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An Upper Carboniferous eurypterid trackway from Mostyn, Wales
An Upper Carboniferous eurypterid trackway from Mostyn, Wales Simon J. Braddy & Lyall I. Anderson BRADDY, S. J. & ANDERSON, L. I. 1996. An Upper Carboniferous eurypterid trackway from Mostyn, Wales. Proceedings of the Geologists' Association, 107, 51-56. A trackway previously attributed to the activities of an ambulatory fish (Buckland (\843) Proceedings of the Geological Society, 4, 204) is reinterpreted as that of a large heteropodous arthropod utilizing a hexapodous gait, the most likely candidate being a eurypterid. The trackway is assigned to Palmichnium pottsae ichnosp. nov.
Department of Geology, University of Manchester, Manchester, M 13 9PL.
1. INTRODUCTION The specimen (BGS GSM 26037), first noted by Buckland (1843), was interpreted as the result of the repeated impressions of three bony processes projecting from the anterior rays of the pectoral fin of an ambulatory fish. However, the disposition of the imprints, in rows of three paired tracks, is clearly characteristic of a large hetero-
External
Width
\ 3
\ I
l'
2
(
1:
,r' .
r
\
. '·'1
, I
2. IDENTITY OF THE TRACKWAY MAKER
I
'I-.------~V'------_.-!
Sel ABC
left
podous arthropod utilizing a hexapodous gait, similar in many respects to other previously described eurypterid trackways. Buckland (1843) noted that the specimen was collected from near the shaft of a coal pit at Mostyn, North Wales. This is the only locality detail, but the slab was probably derived from the adjacent mine. The exploitable coals in the Mostyn area are Westphalian A-B in age (Smith & George, 1961). The presence of a primary current lineation on the same surface as the trackway indicates fairly shallow, fast flowing water, perhaps on the margin of a river channel. The nomenclature adopted in this description follows Hanken & Stprmer (1975). A series is defined as a discrete group of imprints occurring on one side of the midline of the trackway and a set defined as a pair of series. The successive series of imprints are numbered from one to nine in the presumed direction of motion. The term track is defined as a single imprint, whereas the term trackway is used to define a repeated pattern of tracks (Anderson, 1975). External width is defined as the distance between the anterior points of the outer tracks (A) on either side of the trackway. Similarly, internal width is defined as the distance between the anterior points of the inner (C) tracks (Fig. I). Individual tracks are described according to their position in the trackway and expressed as a simple descriptive formula. The first element of the formula denotes the position of the track, either on the left or the right side of the hypothetical medial axis, the second element denoting the position of the track in a series; 'A' for outer, 'B' for middle and 'C' for inner and the third element indicating the series number, a letter 'b' following this number indicating an irregular imprint.
C
B Right
Fig. 1. Trackway terminology.
Proceedings of the Geologists' Associq{ion, 107,51-56.
A
We disagree with the original suggestion (Buckland, 1843) that the trackway was produced by an ambulatory fish. Ichnofossils attributed to fish consist of wavy, horizontal grooves, the result of their swimming motion (Higgs, 1988), e.g. Undichna (Anderson, 1976; Turek, 1989). Heezen & Hollister (1971) reported an abyssal bathypteroid fish 0016-7878196 $07·00 © 1996 Geologists' Association
52
S. J. BRADDY
&
L. I. ANDERSON
Fig. 2. Palmichnium pottsae ichnosp. nov., holotype BGS GSM 26037, British Geological Survey, Mostyn, North Wales. x 0.53.
UPPER CARBONIFEROUS EURYPTERID TRACKWAY FROM MOSTYN, WALES
C
ABC
B
A
The disposition of imprints suggests that a moderately large heteropodous arthropod, with its shortest legs towards its anterior, utilizing a hexapodous gait, made this trackway (Fig. 2). Jeram (1989) stated that in the absence of aquatic morphological characters in scorpion fossils after the Visean, it was likely that all scorpion lineages were terrestrial by the Upper Carboniferous, thus negating a: scorpion producer for this aquatic trackway. The lack of a medial groove, genal or lateral spine drags, and absence of five in-phase imprints, as is commonly identified for Kouphichnium Nopsca, 1923, negates a xiphosuran identity for the arthropod responsible for this trackway. We think that this trackway was produced by a moderately large (c. 20 cm long) eurypterid. The detailed leg morphology of many Carboniferous eurypterids is either unknown (Briggs & Rolfe, 1983), or based on fragmentary material (St~rmer & Waterston, 1968; Jeram & Selden, 1994), although the size of this trackway matches the maximum leg span, approximately 10 cm, of Adelopthalmus Jordan & Von Meyer (1854), a typical Coal Measures form (Briggs & Rolfe, 1983).
I r,
9
8b
.
.{.Q
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7
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, (fa
,/r'
3. BEHAVIOURAL INTERPRETATION
, (Vi
., \
, rf'
o
'
,~ , fl' 'i, ' .'f
LEFT
53
'l, RIGHT
Fig. 3. Interpretative drawing of Palmichnium pottsae ichnosp. nov., holotype BGS GSM 26037, British Geological Survey, Mostyn, North Wales. x 0.35. (PC indicates primary current lineation).
capable of leaving small tracks in soft sediment with elongate extensions of their ray-fins, but even these impressions consist of two lines of tracks with an additional medial line. The gurnard (Trigla) is an extant shallow water fish with three pairs of extensions from its pectoral fins but, as it had not evolved by the Upper Carboniferous, may be discounted.
We interpret the bedding plane upon which the trackway occurs as being an undertracked surface, as the impressions are relatively sharp and well defined, although of variable depth. The trackway was produced subaqueously, deduced from the presence of primary current lineation on the same bedding plane. The eurypterid was walking parallel to the palaeocurrent direction with a degree of curvature to the right, away from the palaeocurrent. It is not possible, however, to determine whether it was walking into, or out of, the current. The eurypterid walked in the direction of convergence of the left and right track series (see Hanken & St~rmer, 1975; Briggs & Rolfe, 1983). The outer two pairs of tracks shallow slightly posteriorly, away from the presumed direction of motion, indicating that the animal bore the majority of its weight on the anterior part of its posterior two appendages. Considerable variation exists in the degree of curvature of the tracks on either side of the trackway. The left rows display greater curvature than the right rows, which are linear in form, suggesting that the appendages responsible followed a curvilinear course in their stepping motion, scraping the surface of the substrate slightly. Not only are the rows on opposite sides of the trackway slightly out of phase but the orientation of the individual imprints is slightly asymmetric. The angle of the individual imprints to the mid-line is greater in the left rows than those of the right. This implies that the appendages of the left side of the body were maintained further from the medial axis of the body than the appendages of the right side, perhaps as the current forced the animal to veer sideways. The inner tracks are interpreted as having been produced by the bifid tarsal claws or spinose projections of an anterior walking appendage. On occasions where these
54
S. 1. BRADDY
&
imprints have a more linear form, the appendage probably just scraped the surface of the substrate. The right track rows retain a constant stride pattern throughout progression whereas the left rows show progressive relative displacement. Despite larger strides on the right side of the animal, the frequency of strides is greater on the left-hand side. We interpret this as being due to the turning motion of the eurypterid, larger steps taken on the right side to compensate for the greater frequency of leg movement on its left side. A greater degree of disorder of footprints on one side is not uncommon in arthropod trackways (Seilacher, 1958, 1962). Often this is indicative of a slightly oblique motion. The relative phase difference between opposite legs of a pair (calculated by the equation y1/(yl + y2), Fig. 1) varies considerably throughout the course of the trackway. The outer (A) and middle (B) tracks gradually alter from an opposite to an alternate arrangement. The inner pair of imprints (C) are largely opposite during the first three sets, until the occurrence of track R3b (Fig. 3), an irregular footfall, where the tracks gradually become alternate. The first row of tracks on the left side are set much further out from, and arranged almost transversely to, the mid-line, indicating they were held far from the body, possibly as part of a swimming stroke. The rapid transition from an in-phase to an out-of-phase motion in the inner tracks may suggest a transition from a swimming to a walking mode of locomotion, perhaps as the eurypterid emerged from shallow water. This transition is less apparent in the outer two tracks, perhaps due to the turning motion of the eurypterid and the occurrence of the irregular imprints, making interpretation of the change in opposite phase difference between the outer tracks complex. The successive phase difference between adjacent legs on one side of the body cannot be commented upon as unknown factors such as the leg length and speed produce variations in the imprint disposition. The left side of the trackway displays an irregular spacing of the successive imprints indicating that the relative phase difference altered throughout the course of the trackway, a feature which may also be due to the turning motion of the arthropod. Although the lack of a median groove may be the result of undertrack fallout (Goldring & Seilacher, 1971), it is suggested that the absence of it here may equally imply different behavioural activity. Buoyancy supplied to the abdomen by the surrounding water or the result of the telson being actively held up off of the substrate in a horizontal plane with the rest of the body, would result in no medial groove. This behaviour would reduce friction, allowing more efficient locomotion and enable the telson to act as a hydrodynamic structure, helping to steer the animal and assist stability (Waterston, 1979). Studies of the functional morphology of eurypterid walking (Waterston, 1979; Selden, 1981) suggests that eurypterids moved opposite legs out-of-phase in order to maximize body stability. A giant trackway from Pennsylvania (Briggs & Rolfe, 1983), attributed to a large eurypterid, however, shows a symmetrical arrangement
L. I. ANDERSON
of tracks, indicating that the legs of a pair were moved in-phase. The morphologically diverse eurypterids probably had a number of methods of walking (Selden, 1984). Assuming hexapody, the range of possible gaits could be discussed on the basis of consideration of stability. Speed could have been increased by reducing the pace duration, as in arachnids, rather than by altering the gait pattern, as in uniramians (Briggs, Dalingwater & Selden, 1991). The eurypterid responsible for this trackway probably used a relatively low gear gait (i.e. the relative duration each appendage was off the substrate, the promotor stroke and on the substrate, the remotor stroke, contributing to motion, usually expressed as a proportion out of ten), perhaps in the order of 2: 8 (see Selden, 1981). This gait and the accompanying short stride would have been relatively stable, enabling the eurypterid to make frequent irregular footfalls, which are observed in the trackway.
4. SYSTEMATIC ICHNOLOGY Ichnogenus PALMICHNIUM Richter, 1954 Diagnosis. Large trackways, usually with symmetrically opposed rows of three (occasionally four) tracks disposed en echelon, each track ranging from subcircular to adaxially concave impressions within the same trackway, the rows at a high angle to the axis of the trackway. Median groove, or ridge, usually present (after Briggs & Rolfe, 1983). Type ichnospecies. Palmichnium palmatum, Richter, 1954, from the Lower Devonian of the Rhine area in Germany. Palmichnium pottsae ichnosp. nov. 1843 [cthyopatolites Buckland, p. 204. Material. BGS GSM 26037. The specimen is housed in the Geological Museum Collections of the British Geological Survey, Keyworth, Nottingham. Diagnosis. Large trackway, with asymmetric rows of three paired tracks, lacking a median groove. Outermost tracks large and curvilinear. Intermediate tracks smaller and curvilinear. Inner tracks small and bifid. External to internal width ratio of 2.7. Description. The trackway occurs on the upper surface of a fine-grained, laminated (1-2 mm) micaceous sandstone, preserved in negative epirelief. It consists of eight complete sets of three paired impressions and several irregular imprints, not assigned to any series. The left series are well preserved, the right series are much fainter. There is no evidence of a medial groove. The total length of the trackway is 370 mm (Fig. 2). There are eleven rows of tracks on the left side of the trackway but only eight on the right side. The trackway has an average external width of 96 mm and internal width is 36 mm. The external to internal width ratio is approximately 2.7. The outer two tracks of each series (A and B) take the form of shallow
UPPER CARBONIFEROUS EURYPTERID TRACKWAY FROM MOSTYN, WALES
curvilinear or linear grooves, the inner tracks (C) being much shorter than the outer tracks and occasionally bifid in appearance. These tracks are arranged in groups of three, disposed slightly asymmetrically, but at regular intervals on either side of the hypothetical mid-line, with no evidence of a medial drag mark. The overall course of the trackway is slightly curved to the right, a displacement from a hypothetical mid-line of approximately 60 mm occurs along its length. The stride length has an average value of 35 rom for the left side of the trackway and 45 mm for the right-hand side. These proportions remain relatively constant throughout the successive series of tracks. The outer (A) tracks consist of gently curvilinear impressions, slightly convex outwards. The average length of these tracks is approximately 24 mm on the left side and 20 mm on the right, each track being deepest on its anterior side and shallowing gradually backwards. The middle pair of tracks (B) are similarly shaped to those of the outer, although slightly smaller, typically around 15 mm on both sides of the trackway. Similarly the middle tracks shallow posteriorly, but to a lesser extent than those of the outer tracks. Some of the inner tracks (C) have a bifid form, diverging in the presumed direction of motion. The individual length of these tracks is approximately 9 mm on the left side of the trackway and only 7 mm on the right side. There is no significant depth variation in these inner tracks. No sediment back push mounds are observed for any of the tracks. A small number of individual tracks, not belonging to any of the regular track rows, represent irregular footfalls of the animal. The majority of these occur on the left side of the trackway, some comprising complete rows, for example LAlb-LCIb and LA8b-LC8b (Fig. 3), whereas most occur only as irregular imprints of the inner tracks. The arrangement of the successive series of imprints display considerable variation on the left and right sides of the trackway. The angle to the mid-line of the left series is highly variable (62° to 105°, with an average value of 87°). The angle to the mid-line on the right-hand side of the trackway shows much less variation (48° at the start of
55
the trackway to 60° by the fifth set, maintaining this value consistently over the remaining right hand rows). Discussion. This trackway resembles most closely P. stoermeri in the general disposition and form of the tracks yet differs in lacking a median groove (possibly due to undertrack fallout), the external to internal width ratio of 2.7 (3.2 in P. SlOermeri) and the overall external width, which is only 60% of that of P. stoermeri. These proportions indicate a different eurypterid producer to the probable mixopterid producer of P. stoermeri. The original name, [cthyopato!ites, was only informally used, and not properly diagnosed, in the original description (Buckland, 1843), avoiding complications of synonymy. A number of other trackways credited to the activities of vertebrates are probably also of arthropod origin. It is almost inevitable that arthropod trackways were initially interpreted as of vertebrate origin (Caster, 1944). A series of trackways from the Potsdam Sandstone (Upper Cambrian) from Potsdam, Canada, attributed to the activity of a chelonian (Owen, 1852) were later suggested as xiphosuran or eurypterid trackways (Salter, 1867). Salter (1867) described trackways from the Downton Sandstone (Upper Silurian) of Kingston, Herefordshire, interpreting them as tracks of the heterostracan fish Pteraspis, although he did consider the possibility that the eurypterid Pterygotus may have been responsible. Salter's (1867) interpretative drawings show a disposition of tracks clearly characteristic of a large arthropod, possibly a eurypterid.
ACKNOWLEDGEMENTS We thank Dr Hugh Ivimey-Cook and Dr Steve Tunnicliffe of the British Geological Survey, Keyworth, Nottingham, for the loan of the specimen. We also thank Dr Jason Dunlop, Dr Paul Selden, Dr John Pollard, Dr Nigel Trewin and an anonymous referee for their constructive criticism. S. J. B. is funded by a NERC studentship. L. I. A. is funded by the W. & E. AIkins' Memorial Scholarship for Palaeontology (University of Manchester).
REFERENCES ANDERSON. A. M. 1975. The "Trilobite" Trackways in the Table Mountain Group (Ordovician) of South Africa. Palaeontologia Africana, 18, 35--45. - - 1976. Fish trails from the early Permian of South Africa. Palaeontology, 19, 397--409. BRIGGS, D. E. G. & ROLFE, W. D. I. 1983. A giant arthropod trackway from the Lower Mississippian of Pennsylvania. Journal of Paleontology, 57. 377-390. - . DALINGWATER, J. E. & SELDEN. P. A. 1991. Biomechanics of locomotion in fossil arthropods. pp. 37-56. In (Rayner, J. M. V. & Wootton. R. 1.; eds) Biomechanics in Evolution. Journal of the Royal Society of Experimelltal Biology Seminar series 36. BUCKLAND, W. 1843. On Icthyopatolites. or petrified trackwings of ambulatory fishes upon sandstone of the Coal formation. Proceedings of the Geological Society. 4. 204. CASTER, K. E. 1944. Limuloid trails from the Upper Triassic
(Chinle) of the Petrified Forest National Monument. Arizona. American Journal of Science, 242, 74-84. GOLDRING. R. & SEILACHER. A. 1971. Limulid undertracks and their sedimentological implications. Neues Jahrbuch fur Geologie und Paliiontologie Abhandlungen Monatschefte. 137, 422--442. HANKEN. N. M. & ST0RMER. L. 1975. The trail of a large Silurian euryplerid. Fossils and Strata. 4. 255-270. HEEZEN. B. C. & HOLLISTER, C. D. 1971. Theface ofthe deep. Ol\ford University Press. HIGGS, R. 1988. Fish trails in the Upper Carboniferous of SouthWest England. Palaeontology, 31, 255-272. JERAM, A. 1989. 'The micropalaeontology of Palaeozoic scorpions.' PhD thesis, University of Manchester. - - & SELDEN. P. A. 1994. Eurypterids from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences. 84, 301-308.
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JORDAN, H. & VON MEYER, H. 1854. Ueber die Crustacean der Steinkohlen formation von Saarbrucken. Palaeontographica, 4,1-15. NOPSCA, F. 1923. Die Familien der Reptilien. Fortschritte del' Geologie und Paliiontologie, 2, 1-210. OWEN, R. 1852. Description of the impressions and footprints of the Protichnites from the Potsdam Sandstone of Canada. Proceedings of the Geological Society, 8, 214-225. RICHTER, R. 1954. Hihrte eines "Reisenkrebses" im Rheinischen Schiefergebirge. Natur und Volk, 84, 261-269. SALTER, J. W. 1867. On some tracks of Pteraspis(?) in the Upper Ludlow Sandstone. Proceedings of the Geologists' Association, 23,333-339. SEILACHER, A. 1958. Von Leben der Trilobiten. Die Naturwiisenschaffen, Springer Verlag, Berlin, 46, 389-393. - - 1962. Form Und Funktion des Trilobiten-Dactylus. Paleontologische Zeitschrift., H. Schmidt Festband, A. Rabien (ed),218-227. SELDEN, P. A. 1981. Functional morphology of the prosoma of Baltoeurypterus tetragonophthalmus Fischer Chelicerata:
L. I. ANDERSON
Eurypterida. Transactions of the Royal Society of Edinburgh: Earth Sciences, 72, 9--48. - - 1984. Autecology of Silurian Eurypterids. In (Bassett, M. G. & Lawson, J. D.; eds) Autecology of Silurian organisms. Special Papers in Palaeontology, 32, 39-54. SMITH, B. & GEORGE, T. N. 196 I. North Wales. British Regional Geology. HMSO, London. ST0RMER, 1. & WATERSTON, C. D. 1968. Cyrtoctenus gen. nov., a large late Palaeozoic arthropod with pectinate appendages. Transactions of the Royal Society of Edinburgh: Earth Sciences, 68, 63-104. TUREK, V. 1989. Fish and Amphibian trace fossils from Westphalian sediments of Bohemia. Palaeontology, 32, 623-643. WATERSTON, C. D. 1979. Problems of functional morphology and classification in stylonurid eurypterids, (Chelicerata, merostomata), with observations on the Scottish Silurian Stylonuroidea. Transactions of the Royal Society of Edinburgh: Earth Sciences, 70, 251-322.
Received 5 January 1995; revised typescript accepted 1 March 1995
Department of Geology, University of Manchester, Manchester, M 13 9PL.
1. INTRODUCTION The specimen (BGS GSM 26037), first noted by Buckland (1843), was interpreted as the result of the repeated impressions of three bony processes projecting from the anterior rays of the pectoral fin of an ambulatory fish. However, the disposition of the imprints, in rows of three paired tracks, is clearly characteristic of a large hetero-
External
Width
\ 3
\ I
l'
2
(
1:
,r' .
r
\
. '·'1
, I
2. IDENTITY OF THE TRACKWAY MAKER
I
'I-.------~V'------_.-!
Sel ABC
left
podous arthropod utilizing a hexapodous gait, similar in many respects to other previously described eurypterid trackways. Buckland (1843) noted that the specimen was collected from near the shaft of a coal pit at Mostyn, North Wales. This is the only locality detail, but the slab was probably derived from the adjacent mine. The exploitable coals in the Mostyn area are Westphalian A-B in age (Smith & George, 1961). The presence of a primary current lineation on the same surface as the trackway indicates fairly shallow, fast flowing water, perhaps on the margin of a river channel. The nomenclature adopted in this description follows Hanken & Stprmer (1975). A series is defined as a discrete group of imprints occurring on one side of the midline of the trackway and a set defined as a pair of series. The successive series of imprints are numbered from one to nine in the presumed direction of motion. The term track is defined as a single imprint, whereas the term trackway is used to define a repeated pattern of tracks (Anderson, 1975). External width is defined as the distance between the anterior points of the outer tracks (A) on either side of the trackway. Similarly, internal width is defined as the distance between the anterior points of the inner (C) tracks (Fig. I). Individual tracks are described according to their position in the trackway and expressed as a simple descriptive formula. The first element of the formula denotes the position of the track, either on the left or the right side of the hypothetical medial axis, the second element denoting the position of the track in a series; 'A' for outer, 'B' for middle and 'C' for inner and the third element indicating the series number, a letter 'b' following this number indicating an irregular imprint.
C
B Right
Fig. 1. Trackway terminology.
Proceedings of the Geologists' Associq{ion, 107,51-56.
A
We disagree with the original suggestion (Buckland, 1843) that the trackway was produced by an ambulatory fish. Ichnofossils attributed to fish consist of wavy, horizontal grooves, the result of their swimming motion (Higgs, 1988), e.g. Undichna (Anderson, 1976; Turek, 1989). Heezen & Hollister (1971) reported an abyssal bathypteroid fish 0016-7878196 $07·00 © 1996 Geologists' Association
52
S. J. BRADDY
&
L. I. ANDERSON
Fig. 2. Palmichnium pottsae ichnosp. nov., holotype BGS GSM 26037, British Geological Survey, Mostyn, North Wales. x 0.53.
UPPER CARBONIFEROUS EURYPTERID TRACKWAY FROM MOSTYN, WALES
C
ABC
B
A
The disposition of imprints suggests that a moderately large heteropodous arthropod, with its shortest legs towards its anterior, utilizing a hexapodous gait, made this trackway (Fig. 2). Jeram (1989) stated that in the absence of aquatic morphological characters in scorpion fossils after the Visean, it was likely that all scorpion lineages were terrestrial by the Upper Carboniferous, thus negating a: scorpion producer for this aquatic trackway. The lack of a medial groove, genal or lateral spine drags, and absence of five in-phase imprints, as is commonly identified for Kouphichnium Nopsca, 1923, negates a xiphosuran identity for the arthropod responsible for this trackway. We think that this trackway was produced by a moderately large (c. 20 cm long) eurypterid. The detailed leg morphology of many Carboniferous eurypterids is either unknown (Briggs & Rolfe, 1983), or based on fragmentary material (St~rmer & Waterston, 1968; Jeram & Selden, 1994), although the size of this trackway matches the maximum leg span, approximately 10 cm, of Adelopthalmus Jordan & Von Meyer (1854), a typical Coal Measures form (Briggs & Rolfe, 1983).
I r,
9
8b
.
.{.Q
(( 8
/ /.. /
7
I/~
, (fa
,/r'
3. BEHAVIOURAL INTERPRETATION
, (Vi
., \
, rf'
o
'
,~ , fl' 'i, ' .'f
LEFT
53
'l, RIGHT
Fig. 3. Interpretative drawing of Palmichnium pottsae ichnosp. nov., holotype BGS GSM 26037, British Geological Survey, Mostyn, North Wales. x 0.35. (PC indicates primary current lineation).
capable of leaving small tracks in soft sediment with elongate extensions of their ray-fins, but even these impressions consist of two lines of tracks with an additional medial line. The gurnard (Trigla) is an extant shallow water fish with three pairs of extensions from its pectoral fins but, as it had not evolved by the Upper Carboniferous, may be discounted.
We interpret the bedding plane upon which the trackway occurs as being an undertracked surface, as the impressions are relatively sharp and well defined, although of variable depth. The trackway was produced subaqueously, deduced from the presence of primary current lineation on the same bedding plane. The eurypterid was walking parallel to the palaeocurrent direction with a degree of curvature to the right, away from the palaeocurrent. It is not possible, however, to determine whether it was walking into, or out of, the current. The eurypterid walked in the direction of convergence of the left and right track series (see Hanken & St~rmer, 1975; Briggs & Rolfe, 1983). The outer two pairs of tracks shallow slightly posteriorly, away from the presumed direction of motion, indicating that the animal bore the majority of its weight on the anterior part of its posterior two appendages. Considerable variation exists in the degree of curvature of the tracks on either side of the trackway. The left rows display greater curvature than the right rows, which are linear in form, suggesting that the appendages responsible followed a curvilinear course in their stepping motion, scraping the surface of the substrate slightly. Not only are the rows on opposite sides of the trackway slightly out of phase but the orientation of the individual imprints is slightly asymmetric. The angle of the individual imprints to the mid-line is greater in the left rows than those of the right. This implies that the appendages of the left side of the body were maintained further from the medial axis of the body than the appendages of the right side, perhaps as the current forced the animal to veer sideways. The inner tracks are interpreted as having been produced by the bifid tarsal claws or spinose projections of an anterior walking appendage. On occasions where these
54
S. 1. BRADDY
&
imprints have a more linear form, the appendage probably just scraped the surface of the substrate. The right track rows retain a constant stride pattern throughout progression whereas the left rows show progressive relative displacement. Despite larger strides on the right side of the animal, the frequency of strides is greater on the left-hand side. We interpret this as being due to the turning motion of the eurypterid, larger steps taken on the right side to compensate for the greater frequency of leg movement on its left side. A greater degree of disorder of footprints on one side is not uncommon in arthropod trackways (Seilacher, 1958, 1962). Often this is indicative of a slightly oblique motion. The relative phase difference between opposite legs of a pair (calculated by the equation y1/(yl + y2), Fig. 1) varies considerably throughout the course of the trackway. The outer (A) and middle (B) tracks gradually alter from an opposite to an alternate arrangement. The inner pair of imprints (C) are largely opposite during the first three sets, until the occurrence of track R3b (Fig. 3), an irregular footfall, where the tracks gradually become alternate. The first row of tracks on the left side are set much further out from, and arranged almost transversely to, the mid-line, indicating they were held far from the body, possibly as part of a swimming stroke. The rapid transition from an in-phase to an out-of-phase motion in the inner tracks may suggest a transition from a swimming to a walking mode of locomotion, perhaps as the eurypterid emerged from shallow water. This transition is less apparent in the outer two tracks, perhaps due to the turning motion of the eurypterid and the occurrence of the irregular imprints, making interpretation of the change in opposite phase difference between the outer tracks complex. The successive phase difference between adjacent legs on one side of the body cannot be commented upon as unknown factors such as the leg length and speed produce variations in the imprint disposition. The left side of the trackway displays an irregular spacing of the successive imprints indicating that the relative phase difference altered throughout the course of the trackway, a feature which may also be due to the turning motion of the arthropod. Although the lack of a median groove may be the result of undertrack fallout (Goldring & Seilacher, 1971), it is suggested that the absence of it here may equally imply different behavioural activity. Buoyancy supplied to the abdomen by the surrounding water or the result of the telson being actively held up off of the substrate in a horizontal plane with the rest of the body, would result in no medial groove. This behaviour would reduce friction, allowing more efficient locomotion and enable the telson to act as a hydrodynamic structure, helping to steer the animal and assist stability (Waterston, 1979). Studies of the functional morphology of eurypterid walking (Waterston, 1979; Selden, 1981) suggests that eurypterids moved opposite legs out-of-phase in order to maximize body stability. A giant trackway from Pennsylvania (Briggs & Rolfe, 1983), attributed to a large eurypterid, however, shows a symmetrical arrangement
L. I. ANDERSON
of tracks, indicating that the legs of a pair were moved in-phase. The morphologically diverse eurypterids probably had a number of methods of walking (Selden, 1984). Assuming hexapody, the range of possible gaits could be discussed on the basis of consideration of stability. Speed could have been increased by reducing the pace duration, as in arachnids, rather than by altering the gait pattern, as in uniramians (Briggs, Dalingwater & Selden, 1991). The eurypterid responsible for this trackway probably used a relatively low gear gait (i.e. the relative duration each appendage was off the substrate, the promotor stroke and on the substrate, the remotor stroke, contributing to motion, usually expressed as a proportion out of ten), perhaps in the order of 2: 8 (see Selden, 1981). This gait and the accompanying short stride would have been relatively stable, enabling the eurypterid to make frequent irregular footfalls, which are observed in the trackway.
4. SYSTEMATIC ICHNOLOGY Ichnogenus PALMICHNIUM Richter, 1954 Diagnosis. Large trackways, usually with symmetrically opposed rows of three (occasionally four) tracks disposed en echelon, each track ranging from subcircular to adaxially concave impressions within the same trackway, the rows at a high angle to the axis of the trackway. Median groove, or ridge, usually present (after Briggs & Rolfe, 1983). Type ichnospecies. Palmichnium palmatum, Richter, 1954, from the Lower Devonian of the Rhine area in Germany. Palmichnium pottsae ichnosp. nov. 1843 [cthyopatolites Buckland, p. 204. Material. BGS GSM 26037. The specimen is housed in the Geological Museum Collections of the British Geological Survey, Keyworth, Nottingham. Diagnosis. Large trackway, with asymmetric rows of three paired tracks, lacking a median groove. Outermost tracks large and curvilinear. Intermediate tracks smaller and curvilinear. Inner tracks small and bifid. External to internal width ratio of 2.7. Description. The trackway occurs on the upper surface of a fine-grained, laminated (1-2 mm) micaceous sandstone, preserved in negative epirelief. It consists of eight complete sets of three paired impressions and several irregular imprints, not assigned to any series. The left series are well preserved, the right series are much fainter. There is no evidence of a medial groove. The total length of the trackway is 370 mm (Fig. 2). There are eleven rows of tracks on the left side of the trackway but only eight on the right side. The trackway has an average external width of 96 mm and internal width is 36 mm. The external to internal width ratio is approximately 2.7. The outer two tracks of each series (A and B) take the form of shallow
UPPER CARBONIFEROUS EURYPTERID TRACKWAY FROM MOSTYN, WALES
curvilinear or linear grooves, the inner tracks (C) being much shorter than the outer tracks and occasionally bifid in appearance. These tracks are arranged in groups of three, disposed slightly asymmetrically, but at regular intervals on either side of the hypothetical mid-line, with no evidence of a medial drag mark. The overall course of the trackway is slightly curved to the right, a displacement from a hypothetical mid-line of approximately 60 mm occurs along its length. The stride length has an average value of 35 rom for the left side of the trackway and 45 mm for the right-hand side. These proportions remain relatively constant throughout the successive series of tracks. The outer (A) tracks consist of gently curvilinear impressions, slightly convex outwards. The average length of these tracks is approximately 24 mm on the left side and 20 mm on the right, each track being deepest on its anterior side and shallowing gradually backwards. The middle pair of tracks (B) are similarly shaped to those of the outer, although slightly smaller, typically around 15 mm on both sides of the trackway. Similarly the middle tracks shallow posteriorly, but to a lesser extent than those of the outer tracks. Some of the inner tracks (C) have a bifid form, diverging in the presumed direction of motion. The individual length of these tracks is approximately 9 mm on the left side of the trackway and only 7 mm on the right side. There is no significant depth variation in these inner tracks. No sediment back push mounds are observed for any of the tracks. A small number of individual tracks, not belonging to any of the regular track rows, represent irregular footfalls of the animal. The majority of these occur on the left side of the trackway, some comprising complete rows, for example LAlb-LCIb and LA8b-LC8b (Fig. 3), whereas most occur only as irregular imprints of the inner tracks. The arrangement of the successive series of imprints display considerable variation on the left and right sides of the trackway. The angle to the mid-line of the left series is highly variable (62° to 105°, with an average value of 87°). The angle to the mid-line on the right-hand side of the trackway shows much less variation (48° at the start of
55
the trackway to 60° by the fifth set, maintaining this value consistently over the remaining right hand rows). Discussion. This trackway resembles most closely P. stoermeri in the general disposition and form of the tracks yet differs in lacking a median groove (possibly due to undertrack fallout), the external to internal width ratio of 2.7 (3.2 in P. SlOermeri) and the overall external width, which is only 60% of that of P. stoermeri. These proportions indicate a different eurypterid producer to the probable mixopterid producer of P. stoermeri. The original name, [cthyopato!ites, was only informally used, and not properly diagnosed, in the original description (Buckland, 1843), avoiding complications of synonymy. A number of other trackways credited to the activities of vertebrates are probably also of arthropod origin. It is almost inevitable that arthropod trackways were initially interpreted as of vertebrate origin (Caster, 1944). A series of trackways from the Potsdam Sandstone (Upper Cambrian) from Potsdam, Canada, attributed to the activity of a chelonian (Owen, 1852) were later suggested as xiphosuran or eurypterid trackways (Salter, 1867). Salter (1867) described trackways from the Downton Sandstone (Upper Silurian) of Kingston, Herefordshire, interpreting them as tracks of the heterostracan fish Pteraspis, although he did consider the possibility that the eurypterid Pterygotus may have been responsible. Salter's (1867) interpretative drawings show a disposition of tracks clearly characteristic of a large arthropod, possibly a eurypterid.
ACKNOWLEDGEMENTS We thank Dr Hugh Ivimey-Cook and Dr Steve Tunnicliffe of the British Geological Survey, Keyworth, Nottingham, for the loan of the specimen. We also thank Dr Jason Dunlop, Dr Paul Selden, Dr John Pollard, Dr Nigel Trewin and an anonymous referee for their constructive criticism. S. J. B. is funded by a NERC studentship. L. I. A. is funded by the W. & E. AIkins' Memorial Scholarship for Palaeontology (University of Manchester).
REFERENCES ANDERSON. A. M. 1975. The "Trilobite" Trackways in the Table Mountain Group (Ordovician) of South Africa. Palaeontologia Africana, 18, 35--45. - - 1976. Fish trails from the early Permian of South Africa. Palaeontology, 19, 397--409. BRIGGS, D. E. G. & ROLFE, W. D. I. 1983. A giant arthropod trackway from the Lower Mississippian of Pennsylvania. Journal of Paleontology, 57. 377-390. - . DALINGWATER, J. E. & SELDEN. P. A. 1991. Biomechanics of locomotion in fossil arthropods. pp. 37-56. In (Rayner, J. M. V. & Wootton. R. 1.; eds) Biomechanics in Evolution. Journal of the Royal Society of Experimelltal Biology Seminar series 36. BUCKLAND, W. 1843. On Icthyopatolites. or petrified trackwings of ambulatory fishes upon sandstone of the Coal formation. Proceedings of the Geological Society. 4. 204. CASTER, K. E. 1944. Limuloid trails from the Upper Triassic
(Chinle) of the Petrified Forest National Monument. Arizona. American Journal of Science, 242, 74-84. GOLDRING. R. & SEILACHER. A. 1971. Limulid undertracks and their sedimentological implications. Neues Jahrbuch fur Geologie und Paliiontologie Abhandlungen Monatschefte. 137, 422--442. HANKEN. N. M. & ST0RMER. L. 1975. The trail of a large Silurian euryplerid. Fossils and Strata. 4. 255-270. HEEZEN. B. C. & HOLLISTER, C. D. 1971. Theface ofthe deep. Ol\ford University Press. HIGGS, R. 1988. Fish trails in the Upper Carboniferous of SouthWest England. Palaeontology, 31, 255-272. JERAM, A. 1989. 'The micropalaeontology of Palaeozoic scorpions.' PhD thesis, University of Manchester. - - & SELDEN. P. A. 1994. Eurypterids from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences. 84, 301-308.
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JORDAN, H. & VON MEYER, H. 1854. Ueber die Crustacean der Steinkohlen formation von Saarbrucken. Palaeontographica, 4,1-15. NOPSCA, F. 1923. Die Familien der Reptilien. Fortschritte del' Geologie und Paliiontologie, 2, 1-210. OWEN, R. 1852. Description of the impressions and footprints of the Protichnites from the Potsdam Sandstone of Canada. Proceedings of the Geological Society, 8, 214-225. RICHTER, R. 1954. Hihrte eines "Reisenkrebses" im Rheinischen Schiefergebirge. Natur und Volk, 84, 261-269. SALTER, J. W. 1867. On some tracks of Pteraspis(?) in the Upper Ludlow Sandstone. Proceedings of the Geologists' Association, 23,333-339. SEILACHER, A. 1958. Von Leben der Trilobiten. Die Naturwiisenschaffen, Springer Verlag, Berlin, 46, 389-393. - - 1962. Form Und Funktion des Trilobiten-Dactylus. Paleontologische Zeitschrift., H. Schmidt Festband, A. Rabien (ed),218-227. SELDEN, P. A. 1981. Functional morphology of the prosoma of Baltoeurypterus tetragonophthalmus Fischer Chelicerata:
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Eurypterida. Transactions of the Royal Society of Edinburgh: Earth Sciences, 72, 9--48. - - 1984. Autecology of Silurian Eurypterids. In (Bassett, M. G. & Lawson, J. D.; eds) Autecology of Silurian organisms. Special Papers in Palaeontology, 32, 39-54. SMITH, B. & GEORGE, T. N. 196 I. North Wales. British Regional Geology. HMSO, London. ST0RMER, 1. & WATERSTON, C. D. 1968. Cyrtoctenus gen. nov., a large late Palaeozoic arthropod with pectinate appendages. Transactions of the Royal Society of Edinburgh: Earth Sciences, 68, 63-104. TUREK, V. 1989. Fish and Amphibian trace fossils from Westphalian sediments of Bohemia. Palaeontology, 32, 623-643. WATERSTON, C. D. 1979. Problems of functional morphology and classification in stylonurid eurypterids, (Chelicerata, merostomata), with observations on the Scottish Silurian Stylonuroidea. Transactions of the Royal Society of Edinburgh: Earth Sciences, 70, 251-322.
Received 5 January 1995; revised typescript accepted 1 March 1995