The adaptation of mesozoic brachiopods to different environments

Palaeogeography, Palaeoclimatology,Palaeoecology Elsevier Publishing C o m p a n y , A m s t e r d a m - Printed in T h e N e t h e r l a n d s

THE ADAPTATION OF MESOZOIC BRACHIOPODS TO DIFFERENT ENVIRONMENTS DEREK VICTOR AGER

Department of Geology, Imperial College of Science and Technology, London (Great Britain) (Received April 27, 1965)

SUMMARY

Seven different general habitats are recognized among the Mesozoic articulate Brachiopoda. Each of these habitats is characterized by particular morphological adaptations as follows: (1) very shallow water sea-floors, usually transgressive and possibly littoral: large reinforced pedicle openings, thickened anterior margins, coarsely-ribbed rhynchonellids, sharply biplicate terebratulids; (2) sublittoral, sand-grade sea-floors without reefs: "ordinary" trilobate rhynchonellids and "unusual" forms (spinose, cynocephalous, etc.), rectimarginate or gently plicate terebratulids, indentate zeilleriids, all usually living in dense colonies; (3) seafloors in the vicinity of reefs: large, thick-shelled forms, large reinforced pedicle openings, sometimes elongate beaks, broadly plicate terebratulids, asymmetrical rhynchonellids; (4) shallow non-depositional sea-floors: very large coarsely ribbed and smooth forms, mergifer crura, opposite folds; (5) sublittoral, mud-grade sea-floors: generally smaller, thin-shelled, compressed forms, finely ribbed, falcifer or prefalcifer crura; (6) deeper (? bathyal) mud-grade sea-floors or their equivalent in very calm, shallower water: axiniform, perforate and sulcate forms, usually smooth; (7) floating weed: very small, thin-shelled rhynchonellids with falcifer crura. Several of these adaptations are repeated in unrelated groups. Functional interpretations are suggested for some, but not all, of the morphological features. A purely uniformitarian attitude is shown to be inadequate. Several taxonomic anomalies are demonstrated where genetic relationships and adaptive changes are apparently inextricably interwoven.

1NTRODUCT1ON

By applying the simple morphological modification of the addition of a tail to Peer Gynt, the Troll King both adapted him to fit his uncomfortable new environment and enabled him to produce troll-like offspring. In years of work on the Mesozoic Brachiopoda it has often proved difficult to distinguish between the Palaeogeography, Palaeoclimatol., Palaeoecol., 1 (1965) 143-172

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true trolls and Peer's progeny. The writer has become increasingly convinced that the brachiopod trolls, like Peer Gynt's, only exist in his own mind, and that when one peels off the layers (both literally and figuratively) the brachiopod taxon-like Peer Gynt's onion--turns out to have no centre. To leave this laboured metaphor, one may say that the Mesozoic brachiopods show certain repeated patterns. Over and over again, apparently unrelated stocks of brachiopods produce similar anatomical features at the same time and in the same place. Several examples of this have been discussed before (AcER, 1960; 1963, pp.49, 133-134). In some cases they have been found to persist in the same general area for very long periods of Mesozoic time. Clearly these are adaptations to similar environments or modes of life, and it would be facile simply to label them as examples of homoeomorphy. In practice, when dealing with stocks of subordinal rank or below, it seems to be almost impossible to disentangle homoeomorphy between independent stocks from persistence of one particular stock in a particular environment. Neither internal nor external characters appear to be fool-proof in this connection and the pattern of evolution may or may not cut across any existing or apparent classificatory system. This paper is therefore not concerned with the evolution of the Mesozoic Brachiopoda (which it is hoped to deal with at another time), but with the brachiopod shapes which appear repeatedly in particular inorganic and organic associations. These shapes must have names, but those names may not necessarily represent true genetically related families, genera or even species. This is essentially a paper of deductions from field observations in many countries. The observations are, for the most part, recorded in the older literature, in the author's systematic papers and in the records of hundreds of brachiopod localities in his field and museum notebooks. It is not possible to place all these observations on record in any readable form.

BRACHIOPOD HABITATS

The different supposed brachiopod habitats will now be considered in turn, though it is not implied that there is always necessarily a sharp break between them.

Very shallow transgressive sea-floors Under stable geographical conditions, the shallowest water environments are probably not normally preserved in the fossil record by reason of their very nature. Sandy beaches, for example, are essentially ephemeral phenomena which come and go with the seasons under normal circumstances. They are not areas of prolonged deposition and any slight change in sea level is likely to result in their imme-

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diate destruction. The only situation under which they may be preserved is that of a rapid marine transgression. This is best seen in the Mesozoic in the classic Cenomanian transgression which is observed all over the world, albeit of slightly varying ages in different areas. It is in this stage that the best examples are seen of brachiopods adapted to very shallow water environments, though it is not implied that there is any sharp distinction from the sublittoral environments which follow. This may be termed the littoral environment, but it must be understood that this term, like the others which follow, must be read as though written in inverted commas. The littoral environment is characteristically one of very high energy levels and coarse detrital sediments. Typical deposits are coarse, bioclastic calcarenites and quartz sands, with pebbles or even boulders. The most noteworthy brachiopod of this environment in the Cenomanian is the rhynchonellid Cyclothyris (sensu stricto). The present author observed in an earlier paper (AGER, 1964, p. 112) that frequent attempts through the years to obtain satisfactory serial sections of this genus had failed because of the invariably coarse nature of the infilling sediment. This was equally true, for example, of specimens from England, Germany, Poland and Texas, so could not be regarded as mere coincidence. It was postulated that the true Cyclothyris was adapted to life on such a substratum, and that certain features of its shell anatomy supported this hypothesis. Notable is its large hypothyrid foramen with a pronounced rim, which presumably helt and supported a thick pedicle to enable it to hang on in the rough, shallow water of its habitat (Fig.lB). Such rims are taken to an extreme in the related form Cretirhynchia retracta (ROEMER) described by STEINICH (1963) from East Germany, and are well seen in the British record in the abundant Cyclothyris (sp. or spp. according to taste) from the Cenomanian of Wilmington in south Devon. In Bohemia, Miss Nekvasilowl has found such cyclothyrids abundantly, but only in the most shallow water facies of the Cenomanian, i.e., the "surf area"

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Fig.1. Typical brachiopods of very shallow transgressive sea-floors. A. Sellithyris sella (J DE C. SOWERBY), Aptian. B. Cyclothyris difformis (LAMARCK),Cenomanian. C. Lobothyris edwardsii (])AVIDSON), Pliensbachian.

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(oblast p~ibojov~i) of DVORAK (1958, pl.1). Here they most characteristically occur in pockets or hollows right at the transgressive base of the Upper Cretaceous, where it rests, with spectacular unconformity, on the Pre-Cambrian rocks of the Bohemian Massif. The pockets often contain great boulders of the ancient rocks beneath, and a varied fauna including oysters, shark's teeth, thecideans, and such notable shallow-water elements as thick-shelled gastropods, limpets, barnacles, boring lamellibranchs and calcareous Algae. Soft-bottom elements such as burrowing lamellibranchs and irregular echinoids are notably absent. The author has suggested, in dealing with the Texan localities (loc. cit.), that Cyclothyris s.str, may in fact be nothing more than "the name which has been given to those variants of the cyclothyrid plexus which happen to have been living in such environments". The distinction from such other related genera as Lamellaerhynchia may be nothing more than an ecological one. It may, in other words, be a phenotype rather than a genotype. In the Texas example, Cyclothyris is associated with coral reefs and with other forms which are discussed below as characteristic of a perireefal environment. Clearly it is undesirable to be too dogmatic about precise habitats, and different species of the same genus (whatever that term means) are likely to range through various shallow water environments. With Cyclothyris in the Bohemian pockets mentioned above (though not equally abundant at any one place) is the very variable and sharply biplicate "Terebratula" phaseolina LAMARCK. This brings to mind the observation that such sharply biplicate terebratulids, whatever their genus and classificatory position, appear frequently in what seem to have been the most shallow water deposits. Obvious examples are the abundant Sellithyris sella (J. DE C. SOWERBY)in the transgressive Aptian Hythe Beds of southern England, (Fig.lA) and the even more abundant Stiphrothyris tumida (DAvIDSON) and Goniothyris phillipsi (MORRIS) of the transgressive Upper Bajocian (Parkinsoni zone) of western England. It may be significant that biplicate terebratulids appear to be virtually absent in Lower Jurassic all over the world, and that--at the same time--brachiopods are virtually absent from Lower Jurassic littoral environments. They do not occur, for instance, in the littoral Lower Lias of south Wales, nor in the sandy facies of the Toarcian and Aalenian. The writer has noted a marked falling off in the numbers of brachiopods in the Upper Pliensbachian Marlstone Rock-Bed of the Cotswolds, as it passes into what he regards as a marginal facies between Stroud and Cheltenham (AGER, 1956b). That this is not just a matter of preservation in a sandy facies is shown by the fact that brachiopods are still present at the very top and bottom of this formation, and other shelly fossils are present throughout. Such cyclothyrid rhynchonellids (sensu AGER, 1956b) as do occur in the Lower Jurassic, are for the most part rare and local in distribution. The most obvious example is Furcirhynchia, with its large hypothyrid foramen. Probably the

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best locality for this genus in Britain is in the pockets of transgressive Sinemurian at Vobster Quarry in Somerset. The most prolific occurrence of the genus known to the author anywhere in the world is in the Peace River area of northern British Columbia (AGER and WESTERMANN, 1963). Its unusual abundance here was specifically noted, and its discovery drew attention to a previously unrecorded transgressive unconformity immediately below, between Sinemurian and Norian deposits. It was noted, however, that Furcirhynchia tends to occur in fine-grained, muddy sediments. It may be that brachiopods, after a traumatic experience at the end of the Triassic, did not become widely adapted again to really shallow water habitats until Middle Jurassic times, and perhaps only prospered there in the Upper Cretaceous. Even so, they could probably never live actually in the depositional parts of the intertidal zone (which is normally a habitat only of burrowers) but clung on, perhaps, around the low tide mark and in rock pools, like certain living brachiopods. There are also slight indications of some degree of adaptability to lower salinities among the Mesozoic brachiopods, at least among the rhynchonellids which show a greater degree of tolerance than other groups. This is best seen in the Bathonian Great Deltaic Series of the English Midlands, where rhynchonellids-Kallirhynchia concinna (J. SOWERBY)--persist farther than any other brachiopods in the transition from fully marine to fully non-marine deposits. HUDSON (1963) demonstrated this very clearly in equivalent deposits in the island of Skye. Nevertheless, all the evidence from the Mesozoic shows that the brachiopods of that era were, like those of the present day, stenohaline and restricted to seas of normal chemical conditions. The almost complete absence of brachiopods from the Swabian facies of the Rhaetian, with the exception of rare inarticulates such as Orbieuloidea townshendi (DAVIDSON), is presumably significant. One may also note the world-wide extreme scarcity of brachiopods in the Lower Triassic deposits, which may have been related to salinity. They continued in a minor role in the Middle Triassic also, and only recovered some of their lost Palaeozoic glory in Upper Triassic times. The connections between early Mesozoic and late Palaeozoic brachiopods are very obvious, in spite of this break, and in spite of the obscurantism of taxonomy and classification. One is tempted to postulate that some of the direct lineages, such as the Wellerellidae and the Camarotoechiidae-Rhynchonellidae among the Rhynchonellida, survived in deeper water basins which are not recorded in the continental stratigraphical record. The only evidence at present available is the apparent scarcity of brachiopods in the shallower water deposits of early Mesozoic times and their later colonization of such areas. It may be significant that most of the supposedly littoral forms do not usually have the very thick valve walls of the perireefal forms discussed below. This may indicate that they were not adapted to the full stresses of the inter-tidal habitat, and lends support to the idea that they lived in semi-protected rock pools. Certain

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terebratulids, however, in very shallow facies, do develop shells which are unusually thick in overall dimensions. That is to say, they show a concentration of growth lines around the anterior margin, due to continuance of growth in thickness after cessation of growth in length. In certain cases this is an example of environment-induced variation within a species, though this is frequently concealed in the taxonomy. Thus Lobothyris edwardsii (DAvIDSON) of the Pliensbachian, which is characteristically flattened around the commissure (Fig. 1C) may well be only a local variant of the ubiquitous L. punctata (J. SOW~RBY),from which it is often only distinguished with difficulty. The best locality for L. edwardsii in England, for example, is the old Redlands Pit near Hook Norton in Oxfordshire, and it seems probable to the writer that it was a local development there in the presumed shallows along the Moreton-inMarsh axis. A remarkable parallel to this was observed by the author at Glozhen6 in central Bulgaria, where L. edwardsii is again found (here called Terebratula lakatnikensis TOULA) as a local development in place of the usual L. punctata. This again is an area of known shallowing, with overlap by the Pliensbachian, and much reworking of the sediment. Comparable examples higher up the column are Rugitela waltoni (DAVIDSON) in the transgressive Upper Bajocian and certain "species" or "varieties" of the genera Sellithyris and Cyrtothyris, for example C. dallasi (WALKER),in the transgressive Aptian. All these examples may be compared with the phenomena observed by DuBoIs (1916) in the Recent terebratulid Terebratalia obsoleta, which he blamed on the effects of wave battering on the mantle margins.

Sublittoral sand-grade sea-floors Sublittoral, non-reef, sand-grade depositional environments probably provided the bulk of the brachiopod-bearing Mesozoic sediments with which one is acquainted in Britain, and generally throughout Europe outside the Alpine and Carpathian chains. The characteristic sediments are bioclastic calcarenites, quartz sands and oolites, often ferruginous, with abundant epibiontic fossils which have been more or less transported. Typical of such sediments in the British succession are the Marlstone Rock Bed of the Upper Pliensbachian and most of the Inferior Oolite, Great Oolite and Cornbrash limestones. The characteristic brachiopods of these sediments are the "ordinarylooking" rhynchonellids and terebratulids (and spiriferinids up to the Toarcian) as recognized by every geologist. The rhynchonellids are usually strongly costate with more or less well-developed folds. Especially characteristic are members of the Tetrarhynchiinae (AG~R, 1965b) such as Tetrarhynchia itself (Fig.2B), Goniorhynchia, Burmirhynchia and several others. Also typical of this general habitat are those rhynchonellids with a more "unusual" external form such as the spinose Acanthothiris (Fig.2C), those with an antidichotomous type of ornament (at first

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Fig.2. Typical brachiopods of sublittoral sand-grade sea-floors. A. Rhynchonella loxiae FISCHER DE WALDHEIM, Volgian. B. Tetrarhynchiatetrahedra(J. SOWERBY),Pliensbachian. C. Acanthothiris spinosa (SCHLOTHEIM), Bathonian. D. Zeilleria quadrifida (LAMARCK),Pliensbachian. E. Lobothyris buckmanii (DAvIDSON), Bajocian. capillate, then costate) such as Rimirhynchia and those with a very sharp plication or "cynocephalous" form such as Homoeorhynchia. Such distinctive characters may be repeated later in the same general environment. Thus the ornament of Rimirhynchia in the Sinemurian is repeated in Cyclothyris antidichotoma (BuvIGNIER) in the Cenomanian. Similarly, the extreme plication of Homoeorhynchia in the Pliensbachian is seen again in Rhynchonella s.s. (Fig. 2A) in the Volgian. Whether or not these genera are directly related is a matter of doubt, but the author thinks they are. The terebratuloids of this environment do not seem to be so distinctive, though it does seem to have been the favourite habitat of the long-looped forms (zeilleriids, ornithellids, etc.), and among these, indentate forms, such as Zeilleria quadrifida (LAMARCK), (Fig.2D) appear to be particularly characteristic. Among the short-looped forms (family Terebratulidae), smooth rectimarginate or slightly plicate species such as Lobothyris buckmannii (Fig.2E) were in the majority. All these brachiopods tended to be of average size compared with the giants of perireefal and rocky sea-shore environments. They also all tended to have comparatively small beaks and pedicle openings, without rims. Inevitably in shallow water environments, much transport of shells must have occurred, and this explains the existence of many brachiopod-rich bands of limited thickness in well-bedded sediment. However, it is thought that the importance of post-mortem transport is usually much exaggerated, and there is little evidence to suggest that the majority of brachiopods has been carried far from where they actually lived, and certainly not out of their general environmental setting. Thus in the interesting study by MIDDLEMISS(1962) of the transport of brachiopod shells after death in the Aptian Bargate Beds of Surrey, they pass from whole joined valves to completely comminuted material within a distance of about 3 km, and in the

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field they are found to be extremely rare within a very short distance of their postulated living place. It is thought that usually the brachiopod shells were little more than scattered and swept to and fro a matter of a few metres or scores of metres on the seafloor. ELL1OTT (1956) has drawn attention to the possible transport of whole brachiopod shells without damage by gentle currents, but the writer agrees with Middlemiss that the field evidence in many cases suggests that the position of life was at least in the immediate vicinity of the present-day deposits. Characteristically the brachiopods of this habitat appear to have lived in close-packed colonies, which are seen in the rocks as "nests" or lenses. In almost every example seen by the author, each nest consists essentially of only one species, in all growth stages and with no signs of mechanical wear. For example, Tetrarhynchia tetrahedra (J. SOWERBY) usually occurs in this way in the British Upper Pliensbachian, and can be seen in the same gregarious associations right across Europe at least as far as Teteven in Bulgaria, where the colonies may perhaps be referrable to the closely related species T. argotinensis (RADOVANOVIC).Such nests have been studied statistically by HALLA~ (1962) who has satisfied himself as to their being "life associations", though generally speaking it is sufficiently evident without the need for the vast labour of quantitative study. Alternatively, the same species may be found spread out along a bedding plane in what is known in the English Midlands as a "jack". In this case the original colonies have clearly been scattered by bottom currents. A familiar British example of a very similar form which has been somewhat transported, is the fantastically abundant Goniorhynchia of the boueti Bed in the Bathonian of Herbury on the Dorset coast. Here, the mode of preservation and occurrence, the admixture with other species, the nature of the epifauna and the absence of juveniles are all simple field observations which make quantitative studies even more unrewarding and unnecessary except to demonstrate the obvious point that only one species is present. Terebratulids occur in the sediments of this environment in exactly the same way. Thus Lobothyrispunctata (J. SOWERBY)is well-known to every British student as discrete and separate "nests" in the same formation as Tetrarhynchia tetrahedra. These two occur right across Europe outside the Alps and Carpathians, as does the earlier zeilleriid Cincta numismalis (LAMARCK). In the Cretaceous, MIDDLEMISS (1962) described similar "nests" of terebratulids in the Aptian of the Isle of Wight, where in fact (as in the Pliensbachian) there are discrete and separate nests of rhynchonellids. On the whole, however, rhynchonellids seem to occur more often in closely packed colonies than do the terebratulids and terebratellids. This seems to be true, for example, in the Bathonian and Callovian of southeastern France. Thus in the southern Jura, Acanthothiris is found abundantly in nests in Bathonian-Callovian calcarenites (ACER and EVAMY, 1964, p.333) whereas the associated terebratuloids

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tend to be more scattered (AGER, 1963, p.302). The same seems to be true for Burmirhynchia hopkinsi (DAvIDSON) and its associated Lower Callovian terebratuloids on the C6te d'Or near Dijon. It must not be thought, however, that all rhynchonellids occur in this way. Some genera appear never to occur in nests, but always as scattered individuals. Sphenorhynchia of the Middle Jurassic, for example, and Austrirhynchia of the Rhaetian seem to be of this kind. In some cases the mode of occurrence varies even between the species of a single genus. Thus most species of Gibbirhynchia occur in nests, but G. curvifrons (QuENSTEDT) and G. tiltonensis AGER are only found scattered (AGER,1954, 1962b, p.108). This cannot always be a matter of postmortem transport, and it may be significant that they are the first and last members of the genus respectively. So far as brachiopods are concerned, the writer is a great believer in the concept that the maximum size attained by a particular species is a guide to its optimum conditions for life, with the obvious proviso about the possibility of mechanical sorting. Many examples of this are seen in the general environment under consideration. Thus in the British Upper Pliensbachian (AGER, 1956a, c) many species consistently attain a larger size at the southern end of the outcrop, for example the rhynchonellid Homoeorhynchia acuta and the terebratulid Lobothyris punctata (both referred to earlier). The latter illustrates the conflicting problems of taxonomy and ecological adaptation referred to in the introduction, since large variants of this species have invariably been called L. subpunctata (DAVlDSON) all over Europe. What is more, such ecological transitions may even cut across the supposed generic limits. Quadratirhynchia, for example, which is particularly abundant in southwest England, may well be only the Tetrarhynchia of elsewhere attaining its maximum potentialities under optimum conditions, even though there are other more subtle differences besides their average dimensions. Certainly as one leaves this general environment, species can often be seen to diminish abruptly in abundance and in maximum size. Thus the Sinemurian and Pliensbachian brachiopod faunas of southern England can be traced right across France to the Jura without much change, but when these stages are followed into the Alps, the brachiopods virtually disappear, at least in the literature! (AGER, 1961). In the very rare localities where they have been found by the author, for example near Bourg d'Oisans in Is~re, they are extremely uncommon, and only attain a fraction of their normal adult size.

Perireefal Reefs and their associated deposits are a special facies of the sublittoral environment. ARKELL (1933, p.567) long ago noted the antipathetic nature of autochthonous corals and ammonites in the British Jurassic. The same appears to be true of corals and articulate brachiopods all through the Mesozoic record. The

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dullest part of the British Jurassic, for example, (in the prejudiced eyes of a brachiopodologist) is the Corallian facies of the Upper Oxfordian, where brachiopods are almost completely absent (AGER, 1959b, p.28). Without being unnecessarily pedantic about what we mean by a "reef", it may be said quite dogmatically that indigenous brachiopods are not normally found with in situ compound corals. At the present day, only one brachiopod (Frenulina) is ever found in this association and ELLIOTT(1950) suggested that this may be due to the eating of the brachiopod larvae by the voracious coral polyps. COOPER (1964) did recently describe two brachiopods --Argyrotheca and Thecidellina--from the boreholes at Eniwetok and Bikini, but these were small and poorly preserved and did not appear to occur in the true reef environment. The absence of brachiopods from Mesozoic reefs may be readily noted in the field, and deduced from the literature, though it has rarely been put on record. YAKOVLEV(1952, p.282) did comment on the general absence of brachiopods from Jurassic reefs (i.e., coral reefs), though he did say that they occur in algal reefs such as those of the Austrian Trias. Nevertheless, there is a very distinctive group of brachiopods which is frequently found associated with coral reefs, though not in the reefs themselves. This was first noted by R6zYcI(I (1948), who observed that certain asymmetric rhynchonellids in the Polish Upper Jurassic always occur just above reef horizons. In the French Jura, similar rhynchonellids and distinctive terebratulids are found in Kimmeridgian limestones immediately below reefs (AGER and EVAMY, 1964, p.337). Lower down the column in the same area (op. cit., p.332) a related rhynchonellid is found in the detritus of Bathonian reefs, but not with the compound corals. These observations appear to hold good all over Europe, providing one makes sure that what have been called "reef limestones" really d o contain compound corals in position of life, and providing one always makes sure of the precise distribution of the two groups within a particular formation. If the Mesozoic is stretched for a moment to include the Danian, there is a very interesting example of the relationship of brachiopods to reefs at the type locality for that stage in Fakse quarry near Copenhagen. The quarry shows a complex bioherm of corals and bryozoans and the whole succession is dominated by the delicately branching coral Dendrophyllia candelabra. Brachiopods occur fairly commonly (both rhynchonellids and terebratuloids) but are invariably stunted. Only in one thin band between the reef layers (unfortunately no longer exposed) do the brachiopods reach their normal adult dimensions. The precise inter-relationships of the different fossils here have not yet been studied, but even the occurrence of stunted brachiopods in reefs appears to be unusual, and may only have been effected very late in Mesozoic times. Generally speaking, in the Cretaceous as in the Jurassic, the best development ofbrachiopods is found in the detrital sediments in front of reefs. This seems to be true, for example, of the "Urgonian" reef facies of mid Cretaceous age,

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Fig.3. Typical brachiopods of perireefal environments (all examples from the Tithonian Stramberk Limestone). A. "Rhynchonella" paehytheca ZEUSCnNER.B. Septaliphoria astieriana (D'ORBIGNV). C. Loboidothyris (?) insignis (ScHi)BLER). D. "Terebratula" moravica GLOCKER. E. "Terebratula" immanis ZEUSCHNER.F. Aulacothyris (?) hoheneggeri (SuEsS).

which is found right across Europe. The same relationship may well have occurred also in Palaeozoic reefs. One of the outstanding characteristics of what are therefore called "perireefal" brachiopods, is their large size, a feature symptomatic of the optimal conditions for shell growth present in this habitat. Once more the necessity for the brachiopods to hold on in rough water is clearly apparent in the massive beaks and large, reinforced pedicle openings in both rhynchonellids and terebratuloids. In some cases the beaks of the latter are excessively elongated, as in the terebratulid "Terebratula" moravica GLOCKER and the terebratellid Aulacothyris (?) hoheneggeri (SUESS) of the Stramberk Limestone (Fig.3D, F), though this feature is also seen in other shallow water habitats. The most distinctive feature of the perireefal brachiopods is the presence of coarsely-ribbed, asymmetrical rhynchonellids, of which Septaliphoria is the outstanding example (Fig.3B). Different species of this genus occur in perireefal habitats all through the Upper Jurassic, though there may well be homoeomorphy Palaeogeography, Palaeoclimatol., Palaeoecol., 1 (1965) 143-172

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by other genera. The distinctive character of the displaced anterior commissure (either left-hand or right-hand) is difficult to explain physiologically if one accepts the standard theory of the trilobed shell being related to two inhalant and one exhalant water current. One can only assume that in the special conditions of the reef biome, one inhalant system may have atrophied, and it may not be too farfetched an analogy to draw a parallel with the giant clam, Tridacna, of modern coral reefs, in which the anterior end of the shell has been foreshortened to an extreme degree, and the body has rotated through 180 o. Many of the cyclothyrids of the Cretaceous (which probably descended from Septaliphoria) are also asymmetric, and some of them still lived in the vicinity of reefs (as in Texas), but most seem to have moved out into related habitats, and some species show a puzzling variation between asymmetric and regularly trilobed shells. The best known asymmetric rhynchonellid in the British Mesozoic is the large "Rhynchonella" inconstans (J. SOWERBY)of the basal Kimmeridgian, which is commonly (though probably wrongly) referred to the related genus Rhactorhynchia. At first sight this does not seem to belong to a reef environment, until it is remembered that its best known locality (and the source of most of the specimens seen in collections) is just a few centimetres above the Ringstead Coral Bed in Ringstead Bay on the Dorset coast. In standard texts the brachiopod rich bed and the coral bed are kept strictly apart, because the latter is regarded as the top layer of the "Corallian", but in the field they are seen to be closely associated both in sedimentation and fauna. The Ringstead Coral Bed clearly represents the detritus from reefs which cannot have been far distant. ARKELL (1933, p.379) suggested that the actual reefs lay under the Chalk downs close by to the east, and once more the large asymmetric rhynchonellids came into the succession very shortly after. The absence of juveniles at this particular locality, the tendency of the valves to gape and the post-mortem nature of the epifauna all suggest that the brachiopods also have been transported. Apart from their similarly large size, the perireefal terebratulids, on the other hand, are fairly "normal "in appearance, though there does seem to be a tendency to produce either rectimarginate forms or those with strong, broad (not sharp) plications (Fig.3C). These perireefal brachiopod faunas undoubtedly reached their acme around the extensive Upper Jurassic coral reefs of Europe. Probably the finest examples are those of the "Tithonian" Stramberk Limestone of Czechoslovakia, which formed the starting point of this paper. The brachiopods in this deposit reach really remarkable sizes, such as the huge "Terebratula" immanis ZEUSCHNER (Fig.3E). Some of the elements here are almost endemic (allegedly about a third of the 600 or so species recorded), for example, the thick-shelled, Palaeozoiclooking "Rhynchonella" pachytheca ZEusCnNER (Fig.3A), but most are identical with, or close to, species which are found in similar habitats elsewhere in the Upper Jurassic. Thus Septaliphoria astieriana (D'ORBIGNY), (Fig.3B), is found in a variety

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of forms at many different levels and localities, whereas Loboidothyris (?) insignis (ScHi3BLER), (Fig.3C), is known in a fairly consistent form but under many different names right across Europe. A good example of a very comparable fauna to that of Stramberk was described by TSAN-HSUNYIN (1931) from the Tithonian of Gard and H6rault in the south of France, though (as with the Polish and German localities discussed below) this French fauna lacked the anomalous species which make the Stramberk association so particularly interesting. These anomalies (which are essentially ecological anomalies) are contained in the older monographs and faunal lists relating to this locality. Noteworthy was the apparent association with this epitome of a perireefal fauna of forms such as Pygope s.1. and Kolhidaella (?) hoheneggeri (SuEss) which are here regarded as characteristic of quite different environments. The solution lies in the disentangling during recent years, by a number of different Czech geologists and palaeontologists, of the diverse limestones and marls which had formerly been associated under the name "Stramberk Limestone" and which, in fact, belong not only to different environments but also to different stages. The perireefal fauna at Stramberk (as illustrated in Fig.3) now seems to belong strictly to the massive white "true" Stramberk Limestone, which is found not only in Czechoslovakia, but also as huge exotic blocks in southern Poland for example at Woinik (KsI,~KIEWICZ, 1963) and at Kelheim in the Franconian Alb of western Germany. In both cases there is ammonite evidence to suggest an earlier age (i.e., Kimmeridgian and Lower Tithonian compared with Upper Tithonian for Stramberk), so the brachiopod facies was clearly persistent. There is disagreement among Czech geologists about the reef nature of the true Stramberk Limestone. Certainly it contains large compound corals, but the nature of the workings makes it extremely difficult to prove whether or not these are in their position of life. GEYER (1955) expressed the opinion that they were not; ELtA~ov~ (1962) thinks t h a t - - at least locally--they are. Even if they are, however, there is no evidence of a direct association with the brachiopods, and clearly the bulk of the limestone is composed of reef detritus. With reefs one naturally expects to find backreef or lagoonal sediments, but no brachiopods appear to have become adapted to this more selective habitat. For example, the well-developed lagoonal deposits behind the reefs in the French Jura (ACER and EVAMY, 1964) do not contain brachiopods. The only faint suggestion this way comes from a comparison by KSI6~KIEWICZ(1963, p.269) of the Stramberk type faunas at Wo2nik with those at Inwald (also in southern Poland) described by ZEUSCHNER (1859). The latter fauna includes abundant nerineid gastropods which seem to be characteristic of Jurassic lagoonal facies, and at the same time shows differences in the brachiopod fauna, notably the presence of many "Terebratula" moravica (Fig.3D) and other forms which have long delicate beaks and may have needed the greater protection of the back of the reef.

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It is possible that such forms lived with their greatly elongated beaks buried in soft sediment, as Pinna lives today, and in the manner suggested by OPII~ (1934) for the Ordovician orthoid Clitambonites. An even closer Palaeozoic parallel is the Silurian triplesiid Onychotreta ULRICHand COOPER.At Stramberk, the comparable terebratellids Terebrirostra neocomiensis (D'ORBIGNY) and T. angustidens (REMEg) only occur (and very rarely) in the quieter, deeper water sediments of Lower Cretaceous age (Miss Nekvasilov~i, personal communication, 1964). In Britain, the same genus, with a fantastically exaggerated beak, is known by the one species T. lyra (J. SOWERBY)in the relatively deeper water faunas of the Albian and Cenomanian. Again it is extremely rare. Reference has already been made to the small rhynchonellid Kolhidaella (?) hoheneggeri as not being regarded as an expected member of a perireefal fauna. This species is usually referred to Lacunosella (of which Moiseev's genus Kolhidaella is certainly a direct descendent), and rhynchonellids of this group are normally associated with muddy and usually deeper sea-floors which will be considered later. There is, however, another much larger Lacunosella of the L. lacunosa group, which does definitely occur in the Stramberk "reef" limestone, and is in fact probably the most common rhynchonellid there. This is mentioned to draw attention to an interesting idea put forward by R6zYcKI (1948) that in the Upper Jurassic of Poland, Lacunosella periodically took the place of Septaliphoria in the supra-reef habitat. He further suggested that when this happened, Lacunosella (which is easily recognizable by its falcifer crura and branching costae) took on the asymmetric external form of Septaliphoria. L. cracoviensis (QuENSTEDT) is an example of a Lacunosella of this type. If R6zycki's theory is true, then it is a really remarkable example of an evolutionary take-over of a particular ecological niche. There are some hints that it may have happened in the French Jura, and this possibility is being investigated at the moment by Mr. Alan Childs. At Stramberk, L. lacunosa (SCHLOTHEIM)is certainly the dominant rhynchonellid of the perireefal association, and the accompanying Septaliphoria is relatively small and less abundant. This contrasts with the usual situation in the Oxfordian and Kimmeridgian reefs, where Septalipttoria is certainly the dominant form. This Lacunosella, however, is not an asymmetric form, and it remains to be seen to what extent the external characters, and to what extent the internal ones are of genotypic rather than phenotypic significance.

Sublittoral non-depositional sea-floors Rocky sea-floors are probably among the most densely populated parts of the earth's surface in terms of numbers of animals present per unit area, but they are probably the least likely habitat to be preserved in the fossil record, since they are places where sediment normally never accumulates. For this reason they have usually been overlooked by palaeontologists as possible fossil habitats.

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To a certain extent this habitat has already been discussed under supposedly littoral environments, but there are certain large, thick-shelled brachiopods in the Mesozoic which occur in anomalous ways and which seem to require a special explanation. Probably the best documented of these are the two largest brachiopods in the British Mesozoic, "Rhynchonella" sutherlandi (DAvIDSON) and "Terebratula'" joassi (DAvIDSON) from Kimmeridgian boulder beds near Helmsdale in Sutherland on the northeast coast of Scotland. The mode of origin of these remarkable deposits was discussed in detail by BAILEYand WEIR (1932) with the conclusion that they owe their origin to repeated movements of a submarine fault scarp just offshore from an Old Red Sandstone landmass. With each movement of the fault, blocks of the sandstone fell into deeper water Kimmeridgian shales and were accompanied by tsunami sweepings of organic material from the rocky shelf above. The brachiopods are accompanied by much broken shell material, wood and occasional small colonial corals. Support for the earthquake theory is provided by the presence of neptunean dykes. The two brachiopods concerned do not appear to be related to any other known British species, though the rhynchonellid (which has a large pedicle opening and very coarse ribs) may belong to Russirhynchia, which is only otherwise known from the Soviet Union. BUCKMAN (1918) referred the terebratulid very doubtfully to his Lower Jurassic genus Lobothyris, but this is not thought to be likely. It is suggested that they may belong to stocks which had for long previously been adapted to shallow water rocky environments, and it was only in the special circumstances of the Helmsdale deposit that they were preserved for the fossil record. A second example may be the large Middle and Upper Triassic rhynchonellids Halorella (which is coarse ribbed) and Halorelloidea (which is smooth). Both are characterized by wide shells with opposite folds and large pedicle openings (Fig.4A, B). These two genera have a very curious distribution, in the Alps, the Pamirs, Oregon and one island in Indonesia (AGER, 1965a). What is more, this

3 cm

Fig.4. Typical brachiopods of sublittoral non-depositional sea-floors. A. Halorelloidea rectifrons (BIxTNER), Norian. B. Halorella arnphitoma (BRoNN), Norian. C. Peregrinella multicarinata (LAMAaCK), Hauterivian.

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very distinctive group has no apparent ancestors or descendents. Detailed information about their mode of occurrence awaits the much-needed revision of the Upper Triassic brachiopod faunas, though the author's own experience has been that they are extremely localized and difficult to find. One very interesting piece of information about them was, however, furnished by Professor A. G. Fischer (personal communication, 1963). During his work on the Upper Trias near Salzburg in the Austrian Tyrol, Prof. Fischer found that Halorella and Halorelloidea only occur in neptunean (clastic) dykes below an erosion level. What is more, the two genera seem to be mutually exclusive, so that one dyke contains the ribbed Halorella while another has the smooth Halorelloidea. This would suggest that they were swept into the fissures from local patches or colonies on the rocky sea-floor. Not unlike Halorella superficially, and probably distantly related to it, is the early Cretaceous rhynchoneUid Peregrinella (Fig.4C). This has the most interesting story of all. It is the largest of all the Mesozoic rhynchonellids; it has a large pedicle opening and is coarse ribbed; it also has very distinctive internal characters (notably mergifer crura) which cannot be mistaken for those of any other genus. Its remarkable distribution was discussed briefly by BIERNAT (1957) and hasbeen further considered by the present author (AGER, 1965a). It has a very discontinuous distribution, being known only from isolated localities in the French Alps, Switzerland, Czechoslovakia, Poland, Roumania, the southern U.S.S.R. and California. The best known locality is at Chatillon-en-Diois in the French Alps, though the brachiopod can no longer be collected in the area. The author was informed by Professor M. Gottis (personal communication, 1964) that it only occurred in loose calcareous blocks in a deeper water deposit. This is also suggested by museum collections, and appears to hold tor the other French locality at Bois de la Valette north of Montpellier. Similarly, in Poland, the specimens described by Mrs. Biernat all apparently came from a single calcareous boulder in an argillaceous succession (Miss J. Burtan, personal communication, 1963). In the Soviet Union, Peregrinella is only known in the northwest Caucasus, near Novorossysk, where it is found abundantly in limestone blocks within a general clay sequence containing aptychi (the late Dr. M. E. Eristavi, personal communication, 1959). All the available evidence therefore seems to indicate that Peregrinella was always carried down into deeper water deposits from a shallow habitat. It had long been thought by the writer that Peregrinella belonged to an ancient separate stock of rhynchonellids, because of the resemblance of its internal structures to those of Palaeozoic genera such as the Silurian Plagiorhyncha (HAvLI~EK, 1961). This recently received remarkable confirmation when the author was able to section a specimen from the Devonian of Morocco which H. and G. Termier had referred to Halorella. This proved to have a perfect Peregrinella type internal struc-

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ture, and has been placed in a new genus Eoperegrinella (AGIER, 1965a). A link is provided by the Rhaetian to Lower Jurassic genus Carapezzia. The species "Rhynchonellina" geyeri, which seems to belong here, was described by B1TTNER (1898) with what are probably the earliest serial sections of a Mesozoic brachiopod. These sections show quite clearly the mergifer type crura which are so characteristic of Peregrinella, and Mr. D. A. B. Pearson informs the author that its mode of occurrence in the Rhaetian of Oberdrauberg in southern Austria is very similar to that described above for Peregrinella in the Lower Cretaceous. It is therefore suggested the Peregrinellinae is another example (and perhaps the best) of a very old lineage which has escaped the fossil record by reason of its adaptation to life on a shallow sea-floor where sediment did not accumulate. This is suggested by the spasmodic occurrence of members of the subfamily both stratigraphically and geographically. It cannot be said that no deposition occurs in their natural habitat, for clearly the specimens are always enclosed in a calcareous matrix, but it is suggested that this sediment was only seasonally deposited and did not continue to accumulate in the current swept rocky shallows of a tectonically unstable coast-line.

Sublittoral muddy sea-floors It is tempting to refer to this environment as the outer sublittoral or circalittoral zone (sensu HEDGPETH, 1957; AGIER, 1963), that is to say, to regard it as a deeper water facies than all those so far discussed. It is so shown on Fig.7, and as a generalization it is reasonable to postulate that the majority of Mesozoic mud-grade sediments (clays, shales, calcilutites) were laid down in deeper water than the sandgrade sediments. Nevertheless, this was not necessarily always so, and the grain size of the substratum is regarded as a more important criterion than the actual depth. The essential point about these sea-floors from a brachiopod's point of view is that they were soft and unconsolidated, and did not provide a firm substratum for pedicle attachment. Faunistically they are characterized by pelagic organisms such as ammonites and belemnites, and by endobenthos such as burrowing lamellibranchs. Brachiopods and other large epibenthos are typically absent except for occasional large oysters which simply lay in the mud. Only one brachiopod is known to be able to live directly attached to such soft bottoms at the present day. This is the terebratellid Chlidonophora (closely related to Terebratulina) which does so by means of a finely branching pedicle, which acts as an anchor. It was recently refigured by MUIR-WOOD (1959) attached to Globigerina ooze from the floor of the Indian Ocean. It may be that certain Mesozoic brachiopods were similarly adapted, but the evidence is hardly likely to be forthcoming. RUDWICK (1961) drew attention to several living brachiopods (all, it may be

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3 CITI

,

Fig.5. Typical brachiopods of sublittoral muddy sea-floors. A. Kolhidaella (?) hoheneggeri (SuEss), Valanginian. B. Terebratulina striata (WArlLENBERG),Campanian. C. Acanthorhynchia spinulosa (OPPEL),Kirnmeridgian. All natural size. noted, terebratellids) which attach themselves to the stems of Algae or other unfossilizable organisms on soft substrata. He suggested that such attachment might explain certain fossil brachiopod occurrences in Mesozoic rocks. MIDDLEMISS (1962) suggested attachment to sponges as the mode of life of certain brachiopods in the English Aptian, and R. E. H. Reid (personal communication, 1962) made the same suggestion for Orbirhynchia in the English Upper Cretaceous. Good confirmation was provided in the southern French Jura (AGER, and EVAMY, 1964, p.334) where specimens of Lacunosella visulica (OPPEL) occur commonly in Upper Oxfordian shales associated with hexactinellid sponges. Mr. Alan Childs has now confirmed this association around the arc of the Jura as far as the Franconian Alb, in western Germany, and the writer found a comparable association at Stramberk in north Moravia. At the latter locality Kolhidaella (?) hoheneggeri (Fig.5A), which is certainly a direct descendent of Lacunosella, occurs not in the white Stramberk Limestone, but in the red Kop~ivnice Limestone which is much finer grained. This red limestone is now known to be later in age (? Valanginian) and was certainly a soft-bottom deposit of a deeper sea. Microcrinoids (Torynocrinus sp.) are also very common in this limestone and may have formed an alternative means of attachment. Acanthorhynchia is a (rare) Mesozoic brachiopod which is always associated with soft bottom sediments. This will be discussed again later, but it is interesting to note here an association of A. spinulosa (Fig.5C) with crinoids in Lower Kimmeridgian calcilutites in the French Jura (AGER et al., 1964, p.490). Terebratulina is another, and comparatively common, brachiopod of sediments which must have been soft muds when first deposited (Fig.5B). It occurs in many Cretaceous clays and marls, and is particularly characteristic of parts of the Chalk. At the present day, Terebratulina is known to live on muddy sea-floors attached to other organisms, for example ascidians (EKMAN, 1896), Algae (in GRAY, 1872) and lamellibranchs (RoWELL, 1960). This was presumably the mode of life of most of the small Mesozoic members of the genus. However, Miss NekvasilovS, has kindly allowed the author to mention a remarkable specimen of Terebratulina, cf., T. chrysalis FRI(~ in her collection from the Coniacian marls of B~ezno near Louny in Bohemia. The specimen shows what seems to be a long frill or flange extending beyond the commissure, reminiscent of the frill sometimes

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found in the Palaeozoic atrypids. It may be that, as in the latter group, the frill could have functioned as a kind of snow-shoe to prevent the shell sinking in the soft sediment. It may be significant that this specimen is much larger than the usual tiny forms discussed above. EKMAN (1896) also noted that the pedicle of Terebratulina sometimes shows a tendency to split like that of Chlidonophora, and may have had a similar mode of life. We therefore here have an example of a genus in which different species may have adapted themselves in three or more different ways to a particular habitat.

Deeper and/or calmer sea-floors Some of the most interesting and puzzling Mesozoic brachiopods are those which occur in what may, for convenience, be called the "bathyal" environments of the Alps and Carpathians. This is essentially the "Alpine Group" of brachiopods as previously defined (AGER, 1960) which normally occur in fine-grained sediments and which are normally associated with pelagic organisms. Clearly it is dangerous to attach a specific epithet like "bathyal" to a fossil environment, but the author does think that, generally speaking, this environment was deeper than the sublittoral environments already discussed. The chief argument for this is that the forms concerned are almost completely absent from the "shelf sea" deposits of Europe outside the Alps and the Carpathians. Coupled with this is the prevalence of pelagic organisms and the absence of coarse landderived sediments. Nevertheless, there are clear indications in some areas that it was not so much depth as the absence of strong currents that was the deciding factor in the environment. The best-known brachiopods of this habitat are the Pygope groul~ of terebratulids in the uppermost Jurassic and early Cretaceous rocks. These forms characteristically occur in very fine-grained limestones and marls with an associated fauna of ammonites (especiallly just aptychi), belemnites, tintinnids and pelagic foraminifers. The tintinnid Calpionella and the aberrant belemnite Duvalia are especially characteristic associates. The author previously commented (AGER, 1963, p.210) on a particularly good example of this assemblage in Valanginian beds near Vratsa in northwest Bulgaria, but many other localities are known from the Balkan Mountains to the Betic cordillera in southeast Spain (AGER, 1963, p. 157). Examples are also known in the Lower Cretaceous of Morocco (Mine. J. Geyssant, personal communication, 1964) and Tunisia (M. Rakus, personal communication, 1964), both in very similar geological settings. Where the palaeogeographical relationships are clearly known, there seems to be clear evidence of relatively deep water, well away from a shoreline. Thus the Tithonian Pygope janitor (Fig.6B) is found at the famous Porte de France locality at Grenoble in southeast France, and can be seen to be far to the seaward side of the reefs and lagoons of the Jura mentioned earlier.

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J 3 ¢rn Fig.6. Typical brachiopods of deeper and/or calmer sea-floors.A. Norellarefractifrons(BITTNER), Anisian. B. Pygopejanitor (PICTET),Tithonian. C. "Terebratula" helenae RENZ, Pliensbachian. D. Glossothyris(?) aspasia(MENEGHINI),Toarcian. All natural size.

The author has, however, seen some seemingly anomalous Pygope (s.l.) localities, notably in Hungary (e.g., in the Tithonian of the Gerecse Mountains near Tardos, and in the Valanginian of the Bakony Mountains near Zirc). The most remarkable locality is probably that at Rogoinik in the Pieniny Klippen Belt of southern Poland. Here abundant examples of Pygope diphya (Columna)1 are found in a thin lumachelle full of organic detritus (described by BIRKENMAJER, 1963). Superficially it looks like a shallow water shell breccia. However, the matrix is a fine-grained calcilutite with pelagic micro-organisms, and the macro-fossils (apart from the Pygope) are mainly ammonites, including many aptychi. Crinoid fragments are also very abundant, but these can be very misleading, partly because of the pelagic nature of certain crinoids and partly because echinoderm debris generally, by reason of the intricate canal systems that penetrate every plate, has different hydrodynamic properties from other shelly material. Dr. G~siorowski has expressed the opinion (personal communication, 1964) that the Rogo2nik limestone appears to have been deposited deeper than the wave base but shallower than the Calpionella limestones where Pygope also occurs in the Polish Tithonian. Referenco has already been made to the anomalous occurrence of Pygope (s.1.) at Stramberk in northern Moravia, where it was recorded in some of the older monographs (e.g., in SuEss, 1858) associated with a typically perireefal fauna. Once more, modern Czech work, notably by ELd,~ and STR/~MK (1963) on the stratigraphical relationships and by Eva Hanzlikovfi on the microfauna, has solved an ecological riddle. The relatively common Pygope--strictly Pygites diphyoides (D'ORBIGNV)--OCCUrS in sediments which differ both in stratigraphical age and in facies from the Tithonian "reefs". Pygope has long been recognized as a descendent of the sulcate terebratulid Glossothyris and is closely associated with it both stratigraphically and ecologically. It is, in fact, itself sulcate (i.e., with a dorsal sulcus and a ventral fold) and the rea Always cited as such, but a pre-Linnaean name and not valid.

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lationship between these two "genera" i s clearly complex. Certainly, for example, G. euthymi (PIcTET) and P. diphyoides (D'ORnIGNY) are closer to each other in the Valanginian than to other species of the same "genera", such as G. bouei (ZEUSCnNER), G. planulata (ZEUSCHNER),P. diphya ( Columna) and P. janitor (PICTET), in the uppermost Jurassic. Nevertheless, the writer does not accept the independent parallel evolution of three Pygope-like stocks (Pygope, Pygites and Antinomia) as propounded by BUCt
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thyris, and the only living sulcate rhynchonellid, Neorhynchia, are both abyssal forms from oceanic troughs (MuR-WOOD, 1961) 1. It is not suggested that this implies abyssal depths for the Mesozoic sulcate brachiopods, but it may imply somewhat comparable ecological factors. There is no obvious functional advantage for a brachiopod to be sulcate rather than uniplicate if it is considered in vacuo. However, if these forms lived-as their shape suggests--with their brachial valves on the substratum, then there may have been considerable advantage in having the exhalant current raised above the sea-floor. This would be especially true in the very quiet waters implied by the sediments in which these brachiopods are characteristically found, since the separation of inhalant and exhalant currents would have been vital in conditions where the waste material was not readily swept away. Pygope and its allies may also be regarded as forms in which the anterior commissure has been considerably expanded (Fig.6B). Bather (in discussion of BUCKMAN, 1906) suggested that this may have served for increased oxygen absorption on a poorly oxygenated sea-floor. In Mesozoic times a whole series of forms were developed, in all three main families, with greatly expanded anterior margins and most of these were restricted, like Pygope, to the Alpine-Carpathian region. Examples have been cited previously (AGER, 1960; 1963, p.133) of species of this kind in the Upper Triassic, e.g., "Rhynchonella" longicollis SOESS, Cruratula damesii BITTNER, and in the Lower Jurassic, e.g., "Terebratula" helenae RENZ (Fig.6C) and Zeilleria hierlatzica (OPPEL). An almost unbelievable extreme development is seen in the Rhaetian rhynchonellid Austrirhychia cornigera (SCHAFHAUTL), but Mr. D. A. B. Pearson thinks that this is a special mechanical adaptation to currents rather than indicative of a particular depth facies. Forms of this shape have been called "axiniform" by the present author (AGER, 1960) from their resemblance to a mediaeval battle axe, and it is particularly noteworthy how this pattern was repeated several times in different families though in the same general facies. It is often argued that poor oxygenation cannot be postulated for the deeper sea-floors because of the known circulation of currents in deep water at the present day. Three points may be argued the other way: (1) the probable absence of polar ice-caps in Mesozoic times may have considerably reduced such circulation; (2) there is a known marked falling off in the oxygen content of deep ocean water within a metre or so of the sea-floors, as was found by the Danish Galathea expedition; (3) there are strong reasons to suppose that the Mesozoic seas of the AlpineCarpathian region, wher.e these forms are chiefly found, were cut off from the open sea, at least by a sill, and were by no means open oceans. Another possibility derived from the observation by LAMONT (1934) that Lower Palaeozoic brachiopods occurring in muddy sediments tend to be laterally 1 M o d e r n sulcate terebratellids such as Megerlina a n d Terebratalia do inhabit relatively shallow waters, as did Mesozoic sulcate terebratellids s u c h as Aulacothyris.

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expanded and compressed, due to the difficulties of oxygen absorption on seafloors of this kind. This could also well be an explanation of the perforate and axiniform brachiopods since they invariably occur in fine-grained sediments such as shales and calcilutites. A further factor may have been simply the desirability of more expanded lophophores in areas of low food concentration, though it must be pointed out that the lophophore supports (i.e., the loops) are quite small in these forms. The rare examples of axiniform brachiopods which are found outside the Alpine-Carpathian region may also fit in with the above explanations. Thus the best known British axiniform brachiopod is Digonella digona (J. SOWERBY), which occurs in the Bathonian Bradford Clay in a rather unusual association of such species as the curious crinoid Apiocrinus elegans (DEFRANCE) ( = A. parkinsoni AUCTT.) and in circumstances which suggest very quiet, muddy deposition. Just as temperature in the sea is a function both of latitude and of depth, so the optimum conditions for this group of brachiopods may have existed both in the deeper waters and in the shallower quieter waters of the Mesozoic seas. There is rarely any direct evidence of water depth in the sediments under consideration, and evidence for this particular group is usually unfortunately lacking. However, returning once more to the manifold ecological problems of Stramberk, there is there an occurrence of Pygites diphyoides in red marls which also yields a diagnostic foraminiferal fauna. The latter has been examined by Eva Hanzlikovfi who has kindly informed the author (personal communication, 1964) that the benthonic forms suggest a depth of not more than about 100 m. This particular occurrence therefore is clearly not bathyal, though it was evidently a very quiet part of the shelf.

Pelagic The author has previously suggested (AGER, 1962a) that certain small, thinshelled rhynchonellids may have been epiplanktonic in habit, that is to say, they may in life have been attached to floating weed in a sort of planktonic friendship. So far as mode of occurrence is concerned, these are forms which are usually found as scattered individuals in sediments such as clays, shales and calcilutites, but also show some disregard of sedimentological facies. Morphologically, besides their small size and thin shells, they are characterized by the possession of falcifer or prefalcifer crura (as defined by AGER, 1965b). Clearly this is a somewhat reckless hypothesis which cannot be proved, but it was put forward to explain certain anomalous occurrences in both Palaeozoic and Mesozoic rocks, for which no other possible mode of lie seems to be fully acceptable. The main criteria are the soft-bottom nature of the original sediment, the absence of other epibenthonic body fossils and the absence of any evidence or likelihood of post-mortem transport. In some cases, such rhynchonellids are the only

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benthonic organisms of any kind present in the rocks, as in the Permian basin environment of Texas and New Mexico (NEWELLet al., 1953, p.67). More commonly they are only accompanied by endobenthos such as burrowing lamellibranchs and irregular echinoids, which imply a soft floor. Some of the examples suggested previously may be explained by other means. Thus the clusters of Calcirhynchia calcaria BUCKMANin the British Hettangian may have been attached in life to fixed Algae (though this implies somewhat shallower depths than is suggested by other evidence). Others such as Orbirhynchia in the Chalk may have been attached to sponges (as discussed earlier). There still remain such cases as the rare Rhynchonelloidella spathica (LAMARCK) in the Oxford Clay, Stolmorhynchia bouchardii (DAvIDSON) in Upper Lias clays and Thurmannella subvariabilis (DAvIDSON) in the Kimmeridge Clay, which cannot be explained by other means. In the French Jura, the small thin-shelled, spinose rhynchonellid Acanthorhynchia occurs only in the argillaceous Oxfordian and in the calcilutitic Kimmeridgian (AGER and EVAMV, 1964). Its occurrence there is particularly interesting because immediately below, in calcarenitic Bathonian-Callovian beds, is found Acanthothiris, the only other Mesozoic spinose rhynchonellid. The two "genera" are very much alike externally (compare Fig.2C and 5C) except that Acanthorhynchia is usually smaller, more compressed and with finer costae. Internally they differ in that Acanthothiris has "normal" radulifer crura, whilst the other has crura that approach the falcifer form. The question here is whether the internal structures are to be regarded as more significant than the external form and mode of occurrence. The present author cannot accept the juxtaposition of the two genera as merely a coincidence, and prefers the theory that the differences in crura represent adaptations in the lophophores to different environments and different modes of life. Acanthorhynchia appears to always occur in clay-grade sediments and as scattered individuals rather than the concentrated nests of Acanthothiris. At one locality in the southern Jura (referred to above), Acanthorhynchia is found associated with crinoid debris, and the possibility of attachment to benthos cannot be ruled out. These problems are currently being investigated by Mr. A. Childs.

COMPARISON OF SHALLOW AND DEEPER WATER BRACHIOPOD ASSEMBLAGES

In England one may readily distinguish the Aptian to Cenomanian brachiopods from those of the Turonian to Maestrichtian simply by the size of their pedicle openings. This applies to all brachiopod groups. Typical examples in the first group are Cyclothyris latissima (J. DE C. SOWERBY), Sellithyris sella (J. DE C. SOWERBY) and Gemmarcula aurea ELLIOTT, whereas in the later sediments Cretirhynchia plicatilis (J. SOWERBY), Carneithyris carnea (J. SOWERBY) and Magas pumilus (J. SOWERBY) are just a few of the many examples with minute pedicle

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openings. This clearly has no evolutionary significance, but is merely a reflection of the increasing depth of water in later Cretaceous times. Such obvious morphological characters readily serve to distinguish "shallow" from "deep" associations, but there are also more subtle differences in the characters of the faunas considered as a whole. It is well-known that at the present day there is a marked decrease in the diversity of marine faunas with increasing depth, and this can be clearly demonstrated among the Mesozoic brachiopods. If one simply compares the numbers of species in the first four general environments discussed above with the last three, the difference is immediately obvious. Thus at Stramberk, about 600 fossil species have been described from the white "reef" limestone, among which there must be at least 20 common brachiopods. On the other hand, in the deeper water red Kopfivnice Limestone there seems to be only the one common rhynchonellid and two or three rare terebratuloids. What is more, within the shallow water faunas there is considerably more lateral variation and diversity than in the deeper habitats. This is partly a matter of the diversity of "microhabitats" available for colonization by brachiopods along the margins of the Mesozoic seas, and partly a matter of the less stable and laterally less constant nature of these habitats. Thus around the reefs at Stramberk there seem to have been some brachiopods adapted to the very rough water high on the talus slope and others which may have sheltered in pools or pockets between the reefs. Clearly "Terebratula" moravica, with its graceful elongated beak, did not live in the same place as the almost sphaeroidal "Rhynchonella" pachytheca. On the other hand, the deeper water forms of different groups appear to have adapted themselves in remarkably similar ways, as in the striking cases of homoeomorphy mentioned earlier. Lateral variation in the composition of brachiopod faunas may be related to local differences in environment, such as are particularly common in shallow seas, or by factors of more regional or even world-wide significance. Thus in the brachiopod faunas of the British uppermost Pliensbachian (AGER, 1956a), the author found that in the clearly unpopular ironstone environments of the English Midlands, the fauna was extremely limited in diversity, though not in numbers. This contrasted with the more diverse faunas both north and south. Besides this local situation, however, the writer also found a marked general increase in diversity southwards, so that Dorset has 24 recognizable species compared with eighteen in Oxfordshire and only nine in Yorkshire. On a broader scale also, changes in the shallow water faunas seem to be mainly in a north-south direction. The Lower Jurassic faunas just discussed can be traced southwards through France in similar sediments, but change in the process. For example, Homoeorhynchia acuta (J. SOWERBY)of Britain and northern France, with its distinctive unicostate fold, gives way laterally to H. meridionalis (EUDES-DESLONGCHAMPS) with a multicostate fold (AGER, 1961) 1. Similar south1 This was denied by G. Dubar in the discussion of this paper, but has since been confirmed by the author in Provence.

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ward increases in costation in this genus are seen also in Slovakia and in Turkey (AGER, 1959a, p.1019). A similar trend in the terebratuloids was demonstrated in the work of DuBAR (1942). Multicostate terebratulids are extremely rare and local in the shallow water environments of northern Europe; Plectothyris fimbria (J. SOWERBY) provides a good example over a few square miles of one zone in the British Bajocian. These forms become notable, however, in the Lower and Middle Jurassic of the general "Tethyan" region of southern Europe and reach a maximum in Morocco. Here again the phenomenon of convergence is seen very clearly, for in the same region multicostate long-looped zeilleriids also reach their best development. This then may be a simple climatic differentiation from north to south, and contrasts markedly with some of the supposedly deeper water faunas which are remarkably constant for long distances. The Upper Triassic rhynchonellid Austriellula, for instance, is found in essentially the same form from the Alps to Laos and Timor in south-east Asia.

GENERAL CONCLUSIONS

Palaeontologists cannot live by uniformitarianism alone. There is ample evidence to suggest that the brachiopods have for the most part--changed both their habits and their distribution since Mesozoic times. Comparison with what little is known about modern brachiopods is full of difficulties and inadequacies. On the whole it is much better to look at the evidence provided by the fossils themselves, both in their structures and in their mode of occurrence. Many observations (such as the coincidence of falcifer crura and soft-bottom sediments) must remain enigmas, but it does not make them invalid as observations. The deductions which are contained in this paper are summarized in Table I and Fig.7. These are undoubt-

clostlc d r°cfkiYoorSe°- ~ \

~

floatlr~ algoe 7 normal muddy sediments

2 1 norrno} coarse sand-grodeI~toral sedimentssediments

reef iogoonoi reef sediments detritus

agi~antly sediments transported shollow water materiol

UNSTABLE COASTLINES

coast

without reefs

NORMAL

coast w i t h reefs

COASTLINES

Fig.7. Diagrammatic representation of the seven main Mesozoic brachiopod habitats.

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169

ADAPTATIONOF MESOZOICBRACHIOPODS TABLE 1 THE SEVEN MAIN MESOZOIC BRACHIOPOD HABITATS

Environment

Typical sediments

Brachiopod characters Examples

Very shallow water (? littoral)

Coarse sands, conglomerates, calcarenites

Coarse costae, large rimmed foramens (R)l, sharp plieae (T)1

Shallow water (sand-grade floors)

Calcarenites, oolites, ironstones

Normal trilobed (R) 1 Tetrarhynchia cynocephalous and Lobothyris spinose (R) l, rectiZeilleria marginate (I")l, indentate (Z)1

Shallow water (perireefal)

Bahamites, bloelastic ealcarenites

All very large, asymmetrical (R) l, broad folds (T)1. elongated beaks (T, Z)l, large foramen

Septaliphoria Loboidothyris

Shallow water (rocky floors)

Normally nil, but fossils preserved in turbidites and elastic dykes

All very large, coarse ribbed or smooth, mergifer crura etc.

Peregrinella Halorella

Deeper water (muddy floors)

Clays, shales, calcilutites

Medium size, thin valves, faleifer crura (R)l, fine costae (R, Z)1

Kolhidaella Lacunosella (partim) Terebratulina

Relatively deep water (? bathyal)

Clays, shales, calcilutites, but only pelagic fossils

Smooth sulcate (R, Norella T) l, axiniform (R, Pygope T, Z)l, perforate (T)1 Glossothyris

Epiplanktonic(?)

All sediments, but Very small, thin especially clays, shales valves, falcifer and calcilutites crura (R only)1

Cyclothyris Stiphrothyris

Stolmorhynchia

1 R = Rhynchonellids, T = Terebratulids, Z = Terebratellids.

edly oversimplified, exaggerated a n d in part inaccurate, b u t they seem to be the best i n t e r p r e t a t i o n possible of the evidence available at the m o m e n t .

ACKNOWLEDGEMENTS This p a p e r is one of the by-products of more t h a n 12 years work o n the Mesozoic Brachiopoda in almost every c o u n t r y o f E u r o p e a n d in N o r t h America, so that it is impossible to acknowledge all the m a n y geologists a n d palaeontologists

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who have helped in various ways, in the field, in the m u s e u m a n d b y c o r r e s p o n d ence. It is h o p e d t h a t any specific piece o f i n f o r m a t i o n t h a t was n o t the a u t h o r ' s own o b s e r v a t i o n is suitably a c k n o w l e d g e d in the text. The writer would, however, like to express special t h a n k s to three Czech friends: Eva Hanzlikovfi, Olga Nekvasilovfi a n d Zden~k R o t h , who, with their colleagues, b y two visits to the classic S t r a m b e r k localities in n o r t h e r n M o r a v i a a n d b y a series o f t h o u g h t - p r o v o k i n g discussions a b o u t the e n v i r o n m e n t s represented there, stimulated the writing o f this paper. I t was, in fact, written d u r i n g the a u t h o r ' s Second visit to their friendly a n d scenically delightful country. H e has also m u c h pleasure in recording his gratitude to his research assistant, M a r y Pugh, for d r a w i n g the figures a n d for the b a c k g r o u n d w o r k o f sectioning a n d b i b l i o g r a p h i c search which f o r m s an essential basis o f all his writings in this field.

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