Tiny individuals attached to a new Silurian arthropod suggest a unique mode of brood care

Derek E. G. Briggsa,1, Derek J. Siveterb,c, David J. Siveterd, Mark D. Suttone and David

Leggb

aDepartment of Geology & Geophysics, and Yale Peabody Museum of Natural History, Yale

University, PO Box 208109, New Haven, CT 06520-8109, USA

bOxford University Museum of Natural History, Oxford OX1 3PW, UK

cDepartment of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK

dDepartment of Geology, University of Leicester, Leicester LE1 7RH, UK

eDepartment of Earth Sciences and Engineering, Imperial College London, London SW7 2BP,

UK

Author contributions: DJS, DJS, DEGB and MS designed research and carried out fieldwork. DL

performed phylogenetic analyses. DEGB wrote the paper with scientific and editorial input from

the other authors.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission

1To whom correspondence should be addressed. E-mail: derek.briggs@yale.edu

This article contains supporting information online at www.pnas.org/lookup/suppl/….

PHYSICAL SCIENCES: Earth, Atmospheric, and Planetary Sciences

BIOLOGICAL SCIENCES: Evolution

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(Abstract) The ~430 myr old Herefordshire, UK, Lagerstätte has yielded a diversity of

remarkably preserved invertebrates many of which provide fundamental insights into the

evolutionary history and ecology of particular taxa. Here we report a new arthropod with 10 tiny

arthropods tethered to its tergites by long individual threads. The head of the host, which is

covered by a shield that projects anteriorly, bears a long stout uniramous antenna and a chelate

limb followed by two biramous appendages. The trunk comprises 11 segments, all bearing limbs

and covered by tergites with long slender lateral spines. A short telson bears long parallel cerci.

Our phylogenetic analysis resolves the new arthropod as a stem-group mandibulate. The

evidence suggests that the tethered individuals are juveniles and the association represents a

complex brooding behavior. Alternative possibilities - that the tethered individuals represent a

different epizoic or parasitic arthropod – appear less likely.

(Key words) arthropod/ brooding strategy/ Herefordshire Lagerstätte

Significance statement: The paper reports a remarkable arthropod from the Silurian

Herefordshire Lagerstätte of England.  The fossil reveals a unique association in an early

Paleozoic arthropod involving tethering of 10 tiny individuals each by a single thread to the

tergites so that their appearance is reminiscent of kites.  The evidence suggests that these are

juveniles and that the specimen records a unique brooding strategy. This is part of a diversity of

complex brooding behaviors in early arthropods heralding the variety that occurs today. The

possibility that the small individuals represent a different arthropod, possibly parasitic, which

colonized the larger individual, seems less likely.

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\body

(Introduction) Evidence of brooding in fossil arthropods is unusual and normally confined to

eggs and early juveniles: later stage juveniles are rarely encountered. Among the highlights

described from the Silurian Herefordshire Lagerstätte are ostracods preserving soft parts,

including evidence of a brooding strategy that persists today: eggs and possible early juveniles

are held within the space at the rear of the carapace (1). Here we report a new larger arthropod

from the same fauna, with smaller arthropods attached to the tergites by means of long threads.

These smaller individuals lie within or are associated with a cuticular capsule, the largest about 2

mm in length, with a gape through which the appendages emerged. They preserve evidence of

~6 pairs of appendages in contrast to 15 (four of them in the head) in the adult. The evidence

suggests that the attached individuals are juveniles which must have added segments during the

transition to an adult morphology, a strategy established in trilobites, eucrustaceans, pycnogonids

and other 'Orsten’ forms, and in short great appendage arthropods by the early Cambrian (2, 3,

4). If so the parent may be a female, although male brood care is known in arthropods (in

pycnogonids eggs are carried by the male, which is equipped with ovigers).

Results

Aquilonifer spinosus is a new genus and species of arthropod from the Herefordshire Lagerstätte,

a late Wenlock (mid-Silurian) volcaniclastic deposit in Herefordshire, U.K. (5, 6). It is

preserved, as are the other fossils from this Lagerstätte, in three dimensions as a calcitic void fill

in a carbonate concretion (7). The name of the new taxon refers to the fancied resemblance

between the tethered individuals and kites, and echoes the title of the 2003 novel The Kite

Runner by Khaled Hosseini (aquila: eagle or kite; -fer: suffix meaning carry; thus aquilonifer:

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kite bearer; spinosus: spiny, referring to the long lateral spines on the tergites). The material is a

single specimen, the holotype OUMNH C.29695, registered at the Oxford University Museum of

Natural History (Fig. 1; S1 video).

Diagnosis. Head shield with rostrum-like anterior projection, large uniramous antenna, chelate

limb and two other biramous appendages in the head, the last similar to those of the trunk;

elongate trunk with long, slender lateral spines on the 11 tergites, all trunk somites bearing limbs

of which all but the last are biramous; short telson and long cerci.

Description. The head shield is subtriangular in dorsal view (Fig. 1 A and J); the margins are

incompletely preserved. The posterior area is raised medially into a broad axial ridge which is

also present along the length of the trunk (Fig. 1 J and K). An anterior rostrum-like projection

extends forward and somewhat ventrally a distance similar to the length of the rest of the head

shield (Fig. 1 J and K). An apparent series of 4 or 5 short slender lateral spines near the base of

this projection are artefacts of preservation (Fig. 1 J). The sides of the head shield bear a paired

series of at least four long slender spines, projecting antero-laterally and curved convex dorsally

(Fig. 1 J). The spines increase slightly in length from anterior to posterior, and are similar in

morphology to those on the trunk tergites. A swelling in the axial area on the ventral side of the

head, which is aligned with the attachment of appendage 3, is interpreted as a hypostome (Fig. 1

B and C).

There is no evidence of eyes. The first three head appendages are morphologically differentiated

whereas the fourth appears very similar to those of the trunk (Fig. 1 B-D, and H).

Due to incomplete preservation proximally, and lack of information on the interior morphology

of the head, it is not possible to determine the sequence in which the first two head appendages

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insert. The relative position of antenna and chelate appendages in other Paleozoic arthropods,

however, suggests that the uniramous non-chelate appendage (the antenna) is anteriormost (8).

Head appendage 1 (green: Fig. 1 A, B, D, G, I, and J), as designated here, is uniramous,

antenniform and large. The right appendage is the better preserved (the reconstruction of the left

is incomplete distally). The angle of the slices (see Methods), subparallel to the length of the

appendage, makes the proximal part difficult to interpret but it may consist of 3 or 4 segments

similar in length to the more distal ones or, perhaps less likely, a long basal segment. The

appendage tapers gradually to a point. The individual podomeres are narrower proximally and

expand distally (Fig. 1 I) to a point about their mid length where they bear two short narrow

spines which project dorsolaterally relative to the orientation of the trunk; more spines may have

been present. The podomeres taper distally beyond the spines to their articulation with the next

podomere. Only the segments in the proximal half of the appendage are easy to enumerate –

neither spines nor podomere boundaries are evident more distally. Spine bases are evident on the

left appendage but not the spines themselves. Extrapolation suggests that the total number of

podomeres is about 25. This first appendage is about the same length as the body, including the

‘rostrum’ but excluding the cerci.

Head appendage 2 (pink: Fig. 1 A-D, K, and L) extends forward but not beyond the anterior

projection of the head shield. Subtle changes in direction along the length of the right limb

suggest that there may be as many as 5 proximal podomeres – but this is not certain (the slices

run along the length of the limb, rather than transverse to it, obscuring details). The appendage

terminates in a laterally directed swollen chela-like structure which terminates distally in two

slender curved finger-like projections. A poorly preserved laterally directed projection from near

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the base of appendage 2 (better preserved on the left example but difficult to reconstruct; Fig. 1

A and B) may represent a slender exopod but its nature is uncertain.

Appendage 3 (blue: Fig. 1 A-D, G, K, and L) is biramous. A broad basis expands adaxially and

may project into a gnathobase. It gives rise to an endopod which extends abaxially and then

curves axially so that the distal and proximal podomeres are subparallel. Geniculations suggest

the presence of 5 or 6 podomeres and a terminal spine. The exopod is much longer, more slender

and projects laterally. That on the right appendage bends sharply ventrally and curves outward

distally; it may end in a series of short podomeres (Fig. 1 L).

Appendage 4 (yellow-green: Fig. 1 B-D, G, H, K, and L) bears an endopod similar to that of

appendage 3, likewise with evidence of 5 or 6 podomeres. The exopod is evident on the left

side, where it is very short and projects just a short distance anteriorly (this ramus is

incompletely preserved, and has been lost on the right limb but the data available are consistent

with the morphology of the trunk appendage exopods).

The trunk consists of 11 divisions (tergites) of similar length; the first two and the last one are

slightly shorter than the rest (Fig. 1 A) which may reflect a gradient in growth rate along the

trunk axis (9). The trunk is near parallel sided, tapering markedly only in the last three tergites

(Fig. 1 M). Each tergite is comprised of a broad, gently convex axial ridge occupying about half

its width (excluding the long slender lateral spines) flanked by lateral areas which are slightly

concave dorsally (Fig. 1 J-L). Two short triangular lateral projections of trunk tergites 1 to 10

bear long slender spines, curved concave dorsally. These lateral spines are approximately evenly

spaced along the length of the trunk. Only the posterior spine is preserved on the left side of

tergite 10, and the anterior spine, together with a hint of the posterior one, on the right side.

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Tergite 11 appears to bear just one spine on each side which projects posteriorly (Fig. 1 M). The

boundaries between the tergites are marked by transverse grooves in the axial area (Fig. 1 J and

K). The position of the maximum height of a tergite lies progressively further posteriorly in

tergites 7-9 (Fig. 1 K).

The first trunk appendage (appendage 5, blue-green) is similar to the posteriormost appendage of

the head (Fig. 1 B-D and H). A broad basis expands adaxially; it may project into a gnathobase

but there is a significant gap between the opposing members of the pair here and in successive

limbs. The basal podomere gives rise to an endopod which extends abaxially and then curves

axially so that the distal and proximal podomeres are subparallel. Geniculations in the right

appendage suggest the presence of six podomeres and a terminal spine. The right appendage

preserves a short incompletely preserved exopod projecting forwards.

Trunk appendages 2-10 (appendages 6-14) are similar in morphology to the first trunk

appendage. They increase slightly in size to trunk appendage 6 and decrease slightly in the more

posterior appendages (Fig. 1 B and D). The basis projects adaxially and the right limb of trunk

appendage 7 preserves delicate spines. Left trunk appendages 8 and 9 preserve possible evidence

of segmentation in the endopod (Fig. 1 N). Four stout proximal podomeres are evident followed

by a distal section of apparently two podomeres (left trunk endopod appendage 9) terminating in

a slender claw (i.e., six podomeres + claw). The exopod is a long flat forward projection. The

orientation of the slices combined with indifferent preservation makes it appear filamentous but

its structure is unknown. This exopod is not evident in appendage 11 (this is unlikely to be a

preservational artefact as the exopod is clearly present in appendage 10).

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The trunk terminates in a small conical projection that extends beyond the last trunk tergite (Fig.

1 M). This projection (referred to here as the telson) bears a pair of slender parallel cerci which

are about three-quarters the length of the rest of the body (Fig. 1 A and G). It is unclear whether

or not these structures are annulated.

The gut is preserved as an impersistent sediment fill; it becomes visible dorso-medially in the

head because it lies too close to the head shield for the intervening material to be visualized (Fig.

1 A, J and K). The position of the anus is unknown.

The length of the body from the tip of the rostrum-like projection of the head shield to the

posterior margin of the telson is 9.5 mm. The large first antenniform appendage is about the

same length (9.5 mm) and the cerci are about 7.3 mm long.

Apart from its unusual morphology, the other remarkable feature of the arthropod is the

attachment of multiple individuals to the trunk tergites (Fig. 1 A, D, J, K, and L). These 10

individuals, which are best seen when the trunk limbs are removed (Fig. 1 J), are enumerated

clockwise in what follows starting from the anteriormost on the right side (Fig. 1 J). They are

shaped like flattened lemons. They consist of an outer ‘shell’ (here referred to as a capsule)

which does not appear to be calcified. The shell is generally ~15-20 µm thick where it is thinnest

(Fig. S1 B) but may be thicker in places perhaps as a result of soft tissue adhering to the inner

surface or the orientation of the capsule to the grinding plane. The capsule opens distally

exposing filamentous structures within. Some capsules, such as that of individual 3, show a

narrow ridge along one margin which may represent a kind of hinge (Figs. 1 J and S1 A). The

largest capsules (individuals 3,6,9) are about 2 mm in length (Fig. 1 J). In some cases the

filamentous internal structures are separated from the capsule, particularly in individuals 1 and 5

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(Fig. 1 J). The smallest capsules (individuals 4,10) are less than 0.6 mm long (Fig. 1 J). Thus the

capsules are characterized by a significant size range (the largest is ~4x the length of the

smallest). Most individuals preserve a mass of tissue associated with the capsule, and individuals

2, 3 and 5 in particular preserve evidence of multiple paired slender projections which represent

limbs (Figs. 1 E, F and J and S1) although the details are difficult to interpret due to their small

size relative to the spacing of slices. Individual 5, which is preserved outside its capsule, shows

at least six pairs, some of them evident as curved lines on a surface exposed during grinding

(Fig. 1 E and F, Fig. S1 B). The body extends and tapers beyond the obvious appendages through

a length similar to the appendage-bearing part. Individual 3 sits within its capsule and shows at

least three pairs of limbs projecting out of the gape (Fig. S1 C). The smallest capsules

(individuals 4 and 10) preserve hints of soft tissue within the capsule but no evidence of specific

structures.

Each capsule is borne by a slender flexible thread (funiculus) which originates where the capsule

tapers to a point. This proximal area of the capsule is thickened (Fig. S1 A, B). The thread

expands abruptly just beyond the capsule and tapers gradually to a long slender portion that

affixes to the host (Fig. 1 J). Some of the threads appear to be discontinuous (e.g., that of

individual 1), but this is interpreted as a reconstruction artefact. The threads vary in length from

about 1.5 mm (that of individual 5) to 3.3 mm (that of individual 9) (Fig. 1 J). The threads are

attached to the slender lateral spines on the tergites except for those of capsules 4 and 10 which

are attached to the main part of a tergite (Fig. 1 J).

Discussion

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Phylogenetic position of the new genus and species. The combination of characters in

Aquilonifer spinosus differs from that in any other known arthropod, living or fossil, and we

therefore assign it to a new genus and species. Aquilonifer shows some similarity to Artiopoda

but when added to the analysis of Legg et al. [(10), with minor modifications: see Methods] it

falls out as a stem-group mandibulate lying above the Marrellomorpha and below those ‘Orsten’

forms that cannot be placed in crown-group Crustacea (Fig. 2). Transposing the order of head

appendages 1 and 2 (see Description) yields longer trees (142.60540 steps versus 142.16612

steps) with a largely unresolved topology. Modes of development are coded in the phylogenetic

analysis (see Methods and citations therein) but more derived brooding strategies are very

diverse, particularly among Eucrustacea (11), and provide little constraint on phylogenetic

position.

The nature of the attached individuals. The very small size and consequent lack of detail

revealed by the grinding technique makes the individuals attached to Aquilonifer difficult to

interpret. However, their size and morphology are inconsistent with protozoan ciliates such as

peritrichs or with epiphytic algae. The outer covering of the capsules resembles a carapace that

encloses the body and opens at one extremity. The absence of a mineralized shell, and presence

of soft tissue beyond the capsule, together with the apparent symmetry, eliminates brachiopods.

The serially arranged paired structures within the capsules, about six in number (Fig. 1 E, F, Fig.

S1) and sometimes projecting out or separated from the capsule, represent segmented

appendages. Thus the evidence indicates that the attached individuals are arthropods.

Arthropods attached by a thread are likely to represent one of three possible strategies: they are

either parasites, epizoans, or brooded juveniles. Comparative behaviors are most readily sought

among living crustaceans because they are by far the most diverse group of aquatic arthropods

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today. Parasitic forms may retain appendages for a motile phase in the life cycle. Living

tantulocarids develop in a sac-like structure derived from the tantalus larva to which they are

connected by a kind of umbilical cord, the larva in turn attached to the host crustacean (12, 13).

Parasitic thoracican barnacles may retain cirri even though they feed by absorption through the

peduncle (14). A variety of parasitic copepods employ a system of rootlets, some threadlike, to

absorb nutrients from a variety of different hosts (13). The individuals attached to Aquilonifer,

however, are unlikely to be parasitic because there would be no advantage in such long threads

for absorbtion, and their most common attachment position, on the slender lateral spines of the

host, is not a favorable site for accessing nutrients.

The gape at the distal end of the capsules attached to Aquilonifer would have facilitated feeding

with the appendages. Among living epizoans thoracican cirripedes such as Octolasmis, which

infest larger crustaceans today (15), are similar to these attached individuals. Some thoracicans,

such as Pagurolepas which lives in association with hermit crabs, have reduced the calcified

plates that armor the capitulum (16). The threads that tether the capsules to Aquilonifer, however,

are much more slender and longer than the robust muscular peduncle of thoracican cirripedes.

The attached individuals are also different to the larval stages of the cirripede Rhamphoverritor

reduncus from the Herefordshire Lagerstätte which are about twice the size, even though they

represent developmental stages prior to attachment to a substrate (17). Given the potential for

diversification among arthropods, as exemplified by living crustaceans, the individuals attached

to Aquilonifer could represent an unknown type of epizoan; other lines of evidence, however,

argue against this possibility. Epizoans have been reported from the Herefordshire biota – on

brachiopods (18, 19) – but similar capsules to those described here have not been observed

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tethered to any other animal from the fauna. Furthermore the ‘host’ arthropod was clearly living

when the capsules became attached: it is unlikely to have tolerated the presence of so many drag-

inducing epizoans and head appendage 1 is long enough to have cleaned the trunk tergites

(‘general body grooming’ as in some living crustaceans: 20). Thus the attached individuals are

more likely to be juveniles.

Tethering of capsule-like structures containing tiny individuals is consistent with a brooding

strategy, albeit one with no exact parallel among living arthropods – it would have protected the

juveniles from predation by keeping them close to the parent. Attachment by a stalk occurs in the

embryos of freshwater crayfish (Astacida), for example, which are tethered to the adult (21, 22).

Some of the individuals attached to Aquilonifer show evidence of limbs: about six pairs are

evident in individual 5, for example (Fig. 1 E, F). The length of the body in individual 5 would

accommodate sufficient pairs to make up the number in the host: they may not be preserved or

have not yet developed fully. Release of the juveniles would have to have occurred within a

molt cycle of the adult, but this may have been extended to avoid them being discarded.

The size of the capsules varies from ~0.5 to 2.00 mm. A diversity of larval sizes is also known

in recent ostracods (23) and eggs and juveniles have been reported together in individuals of the

ostracod Nymphatelina from the Herefordshire Lagerstätte (1) and in ostracods from the

Ordovician Beecher’s Bed (24). Embryos brooded by the living crayfish Procambarus pass

through the earliest stages ‘rather synchronously’ whereas rates of development vary thereafter

so that Stage 3, 4 and 5 juveniles from the same batch may occur together under a mother’s

abdomen (21, p. 573). Similar patterns could explain the variation in size of the capsules attached

to Aquilonifer. Alternatively the range in size may indicate that the breeding adult

accommodated more than one generation by molting at long intervals.

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The correlation between the size of each capsule and the length of its thread is not statistically

significant (Figs. S2, S3, Table S1). The correlation becomes stronger (although it is only

significant at p < 0.10) when the thread length is augmented by the distance between its point of

attachment and the lateral margin of the trunk of the host (i.e., the base of the slender tergal

spines). Thus molting may have included lengthening of the thread in a manner similar to

epizoic thoracican barnacles (25) perhaps to improve access to particulate food (Table S1).

Mode of life. The morphology of the adult Aquilonifer provides limited evidence of mode of life.

The first head appendage shows a superficial similarity to that of the Cambrian arthropod

Kiisortoqia soperi from Sirius Passet, Greenland which is antenna-like but armed with paired

spines along its adaxial margin interpreted as ‘possibly suitable for capturing prey’ (26, p. 495).

The spines on the equivalent appendage in Aquilonifer, however, are relatively short, more

widely spread, and do not face adaxially. Furthermore the appendage in Aquilonifer tapers to a

slender extremity and does not appear suitable for a grasping function. This first appendage may,

in contrast, have been sensory or functioned in sweeping sediment in search of food. The second

appendage is chelate and presumably functioned in manipulating food.

Both right and left (in a less pronounced fashion), biramous limbs curve adaxially at their distal

extremity (Fig. 1 B and D). This position may be a response to burial. However, the segmented

nature and flexibility of the endopods suggest that they could have functioned as walking limbs.

Neither their morphology, nor that of the exopods, appear to be primarily adapted for swimming,

indicating that Aquilonifer was benthic. The basipods were weakly spinose, but there is no

evidence that they met in the midline. Food was presumably transferred directly to the mouth

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rather than transported anteriorly by the trunk limbs. The long cerci that project from the telson

were presumably sensory.

The juveniles would have operated at low Reynolds numbers and likely used movement of the

appendages to elevate them during feeding (27) rather than relying on forward locomotion of the

adult to generate lift. The vast majority of crustacean larvae, for examples, filter phyto- or

zooplankton from the surrounding water (28). It is less likely that the juveniles attached to

Aquilonifer were feeding on the sediment surface as there would be no obvious advantage in a

longer thread once the substrate was reached. Although the threads are preserved curving

ventrally none of them reaches below the appendages; their arrangement may be partly a result

of the parent being overwhelmed by sediment (capsules 7 and 8 are in a position where they

might impede movement of the trunk appendages). There is no evidence of the position of the

oviduct in Aquilonifer or how the arthropod transferred or attached the offspring to its dorsal

side. The long antenna may have been involved or one parent may have attached the eggs to

another.

Brooding in early arthropods. Evidence of parental care is rarely preserved in fossil taxa and is

largely restricted, as here, to brood care of eggs and early juveniles (1, 24, 29). All examples

reported to date in early Paleozoic arthropods involve protection within a bivalved carapace, a

strategy that evolved independently in bradoriids (30), Waptia (29) and myodocope ostracods (1,

24). Extended parental care (31) has yet to be clearly demonstrated in invertebrate fossils.

Analogues for brood care in aquatic arthropods today are found in crustaceans and pycnogonids.

Several strategies exist: enclosure by the thoracopods, by attaching eggs to the pleopods or

ovigers (in the case of pycnogonids), within a dorsal brood pouch, within a marsupium formed

14

by oostegites, and protection using an elongated first pleopod (32). The distribution of these

methods among crustaceans suggests that most or all of them may have evolved independently

(32). Brooding in pycnogonids is different in that the male rather than the female carries the

eggs. Aquilonifer adopted yet another strategy which includes a dorsal position and attachment

by a thread to a tergite. Among living crustaceans a dorsal position for the embryos is confined

to Thermosbaenacea, blind shrimp-like forms which live in caves and other underground systems

(33). Their dorsal brood pouch is formed from an extension of the carapace in the female and the

embryos are transported there by currents generated by the thoracopods, or transferred within a

membrane that subsequently dissolves. The embryos of Thermosbaenacea are free within the

dorsal brood pouch (33). The embryos of freshwater crayfish (Astacida) are tethered to the adult

by a stalk (21, 22). The egg cases are attached to the pleopods by a stalk secreted by cement

glands on the sternum and pleopods. When the hatchling emerges it remains tethered to the egg

case by a telson thread composed of the inner lining of the egg capsule. This maintains the

attachment to the parent until the hatchling can use the hooks on the first pereiopod to grip the

adult. In some crayfish an anal thread performs the same function as the telson thread. Thus

among the diversity of brooding strategies in living aquatic arthropods are devices analogous, but

very different, to that in A. spinosus.

Our interpretation of this remarkable specimen as representing 10 juveniles tethered to the parent

A. spinosus, combined with its phylogenetic position among early arthropods, indicates that a

complexity of brooding strategies evolved early in the history of the group.

Methods

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The holotype of Aquilonifer spinosus (OUMNH C. 29695) was ground at 30 µm intervals, in two

separate pieces. Surfaces were imaged digitally and image stacks used to generate a three-

dimensional ‘virtual fossil’ using the custom SPIERS software suite (www.spiers-software.org)

(34, 35). The virtual fossil (VAXML) was studied on-screen using the manipulation, virtual

dissection and stereoscopic-viewing capabilities of SPIERS. Images in Fig. 1 were rendered as

ray-traced virtual photographs using the open-source Blender package (www.blender.org). The

data are housed at the University Museum of Natural History, Oxford (OUMNH).

The holotype of Aquilonifer spinosus (OUMNH C.29695) was studied as an interactive virtual

model, in VAXML format. VAXML models (36) consist of a series of STL- or PLY-format files

describing morphology, together with an XML-based file providing metadata. They can be

imported into any 3-D graphics package that supports STL/PLY files, or more conveniently can

be viewed directly using the SPIERSview component of the freely available SPIERS software

suite.

In order to understand the affinities of Aquilonifer it was coded into the extensive phylogenetic

data set of Legg et al. (10), including subsequent modifications by Siveter et al. (37), and a

single additional character from Legg (38): the possession of an extensive posterior transverse

ridge on the trunk tergites, which was coded as present in some cheloniellids [see (38) supp. for

discussion]. This new data set of 315 taxa and 754 characters (Dataset S1) was analysed under

general parsimony in TNT v.1.1. (39). All characters were unordered and weighted using implied

weighting with a concavity constant of three. Tree searches employed 100 Random Addition

Sequences with Parsimony Ratchet (40), Sectorial Searches, Tree Drifting, and Tree Fusing (41).

Nodal support was measured using Symmetric Resampling (each search used New Technology

Searches with a change probability of 33 per cent) and is reported as GC values.

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ACKNOWLEDGMENTS. We thank the Natural Environmental Research Council (grant

NE/F018037/1), the John Fell Oxford University Press (OUP) Fund, the Leverhulme Trust (grant

EM-2014-068), and the Yale Peabody Museum of Natural History Invertebrate Paleontology

Division for support. C. Lewis provided technical assistance, and David Edwards and other staff

of Tarmac Western and the late R. Fenn facilitated fieldwork. We are grateful to E. Lazo-Wasem

for discussion and comments and S. McMahon for help in presenting the Supporting

Information. J.B. Solodow assisted in coining the taxon name. The paper benefited from

insightful suggestions offered by the two reviewers.

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Fig. 1. Holotype of Aquilonifer spinosus gen. et sp. nov., “virtual” reconstructions. (A) Dorsal

view, (B) ventral view with juveniles omitted, (C) ventral oblique view of right head appendages

and hypostome (stereo-pair), (D) ventral-oblique (stereo-pair), (E) juvenile 5, oblique view with

associated capsule, (F) juvenile 5, lateral view, (G) lateral view with juveniles removed, (H)

anterior-oblique view (stereo-pair) of posteriormost head appendage and anterior trunk

appendages showing exopods, (I) proximal part of antenna showing spines (stereo-pair), (J)

dorsal view without appendages (stereo-pair) with juveniles numbered as referred to in text, (K)

anterodorsal-oblique view, (L) anterior view (stereo-pair), (M) dorsal view of posterior of trunk

(stereo-pair), (N) anterior view of trunk limb 9 (stereo-pair). Abbreviations: ap, juvenile

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appendages; b, basis; c, claw; ca, capsule; e, endopod; g, gut; h1-4, head appendages; hy,

hypostome; t1-11, trunk appendages; t, telson; x, exopod. Numbers refer to trunk tergite,

attached juveniles, or appendage podomeres as appropriate. All scale bars 1 mm.

Fig. 2. Cladogram showing the phylogenetic position of Aquilonifer spinosus gen. et sp. nov. A

strict consensus of 12 Most Parsimonious Trees of 142.16612 steps (CI = 0.513; RI = 0.870),

produced using New Technology search options in TNT and utilizing implied character

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weighting with a concavity constant of three. Numbers above nodes are GC support values. 1 =

Euarthropoda (crown-group); 2 = Total-group Chelicerata; 3 = Artiopoda; 4 = Total-group

Mandibulata; 5 = Mandibulata (crown-group).

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