Kenrick and Vinther 2006 Chaetocladus

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Chaetocladus gracilis n. sp., a non-calcified Dasycladalesfrom the Upper Silurian of Skne, Sweden

Paul Kenrick a,, Jakob Vintherb

aDepartment of Palaeontology, The Natural History Museum, London SW7 5BD, England, United Kingdom

b Geological Museum, University of Copenhagen, ster Voldgade 5-7, 1350 Copenhagen, Denmark

Received 16 September 2005; accepted 21 March 2006Available online 17 August 2006

Abstract

A rare non-calcified dasycladalean alga Chaetocladus gracilis n. sp. is described from the Upper Silurian (Ludlow) BjrsjlagrdLimestone (Klinta Formation; ved-Ramssa Group), Skne, Sweden. C. gracilis comprises a slender rod-like thallus witheuspondyl, acrophorous laterals arrayed in more or less equally spaced verticils. In its morphology Chaetocladus converges onseveral other groups of contemporaneous flora (e.g., charalean algae, land plants) and fauna (e.g., graptolites), and the potential ofmisattributing dasyclads of this type to other groups such as green algae and in particular to land plants is noted and discussed. The

preservation of such a delicate thallus suggests that special taphonomic conditions prevailed over parts of the BjrsjlagrdLimestone, involving rapid burial in anoxic mud and low levels of bioturbation. The presence ofChaetocladus is also indicative of anunusual floral/faunal assemblage that has been termed a thallophytic-alga-dominated biota. Assemblages of this type are

characterized by thallophytic algae, annelid worms, lightly sclerotized arthropods and a low diversity shelly fauna. 2006 Elsevier B.V. All rights reserved.

Keywords: Dasycladales; Chaetocladus; Silurian; Sweden; Graptolite; Charales

1. Introduction

The affinities of fossils are often puzzling, and this canlead to doubtful taxonomic assignment, fostering contro-

versy over issues such as the time or place of origin ofmajor groups of organisms. This is particularly true inancient rocks and during formative periods when body plans were developing and modern lineages werediverging from a plethora of long extinct types. Oneproblem is convergence, and one common form on whichseveral groups of organisms have converged indepen-

dently is the simple appendage-bearing axis. One sees thistype of organisation in the stipes of graptolites, in landplants where it has been acquired independently in theleafy branches of the true mosses, the leafy liverworts,and

in the distantly related clubmosses, and it is encounteredagain in the green algae particularly among the Dasycla-dales and the Charales. In these groups this simpleappendicular structure belies three fundamentally differ-ent forms of construction. Graptolites are colonialorganisms, land plants are fully integrated multicellularindividuals, and the Dasycladales have a siphonousorganisation. Differences such as these would seem toform a good basis for formulating a series of criteria forassigning fossils to groups. However, when the tapho-nomic history is such that soft tissues are not preserved

Review of Palaeobotany and Palynology 142 (2006) 153160www.elsevier.com/locate/revpalbo

Corresponding author.E-mail addresses: [email protected] (P. Kenrick),

[email protected] (J. Vinther).

0034-6667/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.revpalbo.2006.03.023
mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.revpalbo.2006.03.023http://dx.doi.org/10.1016/j.revpalbo.2006.03.023mailto:[email protected]:[email protected]


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and all that remains is little more than a thin film ofcarbon, distinguishing among these and other alternativescanbe tricky. Graptolites and conodonts have been namedasplants(Gabbott et al., 1995; Kenrick et al., 1999), greenalgae have been assigned to the graptolites (LoDuca,

1990; LoDuca, 1997), and within the green algaeDasycladales can be confused with Charales (Kenrickand Li, 1998; Feist et al., 2003). The affinities of manyother putative early plant fossils are doubtful (Chaloner,1960; Lundblad, 1972; Lemoigne, 1988; Zhang, 1988;Cai et al., 1996; Yang et al., 2004). One consequence ofmisattribution is that it has led to many questionable earlyrecords of land based plant life in the Lower Palaeozoic.

Here we document an enigmatic appendage-bearingaxis from the Silurian of Skne, Sweden. Its affinities aredifficult to determine, and the specimen shows attributes

of several distantly related groups of organisms. Weinterpret this fossil as a new species of Dasycladales inthe genus Chaetocladus (LoDuca, 1997). Dasycladaleshas a long and highly diverse geological history that isdominated by calcareous forms (Berger and Kaever,1992). Chaetocladus is a non-calcified genus. The ab-sence of a robust carbonate skeleton means that thisdelicate siphonous alga is very much rarer in the fossilrecord than its carbonate encrusted relatives, and it isonly preserved under exceptional taphonomic conditions(LoDuca, 1997; LoDuca et al., 2003).

2. Material and methods

The description is based on a single coalified adpres-sion preserved in a fine-grained calcareous sediment. Thespecimen was collected from an old quarry near the

village of Bjrsjlagrd (Fig. 1) (Jeppson and Laufeld,1986). The sediments exposed here comprise a unit ofsilty mudstone with intercalated thin limestone bedsformed between two major biostromal carbonate succes-sions (Fig. 2). In addition to Chaetocladus, other col-lectable fossils include articulated crinoids, trilobites,molluscs, scolecodonts, eurypterids, and brachiopods.These sediments form part of the Bjrsjlagrd Lime-stone, which is the uppermost member of the KlintaFormation. The Bjrsjlagrd Limestone Member hasbeen correlated to the BurgsvikandSundreFormations on

Gotland (Larsson, 1979; Jeppson and Laufeld, 1986;Grahn, 1996) and to the uppermost Eke and BurgsvikFormations also on Gotland (Eriksson, 2002). These areall late Silurian in age (late Whitcliffian: upper part of theLudfordian stage of the Ludlow epoch).

The Klinta Formation andthe vedFormation togetherform the Upper Silurian ved-Ramssa Group (Jeppsonand Laufeld, 1986), which is exposed at various placesthroughout Skne (Scania) in the downfaulted ColonusShale Trough (Erlstrm et al., 1997). These sedimentswere deposited during the closing of the marginal basin

Fig. 1. Map of Scania (Skne) outlining Lower Palaeozoic bedrock of the Colonus Trough (large black area). The white dot marks the Bjrsjlagrdlocality. Modified from Bergman et al. (2004).

154 P. Kenrick, J. Vinther / Review of Palaeobotany and Palynology 142 (2006) 153160


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between the Baltic and the Avalonian continental plates.Shallow-water deposits of mudstone and biostromalcarbonates mark the transition from the underlying thicksuccession of Colonus Shale into the Klinta Formation.The overlying ved Formation is a shallow marine sandydeposit. Above the ved-Ramssa Group there is a majordepositional hiatus until the Triassic Period.

The specimen was imaged using two methods. Lightmicrographswere made usingcross-polarized illuminationto enhance contrast between the adpression of Chaeto-cladus and the surrounding sediment (Rowe, 1999).Scanning electron micrographs were obtained usingbackscatter electrons on an uncoated surface (Collinson,1999). The specimen was scanned in a low-pressurevacuum machine (LEO 1455 VP) at about 1520 Pa and15 kV.

3. Results

3.1. Systematics

Order Dasycladales (Pascher, 1931)Family Triploporellaceae (Berger and Kaever, 1992),emend (LoDuca, 1997)Tribe Salpingoporelleae (Bassoullet et al., 1979), emend(LoDuca, 1997)Subtribe Chaetocladinae (LoDuca, 1997)Genus Chaetocladus (Whitfield, 1894), emend(LoDuca, 1997)Species Chaetocladus gracilis new speciesSpecific diagnosis: Thallus non-calcified, slender (thal-lus b3 mm wide; axis 1.5 mm1.7 mm wide, N80 mmlong), rod-like, with euspondyl, acrophorous laterals;verticils more or less equally spaced at 0.3 mm

intervals, composed of whorls of 3040 laterals, eachat least 1.5 mm long and about 50 m in width. Prob-ably endosporate.Locality: Bjrsjlagrd Quarry, Skne, Sweden. Fordetails see Jeppson and Laufeld (1986).Age: Late Silurian (Ludlow).Repository: The Geological Institute and the GeologicalMuseum, Copenhagen, Denmark.Holotype: MGUH 27641; Plates I and II.Etymology: The specific epithet gracilis refers to the

slender, thin or slim nature of the thallus.

3.2. Description

The new species is based on a single axis measuringover 8 cm in length and 1.5 mm to 1.7 mm in width(Plate I, 1). The specimen is unbranched, but it isincomplete at the apex and at the base. It does not appearto taper appreciably over its length, and obvious repro-ductive structures were not observed. The most strikingaspect of this fossil is its segmented appearance, and this

is caused by whorls of hair-like filaments (laterals) thatare distributed at more or less regular intervals of about0.3 mm (Plate I, 12; Plate II, 1). Viewed in profile, eachlateral has a broad base, estimated to be about 100 mwide, which tapers to a narrow, parallel-sided filamentabout 50mthick(Plate II, 4). The lengths of laterals aredifficult to gauge. Some appear to be very short, whereasothers are much longer. It is likely however that all weremore or less the same length but that most are veryincomplete. Laterals are at least 1.5 mm long, and all areborne at an acute angle. Overall, therefore, the thallus isslender, and its total width (b3 mm) does not greatlyexceed that of the axis itself (1.5 mm1.7 mm).

Fig. 2. Picture (left) of the exposure at the Bjrsjlagrd locality, marked as Bjrsjlagrd 2 in Jeppson and Laufeld (1986). The outline drawing(right) shows the two limestone units and the mudstone unit from which Chaetocladus was collected.

155P. Kenrick, J. Vinther / Review of Palaeobotany and Palynology 142 (2006) 153160


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We estimate that the number of lateral appendages perwhorlisoftheorderof3040 filaments.This is double thenumber that is visible on the axis surface. The reason forthis is that visible filaments actually underlie the axis oremerge from the sides. They therefore represent roughly

half the filaments that one would expect in the completewhorl. The other half would have been borne on thecounterpart, which is missing. We are able to show that thelaterals underlie the axis and are therefore attached to oneside only by comparing scanning electron micrographswith light micrographs of comparable areas(Plate II, 12).The circular attachment points and the filaments are clear-ly visible with light microscopy (Plate II, 1), whereas onlythe circular attachment points are discernable using elec-tron microscopy (Plate II, 23). This implies that thelaterals but not their attachment points are covered by a

thin film of sediment that is partially translucent to lightbut opaque to backscatter electrons. In other words, thevisible lateral filaments all underlie the axis.

4. Discussion

Fossils of the sort recorded here pose problems ofinterpretation because there is so little structure preservedand there are no remnants of internal soft tissues. The formof preservationa thin layer of carbonis itself suggestiveof a relationship with algae or plants rather than animals.Could the specimen be a land plant or perhaps a member of

the closely related charophycean algae? Such fossils areexceptionally rare in Silurian sediments, and any find couldbe of great potential significance. One needs therefore to beespecially cautious in attributing affinity and in criticallyanalysing the available evidence. The scientific literaturecontains examples of organisms attributed to the land plantsthat on further investigation turned out to be algae, hemi-chordates, or animal parts (e.g., Gabbott et al., 1995; Ken-rick et al., 1999). Others are simply too poorly documentedto fully justify a taxonomic assignment (e.g., Kenrick et al.,1999; Lundblad, 1972; Lemoigne, 1988). We therefore feel

that it is important to state in a very explicit way why weattribute this fossil to Dasycladales rather than to landplants, charophycean algae, or indeed the graptolites.

4.1. Comparisons and affinities

Large macroscopic green algae in the Charales (stone-worts) andland plants in the Equisetales (horsetails) have a

highly distinctive jointed appearance with parts borne inwhorls. In thisrespect they resemble the fossil documentedhere, but the similarity is superficial. The leaves andleaflets of both Charales and Equisetales are larger andmuch fewer in number. The leaf-like laterals of Charales

are more properly termed lateral branches, and thesedichotomise in some genera. In living Chara and Lam-prothamnium an additional whorl of stipulesspine-likecellsoccurs just below each whorl of leaves. LivingNitella, Nitellopsis, and Tolypella do not possess stip-ules, yet all Charales have conspicuous external maleand female gametangia borne in the axils of leaves andleaflets. Stipules and external gametangia have notbeen observed in Chaetocladus gracilis. With regard toland plants in the Equisetales, C. gracilis would be mostcomparable to a lateral branch. These bear minute mi-

crophyllous leaves that are united for at least part of theirlengths forming a sheath around the stem. Furthermore,the internodal regions have very clear and distinctivelongitudinal ribs that reflect the underlying vasculature.Neither sheathed leaves nor longitudinal ribs were ob-served in C. gracilis. Modern Equisetales are of coursemulticellular plants with a complex internal anatomyincluding vascular tissues. They possess a robust cuticu-larised epidermis and a particularly rough textured sur-face caused by the deposition of massive quantities ofsilica (opal). Stomata are arrayed in longitudinal rows infurrows between ridges.Careful inspection of the surface

ofC. gracilis using scanning electron microscopy wouldbe expected to yield some evidence for these anatomicalfeatures, however none was observed. Our analysis doesnot provide any evidence to support a relationship withCharales, Equisetales, or land plants in general.

A further possibility is a relationship with graptolites,especially the most comparable forms of dendroid grap-tolites in the family Inocaulidae. On close inspection,however, the lateral filaments do not show any of thefeatures of graptolite thecae. Furthermore, they are bornein distinct whorled arrangements, which is not a char-

acteristic of graptolites. Also, even thoughincomplete ourspecimen is unbranched, which is inconsistent with themorphology of most if not all dendroid graptolites. Sim-ilarities to the unbranchedMedusaegraptus (Ruedemann,1925) and to Diplospirograptus (Ruedemann, 1925) arenot indicative of graptolite affinity because these generahave recently been reinterpreted as dasycladalean algae(LoDuca, 1990, 1997).

Plate I. Chaetocladus gracilis sp. nov. Holotype specimen MGUH 27641.

1. Axis bearing lateral filaments. Note segmented appearance. Scale bar= 3 mm.2. Detail of axis showing underlying filaments. Scale bar =1 mm.



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The new fossil most closely resembles rare non-calcified dasycladalean algae in the genus Chaetocladus(LoDuca, 1997). Key similarities include an unbranchedthallus bearing more or less close-set whorls of un-branched laterals (1040 per whorl). Of the five species

recognised by LoDuca (1997), Chaetocladus gracilismost closely resembles three of the more slender ones. C.ruedemanni (LoDuca, 1997) possess an axis twice asstout and a wider thallus overall with laterals some 4 timesthicker. C. dubius (Spencer, 1884) emend (LoDuca,1997) has a marginally narrower axis but a wider thallus.The laterals are of broadly similar width. In both speciesthe verticils are more extended, being repeated at 1 mmintervals as opposed to the 0.3 mm intervals in C. gracilis.C. capillatus (Heg and Kiaer, 1926) emend (LoDuca,1997) is the closest species. The axis, thallus, and laterals

are all about twice the width of those ofC. gracilis,yetthethallus is somewhat shorter. In summary, C. gracilis isconsidered to represent a distinct species because it is themost slender yet documented bearing the shortest or leastdistinct laterals. The differences noted here are compara-ble to those that have been used in the segregation of otherspecies within the genus (LoDuca, 1997).

4.2. Distribution and palaeoenvironment

In 1997, LoDuca published a critical review of thegenus Chaetocladus recognising a total of six species.

These are known from only a handful of sites in NorthAmerica and northern Europe spanning an age range ofMid Ordovician to Early Devonian. The rarity of Chae-tocladus is attributable partly to the delicate nature of thethallus and partly to the narrowly constrained environ-mental conditions in which it thrived. This distinctivedasyclad seems to be associated with rather unusual floral/faunal assemblages, and these have been termed thallo-phytic-alga-dominated biotas (LoDuca, 1997). Typically,they comprise several species of thallophytic algae, an-nelid worms, lightly sclerotized arthropods (e.g., eur-

ypterids, phyllocarids) and a low diversity shelly fauna.Chaetocladus gracilis is found in association witheurypterids, scolecodonts and lightly sclerotized bivalvedarthropod carapaces indicating that similar associationsprevailed over parts of the Bjrsjlagrd Limestone. Yet, a

diverse brachiopod fauna and the occurrence of variousmolluscs would seem to be atypical. Further collecting isneeded to clarify more precisely the associated faunaldiversity.

Modern dasyclads are abundant in clear, calm, warm,

shallow marine environments, as typically developed intropical lagoons, interreef areas, and embayments. Thishabitat interpretation is consistent with the sediments inwhich Chaetocladus is found. Typically, these are fine-grained laminated limestone, dolostone, and shale that capsharp-based normally graded beds that are essentiallydevoid of bioturbation. At the Bjrsjlagrd Quarry lo-cality, the mudstone containing the fossil is intercalated between two larger carbonate units. The BjrsjlagrdLimestone member is characterized by great lateral vari-ation, and this can be interpreted as biostromes isolated

from each other by stagnant waters comprising muddysediments. Thefossilisation of such a delicate non-calcifiedsiphonous alga is also indicative of a distinctive tapho-nomic environment that in addition to a lack of bioturbationis characterized by poorly oxygenated conditions at thesedimentwater interface and episodic sedimentation in-volving rapid burial in anoxic mud (LoDuca, 1997). Suchconditions might also have occurred in stagnant but pe-riodically storm-agitated lagoons or embayments.

The discovery of rare non-calcified fossil algae such asChaetocladus provides additional insights into thegeological history of groups that are known principally

through biomineralized forms. The fossil record ofDasycladales is dominated by species that develop anencrusting layer of calcite or aragonite. These carbonateminerals are far more resilient than the delicate siphonousthallus, which they envelop preserving the original algalcell as a cast. Despite the fact that the fossil record ofmineralized Dasycladales is lengthy and diverse (Bergerand Kaever, 1992), there is some evidence to suggest thatthe time of origin of certain key features and groups hasbeensignificantlyunderestimated. Based on non-calcifiedspecimens from China, Kenrick and Li (1998) documen-

ted unequivocal evidence of choristospore gametangia inthe Early Devonian, implying a very large range extension(ca. 240 million years) to the family Dasycladaceae.These same data also extended the known range of a modeof reproduction involving operculate cysts from the Early

Plate II. Chaetocladus gracilis sp. nov. Details of holotype specimen MGUH 27641.

1 and 2. Details of axis in same field of view imaged using two different methods:1. Cross-polarized light.2. Scanning electron microscopy (SEM) (backscatter electrons).Scale bars= 1 mm. Comparison of the two images illustrates that all the filaments visible in image 1 actually underlie the axis.

3. SEM (backscatter electrons) of filament attachment points. Scale bar= 200 m.4. SEM (backscatter electrons) of filament (incomplete at apex). Scale bar= 200 m.



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Mesozoic to the mid Palaeozoic. Similarly, the presenceof euspondyl branching in Chaetocladusa key feature indasycladalean systematicsextends the stratigraphicrange of euspondyl taxa from the Early Devonian to themid Ordovician (LoDuca, 1997). Evidence such as this

indicates that there exists a significant representational bias in the fossil record of Dasycladales, favouringheavily mineralized forms. The distribution, diversity, andprobably also the ecological significance of non-miner-alized or weakly mineralized species of Dasycladales inthe warm, shallow marine environments of the Palaeozoicare very probably underestimated.

Acknowledgements

We thank Dr. Ben Williamson and Dr. Alex Ball

(NHM) for assistance with scanning electron microsco-py and Mr. Phil Crabb (NHM) for the light micrographs.Thanks also to Dr. Per Ahlberg (Lund) and Dr. EckartHkansson who provided valuable information onlocality and geology, and Dr. David A. T. Harper whointroduced the authors and provided very helpful crit-icism. Paul Kenrick gratefully acknowledges MurielFairon-Demaret's help and support during his tenure as aRoyal Society Research Fellow in Lige in 1989.

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