Although it has long been recognized that the Graptoloidea constituted a diverse group of planktic organisms, the precise hydrodynamics of the various colony morphotypes has been a source of debate. Recent discoveries of specimens of Cyrtograptus murchisoni with a complex suite of webs or vanes between the central coiled stipe and the cladial branches have shown that the hydrodynamic modifications of at least this taxon were considerably more complex than previously thought. These webs are composed of very thin peridermal tissue and stretch between the first or second order cladial branches and the main stipe, the webs overlapping to give a screw-type morphology to the rhabdosome. The form of the webbing also has implications for the mode of life and mobility of individual zooids within the colony, as the main areas of web construction are in regions in which the zooids were enclosed within restricted thecal apertures

Underwood, Charlie J.

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1INTERSTIPE WEBBING IN THE SILURIAN GRAPTOLITECYRTOGRAPTUS MURCHISONI AND ITS PALAEOBIOLOGICALSIGNIFICANCE.by CHARLIE J. UNDERWOODABSTRACT. Although it has long been recognized that the Graptoloidea constituted a diversegroup of planktic organisms, the precise hydrodynamics of the various colony morphotypes hasbeen a source of debate. Recent discoveries of specimens of Cyrtograptus murchisoni with acomplex suite of webs or vanes between the central coiled stipe and the cladial branches haveshown that the hydrodynamic modifications of at least this taxon were considerably morecomplex than previously thought. These webs are composed of very thin peridermal tissue andstretch between the first or second order cladial branches and the main stipe, the websoverlapping to give a screw-type morphology to the rhabdosome. The form of the webbing alsohas implications for the mode of life and mobility of individual zooids within the colony, as themain areas of web construction are in regions in which the zooids were enclosed within restrictedthecal apertures.2AFTER many years of acceptance of graptolites as passive drifters (e.g. Bulman 1970), it hasbecome evident that graptolites were to some extent motile (e.g. Kirk 1990; Rigby 1991;Underwood 1993). The degree and form of this motility is controlled by the hydrodynamicproperties of the living rhabdosome (Rigby 1991), which may not necessarily be the same asthose of the dead colony as preserved owing to the influence of zooidal and other 'unpreserved'tissues (Underwood 1993). Although no preserved hydrodynamically important parts of zooids(such as lophophores) are known, a number of rarely preserved semi-sclerotized tissues havebeen recorded. These include proximal webbing in several taxa of anisograptids (Bulman 1970),ancoral periderm in retiolitids (Lenz and Melchin 1987; Bates and Kirk 1992) and a singleexample of a proximal web in Cyrtograptus (Lenz 1974). Such structures are of fundamentalimportance to the functional morphology of these taxa.The discovery of three specimens of Cyrtograptus murchisoni Carruthers 1867 withcomplex interstipe webbing has important implications for graptolite functional morphology andlifestyle. They were found by Mr. Brian Beveridge in the murchisoni Biozone (Silurian,Wenlock Series) of Buttington Brick Pit, near Welshpool, Powys, Wales (see Cave and Dixon1993 for locality details). They are preserved as periderm compressions in a grey mudstone,associated with a rich fauna of other C. murchisoni, Monograptus priodon (Bronn) andMonoclimacis sp., along with occasional crinoid material, bivalves and trilobites. None of thespecimens has a counterpart. The presence of web-bearing stipe fragments of other individuals ofCyrtograptus on two of the slabs suggests that interstipe webbing was normal, but is onlypreserved under particularly favourable taphonomic conditions. All three of these specimens areexposed with the sicular or apical surface uppermost, with the sicula being missing out of theplane of the rock. All three specimens show sinistral coiling, but it is unclear whether this ischaracteristic of the taxon. All are deposited in the collection of the Bristol City Museum and ArtGallery, registered as BRSMG Cd2548-2550.WEB MORPHOLOGY3A proximal web was described from a specimen of Cyrtograptus by Lenz (1974), as asimple sheet of periderm stretched between the main stipe and the first two cladial branches. Thefar larger and more mature rhabdosomes of C. murchisoni, however, show that the web was not asingle sheet draped between branches, but a complex series of sheets showing various degrees ofoverlapSpecimen Cd2550 represents a mature rhabdosome, and possesses the most readilyrecognizable web structure (Text-fig. 1A-B). The webs are present as a series of overlappingsheets, which generally became more extensive towards the central part of the rhabdosome. Manyof the webs grade distally from a wide sheet to a narrow flange which becomes difficult todifferentiate from the 'normal' stipe, with some suggestion of these flanges being present alongalmost all of the stipes of the graptolites studied.Although very poorly preserved, there is evidence of a small web at the extremeproximal end, probably stretching from sicula to the first cladium. The webbing over the rest ofthe primary stipe is broad and extends along its concave edge until the origination of at least thefifth cladium. This appears to be in three distinct overlapping layers,with the more distal layersbeing overlapped by the more proximal (Text-fig. 3); the most distal of these has a large lobe orirregularity on its trailing edge. The webs along the first cladial stipe are more extensive thanalong the primary stipe, but also appear to be composed of three overlapping sheets stretchedalong the concave edge. It appears that these are, at least in part, continuous with webs along theprimary stipe. The third major area of webbing is along the second cladial stipe and its secondorder branches. Less easily interpreted than the other two areas of webs, these appear to consistof two main layers. The better preserved of these is present along the concave and convex sidesof the main cladial stipe, as well as forming an extensive web along the second order branch 2.1.This is probably continuous with the second web along the primary stipe. A second, much wider,web is also evident along branch 2.1, behind (asicular to) the forementioned web. Small webs arealso present along the proximal parts of cladial stipes 3, 4 and 5, as well as forming a smallprocess which may mark the position of stipe 6, which is not seen.The smallest of the specimens, Cd2549, shows the simplest web structure (Text-fig.2A). There appear to be two webs along the primary stipe, each of which are in continuity with4webs along the first two cladial stipes. The longer of these is seen to stretch from 1.3 cmalong the cladial stipe 1, passes over the asicular ('upper') surface of the primary stipe beforeextending 3.5 cm along its concave edge, whilst the second, wider, web extends from the concaveedge of cladial stipe 2, passes over the asicular surface of the earlier web and continues as a widersecond web along the primary stipe. A further small area of similar material may indicate anincipient web along the poorly preserved third cladium.Specimen Cd2548 is the most mature and complex rhabdosome (Text-fig. 2B-C).Although the webs are generally well preserved, the proximal end of the primary stipe has beenlargely lost with the counterpart, making interpretation difficult. The most proximal  webs extendalong both sides of cladial stipes 1 and 2 and are then seen to pass across the proximal part of theprimary stipe. Although unclear, this then appears to continue along both sides of cladial stipe 4and branch 4.1. A small area of web is also visible at the base of third order cladial branch 2.1(not seen in figures), showing that in places the web extends over 7 cm from the primary stipe.Cladial stipe 3 appears to provide the origination of two or three webs. One web is present alongthe concave edge of the first order branch, extending along the primary stipe to just past theorigination of cladial stipe 5. A second, wider, web is present along the edge of second orderbranch 3.1. This appears to pass over the asicular surface of all of the previous webs, extendingalong the concave edge of the primary stipe and attaching to its convex edge close to theorigination of cladial stipe 4. It also appears that the main web which extends along the concaveside of the primary stipe, itself probably consisting of two overlapping sheets, may originate inthe area of stipe 3. A small web is also present between the convex edges of the primary stipe andcladial stipes 5 and possibly 6.A straight stipe fragment with some webbing between cladial stipes 3 and 4 appears tobe on a lower lamella in the rock and belong to a separate specimen.WEB STRUCTUREThe webs appear as dark, vaguely striated films. In places, pyrite polyframboids are present, butthese are surface encrustations rather than infills of some hollow structure. The dark materialappears to be normal periderm, but is seen to be far thinner than periderm from the main5rhabdosome. The striations could represent original structure, but their irregular form makethem more likely to be the result of a wrinkled surface texture. Otherwise there is no evidence forthe microstructural composition of the webs. However, as structures obviously post-dating theformation of the thecae, it is likely that they are composed largely or totally of cortical tissue (seeCrowther 1981).The hydrodynamic properties of the web periderm would have been strongly influencedby its rigidity in life. Normal graptolite periderm in life was a rigid material, but far more likely todeform in a ductile than brittle manner (as evidenced by the extreme predominance of buckledover broken stipes in fossil assemblages). The probable wrinkled surface texture of the webssuggests that they were composed of a far more flexible material. This is backed up by the easewith which the webs twist as they pass from the convex edges of the cladial branches to primarystipe. It is unclear whether the webs represented a loose, flexible sheet or an elastic membrane.Small scale scalloping along some of the trailing edges of webs could either be due to contractionof a flexible web or could be original irregularities in the edge. In Cd 2550 these form a smallloose lobe, showing that some original irregularities are present.IMPLICATIONS FOR PERIDERM CONSTRUCTIONThe presence of wide peridermal webs has considerable implications for the manner by whichperiderm is laid down. The general manner of periderm secretion is for the thecae to be built upby the zooids secreting fusellar tissue, with the subsequent addition of cortical bandages to bothouter and inner surfaces (Crowther 1981). Structures such as nemal vanes were built in a similarmanner, being constructed by the most mobile, distal, zooids (Urbanek 1973). In all cases, it wasprobably only the more distal zooids which carried out the construction. The webs in C.murchisoni, however, are concentrated at the proximal ends of mature rhabdosomes and mustthus have been constructed at a considerable distance from developing zooids. This is shown bythe increase in colony size and complexity in the series of specimens Cd2549-2550-2548, with acorresponding increase in area and complexity of the webs. The size of these webs, extending upto 8 mm from the closest zooid, would suggest highly mobile proximal zooids. However, this isnot backed up by the thecal morphology. The thecae of Cyrtograptus, especially at the proximal6end, are hooked with restricted apertures. This modification is almost certainly defensive(Underwood 1993), preventing access by predators but also probably restricting zooid mobility.Each web also stretches across many thecae, and would require coordinated activity from allzooids involved for its construction. There are thus three options for the mode of web secretion:(1) The morphology of the webs could easily be accommodated by secretion within a body ofextrathecal tissue (e.g. Kirk 1972). This, however, would contradact the need for defensiverestricted thecae and the body of tissue would detract from the hydrodynamic advantage given bythe webs and therefore seems unlikely. (2) Zooids were both considerably more mobile thanpreviously thought, and showed extremely highly coordinated behaviour. They were also smallenough to fit through the restricted proximal thecal apertures. (3) It is possible that theunrestricted distal zooids carried out the construction. This would involve high mobility, withzooids needing to travel over 10 cm from the growing stipe ends. This mechanism would also bepossible if the thecae below the webs were empty or filled with non-mobile zooids.IMPLICATIONS FOR HYDRODYNAMICS AND FEEDINGIt is obvious from the general form of the rhabdosome that the hydrodynamics of Cyrtograptuswere both complex and closely constrained. It has long been realized that the spiral of the mainstipe forms a low, open cone (e.g. Bulman 1970), with it now being evident, owing to the mannerin which stipes overlie each other, that the cladial branches diverge at a steeper angle, emphasisingthe general conical form. The spiral pattern of the Cyrtograptus rhabdosome is itself closelyconstrained, following a set geometric pattern within each taxon (Huo et al. 1986). Within thiscontrolled pattern, the morphology of the interstipe sheets is closely constrained by the spiralgeometry of the rhabdosome. Although this was suggested by Lenz (1974) from his observationson a webbed specimen, the small and broken nature of that specimen prevented any firmconclusions.Physical modelling of the mode of movement of graptolites by Rigby and Rickards(1989) included work on large Cyrtograptus, but no allowance was made for the presence ofinterstipe webbing. The effects of interstipe webs on the hydrodynamics of large multiramousdichograptids was modelled, as the presence of these is well known within a range of taxa7(Bulman 1970). Dichograptid webs are not, however, directly functionally analogous to thewebs of Cyrtograptus, as they formed a simple drape between symmetrically spaced stipes of aplanar rhabdosome. This symmetrical webbing in dichograptids would thus have acted simply toincrease surface area and reduce the rate of movement, whilst the complex screw morphology ofCyrtograptus webs would have been capable of controlling the rate of movement, degree ofrotation and probably the rhabdosome orientation.The overlapping nature of the webbing suggests that the hydrodynamic structure wasnot so much a planar screw controlling water flow over a single curved surface, but more a seriesof (probably flexible) 'wings' or 'sails' acting to direct water through distinct channels between thestipes (see Text -fig. 3). Along the leading edge of each of these wings was a stipe filled withzooids. Any water through which the central area of the rhabdosome passed thus had to passthrough the lophophores of the zooids and the channels between the webs. It is therefore likelythat although the webs controlled water flow in a purely passive manner, the water flow passingover the webs could have been controlled by the zooids.As filter feeders, the flow of water over the zooids would have controlled the quantity offood available to the colony. The webs would thus have directly (by directing water over thezooids) and indirectly (by helping control the mobility of the rhabdosome) increased the feedingefficiency of the colony. This would support the idea that Cyrtograptus was adapted for highlyefficient feeding (Rigby, 1991) within deeper, more nutrient poor waters (e.g. Kirk, 1990;Underwood, 1993).The direction of movement of Cyrtograptus was assumed by Underwood (1993) to beopen end first, the colony acting as an open 'trawl net'. On all three specimens examined, however,narrower webs are present on the sicular surface of wider ones. To get a hydrodynamic benefitfrom these narrower webs the colony would have had to move sicular (closed) end first. Thisstyle of orientation with the point of the cone up-current has been noted within other groups offilter feeders, particularly crinoids (Prof. C.R.C. Paul, pers. comm. 1994). On the largestspecimen (Cd 2548), however, one narrow web is seen to pass over the asicular surface of a widerone. It may be therefore, that movement of Cyrtograptus was not simply unidirectional, and thecolony may have been able to move in a variety of orientations.8Acknowledgements. I thank Dr. Peter Crowther for showing me these specimens and later lettingme borrow them and Mr. Brian Beveridge for donating the specimens to Bristol Museum. Prof.Chris Paul and referees provided useful comments.9REFERENCESBATES, D.E.B. and KIRK, N.H. 1992. The ultrastructure, mode of construction and functioning of a number of Llandovery ancorate diplograptid and retiolitid graptolites. Modern Geology, 17, 1-270.BULMAN, O.M.B. 1970. Graptolithina, with sections on Enteropneusta and Pterobranchia. V1-V163. In TIECHERT, C. (ed.) Treatise on Invertebrate Palaeontology.Part V, (2ndEdition). Geological Society of America and University of Kansas Press, Boulder,Colorado and Lawrence, Kansas, i-xxxii +163pp.CAVE, R. and DIXON, R.J. 1993. The Ordovician and Silurian of the Welshpool area. 51-84. In WOODCOCK, N. H. and BASSETT, M. G. (eds). Geological excursions in Powys, Central Wales. University of Wales Press, Cardiff, 366pp.CROWTHER, P.R. 1981. The fine structure of graptolite periderm. Special Papers in Palaeontology, 26, 1-119.HUO, S., FU, L. and SHU, D. 1986. A mathematical study of the Cyrtograptus sakmaricuslineage with discussions of the evolutionary trends in this lineage. 197-205. InHUGHES, C.P. and RICKARDS, R.B. (eds). Palaeoecology and biostratigraphy of graptolites. Geological Society Special Publication  No. 20, Oxford, 277ppKIRK, N.H. 1972. Some thoughts on the construction of the rhabdosomes in the Graptolithina,with special reference to extrathecal tissue and its bearing on the theory of automobility.University College of Wales, Aberystwyth. Department of Geology Publication, 1, 21pp.KIRK, N.H. 1990. Juvenile sessility, vertical automobility and passive lateral transport as factors in graptoloid evolution. Modern Geology, 14, 153-187.LENZ, A.C. 1974. A membrane-bearing Cyrtograptus and the interpretation of the hydrodynamics of cyrtograptids. Special Papers in Palaeontology, 13, 205-214.LENZ, A.C. and MELCHIN, M.J. 1987. Peridermal and interthecal tissue in Silurian retiolitid graptolites : with examples from Sweden and Arctic Canada. Lethaia, 20, 353-359.RIGBY, S. 1991. Feeding strategies of graptolites. Palaeontology 34, 797-815.10RIGBY, S. and RICKARDS, R.B. 1989. New evidence for the life habits of graptoloidsfrom physical modelling. Paleobiology 15, 402-413.UNDERWOOD, C.J. 1993. The position of graptolites within Lower Palaeozoic planktic ecosystems. Lethaia 26, 189-202.URBANEK, A. 1973. Organization and evolution of graptolite colonies. 414-441 In BOARDMAN, R.S., CHEETHAM, A.H. and OLIVER, W.A. (eds). Animal colonies through time. Dowden, Hutchinson and Ross, Stroudsburg, Pa.Charlie J. Underwood, Dept. of Earth Sciences, University of Liverpool, Brownlow St., LiverpoolL69 3BX.11Figure Captions.TEXT-FIG. 1. Cyrtograptus murchisoni, Wenlock Series, Buttington, Powys. BRSMG Cd2550.A, central portion showing the wrinkled and overlapping nature of the dark interstipe webs. B,drawing interpreting the full extent of the webs along the proximal parts of the stipes. Note thesmall proximal web (p) and the lobe of web material (l). The main stipe (m) is labelled, as are thecladial branches 1-5 and second order branch 2.1. The webbed stipe fragment (f) belongs toanother specimen at a lower level in the rock. Scale bars represent 10 mm.TEXT-FIG. 2. Cyrtograptus murchisoni, Wenlock Series, Buttington, Powys. A, the smallrhabdosome, BRSMG Cd2549, with only two webs visible. B, the large and complexrhabdosome, BRSMG Cd 2548. C, drawing of BRSMG Cd2548, showing the complex nature ofthe multiple, overlapping interstipe webs. The main stipe (m) is labelled, as are the cladialbranches 1-6and second order branches 3.1 and 4.1. The webbed stipe upper left (f) probablybelongs to a second specimen. Scale bars represent 10 mm.TEXT-FIG. 3. Cyrtograptus murchisoni, Wenlock Series, Buttington, Powys. A, diagrammaticreconstruction of BRSMG Cd2550 showing the various layers of interstipe webbing recognized.There is no suggestion that all the webs of the same generation are laterally continuous with eachother. B, hypothetical life cross section along line a-a' in above showing the overlappingrelationships of the webs. Scale bars represent 10 mm.

A mathematical study of the Cyrtograptus sakmaricus lineage with discussions of the evolutionary trends in this lineage. 197-205. In

A membrane-bearing Cyrtograptus and the interpretation of the hydrodynamics of cyrtograptids.

Feeding strategies of graptolites.

Graptolithina, with sections on Enteropneusta and Pterobranchia.

Juvenile sessility, vertical automobility and passive lateral transport as factors in graptoloid evolution.

Organization and evolution of graptolite colonies. 414-441 In

Peridermal and interthecal tissue in Silurian retiolitid graptolites : with examples from Sweden and Arctic Canada.

The fine structure of graptolite periderm.

The Ordovician and Silurian of the Welshpool area. 51-84. In

The position of graptolites within Lower Palaeozoic planktic ecosystems.

The ultrastructure, mode of construction and functioning of a number of Llandovery ancorate diplograptid and retiolitid graptolites.

Interstipe webbing in the Silurian graptolite Cyrtograptus murchisoni and its palaeobiological significance.

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Interstipe webbing in the Silurian graptolite Cyrtograptus murchisoni and its palaeobiological significance.

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