Middle Cambrian pterobranchs and the ... - Wiley Online Library

Middle Cambrian pterobranchs and the Question: What is a graptolite? ¨ RG MALETZ, MICHAEL STEINER AND OLDRICH FATKA JO

Maletz, J., Steiner, M. & Fatka, O. 2005 03 15: Middle Cambrian pterobranchs and the Question: What is a graptolite? Lethaia, Vol. 38, pp. 73–85. Oslo. ISSN 0024-1164. The presence of distinct fusellar structure is taken as evidence to include a number of fossils from the Middle Cambrian to the Lower Ordovician of North America and Europe with the Pterobranchia. The dome of the pterobranchs and the prosicula of the planktic graptolites are contrasted and evidence is given for the re-assignment of a number of well known dendroid graptolites to the pterobranchs. A non-destructive method is described to reveal fusellar development of delicate hemichordate exoskeletons from shales. Rhabdotubus robustus n. sp. from the Czech Republic and ?Cephalodiscus sp. from the Wheeler Shale of North America are described as new Middle Cambrian pterobranchs. & Evolution, fuselli, graptolites, preservation, pterobranchs. Jo¨rg Maletz [[email protected]], Department of Geology, State University of New York at Buffalo, SUNY, 772 Natural Sciences and Mathematics Complex, Buffalo, New York 14260-3050, U.S.A.; Michael Steiner [[email protected]], TU Berlin, Sekr. ACK 14, Ackerstrasse 71-76, D-13355 Berlin, Germany; Oldrich Fatka, Institute of Geology and Paleontology, Charles University of Prague, Albertov 6, 128 43 Praha 2, Czech Republic; 11th September 2003, revised 9th December 2004.

The phylum Hemichordata can be divided into four classes, the Pterobranchia, Graptolithina, Enteropneusta, and the questionable monospecific Planctosphaeroidea (Hyman 1958; Bulman 1970), of which the Enteropneusta and Planctosphaeroidea are known from extant members, but have no fossil record. Recently the phylogenetic relationships within the Hemichordata have been questioned (Cripps 1991; Jefferies 1991; Schram 1991) and they have been interpreted as a highly artificial grouping (Christoffersen & Araujo-de-Almeida 1994). The Pterobranchia and the Graptolithina represent the only hemichordates with a preservable organic exoskeleton and, thus, are documented from the fossil record, but the Graptolithina are not known from extant species. The Pterobranchia survived from at least the Middle Cambrian to the present day with a small number of benthic species belonging to the orders Cephalodiscida and Rhabdopleurida. The external skeleton of both groups of skeleton-bearing hemichordates is constructed of fusellar and cortical tissues, but the extant cephalodiscid genus Atubaria is known only from a number of isolated zooids (Sato 1936) and apparently does not secrete a skeleton at all. The phylogenetic relationships of the pterobranchs and graptolites have been discussed exensively over the last decades (Kozlowski 1966; Urbanek 1976; Andres 1977, 1980; Mierzejewski 1986). Especially the ultrastructure of the exoskeleton has been used to infer or

reject phylogenetic relationships (Urbanek & Towe 1974; Urbanek 1976; Andres 1980), but also the ultrastructure of fusellar and cortical tissues were taken as important characters (Crowther 1981; Andres 1977, 1980). The ultrastructure of living pterobranchs and graptolites was documented to differ in certain aspects (Urbanek 1976; Mierzejewski 1984a,b; Urbanek 1986; Dilly 1993). Recently, Mierzejewski and Kulicki (2001) indicated that the ultrastructure of fossil Rhabdopleura and graptolites is closely comparable, resurrecting Beklemishev’s (1951, 1970) idea of a close phylogenetic relationship of graptolites and pterobranchs, referable to one class – the Graptolitoidea, was also argued for by Urbanek (1994). The general problem of identification of unique characters of fossil pterobranchs and graptolites is the poor preservational potential of the colonies and their small zooids. It is often difficult to observe important structural details and material has erroneously been assigned to other groups of organisms, such as algae. Early metazoans often lack identifiable characters, and usually only outlines of organic walled fossils are available for determination. Morphological convergence can be seen in the dimensions and the presence of dichotomous branching divisions, producing very similar outlines. Differences in wall structure are often impossible to recognize in fossil material. The preservation of true fusellar structures and the presence of a stolon system, however, unanimously identify the pterobranch or graptolitic relationship of specimens. DOI 10.1080/00241160510013204 # 2005 Taylor & Francis

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Rhizoids on the other hand can indicate an assignment to algae. As parts of the material described herein were formerly identified as algal remains by the original collectors, it seems to be important to pay attention to small organically preserved fossils identified as fossil algae. In this way new records of early pterobranchs will hopefully surface from the fossil record. Soft parts are generally not preservable in the small zooids of the pterobranchs and can be found only under very special circumstances. An example for the exceptional preservation of minute detail is the documentation of fusellar structures and the presence of possible zooids in the Middle Cambrian Rhabdotubus obuti (Durmann & Sennikov 1993), although identifications of zooidal remains in the Graptolithina (Bjerreskov 1978; Rickards & Stait 1984; Rickards et al. 1991) are less convincing. The aim of this paper is to describe new Middle Cambrian pterobranch remains from Bohemia and the USA and to discuss their relationships to the Graptolithina. The back scatter electron (BSE) investigation of thin-walled, organically preserved material with the scanning electron microscope (SEM) represents a well-established method, which has rarely been applied to specific palaeontological studies. It is a suitable method to study external and internal structures of thin graptolite and pterobranch periderm, which would not be extractable from the rocks by acid dissolution.

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Extant Pterobranchs The knowledge about the exoskeleton variability in extant pterobranchs (Cephalodiscida and Rhabdopleurida) is crucial for the understanding of the fossilization potential and the taxonomic treatment of incomplete remains with identifiable structural details and colony design. The coenecium of Rhabdopleura is a typical colonial structure in which all zooids are interconnected by the stolon (Schepotieff 1907). Structural details of the stolon were described recently by Urbanek & Dilly (2000) for the modern Rhabdopleura compacta. The individual tubes of the colony, the exoskeleton, show a resorption foramen through which the stolon connects the individual zooids (Schepotieff 1905, 1907). The creeping tubes show a conspicuous dorsal zigzag suture where the fuselli meet, enhanced by a moderate development of a collar (Fig. 1A). The basal layer is usually structureless and smooth, as it was formed on a pre-existing surface and the development of conspicuous structures seemed not necessary. In the erect tubes of Rhabdopleura, the fuselli are more irregularly developed and consist of full rings instead of half-rings, with strong collar structures (Fig. 1A, B). This typical development of two extremely different modes of construction of the coenecium seems already to be developed in the Ordovician (Rhabdopleurites and Rhabdopleuroides: Kozlowski 1961, 1967), even though the described material consists of the erect parts of the colonies

Fig. 1. &A, B. Rhabdopleura compacta (Hincks), colony with creeping and erect tubes (A) and aperture of tube (B), also showing the conspicuous collars. &C. Rhabdopleura normanni Allman, light photograph of semi-transparent tubes and dark stolons, fuselli and collars are faintly visible in apertural parts of the colony.

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mostly. Mierzejewski & Kulicki (2001) documented both parts of the colony in Kystodendron ex. gr. longicarpus (Eisenack, 1937) from the early Darriwilian ¨ land, Sweden. Both parts of (Middle Ordovician) of O the colony were also illustrated from the Jurassic of Poland (Kulicki 1969). The shape of the colonies is highly variable in Rhabdopleura due to the surface on which it grows. Runner type colonies (Rhabdopleura normanni, Fig. 1C) and compact colonies (Rhabdopleura compacta, Fig. 1A, B) can be found. Cephalodiscus colonies show a considerable differentiation of colony shapes that is not associated with changes in the morphology of the zooids themselves. Individual tubes forming masses or bushy colonies, complex and elaborate exoskeletons with de-individualized apertures and colonial spines and even meshworks of spines without common enclosures occur in this genus (Hyman 1958). A stolon system is not developed and the zooids are individuals. The highly variable development of the coenecium in Cephalodiscus and Rhabdopleura is quite revealing, especially when the minor variation in shape and development of the zooids in the extant species is considered. Hyman (1958) stated that the zooids of the diverse subgenera of Cephalodiscus are virtually identical! In this light, the considerable infra-generic variation of their coenecia may question the erection of the high number of orders of the extinct Graptolithina (e.g. Kozlowski 1949; Bulman 1970; Bates & Urbanek 2002), based on minute details in colony design. The distinct orderly dorsal zigzag suture in creeping tubes of extant Rhabdopleura is also found in a number of Palaeozoic dendroids, especially in the Camaroidea and Crustoidea (Kozlowski 1949). Andres (1977, 1980) suggested it to be a derived character in the Rhabdopleurida by describing the irregular development of sutures in thecae of Mastigograptus and in erect tubes of Rhabdopleura as primitive. However, the erect tubes of Rhabdopleura have to be interpreted as a derived character. This can be shown by the presence of the orderly zigzag sutures in the creeping, earlier parts of the colonies. The suture types of the erect tubes in Rhabdopleura and the thecae in Mastigograptus, thus, are not strictly homologous structures. The early rhabdopleurids Idiotubus and Rhabdotubus possess only irregular sutures. The regular development of zigzag sutures in the creeping parts of Rhabdopleura species is not known in Cambrian material.

New Middle Cambrian pterobranchs

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repeatedly. The discussion heated up again since the discovery of Cephalodiscus graptolitoides Dilly from the New Caledonian Islands. Dilly (1993, 1994) described his discovery as a living graptolite and received both, support and rejection. Rigby (1993) supported the interpretation as a living graptolite. Urbanek (1994) instead, regarded Cephalodiscus graptolitoides as a pterobranch hemichordate. Most graptolite specialists recently agree that pterobranchs and graptolites are phylogenetically related. A detailed interpretation of the phylogeny of pterobranchs and graptolites appears to be required urgently, but is beyond the scope of this paper. Preliminary results indicate a misinterpretation of supposedly graptolitic characters in some ‘dendroid graptolites’. The investigation of proximal end development for planktic graptolites has been the main focus in graptolite research of recent years as this was shown to bear the phylogenetically most important and useful characters (Fortey & Cooper 1986; Mitchell 1987; Maletz & Mitchell 1996; Melchin 1998). In this respect, the initial development in pterobranchs is reviewed here and compared with the initial colony construction of the Graptolithina. The initial part of Rhabdopleura compacta (Fig. 2A) is the vesicle or dome (Stebbing 1970; Dilly 1986), sometimes misleadingly called a prosicula. It is a structureless oval body, secreted by the larva and completely seals the larva inside. The larva then develops into a zooid and resorbs a hole into one end of the dome (Lester 1988b). The founding zooid constructs the earliest fuselli of the initial tube at this secondary opening, forming already extremely regular dorsal zigzag sutures (Fig. 2A). The origin of the next zooid is through a resorption foramen in the creeping tube of the founding zooid (Schepotieff 1907), but details are not well known. This type of skeletal development is quite different from the prosicular and metasicular stages in planktic

Initial colony growth in pterobranchs and graptolites The question whether a certain fossil can be assigned to the graptolites or pterobranchs has been addressed

Fig. 2. Initial growth of the Rhabdopleurida (Pterobranchia) and Graptoloidea (Graptolithina). The longitudinal rods of the prosicula (Kraft 1926) have been omitted in the Graptolithina.

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Graptolithina (order Graptoloidea) and a direct comparison is difficult (Fig. 2B). The Graptoloidea possess a prosicula with a spiral thread (Kraft 1926: Schraubenlinie), a primary opening (prosicular aperture) and secondary longitudinal strengthening rods (Kraft 1926; Williams & Clarke 1999). In some dendroids the shape of the prosicula is slightly different due to the attachment site (Kozlowski 1949), but the spiral thread and the primary opening in the prosicula are present. None of these characteristics is present in the initial growth of the Pterobranchia. Intermediates between the pterobranch dome and the graptolite prosicula are unknown. The initial development of the coenecium in Rhabdopleura and the graptolites is not homologous with certainty as seen from the difference in their structure. The dome of Rhabdopleura is formed at an early, larval stage in the ontogeny of the founding zooid, before the development of a functional pre-oral lobe (Dilly 1986; Lester 1988a, b). In Cephalodiscus the larva develops into a zooid before settling down, but the stalk and arms are still rudimentary (John 1932, p. 201). Unfortunately, the early development of the coenecium is unknown. In the Graptoloidea and in certain dendroid graptolites, the prosicula is formed in a similar or even identical fashion to the fuselli of the metasicula and the thecae: as a band of fusellar tissue. Thus, the pre-oral lobe of the zooid must have been developed already during the planktonic larval stage. The difference in the initial construction of the coenecium or rhabdosome, therefore, can be interpreted as the result of secretion during different ontogenetic stages in the growth of the initial, founding (in pterobranchs) or sicular zooid (in graptolites). If the rhabdopleurid type is interpreted to be the more primitive one, the graptolite larva adds another developmental stage before the secretion of the coenecium or rhabdosome commences. It develops into a zooid before secreting any part of the coenecium. This simple heterochronic change in the developmental stage at which the initial skeleton is constructed can easily explain the major differences between the dome and the prosicula without the need of a completely independent origin of both structures. It would also explain the lack of intermediates between both types of initial development. Differences in the later colony development can be seen in the formation of a resorption foramen for each new thecal tube in Rhabdopleura (Schepotieff 1905, 1907, 1909). This development is not seen in any graptolites, where even the prosicula bears a primary opening and only the first post-sicular zooid resorbs a foramen into the prosicular wall (Fig. 2B). All later openings are primary and not resorbed into a pre-existing tube.

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The genus Rhabdotubus from the Middle Cambrian of Sweden was regarded as the oldest rhabdopleurid hemichordate by Bengtson & Urbanek (1986), based on the presence of a mosaic of rhabdopleurid and tuboid graptolite characters. Even though the species is known from isolated material, the early development of the colony remains unknown. All parts of the colony show irregular sutures, but some more orderly parts also exist. The erect tubes widen considerably from 0.2 mm to apertural widths of 1 mm. They sometimes show a slight development of collars around the fusellar boundaries, but in general the thecae appear identical to those of many tuboid graptolites. A sclerotized stolon system was not described. Bengtson & Urbanek (1986) assume the presence of resorption foramina for the development of thecal tubes, but were not able to unequivocally show them from their material. The genus Epigraptus Eisenack (1941) (= Idiotubus Kozlowski, 1949; see Mierzejewski 1978 and Urbanek 1986) is well known from isolated fragments of a number of species. Andres (1977, figs 27–28) illustrated a nearly complete small colony of Epigraptus, identified as Idiotubus. The creeping part of the colony appears more or less identical to that of Rhabdopleura compacta, but there is no indication of collars. The upright tubes are formed by normal fuselli with a dorsal and ventral zigzag suture instead of the rhabdopleurid full rings with the conspicuous collar. Andres (1977) also illustrated a dome-shaped ‘prosicula’ typically found in Rhabdopleura as ‘Reste des blasenfo¨rmigen Periderms der Larve’ [remains of the bubble-shaped periderm of the larva]. Based on this evidence, Idiotubus should be regarded as a typical rhabdopleurid. The investigation of structural details in these examples shows the close relationship of some dendroid graptolites with pterobranchs and the difficulty of differentiating both groups. Fossil material was generally included in the Graptolithina when fusellar structure was found, but in Rhabdopleurida when the fusellar structure showed the typical collar and complete fusellar rings of erect tubes of extant Rhabdopleura. Data on the initial colony growth suggests that the differentiation of both groups is less well constrained and that a number of so-called dendroid graptolites should be referred to the Pterobranchia instead. The typical development of the erect tubes in Rhabdopleura is a derived homology differentiating this genus from other (extinct) pterobranch genera. It is unknown whether this change in tube development is connected to changes in the anatomy of the zooids or merely a reflection of the difference of growth on a surface versus unsupported growth. However, the consistent growth of creeping and erect tubes in Rhabdotubus and Epigraptus

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requires a taxonomic and phylogenetic interpretation. Preliminary data suggest that the proximal end structure and the development of diad or triad budding mechanisms may be useful to differentiate pterobranchs and graptolites.

BSE-SEM investigation Identification of pterobranchs was only possible with the documentation of fusellar or stolonal structures in the new material. Otherwise the relationship of these organisms might have stayed unresolved. A nondestructive method to successfully identify fusellar structure is outlined below. Chemical isolation and bleaching or IR-light investigations are the only other way to identify fusellar structure in these forms. However, these would have destroyed the material and failed to reveal the internal structures. Back scatter electron (BSE) investigation with scanning electron microscopes (SEM) is a well-established method. It has been mainly applied for element determinations and element mapping of materials and rocks. BSE-microscopy has so far only rarely been applied to structural investigation of fossils. Due to a considerable penetration of elements of low atomic number by a high-energetic electron beam the method is in principle applicable for a study of internal structures of fossils in organic preservation. The investigations described herein were carried out on a HITACHI S 2700-SEM at ZELMI (TU Berlin, Germany). The fossil samples were glued on sample stubs by a carbon-glue and contacts were covered by a silver suspension to reduce electron charging. The sample was then inserted unsputtered into the


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large-scale vacuum chamber of the SEM. The back scatter electrons result from the collision of electrons of the beam with atomic nuclei of the sample material, which may additionally also release X-ray photons with an element-specific energy. The penetration depth of electrons into the sample is mainly depending on the elemental composition of the sample and the acceleration-voltage of the electron beam. A Monte Carlo simulation (Fig. 3) can be used to estimate the penetration depth of electrons into a coalified organic sample. The following maximum penetration depth of electrons (dmax) and depth of maximum X-ray photon generation (dmed) can be derived considering specific conditions for a pure carbon sample (target density = 2.25 g/cm3; specimen tilt = 0 ): acceleration voltage Eo = 10 kV Eo = 15 kV Eo = 20 kV Eo = 30 kV

dmax 1.3 mm 2.6 mm 4.4 mm 9.2 mm

dmed 0.6 mm 1.2 mm 2.1 mm 5.6 mm

It is also evident, that the majority of back scatter electrons are generated at a medium depth of total electron penetration, which roughly correlates with the number of released X-ray photons. Back scatter electrons from a greater depth are more easily absorbed. The BSE image reflects a cumulative information of internal and surface structures, depending on the ratio from which regions the back scatter electrons are derived. The depth in which most of the back scatter electrons and X-ray photons are generated (dmed) is therefore the determining factor for the BSE image of the investigated fossil sample. The non-destructive

Fig. 3. Monte Carlo Simulation of electron scattering (three cases left side) and X-ray photon generation (right) for an electron beam of SEM within a carbon sample (target density = 2.25 g/cm3; specimen tilt = 0 deg.; trajectory number = 250) at varying beam voltage. Penetration depth is indicated on the left side, depth histogram of X-ray photon generation at 30 kV shown on right side; note arrow indicating depth from which majority of X-ray photons is derived, also correlating with the origin of main information of BSE images (30 kV, tilt = 0).

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Fig. 4. &A–D. BSE images of a detail of ?Cephalodiscus sp. with fusellar structures from the Middle Cambrian Wheeler Shale at different acceleration voltage (A: Eo = 10, B: Eo = 15, C: Eo = 20, D: Eo = 30 kV) (note that with increasing Eo more internal structures from deeper regions become visible). Scale in lower right corner of figs indicates 300 mm.

method is useful for the study of internal structures of thin organic-walled fossils, such as fusellar rings of the skeletons of pterobranchs and graptolites. The limitation of the method lies mainly in the maximum acceleration voltage of conventional electron microscopes of 30 kV. It only allows the penetration of ca. 5 mm thick organic walled fossils and is in the normal technical configuration not suitable for the investigation of thicker walled material. Fig. 4 shows the penetration of pterobranch periderm at different accelerating voltage and resulting various penetration depth in our specimen of ?Cephalodiscus sp. from the Wheeler Shale. The periderm of the specimen is thin and details of the fusellar structure are visible to various degrees, even showing oblique sutures (Fig. 5D, E). Fuselli are nicely visible under the SEM (Figs 4, 5), but are difficult to observe with the light microscope. They are about 45–50 mm high and

evenly spaced, but irregular spacing and more densely spaced fuselli are present. Oblique sutures have been recognized in a few places (Figs 4, 5E). The normal light photograph of the specimen does not show the fuselli (Fig. 5A), but the SEM’s clearly indicate their presence (Fig. 5B). The SEM pictures also reveal the thickness of the wall as they show the thicker periderm at the rims of the tubes (Fig. 5E). We tried to use this method for a variety of shale material and isolated graptolite specimens, mostly planktic graptoloids, including a number of Rhabdinopora, Staurograptus and Anisograptus. In all isolated specimens we found the periderm to be too thick to be penetrated and to show useful results, unfortunately. In shale material of species with thin periderm, as exemplified by Bergstroemograptus crawfordi from the Table Head Group of western Newfoundland (Fig. 5C, F), fusellar structure can be shown, but even in this

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Fig. 5. A, B, D, E. ?Cephalodiscus sp., Wheeler Shale, Utah, TU Berlin collection – ACK, No. Whe 001. &A: light photograph of specimen. &B, D, E. SEM BSE images of details. &C, F. Bergstroemograptus crawfordi (Harris), GSC 10328, Black Cove, Table Head Group, western Newfoundland, SEM-BSE image (C) showing the resolution in the thin parts of the periderm and normal light photograph (F) lacking fusellar details. Magnifications A: 5, B: 8, C, D: 20, E: 45, F: 10.

example the periderm in some parts of the rhabdosome proved to be too thick to be penetrated.

Cambrian pterobranchs The oldest possible Pterobranchia and Graptolithina are described from the Middle and Upper Cambrian of ¨ pik 1933; Bengtson & Urbanek 1986), Scandinavia (O Spain (Sdzuy 1974), Siberia (Obut 1960, 1964, 1974), Australia (Chapman 1919; Chapman & Thomas 1936; Rickards et al. 1990), China (Lin 1985), and North America (Ruedemann 1947), but in few of them fusellar development has been confirmed. They already show a high variability in colony form and, thus, indicate a considerable time of evolution to achieve the expressed diversity in colony shapes. The recently described undetermined rhabdopleurids from the

Lower Cambrian of Zhongnan, Guizhou Province, China (Zhao et al. 1999) cannot be identified as pterobranchs with certainty. We have re-studied a few of these colonial tubular fossils by BSE-SEM. The original substance of the fossils is replaced by secondary iron minerals, which do not support the preservation of internal structures, such as fusellar rings. The Dithecoidea Obut, 1960 include erect growing colonies with a slender stem and long, trumpet-shaped, thin-walled thecae that appear to be branching off at irregular intervals, ranging from the Middle Cambrian to Ordovician. The true relationship of many of the Dithecoidea described by Obut (1960, 1964, 1974) from the Middle Cambrian of Siberia is questionable as structural details are unknown. According to Bulman (1970, p. V54) they may belong to the hydroids. Durman (1992) and Chapman et al. (1996)

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Fig. 6. &A. Tarnagraptus palma Sdzuy, holotype, SMF 30 000. &B. Tarnagraptus cristatus Sdzuy, paratype, SMF 30 023. &C, D. isolated fragments from SMF 30 002, Tarnagraptus palma Sdzuy. &E. On SMF 30008, Tarnagraptus thomasi Sdzuy, detail showing thecal aperture. &F. SMF 30028a, Ovetograptus gracilis Sdzuy, holotype. &G. SMF 30021a, Tarnagraptus cristatus Sdzuy, holotype, showing fuselli. &H. SMF 30021a, Tarnagraptus cristatus Sdzuy, holotype, showing fuselli. Magnifications: C, D: 50, E, G, H: 25, F: 4.

interpreted the Dithecoidea as predecessors of the true Dendroidea. These authors, like Sdzuy (1974), refer the Upper Cambrian to Upper Ordovician genus Mastigograptus to the Dithecoidea. Sdzuy (1974) described material from the Cantabrian Mountains of Spain under the generic names Tarnagraptus, Sotograptus, Ovetograptus and Archaeolafoea. The differences between these genera can be seen mainly in thecal spacing and size of thecae. Internal details are unknown and the genera might partly be synonymous. Our re-investigation does not show any suitable characters for their taxonomic differentiation. Thus, we discuss the material under the umbrellaname Tarnagraptus. The material is not redescribed here, as the original descriptions and illustrations are sufficient to characterize the colony outlines and dimensions (Fig. 6A–H). The reinvestigation showed a

considerable degree of distortion not readily visible in the retouched photographs of the original description. Formerly the inclusion with the graptolites had been questioned and the material has been referred to the Hydroidea (Fernańdez Remolar et al. 1996; Mierzejewski 1986). Even though coronate scyphozoans show a strong resemblance to certain dendroid graptolites (Mierzejweski 1986), a relationship of the Spanish material from the Oville Fm. (Sdzuy 1974) to the scyphozoans can be excluded due to the fusellar structure recognized during this investigation in the material, but already mentioned by Sdzuy (1974). An attempt was undertaken to dissolve fragments of unfigured material to obtain isolated periderm fragments. These isolated fragments show that fusellar structure is present in the material (Fig. 6C, D). The fuselli are about 0.01–0.02 mm wide. They show

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Fig. 7. Rhabdotubus robustus n. sp. Jince Formation, Konıćek, Barrandian, Czech Republic. &A, C–I. Details showing fuselli and stolon system. &B. holotype, Kon 001, largest specimen. &J. smaller specimen, Kon001b representing a fragment of holotype sample Magnifications: A: 10, B: 2.3, C–H: 15, I: 18, J: 4.

oblique sutures visible in a single fragment, found also in the shale material (Fig. 6G, H). Thus, the indication shows a likely relationship to the pterobranchs. The presence of fuselli in Tarnagraptus and of related forms supports a relationship to the pterobranchs. There is no information on the branching style in the remaining dithecoids. The genera Mastigograptus and Micrograptus have recently be separated from the Dithecoidea and referred to the new order Mastigograptida by Bates & Urbanek (2002), based

on the typical triad budding found in isolated material and the ‘distinct non-dendroid construction of the stipe’. The presence of distinct triad budding in the Mastigograptida relates these to the Graptolithina, but similarities to the Dithecoidea may be superficial and non-indicative of a phylogenetic relationship. Acknowledgements. – SEM Photos were taken at ZELMI of TU Berlin. Technical assistance of Jo¨rg Nissen is gratefully

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Fig. 8. Rhabdotubus robustus n. sp. &A. Holotype, No. Kon 001, loc. Konıćek, Barrandian, Czech Republic. &B. Fragment showing fuselli, enlargment from holotype. &C, D. Stolon system, details of holotype. &G. Kon 001b, detail showing stolons. &E, F, H, I. loc. Luh, Skryje, Barrandian, Czech Republic, original Boucek collection at Czech Geological Survey, Prague. &F. specimen showing thick periderm at base of theca, distally theca peeled off. Magnifications: A: 1.5, B–D, F: 40, E, H: 4, G: 20, I: 25. acknowledged. Type material was provided by Jean Dougherty (GSC Canada, Ottawa), Eberhard Schindler (Forschungsinstitut Senckenberg, Frankfurt am Main, Germany), Petr Kraft (Charles Univ. Prague), Petr Budil (Czech Geological Survey, Prague). Work

of Michael Steiner was supported by a Feodor Lynen Scholarship of the Alexander von Humboldt Foundation. The Lethaia referees R.A. Fortey and anonymous provided constructive criticism that improved the manuscript.

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Robison, R.A. 1989: Middle Cambrian biotic diversity: examples from four Utah Lagersta¨tten. In Simonetta, A.M. & Conway Morris, S. (eds): The Early Evolution of Metazoa and the Significance of Problematic Taxa, 77–93. Cambridge University Press, Cambridge. Ruedemann, R. 1947: Graptolites of North America. Geological Society of America, Memoir 19, 1–652. Sato, T. 1936: Vorlaüfige Mitteilung u¨ber Atubaria heterolopha gen. nov. sp. nov., einen in freiem Zustand aufgefundenen Pterobranchier aus dem Stillen Ozean. Zoologischer Anzeiger 15, 97–106. ¨ ber die Stellung der Graptolithen im Schepotieff, A. 1905: U zoologischen System. Neues Jahrbuch fu¨r Mineralogie, Geologie und Palaöntologie 1905, 79–90. Schepotieff, A. 1907: Die Pterobranchier. 2. Teil. Cephalodiscus dodecalophus. 2. Abschnitt. Knospungsprozess von Cephalodiscus. Zoologisches Jahrbuch, Abteilung fu¨r Anatomie 25, 125–150. Schepotieff, A. 1909: Die Pterobranchier des Indischen Ozeans. Zoologisches Jahrbuch, Abteilung fu¨r Systematik 28, 429–448. Schram, F.R. 1991: Cladistic analysis of metazoan phyla and the placement of fossil problematica. In Simonetta, A. & Conway Morris, S. (eds): The Early Evolution of Metazoa and the Significance of Problematic Taxa, 35–46. Cambridge University Press, Cambridge. Sdzuy, K. 1974: Mittelkambrische Graptolithen aus NW-Spanien. Palaöntologische Zeitschrift 48, 110–139. Stebbing, A.R.D. 1970: Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata). Marine Biology 5, 205–212. Urbanek, A. 1976: The problem of graptolite affinities in the light of ultrastructural studies on peridermal derivatives in Pterobranchs. Acta Palaeontologica Polonica 21, 3–36. Urbanek, A. 1986: The enigma of graptolite ancestry: Lessons from a phylogenetic debate. In Hoffman, A. & Nitecki, M.H. (eds): Problematic Fossil Taxa, 184–226. Oxford University Press, New York Clarendon Press, Oxford. Urbanek, A. 1994: Living non-graptolite. Lethaia 27, 18. Urbanek, A. & Dilly, P.N. 2000: The stolon system in Rhabdopleura compacta (Hemichordata) and its phylogenetic implications. Acta Palaeontologica Polonica 45, 201–226. Urbanek, A. & Towe, K.M. 1974: Ultrastructural studies on graptolites, 1: The periderm and its derivatives in the Dendroidea and in Mastigograptus. Smithsonian Contributions to Paleobiology 22, 1–48. Williams, S.H. & Clarke, L.C. 1999: Structure and secretion of the graptolite prosicula, and its application for biostratigraphical and evolutionary studies. Palaeontology 42, 1003–1015. Zhao, Y., Steiner, M., Yang, R., Erdtmann, B.-D., Guo, Q., Zhou, Z. & Wallis, E. 1999: Discovery and significance of the early metazoan biotas from the Lower Cambrian Niutitang Formation, Zunyi, Guizhou, China. Acta Palaeontologica Sinica 38 Supplement, 132–144.

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Material. – One specimen from the Ptychagnostus atavus Zone of upper Wheeler Shale of the House Range, Utah, USA, Middle Cambrian. Description. – The specimen is about 25 mm long and 11 mm wide. It consists of a number of associated tubes, about 0.5–1.0 mm wide and only slightly widening towards their apertures. The apertures reach a maximum width of 1.1 mm. The individual tubes are about 7–22 mm long. As the specimen is a fragment, the origin and connection of the tubes is unknown. Apparent branching of the tubes in the lower part of the colony may be produced by overlapping tubes and is not considered to be true branching. Remarks. – The specimen is here identified as ?Cephalodiscus sp. It is preserved in dark gray shale of Wheeler Shale, where it resembles in colony habitus the thalli of the alga Yuknessia simplex (Robison 1989, fig. 3.3). The investigated specimen was previously identified as a fossil alga, but the presence of fusellar structure indicates the relationships to the pterobranchs. It is very probable that more pterobranch colonies will be discovered at a closer inspection of those algae. The specimen in general appears very similar to Cephalodiscus (Orthoecus) solidus, both in size and in observable structure. It represents the earliest pterobranch remains that resemble Cephalodiscus colonies. Kozlowski (1949) described Eocephalodiscus polonicus from the Ordovician of Poland, based on two chemically isolated specimens. The specimens are quite different from all extant cephalodiscid colonies described so far. Cephalodiscus lutetianus Abrard, Dollfus & Soyer (1950) from the Eocene of France shows very regular zigzag sutures, unusual in any Cephalodiscus.

Order Rhabdopleurida Fowler, 1892 Appendix Phylum Hemichordata Bateson, 1855 Class Pterobranchia Lankester, 1877 Order Cephalodiscida Fowler, 1892 Family Cephalodiscidae Harmer, 1905

Remarks. – The Rhabdopleurida are characterized by a true colonial organization and a creeping mode of growth with the main axis fixed to the substrate. The erect apertural parts of the thecal tubes seem not to branch. They can be differentiated from the Cephalodiscida by the presence of a stolon system and the interconnected tubes. Differences also exist in the development of their zooids (Hyman 1958).

Genus Cephalodiscus McIntosh, 1882

Genus Rhabdotubus Bengtson & Urbanek, 1986

?Cephalodiscus sp. Figs. 4, 5A–E

Remarks. – Rhabdotubus obuti (Durman & Sennikov, 1993) was thought to be the oldest true Rhabdopleura

Taxonomy

LETHAIA 38 (2005)

species. It appears to be closely comparable to those of a runner type colony of Rhabdotubus and differences may largely be ecological. Thus, this species is referred to Rhabdotubus herein. There is no differentiation of the fusellar style between the creeping and the erect tubes as in modern Rhabdopleura, and occasionally branching of erect tubes seems to occur. Rhabdotubus obuti can be interpreted easily as a primitive rhabdopleurid, but cannot be accomodated in Rhabdopleura. Durman & Sennikov (1993, p. 284) indicated the lack of cortical bandaging in Rhabdotubus obuti (Durman & Sennikov), but discussed (p. 287) the presence of secondary thickening due to the proximally darker and distally lighter coloured tubes. This would indicate that cortical thickening occurred in this species. The reconstruction of Rhabdotubus obuti in Durman & Sennikov (1993, text-fig. 4) shows a terminal bud that has not been described or illustrated for the species and appears to be inferred from the description of modern Rhabdopleura (Schepotieff 1907, 1909). Rhabdotubus robustus n. sp. Figs 7, 8 Material. – Two specimens from Konıćek, Czech Republic (Figs 7, 8A–D). The exposed succession at Konıćek in the Prı´bram – Jince basin of the Barrandian area of the Czech Republik is about 3 m thick (Havlıćek 1970). The strata consist mainly of siltstones with shale interlayers of the Jince Formation and belong to the local Middle Cambrian Ellipsocephalus hoffi – Rejkocephalus Assemblage zone, which roughly correlates with the Ptychagnostus punctuosus Biozone of Scandinavia. A few specimens from Luh, Skryje-Tysovice basin, Czech Republic from the historical Boucek collection, housed at Czech Geological Survey, Prague (Fig. 8E, F, H, I). The succession at Luh (Kettner 1923) starts with a basal breccia bed above Proterozoic spillites. The trilobite association of the Skryje shale at Luh suggests a correlation either with the Eccaparadoxides pinus Zone or the Eccaparadoxides insularis Zone of the Middle Cambrian of Scandinavia. This is in so far of interest as the first occurrence of Rhabdotubus johanssoni Bengtson & Urbanek comes from the Eccaparadoxides pinus Zone of central Na¨rke (Sweden).


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Holotype. – Specimen Kon001 (Figs 7B, 8A) from the Jince Formation at Konıćek locality is housed at Charles University Prague, Palaeontology Dept. Description. – The holotype covers a surface of about 7020 mm (Figs 7B, 8A). It is preserved in a number of pieces that might belong to a single colony of a sessile pterobranch with a strong creeping axis and a high number of laterally positioned, slowly widening tubes, projecting to both sides of the axis. The main axis branches at several points to produce lateral, secondary axes. The main axis usually reaches a maximum width of about 1 mm, and has a slightly irregular shape. It is more strongly thickened by cortical tissue than the thinner lateral axes and the erect tubes. The erect tubes or thecae are slowly widening from initially less than 0.4 mm to 1.3 1.5 mm at the simple and straight apertures. Branching was not observed in these up to 10 mm long thecal tubes. The tubes clearly show the presence of fuselli, spaced at about 8 14 fuselli per 1 mm, but oblique sutures have only rarely been found (Fig. 8D, E). A stolon system is preserved in a few places (Fig. 8C, D, G I), but may have been present in the whole colony. The width of the stolons is about 0.02 mm. Bifurcation of the stolons has not been observed due to the high coalification of the main axis. Remarks. – Rhabdotubus robustus n. sp. shows a compact and robust central axis, from which the thecae grow upright. It is not clear from the preservation whether the axis is free or incrusting growing on the sediment surface, but an encrusting growth is assumed, supported by the extremely irregular growth of the axis. The details of these thecal origins are obscured by the cortical cover, but may include considerable prothecal overlap. The colonies can be interpreted as of a runner-type organization, similar to modern Rhabdopleura normanni. Stolons are only found in the creeping parts of the colony. The specimens from Luh (Fig. 8E, F, H, I) do not show the well developed axis and we initially considered the material to represent a separate taxon. The thecal tubes appear to be slightly less wide and more parallel-sided. However, the data are insufficient to understand the exact development of the colonies and we, thus, prefer to leave them with Rhabdotubus robustus n. sp.