Canadian Journal of Earth Sciences
Endocranial morphology of the petalichthyid placoderm Ellopetalichthys scheii (Kiær 1915) from the Middle Devonian of Arctic Canada with remarks on the inner ear and neck joint morphology of placoderms
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2020-0005.R1
Manuscript Type: Article
Date Submitted by the 14-May-2020 Author:
Complete List of Authors: Castiello, Marco; Imperial College London - Silwood Park Campus Jerve, Anna; Uppsala Universitet Burton, Maria;Draft Imperial College London Friedman, Matt; University of Michigan Brazeau, Martin; Imperial College London, Life Sciences
Keyword: placoderm, Petalichthyida, braincase, Devonian, inner ear
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
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1 Endocranial morphology of the petalichthyid placoderm Ellopetalichthys scheii
2 (Kiær 1915) from the Middle Devonian of Arctic Canada, with remarks on the inner
3 ear and neck joint morphology of placoderms
4 Marco Castiello1, Anna Jerve1,2, Maria Grace Burton1, Matt Friedman3, Martin D Brazeau1,4*
5
6 1Department of Life Sciences, Imperial College London, Silwood Park Campus, Ascot, SL5
7 7PY, United Kingdom
8
9 2Department of Organismal Biology, Uppsala University, Norbyvägen 18A, 752 36, Uppsala, 10 Sweden Draft 11
12 3Museum of Paleontology and Department of Earth and Environmental Sciences, University of
13 Michigan, Ann Arbor, Michigan 48109-1079, United States of America
14
15 4Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7
16 5BD, United Kingdom
17
18 * Corresponding author. Department of Life Sciences, Imperial College London, Silwood
19 Campus, Buckhurst Road, Ascot SL5 7PY, United Kingdom; E-mail: [email protected]
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20 Abstract: Petalichthyid and “acanthothoracid” placoderms have taken pivotal positions in the
21 debate on placoderm—and, by extension, jawed vertebrate—relationships owing to perceived
22 similarities with the jawless vertebrates. Neurocranial characters are integral to current
23 hypotheses of early gnathostome relationships. Here, we describe the three-dimensionally
24 preserved neurocranial anatomy of the petalichthyid placoderm Ellopetalichthys scheii, from the
25 Middle Devonian (early Eifelian) of Ellesmere Island, Canada. Using X-ray computed
26 microtomography, we generated three-dimensional reconstructions of the endocranial surfaces,
27 orbital walls, and cranial endocavity. These reconstructions verify the absence of a crus
28 commune of the skeletal labyrinth and the complex shape of the petalichthyid endolympathic
29 duct. Details of the craniothoracic joint and occipital musculature fossae help resolve the
30 problematic comparative anatomy of theDraft occipital surface of petalichthyids. These new data
31 highlight similarities with arthrodire placoderms, consistent with older hypotheses of a sister-
32 group relationship between petalichthyids and that clade.
33
34
35 Key words: placoderm, Petalichthyida, braincase, Devonian, inner ear
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42 Introduction
43 The Macropetalichthyida are a group of petalichthyid placoderm fishes characterized by a wide,
44 flat head, dorsolaterally facing eyes, and a broad, projecting snout. Their outward resemblance to
45 jawless osteostracan fishes—particularly in the structure of the brain cavity and orbit—places
46 them at the centre of debates about the monophyly of placoderms (Janvier 1996, Brazeau 2009,
47 Brazeau and Friedman 2014). However, most of our knowledge of the anatomy of the braincase
48 of petalichthyids relies on Stensiö’s (1925, 1969) descriptions of a single reference taxon,
49 Macropetalichthys Norwood and Owen. Stensiö’s initial (1925) accounts were based on three-
50 dimensionally preserved specimens, including one that was broken open with a hammer. These
51 descriptions were greatly augmented in 1969 with one of these specimens prepared
52 mechanically, revealing the external braincaseDraft and endocast. Braincase morphology and brain
53 endocast details have long been key sources of phylogenetic data on early vertebrates. With the
54 advent of X-ray computed micro-tomography (XR-µCT, simply stated “CT” in text), there is a
55 growing wealth of comparative neurocranial data for early vertebrates. Understanding of
56 petalichthyid endocranial morphology and diversity beyond Macropetalichthys, as well as
57 corroboration of Stensiö’s reconstructions, is important for a robust understanding of both
58 placoderm and deep gnathostome phylogeny.
59 In this paper, we describe the braincase morphology of the type and only specimen of the
60 petalichthyid Ellopetalichthys scheii (Kiær 1915) from the Middle Devonian of Arctic Canada
61 using CT scanning. This specimen was first described by Kiær (1915) as Macropetalichthys
62 scheii and later redescribed by (Ørvig 1957) who placed it in the new genus Ellopetalichthys.
63 Ørvig distinguished Ellopetalichthys from Macropetalichthys on the basis of “the large size of
64 the orbital openings” (1957: p. 301), the posterior restriction of the endocranial ossification, and
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65 the “general shape” (1957: p. 301) of these ossifications. At the same time, Ørvig noted that
66 Ellopetalichthys most closely resembles Macropetalichthys among petalichthyids known at the
67 time. This apparently close relationship between these two genera means that three-dimensional
68 details of the braincase of Ellopetalichthys could provide improved comparative interpretations
69 of Macropetalichthys as well as for other petalichthyid neurocrania.
70 In this paper, we present new details of the endocranial anatomy of Ellopetalichthys
71 which we consider to be of likely phylogenetic utility. These include most of the endocranial
72 cavity and external structures obscured by matrix. The probable close relationships of
73 Macropetalichthys and Ellopetalicthys offer the opportunity to assess key aspects of Stensiö’s
74 (1969) reconstructions of the former. We use these new data to clarify the comparative and
75 functional morphology of the petalichthyidDraft head with a particular emphasis on the cranio-
76 thoracic joint (i.e. ‘neck joint’) and the skeletal labyrinth cavity. The former of these has only
77 seen rudimentary phylogenetic consideration and we add recommendations on how this
78 morphology can be represented as phylogenetic characters.
79
80 Materials and Methods
81 The holotype of Ellopetalychthys scheii (Kiær 1915) is held in the collections of the Natural
82 History Museum at the University of Oslo (NHMO) and catalogued as specimen A13047. As it
83 is the only specimen known, we simply refer to it here as Ellopetalichthys . It consists of an
84 incompletely preserved dermal skull roof and exposed posterior part of the basicranium. Most of
85 the pre-orbital part of the skull is missing, but the rest is well preserved and three dimensional.
86 Skull roof material was cleaved off into the counterpart, which was not scanned in this study.
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87 According to Kiær (1915), the specimen was collected by Per Schei at Goose Fiord,
88 Ellesmere Island from Schei’s Dh zone. At the time of Ørvig’s resdescription, there was some
89 discrepancy in dating this zone and he considered it late Middle Devonian (Givetian). It is now
90 referred to the Baad Fiord Member of the Bird Fiord Formation which, at Goose Fiord, is of
91 latest Emsian or Eifelian age on the basis of conodonts [serotinus to costatus zones in lower part;
92 patulus to kockelianus zones in the upper part, (Mayr and Packard 1994); approximately 397.8 to
93 388.3 Ma, according to Becker et al. (2012)].
94 We scanned the specimen at the Imaging and Analysis Centre at the Natural
95 History Museum in London using a Nikon Metrology HMX ST 225 X-ray micro-CT
96 scanner. The fossil was scanned at 165 kV and 170 µA, with a 0.1 mm thick copper filter (voxel
97 size 26.8 µm), to create 6284 X-ray projections.Draft The resulting tomographic dataset was
98 segmented and rendered in 3D using the software Mimics 18.01 (Materialise Technologielaan,
99 Leuven, Belgium). The endocranial cavities were segmented as both the perichondral ‘shell’ and
100 a virtual endocast. The endocast is presented here because it is more complete, as it was often
101 possible to model the endocast where a differential matrix infill contrasted with the surrounding
102 matrix, but where the perichondral shell itself was unresolved. The 3D models produced with
103 Mimics were imported in Blender (blender.org) for scientific image preparation. Tomographic
104 data, virtual 3D models and a photogrammetric model of NHMO A13047 are available on
105 figshare (10.6084/m9.figshare.11093345)
106
107 Systematic Palaeontology
108 Placodermi M’Coy (1848)
109 Petalichthyida Jaekel (1911)
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110 Macropetalichthyidae Eastman (1898)
111 Ellopetalichthys Ørvig 1957
112 DIAGNOSIS: As for type species.
113
114 Ellopetalichthys scheii (Kiær 1915)
115
116 DIFFERENTIAL DIAGNOSIS: (Emended after Ørvig, 1957) Macropetalichthyid placoderm with the
117 following characters: Distribution of skull roof plates and sensory lines identical to
118 Macropetalichthys from level of the anterior margin of the orbits to the posterior margin of the
119 skull (anterior parts unknown); tubercles are low, rounded as in Macropetalichthys; differs from
120 Epipetalichthys Stensiö, Lunaspis Broili,Draft Shearsbyaspis Young, and Wijdeaspis Obruchev in the
121 absence of any evident pattern of concentric or parallel ornament ridges. Apomorphic character
122 of the species: trapezoidal flanges of parachordal ossifications forming a broad, thin floor to the
123 cucullaris/parabranchial fossa; the flanges widest at anterior, spanning most of the width of the
124 fossa and taper out near the caudal end of the parachordal ossifications.
125
126 REMARKS: Ørvig’s (1957) original diagnosis mentions a suite of characters that are generally
127 found in macropetalichthyids and therefore do not uniquely identify a new genus. However, in
128 the remarks on the taxonomic act, he provides a set of differential characters with
129 Macropetalichthys. These character differentials (referred to in the introduction of the present
130 paper), are vaguely stated or relied on relative dimensions such as the orbit diameter. However,
131 the holotype of Ellopetalichthys is small compared to most specimens of Macropetalichthys
132 (Kiær 1915, Ørvig 1957). Differences in relative orbit diameter (presumably body size-
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133 standardized) as well as the extent and proportion of ossifications could easily be explained as
134 differences in the ontogenetic age of specimens. This potentially undermines the generic
135 distinction proposed by Ørvig. We conclude that only the “general shape of the posterior part of
136 the endocranium” of Ellopetalichthys (Ørvig 1957, p. 301) can be said to be unique among
137 petalichthyids on present evidence. We have retained this character, but described the shape
138 more precisely.
139
140 Description
141 Skull roof and sensory lines
142 The skull is anteroposteriorly elongated,Draft with a high convex profile, and possesses large 143 orbits that open dorsolaterally (Figs. 1, 2). The dorsal side of the skull roof is broken off into the
144 counterpart, leaving only the impression of the internal surface of the bones in many areas as
145 well as exposing some of the underlying endoskeleton (Fig. 1B). Preserved dermal ornament
146 consists of low tubercles, similar to Macropetalichthys. Our CT data reveal for the first time the
147 positions of the endolymphatic duct foramina (Figs. 1B, 2). These are located in the anterior
148 paranuchal plate, immediately medial to the junction of the posterior pit line and the main lateral
149 line (Figs. 1A, 1B), as in most other petalichthyids. CT models corroborate the descriptions of
150 the sensory line system by Kiær (1915) and Ørvig (1957) (Fig. 1), but cannot resolve the dermal
151 bone boundaries and thus offer no contrasts with previous descriptions.
152 Endocranium
153 Apart from the exposed posterior portion of the ventral surface of the endocranium
154 described by Kiær (1915) and Ørvig (1957), endocranial structure in Ellopetalichthys is
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155 otherwise unknown. CT scanning reveals additional details obscured by matrix or ablated by
156 preparation. Most importantly, it provides details of the cranial cavity and canals (Figs. 1-6). The
157 braincase is incompletely to weakly ossified in the areas not exposed by mechanical preparation.
158 As in many petalichthyids (Stensiö 1925, 1969, Young 1978) the posterior and occipital portion
159 of the braincase is more strongly mineralised, while the orbital and antorbital regions are
160 incompletely mineralised.
161 Orbital region
162 The orbits are dorsolaterally oriented (Figs. 1A, 1B, 2), as in other petalichthyids. Only the
163 posterolateral wall of each orbit can be partially reconstructed (Figs. 1-4). Preservation of this 164 area is limited, making it difficult to determineDraft the presence and arrangement of foramina for 165 nerves and blood vessels. Part of the posterior wall of the posteroventral myodome (Castiello and
166 Brazeau 2018) is visible in the left orbit (Myodome 6, Figs. 2, 3). It is pierced by a small canal of
167 uncertain identity (?Pituitary vein canal, Fig. 3).
168 Postorbital region
169 The region of the braincase immediately posterior to the orbits—between the presumed
170 transverse otic processes—is not well preserved. However, posterior to this the endocranial
171 surface is more robustly mineralised (Figs. 1-5). Along the lateral sides of the specimen, the
172 vagal process is bifurcated into anterior and posterior processes (Fig. 3) as in acanthothoracids
173 (Ørvig 1975) and many arthrodires (Stensiö 1963, Goujet 1984a). The process bounds the very
174 large paravagal fossa (Fig. 3), as in Macropetalichthys (Stensiö 1969, Young 1980). The broad
175 paravagal fossa is bisected by a low, transverse accessory ridge originating immediately
176 posterior to the anterior vagus nerve opening (Fig. 3), again corresponding to the condition in
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177 Macropetalichthys. A small foramen located on the anterior wall of the paravagal fossa probably
178 represents that for the glossopharyngeal nerve (Figs. 3-4).
179 As in Macropetalichthys (Stensiö 1925, 1969), there are two broad, posteriorly facing
180 openings at the position where the parachordals meet the basicranium (Fig. 3). These were
181 identified by Stensiö (1969) as having carried both the lateral dorsal aorta and the secondary
182 jugular vein, an interpretation consistent with that of Young (1978, 1980). As there are no
183 perichondrally ossified canals leading from these openings their precise course is not possible to
184 reconstruct. In other macropetalichthyids, the aortic canals are reconstructed as connecting to
185 the paravagal fossa. A candidate efferent opening can be seen on the right side of the specimen 186 (Fig. 3). Although no corresponding openingDraft appears on the left side of the 3D model (Fig. 3), 187 we note that this area is not ossified in the specimen (see photogrammetric model in the data
188 archive linked to in the Methods section).
189 Otico-occipital region
190 CT data partially corroborate Ørvig’s (1957) original interpretation of the otico-occipital
191 region of Ellopetalichthys, but also offer new details. According to Ørvig (1957), the otico-
192 occipital area consists of two distinct structures: a flat and broad anterior median bone (Ørvig
193 1957, text-fig. 7; Oc.a), and a long and narrow, stalk-like posterior ossification (Ørvig 1957, text-
194 fig. 7; Oc.p). We confirm the presence of two distinct ossifications but find that they are joined
195 by thin web of perichondral bone that appears to be punctuated by unmineralized gaps. The two
196 ossifications meet at the level of the posterior paravagal processes. The straight posterior vagal
197 process in Ellopetalichthys projects laterally at a right angle to the long axis of the skull. The
198 parachordals are long and narrow: together they are less than half the width of the basicranium,
199 resulting in a condition described as ‘stalk-shaped’ in phylogenetic character lists (Giles et al.
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200 2015). These ossifications bear broad lateral flanges or laminae that taper caudally towards the
201 occipital glenoids. These flanges form a broad shelf, defining a substantial ‘floor’ to the
202 parabranchial (Goujet 1984a) fossa (Figs. 3-5).
203 The parabranchial fossae are extremely large in comparison to those of other placoderms.
204 They are highly vaulted and so broad they nearly come into contact above the parachordal
205 processes, separated only by a narrow supra-occipital septum (Fig. 2). Ellopetalichthys, like
206 Wijdeaspis warroonensis (Young 1978), differs from Stensiö’s (1969) description of
207 Macropetalichthys in that the parabranchial (or cucullaris) fossa is roofed by perichondral bone
208 instead of dermal bone. However, Young’s (1980) revised reconstruction of Macropetalichthys 209 includes an endochondral roof over thisDraft part of the skull, suggesting doubts about this aspect of 210 Stensiö’s reconstruction. A pair of unidentified grooves is located in the endochondral space
211 above and behind the parabranchial fossa (Fig. 2). These grooves extend anteroposteriorly on the
212 mesial part of the dorsal surface of the fossa (Fig. 2).
213 The morphology of the occipital area and neck joint of Ellopetalichthys in our 3D models
214 (Fig. 5) largely agrees with Stensiö’s reconstruction (1969, fig. 83). Differences in the
215 distribution of dermal and perichondral bone between Stensiö’s figure and our 3D models likely
216 arise from an unclear distinction between these tissues in the tomograms. In posterior view, the
217 dorsal occipital margin has a high, parabolic profile (Fig. 5). Following this margin is a robust,
218 arch-shaped platform, corresponding to the insertion area of the levator muscles (Fig. 5). A series
219 of unidentified canals extends rostro-caudally through the space inside of this arch (?Blood
220 vessel canals, Fig. 2). This arch is bounded laterally by deep para-articular processes (Figs. 1C,
221 3, 5). There is no evidence of a lateral articular facet or articular cotyle as in arthrodires (see
222 Discussion for revised interpretation of lateral articular facets in Wijdeaspis described by Young,
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223 1978). Instead, on the medial face of the occiput, above the foramen magnum, the central part of
224 this surface is deeply recessed (Incompletely ossified recess, Fig. 5). This recess opens into the
225 parabranchial cavities. However, it is unclear if this was completely walled in perichondral bone,
226 was cartilaginous, or was open in life. The rear face of the parachordals is not preserved (Fig. 5),
227 such that a separate notochordal canal opening is not apparent, but its diameter and position are
228 constrained by the glenoid process cavities and diameter of the main spinal nerve canal (Figs. 3-
229 5). The notochordal canal was likely about the same size or smaller than the foramen magnum,
230 as in other placoderms.
231 232 Endocranial cavity Draft 233 The endocranial cavity is almost completely preserved, missing only the area anterior to the orbit
234 despite weak ossification in some areas (Figs. 1-5). As some parts of the endocranial cavity are
235 preserved either on the left or on the right side of the specimen, a reconstruction has been
236 produced to facilitate anatomical interpretations (Fig. 7). The overall morphology of the
237 endocranial cavity of Ellopetalichthys is platybasic and resembles that of Macropetalichthys and
238 Shearsbyaspis, with an elongated and laterally narrow profile that is widest at the level of the
239 trigeminal and vagus nerves (Fig. 7).
240 The telencephalic cavity is incomplete, but fragments of the olfactory lobe cavity are
241 preserved on the left side of the specimen (Figs. 2, 7). The diencephalic portion is fairly
242 complete (Figs. 2-3, 7), with large-diameter canals for the optic tract. The mesencephalic portion
243 is almost complete, extending from behind the optic tract until the cerebellar auricles (Figs. 2-3,
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244 7). The ventral side of this region is poorly resolved in the CT scan, leaving no information about
245 the hypophysial duct (Fig. 3).
246 Cavities for the cerebellar auricles of the metencephalon are well developed, showing
247 two very distinct lobes on the dorsal side (Figs. 2-3, 7). The myelencephalic portion of the
248 endocavity is complete dorsally, as a tubular area that is expanded laterally in its posterior
249 portion where it gives origin to the vagus nerve canals (Figs. 2, 7). The spinal cord canal, the
250 posterior most part of the endocranial cavity, narrows to a long slender canal leading to the
251 foramen magnum (Figs. 4-5).
252 Cranial nerves
253 Only a small portion of the olfactoryDraft tract canal (N.I) is preserved, on the right side of the 254 specimen (Figs. 2-4, 7). This large-diameter, tubular canal would likely have extended further
255 anteriorly, as in other macropetalichthyids (Stensiö 1925, 1969, Castiello and Brazeau 2018).
256 Although not completely preserved on either side of the specimen, the optic tract (N.II) canal can
257 be reconstructed using the partial preservation of the left anterior and right posterior perichondral
258 walls of the orbital cavity (Figs. 2-3, 7). Among the nerves for the eye muscles, only the
259 oculomotor nerve (N.III) canal is visible, exiting from the lateral wall of the metencephalic
260 cavity (Figs. 2-3, 7). The canal for the profundus branch of the trigeminal nerve (N.V) is
261 preserved on the left side of the specimen, branching from the anteroventral edge of the
262 cerebellar auricles (Figs. 2-3, 7). Their openings on the orbital wall are not resolved.
263 The facial nerve (N.VII) canal is incompletely preserved (Figs. 2-3, 7). It originates
264 posterior to the cerebellar auricles, passing anterolaterally along the anterior surface of the
265 saccular cavity. Only two branches can be reconstructed: the ramus buccalis lateralis (Figs. 2-3,
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266 7) and the ramus ophthalmicus superficialis (Figs. 2-5, 7). Their division from the main canal of
267 the facial nerve is visible on the left side (Figs. 2-3, 7). The short canal for the ramus buccalis
268 lateralis opens through a small foramen on the lateral wall of the orbit. The superficial
269 ophthalmic branch canal extends anterodorsally towards the snout, following the supraorbital
270 sensory line (Figs. 1-3, 7).
271 The acoustic (N.VIII) and glossopharyngeal (N.IX) nerve canals are partially preserved
272 and exit from the anterior myelencephalic portion of the endocavity (Figs. 2-3, 7). The acoustic
273 nerve canal extends laterally to meet the labyrinth (Figs. 2-4, 7) and the glossopharyngeal nerve
274 canal extends posteroventrally, passing below the labyrinth (Fig. 4). The entire course of the 275 glossopharyngeal nerve is not preserved,Draft but a short segment of its posterior portion exits on the 276 anterior area of the paravagal fossa (Fig. 4).
277 The vagus nerve (N.X) canal is very well preserved on both sides and has a complex
278 morphology. Two branches diverge from the vagal recess: an anterior branch extends
279 transversely towards the paravagal fossa (Fig. 2-4, 7), and a posterior branch extends
280 posterolaterally towards the posterior vagal process to openin the occipital (parabranchial) fossa
281 (Figs. 2-4, 7). Several small canals extend from both the anterior and posterior branch of the
282 vagus nerve, projecting dorsolaterally to innervate the main sensory line canals (Figs. 2-4, 7).
283 There is no evidence of any series of spino-occipital nerve canals that normally pierce the
284 occipital arch in other early gnathostomes.
285 Labyrinth cavity
286 Both labyrinth cavities are incompletely preserved (Figs. 2-4, 6). The right labyrinth preserves
287 the anterior semi-circular canal, the medial part of the sacculus, and the posterior semi-circular
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288 canal and ampulla. The left labyrinth shows the anatomy and position of the anterior ampulla,
289 external utriculus, external ampulla, and part of the posterior semicircular canal. Preservation is
290 sufficient to permit reconstruction of the overall structure of the inner ear in Ellopetalichthys
291 (Figs. 6, 7).
292 In dorsal view, the anterior part of the labyrinth can be seen to be almost in contact with
293 the posterior orbital wall (Figs. 1-4). The anterior ampulla lies just behind the posterior orbital
294 wall (Figs. 2, 6). The anterior semi-circular canal extends from the centre of the dorsal side of the
295 sacculus towards the anterior ampulla. The external ampulla extends parallel to the sacculus and
296 connects with the anterior ampulla by a ventrally bent utriculus (Figs. 2-3, 6). The posterior 297 ampulla is located at the posteromesial endDraft of the sacculus, near the canal for the vagus nerve 298 (Figs. 2, 6). The posterior semi-circular canal crosses the sacculus at the level of the origin of the
299 glossopharyngeal nerve and continues laterally to the posterior ampulla. The posterior semi-
300 circular canal is shorter than the anterior one. The anterior and posterior semicircular canals are
301 not in contact and so a crus commune and sinus superior can both be considered absent (Figs. 2,
302 7).
303 The endolymphatic canal has a complex shape (Figs. 2, 6-8). It extends posteriorly from
304 the medial side of the saccular chamber towards the anterior wall of the parabranchial fossa.
305 Along this course, the canal joins with the trunk of the vagus nerve canal (Fig. 2). Immediately
306 anterior to the face of that fossa, the canal makes a sharp dorsolateral twist before expanding into
307 a flask-shaped fossa (Figs. 2, 7). This flask-shaped fossa narrows towards its opening in the skull
308 roof (Fig. 1B-C) via a short duct (Figs. 2, 7).
309
310 Discussion
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311 The inner ear morphology of petalichthyids: revision and comparison
312 The skeletal labyrinth is a source of important phylogenetic and functional anatomical
313 information in early gnathostomes. We are able to reconstruct the labyrinth cavity of
314 Ellopetalichthys by reference to the left and right sides of the specimen (Fig. 6-8). The most
315 completely known skeletal labyrinth of a petalichthyid is that of Macropetalichthys described by
316 Stensiö (1969). However, even the example used by Stensiö is incomplete and required some
317 questionable reconstruction. The position of the anterior ampulla and course of the anterior
318 semicircular canals are the same in Ellopetalichthys and Shearsbyaspis (Fig. 8), corroborating
319 the conclusion by Castiello and Brazeau (2018) that the unusual structure and orientation 320 reconstructed by Stensiö was in error (Fig.Draft 8). We provide further possible systematic characters 321 relevant to placoderm and stem-gnathostome phylogeny.
322 Inner ear characters of macropetalichthyids
323 Although the floor of the saccular chamber is not well preserved in Ellopetalichthys, the
324 left side of the specimen shows that the posterior ampulla joins to the underside of the saccular
325 chamber (Figs. 4, 6). This configuration is unusual among early gnathostomes, but compares
326 well with Stensiö’s (1969) reconstruction of the same structure in Macropetalichthys. In
327 arthrodires, the posterior ampulla sits at about the same dorsoventral level as the anterior ampulla
328 and is above the level of the saccular chamber floor. In Romundina Ørvig (Dupret et al. 2017)
329 and osteichthyans (Jarvik 1980, Giles and Friedman 2014) the ampullae all sit in a line across the
330 top of the saccular cavity. In early chondrichthyans(Maisey 2005, Davis et al. 2012) and the
331 osteichthyan braincase assigned to Ligulalepis Schultze (Clement et al. 2018), the posterior
332 ampulla sits below the level of the anterior and external ampullae. However, the posterior
333 ampulla connects with the posterior part of the saccular cavity in these latter taxa, rather than on
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334 its ventral side. The unusual connection in Macropetalichthys and Ellopetalichthys could
335 therefore be a synapomorphy of petalichthyids.
336 In Macropetalichthys, Ellopetalichthys, and Shearsbyaspis (see Castiello & Brazeau
337 2018), there is a well-developed anteromedial recess of the saccular cavity occupying much of
338 the space between the endocranial cavity and the rear orbital wall (Figs. 2, 7, 8). Although
339 incompletely preserved in Ellopetalichthys, the medial wall of this cavity is observed in the right
340 labyrinth (Figs. 2-4). In arthrodires (Stensiö 1963, Young 1979, Goujet 1984a), the medial
341 margin of the utriculus slopes back posteriorly to meet the mesial wall of the sacculus. The
342 overall shape of the saccular and utricular cavities in Romundina are much more rounded in all 343 dimensions (Dupret et al. 2017), but thereDraft is no antero-medial cavity. Based on this evidence, the 344 anteriomedial cavity of the utriculus can be reasonably interpreted as a synapomorphy of at least
345 some macropetalichthyids.
346 Inner ear characters bearing on broader interrelationships of ‘placoderms’
347 The external ampulla is preserved in Ellopetalichthys (Figs. 2-4, 6), Shearsbyaspis, and
348 Macropetalichthys (Fig. 7) and is connected with the anterior ampulla at a well-developed
349 utricular cavity in all three genera. The external ampulla occupies the same position in these
350 petalichthyids: lateral to the external margin of the sacculus and posterodorsal to the ampulla
351 (Fig. 7). This laterally ‘out-turned’ nexus of the anterior and external ampullae is found in
352 arthrodires, as well as crown-group gnathostomes. The same condition is not observed in
353 Romundina (see Dupret et al. 2017) or Brindabellaspis Young (1980), where the nexus of
354 ampullae is situated on the anterior face of the saccular chamber. In osteostracans, the condition
355 is different from either arrangement because the anterior ampulla lies in a medial position
356 relative to the entire vestibule and no external ampulla is present because there is no external
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357 semi-circular canal (Janvier 1985). However, if one compares only the position of the anterior
358 ampulla across these taxa, a cline can be described with the anterior ampulla shifting from a
359 medial to increasingly lateral position relative to the sacculus between osteostracans, Romundina
360 and Brindabellaspis, and finally petalichthyids, arthrodires, and crown-group gnathostomes.
361 Such a cline would appear to support a grade of placoderms in which Romundina and
362 Brindabellaspis are in outgroup positions relative to the other placoderms and the gnathostome
363 crown. This would be consistent with proposals that so-called ‘acanthothoracid’ placoderms are
364 more phylogenetically removed from crown-group gnathostomes than most other placoderms
365 (Dupret et al. 2014). 366 Our observations confirm the absenceDraft of a sinus superior and skeletal crus commune in 367 macropetalichthyids (Figs. 2, 7). A skeletal crus commune is present in galeaspids, osteostracans,
368 osteichthyans, and non-elasmobranch chondrichthyans (Maisey 2001), Acanthodes Owen (Davis
369 et al. 2012), Romundina (Dupret et al., 2017) and likely also Jagorina Jaeckel (Stensiö 1950).
370 Given these consistent outgroup conditions, we consider the absence of a skeletal crus commune
371 as possible evidence of a close relationship between petalichthyids and arthrodires. The skeletal
372 crus commune is tentatively reconstructed in the antiarch Minicrania by Zhu and Janvier (1996)
373 based on impressions in the underside of the dermatocranium. We caution against inferring a
374 crus commune based on such impressions. Similar impressions are also known in arthrodires (see
375 Goujet 1984a, fig. 4; plate 3, figs. 1,2), where a skeletal crus commune is absent. These
376 impressions instead appear to reflect a ridge inferred to have carried a dorsomedial extension of
377 the external semi-circular canal (Stensiö 1963).
378 Comparative morphology of the endolymphatic ducts
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379 The complete endolymphatic canals of Ellopetalichthys allow clarification of their
380 apparently unusual structure in macropetalichthyids. Stensiö’s (1969) illustrations of the
381 endolymphatic ducts of Macropetalichthys show them as having oblate, large-diameter openings
382 in the top of the braincase. In Ellopetalichthys, it is evident that these are not openings, but are
383 oblate sac-like expansions of the endolymphatic duct. Brindabellapsis has similar oblate
384 endolymphatic sacs (Young 1980), but these differ in extending parallel to the long axis of the
385 brain endocavity. In petalichthyids, the sac is separated from the labyrinth and oriented laterally
386 at a right angle to the long axis of the endocavity (Fig. 2, 6). When broken through dorsally,
387 these would appear to be vertically oriented openings similar to Brindabellapsis. The unusual
388 shape in petalichthyids can be interpreted as accommodating the large parabranchial fossae, as
389 their sharp lateral twist follows the anteriorDraft wall of these cavities (Fig. 2). By contrast, such
390 fossae are absent in Brindabellaspis (Young 1980).
391
392 Petalichthyid occipital and neck joint morphology
393 The craniothoracic neck joint of placoderms carries important phylogenetic information (Goujet
394 1984b, Goujet and Young 1995, Zhu et al. 2019). The occipital region and neck joint
395 morphology of petalichthyids exhibits several, likely apomorphic, conditions. Some aspects of
396 the neck joint have been used to propose close relationship between petalichthyids and
397 ptyctodontid placoderms (Goujet 1984b, Goujet and Young 1995). However, a clear
398 understanding of the comparative morphology of this area is still lacking. This is due to the few
399 specimens in which the occipital joint is well preserved and the rarity of confidently assigned
400 petalichthyid anterior dorsolateral (ADL) plates bearing the complementary articulation for the
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401 joint. The cranial component of the joint is well-preserved in Ellopetalichthys and can help
402 resolve some comparative anatomical problems.
403 Comparative anatomy of cranio-thoracic joints
404 Young (1978) and Goujet (1984b) noted difficulties in interpreting the comparative
405 morphology of the petalichthyid occiput and neck joint. Young tentatively interpreted a pair of
406 circular depressions on the occipital surface of Wijdeaspis as lateral articular facets (1978).
407 These facets were situated immediately above and lateral to the foramen magnum. He argued
408 that they compare well in shape and position to similarly named facets in arthrodires. The
409 problem with this interpretation is that these depressions occupy nearly the entire space between 410 the para-articular processes and leave noDraft room for head levator muscles, creating a functionally 411 dubious arrangement. These depressions instead correspond positionally to the inferred
412 insertions of the epaxial or levator capitis muscles of other placoderms (Fig. 9). By this
413 interpretation, lateral articular facets in petalichthyids are undeveloped.
414 Nevertheless, we accept Young’s (1978) identification of the deep dermal blades flanking
415 the occiput as para-articular processes (Figs. 1, 3, 5, 9). This process sits immediately below the
416 main lateral line canal in both arthrodires and petalichthyids (Fig. 9). The terminology used here
417 and by Young follows that of Stensiö (1963), but the same structure has been variably described
418 as the “glenoid process” of other authors (White and Toombs 1972). This process occurs widely
419 among arthrodires and is in some cases it is well developed as in Mulgaspis (Ritchie 2004). The
420 chief difference between arthrodires and petalichthyids, therefore, is that in the former the blades
421 are turned outwards laterally, whereas in petalichthyids they are vertically oriented, and their
422 long axis is longitudinal (Fig. 9).
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423 In phylogenetic character lists of placoderms and early gnathostomes, the cranio-thoracic
424 joint is treated as a single character and subdivided into five states. Goujet and Young (1995)
425 identify the shared condition between petalichthyids and ptyctodonts as a “longitudinal” state,
426 while Dupret et al. (2014) identify it as a “spoon-like” state and shared with Romundina (we see
427 no compelling similarities between the neck joints of petalichthyids or ptyctodonts, on the one
428 hand, and Romundina, on the other, and suggest this may simply have been coded in error).
429 Neither state is clearly defined. We argue that the use of a single, unordered multi-state character
430 inadequately captures neck joint diversity. The para-articular process is present in petalichthyids
431 and many arthrodires. Thus, the “longitudinal” state is only applicable to those taxa with a para-
432 articular process, which in turn can be treated as its own binary presence/absence character. Draft 433 The presence of a para-articular process in ptyctodontids has not, in fact, been clearly
434 illustrated in any indisputable ptyctodontid taxa, leaving open the significance of this trait in
435 uniting petalichthyids and ptyctodonts. Comparisons between ptyctodontid and petalichthyid
436 neck joints rely on Mark-Kurik’s (1977) description of Tollodus Mark-Kurik. However, the
437 specimen showing a petalichthyid-like corresponding skull roof and dorsal shoulder arcade
438 (Mark-Kurik, 1977, fig. 6) is not assigned to Tollodus and is not unequivocally referable to a
439 ptyctodontid as it is incomplete. However, Mark-Kurik’s comparison likened the very similar
440 ADL plates of this specimen with those of Tollodus, suggesting a nearly identical mode of
441 articulation. The specimen bears a petalichthyid-like neck joint with longitudinally oriented para-
442 articular processes, clasped laterally by the articular process of the ADL plate. In well-preserved,
443 indisputable ptyctodont fossils (e.g. Long 1997, Trinajstic et al. 2012), there is no set of
444 processes that have yet been identified as para-articular processes. Given these uncertainties,
445 more detailed analyses of this anatomy in ptyctodontids seem warranted.
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446 We suggest that the craniothoracic joints should be broken into multiple characters, rather
447 than a single multi-state character (Zhu et al. 2019). A para-articular process may unite
448 arthrodires and petalichthyids; ptyctodontids could join these groups if the presence of a process
449 could be more robustly documented. The so-called ‘ginglymoid’ neck joint is seen in arthrodires
450 refers to a well-developed, nearly hemi-cylindrical lateral articular facet (Fig. 9) that receives a
451 complementary condyle on the ADL should be its own binary character. The presence of such a
452 facet has no bearing on the presence of a para-articular process or its shape. The shape of the
453 para-articular process may be either vertically oriented as in petalichthyids,or laterally splayed as
454 in arthrodires (Fig. 9). These conditions should therefore be represented as a set of reductively
455 coded characters (Strong and Lipscomb 1999, Brazeau 2011). Draft 456
457 Biomechanical reinterpretation of petalichthyid cranio-thoracic joints
458 Our reinterpretation of the facets in Wijdeaspis as levator muscle insertions has
459 biomechanical implications. This revised identification is consistent with how the head levator
460 muscles would be expected to function: lifting the head, using the occipital glenoids as a
461 fulcrum. Young (1978) argued that this was functionally sub-optimal, as it places the hinge of
462 the dermal neck joint above the fulcrum formed by the occipital glenoids. He suggested that if
463 the joints formed a “guiding” surface (Young 1978, p. 113), rather than a pivot this position is
464 less problematic (note that the “gliding” type of neck used in phylogenetic character lists differs
465 from the condition described by Young’s confusingly similar term). We presume this guiding
466 function referred to the condition of having been clasped laterally by processes of the ADL plate,
467 as in Mark-Kurik’s (1977) inferred ptyctodont example. Our reinterpretation of the facets in
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468 Wijdeaspis as epaxial muscle insertions resolves Young’s dilemma: the head-elevating muscles,
469 which form the in-lever, now sit above the fulcrum formed by the occipital glenoids (Fig. 9).
470 Conclusions
471 The results of our examination of the three-dimensionally preserved holotype of Ellopetalichthys
472 represent the first description of the neurocranial anatomy for this taxon, including details on the
473 orbital wall, brain cavity, cranial nerve canals, inner ear and occipital joint morphology. We have
474 clarified a number of phylogenetic character states, proposing how they can be described and
475 encoded for phylogenetic analyses. However, further work must be done to better characterise
476 neck joints in placoderms—in particular, ptyctodontids—both in terms of comparative anatomy 477 and character coding procedures. RecentDraft studies of placoderm relationships might be overly 478 focused on the proximity of certain placoderm groups to the gnathostome crown. This possibly
479 leads trees overly imbalanced towards the gnathostome crown and could explain the high
480 instability of many placoderm subgroups between the trees of different phylogenetic studies.
481 Addressing this requires better understanding of characters that could potentially link the
482 subgroups of this assemblage. Petalichthyids have been of particular importance because of their
483 perceived osteostracan-like characters. However, as we have shown here and elsewhere
484 (Castiello and Brazeau 2018), new anatomical data for petalichthyids could restore the earlier
485 hypotheses of their close relationships with arthrodires and the secondary derivation of their
486 osteostracan-like characters.
487
488 Acknowledgments
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489 We thank Franz-Josef Lindemann at the NHMO for loan of the Ellopetalichthys holotype. Farah
490 Ahmed at the NHM Imaging and Analysis Centre is thanked for assistance with conducting the
491 computed tomography scanning. Sam Giles (University of Birmingham) is thanked for
492 discussions on inner ear morphology. We thank Zerina Johanson (Natural History Museum,
493 UK), an anonymous referee, and Associate Editor Craig Scott for their detailed comments which
494 greatly improved the manuscript. The work was supported by a European Research Council
495 Starting Grant under European Union’s Seventh Framework Programme (FP/2007–2013)/ERC
496 Grant Agreement number 311092 to M.D.B. This work began when M. F. was at the University
497 of Oxford, where he was supported by St Hugh's College, the John Fell Fund, and a Philip
498 Leverhulme Prize. Draft 499
500 Figure Captions
501 Fig. 1. NHMO A13047, Ellopetalichthys scheii: (A) Skull reconstruction adapted from Ørvig
502 (1957); Virtual 3D models of skull from segmented CT data in (B) dorsal and (C) lateral views.
503 Note that artificial straight edges in the model and illustration reflect areas unresolved in the scan
504 and which could not be confidently segmented. APaN, anterior paranuchal plate; Mg, marginal
505 plate; Nu, nuchal plate; PaN, paranuchal plate; Po, postorbital plate.
506
507 Fig. 2. NHMO A13047, Ellopetalichthys scheii braincase and endocranial surface in dorsal view:
508 (A) virtual 3D model of skull from segmented CT data; (B) interpretive drawing. Note that
509 artificial straight edges in the model and illustration reflect areas unresolved in the scan and
510 which could not be confidently segmented. Dashed lines indicate incomplete margins. Roman
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511 numerals indicate canals for cranial nerves. Vpf?, possible canal of the profundus branch of
512 trigeminal nerve; VIIopht, superficial ophthalmic nerve canal. Colour scheme legend same as
513 Fig. 1.
514
515 Fig. 3. NHMO A13047, Ellopetalichthys scheii braincase and endocranial surface of in ventral
516 view: (A) virtual 3D model of skull from segmented CT data; (B) interpretive drawing. Note that
517 artificial straight edges in the model and illustration reflect areas unresolved in the scan and
518 which could not be confidently segmented. Roman numerals indicate canals for cranial nerves.
519 Vpf?, possible course of the profundus branch of trigeminal nerve; VIIbuc.lat, buccalis lateralis 520 branch of the facial nerve; Xa, anterior branchDraft of vagus nerve, Xp, posterior branch of vagus 521 nerve. Dashed lines indicate incomplete margins; Colour scheme legend for A same as Fig. 1;
522 colour scheme legend for B same as Fig. 2.
523
524 Fig. 4. NHMO A13047, Ellopetalichthys scheii transparent interpretive drawing of endocranium
525 and cranial endocavity in ventral view with transparent overlay of basicranial surface to show
526 positional relationships between nerve canals and superficial structures. Note that artificial (i.e.
527 straight) edges in the illustration reflect areas unresolved in the scan and which could not be
528 confidently segmented. Xa, anterior branch of vagus nerve, Xp, posterior branch of vagus nerve.
529 Dashed lines indicate incomplete margins; cross-hatching indicates incomplete or weakly
530 ossified surfaces. Colour scheme and hatching legends same as Figs. 2 and 3.
531
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532 Fig. 5. NHMO A13047, Ellopetalichthys scheii braincase and endocranial surface of in posterior
533 view: (A) virtual 3D model from segmented CT data; (B) interpretive drawing. Colour scheme
534 legend for A same as Fig. 1; colour scheme legend for B same as Fig. 2.
535
536 Fig. 6. NHMO A13047, Ellopetalichthys scheii skeletal labyrinths in lateral views: (A) left
537 lateral view interpretive drawing; (B) virtual 3D model of left lateral view from segmented CT
538 data; (D) right lateral view interpretive drawing; (B) virtual 3D model of right lateral view from
539 segmented CT data; (E) composite reconstruction in left lateral view. Dashed lines indicate
540 incomplete or inferred margins; Hatching convention same as Fig. 3. X, vagus nerve.
541 Draft
542 Fig. 7. Reconstruction of the braincase and nerve canal endocast of Ellopetalichthys scheii.
543 Roman numerals indicate canals for cranial nerves. Vpf, profundus branch of trigeminal nerve;
544 VIIbuc.lat, ramus buccalis lateralis of facial nerve; VIIopht, superficial ophthalmic nerve canal;
545 Xa, anterior branch of vagus nerve, Xp, posterior branch of vagus nerve.
546
547 Fig. 8. Reconstruction and comparison of the skeletal labyrinth of three petalichthyid taxa
548 showing a new interpretation of the likely position of the ampulla and course of anterior semi-
549 circular canal in Macropetalichthys. Macropetalichthys drawings are re-drafted and modified
550 based on Stensiö (1969). Shearsbyaspis is redrafted from Castiello and Brazeau (2018). Colour
551 scheme legend for A same as Fig. 1.
552
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553 Fig. 9. Comparative anatomy of the occipital surface and cranial side of the cranio-thoracic joint
554 of four placoderm taxa: (A) Arabosteus (‘Acanthothoraci’); (B) Parabuchanosteus (Arthrodira);
555 (C) Ellopetalichthys (Petalichthyida); (D) Wijdeaspis (Petalichthyida). The para-articular process
556 is shown to consistently join below the course of the main lateral line canal in petalichthyids and
557 the arthrodire Parabuchanosteus [redrawn and modified from White and Toombs (1972) and
558 modified after (Long et al. 2014)]. Wijdeaspsis is redrawn from Young (1978). Arabosteus is
559 original artwork drawn from photograph in Olive et al. (2011).
560
561 Literature cited 562 Becker, R.T., Gradstein, F.M., and Hammer,Draft O. 2012. The Devonian Period. In The Geologic 563 Time Scale. Edited by F.M. Gradstein, J.G. Ogg, M.D. Schmitz and G.M. Ogg. Elsevier. pp.
564 559–601. doi:10.1016/B978-0-444-59425-9.00022-6.
565 Brazeau, M.D. 2009. The braincase and jaws of a Devonian “acanthodian” and modern
566 gnathostome origins. Nature, 457: 305–308. doi:10.1038/nature07436.
567 Brazeau, M.D. 2011. Problematic character coding methods in morphology and their effects.
568 Biological Journal of the Linnean Society, 104: 489–498. Wiley Online Library.
569 Brazeau, M.D., and Friedman, M. 2014. The characters of Palaeozoic jawed vertebrates.
570 Zoological Journal of the Linnean Society, 170: 779–821. doi:10.1111/zoj.12111.
571 Castiello, M., and Brazeau, M.D. 2018. Neurocranial anatomy of the petalichthyid placoderm
572 Shearsbyaspis oepiki Young revealed by X‐ray computed microtomography. Palaeontology,
573 61: 369-389. doi:10.1111/pala.12345.
26 https://mc06.manuscriptcentral.com/cjes-pubs Page 27 of 39 Canadian Journal of Earth Sciences
574 Clement, A.M., King, B., Giles, S., Choo, B., Ahlberg, P.E., Young, G.C., and Long, J.A. 2018.
575 Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan
576 evolution. eLife, 7: e34349. eLife Sciences Publications Limited. doi:10.7554/eLife.34349.
577 Davis, S.P., Finarelli, J.A., and Coates, M.I. 2012. Acanthodes and shark-like conditions in the
578 last common ancestor of modern gnathostomes. Nature, 486: 247–250.
579 doi:10.1038/nature11080.
580 Dupret, V., Sanchez, S., Goujet, D., and Ahlberg, P.E. 2017. The internal cranial anatomy of
581 Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci) and the origin of
582 jawed vertebrates—Anatomical atlas of a primitive gnathostome. PLoS ONE, 12: e0171241.
583 doi:10.1371/journal.pone.0171241.
584 Eastman, C.R. 1898. Some new points inDraft dinichthyid osteology. Proceedings of the American
585 Association for the Advancement of Science, 47: 371–372.
586 Giles, S., and Friedman, M. 2014. Virtual reconstruction of endocast anatomy in early ray-finned
587 fishes (Osteichthyes, Actinopterygii). Journal of Paleontology, 88: 636–651. doi:10.1666/13-
588 094.
589 Giles, S., Friedman, M., and Brazeau, M.D. 2015. Osteichthyan-like cranial conditions in an
590 Early Devonian stem gnathostome. Nature, 520: 82–85. doi:10.1038/nature14065.
591 Goujet, D. 1984a. Les poissons placodermes du Spitsberg. Cahiers de Paléontologie, Editions du
592 CNRS.
593 Goujet, D. 1984b. Placoderm interrelationships: a new interpretation, with a short review of
594 placoderm classifications. Proceedings of the Linnean Society of New South Wales, 107:
595 211–243.
596 Goujet, D., and Young, G. 1995. Interrelationships of placoderms revisited. Geobios, 28: 89–95.
27 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 28 of 39
597 Jaekel, O. 1911. Die Wirbeltiere: eine Übersicht über die fossilen und lebenden Formen. Verlag
598 Gebrüder Borntraeger, Berlin.
599 Janvier, P. 1985. Les céphalaspides du Spitsberg. Éditions du Centre National de la Recherche
600 Scientifique, Paris.
601 Janvier, P. 1996. The dawn of the vertebrates: Characters versus common ascent in the rise of
602 current vertebrate phylogenies. Palaeontology, 39: 259–287.
603 Jarvik, E. 1980. Basic Structure and Evolution of Vertebrates. Academic Press, London.
604 Kiær, J. 1915. Upper Devonian fish remains from Ellesemere Land with remarks on
605 Drepanaspis. In Report of the Second Norwegian Arctic Expedition in the "Fram". pp. 1–56.
606 Long, J.A., Mark-Kurik, E., and Young, G.C. 2014. Taxonomic revision of buchanosteoid
607 placoderms (Arthrodira) from the EarlyDraft Devonian of south-eastern Australia and Arctic
608 Russia. Australian Journal of Zoology, 62: 26. doi:10.1071/ZO13081.
609 Maisey, J. 2005. Braincase of the Upper Devonian shark Cladodoides wildungensis
610 (Chondrichthyes, Elasmobranchii), with observations on the braincase in early
611 chondrichthyans. Bulletin of the American Museum of Natural History, 288: 1–103.
612 Maisey, J.G. 2001. Remarks on the inner ear of elasmobranchs and its interpretation from
613 skeletal labyrinth morphology. Journal of Morphology, 250: 236–264.
614 doi:10.1002/jmor.1068.
615 Mark-Kurik, E. 1977. Shoulder girdle structure of early ptycotodonts. In Ocherki po filogenii i
616 sistematike iskopaemykh ryb i beschelyustnykh [Essays on Phylogeny and Systematics of
617 Fossil Agnathans and Fishes]. Edited by V.V. Menner. Akademia Nauk. pp. 61–70.
618 Mayr, U., and Packard, J.J. 1994. Upper Ordovician to Lower Devonian Carbonate Platform.
619 Geological Survey of Canada Bulletin, Bulletin 470: 68–95.
28 https://mc06.manuscriptcentral.com/cjes-pubs Page 29 of 39 Canadian Journal of Earth Sciences
620 M’Coy, F. 1848. On some new fossil fish of the Carboniferous Period. The Annals and
621 Magazine of Natural History, 2: 1–10.
622 Ørvig, T. 1957. Notes on some Paleozoic lower vertebrates from Spitsbergen and North
623 America. Norsk Geologisk Tidsskrift, 37: 285–353.
624 Ørvig, T. 1975. Description, with special reference to the dermal skeleton, of a new radotinid
625 arthrodire from the Gedinnian of Arctic Canada. Colloque international C.N.R.S. no. 218,:
626 43–71.
627 Ritchie, A. 2004. A new genus and two new species of groenlandaspidid arthrodire (Pisces:
628 Placodermi) from the Early-Middle Devonian Mulga Downs Group of western New South
629 Wales, Australia. Fossils and Strata 50: 56–81.
630 Stensiö, E.A. 1925. On the head of the macropetalichthyids.Draft Field Museum of Natural History
631 Publication Geological Series, 4: 87–197.
632 Stensiö, E.A. 1950. La cavité labyrinthique, l’ossification sclérotique et l’orbite de Jagorina.
633 Colloques internationaux du Centre national de la Recherche scientifique, 21: 9–43.
634 Stensiö, E.A. 1963. Anatomical studies on the arthrodiran head. Kunliga Svenksa
635 Vetenskapsakademiens Handlingar, 9: 1–419. Almqvist & Wiksell.
636 Stensiö, E.A. 1969. Elasmobranchiomorphi Placodermata Arthrodires. In Traité de
637 Paléontologie, vol 4. Edited by J. Piveteau. Masson, Paris. pp. 71–692.
638 Strong, E.E., and Lipscomb, D. 1999. Character coding and inapplicable data. Cladistics, 15:
639 363–371. doi:10.1006/clad.1999.0114.
640 White, E.I., and Toombs, H.A. 1972. The buchanosteid arthrodires of Australia. Bulletin of the
641 British Museum (Natural History) Geology, 22: 377–419. BM(NH).
29 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 30 of 39
642 Young, G.C. 1978. A new Early Devonian petalichthyid fish from the Taemas/Wee Jasper region
643 of New South Wales. Alcheringa, 2: 103–116. doi:10.1080/03115517808619082.
644 Young, G.C. 1979. New information on the structure and relationships of Buchanosteus
645 (Placodermi: Euarthrodira) from the Early Devonian of New South Wales. Zoological
646 Journal of the Linnean Society, 66: 309–352. Wiley Online Library.
647 Young, G.C. 1980. A new Early Devonian placoderm from New South Wales, Australia, with a
648 discussion of placoderm phylogeny. Palaeontographica, Abteilung A, 167: 10–76.
649 Palaeontographica, Abteilung A.
650 Zhu, M., and Janvier, P. 1996. A small antiarch, Minicrania lirouyii gen. et sp. nov., from the
651 Early Devonian of Qujing, Yunnan (China), with remarks on antiarch phylogeny. Journal of
652 Vertebrate Paleontology, 16: 1–15. Draft
653 Zhu, Y.-A., Lu, J., and Zhu, M. 2019. Reappraisal of the Silurian placoderm Silurolepis and
654 insights into the dermal neck joint evolution. Royal Society Open Science, 6: 191181.
655 doi:10.1098/rsos.191181.
30 https://mc06.manuscriptcentral.com/cjes-pubs Page 31 of 39 Canadian Journal of Earth Sciences
Draft
Fig. 1. NHMO A13047, Ellopetalichthys scheii: (A) Skull reconstruction adapted from Ørvig (1957); Virtual 3D models of skull from segmented CT data in (B) dorsal and (C) lateral views. Note that artificial straight edges in the model and illustration reflect areas unresolved in the scan and which could not be confidently segmented. APaN, anterior paranuchal plate; Mg, marginal plate; Nu, nuchal plate; PaN, paranuchal plate; Po, postorbital plate.
181x133mm (300 x 300 DPI)
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Fig. 2. NHMO A13047, Ellopetalichthys scheii braincase and endocranial surface in dorsal view: (A) virtual 3D model of skull from segmented CT data;Draft (B) interpretive drawing. Note that artificial straight edges in the model and illustration reflect areas unresolved in the scan and which could not be confidently segmented. Dashed lines indicate incomplete margins. Roman numerals indicate canals for cranial nerves. Vpf?, possible canal of the profundus branch of trigeminal nerve; VIIopht, superficial ophthalmic nerve canal. Colour scheme legend same as Fig. 1.
181x105mm (300 x 300 DPI)
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Fig. 3. NHMO A13047, Ellopetalichthys scheii braincase and endocranial surface of in ventral view: (A) virtual 3D model of skull from segmented CT data; (B) interpretive drawing. Note that artificial straight edges in the model and illustration reflect areasDraft unresolved in the scan and which could not be confidently segmented. Roman numerals indicate canals for cranial nerves. Vpf?, possible course of the profundus branch of trigeminal nerve; VIIbuc.lat, buccalis lateralis branch of the facial nerve; Xa, anterior branch of vagus nerve, Xp, posterior branch of vagus nerve. Dashed lines indicate incomplete margins; Colour scheme legend for A same as Fig. 1; colour scheme legend for B same as Fig. 2.
176x99mm (300 x 300 DPI)
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Fig. 4. NHMO A13047, Ellopetalichthys scheii transparent interpretive drawing of endocranium and cranial endocavity in ventral view with transparent overlay of basicranial surface to show positional relationships between nerve canals and superficial structures. Note that artificial (i.e. straight) edges in the illustration reflect areas unresolved in the scan and which could not be confidently segmented. Xa, anterior branch of vagus nerve, Xp, posterior branch of vagus nerve. Dashed lines indicate incomplete margins; cross-hatching indicates incomplete or weakly ossified surfaces. Colour scheme and hatching legends same as Figs. 2 and 3.
85x93mm (300 x 300 DPI)
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Fig. 5. NHMO A13047, Ellopetalichthys scheii braincase and endocranial surface of in posterior view: (A) virtual 3D model from segmented CT data; (B) interpretive drawing. Colour scheme legend for A same as Fig. 1; colour scheme legend for B same as Fig. 2.
97x101mm (300 x 300 DPI)
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Fig. 6. NHMO A13047, Ellopetalichthys scheii skeletal labyrinths in lateral views: (A) left lateral view interpretive drawing; (B) virtual 3D model of left lateral view from segmented CT data; (D) right lateral view interpretive drawing; (B) virtual 3D model of right lateral view from segmented CT data; (E) composite reconstruction in left lateral view. Dashed lines indicate incomplete or inferred margins; Hatching convention same as Fig. 3. X, vagus nerve.
85x173mm (300 x 300 DPI)
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Fig. 7. Reconstruction of the braincase and nerve canal endocast of Ellopetalichthys scheii. Roman numerals indicate canals for cranial nerves. Vpf, profundus branch of trigeminal nerve; VIIbuc.lat, ramus buccalis lateralis of facial nerve; VIIopht, superficial ophthalmic nerve canal; Xa, anterior branch of vagus nerve, Xp, posterior branch of vagus nerve.
88x104mm (300 x 300 DPI)
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Fig. 8. Reconstruction and comparison of the skeletal labyrinth of three petalichthyid taxa showing a new interpretation of the likely position of the ampulla and course of anterior semi-circular canal in Macropetalichthys. Macropetalichthys drawings are re-drafted and modified based on Stensiö (1969). Shearsbyaspis is redrafted from Castiello andDraft Brazeau (2018). Colour scheme legend for A same as Fig. 1. 134x68mm (300 x 300 DPI)
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Fig. 9. Comparative anatomy of the occipital surface and cranial side of the cranio-thoracic joint of four placoderm taxa: (A) Arabosteus (‘Acanthothoraci’); (B) Parabuchanosteus (Arthrodira); (C) Ellopetalichthys (Petalichthyida); (D) Wijdeaspis (Petalichthyida). The para-articular process is shown to consistently join below the course of the main lateral line canal in petalichthyids and the arthrodire Parabuchanosteus [redrawn and modified from White and Toombs (1972) and modified after (Long et al. 2014)]. Wijdeaspsis is redrawn from Young (1978). Arabosteus is drawn from photograph in Olive et al. (2011).
134x143mm (300 x 300 DPI)
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