Canadian Journal of Earth Sciences
Cretaceous flora and fauna of the Sustut Group near the Sustut River, northern British Columbia, Canada
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2019-0031.R2
Manuscript Type: Article
Date Submitted by the 26-Jul-2019 Author:
Complete List of Authors: Arbour, Victoria; Royal BC Museum, ; Evans, David; Royal Ontario Museum, Department of Natural History; University of Toronto, Ecology and Evolutionary Biology Simon, D.; University of Toronto, Ecology & Evolutionary Biology Cullen, Thomas;Draft Field Museum of Natural History Braman, Dennis R.; Royal Tyrrell Museum of Palaeontology,
Cretaceous, Sustut Group, Nanhsiungchelyidae, Metasequoia, Sustut Keyword: Basin, Ornithischia
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
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1 Cretaceous flora and fauna of the Sustut Group near the Sustut River, northern British
2 Columbia, Canada
3
4 Victoria M. Arbour1,*, David C. Evans2, D. Jade Simon2,3, Thomas M. Cullen4, Dennis Braman5
5
6 Affiliations
7 1Department of Knowledge, Royal BC Museum, 675 Belleville St, Victoria, BC, V8V 9W2, Canada
8 2Department of Natural History, Royal Ontario Museum, 100 Queens Park, Toronto, ON, M5S
9 2C6 Canada 10 3Department of Ecology & EvolutionaryDraft Biology, 25 Willcocks St, Toronto, ON M5S 3B2 Canada 11 4Department of Geology, Field Museum of Natural History, 1400 South Lake Shore Dr, Chicago,
12 IL, 60605, USA
13 5Royal Tyrrell Museum of Palaeontology, 1500 North Dinosaur Trail, Drumheller, AB, T0J 0Y0,
14 Canada
16
17
18
19
20
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22
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23 ABSTRACT: A partial ornithischian dinosaur skeleton discovered near the Sustut River in 1971
24 has, to date, represented the only vertebrate fossil remains recovered from the Sustut Basin in
25 northern British Columbia, Canada, but the geological provenance and age of this specimen has
26 remained unclear. We provide new data on the age of this dinosaur specimen based on
27 reconnaissance palaeontological prospecting along the Sustut River, and also report new
28 vertebrate and plant fossils from this region. A skeletal fragment of the turtle Basilemys sp. was
29 discovered at a site closely matching field notes describing the initial collection of the
30 ornithischian dinosaur, suggesting that the new turtle fossil derives from the same locality as
31 the dinosaur. Palynomorphs collected from this site include the marker taxon 32 Pseudoaquilapollenites bertillonites, foundDraft in the lower Hell Creek Formation, and suggesting an 33 age range of between 68.2 and 67.2 Ma for the locality. To the west of this locality we
34 discovered multiple new fossil plant sites preserving wood and the leaves of Metasequoia and
35 several angiosperms, and one site preserved fronds resembling the tree fern Coniopteris sp.,
36 suggesting a Cenomanian or older age for sites the area. The complex translational history of
37 the Intermontane Terrane means that the newly discovered turtle may not represent a
38 northern range extension for Basilemys, but it does represent one of the westernmost
39 occurrences of this genus. The discovery of new vertebrate fossil remains in a region with
40 relatively little accessible outcrop at present indicates for the potential for future discoveries in
41 the higher elevation outcrops of the Sustut Basin in this mountainous region of British
42 Columbia.
43 Keywords: Cretaceous, Sustut Group, Nanhsiungchelyidae, Ornithischia, Metasequoia, Sustut
44 Basin
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45 INTRODUCTION
46 The Sustut Basin is a vast terrestrial Cretaceous basin in the northern interior of British
47 Columbia (Fig. 1). The thickness of its accumulated sediments, and its exposed surface area,
48 would seem to make it a prime candidate for vertebrate fossil exploration, given interest in the
49 terrestrial basins of the North American western interior, but its remote location and rugged
50 terrain have resulted in relatively little attention from palaeontologists. A well preserved, but
51 fragmentary specimen of a small ornithischian dinosaur (RBCM P900) discovered in 1971 near
52 the confluence of Birdflat Creek and the Sustut River in the southern portion of the Sustut Basin
53 by Kenny F. Larsen (Arbour and Graves 2008) points to the potential for significant new fossil 54 discoveries in this region. This specimenDraft was collected prior to extensive geological fieldwork 55 and data collection undertaken in the 1980s through the 2000s (Evenchick et al. 2003,
56 Evenchick and Thorkelson 2005, McMechan et al. 2007) and consequently the precise location
57 and geological age of the original collection site has been unknown. A one-day reconnaissance
58 trip to the area by the Peace Region Palaeontological Research Centre in 2013 did not recover
59 additional vertebrate skeletal fossils or identify the original collection site, but reported
60 dinosaur footprints at a site along the Sustut River (R. McCrea, pers. comm. 2019 ). Given the
61 rarity of vertebrate fossils from the region, a detailed understanding of the stratigraphic
62 context for RBCM P900 is crucial for interpreting its significance to broader patterns of dinosaur
63 biogeography in western North America during the Late Cretaceous. To that end, we undertook
64 a reconnaissance field expedition to the Sustut River area near its confluence with Birdflat
65 Creek in 2017 with the aims of 1) relocating the original collection site of RBCM P900, 2)
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66 searching for additional fossils in the area, and 3) documenting the geologic context and age of
67 RBCM P900 and any newly discovered fossils.
68
69 Geographical and geological setting
70 Interviews with K. Larsen by one of us (VMA) in February 2005, as well as copies of
71 Larsen’s 1971 field notes (SI 2), provided the initial geographic information for where RBCM
72 P900 was discovered near the confluence of the Sustut River and Birdflat Creek. From the 1960s
73 to 1970s, BC Rail constructed the Dease Lake Extension through much of the Sustut Basin,
74 reaching the extent of the Sustut River in 1971 (and providing the access that would ultimately 75 result in the discovery of RBCM P900). ThisDraft part of the Sustut Basin was also heavily logged 76 from the early 1990s to 2007 and, as such, several dirt roads and airstrips were developed, and
77 the railway reopened for use after having been abandoned since the late 1970s (Rabnett and
78 Wilson 2008). Although the roads and railway are visible on Google Earth satellite images from
79 2005, the railway has now been inactive for several years and is densely vegetated, and only a
80 few airstrips and roads are maintained for use by two fishing lodges on the river. Access to the
81 area by our field team in August 2017 was via a fixed-wing aircraft at the Minaret grass airstrip,
82 originating from Smithers, BC. We prospected 14 km on foot along the abandoned BC Rail
83 Dease Lake Extension, about 1.5 km on foot along the Sustut River near the Birdflat Creek
84 confluence and under the Surespan Bridge, 800 m on foot along Birdflat Creek, and 6.5 km by
85 jet boat along the Sustut River. The extensive outcrops visible in photographs taken during the
86 construction of the railway line (Rabnett and Wilson 2008) are now much more limited, having
87 been revegetated in the intervening five decades.
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88 The Sustut Basin lies within the Stikinia terrane of the Intermontane Superterrane
89 (Ricketts 2008). Stikinia was an ancient Pacific Ocean volcanic arc that, along with several other
90 island arcs, slowly collided with the western edge of the continent around 170 million years ago
91 (Ricketts 2008). As additional terranes collided to the west of Stikinia and the Intermontane
92 Superterrane, Stikinia flexed and became a basin. The rising mountains to the west deposited
93 sediments into the Sustut Basin, forming the Sustut Group (Riddell 2011). Although the Sustut
94 River is today located at about 56.6°N, palaeomagnetic data suggest that Wrangellia and the
95 Intermontane Superterrane were locked together and located around 1700 km south of their
96 present position 75 million years ago (Shaw and Johnston 2016, Enkin et al. 2003, Enkin 2006), 97 although others argue for a less extremeDraft amount of displacement (e.g. Butler and Gehrels 2001, 98 Symons et al. 2005).
99 The Sustut Group is a succession of clastic terrestrial rocks first defined by Lord (1948)
100 based on strata in the McConnell Creek map area (National Topographic Service map sheet
101 94D), and further mapped and described by Eisbacher (1971, 1974), Bustin and McKenzie
102 (1989), and Evenchick and Thorkelson (2005). It is subdivided into two formations, the lower
103 Tango Creek Formation and upper Brothers Peak Formation (Eisbacher 1971, 1974; Fig. 2). The
104 Tango Creek Formation consists of a series of alternating, fining-upwards sandstones and
105 mudstones. Eisbacher (1974) further subdivided the Tango Creek Formation into the informal
106 Niven and Tatlatui members, and subdivided the Brothers Peak Formation into the informal
107 Laslui and Spatsizi members. The Niven member of the Tango Creek Formation is a sandstone-
108 mudstone succession with sandstone more abundant, and green and red mudstones. The
109 Tatlatui member includes dark grey mudstones, chert-pebble-bearing arenites, and local thin
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110 seams of lignite. The overlying Brothers Peak Formation consists of cobble conglomerate,
111 conglomeratic sandstone, and silicic tuffs. The base of the Brothers Peak Formation is defined
112 as the base of the conglomerate or conglomeratic sandstone bed below the first tuff horizon.
113 Evenchick et al. (2003) note that the Tango Creek Formation - Brothers Peak Formation contact
114 is difficult to discern in the McConnell Creek map area because the lowest tuff is difficult to
115 recognize in low-lying areas. Plants and animals living in the Sustut Basin during the Cretaceous
116 would have inhabited a low-energy alluvial floodplain with lakes and ponds, bounded by rising
117 mountains to the east and west, an active volcanic center to the southwest (Bustin and
118 Mackenzie 1989), and possibly the ocean to the north (Evenchick and Thorkelson 2005). 119 In a comprehensive analysis of SustutDraft Group strata in the Spatsizi River map area (to the 120 north of our study area), Evenchick et al. (2003) suggested that the Tango Creek Formation is
121 Barremian or early Albian to late Campanian in age, while assigning a late Campanian to late
122 early Maastrichtian age to the Brothers Peak Formation , based on palynomorphs. Although the
123 sampled age of the base of the Tango Creek Formation was variable, the top of the formation
124 was never younger than late Campanian in all sampled sites. Lord (1948) reported the presence
125 of plant macrofossils in the McConnell Creek map area (the western boundary of which lies just
126 east of our main study area), including wood fragments and leaf impressions, and estimated an
127 age of approximately Cenomanian to Turonian for the lower part of the Sustut Group,
128 equivalent to the Dunvegan Formation of Alberta. In the southern portion of the Sustut Basin,
129 Bustin and McKenzie (1987) and Moffat et al. (1988) considered the palynomorph assemblage
130 of the Sustut Group to be Campanian-Maastrichtian in age based on the presence of
131 Aquillapollenites. Eisbacher (1971) reported K-Ar dates of Brothers Peak Formation tuffs of 53
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132 +- 6 my and 49 +- 5 my, suggesting that the Brothers Peak Formation was Palaeocene-Eocene in
133 age; Evenchick et al. (2003) considered the discrepancy between K-Ar and palynological dates
134 for the Brothers Peak Formation to possibly be the result of a Tertiary thermal event which
135 reset the K-Ar ages.
136
137 METHODS
138 Specimens were collected under a research authorization to VMA by the Heritage
139 Branch of the BC Ministry of Forests, Land, Natural Resource Operations, and Rural
140 Development. Sites of interest were marked with handheld GPS units, and GPS coordinates are 141 available from the Royal BC Museum (RBCM)Draft upon request. All specimens are permanently 142 accessioned at the RBCM (SI 1). Where applicable, we retained site names for points along the
143 Sustut River used by the operators of Suskeena Lodge to reference outcrops.
144 Mudstone samples (RBCM P29, RBCM P88, RBCM P114) were collected from beneath
145 the weathered surfaces of several fossiliferous outcrops in the study area. We selected three
146 sites for palynological sampling, and these samples were processed by Global Geolab Ltd
147 (Medicine Hat, Alberta) using standard palynological sampling techniques. A 10 to 20 gram
148 sample was processed by dissolving the mineral matter with the use of hydrofluoric acids, the
149 transparency of the fossils was adjusted using an oxidation agent, the organic portion of the
150 resulting residue was concentrated using heavy liquid separation and sieving techniques, a stain
151 was applied to enhance the visibility of the wall structure of the palynomorphs, the resulting
152 organic residue was mounted on a glass cover slip using polyvinyl alcohol, and the cover slip
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153 was permanently mounted to a glass slide using bioplastics. The resulting glass slide was
154 studied using a biological microscope and the observed specimens identified.
155
156 RESULTS
157 Dinosaur site relocation
158 K. Larsen’s 1971 field notes (SI 2) and February 2005 interviews with VMA indicated that
159 the dinosaur specimen was discovered in a talus slope on the BC Rail grade near the confluence
160 of Birdflat Creek and the Sustut River. The specimens were first discovered on August 3, 1971
161 and Larsen returned to the site one additional time on August 4, when his field notes indicate 162 he collected the articulated toe of RBCMDraft P900. Larsen noted the presence of petrified logs and 163 carbonized plant remains in the area as well, and he recalled being able to see the Sustut River
164 from the dinosaur site. The specimens were described as weakly radioactive, but no formal
165 readings were recorded.
166 In total we discovered six localities yielding fossil leaves to the west of the Birdflat
167 Creek-Sustut River confluence, and one locality that produced a single fragment of vertebrate
168 bone to the east of the confluence (Fig. 3); we did not observe any dinosaur footprints as
169 observed by R. McCrea and L. Buckley (McCrea pers. comm. 2019) but flat outcrops along the
170 Sustut River in this area were largely obscured by fallen logs in 2017. The fragmentary turtle
171 specimen (Fig. 3) was discovered in the talus at the base of a small outcrop a few hundred
172 meters east of Birdflat Creek along the BC Rail line. The specimen shares the same charcoal
173 grey, glossy, recrystallized preservation style as RBCM P900. Additionally, matrix attached to
174 the tibia and phalanges of RBCM P900 matches the colour and texture of the siltstones at this
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175 outcrop. The railway is very close to the Sustut River at this locality, and this site closely
176 matches Larsen’s notes and recollections; although the river is not visible from this site, no
177 outcrops in this area had a clear view of the river, and it is possible that new vegetation
178 currently blocks a view that was unimpeded during the initial construction of the railway in the
179 1970s. No fossils of any kind were encountered for an additional 7 km farther east on the
180 railway line from this site. West of Birdflat Creek, the railway line does not follow the Sustut
181 River as closely, and only plant fossils were recovered. Taken together, these clues indicate that
182 the Turtle Site just east of Birdflat Creek is the most likely provenance of RBCM P900, or at least
183 from the same fossiliferous stratigraphic horizon nearby. 184 Draft 185
186 Flora and fauna
187 We recovered a single vertebrate fossil, a fragment of a turtle carapace (RBCM P1), near
188 the confluence of Birdflat Creek and the Sustut River (Fig. 4). RBCM P1 has a maximum
189 mediolateral width of 485 mm, a maximum dorsoventral depth of 285 mm, and a maximum
190 anteroposterior length of 205 mm. The fragment does not preserve any internal bony structure;
191 a faint demarcation between an outer ‘cortex’ and the interior of this solid fragment is visible,
192 but similar rinds were noted on other glossy mudstone pieces encountered during prospecting
193 in the Sustut River area, and thus may not represent original anatomical features. The fragment
194 represents part of the lateral edge of the shell: the dorsal surface is strongly convex, and the
195 ventral surface is nearly flat. The surface sculpturing consists of a series of small pyramidal
196 peaks arranged in rough rows separated by shallow pits, a texture found on the carapace of the
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197 nanhsiungchelyid turtle Basilemys (Brinkman 2005). Basilemys is known from the Santonian to
198 Maastrichtian, and includes five species that are distinguished from one another by features of
199 the shell (Mallon and Brinkman 2018); unfortunately, RBCM P1 is too fragmentary to refer to
200 any known species.
201 We collected plant fossils from five sites along the Sustut River and BC Rail line and
202 recorded plant remains (but made no collections) at an additional three sites (Fig. 3, Table 1).
203 Leaf impressions and compressions were the most commonly encountered remains, but several
204 sites along the river and BC Rail line near the Surespan Bridge preserved abundant large wood
205 fragments of indeterminate taxonomic affinity, and one site included a large (~80 cm long) 206 coalified log. One potential seed (RBCMDraft P107) was recovered from a concretion at Suskeena 207 Lodge Hill. We follow the Leaf Architecture Working Group’s Manual of Leaf Architecture (Ash
208 et al. 1999) for terminology describing leaf morphology and morphotypes here.
209 Approximately 33% (40 out of 119) of the specimens we collected from all sites are
210 identified as leaves and shoots of the dawn redwood Metasequoia (Fig. 5). All of the leaves in
211 our collection show an opposite arrangement, and thus are referable to Metasequoia rather
212 than Taxodium, which have alternately arranged leaves. Metasequoia was recorded at four
213 sites (Amel’s, Suskeena Lodge Hill, Hickory Sticks, and immediately west of the Surespan
214 Bridge), and comprised at least 40-50% of the total specimens collected from two of these sites.
215 Although over 20 species have been proposed for this genus, recent taxonomic revisions have
216 argued for the validity of only three extinct species (M. occidentalis, M. milleri, and M. foxii)
217 plus the living species M. glyptostroboides (Liu et al. 1999, Stockey et al. 2001). These species
218 are most easily distinguished by the morphology of the reproductive structures (Stockey et al.
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219 2001), which are not preserved in our sample, so we conservatively refer our specimens to
220 Metasequoia sp.
221 Abundant remains of a fern (Fig. 5) were recovered at one site (Amel’s), where they
222 constitute approximately 37% (33 out of 89) of the specimens. The specimens have a pinnate-
223 pinnatifid appearance with deltoid frond segments. The fronds have rounded, asymmetric,
224 tapering pinnules with parallel venation. The absence of reproductive structures makes these
225 pinnae difficult to identify to lower taxonomic levels, but they bear a strong resemblance to the
226 tree fern Coniopteris (Cyatheales, Dicksoniaceae) (Tidwell 1998). Coniopteris is present at
227 several localities of Early Cretaceous age in western and northwestern North America, including 228 the adjacent Bowser Basin (MacLeod andDraft Hills 1991), and appears to be absent after the 229 Cenomanian (Skog 2001). Ferns were not recovered from any other sites in the study area.
230 Angiosperm fossils (Fig. 5, SI 1) were discovered at five localities along the Sustut River
231 (Lindy’s Lane, Amel’s, Suskeena Lodge Hill, Hickory Sticks, and immediately to the west of the
232 Surespan Bridge on the north side of the river, and to the east of the bridge on the south side of
233 the river), comprising between 25% and 100% of the specimens collected at each site, and
234 about 33% of the plant specimens overall.
235 Leaves were preserved as impressions or compressions. In many cases angiosperm
236 leaves occurred in leaf mats with many fragmentary and overlapping pieces; as such, very few
237 petioles or leaf margins are preserved, making it difficult to assign leaves to specific
238 morphotypes or taxa. All observed leaves had reticulate rather than parallel venation, and as
239 such represent eudicot angiosperms. Most specimens have uniformly spaced secondary veins
240 with convex, opposite percurrent tertiaries (e.g. RBCM P96). A few leaves with opposite
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241 percurrent tertiaries have secondary veins that abruptly decrease in angle towards the base
242 (e.g. RBCM P115). A less common morphotype includes uniformly spaced secondary veins with
243 alternate percurrent tertiary venation (e.g. RBCM P82). A few morphotypes are represented by
244 single specimens: 1) a leaf with a peltate petiole attachment and smooth margin (RBCM P46),
245 and 2) a distinctive long leaf with a tapering tip and only a single central primary vein visible
246 (RBCM P73).
247
248 Geologic age of the Sustut River dinosaur
249 Arbour and Graves (2008) suggested that the Sustut dinosaur was most likely recovered 250 from the lower portion of the Brothers DraftPeak Formation (Bustin and Mackenzie 1989), based on 251 mineralogical characteristics of geological thin sections derived from matrix remaining on the
252 bones. New geological and palynological data from our field survey allow us to refine this
253 interpretation.
254 We measured a 22 m thick section of outcrop along the railway to the east of Birdflat
255 Creek, beginning at the outcrop (the Turtle Site) at which RBCM P1 was discovered (Fig. 6). The
256 base of the outcrop is buried, and the top is obscured by thick vegetation. The beds strike 110°
257 E-W and dip 21° N-NE. Three metres of a litharenite are exposed at the base. Disorganized,
258 subrounded to rounded granules and pebbles ranging from 4 – 15 mm in size are present at the
259 base, and become sparser and smaller towards the top. This unit is overlain by a 4.75 m thick
260 sandy siltstone with only a few sparse granules and pebbles, 6 mm in size or less, at the very
261 base. The turtle fragment (RBCM P1) was found in a scree pile along this section of the outcrop
262 (Fig. 6). A 3 m thick paraconglomerate has scoured the top of the sandy siltstone and contains
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263 1-2 cm poorly sorted pebbles throughout the entire unit; the matrix fines upwards from sand to
264 silt. A 2 m thick mudstone conformably overlies the silty paraconglomerate and breaks into
265 characteristic fine chips. Slickensides are abundant in this unit and micas appear to have been
266 reoriented from low-grade metamorphism. This unit is scoured by a 4 m thick complex unit
267 containing 5-10 sequences of fining-upward pebbly sandstones for the first two meters, and
268 overall fining upward into a siltstone. This unit is overlain by a 1.5 m siltstone lacking pebbles.
269 The siltstone is scoured by a 1.75 m conglomerate, 80% of which is moderately sorted,
270 moderately rounded clasts. The top of the section is a 2 m thick coarse-grained sandstone with
271 basal 1 cm thick laminations; vegetation obscures the top of the section and its relationship to 272 outcrops to the east. These features areDraft consistent with previous descriptions of the geology of 273 the Tango Creek Formation (Eisbacher 1971, Bustin and McKenzie 1989).
274 To the west of Birdflat Creek, along both the Sustut River and BC Rail line, we found dark
275 grey fining-upwards sandstone-mudstone successions, consistent with descriptions of the
276 Tango Creek Formation by Evenchick et al. (2003, 2005). Approximately 300 m upstream along
277 Birdflat Creek the geology changed abruptly to thick sequences of resistant conglomerates
278 forming vertical walls, matching the description for the Brothers Peak Formation, although we
279 were unable to identify a basal tuff. Along the BC Rail line east of Birdflat Creek and west of
280 February Creek there are large swathes without outcrop or in which outcrop that was exposed
281 during railway construction has been re-vegetated. To the east of February Creek we
282 encountered a 1 km stretch of unfossiliferous outcrop that matches descriptions of the
283 Brothers Peak Formation. Our observations are generally consistent with features on the
284 geological map compiled by McMechan et al. (2007), which indicates outcrops of Tango Creek
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285 Formation on the Sustut River to the west of Birdflat Creek, and outcrops of Brothers Peak
286 Formation along Birdflat Creek and to the east of the creek along the Sustut River and BC Rail
287 line, with one small modification: the Turtle Site immediately to the east of Birdflat Creek
288 appears to represent Tango Creek Formation strata. All fossils encountered during our
289 fieldwork appear to come from the Tango Creek Formation.
290 We collected mudstone samples for palynological analysis from multiple sites (Table 2).
291 Two samples, one from the Hickory Sticks site along the BC Rail line, and the most productive
292 plant site along the Sustut River (Amel’s), were essentially barren of pollen. A third sample
293 (RBCM P114), collected from the Turtle Site (Fig. 5), contained the marker taxon 294 Pseudoaquilapollenites bertillonites, whichDraft has a restricted occurrence in the lower Hell Creek 295 Formation in the type area, which is roughly equivalent to the unconformity between the
296 Whitemud Member of the Horseshoe Canyon Formation and the Battle Formation in Alberta
297 (Braman 2018). Using radioisotopic data and estimated sedimentation rates for the Battle
298 Formation of Alberta, Eberth and Kamo (in press) suggested that the Pseudoaquilapollenites
299 bertillonites biozone proposed by Braman (2018) ranges from 68.2 to 67.2 Ma. The Basilemys
300 fragment (RBCM P1), and most likely the ornithischian dinosaur (RBCM P900), are derived from
301 this interval. Plant macrofossils are consistent with a Late Cretaceous age for our study area,
302 although it is worth noting that the presence of a Coniopteris-like tree fern towards the western
303 edge of our study area suggests this site (Amel’s) may be Cenomanian in age (or older).
304
305 DISCUSSION
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306 The Turtle Site to the east of Birdflat Creek is in an area currently mapped as Brothers
307 Peak Formation (McMechan et al. 2007), but the lithology of this specific outcrop more closely
308 matches descriptions for the Tango Creek Formation. The late Maastrichtian age of this site, as
309 suggested by palynomorphs, is younger than that reported for the Tango Creek Formation
310 anywhere in the basin, but is closer to reported ages for the Brothers Peak Formation (late early
311 Maastrichtian) by Evenchick et al. (2003). This suggests that the Turtle Site is either 1) in the
312 Brothers Peak Formation rather than the Tango Creek Formation, or 2) the Tango Creek
313 Formation extends into the Maastrichtian in this part of the basin. We prefer the second
314 hypothesis, because the lithology at this site more closely matches that of the Tatlatui Member 315 of the Tango Creek Formation (dark greyDraft mudstones and chert-pebble-bearing arenites with 316 thin seams of lignite) compared to the Laslui member of the Brothers Peak Formation (coarse
317 conglomerate and tuffs) (Bustin and McKenzie 1989). Interpretations of the age of the Brothers
318 Peak Formation have varied substantially over the years, ranging from Maastrichtian to Eocene
319 depending on the methodology used. The new pollen samples provided here contain
320 Pseudoaquilapollenites bertillonites, which has a very restricted occurrence in the upper
321 Maastrichtian of western North America (Braman 2018). If our assessment of the stratigraphic
322 position of the locality in the uppermost Tango Creek Formation is correct, this suggests that
323 the Brothers Peak Formation was probably deposited during the Paleocene - at the earliest - in
324 the Sustut River field area.
325 Basilemys was a large, herbivorous turtle with robust limbs bearing osteoderms
326 (Holroyd and Hutchison 2002) and short manual and pedal digits, which are correlated with a
327 terrestrial mode of life in extant turtles (Joyce and Gauthier 2004). It is a common constituent
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328 of Campanian-Maastrichtian vertebrate fossil assemblages in western North America, having
329 been reported from the Aguja (Tomlinson 1997), Fruitland/Kirtland (Lehman 1981), Kaiparowits
330 (Hutchison et al. 1998), Judith River (Hutchison and Archibald 1986, Hutchison et al. 1998),
331 Dinosaur Park (Brinkman 2003), Horseshoe Canyon (Mallon and Brinkman 2018), and Scollard
332 formations (Brinkman 2003). Nanhsiungchelyidae is one of only a few turtle clades to have gone
333 extinct during the end-Cretaceous mass extinction, despite the abundance of Basilemys at
334 many latest Cretaceous fossil localities in North America (Holroyd and Hutchison 2002, Holroyd
335 et al. 2014). As such, the presence of Basilemys provides further support for an age no younger
336 than Maastrichtian at the Turtle Site, a contention further supported by palynomorph data. 337 Although its presence in the Sustut BasinDraft may seem to indicate a significant northern range 338 extension for this taxon, the complex translational history of the Intermontane Superterrane
339 means that the Sustut River might have been located as much as 1600 km to the south of its
340 current position with respect to cratonic North America during the Maastrichtian (Enkin et al.
341 2003). In other words, the Sustut River study area may have been located at about the same
342 latitude as the southern border of Oregon and Idaho (having a palaeolatitude of perhaps
343 around 48°N; van Hinsbergen et al. 2015), and well within the latitudinal range of Lancian-aged
344 Basilemys specimens (Mallon and Brinkman 2018). However, both the Basilemys (RBCM P1) and
345 small ornithischian dinosaur (RBCM P900) from the Sustut River represent some of the
346 westernmost occurrences of these clades from this time period.
347 The absence of complete plant macrofossils or angiosperm leaf margins makes it
348 difficult to comment on the diversity of angiosperms in the study area, but it is noteworthy that
349 angiosperms appear to make up a lower percentage of the macroflora (50% or less) compared
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350 to sites in the potentially stratigraphically equivalent Hell Creek Formation to the east
351 (considering the latitudinal displacement of the Sustut Basin relative to its current northern
352 location), where angiosperms make up 90% of the macroflora (Johnson 2002); Metasequoia is
353 relatively more abundant in the Sustut Basin. Coniopteris ferns, abundant at one site along the
354 Sustut River (Amel’s), are not present in the Hell Creek Formation; a search of the Paleobiology
355 Database for Coniopteris in January 2019 did not recover any occurrences after the
356 Cenomanian,, which may indicate that the age of the westernmost sites in our study area is
357 significantly older than the Maastrichtian.
358 359 Draft 360 Summary and conclusions
361 Reconnaissance fieldwork along the Sustut River resulted in the discovery of new fossil
362 turtle and plant remains. The preservation of the Basilemys shell fragment matches that of the
363 dinosaur collected in 1971, matrix remaining on the dinosaur bones matches the lithology
364 observed at the Turtle Site, and the Turtle Site closely matches observations of the collection
365 site for RBCM P900 recorded by KF Larsen in his field notes. Although we did not encounter
366 additional dinosaur fossils during this field study, we consider the presence of vertebrate fossils
367 at a location matching the 1971 field notes to be compelling evidence in favour of the Turtle
368 Site as the original collection site for RBCM P900. Palynomorphs recovered from this site are
369 similar to those correlated with rocks dated to between 68.2 to 67.2 Ma elsewhere in North
370 America. Outcrops along the BC Rail line and Sustut River in the study area included the
371 remains of angiosperms, Metasequoia, and cf. Coniopteris; however, several outcrops observed
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372 along the Sustut River are strongly inclined, so the sites and specimens described here should
373 not be considered to necessarily represent a single ecosystem.
374 Dinosaur body fossils from the western portion of the North American Cordillera are
375 uncommon, with only a few specimens reported from Baja California (Prieto-Marquez et al.
376 2012), California (Ford and Kirkland 2001), Washington (Peecook and Sidor 2015), and Oregon
377 (Retallack et al. in press). Additionally, terrestrial vertebrate skeletal remains are extremely rare
378 in British Columbia, and RBCM P900 and RBCM P1 represent the only vertebrate fossils yet
379 reported from the 6000 km2 Sustut Basin. The rarity of vertebrate skeletal fossils in northern
380 British Columbia most likely reflects the difficulty of accessing outcrops in this remote 381 mountainous region, rather than any preservationalDraft biases. Continued prospecting and 382 collecting in the Sustut Basin therefore has great potential for revealing additional data about
383 the flora and fauna of this region during the Cretaceous Period.
384
385 ACKNOWLEDGEMENTS
386 Field sites are located on the unceded traditional territory of the Gitxsan peoples. Elisabeth
387 Deom and Richard Linzey (Heritage Branch, BC Ministry of Forests, Lands, Natural Resource
388 Operations, and Rural Development) provided authorization to conduct this fieldwork. Funding
389 for this project was provided by an NSERC postdoctoral fellowship, an NSERC L’Oréal-UNESCO
390 for Women in Science fellowship supplement, a National Geographic Society Waitt Grant, and a
391 Dinosaur Research Institute grant to VMA, and an NSERC Discovery Grant to DCE (NSERC Grant
392 File Number: RGPIN 355845). Randall Mindel provided assistance with plant identification.
393 Many thanks to Angela and Eric Van Velzen of Suskeena Lodge for their hospitality and logistical
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394 support during our 2017 fieldwork. Thanks are also owed to Kenny Larsen for his discovery of
395 the dinosaur material in 1971 and subsequent donation to the Royal BC Museum, and to Milton
396 Graves & Grant Wach (Dalhousie University), Phil Currie (University of Alberta), Lisa Buckley
397 (Peace Region Palaeontological Research Centre), Don Brinkman (Royal Tyrrell Museum) and
398 Peter Mustard (Simon Fraser University) for advice and assistance during the early days of this
399 project beginning in 2005. Comments from two anonymous reviewers and editor Ali Polat
400 improved the manuscript.
401
402 403 Draft 404
405
406
407
408
409
410
411
412
413
414
415
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546 TABLES
547 Table 1. Summary of fossil samples collected at various localities along the Sustut River in the
548 Sustut Basin, northern British Columbia. Many of the plant samples include more than one leaf
549 per specimen and, especially for cf. Coniopteris, it is difficult to determine whether or not
550 fragmentary portions represent one or multiple leaves. In many cases leaves also overlap each
551 other. As such, the counts provided here are meant to only be a representative snapshot of the
552 general fossil diversity at different localities, not an exact abundance count.
553
Site Basilemys cf. Metasequoia Angiosperm Other plant Totals Coniopteris Draft remains Lindy’s Lane 9 9
Amel’s 33 35 19 2 89
Suskeena 1 4 1 6 Lodge Hill
Hickory 4 4 8 Sticks
Surespan 4 2 6 South Side
BC Rail 1 1 Wood Site
Turtle Site 1 1
Totals 1 33 40 40 6 120
554
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555 Table 2. Palynomorphs sampled from sites along the Sustut River.
Sample Number Site Palynomorphs
RBCM P114 Turtle Site Alisporites grandis
Alnipollenites vera
Aquilapollenites quadrilobus
Cyathidites minor
Laevigatosporites haardtii
Liliacidites sp.
Lusatisporis dettmannae
Pityosporites constrictus PseudoaquilapollenitesDraft bertillonites Retitriletes sp.
Retroprojectus notabile
Siberiapollis sp.
Taxodaceaepollenites hiatus
Tricolpites sp.
Triporopollenites sp.
Triprojectus augustus
Tsugaepollenites sp.
RBCM P88 Hickory Integricorpus clarireticulatus Sticks
RBCM P29 Amel’s Barren 556
557
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558 FIGURE CAPTIONS
559 Figure 1. Generalized location map of the Sustut River study area in the Sustut Basin (inset) of
560 north-central British Columbia, Canada. Map modified from Evenchick et al. (2003). [formatted
561 for column width]
562
563 Figure 2. Lithostratigraphy of the Sustut Group, modified after Evenchick and Thorkelson (2005)
564 and Cohen et al. (2018). The Sustut Group includes the Tango Creek Formation and Brothers
565 Peak Formation, both of which are subdivided into informal members. The age of these
566 formations may vary from the northern to southern ends of the basin, and the base of the 567 Tango Creek Formation is currently poorlyDraft constrained (Evenchick and Thorkelson 2005). The 568 Sustut Group unconformably overlies the Bowser Lake Group or Hazelton Group depending on
569 location within the Sustut Basin (Evenchick and Thorkelson 2005). Our results suggest that the
570 Tango Creek Formation along the Sustut River may extend into the Maastrichtian. [formatted
571 for page width]
572
573 Figure 3. Fossil localities encountered during 2017 fieldwork along the Sustut River near its
574 intersection with Birdflat Creek. No fossils were found for an additional 7 km east of the turtle
575 fossil site. [formatted for page width]
576
577 Figure 4. Carapace fragment of Basilemys sp., RBCM P1, in A) dorsal, B) ventral, and C) cross-
578 sectional view. The specimen is completely recrystallized internally. [formatted for page width]
579
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580 Figure 5. Selected plant specimens collected from various sites in the Tango Creek Formation
581 along the Sustut River: A) RBCM P35, cf. Coniopteris (Amel’s); B) RBCM P31, cf. Coniopteris
582 (Amel’s); C) RBCM P39, Metasequoia sp. (Amel’s); D) RBCM P92, multiple layers of
583 indeterminate angiosperm leaves (Lindy’s Lane); E) RBCM P99, multiple layers of indeterminate
584 angiosperm leaves (Lindy’s Lane); F) RBCM P97, indeterminate angiosperm leaf with alternate
585 percurrent tertiary veins (Lindy’s Lane); G) RBCM P115, overlapping angiosperm leaves (Hickory
586 Sticks), the largest leaf specimens recovered during this study; H) RBCM P106, angiosperm leaf
587 preserved in hard nodule (Suskeena Lodge Hill). [formatted for page width]
588 589 Figure 6. A) Stratigraphic log of the outcropDraft from which RBCM P1 (Basilemys sp.) was 590 recovered. This sedimentary sequence more closely matches descriptions of the Tango Creek
591 Formation than the Brothers Peak Formation. The strata strike 110° E-W and dip 21° N-NE .
592 RBCM P1 was found in the talus below the outcrop, and most likely derives from sediments
593 from about 3-5 m above the base of the section. B) Photograph of the Turtle Site outcrop along
594 the BC Rail line to the east of Birdflat Creek. VMA, DCE and DJS are standing at the spot where
595 RBCM P1 was discovered. [formatted for page width]
596
597
598 Supplementary Information
599 SI 1. Specimens collected from the Sustut River field area (.csv)
600 SI 2. Copy of K. Larsen’s field notes from 1971 (.pdf)
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Draft
Figure 1. Generalized location map of the Sustut River study area in the Sustut Basin (inset) of north-central British Columbia, Canada. Map modified from Evenchick et al. (2003).
134x124mm (300 x 300 DPI)
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Draft
Figure 2. Lithostratigraphy of the Sustut Group, modified after Evenchick and Thorkelson (2005) and Cohen et al. (2018). The Sustut Group includes the Tango Creek Formation and Brothers Peak Formation, both of which are subdivided into informal members. The age of these formations may vary from the northern to southern ends of the basin, and the base of the Tango Creek Formation is currently poorly constrained (Evenchick and Thorkelson 2005). The Sustut Group unconformably overlies the Bowser Lake Group or Hazelton Group depending on location within the Sustut Basin (Evenchick and Thorkelson 2005). Our results suggest that the Tango Creek Formation along the Sustut River may extend into the Maastrichtian.
116x83mm (300 x 300 DPI)
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Figure 3. Fossil localities encountered during 2017 fieldwork along the Sustut River near its intersection with Birdflat Creek. No fossils were found for an additional 7 km east of the turtle fossil site. [formatted for page width]
215x90mm (150 x 150 DPI) Draft
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Figure 4. Carapace fragment of Basilemys sp., RBCM P1, in A) dorsal, B) ventral, and C) cross-sectional view. The specimen is completely recrystallized internally. [formatted for page width]
215x63mm (300 x 300 DPI)
Draft
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Draft
Figure 5. Selected plant specimens collected from various sites in the Tango Creek Formation along the Sustut River: A) RBCM P35, cf. Coniopteris (Amel’s); B) RBCM P31, cf. Coniopteris (Amel’s); C) RBCM P39, Metasequoia sp. (Amel’s); D) RBCM P92, multiple layers of indeterminate angiosperm leaves (Lindy’s Lane); E) RBCM P99, multiple layers of indeterminate angiosperm leaves (Lindy’s Lane); F) RBCM P97, indeterminate angiosperm leaf with alternate percurrent tertiary veins (Lindy’s Lane); G) RBCM P115, overlapping angiosperm leaves (Hickory Sticks), the largest leaf specimens recovered during this study; H) RBCM P106, angiosperm leaf preserved in hard nodule (Suskeena Lodge Hill).
215x279mm (300 x 300 DPI)
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Draft
Figure 6. A) Stratigraphic log of the outcrop from which RBCM P1 (Basilemys sp.) was recovered. This sedimentary sequence more closely matches descriptions of the Tango Creek Formation than the Brothers Peak Formation. The strata strike 110° E-W and dip 21° N-NE . RBCM P1 was found in the talus below the outcrop, and most likely derives from sediments from about 3-5 m above the base of the section. B) Photograph of the Turtle Site outcrop along the BC Rail line to the east of Birdflat Creek. VMA, DCE and DJS are standing at the spot where RBCM P1 was discovered.
177x155mm (300 x 300 DPI)
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