Non-marine bivalves from Writhlington Geological Nature Reserve, Avon

Non-marine bivalves from Writhlington Geological Nature Reserve, Avon R. M. C. Eagar EAGAR, R. M. C. 1994. Non-marine bivalves from Writhlington Geological Nature Reserve , Avon . Proceedings of the Geologists' Association, lOS, 251-264. Fossiliferous material from the rock store of Writhlington Geological Nature Reserve has come from two horizons , the lower one above No. IO Coal in the lower part of the Farrington Formation and the second one at its top, 200m higher, above the Rock or Badger Coal. Both horizons lie in the Chronozone of Anthraconauta tenuis, in the Zone of Lobatopteris vestita, and the Subzone of Dicksonites plueckeneti, Upper Westphalian D . All the material from the Nature Reserve store which has received published descriptions has come from above No . IO Coal. This horizon, which is rich in plants and land arthropods, has yielded uncommon, poorly preserved small short shells of Anthraconaia which are hitherto unknown in the Westphalian D . They are biometrically defined , illustrated and referred collectively to Anthraconaia cf. saravana (Schmidt). In the fauna above the Rock Coal the elongate biospecies Anthraconaia pringlei (Dix & Trueman) is also biometrically defined and is shown to have a posterior gape, a feature characteristic of a deep burrower and not previously recorded in the genus . Anthraconauta is represented by an apparent single community of complete shells and by many originally broken fragments. A study of the growth and variation of over 70 shells of Anthraconauta from Writhlington constitutes the first quantitative biometrical approach to this genus made in the UK. The Manchester Museum , University of Manchester, Manchester M13 9PL (address for correspondence : 23 High Bond End, Knaresborough, North Yorkshire , HG59BT) .

1. INTRODUCTION: EARLIER WORK

Bolton (1911), Dix & Trueman (1929, 1931) and Weir (1960, 1967) described a few shells from Writhlington Colliery, Lower Writhlington, Bolton having established the horizon as 'above the Rock or Badger Vein ' . Hereinafter the name Rock Coal is used. It has thus been known for many years that the non-marine fauna above the Rock Coal formerly worked at Writhlington Colliery includes Anthraconaia pringlei (Dix & Trueman) (Fig . Ih) and Anthraconauta tenuis (Davies & Trueman) (Fig . 8a). Dix & Trueman (1931) illustrated one other shell they referred to A. cf. pringlei (Fig. Ij). No other horizon yielding non-marine bivalves was recorded at Writhlington until Jarzembowski (1989) reported occasional non-marine bivalves from a horizon deduced to be above the No. 10 Coal, 200 m below the Rock Vein. However, Allen (1977) had noted occasional bivalves from the same horizon on the tip of the nearby Kilmersdon Colliery , 1 km south of Radstock . Both the horizons lie in the Farrington Formation, the former at the top and the latter near its base. Both lie within the Chronozone of Anthraconauta tenuis , the Zone of Lobatopteris vestita and in the Subzone of Dicksonites plueckeneti (Thomas & Cleal, 1994; Cleal, pers. comm .) . The horizon is, therefore, Upper Westphalian D . The present work, on extensive collections of nonmarine bivalves from Writhlington Colliery tip, has been made possible by the preservation of its material in the store of Writhlington Nature Reserve in 1987 (JarzemProceedings ofthe Geologists' Association , lOS, 251-264.

bowski, 1989, fig. 1). The store has been visited by many collectors since this date and the non-marine bivalves , sent to me through Dr Jarzembowski , have been subsequently deposited in the Manchester Museum (numbers prefixed by MM). In this paper reference is also made to earlier collected specimens in the City Museum , Bristol (BM) , the National Museum of Wales (NMW) and the Natural History Museum, London (BMNH). 2. DISCRIMINATION OF THE TWO NON-MARINE HORIZONS Mixture of material on the tip Writhlington Colliery was sunk in 1829. The shallower coals in the Radstock Formation (Thomas & Cleal, 1994, fig. 1) were worked before those in the underlying Farrington Formation, but coals from both formations were being worked in the first quarter of this century (Down & Warrington , 1971). By 1960 only No . 10 Coal was being worked (Allen, 1977). Jarzembowski (1989) and Thomas & Cleal (1994) give cogent reasons for supposing that all the plants and the rich arthropod faunas, including trace fossils , collected from both the tip and the later rock store of the Nature Reserve came from above No. 10 Coal. Moreoever, there was some evidence (Jarzembowski , 1989, p. 219) to suggest that earlier buried material from above the Rock Coal had been destroyed on the tip by combustion . Consequently Jarzembowski (1989, p. 221, e .g. fig. 3) regarded all 0016-7878/94 $07·00© 1994 Geolog ists' Association

252

R . M . C. EAGAR

a

c

b

m

d

e

k

Fig. 1. (a-e) Group of Anthraconaia ct. saravana from above No. 10 Coal ; (f-m) Anthracona ia pringleifrom above the Rock Coal, Writhlington. All shells x 2.4, except (e), which is x 7, and I and m, which are x8. (a) - (d) in order MM LL.lll00-11104. (f-g) A . aff. pringlei , MM LL .11lO5-6; (h) A . prin glei , holot ype , BMNH L.56279; (i) A . aff. pringlei , MM LL.I1107; (j) A. aff. pringlei, figured by Dix & Trueman , 1931, pI. xvii, fig. 11, BMNH L.56280; (k) A . aff. pringlei, MM LL.11108; (I,m) , dors al and ventral views of a stein kern of A . pringlei , showing posterior gape of the valves; figured by Bolton (1911, pI. XXVII. fig. 7), 8M C.960; both views x 8.

non-marine bivalves collected from the store as also having come from above No . 10 Coal. Faunal and sedimentary associations of the two faunas

Abov e No. 10 Coal Jarz embowski (1989) and others

(pap ers in this volume) found a great abundance of plants and Blattodea and, in order of much diminishing numbers, Protorthoptera, Eophrynus, Euproops , Kouphichnium and Cochlichnus, the last trace fossils being almost equally uncommon with non -marine bivalves and conchostracans (Jarzembowski , 1989, fig.

253

WRITHLINGTON GEOLOGICAL NATURE RESERVE. AVON

19). Still rarer occurrences were ostracod traces , Palaeodictyopteroidea, a myriapod and Araneida. Well-preserved plants with this terrestial fauna occur in 'grey laminated mudstone'. In the same paper nonmarine shells were reported to be occasionally associated with Cyperites (strap-like leaves of lycopsid, e.g. Thomas & Cleal, 1994, fig. 5e) and with insects. Pollard & Hardy (1991, figs 4, 5) described and illustrated small ovoid 'resting places' on planes within the laminated mudstones and referred them to Lockeia or Pelecypodichnus but attributed their origin to conchostracans. Non-marine bivalves occur sparsely within this facies and are referable to small, short Anthraconaia. By contrast, the fauna known to occur above the Rock Coal in collections made before 1931consist only of nonmarine bivalves without recognizable plant debris and with no other associated animal fossils. The shells occur in dark grey unlaminated carbonaceous, typically carbonate-rich shaly mudstone and include common elongate Anthraconaia and abundant Anthraconauta with originally fragmented shells.

In summary, faunally, florally and lithologically the grey mudstones which bear the small short Anthraconaia correspond fully to the description of the material concluded to lie above No. 10 Coal. They are separable without doubt from the material known to occur above the Rock Coal. However, of the material sent to me from the rock store at Writhlington, more than nine-tenths consist of elongate Anthraconaia and of Anthraconauta including many shell fragments of this genus in dark unlaminated shale. This material perfectly matches collections made above the Rock Coal from Writhlington Pit by Bolton (1911, pI. XXVII, figs 7, 14), now in Bristol City Museum, by Dix & Trueman (1929, figs. la-c) now in the National Museum of Wales, and by Dix & Trueman (1931, pI. XVII , figs 10,11), now in the Natural History Museum. It must, therefore, be concluded that, contrary to previous belief, much ofthe materialfrom above the Rock Coal escaped combustion on Writhlington Colliery tip . When height (H) and length (L) of shells of elongate Anthraconaia known to occur above the Rock Coal are

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a

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----- C>· ~ j

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-,

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Fig. 2. Representative shells of the group Anthraconaia ef. saravana , from above No. 10 Coal, Writhlington, arranged to show variational trends. Broken lines denote absence of intermediates. All shells x 2.4 . (a) MM LL.1112l, reversed; (b) shell figured by Dix & Trueman (1929, fig. ld), who assumed that it came from the horizon of the Rock Coal- the matrix suggests that this shell came from above No. 10 Coal, but it is darker than usual, NMW 70.l7G.325; (c) shell shown also as Fig. l c, MM LL.ll102; (d) shown also as Fig. lb, MM LL.lllOl; (e) MM LL.11118; (f) MM LL.11119 ; (g) also as Fig. la, reversed; (h) also as Fig. ld, MM LL.l 1104; (i) also as Fig. Ie , MM LL.lll04; (j) MM LL.11120 .

254

R . M. C. EAGAR

plotted with those of elongate Anthraconaia from the store they show the continuous variation of the species, Anthraconaia pringlei (Dix & Trueman) (see p. 257). The measurements contrast with those of Anthraconaia from the horizon over No. 10 Coal (see Fig. 3, where measurements are shown as logarithms) . When heights and lengths of Anthraconauta which have come from above the Rock Coal are compared with those of Anthraconauta from the store (Fig. 7), it will be seen that the coverage of the points, including the ' kink' in the measurements between L = 11 and L = 13, is the same for both collections.

3. NON·MARINE SHELLS FROM ABOVE No. 10 COAL

Small, short bivalves (Fig. Ia--e; Fig. 2) are represented by the remains of shells or their moulds, which indicate that both valves were attached in most cases . Valves are closed and have been buried with little displacement . Median planes lie parallel or subparallel to the bedding of the mudstone . Where preservation is adequate (Fig . la , b) the bivalves appear to have had thin shells with finely spaced regular growth lines and with no indications of anterior adductor scars. The latter features are part ofthe characteristics ofthe genus Anthraconaia, but the internal mould of Fig. 2j has the semblance of a pallial line. In the case of the largest shell (Fig. 2b), which lies in the darkest of the sediments associated with these shells (see the description of the figure) , growth lines are equally fine but the thin shell has suffered creasing more or less parallel to the growth lines . The smallest shells (Figs Id, e ; 2h, i) have expansion to the posterior in lateral outline, and thus a more characteristically Anthraconaia-like appearance , than the remainder. Pattern of growth When shell length (L) is plotted as abscissa against shell height (H) and length of anterior end (A) (see Fig. 3b) the paths of points form curves convex to the ordinates. The paths straighten when logarithms are plotted (Fig. 3) to give the following fitted lines : where

r = 0.98 and N

=

where

r

= 0.93 and

N

(i)

11, and

Log A = 0.687 Log L - 0.504

Log H = 1.012 Log L - 1.037 where

r

= 0.96 and

N

Log A = I.l45 Log L - 1.934 where

(iii)

= 15, and (iv)

r = 0.95 and N = 14.

Species of Anthraconaia with early growth trends towards subcircularity, as in Figs 2j, e, are very unusual in the Upper Westphalian. The trends are seen , however , in many shell faunas of Carbonicola and Anthracosphaerium of Westphalian A and B (see below). Naming

Occurrence and generic features

Log H = 0.825 Log L - 0.227

the Rock Coal (Figs If-k; Fig. 3, Fig. 4) . For the latter species,

(ii)

= 8.

Although there are few shells available , the trend of their directional growth was sufficiently uniform to reveal a high coefficient of correlation (r) between the logarithms of Hand L. The coefficients may be compared with those of Anthraconaia pringlei from above

Despite some of the apparent discontinuities in the range of variation of the shells of Fig. 2, the group as a whole appears comparable with a biospecies, or to belong in greater part to a biospecies. However, preservation is questionably complete enough for any of its members to serve as type material. The group is, therefore, named tentatively and by comparison with an already described well preserved morphological species , Anthraconaia saravana (Schmidt) (Fig . 3a) of Stephanian A of the Saarbriicken Coalfield. It may be significant that Weir (1960) described two very small shells from the Phillipsii Chronozone of North Staffordshire also as Anthraconaia ct. saravana. Weir (1960, pp. 194-5) suspected that A. saravana (Schmidt , 1907, pI. i, fig. 23) represented a population of 'facies dwarfs', and this conclusion is of particular interest in view of the palaeoecology of these very small shells (described below) . The small Anthraconaia of Westphalian C and D (Phillipsii and Tenuis Chronozones) of the UK and of Stephanian A of northern Europe may all be derivatives of the larger, commoner and stratigraphically long-ranging stock of Anthraconaia of the late Westphalian and early Stephanian. The naming, therefore, supports current supposition that the small shells from the three horizons may be related . Dimensions Standard dimensions of Anthraconaia ct. saravana from Writhlington are shown in Table 1. The shells have been crushed laterally so that thickness or obesity cannot be measured . However, several of the shortest outlines in particular are closely comparable with those of varieties found in the Anthracosiidae, especially Anthracosphaerium (e.g. Figs 2d, e and j) . In the Anthracosiidae all these varieties have high obesities (e.g. 40% or over). It is likely that the comparable varieties of Anthraconaia also had high obesities both on the basis of known variational ranges in smoothshelled bivalves and on theoretical considerations of surface area and shell volume (Eagar, 1978, p. 193).

255

WRITHLIN G T ON GEOLO G I C A L NAT UR E R E S ERVE . AVON

Log & Log

H

o Anthraconaia cf.saravana group

A

+ A. saravana , syntypes

• Group of A. pring lei

*

A. pringlei, holotype

Dix & Trueman 1929, fig1c

X "

2·0

*

+

e " " Bolton 1911, pl.27, fig.?

e•

• •

X

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•

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A.pringlei

,

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Eq iv

0'5

A.cf.saravana

o

0

o

o

0

'

0 '-o

'7

"-E.q .1I.. Log 1'4

L

3 ·0

2·0

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o

a o

1 em

Fig. 3. Logarithms of H (shell height) and A (length of anterior end) plotted against logarithms of L (shell length) for two groups of Anthraconaia at Writhlington, A . cf. saravana from above No . 10 Coal , and A. pringlei from above the Rock Coal. The equations are written in the text. (a) Anthracona iasaravana (Schmidt), syntype , from the Gottelborn Shale s, upper Steph anian A , Wemmetsweiler , Saar Coalfield ; (b) A . aff. pringlei (Dix & Trueman) , also shown as Figs Ii and4g. MM LL.I1107. (a) was figured by Waterlot, 1934, pI. IV, figs 3, 7, and by Eagar, 1984, pI. 1, fig. G,; Von der Heydt Museum , Saarbriicken.

Comparisons The difference between A. cf. saravana from Writhlington and the syntypes of Anthraconaia saravana is best exemplified in the comparison of Fig. 3a with Figs Id and 2h. In the former , the parallelism of the dorsal

and ventral margins in a very short shell produces a most distinctive outline. In the latter, the shell is more elongate and has a shorter anterior end (see the graphs of Fig . 3). Varieties such as Fig. 2a and c may be represented in the Stephanian A , mainly among slightl y

256

R. M. C. EAGAR

Table 1. Standard dimensions of Anthraconaia ct. saravana and A. pringlei from above No. 10 Coal and the Rock Coal respectively at Lower Writhlington. OR is the observed range and N the number of specimens measured Length (L) in mm OR

A. ct. saravana

3.05-12.2

A.pringlei N

Mean

OR

8.81

47.2--66.0

7.10-28.4

Mean

OR

54.47

14.6-15.5

16.48 15

larger shells (Eagar, 1984, fig. 4E 1. E 2 ) . Some members of the Anthraconaia puella group found in silty facies are closely comparable with the A. cf. saravana group in H and L dimensions (Eagar, 1984, fig. 3, 'Querfurt 2' fauna) but the A. puella group have shorter anterior ends and a quite different total range of variation from A. cf. saravana (Eagar, 1984, fig. 4, right-hand side). Palaeoecology and shape of shell

The plants in the laminated mudstones appear to have originated from levee vegetation (Thomas & Cleal, 1994). More detail about probable interrelationships and food chains among the arthropods has been added by Jarzembowski (1989). Non-marine bivalves and arthropods are rarely associated in the geological record (e.g. Baldwin & Sutcliffe, 1904), and it has been commonly assumed that contacts between land areas of the forested delta and non-marine lakes were ephemeral and unlikely to be preserved in the deltaic depositional regime. Pollard & Hardy (1991, with an Appendix by Goldring) were able to define the sedimentary environment of the laminated mudstones fairly closely. They write 'The presence of laminites, or rhythmic siltstones with loaded bases, escape structures and mud partings indicate periodic sedimentation from suspension with the development of mud-draped omission surfaces'. 'The lack of infaunal sediment feeding burrows (e.g. Planolites montanus) reflects the shallow temporary freshwater nature of the environment as well as possible low oxygen levels below the sediment-water interface, usually indicated by sideritic mudstones. A shallow lake or pool on a fluvial floodplain (flood basin lake) seems a likely environment where marginal mudflats may have been temporarily exposed under humid conditions, but not dried or mudcracked'. Conditions in which the waters of non-marine shells were periodically slowed or stilled have been investigated particularly amongst the anthracosiid faunas of the North Midlands coalfields. It was found in these faunas that persistent changes in shape of shell took place in response to changes in past water velocities.

37.78

28.6-42.5

Mean 18.38 8

11

11

N

A/L(%)

HlL(%)

15

16.7-25.3

21.60 14

Increases in HIL and AIL ratios of the shells accompanied decreases in water velocity (e.g. Eagar, 1973, fig. 13a, b). Moreover, such changes, including the further expression of the shortening trend towards subcircular shell form and increased obesity, have been demonstrated among recent freshwater unspecialized unionid stocks where faunas have been followed into slowed and ponded water (Agrell, 1949; Eagar, 1978). The reasons for these persistent changes in shape of shell have also been discussed in detail (Eagar, 1978, pp. 290-3). Thus the particular characteristics of the Anthraconaia cf. saravana fauna may have been caused by the unusual sedimentological conditions under which the laminated mudstone above Coal No. 10 was laid down. Furthermore, small size of shell, cateris paribus, points to a shortened bivalve life span. It is possible that these thinshelled bivalves, which appear to have been members of a stock of longer, larger and more widespread Anthraconaia, found a minor ecological niche as 'floaters' in already well-populated short-lived pools or lakes. 4. THE FAUNA ABOVE THE ROCK COAL Anthraconaia

Earlier collected material; shape of shell, function and variation Dix & Trueman (1931) and Weir (1967) gave a broad description of Anthraconaia pringlei, which they recognized, in related forms, in the Westphalian C and D from Scotland to South Wales. However, they did not describe the type assemblage of A. pringlei from Writhlington. Dix & Trueman (1931) included amongst the species a specimen from Writhlington Colliery, shown here as Fig. 4e, much enlarged, as Fig. 11, m. In this small superbly-preserved steinkern the shell is represented by a black, richly carbonaceous rind on which the growth lines are perfectly preserved. The right valve has slipped ventrally 0.2 mm with respect to the left one, but without any rotatory movement; the hinge margins of the two valves are precisely parallel. The ventral view shows a

257

WRITHLINGTON GEOLOGICAL NATURE R ESERVE . AVON

\

I

~ '--- - - - -

.::--- -:--- -----k

~0 J) b

~-)

m"

C-~ a

~ 9 o

/

1 em

Fig.4. Variation diagram of Anthraconaia pringlei from above the Rock Coal at Writhlington. All shells x 1.6. (a) A . sp. juv. MM LL.I1109; the remainder, except (e) and (f), the holotype, are referred to A . aff. pringlei; (b) MM 111110; (c) LL.11108; (d) BMNH L.52680, see Fig. Ij , reversed ; (e) composite , drawn from both valves, and seen also in Fig. 11 , m; figured by Bolton (1911, pI. XXVII, fig. 7), BM C.690; (I) holotype, see Fig. Ih ; (g) see also Fig. n, LL.11107 ; (h) NMW 70. 17G.585; (i) MM LL.11112; (j) LL.I1113 ; (k) drawing designated Anthraconaia (Anthracomya) sp. nov. (d. A. Ianceolata) by Dix & Trueman (1929, fig. lc); (I) MM LL.11114 ; (m) also shown as Fig. Ig , LL. 11106; (n) LL.11115 .

considerable posterior gape arising from a divergence of the ventral margins of the valves at 7 mm from the posterior end and at nearly half the length of the bivalve (Fig. 1m). The dorsal view (Fig. 11) shows the divergence of the margins at about 2.5 mm from the posterior end. A gape of this type and proportion is a familiar characteristic of marine shells which burrow steeply and maintain siphonal contact with the mud-water interface, for example the Myacea. Thus the gape suggests that the life position of Anthraconaia pringlei may have been somewhat similar; but in this case there is no faunal association which suggests other than fresh , or possibly brackish water. The variational range of the type assemblage of A. pringlei is shown in Fig. 4. There is emphasis on elongation and parallelism to subparallelism of the dorsal and ventral margins (Fig. 4b, e, I, m, i, k). The fauna is unique in the combination of expansion of the posterior end ('ENE' and 'E ' Series) with a straight to arched dorsal margin (shells c, I, j, n) andin the inclusion of a trend in which the curvature of the anterior ventral

margin shows a small reflection (shells e, f, h, k). The pattern of variation is also conformable with the evidence that A. pringlei could have been a siphonate burrower. Elongate, supposedly steep-burrowing bivalves are known from lower horizons in the Westphalian C of Britain, for example Anthraconaia spathulata Weir , with which there are associations suggestive of near-marine or brackish water conditions (Weir, 1967, p. 395). However, in no other Anthraconaia has there been any clear evidence of a posterior gape. Indications ofsequence and life conditions abo ve the coal Slabs of mudstone with Anthraconaia pringlei subse-

quently collected from the Writhlington rock store and used in Figs 3 and 4, suggest that this species was confined to a band little more than 1.5 cm thick . Shells occur commonly through this thickness and without other associates. Valves are invariably attached, more than 70% are closed and 11% are inclined at a small angle to

258

R. M. C. EAGAR

the bedding (Fig. 1£). Some 8% of the attached valves are open, up to an angle of about 60°. Their orientation, which is very likely to be with ligament uppermost, suggests, from a few slabs which include both genera, that the band of Anthraconaia pringlei lay above that of Anthraconauta and overlapped it by a few millimetres; so that a few large shells, including the holotype of Anthraconaia pringlei, share bedding planes with broken pieces of Anthraconauta shell, including the heavily wrinkled periostracum. The sequence of the genera suggested is the reverse of that commonly found in late Westphalian - Stephanian cyclothems of the UK and NW Spain (Eagar, 1985) and Nova Scotia (Vasey, 1985). The thin band of A. pringlei, with several shells having their median planes vertical or near-vertical to the bedding plane of the mudstone and their long axes variably inclined to it, indicates a life assemblage in the sense of Boucot (1953), or a community (Craig, 1953). Anthraconauta

Earlier collected shells: 'solid' specimens and lateral crushing Bolton (1911, pI. XXVII, fig. 14) illustrated a steinkern of large Anthraconauta which was found above the Rock Coal at Writhlington and was then thought to be of Naiadites. Such uncrushed specimens of Anthraconauta are extremely rare. This example (Fig. 5a--e) clearly shows that Anthraconauta lacks the carina of Naiadites, as was contended but rarely indicated by Weir (1960). The steinkern is partly embedded in mudstone,

~~------------_

so that its convexity can only be estimated, and appears to lie between 28 and 32%. In Naiadites, as a result of the carina, convexity is commonly 40% or more (Weir 1960, p. 284). As in typical communities of Anthraconauta, virtually all the shells lie parallel to bedding planes. Those which lie oblique to them are usually incomplete and their distortion is readily recognizable. When compression takes place perpendicular to the median plane of the valves, by far the commonest situation in assemblages of Anthraconauta, work on the nearest related thin-shelled myalinid, Anthraconaia, has indicated that the shell has no more strength than the surrounding mud. Major crumpling, with a variable amount of small breaks in the shell takes place in the parts where the valve is most convex (Eagar, 1987, p. 178). No evidence was found that the dimensions of Anthraconaia were significantly altered by lateral crushing, except in the immediate vicinity of the umbo. The equally thinshelled Anthraconauta evidently behaves in a similar manner in this situation. However, the thicker periostracum in Anthraconauta, also found in Naiadites, records the degree of crumpling and foreshortening by patterns of wrinkles (Figs 8a, e, h, k). The virtual absence of wrinkling in the small shell of Fig. 8g suggests that this variety had low obesity and that foreshortening left wrinkling parallel to the lines of concentric shell growth. By contrast, the extensive wrinkling in the equally small shell of Fig. 8k suggests an originally more convex shell. The shell of Figure 5a is also remarkable because most of it has resisted lateral compression, whereas the posterior dorsal part, where the shell was presumably thinnest, has been crushed, sheared and pushed obliquely upward,

o I

C~

1 em I

b Fig. 5. A steinkern of Anthraconauta, laterally crushed to the posterior, with displacement of the posterodorsal margin, where part of the shell remains. Lateral (a), dorsal (b) and anterior (c) views, from photographs with simplification of detail. From above the Rock Coal at Writhlington Colliery. The shell was interpreted differently by Bolton (1911, pI. XXVII, fig. 14), BM C.9?!. x 2.5.

WRITHLINGTON GEOLOGICAL NATURE RESERVE. AVON

over the uncrushed part. The resultant lateral outline is a rare distortional accident. In summary, crushing by pressure oblique to the median plane o~ the bivalve is recognizable and the few shells so distorted can be eliminated from biometrical studies of Anthraconauta. The vast majority of Anthraconauta have been crushed perpendicular to their median planes and have suffered little or no significant change in lateral outline. As in previous work on Anthraconauta, these laterally flattened shells are alone used for the biometrical data in the present study. Fossillferous bands and sequence On the evidence from some ten large collected slabs there is probably no fauna immediately overlying the coal. The sequence begins at a short but uncertain distance above the top of the coal, probably only a few centimetres. Shells of Anthraconauta with small occasional plant fragments are mingled with broken shells of this genus. Anthraconauta continues upward for a maximum thickness of 5 cm when broken shells are overlapped, over a few millimetres, by complete specimens of Anthraconaia pringlei. Size and general growth pattern of the shells in terms ofH and L The shells reached smaller size than many figured examples of Anthraconauta from bands in the A. tenuis Chronozone. A plot of shell height against length (Fig. 7) shows a partial interval or 'kink' in the path of points between the lengths of 11 to 13mm. The few shells in the interval differ from the bulk of the remainder in having a larger angle y (Fig. 6) and thus

259

a different orientation with respect to their hinges. Several lie on or near Series E of Fig. 9. It is likely that in an Anthraconauta community such different varieties would have different hydrodynamic characteristics, as suggested by the results of Eagar (1985) and Vasey (1985), who noted consistent sedimentological changes accompanied particular variational changes in communities of this genus. Thus the limited variational trends seen in this interval of shell length may indicate a temporary switch in current, such as would affect a single generation of bivalves in the immediate area. This suggests that the shells probably reflect a life assemblage in the area rather than protracted colonization. Shell growth and variation The range of variation (Fig. 9 and Table 2), is very great. Numerical representation is indicated within the series by the degree to which each series is filled up with intermediate, progressively larger shells from the centre. Thus the dominance of Anthraconaia tenuis, a species which has been broadly interpreted in accordance with its stratigraphical utility since its description, is apparent at once in the Series A cA 3 (see also Table 2). Series B shows the smaller numerical incidence of shells with outlines approaching A. calveri, a morphological species which Vasey & Bowes (1985) separated from shells which for many years have been called A. cf. phillipsii. In Series C and D the general form of Anthraconauta wrighti, grading closely with A. tenuis, is approached, but not reached in shell No. 23. Varieties similar to those of Series E have been compared with A. phillipsii, but the obliquely ovate

I+---DM - - - - . I

o

cm

Fig. 6. Standard linear and angular measurements used for Anthraconauta. The shell to the left is A. tenuis , the lateral outline being based on the specimen NMW 70.17G.84, Dix & Trueman (1929, fig. lb). The right-hand shell is shown in Fig. 8h. L: the greatest shell length measured parallel to the line of hinge - see also Fig. 3b; H: measured at right angles to length; m: line of maximum growth; w: width at right angles to m (Eagar, 1973, fig. 6a); DM: dorsal margin; D: length tangential to point of downward bulge (Leitch, 1940; Vasey & Bowes, 1985); ax. 0: axis of obliquity; ax. p.e: axis of posterior extension - see Trueman and Weir (1955, text-fig. 29). In the left-hand shell the line of maximum growth (m) coincides with the axis of posterior extension.

260

R. M. C. EAGAR

H in mm o 10

o o

o o

0

0

o

0

0

00

o 0

o

0

o 00

5

o o o

y

:

. -·:0

o

o •

Shells from the store of Writhlington Nature Reserve

o

Shells above the Rock Coal,

*

00

10

5

Wrlthlington Colliery

Anthraconauta fenws (Davies and Trueman), hototype

15

25

20

L

10 mm

Fig. 7. Height and length measurements of Anthraconauta from above the Rock Coal at Lower Writhlington.

shells of Series F and G, the mytiliform series of Series H, including No. 40, and the bizarre small shells of Series I and J have not previously been shown associated with Anthraconauta in the UK. The method of building the pictograph, on the principle of minimizing the number of varieties which are unplaceable because they fall between distant 'series' (Eagar, 1972), has revealed that considerable

differences in variation can be traced back to shells little larger than spat size. These have been placed on the broken circular line round the central norm. Comparisons Studies of the total variation of Anthraconauta on two horizons in the late Westphalian D of Nova Scotia were made by Vasey & Bowes (1985) who used cluster

Table 2. Standard dimensions of a community of Anthraconauta tenuis from above the Rock Coal of Lower Writhlington. OR is the observed range and N the number of specimens measured Length (L) in mm OR 2.10-25.90

Mean

OR

10.08

1.50-10.50

70

N

OR 33.70-93.2

Antr end (A) in mm

Mean

OR

5.67

0.40-2.25

70

HlL(%)

N

Height (H) in mm

OR

59.89

8.1-32.0

70

w/m(%)

Mean

OR

12.27

34.1-92.9

58

Mean 53.51 58

y

OR

1.17 58

A/L(%)

Mean

Mean

Mean

12°_50° 27.54 N 57

261

WRITHLINGTON GEOLO G ICA L NATURE R E S ER V E , AVON

b

a

c

e

d

f

9

h

k Fig. 8. Varieties from a community of Anthraconauta with domin ant A . tenuis from above the Rock Vein , Writhlington . All specimen s are laterall y crushed. All she lls x 3, except (g) and (k) , which are x 10. (a) A. tenuis (Davies & Truem an) , MM LL.11067; (b) A . tenuis LL.ll068; (c) A. cf. wrighti (Dix & Trueman ), LL.1 1069A; (d) A . aff. phillipsii (Williamson) cf. Weir (1960, pI. XXXII, fig. 7, young stage) , LL.ll 069B; (e) A . cf. calveri Vasey & Bowes, LL.ll070; (f)A . aff. calveri, LL.11071 (g) Anthraconauta sp., a shell near spat size , with regular growth lines , shown also as Fig. 9.29, LL.ll0n x 10; (h) A . cf. calveri, a shell with a large wlm ratio which is just comparable with that of fig. 7, No. 2051 of Vasey & Bowes 1985, BM Cd:3349; (i) Anthracon auta sp. nov. shown also as Fig. 9.43, LL.11073; U) A . sp. nov. , a mytiliform variet y, shown also as Fig. 9.39, LL.11074 ; (k) A . tenuis sp. juv., with a strongl y wrinkled peri ostracum , shown also as Fig. 9.9, LL.11075.

262

R. M. C. EAGAR

B

45

J

o I

20

H

c '0

23

D

31

F 33

Fig. 9. Variation diagram of a community of Anthraconauta tenuis and associated varieties, showing growth. Lines A to J denote series which are described in the text. All shells x 1.4; R denotes a reverse image. (1) BM Cd.3348A, a, b; (2) MM LL. I 1076; (3) also Fig. 8a, LL.1I067 R; (4) LL.lI077; (5) LL.7986; (6) LL.I1069C; (7) LL.1l078; (8) LL.1l079; (9) also asFig. 8k, enlarged, LL.I1075; (10) LL.lI080; (11) LL.lI081 R; (12) also as Fig. 8b, LL.lI068; (13) NMW 70.17G.323A; (14) BM Cd.3351; (15) MM LL. I 1082C; (16) also as Fig. 8e, LL.lI070 R; (17) LL.lI082A; (18) LL.lI085 R; (19) LL.lI086 R; (20) LL.1l087 R; (21) LL.11084; (23) BM Cd.3350 R; (24) Cd.3348a, b R; (25) MM LL.11088; (26) LL.1l089 R; (27) BMNH PL.4439A; (28) NMW 70.17G.616 R; (29) also Fig. 8g, enlarged, MM LL.1l072; (30) LL.11090 R; (31) LL.l1091; (32) 11092 R; (33) LL.11093; (34) 70. I7G .586 R; (35) L.I 1094 R; (36) also Fig. sr, LL.l107I R; (37) also Fig. 8h, BM Cd.3349; (38) MM LL. 11095 R; (39) also Fig. 8j, LL.11074; (40) BMNH L.24491 R; (41) MM LL.1l096 R; (42) LL.11097; (43) also Fig. 8i, LL.11073; (44) LL.11098; (45) LL.II099.

WRITHLINGTON GEOLOGICAL NATURE RESERVE . AVON

analysis . The faunas above the Phalen Seam and between the Harbour and Hub Seams of Sydney Coalfield were compared using the ratios HlL, AIL, DMIL, DlL, and wlL (see Fig. 6), when Nwas89 and 91, respectively. Significant differences were found in four clusters representing A . calveri, A. phillipsii, A. tenuis, and A. sp. cf. A. wrighti. A detailed fully objective comparison of the Writhlington and Nova Scotian faunas requires the tabulation of data from all three faunas but the Nova Scotian measurements are not yet available. The Writhlington and Nova Scotian faunas have similar elements, as is apparent in the naming and comparison of morphological species common to both, but on the basis of the comparison of figured varieties there appear to be a few significant differences between them. Anthraconauta tenuis of Nova Scotia has a higher wlm ratio than this morphological species at Writhlington . The shape of A. calveri, as represented by the types, is not reached in Writhlington, where comparable specimens have a longer hinge line (DMIL ratio). It appears very unlikely that the 'narrow' varieties of Anthraconauta with low angle {J (Fig. 9, Series E) are represented in Nova Scotia . It should finally be stressed that although the method of cluster analysis has the advantage of objectivity, which has been denied in some pictographic comparisons, the ratios used in Vasey & Bowes' work imply that such ratios have

263

remained unchanged during shell growth. The present growth study does in fact indicate comparatively small change in shell ratios in some Writhlington trends within Anthraconauta (Fig. 9, Series A I-A3 ) although in several other series (e.g. B, E, H) dimensional changes with growth are considerable.

ACKNOWLEDGEMENTS I am very grateful to Dr E. A. Jarzembowski who first suggested that I should undertake the work and very kindly arranged the transfer of non-marine bivalves collected at Writhlington Geological Nature Reserve to me at Knaresborough. For the loan of earlier collected material from Lower Writhlington I am indebted to Bristol City Museum, and especially to Mr Roger Clark who went to much trouble to help me. I am also indebted for the loan of further earlier collected material to the National Museum of Wales, where Dr Tom Sharpe searched for me, and to the Natural History Museum, where Mr Paul Jefferies found type and other material as well as specimens originally in the Museum's Department of Entomology. Finally I am grateful to Dr John Nudds , of the Manchester Museum, both for the loan of material and for his ready provision of facilities for work there.

REFERENCES AGRELL, I. 1949. The shell morphology of some Swedish Union ids as affected by ecological conditions . Arkiv for Zoologi , 41A, 1-30. ALLEN, K. C. 1977. Fossil plants from the Somerset Coalfield. In (Savage , R. J. G .; ed.) Geological excursions in the Bristol district, University of Bristol , 73-78 . BALDWIN, W. & SUTCLIFFE, W. H . 1904. Eoscorpius sparthensis, sp. nov., from the Middle Coal Measures of Lancashire. Quarterly Journal of the Geological Society of London, 60, 394-399. BOLTON, H. 1911. Faunal horizons in the Bristol Coalfield. Quarterly Journal of the Geological Society of London, 67, 316--341. BOUCOT, A . 1. 1953. Life and death assemblages amongst fossils. American Journal of Science, 251, 25-40. CRAIG, G. Y. 1953. Fossil communities and assemblages. American Journal of Science, 251, 547-548 . DIX, E . & TRUEMAN, A. E . 1929. The Zone of Anthracomya tenuis in the Somerset Coalfield . Geological Magazine, 65, 499-503. - - & - - 1931. Some non-marine lamellibranchs in the upper part of the Coal Measures . Quarterly Journal of the Geological Society of London , 87, 18G--211 . DOWN, C. G . & WARRINGTON, A, 1. 1971. The History of the Somerset Coalfield. David & Charles, Newton Abbot . EAGAR, R . M. C. 1972. Use of the pictograph. Palaeontology, 15,378-380. - - 1973. Variation in shape of shell in relation to palaeoecological station in some non-marine Bivalvia of the Coal Measures of south-east Kentucky and of Britain. 7th

International Carboniferous Congress, Compte Rendu, Krefeld, 1971,2,387-413. - - 1978. Shape and function of shell: a comparison of some living and fossil bivalve molluscs. Biological Reviews , 53, 169-210. - - 1984. Late Carboniferous - Early Perm ian non-marine bivalve faunas of northern Europe and eastern North America. 9th International Carboniferous Congress, Compte Rendu, Washington and Champaign - Urbana, 1979, 2, 559-576 . --1985. The stratigraphical and palaeoecological distribution of non-marine bivalves in the coalfields of northwest Spain. 10th International Carboniferous Congress, Compte Rendu, Madrid, 1983,2,455-476. - - 1987. The shape of the Upper Carboniferous non-marine bivalve Anthraconaia in relation to the organic carbon content of the host sediment. Transactions of the Royal Society of Edinburgh: Earth Sciences, 78,177-195. 1ARZEMBOWSKI, E . A, 1989. Writhlington Geological Nature Reserve . Proceedings of the Geologists ' Association , 100,219-234. LEITCH, D. 1940. A statistical investigation of the Anthracomyas of the basal Similis-Pulchra Zone in Scotland. Quarterly Journal of the Geological Society of London, 96, 13-37. POLLARD , 1. E. & HARDY, P. G. 1991. Trace fossils from the Westphalian D of Writhlington Nature Reserve, nr Radstock , Avon. Proceedings ofthe Geologists ' Association, 102, 169-178. THOMAS , B. A. & CLEAL, C. J. 1994. Plant fossils from

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Writhlington Nature Reserve. Proceedings ofthe Geologists' Association, 105, 15-32. TRUEMAN, A. E. & WEIR, J. 1955. A Monograph of British Carboniferous non-marine Lamellibranchia, Palaeontographical Society Monograph, Part 8, 207-242. VASEY, G. M. 1985. The nonmarine fauna of the Sydney Coalfield (Morien Group), Canada: palaeoecology and correlation. 10th International Carboniferous Congress, Compte Rendu, Madrid, 1983,2,477-490. - - & BOWES, G. E. 1985. The use of cluster analysis in the study of some non-marine bivalvia from the Westphalian D

of the Sydney Coalfield, Nova Scotia, Canada. Journal ofthe Geological Society, London, 142,397-410. WATERLOT, G. 1934. Etude de la faune continentale du terrain houiller Sarro-Lorraine. In Etudes Gftes mineraux France, Bassin houiller de la Sarre et de La Lorraine. II, Faune fossile, 9-31. WEIR, J. 1960,1967. A monograph of British Carboniferous non-marine Lamellibranchia. Palaeontographical Society Monograph, Part 10, 273-320, pis 32, 33; Part 12, 373-413, pis 42-47.

Received 7 July 1994; revised typescript accepted 15 July 1994.