Canadian Journal of Earth Sciences
New Crassigyrinus-like fibula from the Tournaisian (earliest Carboniferous) of Nova Scotia
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2019-0128.R3
Manuscript Type: Communication
Date Submitted by the 27-Jan-2020 Author:
Complete List of Authors: Lennie, Kendra; University of Calgary, Biological Sciences Mansky, Chris; Blue Beach Fossil Museum Anderson, Jason S.; University of Calgary, Comparative Biology and ExperimentalDraft Medicine Fin-to-Limb, Carboniferous, Vertebrate Evolution, Romer's Gap, Early Keyword: Tetrapod, Crassigyrinidae
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
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1 New Crassigyrinus-like fibula from the Tournaisian (earliest
2 Carboniferous) of Nova Scotia
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6 Kendra I. Lennie1,2
7 Chris F. Mansky3
8 Jason S. Anderson2,4
9 1. University of Calgary, Biological Sciences. 507 Campus Drive N.W. University of Calgary. Calgary,
10 Alberta. Canada, T2N 1N4. [email protected]
11 2. University of Calgary, McCaig Bone and Joint institute
12 3. Blue Beach Fossil Museum, 127 Blue Beach Road, Hantsport, Nova Scotia, B0P 1P0.
14 4. University of Calgary, Comparative Biology and Experimental Medicine. 3330 Hospital Dr. NW,
15 Calgary, Alberta. Canada, T2N 4N1. [email protected]
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25 Abstract:
26 The transition between the Devonian and Carboniferous Periods is important for tetrapod vertebrates.
27 By the end of the Devonian the first limbs are present in aquatic animals, and by the mid Carboniferous
28 fully terrestrial tetrapods have diversified.
29 Knowledge of the fin-to-limb transition is sparse because few fossils from the earliest Carboniferous
30 (Tournaisian) are known. Blue Beach Nova Scotia, in addition to a small number of sites in Scotland and
31 Australia, is an exception to this global trend. Previous reports from Blue Beach identified fossils from a
32 variety of Devonian-like and Carboniferous-like tetrapod body forms, making it a valuable site for 33 studying the fin-to-limb transition. Here weDraft report on a new left fibula from Blue Beach, which we 34 attribute to the later occurring Visean-aged (Early Carboniferous) Crassigyrinidae. Recent investigations
35 of deposits in Scotland, similar in age to the Tournaisian exposed at Blue Beach, have found
36 Crassigyrinus-like elements as well, reinforcing this 20 million year lineage extension.
37
38 Keywords:
39 Fin-to-limb, Carboniferous, Crassigyrinidae, Early Tetrapod, Vertebrate Evolution, Romer’s Gap
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47 Introduction
48 Crassigyrinus scoticus is an unusual tetrapod first described from the Visean (Early
49 Carboniferous) and Namurian (Late Carboniferous; Panchen 1985; Panchen and Smithson 1990). The
50 holotype, described by Lydekker (1890), was not formally ascribed to Crassigyrinus scoticus until
51 a redescription by Panchen (1985), followed by Panchen and Smithson (1990). The material
52 is from the Visean-aged Gilmerton Ironstone in Scotland, and preserves only cranial material. Cranial,
53 pelvic girdle, and limb material also are known from the Namurian (Late Carboniferous) of the
54 Dora bone bed in Cowdenbeath, Scotland (Panchen 1985; Panchen and Smithson 1990). One reason for
55 its common description as an aberrant species is the minute size of the forelimbs, so small they were
56 originally thought to be part of the hyobranchial skeleton (Panchen 1985). As with many of the early
57 tetrapods, Crassigyrinus scoticus shows an Draftunusual range of “primitive” fish-like and “derived”
58 anthracosaur-like characteristics. The heavily reduced limbs, retention of fish-like foramina in the
59 humerus, torsion of hindlimb elements, and proportionately large, fish-like skull have led many
60 researchers to conclude that Crassigyrinus was an aquatic tetrapod (Panchen 1985; Panchen and
61 Smithson 1990; Herbst and Hutchinson 2019), with Carroll (1992) suggesting it may have had a
62 terrestrial ancestor and itself been secondarily aquatic.
63 Crassigyrinus is an unusual tetrapod in having a combination of features that suggest it is a very
64 derived form of an early tetrapod, and until recently no specimen from Tournaisian deposits, in which
65 most stem tetrapods are found, was known. Earlier reports from the earliest Tournaisian-aged Horton
66 Bluff Formation exposed at Blue Beach near Hantsport, Nova Scotia, suggested the presence of
67 Crassigyrinus-like fossils (Mansky and Lucas 2013), but these were briefly mentioned without detailed
68 description. Increased effort to sample the Tournaisian by Clack and colleagues (Clack, Porro, and
69 Bennett 2019), examining the Burnmouth Locality in Scotland, revealed a Crassigyrinus-like jaw 20 myr
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70 older than the holotype Crassigyrinus scoticus (Clack, Porro, and Bennett 2019; Clack et al. 2019). Here
71 we report on a new Crassigyrinus-like element from Blue Beach, which increases our confidence in this
72 long range extension into antiquity for the family, and further supports the stem tetrapod placement of
73 the taxon.
74 Methods
75 The specimen NSM 005.GF.045.112 was collected by CM and prepared by KL using Paleotools
76 Microjacks 1-3 (Brigham, Utah) pin vice, carbowax, and PVA. Once fully prepared out of the matrix it was
77 Scanned in an Xradia Versa 510 (Carl Zeiss, Germany) at the University of Calgary with settings of 140kV,
78 72µA, no filter, and magnification 0.4x. The scan resulted in an image stack 1001 by 1024 pixels totaling 79 1823 images with a voxel size of 23.35µm. DraftThe resulting image stack was imported into Dragonfly 80 (Dragonfly 4.0. Object Research Systems (ORS) Inc, Montreal, Canada, 2018; software available at
81 http://www.theobjects.com/dragonfly) image intensity settings (look up tables or LUTs) were modified
82 to create high contrast images.
83 Locality
84 Blue Beach is one of a small number of localities worldwide that represent the Tournaisian Age
85 of the Early Carboniferous (Anderson et al. 2015). Blue Beach, along with Burnmouth in the Scottish
86 Borders Region, is one of the oldest localities within what was previously a global fossil hiatus known as
87 Romer’s Gap (Romer 1956; Coates and Clack 1995; Smithson et al. 2012). Disarticulated fish and
88 tetrapod elements are present at Blue Beach, but until recently little research has been done on the
89 elements. Sufficient numbers of more complete specimens have now been described, for interpretation
90 and identification of the isolated elements to be possible. Located between Hantsport and Avonport,
91 Nova Scotia, Blue Beach is near the mouth of the Avon River where it meets the Bay of Fundy. The Bay
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92 of Fundy is known for record daily tides, and this tidal action exposes fossil material on a daily basis.
93 Whereas the tides aid in extracting material they also result in considerable wear, and potential that
94 material gets washed out with the tide. Wear from recent tidal action is not the only taphonomic factor
95 acting on these bones, as some show evidence of predepositional wear. This includes cracks and areas
96 of cortical erosion that are encased in matrix upon collection and are exposed after manual preparation.
97 The element comes from the Lighthouse Sandstone layer of the Hurd Creek Member of the
98 Horton Bluff formation (Anderson et al. 2015). It is mid to early late Tournaisian in age (late Tn2 to early
99 Tn3; Utting, et al. 1989; Martel, McGregor, and Utting 1993), probably slightly below the CM (claviger-
100 macra) palynozone of the Burnmouth specimen. However, the Tournaisian palynozonation of Scotland is
101 currently under review so the exact temporal relationship between the specimens remains
102 indeterminate (Anderson et al. 2015; Clack,Draft Porro, and Bennet 2018; Otoo et al. 2018). The depositional
103 environment of Horton Bluff is believed to be a high energy lagoonal site exhibiting marginal marine
104 conditions (Mansky and Lucas 2013). As such, the bones were disarticulated before or at the time of
105 deposition, when they were likely transported into the lagoonal environment at Blue Beach during
106 storms and under high energy conditions. It is unclear if the remains were originally from terrestrial or
107 aquatic environments. Most of the material from Blue Beach is preserved quite well in three
108 dimensions, and rugosity such as pitting and scarring often remains.
109
110 Systematic Paleontology
111 Osteichthyes Huxley, 1880
112 Sarcopterygii Romer, 1955
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113 Tetrapodomorpha Ahlberg, 1991
114 Crassigyrinidae Huene, 1948
115 Referred specimen: NSM.005.GF.045.112.
116 Locality: Lighthouse Sandstone, Hurd Creek Member, Horton Bluff Formation, exposed below
117 the lighthouse (“Lighthouse Cove”) between Hantsport and Avonport, Nova Scotia.
118 Description
119 The element, a left fibula (Figure 1A and 2A-G), is 40mm in length from the middle of the 120 proximal surface to the middle of the distalDraft surface. The specimen appears to be dorsoventrally 121 flattened although what remains of the internal structure does not suggest crushing (Figure 1B-D).
122 Therefore the dorsoventral flattening appears to be a true morphological feature rather than an artifact
123 of taphonomy. The element is narrowest at the midshaft, and the distal end flares more than the
124 proximal end. There is no prominent scarring on either the flexor or extensor surface, although small
125 pits are visible on the extensor surface. The fibula described here shows an axial twist of about
126 25⁰between the proximal and distal articulating surfaces. The flexor surface (Figure 2B) of
127 NSM.005.GF.045.112 has a ridge that runs along the posterior edge from the proximal end to the distal
128 end (flexor ridge; Herbst and Hutchinson 2019; figure 11, b, b’). At the distal end of the fibula the ridge
129 bifurcates (red highlight; Figure 2B). One bifurcation of the ridge curves medially and the other remains
130 along the posterior edge of the bone, creating a triangular fossa similar to the one described in Herbst
131 and Hutchinson (2019). The fibulae of other early teterapods such as Tulerpeton, Pederpes,
132 Proterogyrinus, which this element resembles in shape, have no such a fossa described or figured
133 (Holmes 1984; Lebedev and Coates 1995; Clack and Finney 2005). Further examination of the material
134 may reveal the presence of a triangular fossa in other early tetrapod taxa. The extensor surface (Figure
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135 2A) is remarkably smooth and cylindrical. However, CT images enhance the contrast of the bone surface
136 to reveal a great number of small pits. The proximal articulation surface (Figure 2E) is a sub oval to
137 almost diamond shape, and the distal articulating surface (Figure 2F) is elongate and rectangular. At the
138 midshaft, the posterior edge (Figure 2D) of the element comes to a point rather than a rounded curve.
139 The rounded edges of the proximal and distal articulating surfaces can be attributed to the wear both at
140 the time of deposition and due to current tidal erosion.
141 Discussion
142 Our identification of this element as a member of Crassigyrinidae is based on the angle between
143 the planes of the proximal and distal articulating surfaces, and the presence of the unique triangular 144 fossa on the distal part of the flexor surface.Draft Among other early tetrapods, Tulerpeton (Figure 3 A-F) 145 shows a similar degree of axial rotation between the proximal and distal articulating surfaces.
146 Tulerpeton has a more sigmoidal shape to the distal end when viewed posteriorly (Figure 3D) whereas
147 NSM.005.GF.045.112 and Crassigyrinus have a more rectangular shape (Figure 2 D,J,P). Additionally, the
148 proximoextensor edge of the sigmoidal curve in Tulerpeton comes to a sharp angle approaching the
149 horizontal whereas those in NSM.005.GF.045.112 and Crassigyrinus do not. It is unclear if this is due to
150 wear or is a true biological feature. Tulerpeton-like elements have already been described from Blue
151 Beach (Anderson et al. 2015), so the discovery of a Tulerpeton-like fibula would be unsurprising;
152 however, the additional features outlined below provide greater support for identification as a
153 Crassigyrinus-like form. The element is waisted in a similar manner to Pederpes (Figure 3 G-H), but does
154 not have any prominent pitting or scarring as Pederpes does on the posterior extensor surface (Clack
155 and Finney 2005). Indeed, the whole of NSM.005.GF.045.112 lacks any large prominent rugosity,
156 although pitting is present along the extensor surface. Pederpes also differs from NSM.005.GF.045.112
157 in that the proximal and distal articulating surfaces are aligned in the same plane in the fibula, whereas
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158 in NSM.005.GF.045.112 there is a 25⁰ offset between the planes. The angle between the planes of the
159 proximal and distal expansions is even greater in Crassigyrinus scoticus, (45⁰ in Crassigyrinus scoticus vs
160 25⁰ in NSM.005.GF.045.112). Panchen and Smithson (1990) observed this would presumably rotate the
161 plantar surface of the foot into a vertical orientation, as opposed to the horizontal one common to other
162 early tetrapods. We interpret this difference in “axial twist” between in NSM.005.GF.045.112 and
163 Crassigyrinus scoticus to be due to a greater degree of specialization in the later occurring species,
164 suggesting that it was an active kicking swimmer as opposed to a more primitive animal which primarily
165 used tail-based propulsion in an aquatic environment. NSM.005.GF.045.112 may have had a hindlimb
166 orientation somewhere between Crassigyrinus scoticus, and earlier Devonian tetrapods such as
167 Acanthostega or Tulerpeton. The fibulae of Acanthostega and Tulerpeton both exhibit a 25⁰ axial twist 168 (Lebedev and Coates 1995; Coates 1996). CoatesDraft (1996) noted that whereas the femur of Acanthostega 169 had 25⁰of axial twist, an even greater amount would be beneficial for the function of a paddling limb as
170 in Crassigyrinus (Panchen and Smithson 1990). Ichthyostega, which does not have torsion of the fibula,
171 is proposed to have moved more like a seal (Pierce, Clack and Hutchinson 2012) although this mode of
172 locomotion is not inferred from its fibular morphology. As discussed above, the element is similar to the
173 Carboniferous tetrapod Pederpes, but there are also similarities to Proterogyrinus. Proterogyrinus,
174 however, can be quickly discounted as a possible identification, as the fibula figured and described by
175 Holmes (1984) is much more waisted and gracile. Proterogyrinus (Figure 3 I-N) has an oval shape to the
176 distal end in posterior view, and the anterior curvature of the fibula of Proterogyrinus is abrupt enough
177 to make this bone more L-shaped as opposed to the gentle concave curve seen in Pederpes,
178 Crassigyrinus, and NSM.005.GF.045.112. In Tulerpeton the distal end of the anterior surface appears to
179 be approaching L-shaped albeit at a much lesser angle than in Proterogyrinus. Proterogyrinus has not
180 been described or figured as having a triangular fossa. Even if a fossa is in fact present, the posterior
181 ridge along the flexor surface remains along the posterior edge to the distal end. In Crassigyrinus and
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182 NSM.005.GF.045.112 the flexor ridge (Herbst and Hutchinson 2019) bifurcates distally, creating the
183 anterior and posterior boundaries of the triangular fossa. An additional ridge along the anteriodistal
184 edge of the fibula of Proterogyrinus makes the fossa visible in Holmes’ figure much larger than the one
185 in Crassigyrinus and in NSM.005.GF.045.112. Importantly, in Crassigyrinus and NSM.005.GF.045.112 the
186 triangular fossa does not cross the midline of the bone. In Pederpes there is also a ridge on the posterior
187 edge of the flexor surface, but the ridge is quite rugose. The conditions in both Proterogyrinus and
188 Pederpes are unlike that in NSM.005.GF.045.112 and Crassigyrinus, in which the ridge is not associated
189 with heavy rugosity and curves medially at the distal end to create the triangular fossa.
190 Interestingly this element is larger than the later known Crassigyrinus scoticus specimens from
191 Scotland. This could indicate the more primitive nature of NSM.005.GF.045.112. In contrast, a
192 Crassigyrinus-like jaw from the TournaisianDraft of Scotland (Clack, Porro and Bennett 2018) is observed to
193 be smaller than Crassigyrinus scoticus. The presence of such disparate body sizes in similar taxa at
194 similar times in the fossil record indicates that crassigyrinids exhibited a range of sizes and morphologies
195 in the early Carboniferous, and that these animals likely diverged much earlier than the Tournaisian.
196 Variation in size and morphology may be influenced by factors such as ontogeny and individual
197 variation, and with future recognition of crassigyrinid material, conclusions about the influence of such
198 factors may be made with more certainty. This specimen highlights the importance of studying other
199 material previously attributed to the taxon at Blue Beach (Mansky and Lucas 2013), which will be the
200 subject of future work.
201 This fibula is one of a small number of recently recognized Crassigyrinus-like elements dating to
202 the Tournaisian, in deposits 20 million years older than those containing Crassigyrinus scoticus (Mansky
203 and Lucas 2013; Clack, Porro, and Bennett 2019). This contributes to a growing body of information that
204 indicates the Tournaisian was a period of high tetrapod diversity. Tetrapod limbs, and by extension
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205 locomotor strategies, were also diversifying. Carroll (1992) suggested based on vertebral characters that
206 Crassigyrinus may have been a secondarily aquatic animal. If this were the case, a terrestrial
207 morphotype would have predated the holotype of Crassigyrinus scoticus and possibly also the animal
208 described here, placing it in a time interval from which few terrestrial vertebrates are known. There is an
209 emerging view that a much greater number of stem tetrapods retained internal gills (Coates and Clack
210 1991; Pardo et al. 2019), suggesting that many early tetrapods remained obligatorily aquatic. Future
211 discoveries and recognition of Tournaisian crassigyrinid material will provide critical information to more
212 confidently draw conclusions about the primarily or secondarily aquatic nature of Crassigyrinus scoticus.
213 This fibula likely represents an animal transitioning from a tail-based method of propulsion to a limb-
214 based one. While we may not be able to draw firm conclusions on its terrestrial locomotory capabilities 215 based on the surface morphology of the bone,Draft it appears this animal had limbs that potentially acted as 216 early paddles, as opposed to stabilizing the body or interacting with the substrate, as suggested in
217 Ichthyostega (Pierce, Clack and Hutchinson 2012).
218 The variety and amount of material coming from Blue Beach and contemporaneous localities in
219 Scotland illustrate the importance of collecting fragmentary fossils. Previously, collection bias has led to
220 inferences of low diversity during "Romer's Gap". However, this new fibula, together with other recent
221 discoveries from the Tournaisian, reinforces that absence of evidence is not evidence of absence.
222 Emerging discoveries of early tetrapod material from Blue Beach and Scottish Tournaisian localities
223 demonstrate that tetrapods displayed a diversity of body forms even before the emergence of
224 numerous terrestrial taxa later in the Carboniferous.
225 Acknowledgments
226 We thank Katherine Ogdon and Tim Fedak (Nova Scotia Museum) for access to the specimen, and Sonja Wood (Blue
227 Beach Fossil Museum) and Eva Herbst for discussions. This study was supported by a Discovery Grant from the
228 Natural Sciences and Engineering Research Council of Canada to JSA.
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229
230
231 References
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Draft
Left fibula NSM.005.GF.045.112 (A) with red bars indicating transaxial proximal (B), midshaft (C), and distal (D) slices. The transaxial computer tomographic images illustrate that dorsoventral flattening does not appear to be the result of taphonomy, crushing is minimal, and the internal structure is well preserved throughout the element. Scale bar represents 10mm
257x187mm (300 x 300 DPI)
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Draft
NSM.005.GF.045.112, a left fibula (A-F) in extensor (A), flexor (B), anterior (C), posterior (D), proximal (E), and distal (F) views. Crassigyrinus, right (G-L) and left (M-R) fibula in extensor (G,M), flexor (H,N), anterior (I,O), posterior (J,P), proximal (K,Q), and distal (L,R) views (line drawings modified from Herbst and Hutchinson 2018).Red highlight illustrates bifurcation of the posterior flexor ridge. Scale bar represents 10mm.
257x206mm (300 x 300 DPI)
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Draft
Fibulae of Tulurpeton (A-F), Pederpes (G-H), and Proterogyrinus (I-N) in extensor (A,G,I), flexor (B,H,J), anterior (C,K), posterior (D,L), proximal (E,M), and distal (F,N) views. Scale bar represents 10mm (line drawings modified from Lebedev and Coates 1995, Clack and Finney 2005, and Holmes 1984).
236x223mm (300 x 300 DPI)
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