Earth's oldest 'Bobbit worm' - Gigantism in a Devonian eunicidan polychaete

Abstract

Whilst the fossil record of polychaete worms extends to the early Cambrian, much data on this group derive from microfossils known as scolecodonts. These are sclerotized jaw elements, which generally range from 0.1–2 mm in size, and which, in contrast to the soft-body anatomy, have good preservation potential and a continuous fossil record. Here we describe a new eunicidan polychaete, Websteroprion armstrongi gen. et sp. nov., based primarily on monospecific bedding plane assemblages from the Lower-Middle Devonian Kwataboahegan Formation of Ontario, Canada. The specimens are preserved mainly as three-dimensional moulds in the calcareous host rock, with only parts of the original sclerotized jaw walls occasionally present. This new taxon has a unique morphology and is characterized by an unexpected combination of features seen in several different Palaeozoic polychaete families. Websteroprion armstrongi was a raptorial feeder and possessed the largest jaws recorded in polychaetes from the fossil record, with maxillae reaching over one centimetre in length. Total body length of the species is estimated to have reached over one metre, which is comparable to that of extant 'giant eunicid' species colloquially referred to as 'Bobbit worms'. This demonstrates that polychaete gigantism was already a phenomenon in the Palaeozoic, some 400 million years ago.

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Scientific RepoRts | 7:43061 | DOI: 10.1038/srep43061
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Earth’s oldest ‘Bobbit worm’ –
gigantism in a Devonian eunicidan
polychaete
Mats E. Eriksson1, Luke A. Parry2,3 & David M. Rudkin4
Whilst the fossil record of polychaete worms extends to the early Cambrian, much data on this group
derive from microfossils known as scolecodonts. These are sclerotized jaw elements, which generally
range from 0.1–2 mm in size, and which, in contrast to the soft-body anatomy, have good preservation
potential and a continuous fossil record. Here we describe a new eunicidan polychaete, Websteroprion
armstrongi gen. et sp. nov., based primarily on monospecic bedding plane assemblages from the
Lower-Middle Devonian Kwataboahegan Formation of Ontario, Canada. The specimens are preserved
mainly as three-dimensional moulds in the calcareous host rock, with only parts of the original
sclerotized jaw walls occasionally present. This new taxon has a unique morphology and is characterized
by an unexpected combination of features seen in several dierent Palaeozoic polychaete families.
Websteroprion armstrongi was a raptorial feeder and possessed the largest jaws recorded in polychaetes
from the fossil record, with maxillae reaching over one centimetre in length. Total body length of the
species is estimated to have reached over one metre, which is comparable to that of extant ‘giant
eunicid’ species colloquially referred to as ‘Bobbit worms’. This demonstrates that polychaete gigantism
was already a phenomenon in the Palaeozoic, some 400 million years ago.
Polychaete worms comprise one of the most abundant and diverse invertebrate groups in modern oceans, occu-
pying habitats from beach environments to abyssal plains1. ey range from millimetre-sized parasitic, pelagic
and interstitial forms to gigantic benthic representatives measuring some metres in total body length1,2. ey
vary considerably in mode of reproduction and feeding habits, encompassing sessile lter feeders through agile
hunters1–3.
Despite their primarily so anatomy and resulting low preservation potential4, the fossil record of polychae-
tes extends to the beginning of the Palaeozoic. Recorded occurrences include a wide array of preservational
modes, ranging from whole bodies with so tissues, through abundant calcareous tubes of serpulids from the
mid-Triassic or putatively the Permian onwards5, to isolated sclerotized jaws (scolecodonts) and jaw apparatuses.
e oldest stem group annelids are of early Cambrian age, with full body carbonaceous compression fossils known
from the Sirius Passet Lagerstätte of Greenland6–9, and the Guanshan biota in China10. e best-known and most
abundant Cambrian polychaetes, however, are from the Burgess Shale Lagerstätte of British Columbia11,12.
Despite their relative rarity compared to many other fossil groups, the fossil record unambiguously shows that
polychaete annelids, like today, were an abundant and diverse group of invertebrates in ancient oceans. Crucially,
the fossil record reveals long extinct body plans, such as the armoured machaeridians13, and shows that some
extant groups had a much higher diversity in the past. e latter is the case for polychaetes of the order Eunicida,
in which there are 15–20 known fossil families compared with only seven today1,14. is diversity is demon-
strated by scolecodonts, which can be extracted in large numbers from ancient sea oor sediments, particularly
by acid maceration of rocks. As well as Eunicida, sclerotized jaws are also produced by the Phyllodocida, and in
both groups these are utilized in feeding and in manipulating the environment (e.g., excavating burrows in the
substrate). Despite Eunicida and Phyllodocida being closely related (and probably sister taxa), based on analyses
using morphological15 and molecular16 data, the jaws of the two groups function dierently; they also dier in
1Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden. 2Life Sciences Building,
University of Bristol, 24 Tyndall Avenue, Bristol BS8 1TH, UK. 3Department of Earth Sciences, Natural History
Museum, Cromwell Road, London SW7 5BD, UK. 4Department of Natural History, Royal Ontario Museum, 100
Queen’s Park, Toronto, Ontario, M5S 2C6, Canada. Correspondence and requests for materials should be addressed
to M.E.E. (email: mats.eriksson@geol.lu.se)
Received: 14 November 2016
Accepted: 17 January 2017
Published: 21 February 2017
OPEN

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composition1,17,18, grow continuously in Phyllodocida and by moulting in Eunicida19, and consequently are prob-
ably not homologous.
Whereas phyllodocidans rst appeared in the Middle Devonian or possibly earlier, and have a rather mea-
gre fossil record, eunicidans are present in the latest Cambrian and are common from the Middle Ordovician
onwards20. Eunicidan scolecodonts are particularly well-known in marine microfossil assemblages of the
Early-middle Palaeozoic14,20–26.
Although Ordovician and Silurian jawed polychaete faunas are better known than those of the remainder of
the Palaeozoic24, the Devonian seems to have been an important time in polychaete evolution. In addition to the
putative appearance of phyllodocidans, new characteristic eunicidan forms appeared, such as kielanoprionids
and their allies8,22,26–28.
Here we describe and discuss an extraordinary new taxon from the Devonian of Canada. e jaws of this
taxon achieve a remarkable size and are the largest known fossil scolecodonts from any time period. Investigation
of the relationship between body and jaw size suggests that this animal achieved a body length in excess of a
metre and thus is the largest known fossil eunicidan annelid and represents a unique case of ancient polychaete
gigantism.
Geology and Age
The study material is of Late Emsian-Early Eifelian (Early-Middle Devonian) age and derives from the
Kwataboahegan Formation29–32 of the Hudson Platform, Hudson Bay Basin (Moose River Satellite Basin)
Ontario, Canada. All specimens were collected at Rabbit (Askaskawayau) Ridge, approximately 11 km southeast
of Moosonee townsite, centred on N51°11′ W080° 30′ (Canada NTS 1:50 000 42 P/2: Bushy Island, Cochrane
District). e area was accessed by helicopter from Moosonee and limited surface sampling was undertaken over
several hours on a single day. Less than 0.5 m of section are exposed at this isolated outcrop, which is developed
in shallow gravel extraction scrapes exploiting Holocene raised beach ridges. Excavation has revealed glacially
scoured bedding planes in limestones comprising thin- to medium-bedded, peloidal bioclastic wackestones
to packstones with fossiliferous bioclastic grainstone interbeds. e Rabbit Ridge grainstones yield a diverse,
well-preserved assemblage of trilobites representative of the Paciphacops-Proetid Association recognized at other
Kwataboahegan Formation sections in the Moose River Basin33. Overall, the lithologies and fossils here are simi-
lar to those in the upper 3 m of a 10 m section in the Labelle Quarry south of the Moosonee townsite, about 15 km
to the southwest34.
Kwataboahegan Formation is the most conspicuously fossiliferous Devonian unit in the Moose River Basin
and contains a macrofauna dominated, in abundance and diversity, by rugosan and tabulate corals, stromatoporo-
ids, brachiopods, and molluscs35, with trilobites comprising an important secondary component33. e docu-
mented microfossil record includes spores36, acritarchs37, and conodonts29, used also for age determinations.
Scolecodonts are noted, but not identied, in earlier regional reports35 and more recently in studies on thermal
maturation38.
Deposition of the largely subtidal carbonate succession took place in a shallow shelf and, in the lower part of
the unit, coral-stromatoporoid biostromes and bioherms are developed locally over topographic highs that reect
underlying basement relief. Away from these relatively massive skeletal build-ups, the Kwataboahegan is generally
thinner bedded and more bituminous.
Materials and Methods
The scolecodont material is preserved as bedding plane specimens and includes three rock slabs with
semi-articulated clusters. e specimens are preserved in a pale beige-greyish mud/wackstone host rock in
which cm-sized rugosan corals also occur. Most jaw elements are preserved as (negative) moulds whereas in
some specimens parts of the original organic jaw walls are still present. Many specimens show some post mortem
deformation, and where original jaw walls are preserved those are usually brittle and fragmented. However, the
assemblages seem to have undergone relatively little post mortem transport; some paired elements are preserved
together or in close proximity, which also facilitates assessment of general jaw apparatus architecture.
e material is housed in the Invertebrate Palaeontology Section, Department of Natural History at the
Royal Ontario Museum, Toronto, Canada (repository; ROM, followed by digits). This published work and
the nomenclatural acts it contains have been registered in ZooBank (http://zoobank.org/urn:lsid:zoobank.
org:pub:54C9B58F-1F96-4543-BCC8-D0D7FFEBCBD1).
Specimens were photographed using a Canon EOS 550D digital camera, with a EFS 60 mm f/2.8 Macro USM
objective, mounted on a table set-up with four external light sources, and through a Olympus SZX16 microscope
equipped with an Olympus SC30 digital camera and operated by cell Sens Standard soware.
Micro-CT scanning, using a Nikon HMX ST 225 system, housed at the University of Bristol, UK, was
employed and allowed detection of additional scolecodonts concealed in the host rock. 3141 projections were
collected for each scan and reconstructed using a modied Feldkamp back projection algorithm39 in CT Pro
(Nikon Metrology, Tring, UK). e data were then visualised using volume rendering in Drishti40.
Systematic Palaeontology
Phylum ANNELIDA Lamarck, 1809
Subclass ACICULATA Rouse & Fauchald, 1997
Family Incertae Familiae
Discussion. e family anity of the new taxon described below remains uncertain for the time being (see also
species remarks below)
Genus WEBSTEROPRION gen. nov.
Type and only species. Websteroprion armstrongi gen. et sp. nov.

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Diagnosis. Asymmetrical jaw apparatus with maxillae that can grow to >10 mm in size; forceps-like,
sub-symmetrical, denticulated MI, with prominent fang and anteriorly spaced, large denticles; MII with
shank representing c. 1/2 of jaw length, anteriorly paucidentate dentary and pointed, sub-triangular ramus.
Denticulated, short, sub-triangular basal plate.
Figure 1. Photographs of Websteroprion armstrongi gen. et sp. nov. (a) Cluster ROM63122, holotype,
bedding plane specimens preserved in dorso-ventral view as negative moulds (except for small black pieces of
original jaw wall, see posteriormost tip of right MI); (b) Same specimen (ROM63122) ipped 180 degrees and
with alternate lighting making the specimens appear positive (as an alternative to making peels with the risk of
damaging the fragile specimens); (c–i) ROM63121; (c) Slab overview with white arrows indicating specimens
enlarged in (d–i); (d) Part of cluster, note basal plate situated in bight of right MI (indicated by arrow); (e) Right
MI in lateral view. (f) Fragmentary right MII in dorsal view and right(?) MI in ventro-lateral view; (g) Imprint
of MII (next to MI in (d)); (h) Crushed maxillae; (i) Fang with cutting edge of right MI (same as in (e)); (j–n)
ROM63120; (j) Slab overview with white arrows indicating specimens enlarged in (k–n); (k) Right MI; (l) Le
MI; (m) Right MII in ventral view; (n) Unknown maxillae and to the right an anterior maxilla. Scale bars 1 mm
except (c,j) 5 mm.

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Discussion. In addition to the diagnosed elements, some anterior maxillae and accessory elements were
recorded (see description).
Etymology. Named aer Alex Webster – a ‘giant’ of a bass player – combined with ‘prion’ meaning saw.
Websteroprion armstrongi sp. nov. (Figs1,2 and 3)
Diagnosis. As for genus.
Description. Right MI, dorsal view. Length 11.1–13.5 mm (note that specimen in Fig.2q is incomplete and
might be even larger than this size spectrum), width c. 1/4 to slightly less of jaw length; maximum width in
posterior 1/3 of jaw. Strongly elongate jaw tapering anteriorly and terminating in well-developed, gently curved
and dorso-laterally directed fang (or hook) that is nearly circular in anterior cross-section. Distinct cutting edge
visible on outer face of fang when original jaw wall is preserved (Fig.1i). Posterior part of jaw sub-rectangular
and bulky (Figs1d and 2q). Inner margin runs nearly straight posteriorly and sub-parallel to outer margin but
bends gently inwards at approximately mid-length only to continue posteriorly and terminating in short shank
with postero-sinistal directed end. Shank represents 1/5 or less of jaw length. Inner wing thin to insignicant.
Posterior jaw termination characterized by sub-triangular bight for tting of basal plate. Relatively deep basal fur-
row, widest in posterior and tapering o anteriorly. Dentary, which consists of c. 15–17 sub-conical and posteri-
orly slanting denticles, occupies anterior c. 0.75 or more of jaw length (fang included). Anteriorly, dentary situated
along inner margin, posteriorly from c. 1/4 it curves gently onto slightly elevated ridge on dorsal face and contin-
ues sub-parallel to inner margin. Anteriormost 6 denticles relatively large, evenly distributed and paucidentate.
Posterior remaining denticles smaller, more tightly packed, and gently and evenly decrease in size posteriorly,
ending at undenticulated ridge, transition to which is indistinct in specimens at hand (which hampers precise
measurement of extension of dentary). Outer margin runs almost straight posteriorly from fang; at approximately
mid-length curves gently outwards, bends and continues postero-dextrally and then antero-laterally into bight;
giving rise to characteristic, sub-angular ramus. In ventral view precise morphology of myocoele opening dicult
to assess but seems strongly enclosed, representing c. 0.22 of jaw length, based on CT data (Fig.2o).
Le MI, dorsal view. Jaw similar to mirror image of right MI with some features diering, particularly pos-
teriorly. Length 10.0–13.2 mm, width c. 1/4 of jaw length; maximum width in posterior c. 1/4–1/5 portion.
Strongly elongate, sub-triangular jaw tapering anteriorly and terminating in well-developed, gently curved and
dorso-laterally directed fang (or hook). Inner margin runs nearly straight and sub-parallel to outer margin poste-
riorly from falcal arch. Posteriormost jaw termination lacks bight and is transversely cut forming a nearly straight
to slightly undulating posterior margin with basal angle of c. 35° against length axis. Paucidentate dentary,
Figure 2. Websteroprion armstrongi gen. et sp. nov. CT-scanning reconstructions of specimens detected
on (n,o) and concealed within (remaining gures) slab ROM 63120; (a–d) Laterally strongly compressed le
MII in (a) le lateral, (b) right lateral, (c) ventral, (d) dorsal view; (e–h) Right MII (posteriormost tip might
be missing) in (e) dorsal, (f) ventral, (g) le lateral, (h) right lateral view; (i–l) Lateral or intercalary tooth in
dierent views; (m) Right MII (same specimen as in (e–h)) on top of unknown element and an anterior right
maxilla; (n) Le MI in ventral view (same specimen as in Fig.1l); (o) Right MI in ventro-lateral view (same
specimen as in Fig.1k); (p) Right MI (same specimen as in (o)) and le MII (same specimen as in (a–d));
(q) Right MI in dorsal view. Scale bars 1 mm.

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housing 14–16(?) rather large, gently posteriorly slanting, sub-conical and evenly spaced denticles spread near
inner margin in anterior portion of jaw. Posteriorly dentary seems to move inwards onto dorsal face and slightly
further away from inner margin. Anteriormost denticle, c. 0.5 of fang length, followed by second, usually largest,
denticle. Posteriorly, denticles decrease very gently and evenly in size. Posteriormost denticles tightly packed,
elevated on ridge, which anteriorly becomes very discrete. As for right MI transition to undenticulated ridge di-
cult to discern. Based on CT data (Fig.2n) posterior c. 1/6 part of jaw characterized by short, anteriorly rounded
sub-triangular inner wing, tapering posteriorly. Similar ‘outer wing’ present on opposite side. Distinct but short
basal furrow le of posterior denticulated ridge. Outer margin subparallel to inner margin anterior of outer wing
and continues to form part of fang. In ventral view, based on CT-data, myocoele opening is strongly enclosed
(Fig.2n), representing c. 0.2 of jaw length.
Right MII, dorsal view. Length 6.6–6.8 mm, width c. 0.45 of jaw length. Dentary arranged along inner mar-
gin in straight to vaguely and convexly curved fashion, holding 19 sub-triangular, posteriorly slanted denticles
(including cusp). Relatively large sickle-shaped, single cusp pointing laterally and slightly dorsally, followed by
2–3 widely spaced, sub-triangular denticles. Following these are normal-sized denticles that rapidly decrease in
size and become more tightly packed posteriorly. Posteriormost 7–8 denticles extremely small and tightly packed
(see broken o posteriormost extremity; Fig.1m). Shank occupying c. 1/2 of jaw length with sub-parallel sides
and tapering slightly posteriorly. Bight shallow, anteriorly straight to slightly concave; bight angle near 90°. Ramus
moderately long, terminating laterally in sub-triangular extremity (partly concealed in Fig.1m). Undenticulated
ridge insignicant. Anterior outer margin straight to slightly sigmoidal, curves and continues into cusp. Inner
wing insignicant. In lateral view dentary straight and maximum jaw width at approximately mid-length. In ven-
tral view extension of myocoele opening dicult to discern but seemingly follows anterior part of ramus across
jaw, thus representing slightly more than 1/2 of jaw length (Fig.2f).
Le MII, dorsal view. Only one, laterally strongly compressed, le MII was identied using CT-scanning,
i.e., element fully concealed in host rock (Fig.2a–d). Length c. 10.8 mm, measured width 0.12 of jaw length (but
estimated as > 0.25 of length in laterally uncompressed elements); maximum width at tip of ramus. Elongate ele-
ment with dentary arranged on elevated ridge, particularly pronounced in middle of jaw, close to outer margin.
Moderately large, postero-dorsally curved single cusp followed by 11(?) denticles that decrease evenly and gently
in size posteriorly. First 3 post-cuspidal denticles rather widely spaced, posteriorly denticles become more tightly
packed. Transition to undenticulated ridge indistinct and minute denticles might be present in posteriormost c.
1/4 of jaw, otherwise this part is undenticulated. Triangular and pointed ramus with straight posterior margin;
bight represents c. 0.47 of jaw length; shank long, slender tapering posteriorly. In lateral view jaw gently sigmoi-
dally curved. In ventral view extension of myocoele opening dicult to discern but seemingly follows anterior
part of ramus across jaw, thus representing c. 0.49 of jaw length.
Anterior maxillae. Cluster 63120 (Fig.1n) hosts partial anterior element, possibly right MIV. Sickle-shaped
as preserved with c.7 denticles decreasing in size posteriorly. CT-scanning revealed another anterior element
(Fig.2m), possibly right MIV; Posteriorly bent cusp followed by diastema and 6 (visible) rather tightly packed
denticles, seemingly 1/2 cusp length. Ramus seems short and anteriorly situated.
Figure 3. (a) Phylogenetic position of Websteroprion armstrongi gen. et sp. nov., based on the discussion in
Paxton44; (b) Schematic drawing of the dorsal maxillary apparatus of W. armstrongi, showing the main elements.

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Basal plate. Small, sub-triangular (partly crushed) element situated in bight of right MI in ROM 63121
(Fig.1d). Representing c. 0.17 of jaw length of right MI this element holds 6 relatively large, conical denticles of
near equal size, distributed along straight inner margin. Outer margin runs straight postero-dextrally from cusp,
bends sharply at approximately mid-length and continues towards, and nally merges with, posteriormost part of
dentary, giving rise to overall sub-triangular element outline.
Lateral/intercalary tooth. CT-scanning revealed one small element probably representing lateral or interca-
lary tooth (Fig.2i–l); its relative size and morphology correspond well to remaining maxillae. Length c. 2.6 mm,
width c. 0.3 of jaw length; maximum width in posteriormost 1/3 of jaw. Sub-conical, slender and vaguely curved
element tapering towards and terminating with pointed, dorsally bent apex.
Etymology. Named aer Derek K. Armstrong, who found and collected the rst specimens in the eld.
Holotype. ROM63122 (Fig.1a,b); incomplete jaw apparatus, collected July 7, 1994. Remaining illustrated spec-
imens (ROM63120 and ROM63121; Figs1 and 2) designated paratypes.
Occurrence. Upper Emsian-Lower Eifelian Kwataboahegan Formation at Rabbit (Askaskawayau) Ridge,
Ontario, Canada.
Remarks. Despite diculties in determining the full morphology of the jaw apparatus and individual jaws
because of the state of preservation, this unambiguously represents a new taxon based on the unique combination
of characters. It cannot be excluded that the element designated as the le MII might constitute a single le MIII.
Most likely W. armstrongi deserves the establishment also of a unique family; however, pending the discovery of
additional material, it is le in open nomenclature at the family level.
Whereas the combination of element morphologies is unique for Websteroprion, the MI and MII in particular
share some characters with those of hadoprionids, paulinitids, ramphoprionids, and kielanoprionids21,41–43. For
example, whilst the MI of hadoprionids are reminiscent of those of Websteroprion in being elongate and sturdy
with anterior hooks and paucidentate dentary, the remaining anterior maxillae are very dierent with their long,
straddling denticles. e MI of paulinitids are more forceps-like and dorso-ventrally attened and the right MI
has a shallower bight with a fused, small basal plate compared to that of Websteroprion. e paulinitid MII are,
however, similar in general appearance to those of Websteroprion. Among ramphoprionids the similarities pertain
to the larger-sized genera Ramphoprion and Megaramphoprion, with which Websteroprion shares the prominent,
elongate MI with a posteriorly truncated termination of the le MI and also, to some extent, the dentary. e
anterior dentary of the MI of kielanoprionids can be paucidentate, resembling that of Websteroprion. However,
the elements are overall bulkier and, similar to those of paulinitids, have a dentary that is more inwards projected,
or facing the opposing element. Moreover, the right MI of kielanoprionids lacks a distinct bight and basal plate.
us W. armstrongi is intermediate also between the eulabidognath and labidognath type of jaw apparatus
architecture sensu Paxton44. Additional jaw elements (particularly the carriers), and in better state of preservation,
are needed in order to unambiguously designate the apparatus type. However, the denticulated and unfused basal
plate and accessory element (putative intercalary/lateral tooth) suggest that W. armstrongi belongs to the labidog-
nath type (Fig.3a,b). Extant eulabidogaths, Onuphidae and Eunicidae, are generally accepted as descending from
the extinct family Paulinitidae21,45, an ancestral eulabidognath sensu Paxton44.
It is noteworthy also that the MI of W. armstrongi is similar to the larval MI of extant onuphids and eunicids
(see Paxton & Safarik46, Fig.1F; Paxton & Eriksson47, Figs1b and 3). Whilst adult onuphids and eunicids possess
an undeticulated MI46, the larval le MI possesses prominent denticles and a similar gross morphology to the
MI of W. armstrongi. us the appearance of the larval MI of these two modern eunicidan families appears to
recapitulate the ancestral adult morphology47, which also strengthens the case of Websteroprion being related to
the extant Eunicidae and Onuphidae (Fig.3).
e serendipitous discovery of multiple specimens from a monospecic assemblage in limited sample vol-
ume may suggest that W. armstrongi was a common species at this particular locality of the Kwataboahegan
Formation. e specimens likely represent ‘snap-shot assemblages’ of deceased individuals that were rapidly
entombed by sediment and subjected to limited transport. It is, however, puzzling that no mandibles (which also
are expected to have been huge) were found associated with the maxillary clusters. is might be related to struc-
tural and/or slight biochemical dierences that were less suitable for preservation in this particular environment.
Alternatively, this indicates that the maxillae represent shed specimens and the animal (with the continuously
growing mandibles) died elsewhere.
W. armstrongi adds to the list known Devonian polychaete taxa and also suggests that eunicidan diversity and
disparity were high during this time.
Feeding habits. Websteroprion armstrongi was a raptorial feeder sensu Fauchald & Jumars3. Such polychaetes
use their buccal apparatus, consisting of a muscular ventral or axial pharynx, to snatch food items1,3. Eunicidans
and phyllodocidans use their jaws to capture live animals as carnivores, to rip o pieces of algae, as herbivores,
or to grasp dead and decaying organic matter, as scavengers. Although it would perhaps be easy to assume that
W. armstrongi had a predatory, carnivorous mode of feeding based on the ‘erce’ appearance of the jaws, it has
been shown that extant jaw-bearing polychaetes exhibit a wide range of feeding habits1,3 and, thus, that jaw mor-
phology does not necessarily reect specic modes of feeding. Moreover, in many active predators antennae and
palps are present on the prostomium1 and such so-body features are obviously not known for W. armstrongi,
although they are assumed to have been present based on their presence in the extant eulabidognaths Eunicidae
and Onuphidae2. erefore, without evidence of preserved gut content and/or so body structures, more precise
knowledge of the feeding habits of W. armstrongi remains elusive for the time being.
Size estimates of Websteroprion and polychaete gigantism. e jaws of Websteroprion are colossal
in size compared to most scolecodonts known from the fossil record and also compared to the jaws of most extant
aciculates. Scolecodonts are typically found in the size range of 0.1–2 mm, although exceptions beyond both

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ends of this spectrum exist. Only some 30 specimens are known from the fossil record – distributed primarily
among ramphoprionids, paulinitids, polychaetaspids, hadoprionids, and atraktoprionids – with a size of 3.5 mm
(arbitrarily chosen upper limit) or more (electronic SupplementaryMaterial, TableS1). By contrast, the MI of W.
armstrongi reach > 13 mm in length. In the published literature (TableS1) there is an interesting record of one
partial specimen, described by Eller48 as Arabellites longiformis from the Devonian of Ontario Co., New York.
Eller measured the specimen to 8 mm and estimated that if intact it could have reached 14 mm in length. Based
on the rudimentary drawing it shows similarities to the (anterior part of) the MI of W. armstrongi. However,
Eller’s specimen only comprises the anterior portion of an MI which are undiagnostic and usually homeomorphic
between many taxa, even at genus and family level. erefore, as the specimen could belong to any number of
taxa, A. longiformis should be regarded as a nomen dubium.
Based on the fossil record data (TableS1) there seems to be no general trend towards a maximum size increase
with time towards the Devonian, suggesting that W. armstrongi is a genuine aberration. Whilst it is uncertain if
the exceptional size of the jaws of W. armstrongi represents pathological or gerontic stages, the presence of mul-
tiple individuals of similar size is congruent with the interpretation that attaining large size was characteristic of
the species. Size and morphology of the maxillae indicate that the individuals are adults44,47 and given that that
some polychaetes grow continuously throughout life it is possible that adults could have grown bigger still. e
absence of smaller individuals may indicate that adults and juveniles had dierent environmental preferences, as
is the case in some extant eunicidans. Nevertheless, it is clear that the maxillae of W. armstrongi are the largest
known from the fossil record.
In order to assess the full body size for W. armstrongi based on the size of its jaws, comparisons with
now-living, jaw-bearing polychaetes are needed. is is, however, not an entirely straight-forward task. Firstly,
W. armstrongi is extinct and the precise phylogenetic relationships of Palaeozoic scolecodont taxa to the mod-
ern eunicidan diversity are currently poorly constrained. Secondly, the relationship between the size of the jaws
(maxillae and/or mandibles) and the full worm body size (length and/or width) is unclear or ambiguous even
from the neontological literature. When the jaw apparatuses in extant worms are at all measured and described
in conjunction with the systematic descriptions of so tissue characters, they are usually related to the number
of chaetigers (or setigers) and not necessarily to body length. In many eunicidan polychaetes replacement of the
maxillary jaws have been suggested and/or recorded from more or less frequent moulting during ontogeny19,44.
Moreover, the moult increments during continuous shedding are not necessarily consistent within a single taxon,
and it may vary between species. For example, Paxton & Safarik46 noted that the jaw apparatus growth rate and
moult frequency in the onuphid Diopatra aciculata slow down with increasing age.
Despite these problems there are some published data that can be used for comparison, in order to acquire
an estimate of the body size of W. armstrongi. Ieno et al.49 found a positive correlation between jaw size and body
length for the ragworm Laeonereis acuta. Extrapolating their results would result in a total body length of W.
armstrongi in the excess of 4.1 m. By sharp contrast, however, using the data of Brenchley50 (p. 308, Table 6) for
Platynereis bicanaliculata and Nereis brandti, results in an extrapolated body length of W. armstrongi of 0.42 m
and 0.73 m, respectively. is highlights the diculties in assessing the body length based on jaws using modern
examples. Note, moreover, that these examples are phyllodocidans, whose continuously growing jaws are of a
dierent composition than those of the eunicidan W. armstrongi.
Among extant eunicidans we nd some of the largest and smallest polychaete species known51. Particularly
within the family Eunicidae and the genus Eunice there are taxa commonly referred to as ‘giant eunicids’52–55,
but gigantic worms also are found in Onuphidae, the sister taxon of Eunicidae56,57. Members of the Eunicidae
and Onuphidae have jaw apparatuses of eulabidognath type sensu Paxton44, similar in architecture to that of W.
armstrongi (Fig.3). Leland57 studied the relationship between the size of the mandibles (which by contrast to the
maxillae grow continuously throughout life and thereby form a more reliable aging structure) and body param-
eters of giant Australian beach worms (species of the onuphid Australonuphis) and found a positive correlation.
Australonuphis mandibles correspond in length approximately to that of the carriers and rst maxilla combined
(H. Paxton, Sydney, pers. commun., 2016). us a conservative conversion for the length of our MI (c. 2/3 of
putative mandible size) and using the calculations of Leland57 (Figs. 15, 17, Table 3) for A. teres and A. parateres
would result in a massive body length of W. armstrongi in excess of 4.8 and 8.3 m, respectively. However, as for
the phyllodocidan examples above, these are (perhaps unlikely) extrapolated values and neglect the signicant
possibility of changes in trajectory in the relationship between mandible length and body length in considerably
larger/older specimens.
For the species-rich genus Eunice, where the largest forms are found today, there are published claims of indi-
viduals reaching 6 m in length53,58; however, some authors have argued that they are rather in the order of 3 m54.
ese large eunicids have a wide distribution in modern tropical and temperate seas55. e largest species known
is the famous ambush predator Eunice aphroditois (colloquially and rather infamously referred to as the ‘Bobbit
worm’), but it must be emphasized that the taxonomy of this species (or species complex) is controversial and in
need of revision55,58,59. For comparison, we received some size data measured (by Luis F. Carrera-Parra, Mexico –
pers. commun., 2016) on two giant eunicid species, E. ‘aphroditois’ and E. roussaei, stored at the National Museum
of Natural History (Washington DC, USA), in which the jaws are directly related to full body length. In these
specimens, an MI length of 0.92–1.2 cm corresponds to body a length of 0.79 to > 1.2 m (TableS2). Using these as
modern analogues suggests again that W. armstrongi could have attained a full body length in excess of one metre.
Although the available data allow for a broad range of inferred sizes, comparison with closely related extant
taxa indicates a body length in the region of 1–2 metres. Body sizes much smaller or much in excess of this range
are derived either from outgroup taxa with jaws that are not homologous (e.g. Nereididae) or extrapolation from
close relatives whose maxillae do not attain such a large size (e.g. Australonuphis).
e extraordinarily large jaws and inferred body length of W. armstrongi demonstrate that eunicidan worm
gigantism had appeared already in the Devonian, some 400 million years ago. Very large body size, or gigantism,

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in animals is an alluring and ecologically important trait, usually associated with advantages and competitive
dominance60. It is known among several living clades as well as throughout the fossil record, with an increase in
body size trajectory towards the present61, but driving mechanism/s are dicult to discern and vary60–62. For W.
armstrongi, it could be related to intrinsic factors (e.g., unique physiology, predation pressure and/or interspecic
competition) and/or extrinsic (physical/chemical environmental) factors (e.g., the availably of oxygen, nutrients,
food resources) during the time of deposition of the Kwataboahegan Formation in the Rabbit Ridge area. Precise
knowledge of the palaeoenvironmental and palaeoecological scenario at Rabbit Ridge is unfortunately limited
and the remote locality is dicult to access. By comparison, it is noteworthy that examples of giant conodonts
(with elements reaching centimetre-size rather than sub-millimetre-size) are known from unusual environmen-
tal (and ecological) settings, now preserved as Lagerstätten; e.g., the Upper Ordovician Soom Shale Lagerstätte
of South Africa63 and more recently the Middle Ordovician Winneshiek Lagerstätte of Iowa, USA, the strata of
which were deposited in a meteorite impact crater64. However, while representing an ancient ‘Bobbit worm’ and a
case of primordial eunicidan worm gigantism, the specic driving mechanism/s for W. armstrongi to reach such
a size remains ambiguous.
Conclusion
Large body size is present in many living so bodied invertebrate taxa, but establishing if similar sizes were
attained in their extinct relatives is oen dicult, if not impossible. e specimens of W. armstrongi described
herein demonstrate gigantism in a group of primarily so bodied polychaetes, a phenomenon which is also
known in its extant closest relatives, but hitherto only among the eulabidognathans Onuphidae and Eunicidae.
us, this new nding reveals gigantism also in the stem group (the Labidognatha) and that this feature occurs
only in one particular clade within the Eunicida (Fig.3a). is discovery highlights that although comparatively
rare, fossils of annelid worms provide important insights into the past diversity, disparity and evolutionary history
of the group.
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Acknowledgements
We thank D.K. Armstrong, S.R. Westrop, and P. Fenton for able assistance in the eld, and T. Davies for assistance
with CT scanning, L.F. Carrera-Parra and S.I. Salazar-Vallejo for helpful discussions and data on extant giant
eunicids, C. Tell for help in assembling size data on fossil scolecodonts, P. Ahlberg and J. Lindgren for assistance
with, and loan of, photography equipment. J. Lindgren kindly reviewed a dra of the manuscript. M.E.E. thanks
the Swedish Research Council (grant no. 2015-05084) and L.A.P. thanks the Palaeontological Association for a

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Sylvester Bradley award (PA-SB201404) for research funding. Fieldwork was funded in part by a grant to D.M.R.
from the Royal Ontario Museum Foundation and by the Ontario Geological Survey. e manuscript benetted
from helpful comments by two anonymous reviewers.
Author Contributions
M.E.E. identied and described the jaw elements, M.E.E. and L.A.P. imaged the specimens and draed the
gures, L.A.P., C.T. scanned the specimens and made the 3D reconstructions, D.M.R. collected the specimens
and performed the initial identication. All authors contributed to writing and approved the nal manuscript.
Additional Information
Supplementary information accompanies this paper at http://www.nature.com/srep
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Eriksson, M. E. et al. Earth’s oldest ‘Bobbit worm’ – gigantism in a Devonian eunicidan
polychaete. Sci. Rep. 7, 43061; doi: 10.1038/srep43061 (2017).
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