejowski et al., 2017 Figure 29B
Limulitella vicensis
(Bleicher, 1897)
Limulidae Keuper Formation, France Triassic Marine Bleicher, 1897; Fisher, 1984 Figure 29A
(Continued)
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
TABLE 7 | Continued
Taxon Family Geological information
(where detailed) and locality
Time period Environment Citation for figured specimens Figured here
Limulitella volgensis
Ponomarenko,1985
Limulidae Rybinsk Formation, Russia Triassic Marine Ponomarenko, 1985 Figure 30E
Limulitella
liasokeuperinus
(Braun,1860)
Limulidae ?Exter Formation, Germany Triassic Freshwater Braun, 1860; Hauschke and Wilde,
1984
Figure 30D
Limulus nathorsti
Jackson,1906
Limulidae Höör Sandstone, Sweden Triassic Marine Jackson, 1906 Figure 31E
Limulus priscus
Münster,1839
Limulidae Muschelkalk Limestone,
Germany
Triassic Marine Münster, 1839 Figure 32F
Mesolimulus crespelli
Vía Boada, 1987
Limulidae Alcover Limestone Formation,
Spain
Triassic Marine Vía Boada, 1987a,b; Martí, 1994 Figure 31B
Sloveniolimulus rudkini
Bicknell et al.,2019e
Limulidae Strelovec Formation, Slovenia Triassic Marine Križnar and Hitij, 2010; Bicknell
et al., 2019e
Figure 32C
Tachypleus gadeai
(Vía Boada and Villalta,
1966)
Limulidae Alcover Limestone Formation,
Spain
Triassic Marine Vía Boada and Villalta, 1966;
Romero and Vía Boada, 1977; Vía
Boada et al., 1977; Martí, 1993,
1994; Diedrich, 2011; Bicknell
et al., 2019e
Figure 31A
Tarracolimulus rieki
Romero and Vía
Boada,1977
Limulidae Alcover Limestone Formation,
Spain
Triassic Marine Romero and Vía Boada, 1977; Vía
Boada et al., 1977
Figure 31C
Yunnanolimulus
luopingensis Zhang
et al.,2009
Limulidae Guanling Formation, Luoping,
China
Triassic Marine Zhang et al., 2009; Hu et al., 2011,
2017; Bicknell et al., 2019b
Figures 32A,B
Limulidae gen. et sp.
indet, previously
Limulus kieri
Limulidae Muschelkalk Limestone,
Germany
Triassic Marine Hauschke et al., 1992 Figure 31D
Limulidae gen. et sp.
indet
Limulidae Bernburg Formation, Germany Triassic Freshwater Hauschke, 2014 Figure 32E
Limulidae gen. et sp.
indet
Limulidae Volpriehausen Formation,
Germany
Triassic Freshwater Hauschke, 2014 Figure 32D
Crenatolimulus sp. Limulidae Kcynia Formation, Poland Jurassic Marine Kin et al., 2013; Błazejowski, 2015;
Błazejowski et al., 2015, 2016
Figure 33A
“Limulus” darwini Kin
and Błazejowski,2014
Limulidae Kcynia Formation, Poland Jurassic Marine Kin and Błazejowski, 2014;
Tashman, 2014; Błazejowski,
2015; Błazejowski et al., 2016,
2019
Figure 33B
Limulus woodwardi
Watson,1909
Limulidae Northampton Sand
Formation(?), England, UK
Jurassic Marine Watson, 1909 Figure 33C
Mesolimulus sibiricus
Ponomarenko,1985
Limulidae Talynzhansk Formation, Russia Jurassic Marginal marine Ponomarenko, 1985 Figure 33E
Mesolimulus sp. Limulidae Purbeck Limestone Group,
England, UK
Jurassic Marine Ross and Vannier, 2002 Figure 33D
Mesolimulus walchi
(Desmarest,1822)
Limulidae Konservat-Lagerstätte of
Ettling, Germany; Solnhofen
Limestone, Germany
Jurassic Marine Desmarest, 1822; Koenig, 1825;
Zittel, 1881; Malz, 1964; Fisher,
1984; Briggs and Wilby, 1996;
Shuster, 2001; Itow et al., 2003;
Shuster and Anderson, 2003;
Briggs et al., 2005; Novitsky,
2009; Rudkin and Young, 2009;
Sekiguchi and Shuster, 2009;
Diedrich, 2011; Haug et al., 2011;
Ebert et al., 2015; Hauschke and
Mertmann, 2016; Bicknell et al.,
2018d, 2019b
Figure 34
(Continued)
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
TABLE 7 | Continued
Taxon Family Geological information
(where detailed) and locality
Time period Environment Citation for figured specimens Figured here
Crenatolimulus
paluxyensis Feldmann
et al., 2011
Limulidae Glen Rose Formation, Texas,
USA
Cretaceous Marine Feldmann et al., 2011; Bicknell
et al., 2019b
Figure 35D
Limulus coffini
Reeside and Harris,
1952
Limulidae Pierre Shale, Colorado, USA Cretaceous Marine Reeside and Harris, 1952; Shuster,
2001; Shuster et al., 2003;
Sekiguchi and Shuster, 2009
Figure 35F
Mesolimulus
tafraoutensis Lamsdell
et al., 2020
Limulidae Gara Sbaa
Konservat-Lagerstätte, Kem
Kem Beds, Morocco
Cretaceous Marine Garassino et al., 2008; Lamsdell
et al., 2020
Figure 35E
Tachypleus syriacus
(Woodward, 1879)
Limulidae Haqel and Hadjoula
Konservat-Lagerstätten,
Lebanon
Cretaceous Marine Woodward, 1879; Novozhilov,
1991; Lamsdell and Mckenzie,
2015; Bicknell et al., 2019b
Figures 35C,G
Victalimulus mcqueeni
Riek and Gill, 1971
Limulidae Korumburra Group, NSW,
Australia
Cretaceous Freshwater Riek and Gill, 1971; Itow et al.,
2003; Poropat et al., 2018;
Bicknell et al., 2019b,e
Figures 35A,B
Limulus decheni
Zincken, 1862
Limulidae Braunkohlen Formation,
Germany; Domsen Sands,
Weißelster Basin, Germany
Eocene Marine Zincken, 1862; Giebel, 1863;
Fiebelkorn, 1895; Böhm, 1908;
Vetter, 1933; Novozhilov, 1991;
Bellmann, 1997; Hauschke and
Wilde, 2004; Dunlop et al., 2012;
Hauschke, 2013, 2018; Hauschke
and Mertmann, 2015; Schimpf
et al., 2017
Figures 36C–E
Unnamed specimen Unspecified Zechstein, Germany Permian Marine Hauschke and Wilde, 1989 Figures 36A,B
Unnamed specimen Unspecified Trochitenkalk Formation,
Germany
Triassic Marine Krause et al., 2009; Diedrich, 2011 Figures 30B,C
Valloisella lievinensis
Racheboeuf, 1992
Unspecified Bickershaw Complex, England
UK; Westphalian B Coal
Measures, England, UK;
Westphalian C Coal Measures,
France
Carboniferous Freshwater Dix and Jones, 1932; Racheboeuf,
1992; Anderson and Horrocks,
1995
Figure 36F
The taxa are order by family, time-period and then alphabetically by genus and species. Synonyms mentioned in Dunlop et al. (2019):Limulidae =Mesolimulidae;Limulitella =Limulites.
Tachypleus =Heterolimulus. Note that due to the paraphyletic status of Paleolimulus, taxa in this genus have been placed into Austrolimulidae. These taxa require revision. ? denotes
uncertain taxonomic affinities or formation assignment.
sufficient to erect a new family (Selden and Siveter, 1987; Siveter
and Selden, 1987).
Bellinuroopsis
This Devonian-aged, monospecific genus (Bellinuroopsis
rossicus) is known from one Russian specimen (Lebedjan
Formation, Table 6,Figure 23;Chernyshev, 1933; Moore et al.,
2007). The main characteristics that distinguishes Bel. rossicus
from other taxa in Limulina are the following: a wedge-shaped
cardiac lobe (Størmer, 1955); and an oblong thoracetron with
eight, free moving, expressed tergites, tapering slightly to a
telson. Furthermore, an expressed opercular (VIII) tergite
that is more pronounced than in Rolfeiidae (Størmer, 1955;
Novozhilov, 1991). These unique features potentially warrant the
erection of a separate family, as suggested by Størmer (1955).
Limuloidea
Taxa in this superfamily are Austrolimulidae, Limulidae,
and Valloisella. The diagnostic features of these taxa are a
“thoracetron showing no lateral expression of individual tergites”
(Lamsdell, 2016, p. 190).
Austrolimulidae
This family ranges from at least the Permian to the Cretaceous
(Figure 1). There are at least seven monospecific genera:
Austrolimulus, Casterolimulus, Dubbolimulus, Panduralimulus,
Psammolimulus, Tasmaniolimulus, and Vaderlimulus (Table 7,
Figures 24–26). Austrolimulids have domed cephalothoraxes,
with overdeveloped genal spines that terminate as far back as
the telson onset. Thoracetrons are mostly fused; occasionally
preserve apodemal pits with highly reduced or vestigial moveable
spines and styliform telsons (Riek, 1955, 1968; Lerner et al.,
2017; Bicknell, 2019). Swallow-tailed thoracetrons are observed
in A. fletcheri (Beacon Hill Shale, NSW, Australia, Triassic)
and V. tricki (Thaynes Group, Idaho, USA, Triassic; Lerner
et al., 2017), but this character is not known from all taxa
in the family, including T. patersoni (Jackey Shale, Tasmania,
Australia, Permain; Bicknell, 2019). Furthermore, A. fletcheri
has a thoracetron with two sections, the posterior section of
which has three exposed tergites (Riek, 1955; Pickett, 1984;
Novozhilov, 1991; Itow et al., 2003). Lamsdell (2016) described
a dorsal thoracetronic keel in Austrolimulidae. This feature
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
is noted in D. peetae (Ballimore Formation, NSW, Australia,
Triassic) and T. patersoni, but is not known to the other taxa
(Riek, 1955; Pickett, 1984; Allen and Feldmann, 2005; Feldmann
et al., 2011; Lerner et al., 2017; Bicknell, 2019). Appendages
are known from T. patersoni, in which the distal portions of
walking legs are observed (Ewington et al., 1989; Bicknell, 2019),
and P. gottingensis (Solling Formation, Germany, Triassic) shows
evidence of pushing legs (Meischner, 1962; Bicknell et al., 2019b).
The large genal spine splay and abnormal forms of
austrolimulids represent the strangest and most extreme
xiphosurid morphologies (they have been considered odd-
ball taxa, Eldredge, 1976; Bicknell, 2019). Their morphologies
likely reflect the freshwater and marginal conditions that were
exploited by the group, and provide evidence against the highly
conserved nature of Xiphosurida (Fisher, 1984; Bicknell, 2019).
The hypertrophied spines may have permitted more effective
motion within unidirectional fluid-flow in rivers (Bicknell, 2019;
Bicknell and Pates, 2019b). As discussed above, Lamsdell (2016)
and Lerner et al. (2017) suggested that species in Paleolimulus
belong in Austrolimulidae (e.g., Pa. fuchsbergensis,Pa. jakovlevi,
and Pa. longispinus) using phylogenetic and linear morphometric
arguments respectively. These taxa require revision; a direction of
research that will begin to uncover the true diversity of these taxa
and their interesting morphologies.
Limulidae
This is the most long-lived and most generically diverse
xiphosurid family, with a fossil record that spans possibly from
the Carboniferous to Recent (Figure 1). There are 10 limulid
genera: Albalimulus, Crenatolimulus, Limulitella, Limulus,
Mesolimulus, Sloveniolimulus, Tachypleus, Tarracolimulus,
Victalimulus, and Yunnanolimulus with 24 species (Table 7,
Figures 27–38;Lamsdell, 2016). Limulids have a domed,
horseshoe-shaped cephalothoraces with genal spines that can
extend posteriorly up to the first third of the thoracetron
(Novozhilov, 1991). Ophthalmic ridges are known from all
taxa and the lateral compound eyes are located along these
ridges (Størmer, 1955; Novozhilov, 1991). Ophthalmic ridges do
not converge anteriorly. The thoracetron is completely fused,
unsegmented, trapezoidal to sub-hexagonal, often displaying
movable spines, with small fixed spines, and a styliform telson
(Størmer, 1955; Tiegs and Manton, 1958; Siveter and Selden,
1987; Lamsdell, 2016). Appendages and soft-bodied material are
occasionally preserved in fossil limulids. Victalimulus mcqueeni
(Latrobe Group, NSW, Australia, Cretaceous), T. syriacus (Haqel
and Hadjoula Konservat-Lagerstätten, Lebanon, Cretaceous)
and Y. luopingensis (Member II, Guanling Formation,
Luoping, China, Triassic) all preserved cephalothoracic and
thoracetronic appendages (Riek and Gill, 1971; Hu et al.,
2011, 2017; Lamsdell and Mckenzie, 2015; Bicknell et al.,
2019b). Limulitella bronnii (Grés á Voltzia Formation, France,
Triassic) only preserved cephalothoracic appendages (Wincierz,
1960). Mesolimulus walchi preserved muscle fibers, and
cephalothoracic and thoracetronic appendages (Zittel, 1881;
Briggs et al., 2005; Bicknell et al., 2019b). Finally, muscle
insertions were identified using and augmented laminography
on a Limulitella sp. specimen from the Triassic-aged Lower
Wellenkalk Member, Muschelkalk, Netherlands (Zuber et al.,
2017). Sexual dimorphism has been suggested for select fossil
taxa (Bicknell et al., 2019b): Limulus decheni (females have
longer cephalothoraces; Hauschke and Wilde, 2004), T. syriacus
(females have broader thoracetrons and males have scalloped
anterior cephalothoraces; Lamsdell and Mckenzie, 2015) and
Y. luopingensis (females have shorter posterior thoracetronic
moveable spines and males have modified anterior walking legs;
Hu et al., 2017). Most limulids were marine, but V. mcqueeni,
Lim. bronnii, and Lim. tejraensis are considered freshwater
species, while Lim. liasokeuperinus is considered a marginal
marine taxon.
Limulids are thought to represent bradytelic evolution and
exhibit strong morphological conservation between extant and
fossil taxa. As such, they have been the focus of evolutionary and
morphological research (Fisher, 1984; Bicknell and Pates, 2019b;
Bicknell et al., 2019b). The limited morphological difference
between the 148 Mya Jurassic “Limulus”darwini (Kcynia
Formation, Poland) and modern juvenile L. polyphemus has
been used to assert stabilomorphism; the “relative morphological
stability of organisms in time and spatial distribution, the
taxonomic status of which does not exceed genus level”
(Błazejowski, 2015, p. 11). The conservation may reflect
habitation of similar marine conditions, or convergence on an
effective morphology.
Extant limulids have distributions across the east coast of
the USA and Asia, with their common names reflecting said
distribution (Shuster, 2001; Bicknell and Pates, 2019a): the
American, or Atlantic, horseshoe crab, Limulus polyphemus;
the Indonesian horseshoe crab, Carcinoscorpius rotundicauda;
the Chinese horseshoe crab, Tachypleus gigas; and the Japanese
horseshoe crab, T. tridentatus (Figures 35,36;Itow et al., 2003;
Zhou and Morton, 2004; Sekiguchi and Shuster, 2009). The
ontogeny and morphology of these taxa has been documented
thoroughly across the past two centuries (Shuster, 1982; Haug
and Rötzer, 2018a) and the morphological similarities are
depicted in Figures 35 and 36. Extant limulids occupy many
environmental conditions and can exploit brackish, freshwater,
shallow water, and fully-marine conditions (Siveter and Selden,
1987). Limulus polyphemus,T. gigas, and T. tridentatus are
mostly shallow marine, bottom-dwelling taxa that spawn on
beaches and inhabit a combination of marine sub-habits during
ontogeny (Fisher, 1984). Conversely, C. rotundicauda migrates
into completely freshwater (Størmer, 1952; Fisher, 1984; Crônier
and Courville, 2005; Sekiguchi and Shuster, 2009; Lamsdell,
2016). Despite representing the descendants of a long fossil
lineage, they now face an extinction event. Extensive harvesting
of specimens for their blood, and as a food source, as well as
habitat modification have majorly impacted populations (Botton,
2001; Hsieh and Chen, 2009; Shin et al., 2009; Akbar John
et al., 2011; Cartwright-Taylor et al., 2011; Carmichael and
Brush, 2012; Nelson et al., 2015; Kwan et al., 2016; Fairuz-
Fozi et al., 2018). Measures therefore need to be taken to
prevent this group from an extinction event. To this end, L.
polyphemus and its kin have now been suggested as world
heritage species (Tanacredi et al., 2009) and T. tridentatus was
recently listed as an endangered taxon (Laurie et al., 2019)
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FIGURE 13 | Belinurid species in the genera Alanops and Bellinurus.(A,B) Alanops magnifica from the Carboniferous-aged Montceau-les-Mines
Konservat-Lagerstätte, Great Seams Formation, France. (A) MNHN SOT001784, paratype, ventral view. Note appendages. (B) MNHN SOT002154, paratype, dorsal
view. (C) Bellinurus arcuatus from the Pennine Middle Coal Measures Formation, England, UK. AM F29886. (D) Bellinurus bellulus from the Carboniferous-aged South
Wales Lower Coal Measures Formation, Wales, UK. NMW 70.17. G9. (E) Bellinurus baldwini from the Carboniferous-aged Pennine Middle Coal Measures Formation,
England, UK. NHMUK PI. In. 18572, holotype. (F) Bellinurus carwayensis from the Carboniferous-aged South Wales Lower Coal Measures Formation, Wales, UK.
NMW 29.197.G3, holotype. (B,C) Converted to gray scale. (C) Coated in ammonium chloride sublimate. Photo credit: (A,B) Dominique Chabard; (C) Patrick Smith,
(D,F) Stephen Pates; (E) Lucie Goodayle, NHM, London.
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FIGURE 14 | Bellinurus species from Canada, UK, Ukraine, and USA. (A) Bellinurus lacoei from the Carboniferous-aged Mazon Creek Konservat-Lagerstätte,
Carbondale Formation, Illinois, USA. USNM 38861, cotype. (B) Bellinurus concinnus from the Carboniferous-aged South Wales Lower Coal Measures Formation,
Wales, UK. BGS.GSE 48775, holotype. (C) Bellinurus iswariensis from the Carboniferous-aged Almaznaya Formation, Ukraine. TsNIGR 3/2095. (D) Bellinurus
grandaevus from the Carboniferous-aged Canso Group, Nova Scotia, Canada. GSC 12806, hypotype. (E) Bellinurus koenigianus from the Carboniferous-aged Coal
Measures Formation, England, UK. CM 11066. (F) Bellinurus kiltorkensis from the Devonian to Carboniferous-aged Kiltorcan Formation, Ireland. NHMUK PI. In.
25931, cast of original specimen. (D,E) Converted to gray scale. Photo credit: (A,C,E) Russell Bicknell; (B) GB3D image, permission given by Mike Howe ©2018
JISC GB3D Type Fossils Online project partners (Amgueddfa Cymru–National Museum Wales); (D) Jodie Francis; (F) Lucie Goodayle, NHM, London.
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FIGURE 15 | Bellinurus species from the Czech Republic, UK, and Ukraine. (A,B) Bellinurus lunatus. (A) Specimen from Carboniferous-aged Upper Silesia Coal
Basin, Czech Republic. GIUS 5-845/7. (B) Specimen from Pennine Middle Coal Measures Formation, England, UK. NHMUK PI. I. 2754. (C) Bellinurus longicaudatus
from Carboniferous-aged Pennine Middle Coal Measures Formation, England, UK. NHMUK PI. In. 18563, holotype. (D) Bellinurus metschetnensis from
Carboniferous-aged Belaya Kalitva Formation, Ukraine. TsNIGR 8/2095. (E) Bellinurus morgani from Carboniferous-aged South Wales Lower Coal Measures
Formation, Wales, UK. BGS.GSE 49362, holotype. (D,E) Converted to gray scale. Photo credit: (A) Bła˙
ze Bła˙
zejowski; (B,C) Stephen Pates; (D) Russell Bicknell; (E)
GB3D image, permission given by Mike Howe ©2018 JISC GB3D Type Fossils Online project partners (Amgueddfa Cymru – National Museum Wales).
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FIGURE 16 | Bellinurus species from Canada, Poland, UK, and Ukraine. (A) Bellinurus silesiacus from the Carboniferous Upper Silesia Coal Basin, Poland.
MB.A.1091, cast of original. (B) Bellinurus stepanowi from the Carboniferous-aged Almaznaya Formation, Ukraine. TsNIGR 6/2095. (C,E) Bellinurus reginae.(C)
Specimen from Karviná Formation(?), Upper Silesia, Poland. MB.A.1090. (E) Specimen from Carboniferous-aged Canso Group, Nova Scotia, Canada. GSC 12803.
(D) Bellinurus pustulosus from Carboniferous-aged South Wales Lower Coal Measures Formation, Wales, UK. NMW 29.197.G2, holotype. ? denotes uncertain
formation assignment. (A–C,E) Converted to gray scale. Photo credit: (A) Andreas Abele; (B) Russell Bicknell; (C) Christian Neumann; (D) Stephen Pates; (E) Matt
Stimson. (A,B,C,E) Converted to gray scale.
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FIGURE 17 | Bellinurus species from the Czech Republic, Germany, and UK. (A) Bellinurus šustai from the Carboniferous-aged Karviná Formation, Czech Republic.
MMO B 976, holotype. (B) Bellinurus. cf. truemani from the Carboniferous-aged Sprockhövel Formation, Germany. SMF Viii 314. (C) Bellinurus trechmanni from the
Carboniferous-aged Pennine Upper Coal Measures Formation, England, UK. NHMUK PI. In. 18487, holotype. (D) Bellinurus trilobitoides from the Carboniferous-aged
?Pennine Upper Coal Measures Formation, England, UK. LL.111267a. (A) Converted to gray scale. ? denotes uncertain formation assignment. Photo credit: (A)
Mertová Eva; (B) Monica Solorzano-Kraemer; (C) Lucie Goodayle; (D) Russell Bicknell.
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FIGURE 18 | Euproops species from France, Germany, UK, and USA. (A,B) Euproops longispina from the Carboniferous-aged Allegheny Formation, Pennsylvania,
USA. (A) USNM 38857, cotype. (B) USNM 38858, cotype. (C) Euproops cambrensis from the Carboniferous-aged South Wales Lower Coal Measures Formation,
(Continued)
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FIGURE 18 | Wales, UK. NMW 29.198.G1, holotype. (D) Euproops bifidus from the Carboniferous-aged Flöz Dreibänke Formation, Germany. B7.135 holotype. (E)
Euproops mariae from the Carboniferous-aged Graissessac Shale and Coal, Graissessac Basin, France. USTL-CC026, holotype. (F) Euproops cf. anthrax from the
Carboniferous-aged South Wales Upper Coal Measures Formation, Wales, UK. NMW 27.177.G3. Photo credit: (A,B) Russell Bicknell; (C,F) Stephen Pates; (D)
Markus Bertling; (E) Jessie Cuvelier.
showing that progress is being made in preventing the human-
driven extinction of Xiphosurida.
Valloisella
This monospecific genus from the Carboniferous Coal Measures
in England and France (Figure 36) was originally considered a
belinurid (Anderson and Horrocks, 1995) but has since been
placed at the base of Limuloidea by recent phylogenetic analyses
(Lamsdell, 2016). The genus is defined by an almond-shaped
cephalothorax, genal spines that extend almost to the thoracetron
terminus, and a flange located along the thoracetronic margin
(Anderson and Horrocks, 1995). The fused thoracetron is
trapezoidal with expressed tergal divisions, contrasting most
other species in Limuloidea. No appendages are known from
this genus.
HORSESHOE CRAB EVOLUTIONARY
HISTORY AND DIVERSITY
Horseshoe crabs experienced three major evolutionary events
across the Phanerozoic (Figure 1). The Palaeozoic horseshoe crab
record was the most exploratory anatomically and evolutionarily
(Bła˙
zejowski et al., 2017). The rise of synziphosurines began in
the Lower Ordovician (Rudkin et al., 2008; Rudkin and Young,
2009; Dunlop, 2010; Van Roy et al., 2010, 2015). Across the
Silurian and Devonian, the marine and marginal marine forms
were abundant and represent the first evolutionary radiation of
this group, before the diversification of Xiphosurida (Størmer,
1955). Synziphosurine diversity declined heavily, reducing to one
taxon in the Carboniferous, when they subsequently went extinct
(Selden and Drygant, 1987; Selden and Siveter, 1987; Babcock
et al., 1995; Anderson and Selden, 1997; Moore et al., 2007; Lucas
et al., 2014). Non-xiphosurid xiphosurans also arose in the Upper
Ordovician, potentially even the Lower Ordovician, and are
unknown after the Devonian (Bicknell et al., 2019c). Xiphosurida
arose in the late Devonian with Bellinuroopsis (Moore et al.,
2007). After this, at least four xiphosurid families arose in the
Carboniferous: the Belinuridae, Limulidae, Paleolimulidae and
Rolfeiidae (Selden and Drygant, 1987; Selden and Siveter, 1987;
Babcock et al., 1995; Anderson and Selden, 1997; Lucas et al.,
2014; Bicknell, 2019; Bicknell and Pates, 2019b; Bicknell et al.,
2019e), with evidence suggesting that Austrolimulidae may also
have arisen at this time (Lamsdell, 2016). Carboniferous Coal
Measures and Konservat-Lagerstätten record the highest specific
diversity and first radiation of Xiphosurida (Anderson, 1997;
Moore et al., 2007; Rudkin and Young, 2009). Exploitation
of brackish and freshwater conditions by the late Palaeozoic
Xiphosurida may reflect adaptation to inconsistent coastlines and
fluctuating shallow-marine conditions (Bła˙
zejowski et al., 2017).
Xiphosurid diversity apparently decreased drastically during
the Permian, reflecting the closure of exceptional preservation
windows and an increase in xiphosurids inhabiting marginal
environments that are poorly preserved in the geological record
(Rudkin and Young, 2009). At the end of the Carboniferous,
there is no further record of Rolfeiidae, while the first definite
austrolimulid species arose in the Permian (Bicknell, 2019).
The Permian-Triassic “Great Dying” drove belinurids and
paleolimulids to extinction, while austrolimulids and limulids
survived into the Mesozoic (Bicknell and Pates, 2019b). The
Triassic was a period of extensive exploration in morphology and
the second radiation of xiphosurids and the third evolutionary
pulse in horseshoe crabs (Bicknell and Pates, 2019b; Bicknell
et al., 2019e). An aspect of this radiation was size increase:
Mesozoic taxa were much larger (30–60 cm long, including
telson) than the Palaeozoic counterparts (3–5 cm) (Størmer,
1955; Bicknell and Pates, 2019b). Austrolimulid diversity peaked
in the Triassic (Figure 1) but then decreased into the Cretaceous,
during which time the group went extinct. Limulid diversity
peaked in the Triassic with 12 species and decreased to
five during the Cretaceous (Bicknell et al., 2019e). Only
limulids survived into the Tertiary with one named Cenozoic
species: the Eocene Limulus decheni (Rudkin and Young, 2009;
Schimpf et al., 2017), a suggested “missing link” between extant
Asian and American taxa (Hauschke and Wilde, 2004). This
evolutionary history is one of generally low generic diversity,
such as in the four extant species (Anderson and Selden,
1997; Anderson, 1999; Shuster et al., 2003; Sekiguchi and
Shuster, 2009; Dunlop et al., 2012). However, the habitation
of marginal environments with poor conditions for exceptional
preservation of un-biomineralised exoskeleton cuticle also may
have impacted this observed low diversity (Babcock, 1998;
Anderson, 1999; Babcock and Merriam, 2000; Lamsdell and
Mckenzie, 2015).
GEOGRAPHICAL DISTRIBUTION OF
XIPHOSURAN MATERIAL
Distribution of horseshoe crab fossils is uneven in space and
time; reflecting historical biases in collecting that favored North
America and Western Europe. The UK has the highest number
of taxa (n=35), followed by the USA (n=23) and Germany
(n=22). Other areas with much larger landmasses have far
fewer known taxa: South America (n=1), Australia (n=7),
Asia (n=5), and Africa (n=6). This uneven geographical
sampling also partly reflects uneven temporal sampling (e.g., 25
UK taxa are Carboniferous, and eight are Silurian and 11 of
22 German taxa are Triassic). Within countries, well-explored
horizons or formations also provide apparent diversity peaks.
Notably the South Wales Coal Measures formations (South
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 19 | Euproops danae from Carboniferous-aged deposits and select species that have been synonymised with E. danae.(A) Specimen from
Carboniferous-aged lower Mercer Shale, Pennsylvania, USA. USNM 697642. (B–H,J) Specimens from the Carboniferous-aged Mazon Creek Konservat-Lagerstätte,
(Continued)
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 19 | Carbondale Formation, Illinois, USA. (B) YPM IP 16912. (C) YPM IP 25590. (D) Specimen that is completely enrolled, ideas mentioned in Fisher (1977)
and Anderson (1994) and discussed in Haug et al. (2012). YPM IP 50963. (E–G) Specimen with cephalothoracic appendages preserved. YPM IP 28514 (E) Complete
specimen. (F) Close up of left appendage. (G) Close up of right appendage. (H) USNM 38855, hypotype. (J) CM 11061. (M) Specimen from the Carboniferous-aged
South Wales Upper Coal Measures Formation, Wales, UK. NMW 70.17.G11 (I) Euproops darrahi=E. danae from the Carboniferous-aged Conemaugh Formation,
Pennsylvania, USA. MCZ 109528, holotype. (K) Euproops gwenti=E. danae from the Carboniferous-aged South Wales Upper Coal Measures Formation, Wales, UK.
BGS.GSE 48524, holotype. (L) Euproops graigola =E. danae from the Carboniferous-aged South Wales Upper Coal Measures Formation, Wales, UK. BGS.GSE
25424, holotype. (I) Converted to gray scale. (I) Coated with ammonium chloride sublimate. Photo credit (A–H,J) Russell Bicknell; (I,M) Stephen Pates; (K,L) GB3D
image, permission given by Mike Howe ©2018 JISC GB3D Type Fossils Online project partners (Amgueddfa Cymru – National Museum Wales).
Wales, UK) where six belinurids are known from the South
Wales Lower Coal Measures Formation and three belinurids
from the South Wales Upper Coal Measures Formation. These
nine taxa, within a limited geographic and temporal sample,
provide an apparently high Carboniferous diversity skewing
the understanding of overall belinurid diversity and geographic
spread as well as reflecting an over-splitting of the group. To
address these sampling issues (which are by no means limited
to horseshoe crabs) further exploration needs to be targeted
to under-sampled regions (Africa, Asia, South America) and
time periods (Jurassic and younger), as well as reassessing the
apparent high diversity of taxa that have not been recently
studied. Such efforts, combined with a concerted effort to
redescribe and refine horseshoe crab taxonomy will allow
ranges of different groups to be compared without the current
underlying biases.
FUTURE DIRECTIONS
Horseshoe crabs are an iconic group of chelicerates and, as
depicted here, have been thoroughly, if somewhat sporadically,
scientifically explored over the past two centuries. However, in
conducting this review we highlighted four main research areas
that should be addressed. To conclude this review, potential
future directions for horseshoe crab research are presented.
(1) Bicknell (2019), and Bicknell et al. (2019e) highlighted
that the traditional views that horseshoe crabs represent
evolutionary conservatism, stasis, and bradytelic evolution
(Fisher, 1984; Selden and Siveter, 1987; Rudkin et al., 2008)
is overstated. In reality, the group experienced three major
changes across the Phanerozoic: increased size, thoracetronic
fusion, and restriction to marine habitats (Størmer, 1955;
Crônier and Courville, 2005; Bicknell and Pates, 2019b).
Lamsdell (2016) thoroughly explored the record of habitat
change, but the remaining two points should be considered.
Thoracetronic fusion has been attributed to a change in
ecology, from enrolment to burrowing, but this remains
fairly unexplored (Fisher, 1977, 1981, 1982; Waterston,
1985; Lamsdell, 2016; Bła˙
zejowski et al., 2017). A study
considering when complete fusion developed in the context
of palaeoenvironmental and palaeoecological conditions
may confirm this hypothesis. Size change is likely associated
with exploitation of different niches: smaller Xiphosurida
likely preferred freshwater conditions, reflected today
in the smallest taxon—Carcinoscorpius rotundicauda
(Hauschke and Wilde, 1991; Dunlop et al., 2012). A study
considering shape and size change through time would
allow this hypothesis to be tested. In addition, modern
descriptive and statistical tools, such as multivariate
geometric morphometrics, semilandmark, and landmark
analyses could be employed to explore this topic in more
detail (Bicknell, 2019; Bicknell and Pates, 2019b; Bicknell
et al., 2019e).
(2) Rates of morphometric change in horseshoe crabs have
not been thoroughly explored (Fisher, 1984). The same
morphometric data outlined above could be used to address
possible evolutionary rates and quantify whether the group,
especially limulids, represent arrested evolution. Time series
analyses can also be conducted with these data to study
modes and models of evolution (Hunt and Carrano, 2010;
Hunt et al., 2015; Bicknell et al., 2018a).
(3) As Tables 6 and 7outline there are many specimens have
been identified as xiphosurids but not formally (re)described
in light of recent progress in the field (Lamsdell et al., 2020).
Formally describing these specimens would thoroughly aid
understanding patterns of horseshoe crab diversity through
time. Similarly, new collecting efforts should be focussed
on under-represented parts of the globe such as Asia,
Africa and South America, as well as Jurassic and younger
deposits, where knowledge of this group is hindered by a lack
of specimens.
(4) Computer tomography (CT) scanning to document fossil
and extant species has become a major tool over the past
decade, which has started to positively impact horseshoe
crab research. Schimpf et al. (2017) CT scanned Limulus
decheni specimens to accelerate digital transfer of important
morphological information (Figure 35). Zuber et al. (2017)
used CT scans and augmented laminography to document
muscle detail in a Limulitella sp. specimen (Figure 24),
and Bicknell et al. (2018b) conducted micro-CT scans
of iodine stained appendages to show L. polyphemus
muscles in situ. Scanning and 3D reconstructions of
specimens are still developing and therefore ripe for research,
especially for documenting and disseminating information
on holotypes.
CONCLUSIONS
The atlas presented here is the first comprehensive collation of
named taxa and other unnamed specimens considered horseshoe
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FIGURE 20 | Euproops species from Germany, Korea (formerly the Ch ¯
osen region) and UK, and Xiphosuroides.(A) Euproops sp., so call “Piesproops”, from the
Carboniferous-aged Osnabrück Formation, Germany. MAS Pal. 1308. (B) Euproops rotundatus specimens from the Carboniferous-aged Pennine Upper Coal
Measures Formation (?) England, UK. YPM IP 428963. (C) Euproops orientalis from the Carboniferous-aged Jido Series, Korea. UMUT PA 00433, holotype. (D)
Euproops meeki from the Carboniferous-aged South Wales Upper Coal Measures Formation, Wales, UK. BGS.GSE 48529, holotype. (E) Xiphosuroides khakassicus
from the Carboniferous-aged Sarskaya Formation, Khakassia, Russia. Scanning electron microscope image. PIN 384/211, holotype. (E) Converted to gray scale.
? denotes uncertain formation assignment. Photo credit (A) Angelika Leipner; (B) Russell Bicknell; (C) Tai Kubo; (D) GB3D image, permission given by Mike Howe ©
2018 JISC GB3D Type Fossils Online project partners (Amgueddfa Cymru – National Museum Wales); (E) Constantine Tarásenko.
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 21 | Belinurids in the genera Anacontium, Liomesaspis, and Prolimulus.(A,B) Liomesaspis laevis specimens from the Carboniferous-aged Mazon Creek
Lagerstätte, Illinois, USA. (A) MCZ 109536, holotype. (B) YPM IP 16913, paratype. (C) ?Liomesaspis birtwelli from the Carboniferous-aged Pennine Middle Coal
Measures Formation, England, UK. NHMUK PI. I. 13882. (D–F) Prolimulus woodwardi from the Carboniferous-aged Kladno Formation, Czech Republic. (D) NHMUK
PI. In. 18588, syntype. (E) MCZ 109537, hypotype. (F) MB.A.1989. (G) Anacontium carpenteri from the Wellington Formation, Oklahoma, USA. MCZ 109531,
paratype. (H) Anacontium brevis from the Permian-aged Wellington Formation, Oklahoma, USA. MCZ 109533, holotype. (I) Liomesaspis leonardensis from the
Permian-aged Wellington Formation, Kansas, USA. Image reproduced from Tasch (1961) as the specimen has been lost (C.D. Burke, pers. comms. 2018). W.U. 200,
holotype. (A,F) Converted to gray scale. ? denotes uncertain taxonomic assignment. (G,H) Coated with ammonium chloride sublimate and converted to gray scale.
Photo credit: (A,B) Russell Bicknell; (C) Monica (C,E,G,H) Stephen Pates; (D) Lucie Goodayle, NHM, London; (F) Lorenzo Lustri; (G) Mark Renczkowski; (I)
Permission to reproduce holotype granted by Kathleen Huber.
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FIGURE 22 | Examples of Carboniferous and Permian paleolimulids. (A) Xaniopyramis linseyi from the Carboniferous-aged Upper Limestone Group, Weardale,
England, UK. OUMNH E.03994, rubber cast of holotype. (B) Paleolimulus woodae from the Carboniferous-aged Horton Bluff Formation, Nova Scotia, Canada.
(Continued)
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 22 | NSM005GF045.374, paratype. (C) Moravurus rehori from the Carboniferous-aged Kyjovice Formation, Czech Republic. MMO B 8169, holotype. (D,F)
Paleolimulus signatus from the Carboniferous-aged Pony Creek Shale Konservat-Lagerstätte, Wood Siding Formation, Kansas, USA. (D) USNM 484411, hypotype.
(F) USNM PAL 484408, hypotype. (E) ?Paleolimulus juresanensis from the Permian-aged Maltchev or Belogor Limestone Beds. CCMGE CM2/3694, holotype. (G)
Paleolimulus kunguricus from the Permian-aged Philippovian Formation, Russia. GIN PH-18, holotype. ? denotes uncertain taxonomic assignment. Photo credit: (A)
GB3D image, permission given by Mike Howe ©2018 JISC GB3D Type Fossils Online project partners (Amgueddfa Cymru – National Museum Wales); (B) Allan
Lerner; (C) Mertová Eva; (D–F) Russell Bicknell; (G) Serge Naugolnykh.
FIGURE 23 | Bellinuroopsis rossicus and Rolfeia fouldenensis.(A) Bellinuroopsis rossicus from the Devonian-aged Lebedjan Formation, Russia. CCMGE CM1/3694,
holotype. (B) Rolfeia fouldenensis from the Carboniferous-aged Cementstones Group, Scotland, UK. NMS 1984.67.1A, holotype. Photo credit: (A) Russell Bicknell;
(B) Bill Crighton.
crabs. The work builds on research presented during the early- to
middle-twentieth century and, its presentation in an open-access
environment will allow all researchers interested in horseshoe
crabs access to key anatomical information needed for new
taxonomic studies. Brief notes detailing the characteristic features
and supposed life modes of families within Xiphosurida are
presented, synthesizing other key works on the group. A brief
evolutionary history of horseshoe crabs is presented, which
outlines diversity changes from the Lower Ordovician to today.
Finally, we highlight four major avenues for future research:
most notably analyses of morphometric data of horseshoe crabs
to mathematically probe the evolutionary history of the group.
These same data may represent an important step toward
reconciling synziphosurines with true horseshoe crabs.
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
FIGURE 24 | Austrolimulids from Australia. (A) Austrolimulus fletcheri from the Triassic-aged Beacon Hill Shale, NSW, Australia. AM F38274, holotype. (B)
Tasmaniolimulus patersoni from the Permian-aged Jackey Shale, Tasmania, Australia. UTGD 123979, holotype. (C) Dubbolimulus peetae from the Triassic-aged
Ballimore Formation, NSW, Australia. MMF 27693, holotype. (B,C) Converted to gray scale. Photo credit: (A) Josh White; (B) Russell Bicknell; (C) David Barnes. (B)
Coated in ammonium chloride sublimate.
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FIGURE 25 | Austrolimulids from the USA. (A,F) Panduralimulus babcocki from the Permian-aged Maybelle Limestone, Texas, USA. (A) USNM 520723, holotype. (F)
USNM 520724, paratype. (B,C) Paleolimulus longispinus specimens from the Carboniferous-aged Bear Gulch Limestone, Montana, USA. (B) UM 81-8-5-1. (C) CM
54050. (D) Casterolimulus kletti from the Cretaceous-aged Fox Hills Formation, North Dakota, USA. USNM 206801, holotype. (E) Vaderlimulus tricki from the
Triassic-aged Thaynes Group, Idaho, USA. UCM 140.25, holotype. (C) Converted to gray scale. Photo credit: (A,C,D,F) Russell Bicknell; (B) Kallie Moore;
(E) Allan Lerner.
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FIGURE 26 | Austrolimulids from Europe. (A–C) Psammolimulus gottingensis from the Triassic-aged Solling Formation, Germany. (A) Complete specimen, GZG INV
15356a. (B) Specimen with pushing leg preserved (black arrow). GZG INV 15376a. (C) Complete specimen with appendage impressions in cephalothorax,
GZG.INV.45730a. (D) ?Paleolimulus fuchsbergensis from the Triassic-aged Exter Formation, Germany. SMF VII I 311, holotype. (E) ?Paleolimulus jakovlevi from
Permian-aged Araukaritovaya Formation Novoselovka, Ukraine. CCMGE CM1/8886, holotype. ? denotes uncertain taxonomic assignment. Photo credit: (A–C)
Gerhart Hundertmark; (D) Norbert Hauschke; (E) Russell Bicknell.
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FIGURE 27 | The oldest suggested limulid from the lower Carbonifeous-aged Ballagan Formation, Scotland, UK; Albalimulus bottoni.(A) BSG.GSE2028, holotype,
part. (B) BGS.GSE9680, holotype, counter-part. Image mirrored to align with (A) Phylogenetic analyses of Xiphosurida placed this taxon close to the base of
Limulidae (Bicknell and Pates, 2019b). Specimens were coated with ammonium chloride sublimate and converted to gray-scale. Photo credit: Russell Bicknell.
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FIGURE 28 | Triassic-aged Limulitella species from France, Germany, and the Netherlands. (A) Limulitella bronnii from the Triassic-aged Grés á Voltzia Formation,
France. State Museum of Natural History Stuttgart specimen in Grauvogel collection, LIM 68. (B) Limulitella henkeli from the Triassic-aged Jena Formation, Germany.
(Continued)
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FIGURE 28 | Slg-TC-4/MLU.Fri1906.VII/5, holotype. (C) Limulitella sp. from the Triassic-aged Lower Wellenkalk Member, Muschelkalk, Netherlands. Specimen within
Oosterink private collection. (D) ?Limulitella sp. from the Triassic-aged Lower Muschelkalk, Netherlands, no specimen number. (A,B,D) Converted to gray scale. ?
denotes uncertain taxonomic assignment. Photo credit: (A) Dieter Seegis; (B) Norbert Hauschke; (C) Thomas König; (D) Martien Oosterink.
FIGURE 29 | Triassic-aged Limulitella species from France, Germany, Madagascar, and Tunisia. (A) Limulitella vicensis from the Triassic-aged Keuper Formation,
France. MAN 8240, holotype. (B) Limulitella tejraensis from the Triassic-aged Ouled Chebbi Formation, Tunisia. ZPAL V. a6/101, holotype. (C,D) ?Limulitella sp. from
the Triassic-aged Buntsandstein, Germany. (C) Exemplar 2 figured in Hauschke and Wilde (2008).(D) Exemplar 1 figured in Hauschke and Wilde (2008).(C,D)
Geologisch-Paläontologischen Instituts der Ruprecht-Karls-Universität Heidelberg specimens and associated with Ph.D. thesis No. 3R.8.34-4. Specimens are likely
lost as they were not found again in the collection. (E) Limulitella sp. from the Triassic-aged Sakamena Group, Madagascar. MSNMi11170, counterpart. ? denotes
uncertain taxonomic assignment. Photo credit: (A) Lukáš Laibl; (B) Bła˙
zej Bła˙
zejowski; (C,D) Permission to reproduce photographs granted by Norbert Hauschke; (E)
Giorgio Teruzzi.
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FIGURE 30 | Triassic and Jurassic Limulitella from Germany and Russia. (A) ?Paleolimulus sp., likely Limulitella sp., from the Triassic-aged Bernburg Formation,
Germany. HAU-WIL2000. (B,C) Unnamed specimen from the Triassic-aged Trochitenkalk Formation, Germany. (B) Part of specimen. NME 07-56a. (C) Counter-part
of specimen. NME 07-56b. (A) may have been lost. (B,C) May be lost (Hartmann pers. comms.). (D) Limulitella cf. liasokeuperinus from the Triassic-aged ?Exter
Formation Germany. SNSB-BSPG 1967 XVI 27. Note: holotype lost in World War II. (E) Limulitella volgensis from the Triassic-aged Parshinskaya Formation, Russia.
PIN 4048/7. (A–C) Converted to gray scale. ? denotes uncertain taxonomic or formation assignment. Photo credit: (A–C) Permission to reproduce photographs
granted by Norbert Hauschke; (D) Mike Reich; (E) Constantine Tarásenko.
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FIGURE 31 | Triassic-aged limulids from Germany, Spain, and Sweden. (A) Tachypleus gadeai from the Triassic-aged Alcover Limestone Formation, Spain. MGSB
19195, holotype. (B) Mesolimulus crespelli from the Triassic-aged Alcover Limestone Formation, Spain. MGSB 35088, holotype. (C) Tarracolimulus rieki from the
Triassic-aged Alcover Limestone Formation, Spain. MGSB M 262, holotype. (D) Limulidae gen. et sp. indet, previously Limulus kieri from the Triassic-aged
Muschelkalk Limestone, Germany. MB.A.0207. (E) Limulus nathorsti from the Triassic-aged Höör Sandstone, Sweden. SMNH Ar33179, holotype. (D) Converted to
gray scale. Photo credit: (A–C) Pedro Adserà; (D) Lorenzo Lustri; (E) Liping Liu.
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FIGURE 32 | Triassic-aged limulids from China and Europe. (A,B) Yunnanolimulus luopingensis from the Triassic-aged Member II, Guanling Formation, Luoping,
China. (A) LPI-61299, holotype. (B) Specimen displaying walking legs and book gills. LPI-61734. (C) Sloveniolimulus rudkini from the Triassic-aged Strelovec
Formation, Slovenia. PMSL T-993, holotype. (D,E) Limulidae gen. et sp. indet from the Triassic-aged Volpriehausen Formation, Germany. GPS. MLU 2018.23. (E)
Limulidae gen. et sp. indet from the Triassic-aged Bernburg Formation, Germany. GPS. MLU 2018.24. (F) Limulus priscus from the Triassic-aged Muschelkalk
Limestone, Germany. SNSB-BSPG AS I 939, holotype. (D,E) Converted to gray scale. Photo credit: (A,B) Shixue Hu; (C) Tomaž Hitij; (D,E) Permission to reproduce
photographs granted by Norbert Hauschke; (F) Mike Reich.
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FIGURE 33 | Jurassic-aged limulids from Poland, Russia, and UK. (A) Crenatolimulus sp. from the Jurassic-aged Kcynia Formation, Poland. ZPAL X.1/O-B/XA 13.B.
(B) “Limulus” darwini from the Jurassic-aged Kcynia Formation, Poland. ZPAL X.1O-BXA, holotype. (C) Limulus woodwardi from the Jurassic-aged Northampton
Sand Formation (?), England, UK. L8627, holotype. (D) Mesolimulus sp. from the Jurassic-aged Purbeck Limestone Group, England, UK. NHMUK PI. I. 3042. (E)
Mesolimulus sibiricus from the Jurassic-aged Talynzhansk Formation, Russia. PIN 3290-21, holotype. (A) Converted to gray scale. Photo credit: (A,B) Bła˙
zej
Bła˙
zejowski; (C) Russell Bicknell; (D) Lucie Goodayle, NHM, London; (E) Sergey Bagirov.
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FIGURE 34 | Examples of the iconic Jurassic-aged Mesolimulus walchi from Germany. (A–H, J–L) Specimens from the Solnhofen Limestone, Germany. (A)
MNHN.F.A33516. (B) TMP 1984.69.5. (C) YPM IP 9011. (D) SMNS 27585. (E) CM 28515. (F) USNM 706404. (G) MCZ 106368. (H) OUMNH F11569. (J) Specimen
preserving gut tract, YPM IP 8975. (K) SMNS 694513. (L) Specimen preserving gut tract, YPM IP 10183. (I) Specimen from the Nusplingen Plattenkalk, Germany,
SMNS 70204. Photo credit: (A) Lilian Cazes; (B,C,E–G,J,L) Russell Bicknell; (D,I,K) Guenter Schweigert; (H) Javier Ortega Hernández.
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FIGURE 35 | Cretaceous-aged limulids. (A,B) Victalimulus mcqueeni from the Korumburra Group, NSW, Australia. (A) Part, NMV P22410B, holotype. (B)
Counter-part showing appendage impressions, NMV P22410A. (C,G) Tachypleus syriacus from the Haqel Konservat-Lagerstätte, Lebanon. (C) NHMUK PI. OR.
59783, holotype. (G) Specimen showing possible sexual dimorphic trait of scalloped anterior cephalothorax, NHMUK PI. OR. 187. (D) Crenatolimulus paluxyensis
from the Glen Rose Formation, Texas, USA. (D) USNM 545241, cast of holotype. (E) Mesolimulus tafraoutensis from the Gara Sbaa Lagerstätte, Morocco. MSNM
i26844, holotype. (F) Limulus coffini from the Pierre Shale, Colorado, USA, USNM 129043, holotype. Photo credit: (A,B) Frank Holmes; (C,G) Stephen Pates; (D,F)
Russell Bicknell; (E) Giorgio Teruzzi.
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FIGURE 36 | Unnamed Permian xiphosurid, Limulus decheni and Valloisella lievinensis.(A,B) Unnamed xiphosurid from the Permian-aged Zechstein, Germany. (A)
Counterpart showing thoracetron, NMK D2.11b. (B) Part showing thoracetron and telson, NMK D2.11a. (C–E) Limulus decheni from the Eocene-aged Domsen
Sands, Germany. (C) 3D reconstruction of a surface scan, VET1931.1.MLU. (D) 3D reconstruction of a surface scan, GIE1863.1a.MLU, holotype. (E) Specimen with
part of telson preserved, MB.A.1901. (F) Valloisella lievinensis from the Carboniferous-aged Bickershaw Complex, England, UK; LL11133. Photo credit: (A,B) Peter
Mansfeld; (C,D) Permission to use 3D reconstructions granted by Lars Schimpf; (E) Andreas Abele; (F) Russell Bicknell.
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FIGURE 37 | Examples of extant male and female Tachypleus species. (A,B) Male T. tridentatus, YPM IZ 55603. (A) Dorsal view. (B) Ventral view. (C,D) Male T. gigas,
YPM IZ 55578. (C) Dorsal view. (D) Ventral view. (E,F) Female T. tridentatus, YPM IZ 55576. (E) Dorsal view. (F) Ventral view. (G,H) Female T. gigas, YPM IZ 103393.
(G) Dorsal view. (H) Ventral view. Photo credit: Russell Bicknell.
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FIGURE 38 | Examples of extant male and female Limulus polyphemus and Carcinoscorpius rotundicauda.(A,B) Male C. rotundicauda, YPM IZ 55595. (A) Dorsal
view. (B) Ventral view. (C,D) Male L. polyphemus, YPM IZ 55605. (C) Ventral view. (D) Dorsal view. (E,F) Female C. rotundicauda, YPM IZ 55574. (E) Dorsal view. (F)
Ventral view. (G,H) Female L. polyphemus YPM IZ 55601. (G) Ventral view. (H) Dorsal view. Photo credit: Russell Bicknell.
AUTHOR CONTRIBUTIONS
RB designed the study and made the figures, with input
from SP. RB and SP photographed material and wrote
the manuscript.
FUNDING
This research was supported by funding from an Australian
Postgraduate Award (to RB), a University of New England
Postdoctoral Research Fellowship (to RB), a Charles Schuchert
and Carl O. Dunbar Grants-in-Aid award (to RB), a James
R. Welch Scholarship (to RB), and an Alexander Agassiz
Postdoctoral Fellowship (to SP). There is no funding for open
access publication.
ACKNOWLEDGMENTS
We thank the following people for providing images of
specimens: Alexander S. Alekseev, Allan Lerner, Andreas Abele,
Bill Crighton, Bła˙
zej Bła˙
zejowski, Carrie A. Eaton, Carsten
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Bicknell and Pates Pictorial Atlas of Horseshoe Crabs
Brauckmann, Christian Neumann, Constantine Tarásenko,
David Barnes, David Marshall, Dieter Seegis, Dmitry E.
Shcherbakov, Dominique Chabard, Ewa Krzeminska, Frank
Holmes, G. Hundertmark, Giorgio Teruzzi, Graham Young,
Gregory Edgecombe, Guenter Schweigert, Hans Arne Nakrem,
Martien Oosterink, Javier Ortega Hernández, Jessica Utrup, Jessie
Cuvelier, Josh White, Kallie Moore, K. C. Kratt, Lars Schimpf,
Lilian Caze, Liping Liu, Lorenzo Lustri, Lucie Goodayle, Lukáš
Laibl, Mark Renczkowski, Markus Bertling, Markus Poschmann,
Matt Stimson, Mertová Eva, Michelle Coyne, Mike Howe, Mike
Reich, Monica Solorzano-Kraemer, Norbert Hauschke, Patrick
Smith, Paul Selden, Pedro Adserà, Peter Mansfeld, Rodney
Feldman, Russell Garwood, Serge Naugolnykh, Sergey Bagirov,
Shixue Hu, Tai Kubo, Thomas König, and Tomaž Hitij.
We thank the following people for their help with collections:
Albert Kollar, Andreas Abele, Andrew Ross, Angelika Leipner,
Brandon Strilisky, Bushra M. Hussaini, Conrad Labandeira,
Constantine Tarásenko, Cornelia Kurz, David Gelsthorpe, David
Holloway, Hans Arne Nakrem, Isabella von Lichtan, Jessica
Cundiff, Jessica Utrup, Jodie Francis, Julien Kimmig, Kacey
Page, Lisa Amati, Mark Florence, Matt Riley, Matthew McCurry,
Melanie Hopkins, Mertová Eva, Michelle Coyne, Mike Reich,
Susan Butts, Sylvain Charbonnier, Takenori Sasaki, Tatiana
Tolmacheva, Thomas Servais, and Yong-Yi Zhen.
Finally, we thank Allan Lerner, Bła˙
zej Bła˙
zejowski, Norbert
Hauschke, and Rachel Wade for their support and discussions
around this work and the editor PS and two reviewers for their
detailed input that thoroughly improved this work.
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Frontiers in Earth Science | www.frontiersin.org 60 July 2020 | Volume 8 | Article 98