ELECTRONIC APPENDIX
 
ThisistheElectronicAppendixtothearticle 
 
Revisedsystematics ofPalaeozoic ‘horseshoe crabs’and themyth of
monophyleticXiphosura
 
by 
James C.Lamsdell
 
Comprisingthemorphologicalcharacterlistandcharactermatrixusedin the 
phylogeneticanalysis and the fullstrictconsensus tree with branch support
  Character list 
 
1. Cephalization in adult: 1+0 (0); 1+3 (1); 1+5 (2). In Fuxianhuia the head 
consists solely of the deuterocerebral (antennula) appendage. Most of the non-chelicerate 
euarthropods in the analysis possess a head incorporating the antennula plus three 
postantennular appendages. The euchelicerate prosoma consists of the antennula 
(chelicera) plus five postantennular appendages. Pycnogonids, despite possessing six 
postantennular appendages that are likely homologous to the appendages in the 
euchelicerate prosoma, only have three postantennular appendages incorporated into the 
head region. 
2. Anterior projection from carapace: absent (0); present (1). The synziphosurines 
Legrandella, Cyamocephalus and Pseudoniscus all have the prosomal carapace 
developed into an anterior angular projection, as does Leanchoilia. This character is 
inapplicable for pycnogonids, which do not possess an enlarged dorsal carapace. 
3. Segmentation in cephalon fully expressed: absent (0); present (1). Pycnogonids, 
without an enlarged dorsal carapace, retain the dorsal expression of their cephalic 
segmentation. This character is inapplicable for Fuxianhuia, which only incorporates a 
single segment into the head. 
4. Sagittal crease or keel of tergites of encephalized segments: absent (0); crest or 
crease (1); keel (2). Fuxianhuia displays a median crease in its head tergite, as does the 
carapace of the scorpion Palaeophonus. The xiphosurids Euproops, Paleolimulus and 
Limulus all have the head shield developed into a median keel.
15. Ancillary eyes: present (0); absent (1). Ancillary eyes are simple, median eyes 
found on a number of arthropods. In chelicerates they are termed the ocelli and are 
incorporated dorsally into the carapace. Simple ancillary eyes are also found in 
Fuxianhuia and Alalcomenaeus, where they are positioned between the deuterocerebral 
appendages. Ancillary eyes absent from Sidneyia and Olenoides 
6. Ancillary eyes incorporated dorsally into head shield: absent (0); present (1). The 
ancillary eyes are incorporated into the dorsal head shield in all chelicerates, where they 
form the ocelli. This character is inapplicable for taxa without ancillary eyes (character 
5). 
7. Unpaired median node: absent (0); between hypostome and doublure (1). An 
unpaired median node located ventrally on the cephalic region between the hypostome 
and doublure is present in Leanchoilia and Alalcomenaeus as well as Limulus and 
Paleolimulus and is absent in the extant arachnids. This character is unknown for the 
majority of other taxa. In Limulus this node bears light-sensitive sense organs. 
8. Lateral eyes: anteroventral (0); dorsal (1); absent (2). Lateral compound eyes are 
known from the majority of arthropods in the analysis, although they are absent in 
pycnogonids, Galeodes and potentially Offacolus while they are reduced to individual 
lenses in scorpions and Mastigoproctus. In euchelicerates and Olenoides the lateral eyes 
are positioned dorsally on the carapace, while in megacheirans, xenopods and Fuxianhuia 
they are located anteroventrally on stalks.
9. Visual surface of lateral eyes part of dorsal cuticle of head shield: absent (0); 
present (1). The visual surface of the lateral eyes is part of the dorsal cuticle in 
2chelicerates and trilobites, however in some taxa like Xandarella the eyes are 
incorporated in carapace slits and remain distinct from the carapace cuticle.
10. Eye shaded dorsally by palpebral lobe forming ophthalmic ridge: absent (0); 
present (1). An ophthalmic ridge is found in most synziphosurines and xiphosurids as 
well as a number of chasmataspidids including Octoberaspis. This character is 
inapplicable for taxa without dorsal lateral eyes (character 8).
11. Well-developed m-shaped ophthalmic ridge: absent (0); present (1). In 
xiphosurids the ophthalmic ridges merge anteriorly with the median ridge, forming a 
double arch shape. This morphology is also seen in Willwerathia. This character is 
inapplicable for taxa without anophthalmic ridge (character 10).
12. Extraophthalmic ridges: absent (0); present (1). Extraophthalmic ridges are 
located on the outer regions of the carapace in Limuloides and Bunodes. This character is 
inapplicable for pycnogonids. 
13. Interophthalmic ridges: absent (0); present (1). Interophthalmic ridgesare located 
on the inner regions of the carapace and are present inEuproops, Weinbergina, 
Venustulus, Camanchia and Legrandella. This character is inapplicable for pycnogonids.
14. Marginal rim: absent (0); independent of doublure (1); congruent with doublure 
(2). A marginal rim congruent with the doublure is seen in Olenoides. Euchelicerates 
possess a marginal rim independent of the doublure, with the exception of Offacolus, 
Weinbergina, Venustulus and Camanchia, which lack a marginal rim. 
15. Extensive (>10%of width of head shield) doublure in headshield: absent (0); 
present (1). An extensive doublure in the head is absent in xenopods and arachnids. This 
character is inapplicable for pycnogonids.
316. Transverse suture on cephalic doublure: absent (0); present (1). A transverse 
suture on the cephalic doublure is present in stylonurine eurypterids. This character is 
inapplicable for taxa without a doublure (character 15).
17. Connective sutures on cephalic doublure: paired (0); single median (1); absent 
(2). Paired connective sutures on the doublure are present in Fuxianhuia, Olenoides, 
Alalcomenaeus and Stoermeropterus. Connective sutures are absent in xiphosurids and 
synziphosurines where known. This character is inapplicable for taxa without a doublure 
(character 15). 
18. Cardiac lobe: absent (0); present (1). A cardiac lobe is present on the carapace in 
most euchelicerates, with the exception of arachnids and Offacolus. 
19. Cardiac lobe extends anteriorly beyond posterior half of carapace: absent (0); 
present (1). The cardiac lobe extends onto the anterior half of the carapace in 
Willwerathia, Kasibelinurus and xiphosurids. This character is inapplicable for taxa 
without a cardiac lobe (character 18).
20. Well-developed H-shaped cardiac lobe: absent (0); present (1). A H-shaped 
cardiac lobe is present in bunodids, pseudoniscids and Kasibelinurus. This character is 
inapplicable for taxa without a cardiac lobe (character 18).
21. Carapace width: wider than long (0); longer than wide or equal (1). The carapace 
is longer than wide or of equal width in Offacolus, Pasternakevia, pseudoniscids, 
eurypterids, Octoberaspis and arachnids. This character is inapplicable for pycnogonids.
22. Vaulted carapace absent (0); present (1). A vaulted carapace is present in 
synziphosurines and xiphosurids. This character is inapplicable for pycnogonids.
423. Vaulted tergopleura of the head completely covering appendages dorsally and 
laterally: absent (0); present (1). A carapace with the tergopleura completely covering 
the cephalic appendages dorsally and laterally is present in xiphosurids. This character is 
inapplicable for pycnogonids. 
24. Carapace genal spines: absent (0); present (1); reduced cornua (2). Genal spines 
are absent in eurypterids and arachnids. The genalspines of Weinbergina, Camanchia, 
Venustulus and Bembicosoma are reduced to blunt cornua. This character is inapplicable 
for pycnogonids. 
25. Posterior margin of carapace: flat (0); convex (1). The posterior margin of the 
carapace is convex in Cyamocephalus and Pseudoniscus. This character is inapplicable 
for pycnogonids. 
26. Configuration of fully ventral prehypostomal plate ('rostral plate'): section of 
doublure (0); complete fusion with doublure (1). In sampled eurypterids the 
prehypostomal plate has completelyfused with the doublure, with the exception of 
Stoermeropterus. This character is inapplicable for pycnogonids and arachnids, which 
lack a prehypostomal plate. 
27. Proboscis: absent (0); present (1).  A proboscis is present in pycnogonids. 
28. Presence of a hypostome as sclerotized plate: present (0); absent (1). The 
hypostome is present as a sclerotized plate in all taxa except pycnogonids and arachnids.
29. Composition of prehypostomal sclerite ('rostral plate'): with dorsal portion (0); 
only ventral ('rostral plate') (1); absent (2). The prehypostomal sclerite is visible dorsally 
in Fuxianhuia and is completely absent in pycnogonids and arachnids.
530. Number of segments incorporated into sternum: postantennular segments 2–3 (0); 
postantennular segments 2–4 (1). Limulus incorporates the sternites of postantennular 
segments 2–4 into the sternum, while other extant chelicerates along with Eurypterus 
incorporate only segments 2–3. This character is inapplicable for Galeodes, which lacks a 
sternum. 
31. Total expressed segment count: 28 (0); 15 (1); 17 (2); 16 (3); 13 (4); 10 (5); 12 
(6); 19 (7). Eurypterids, chasmataspidids, scorpions and Mastigoproctus express 
seventeen segments. Synziphosurines and xiphosurids, where known, express seventeen, 
as does Galeodes, Yohoia and Olenoides. 
32. Morphology of tergite of sixth postantennular segment: fully expressed (0); 
anteroposteriorly reduced (1). The tergite of the sixth postantennular segment is at least 
partially anteroposteriorly reduced in all euchelicerate taxa.
33. Reduced tergite of sixth postantennular segment: retained in reduced form or 
microtergite (0); complete loss (1). The tergite of somite VII is retained in various stages 
of reduction in most chelicerates but is completely lost in limulids. This character is 
inapplicable for taxa without a reduced sixth postantennular tergite (character 32).
34. Form of reduced tergite of sixth postantennular segment: dorsal tergite (0); 
subsumed under carapace (1). The tergite of somite VII is retained as a dorsal tergite in 
chasmataspidids and synziphosurines but appears to be subsumed under the carapace in 
eurypterids. This character is inapplicable for taxa without reduced sixth postantennular 
tergite (character 32) or that have completely lost the tergite (character 33).
35. Morphology of sternite of sixth postantennular segment: fully expressed (0); 
narrowing towards centre (1); medially divided (2). The sternite of somite VII is fully 
6expressed in most taxa. In stylonurine eurypterids it is narrowing towards its centre, 
becoming medially divided in eurypterine eurypterids. 
36. Morphology of tergite of seventh postantennular segment: undifferentiated (0); 
partially reduced (1); macrotergite (2); axially reduced, lateral portions forming free 
lobes (3). The tergite of somite VIII is partially reduced in eurypterids, Octoberaspis and 
arachnids. In Limuloides, Bunodes, Bembicosoma and Pasternakevia it is expanded into a 
microtergite, which in limulids it is axially reduced with the lateral portions forming the 
free lobes of the thoracetron. 
37. Trilobation of trunk: present (0); absent (1). Trilobation of the trunk is present in 
all taxa except pycnogonids. 
38. Ventral plate underlying tergites: absent (0); present (1). Chasmataspidids have a 
ventral plate underlying the tergites of the buckler. This character is inapplicable for 
pycnogonids. 
39. Morphological division of trunk: undifferentiated (0); limbless abdomen of 
segments with ankylosed tergites and sternites retaining tergopleurae (1); reduced 
abdomen (2). Fuxianhuia has a limbless abdomen of ankylosed segments, as does 
Yohoia, Sidneyia, euchelicerates and the pycnogonid Palaeoisopus. The remaining 
pycnogonids have a reduced abdomen. 
40. Anterior trunk segments fused into thoracetron: absent (0); segments VIII-XIV 
(1); segments VII-X (2). Xiphosurids have a thoracetron consisting primarily of trunk 
segments VIII–XIV while chasmataspidids possess a buckler made up of the first four 
segments. 
741. Pleural groove in anterior tergopleurae: absent (0); present (1). Pasternakevia, 
Limuloides, Bunodes and Kasibelinurus all have a pleural groove in their anterior 
tergopleurae. This character is inapplicable for pycnogonids, eurypterine eurypterids, 
xiphosurids and arachnids where the anterior tergopleurae are either fused or lost.
42. Tergopleurae of anterior tergites: tergopleurae expressed (0); tergopleurae 
reduced (1). The anterior tergopleurae are reduced in eurypterids and arachnids. This 
character is inapplicable for pycnogonids.
43. Form of tergopleurae on anterior tergites: angular (pointed termination) (0); 
quadrate (flat termination) (1). Venustulus and Camanchia both have quadrate anterior 
tergopleurae. This character is inapplicable for pycnogonids and any taxa where the 
anterior tergopleurae have been reduced (character 42).
44. Caudal abdomen defined by marked taper (undergoes a sudden constriction and 
then does not narrow further until the telson): absent (0); present (1). A constricted 
abdomen is present in Fuxianhuia, Rhenopterus and scorpions. This character is 
inapplicable for taxa without a limbless abdomen composed of ankylosed segments 
(character 39). 
45. Number of segments in constricted abdomen: 3 (0); 5 (1); 6 (2); 3+10 (3); 1 (4); 
2 (5); 9 (6). This character is independent ofthe number of limbless segments in the 
abdomen, instead marking the point of dorsal differentiation. Synziphosurines, 
Lunataspis, Yohoia and Mastigoproctus all have a constricted abdomen of three visible 
segments (although these segments are fused in Lunataspis). Pasternakevia and 
Kasibelinurus show only two segments, while in the remaining xiphosurids the 
abdominal segments have fused into one. Scorpions and most eurypterids have the last 
8five segments constricted. Chasmataspidids exhibit constriction of the last nine segments. 
This character is inapplicable for taxa without a limbless abdomen composed of 
ankylosed segments (character 39). 
46. Moveable lateral spines associated with opisthosomal segments IX-XIII 
(postantennular segments 8-15): absent (0); present (1). Moveable lateral spines are 
present in limulids and the eurypterid Stoermeropterus. 
47. Somites XV-XVII (postantennular segments 14-16) fused: absent (0); present (1). 
The segments of somites XV-XVII are fused in xiphosurids. 
48. Tergopleurae on posterior tergites other than pretelson: tergopleurae present (0); 
tergopleurae reduced (1); tergopleurae absent (2). The tergopleurae of the posterior 
tergites are reduced in Fuxianhuia, Yohoia, Sidneyia, Chasmataspis, most eurypterids and 
the synziphosurines Legrandella, Limuloides, Bembicosoma and Kasibelinurus. In 
Octoberaspis, arachnids, Eurypterus, Bunodes, Camanchia, Venustulus and the 
xiphosurids the tergopleurae are absent. This character is inapplicable for pycnogonids.
49. Posterior tergopleurae crescentic, with those of pretelson engulfing the proximal 
portion of the telson: absent (0); present (1). The posterior tergopleurae are crescentic in 
Leanchoilia and Alalcomenaeus as well as Pasternakevia, Pseudoniscus and 
Cyamocephalus. This character is inapplicable for pycnogonids and any taxa where the 
posterior tergopleurae are absent (character 48).
50. Main articulating device: anterior axial recess for arthrodial membrane (0); no 
obvious articulating device (1); articulating ridge and/or shelf functioning withnext 
anterior segment (2); articulating half-ring and furrow (3). Most taxa possess an 
articulating ridge, however Olenoides has true half-rings, Fuxianhuia possesses an axial 
9recess, and pycnogonids, Mastigoproctus and Galeodes show no obvious articulating 
device. 
51. Articulating ridge adaxially projecting as a pseudo-half-ring: absent (0); present 
(1). The articulating ridge is adaxially projected into a pseudo-half-ring in xiphosurids 
and Willwerathia. This character is inapplicable for any taxa without anarticulating ridge 
(character 50). 
52. Articulation of tergopleurae: overlap (0); edge-to-edge (1); gape (2). The 
tergopleurae of most chelicerates gape laterally, with the exception of Weinbergina, 
Camanchia, Venustulus and Offacolus where they overlap as in most other arthropods. 
The character is inapplicable for pycnogonids and arachnids where the tergopleurae are 
reduced. 
53. Articulating flanges on tergopleural region: absent (0); present (1). Articulating 
flanges are present in xiphosurids. 
54. Extensive doublure in trunk tergites: absent (0); present (1). Extensive doublure 
in the trunk tergites is present in Olenoides, xiphosurids and chasmataspidids. This 
character is inapplicable for pycnogonids.
55. Trunk width: narrowing from first segment (0); constant for first few segments 
(1). Among most synziphosurines and xiphosurids the trunk begins narrowing 
immediately from the first segment, however in bunodids and pseudoniscids the trunk 
width remains constant for a number of segments before narrowing, as it does in 
chasmataspidids, eurypterids and arachnids. 
56. Width of axis compared to cardiac lobe: wider, in line with ophthalmic ridges (0); 
same width as cardiac lobe (1); effaced (2). The axis is effaced in Willwerathia, 
10eurypterids and chasmataspidids, and in line with the cardiac lobe in xiphosurids, 
bunodids and pseudoniscids. In all other taxa the axis corresponds to the position of the 
ophthalmic ridges. This character is inapplicable for taxa without a cardiac lobe 
(character 18). 
57. Enlarged axial nodes: present (0); absent (1). Distinctive, enlarged axial nodes 
are found on the tergites of most synziphosurines and xiphosurids, as well as Offacolus. 
This nodes are lost in eurypterids, chasmataspidids and arachnids, and are also absent 
from bunodids and pseudoniscids, with the exception of Limuloides. This character is 
inapplicable for pycnogonids. 
58. Subaxial nodes: absent (0); present (1). Subaxial nodes are present in 
Weinbergina, Legrandella, Willwerathia, Lunataspis and Limuloides. This character is 
inapplicable for pycnogonids. 
59. Somites XII-XIII fused: absent (0); present (1). Somites XII–XIII are fused into a 
diplotergite in Pseudoniscus and Cyamocephalus. These tergites are also fused as part of 
the Offacolus pygidium and the xiphosurid thoracetron. This character is inapplicable for 
pycnogonids. 
60. Somites XIII-XIV fused: absent (0); present (1). Somites XII–XIII are fused into a 
diplotergite in Bunodes and Limuloides. These tergites are also fused as part of the 
Offacolus pygidium and the xiphosurid thoracetron. This character is inapplicable for 
pycnogonids. 
61. Gape in tergopleurae increasing posteriorly: absent (0); present (1). The lateral 
gape in the tergopleurae increases posteriorly in pseudoniscids and eurypterids. This 
11character is inapplicable for taxa that to not exhibit a lateral tergopleural gape (character 
52). 
62. Antennular articles elongated and as robust than podomeres of anterior 
endopods: absent (0); present (1). The individual antennular articles of Yohoia, 
Offacolus, Weinbergina and pycnogonids are elongated. This character is inapplicable for 
Pycnogonum which has reduced chelifores in its adult instar.
63. Number of aspiniferous segments in spiniferous short (6 or less segments) 
antennula: 1 (0); 2 (1). In megacheirans, pycnogonids and Offacolus there are two 
aspiniferous segments in the antennula. Larval instars of Pycnogonum, which retain the 
chelifores, also show this condition.. This character is inapplicable for taxa without a 
short antennula. 
64. Antennular peduncle: simple (0); bipartite (1). The antennular peduncle is 
bipartite in megacheirans and pycnogonids. All other taxa possess a simple peduncle. 
This character is inapplicable for Pycnogonum which has reduced chelifores in its adult 
instar. 
65. Number of articles forming fingers on antennula: four (0); three (1); two (2). 
Yohoia has four fingers, Leanchoilia and Alalcomenaeus three, and all included 
chelicerates two. This is inapplicable for taxa which do not have the spines on the 
antennula modified into fingers. 
66. Arrangement of spines on antennula: absent (0); biserial (1); mediodistally 
extended into fingers (2). Megacheirans and chelicerates have an armature of 
mediodistally extended fingers. 
1267. Number of antennular articles: 7–20 (0); ≤6 (1); >20 (2). Megacheirans and 
chelicerates have six or less antennular articles, Olenoides and the xenopods have more 
than twenty. 
68. Filiform antennula: absent (0); present (1). A filiform antennula is present in 
Olenoides, Emeraldella and Sidneyia. 
69. Exopod on second post-antennular limb: simple flap-like (0); bilobate flap-like 
(1); multiarticulate pediform (2); reduced (3). The exopod of somite III is a simple flap 
in megacheirans, pediform in Offacolus, a bilobate flap in Olenoides, and reduced in the 
other euchelicerates. This character is inapplicable for pycnogonids, which have 
completely reduced all their exopods.
70. Endopod of postantennular limbs 1–4: chela absent (0); chelate (1); subchelate 
(2). The endopods of pycnogonids are subchelate, while in xiphosurids and Offacolus 
they are chelate.  
71. Exopod on postantennular appendages 3–4: present (0); lacking (1). Exopods are 
lacking in all euchelicerates except Offacolus. This character is inapplicable for 
pycnogonids, which have completely reduced all their exopods.
72. Terminal podomere of endopod: conical podomere (0); spinose podomere (1). 
The terminal endopod podomere is conical in Fuxianhuia and pycnogonids and spinose 
in all other taxa. 
73. Terminal podomere of fifth postantennular limb: prong-like or blunt (0); chelate 
(1); part of pusher (2); subchelate (3). The terminal podomore of the endopod of somite 
VI is subchelate in pycnogonids, chelate in Offacolus and Chasmataspis, part of a pusher 
in Limulus and Paleolimulus, and prong-like or blunt in all other taxa.
1374. Exopod of fifth postantennular limb: as in postantennular limbs 3–n (0); 
flabellum (1); absent (2). The exopod of somite VI is absent in most chelicerates where 
known, but forms the flabellum in Offacolus, xiphosurids and possibly Chasmataspis. 
This character is inapplicable for pycnogonids, which have completely reduced all their 
exopods. 
75. Basipod of fifth postantennular limb expanded into dorsal 'ear': absent (0); 
present (1). The basipod (or coxa in traditional chelicerate terminology) of the sixth 
appendage in eurypterine eurypterids in dorsally expanded into an ‘ear’. This may 
represent the remnants of the exopod. This character is inapplicable in Fuxianhuia as the 
dorsal region of the proximal podomere is disconnected from the exopod.
76. Modified spine (podomere 7a) at the distal part of the 'basitarsus' of fifth 
postantennular limb: absent (0); present (1). A podomere 7a is found only in eurypterine 
eurypterids. 
77. Form of sixth postantennular limb: pediform (0); reduced, masticatory function 
(1); flap-like (2). The sixth postantennular limb is pediform in the majority of arthropods, 
including Weinbergina. In Offacolus it is large and flap-like, while in xiphosurids and the 
other euchelicerates it is reduced.
78. Fusion of sixth postantennular limbs: absent (0); present (1). In eurypterids and 
chasmataspidids the sixth postantennular limbs are fused to form the metastoma.
79. Limb-bearing segments enclosed by fully sclerotized rings: absent (0); fusion 
product of tergites, sternites, limb-bases (1). In pycnogonids all limb-bearing segments 
are fully enclosed within sclerotized rings, while other arthropods retain a separate tergite 
and sternite. 
1480. Body-limb joint: short, sclerotized, pivot-jointed rings (0); lightly sclerotized 
rings or half-rings in arthrodial membrane ['corm'] (1); arthrodial membrane only (2). 
The body-limb joint for Fuxianhuia and Palaeoisopus consists of sclerotized pivot-
jointed rings, while in the megacheirans, Olenoides and Emeraldella the joint consists of 
a series of lightly sclerotized half-rings in arthrodial memberane. In the remaining taxa 
the joint consists of arthrodial membrane only when known.
81. Basipod with elongate lateroproximal extension: absent (0); present (1). The 
basipod is extended dorsally into an elongate lateroproximal process in Sidneyia, Limulus 
and Paleolimulus. This character is inapplicable in Fuxianhuia as the dorsal region of the 
proximal podomere is disconnected from the exopod.
82. Endites on basipod: absent (0); single, medially drawn out endite (1); multiple 
endites [four to five] (2). Pycnogonids, Emeraldella and Fuxianhuia lack any endites on 
the proximal podomere. Olenoides, Sidneyia and the euchelicerates possess a single, 
medially drawn out endite; the remaining arthropods bear multiple endites.
83. Endites on basipod of first postantennular limb: absent (0); present (1). The 
euchelicerates have an enditic basipod for limb II, in contrast to the other arthropods in 
the analysis. This character is inapplicable for taxa that do not possess any basipod 
endites (character 82).  
84. Insertion of endopod on basipod: distally (0); laterally (1). The endopods of the 
majority of taxa insert distally on the basipod; the exceptions are eurypterids, 
chasmataspidids and arachnids, the prosomal limbs of which insert laterally. This 
character is inapplicable in Fuxianhuia as the dorsal region of the proximal podomere is 
15disconnected from the exopod so there is no way to resolve the orientation of the limb 
insertion. 
85. Moveable endite (epicoxa) dorsally on gnathal edge of basipod of prosomal 
appendages: absent (0); present (1). An epicoxa is present in Eurypterus and Limulus 
and absent in all other taxa where known. This character is inapplicable in Fuxianhuia. 
86. Maximum number of endopod podomeres: nine (0); eight (1); seven (2); six (3). 
Endopod podomere count does not include the basipod but does include the terminal 
spine. Megacheirans possess ten endopod podomeres, eurypterids eight. Arachnids, 
chasmataspidids and Weinbergina share seven podomeres with the xenopods and 
Olenoides. The xiphosurids only possess six podomeres. The podomere count in 
pycnogonids is uncertain as it is not clear how many podomores have fused with the body 
segments. This character is inapplicable in Fuxianhuia as it is unclear whether the 
basipod would be homologous to the first proximal podomere or whether the exopod and 
endopods fuse behind this to form a new distinct unit; without knowing this it is 
impossible to be sure that homologous podomeres are being included in the count.
87. First podomere with gnathal edge: absent (0); present (1). The first podomere 
bears a gnathal edge in Olenoides, the xenopods and Limulus. This character is 
inapplicable in Fuxianhuia as it is unclear whether the basipod would be homologous to 
the first proximal podomere or whether the exopod and endopods fuse behind this to form 
a new distinct unit; without knowing this it is impossible to be sure that homologous 
podomeres are being included in the count.
1688. Armature on distal margin of podomeres other than distal spines: absent (0); 
fringe of bristles (1); denticles (2). Megacheirans have a fringe of bristles around the 
distal margin of their podomeres, while eurypterids bear denticles in this position.
89. Presence of mediodistal spines on podomeres: absent (0); present (1). 
Mediodistal spines are present on the podomeres of most taxa, but are absent in 
Fuxianhuia, Pycnogonum, Euproops, Limulus, Mastigoproctus and Galeodes. 
90. Reduction of mediodistal spines on podomeres: not reduced (0); reduced on 
postantennular limbs 4 and 5 (1); reduced on postantennular limbs 3, 4, and 5 (2). 
Eurypterus and Parastylonurus have the mediodistal spines on limbs V and VI reduced, 
while Stoermeropterus and Rhenopterus show reduction on limbs IV, V and VI. All other 
arthropods show no reduction. This character is inapplicable for taxa lacking mediodistal 
spines (character 89). 
91. Presence of laterodistal spines on podomeres: absent (0); present (1). 
Laterodistal spines are present in Sidneyia, Olenoides and the pycnogonids. 
92. Laterodistal spines on podomeres: present on all podomeres (0); on non-enditic 
or two penultimate podomeres only (1). Laterodistal spines are present only on non-
enditic podomeres in Olenoides and Sidneyia. This character is inapplicable for taxa 
lacking laterodistal spines (character 91).
93. Morphology of distal spines on podomeres: socketed (seta type) (0); projections 
of podomeres (1). The distal spines are socketed in most taxa but projections of the 
podomeres in Pycnogonum, Stoermeropterus and Rhenopterus. This character is 
inapplicable for taxa lacking either laterodistal or mediodistal spines (characters 89 and 
91). 
1794. Spines or endites on median edges of podomeres other than the first: absent (0); 
present (1). Spines or endites on the succeeding podomeres are present in Emeraldella, 
Sidneyia, Olenoides, Haliestes and Palaeoisopus. 
95. Gradual change of stance of endopods from splayed anteriorly to dangling 
posteriorly: absent, all limbs dangling (0); rotation in proximal podomeres (1); rotation 
in basipod insertion (2); absent, all limbs splayed (3). There is no differentiation in 
megacheirans or chelicerates, which are coded based on their opisthosomal appendages. 
Emeraldella shows rotation in the proximal podomeres, while in Olenoides it is the 
basipod insertion that rotates. 
96. Reduction of count of podomere in first postantennular limb: absent (0); present 
(1). Emeraldella, Pycnogonum, Haliestes, xiphosurids, eurypterids and arachnids all 
show a reduction in podomere count of appendage II. This character is inapplicable for 
Fuxianhuia. 
97. Podomere count in endopod of first postantennular limb compared to max count 
in limbs: n-1 (0); n-2 (1); n-n (2); n-3–4 (3). The limb in Haliestes is reduced by two 
podomeres as is the limb in eurypterids, while the limb in Pycnogonum is completely 
lost. Xiphosurids and scorpions lose a single podomere; Galeodes and Mastigoproctus 
each lose three or four podomeres. This character is inapplicable for taxa that show no 
reduction in podomere count (character 96).
98. Exopods: present (0); absent (1). Exopods are present in all taxa except 
pycnogonids. Euchelicerates retain exopods in their opisthosomal limbs, and are thus 
coded as being present. 
1899. Exopod armature other than lamellae: none (0); setae or spines (1). Only 
arachnids possess no armature on their exopods. This character in inapplicable for taxa 
without exopods (character 98). 
100. Exopod of first postantennular limb: simple flap-like (0); multiarticulate, 
pediform (1); reduced (2). The exopod of somite II is a simple flap in Alalcomenaeus, 
pediform in Offacolus, and reduced in the other euchelicerates and Leanchoilia. This 
character is inapplicable for pycnogonids, which have completely reduced all their 
exopods. 
101. Podomere count in endopod of second postantennular limb reduced compared to 
max count in limbs: not reduced (0); reduced (1). The second postantennular limb is 
reduced in limulids, eurypterids, Galeodes and Mastigoproctus. This character is 
inapplicable for Fuxianhuia. 
102. Second postantennular appendage modified into ovigers: absent (0); present (1). 
Ovigers are found only in pycnogonids. 
103. Trunk limb medial fusion: separate (0); medially fused (1). The trunk limbs are 
medially fused in chasmataspidids, eurypterids and arachnids. This character is 
inapplicable for pycnogonids. 
104. Eighth postantennular limb fused with limb of genital segment (7th 
postantennular limb): absent (0); present (1). The limbs of somites VIII and IX are fused 
in eurypterids. This character is inapplicable for pycnogonids.
105. Endopods or limb axis of seventh to thirteenth postantennular limbs: present on 
all segments (0); present on seventh only (1); absent on all segments (2). Endopods are 
generally present on all trunk limb segments. Exceptions are chasmataspidids and 
19eurypterids, where only the limb of somite VIII is present, and arachnids where all limbs 
are reduced. This character is inapplicable for pycnogonids. 
106. Endopods or limb axis of seventh postantennular limbs fused medially into genital 
appendage: separate (0); fused (1). The endopods of somite VIII are fused into a genital 
appendage in chasmataspidids and eurypterids. This character is inapplicable for taxa 
where the endopod is reduced (character 105).
107. Gross morphology of exopods of postcephalic appendages: simple flap-like (0); 
bilobate flap-like (1). The exopods of Olenoides and Emeraldella are bilobate. This 
character is inapplicable for pycnogonids.
108. Lamellar blades on exopod: absent (0); present (1). Lamellar blades are present 
on the exopods of Sidneyia, Emeraldella and Olenoides. This character is inapplicable for 
pycnogonids. 
109. Structure of caudal appendages: uniramous paddle (0); filiform cerci (1); absent 
(2). The caudal appendages are absent from most taxa, but in Olenoides they form 
filiform cerci and are uniramous paddles in Fuxianhuia, Sidneyia and Emeraldella. 
110. Form of telson: plate-like (0); styliform keeled (with triangular cross-section) (1); 
flagelliform (2); expanded with venom sack (3); styliform tubular (with circular or 
oval/flat cross-section) (4); reduced (5). The telson is plate-like in Fuxianhuia, 
megacheirans , Sidneyia and Olenoides. It is expanded with a venom sack in scorpions, 
and styliform with a flat cross-section in Emeraldella. The remaining chelicerates possess 
a styliform telson with a triangular cross section with the exception of Mastigoproctus, 
which has a flagelliform telson and Haliestes, Pycnogonum and Galeodes, which have a 
reduced telson. 
20111. Telson fused with terminal segment (pretelson): absent (0); present (1). The 
telson is fused to the pretelson in Olenoides and Palaeoisopus. This character is 
inapplicable for taxa in which the telson is reduced (character 110).
112. Telson with joints along its length: absent (0); present (1). Emeraldella possesses 
a jointed telson that retains its styliform shape andtherefore is not flagelliform.  
Offacolus also has at least one joint along its telson. This character is inapplicable for 
taxa in which the telson is reduced (character 110).
113. Cuticular terrace lines: absent (0); present (1). Cuticular terrace lines are present 
in Stoermeropterus, Eurypterus and Olenoides, however their absence in other taxa may 
be preservational. 
114. Tuberculate ornamentation: absent (0); present (1). Bembicosoma and Bunodes 
possess a distinct ornament of dense, peg-like tubercles that are unlike the pustulation 
seen in other taxa.
21Character matrix
Taxon 
 10 
70 
 20 
80 
 30 
90 
 40 
100 
 50 
110 
 60 
Fuxianhuiaprotensa 
00-10 0?00- -0001 000-- 00000 0000? 00--? 00010 00013 00100 -0001 -1000 
-0-0- 000?0 ?0??- ?0000 -0--- --00- 0--0? --0?? -0000 00000 0000  
Leanchoiliaillecebrosa
1100? ?100- -0001 0?0-- 00000 ??0?? 10--? 00000 000-- 00012 000?1 -1000 
00111 21000 0100? 00001 02?00 00110 0-?00 ??012 ?0000 000?0 00?0  
Alalcomenaeuscambricus 
10000 0100- -0001 000-- 00000 0001? 10--? 00000 000-- 00?1? ?0001 -1000 
00111 21000 0100? 00001 02?00 00110 0-000 ??010 00000 00020 0000  
Yohoiatenuis
1000? ??00- -00?0 --0-- 00000 ????? 20--? 00010 00000 00102 02001 -1000 
-1110 21000 0100? ?0?0? ???0? ???10 0-0?0 ??01? ?0000 00020 00?0  
Emeraldellabrocki
?0001 -??0- -0000 --0-- 00000 ?001? 30--? 00000 000-- 00002 00001 -1000 
00-0- 121?0 01000 00001 00000 21010 0-011 1?01? ?0000 01104 01?0  
Sidneyia inexpectans
?0001 -000- -0000 --0-- 00000 ?0??? 40--? 00010 00000 00102 00001 -1000 
-0-0- 121?0 ?1000 00002 11?00 21010 11013 ??01? ??000 00100 00?0  
Olenoidesserratus
10001 -0111 00021 000-- 00010 0001? 20--0 00000 000-- 00003 -1111 -1000 
00-0- 02110 01000 00001 01000 21010 11012 0-01? 00000 01110 1010  
Pycnogonum litorale 
1-100 1020- ---0- --0-- ----- -1120 50--0 01-20 ---0- 00--1 --0-- ----- 
--0-1 210-2 -03-0 00012 ?00?0 ?000- 10103 121-- 01--- ----5 --00  
Haliestesdasos
1-10? ??20- ---0- --0-- ----- -112? 50--0 01-20 ---0- 00--1 --0-- ----- 
-1112 210-2 -03-0 0001? ?0??0 ?0??? 10013 101-- 01--- ----5 --?0  
Palaeoisopusproblematicus 
1-100 1?20- ---0- --0-- ----- -112? 60--0 01-10 ---01 00--1 --0-- ----- 
-1112 210-2 -03-0 00011 ?00?0 ?0010 10013 0-1-- ?1--- ---21 10?0  
22Offacoluskingi
2002? ??20- -000? ??0-0 10010 ?0?1? 11??0 00010 0000? 0-00? ?0??0 -0011 
01102 21021 01110 0200? ????? ????? ????0 ??0?1 ?000? 0002? 01?0  
Weinberginaopitzi
2000? ????1 00101 0?100 01020 ???1? 21??? 00010 00000 00002 000?0 00100 
01?02 21?30 110?? 0000? 0110? 2??10 ????0 0-012 00?0? 00021 00?0  
Camanchiagrovensis 
2000? ????? ?0101 02100 01020 ???1? ????? 00010 00100 002-2 00000 01000 
-000? ????? ????? ?1?0? ????? ????? ????? ????? ????? ????? 0??0  
Venustuluswaukeshaensis
20000 1???1 00101 0??00 01020 ???1? ????? 00010 00100 002-2 00?00 01000 
-000? ???3? 1???? 01?0? ????? ????? ????0 ????2 ???0? ????1 00?0  
Willwerathialaticeps
?0000 1???1 1001? ??110 0??10 ???1? ?100? 00010 00000 00002 12000 20100 
0???? ????? ????? ???0? ????? ????? ????? ????? ????? ????1 0000  
Legrandellalombardi
?1000 1?111 00111 0?100 01?20 ?0??? 2100? 00010 00000 00102 02000 00100 
0???? ????? ????? ????? ????? ????? ????? ????? ????? ???21 00?0  
Bembicosomapomphicus 
?000? ????? ?001? ??101 01?20 ????? ?100? 20?10 00000 0010? 020?1 01000 
0???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?1  
Bunodeslunula 
?000? 1???? ?100? ??101 01?10 ????? ?100? 20?10 10000 002-2 020?1 01001 
-???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?1  
Limuloideslimuloides
?0000 1???1 0101? ??101 01?10 ????? 21??? 20010 10000 00102 020?1 00101 
0???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?0  
Pasternakeviapodolica 
?000? ????? ?001? ??101 11?10 ????? ?100? 20010 10005 0?012 020?1 01000 
0???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?0  
Cyamocephalusloganensis 
?1000 ??111 000?? ??101 11?11 ????? ?1??? 00010 00000 00012 020?1 01010 
1???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?0  
Pseudoniscusroosevelti
?100? ????1 0001? ??101 11?11 ????? ?1??? 00010 00000 0001? 020?1 11010 
1???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?0  
23Kasibelinurusamicorum 
?000? ??111 0001? ??111 01?10 ????? ?1??? 00010 10005 00102 020?0 10000 
0???? ????? ????? ????? ????? ????? ????? ????? ????? ????1 00?0  
Euproopsanthrax 
20020 1?111 10111 0?110 01110 ?0?1? 21??? 00011 -00-4 01002 12?10 10011 
-0?0? ??031 11?1? 0??0? ????? ???0- 0--0? ????2 ?0??? ???21 0000  
Lunataspisaurora 
?0000 1?111 ?001? ??110 01110 ???1? ?1??? 00?11 -0000 012-2 121?0 10111 
-???? ????? ????? ???0? ????? ????? ????? ????? ????? ????1 00?0  
Limuluspolyphemus
20020 11111 10011 02110 01110 10011 211-0 30011 -00-4 112-2 12110 10011 
-0002 21031 11211 01002 11101 3100- 0--00 10012 10000 00021 0000  
Paleolimulussignatus 
2002? ?1111 10111 02110 01110 ?0?11 211-0 30011 -00-4 112-2 12110 10011 
-0?0? ??031 11211 0100? 1???? 3???? ????0 100?2 10?0? 00021 0000  
Stoermeropterusconicus 
2000? ??110 -0011 00100 10000 0001? 71??2 10010 -1-01 10102 02001 21000 
10?02 21030 11021 11102 0111? 10?12 0-100 110?2 10111 10021 0010  
Eurypterus
tetragonophthalmus 
20000 1?110 -0011 01100 10000 10010 71012 10010 -1-01 002-2 0-0-1 21000 
-0002 21030 11021 11102 01111 10211 0-000 11012 10111 10021 0010  
Rhenopterusdiensti
20000 1?110 -0011 11100 10000 1001? 71??1 10010 01-12 00102 02001 21000 
10?02 21030 11020 01102 0111? 10?12 0-100 110?2 10111 10021 0000  
Parastylonurusornatus 
2000? ??110 -0011 11?00 10000 1001? 71??1 10010 01-01 00102 02001 21000 
10?02 21030 11020 0110? 0111? 10211 0-000 1?0?2 ?0111 10021 00?0  
Chasmataspislaurencii
?0000 1?11? ?00?? ??100 00?10 ????? 7100? ?0112 00006 00102 ?-0?1 ?1000 
1???? ????? ??111 0??0? 01?1? 2000- 0--0? ??0?? ????? ???2? 00?1  
Octoberaspisushakovi
20000 1?111 000?1 0?100 10010 ????? 71000 10112 00006 002-2 0-011 21000 
-???? ????0 ?10?? 1110? ????? ??00- 0--00 ??0?? ?0101 10?21 0001  
Centruroidesvittatus
20000 10110 -0010 --0-- 10000 -0120 71010 10010 -1-11 002-2 0-0-1 -1000 
-0002 21030 11020 01102 0001? 20010 0-000 10002 00102 -0023 0000  
24Palaeophonusnuncius 
20010 1?110 -00?0 --0-- 10000 ?0??? 7???0 10?10 -1-11 002-2 0-0-1 -1000 
-0?02 21?30 110?? 01?0? ????? 2?010 0-000 100?2 00102 -0023 0000  
Mastigoproctusgiganteus
20000 10110 -0010 --0-- 10000 -0120 7???0 10010 -1-00 002-1 0-0-1 -1000 
-0002 21030 11020 01102 0001? 2000- 0--00 13002 10102 -0022 0100  
Galeodesarmeniacus
[1,2]0000 1020- -0010 --0-- 10000 -012- 2???0 10010 -1-0? 002-1 0-0-1 -1000 
-0002 21030 11020 0??02 0001? 2000- 0--00 13??2 10??2 -??25 --00  
 
25Haliestes dasos
Willwerathia laticeps
Octoberaspis ushakovi
Kasibelinurus amicorum
Stoermeropterus conicus
Bunodes lunula
Palaeoisopus problematicus
Palaeophonus nuncius
Olenoides serratus
Camanchia grovensis
Weinbergina opitzi
Centruroides vittatus
Pycnogonum litorale
Alalcomenaeus cambricus
Venustulus waukeshaensis
Rhenopterus diensti
Fuxianhuia protensa
Bembicosoma pomphicus
Pseudoniscus roosevelti
Yohoia tenuis
Paleolimulus signatus
Parastylonurus ornatus
Mastigoproctus giganteus
Cyamocephalus loganensis
Leanchoilia illecebrosa
Sydneyia inexpectans
Lunataspis aurora
Legrandella lombardi
Euproops anthrax
Eurypterus tetragonophthalmus
Limulus polyphemus
Pasternakevia podolica
Offacolus kingi
Emeraldella brocki
Limuloides limuloides
Chasmataspis laurencii
Galeodes armeniacus
91 (81)
3
26 (24)
1 98 (97)
4
100 (99)
9
54 (50)
1
36 (30)
2
85 (71)
3
48 (42)
3
7 (0)
1
83 (83)
2
90 (87)
2
11 (2)
1
84 (67)
3
27 (8)
1
20 (8)
2
85 (78)
2
85 (84)
2
91 (82)
6
85 (69)
4
89 (78)
5
81 (75)
2
96 (92)
4
66 (41)
2
58 (45)
1
22 (18)
1
46 (41)
2 86 (80)
4 44 (41)
1 90 (88)
2
Supplementary figure 1:
Strict consensus of 12 most parsimonious trees.
CI = 0.580, RI = 0.768, RC = 0.445.
Numbers beneath the nodes are Bremer support
values. Jacknife values (33% deletion, 1000
repetitions) are shown above the nodes with
Bootstrap values (50% deletion, 1000 repetitions)
shown in parentheses.