Realistic interpretation of ichnofabrics and palaeoecology ofthe pipe-rock biotope

DUNCAN MCILROY AND MICHAEL GARTON

The meticulous study of ichnofabrics and palaeobio-logical interpretation of behaviour are challenges thatlie at the heart of rigorous ichnological analysis.Recent study of Cambrian Skolithos ichnofabrics fromthe north-west Scottish Highlands has attempted toshed light on the palaeoecology and evolution of theearly Cambrian endo-benthos (Davies et al. 2009).The work presented herein highlights some of thedifficulties in attempting nearest neighbour analysison ancient bedding surfaces, and provides some sug-gestions for how palaeoecological analyses might beapproached in pipe-rock facies in future.

The greatest challenge to serious study of ichnofa-brics in the Eriboll Formation is the lack of lithologicalcontrast between the burrows and host sediment(Fig. 1A). This is less pronounced in the ‘trumpetpipe’ (Monocraterion-rich) interval due to diageneticenhancement of the fabric (Fig. 1B). At most strati-graphic levels in the Eriboll Formation, pervasivequartz cementation requires that special techniques beemployed to study ichnofabric in detail (Garton &McIlroy 2006; Fig. 1D, E). The recognition of theupper burrow terminations that could be consideredcolonization surfaces is, in our experience, compli-cated by the observed tendency of some Skolithos inthe Eriboll Formation to deviate from the verticalthrough short oblique kinks (cf. Fig. 1C), and for out-crop surfaces to be variably and irregularly oblique tothe vertical plane (see Davies et al. 2009, fig. 3C). Inour experience, upper and lower terminations of bur-rows within pipe-rock ichnofabrics are difficult todetermine in the field (cf. Davies et al. 2009, fig. 3A,C) but may be determined with careful observation(Bromley 1996) and can be demonstrated in the labo-ratory through creation of large thin slices (Garton &McIlroy 2006; Fig. 1F). At sites with net deposition,the Skolithos trace makers are forced to extend theirburrow upward to compensate for aggradation of thesediment–water interface.

Lower Palaeozoic pipe-rock ichnofabrics have beenstudied by a number of authors (e.g. Pemberton &Frey 1984; Droser & Bottjer 1989; Droser 1991;McIlroy 2004; McIlroy & Garton 2004). These studiesinclude discussion of bioturbation intensity and its

estimation in Skolithos ichnofabrics using flashcards(Droser & Bottjer 1986, 1989). The importance ofSkolithos pipe-rock stems from: (1) its potential toprovide evidence for the early colonization of mar-ginal marine settings; but also (2) the presence of sig-nificant hydrocarbon reserves in such faciesthroughout northern Africa, the Middle East and inother peri-Gondwanan regions (McIlroy & Garton2004).

Conceptual models for the interpretation of pipe-rock ichnofabrics have been proposed to focus obser-vations of ichnofabric development with respect tocolonization, sedimentation rate and changing palaeo-environmental stress and model ichnofabrics (McIlroy2004; McIlroy & Garton 2004; Figs 2, 3). These mod-els can be used to augment descriptions of bioturba-tion intensity (cf. Droser & Bottjer 1989). Applicationof these ichnofabric-based models to the Eriboll For-mation requires careful documentation of ichnofaunaand sedimentary facies over large lateral distances, andis a work in progress.

Ichnofabrics, ichnocoenoses andpalaeoecology

One of the great challenges to ichnologists and pal-aeoecologists is that the ideal unit of study, the eco-logical community, is difficult to unequivocallydetermine. In the analysis of shelly faunas, a numberof factors including: differential preservation, strati-graphical condensation, winnowing and transport ofbioclasts work against the study of true communitystructure (e.g. Kidwell & Flessa 1996; Kidwell 2002;Olszewski & Kidwell 2007). Ichnological studiesbenefit from the fact that re-worked trace fossils aregenerally obvious (Bromley 1996). Unfortunately,however, the trace fossils present in a single bed orrelated to a single colonization surface seldom repre-sent the work of a single contemporaneous com-munity (see discussion in McIlroy 2004). Ichnologicalanalysis of ichnofabrics commonly aims to studythe fundamental unit of ichnocoenosis (see Lessertis-seur 1955; Radwanski & Roniewicz 1970; McIlroy

DOI 10.1111/j.1502-3931.2009.00199.x � 2009 The Authors, Journal compilation � 2009 The Lethaia Foundation

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Lethaia, Vol. 43, 420–426. Copenhagen, 2010

2004), which is generally considered to be, at best,an approximation of the ecological compositionof the behaviours represented in the benthic com-munity (McIlroy 2004, 2008). Ichnologists do notgenerally consider ichnocoenoses to represent truly

ecologically pure communities but accept the limita-tions of the ichnological record. This is especially trueof low diversity to monospecific ichnofaunal assem-blages that characterize the pipe-rocks of the EribollFormation.

A

D

2 cm 2 cm 1 cm

1 cm 1 cm

1 cm 1 cm 1 cm 2 cm

F G H I

E

B C

Fig. 1. Photographs of Pipe-Rock Member ichnofabrics. A, Skolithos from the Pipe-Rock Member showing upper bedding termination (left)and oblique cross-section of a Skolithos pipe (right) (from south-east of An Teallach, south of Little Loch Broom). B, pipe-rock ichnofabricshowing well-developed Monocraterion ‘trumpet pipe’ (from Coir a Ghiubhsachhaim, south of Little Loch Broom). C, Skolithos showing ashort horizontal section found in association with a mudstone parting. D, cut and polished surface of a laboratory sample of the Pipe-RockMember. Most ichnofabrics become clearer on slabbed surfaces, but the lithological homogeneity and pervasive cementations make such anapproach un-rewarding (from 3 m above the base of ‘Trumpet Pipes’, Loch Dubh, Beinn Dearg, Ullapool). E, large thin slice of the samespecimen as in (D) showing how the technique of Garton & McIlroy (2006) can aid in the study of pipe-rock ichnofabrics by enhancing con-trast. F, a rare example of the lower end of Skolithos showing a bulb-like termination, as seen in large thin slice (from 3 m above base of‘Trumpet Pipes’, Loch Dubh, Beinn Dearg, Ullapool). G, H, example of cross-cutting of two Skolithos burrows, such an interaction could bedue to rare downward branching, a predatory interaction between two adjacent trace makers or cross-cutting from higher stratigraphic levels(Road cutting east of Quinag). I, bedding plane expression of a pipe-rock ichnofabric (from 12 m above the base of the ‘Trumpet Pipes’ LochDubh, Beinn Dearg, Ullapool) showing several specimens of Monocraterion showing mutual interference ⁄ cross-cutting of the cones. Conesare seen to show cross-sections of various levels of the Monocraterion cones, which are also seen to be cut by Skolithos of various diameters.

LETHAIA 43 (2010) Lethaia Discussion 421

Attempts at ecological analysis of pipe-rock beddingplanes using nearest neighbour analysis on beddingplanes (Pemberton & Frey 1984; Davies et al. 2009)need to unequivocally demonstrate an absence ofmultiple colonization surfaces (see McIlroy 2004, fig.9). This is a particular problem when consideringbedding plane expressions of ichnofabrics dominatedby deep burrowing ichnotaxa such as Skolithos inwhich the depth of burrowing is commonly greaterthan the thickness of bedding. The recent study ofSkolithos palaeoecology from the Eriboll Formation(Davies et al. 2009) involves nearest neighbouranalysis on small bedding plane surfaces but does notpresent any fabric data to demonstrate the absence ofcross-cutting of the sediment surface under study bySkolithos from the overlying bed. The argument that amore uniform distribution of burrows is good evi-dence for a single generation of bioturbators (Davieset al. 2009) does not take into account the mutualavoidance behaviour of the Skolithos trace maker. Inour extensive experience of pipe-rock ichnofabrics,cross-cutting is extremely rare (Fig. 1G, H). Skolithospipes may cross the outer margins of Monocraterioncones, and Monocraterion cones rarely cross-cut oneanother (in such cases there is usually evidence for aphase of erosion separating the two cones), but thedistribution of Skolithos (and Monocraterion) pipes isstrongly controlled by mutual avoidance (Fig. 1I).This means that, as successive generations of Skolithosshow mutual avoidance, nearest neighbour studiesthat only consider bedding plane relationships would

generally appear to represent the work of a singleichnocoenosis even if multiple colonization events arepresent. The statistical validation of the presence of asingle community showing a tendency towardsuniform spacing could be the result of a combinationof cross-cutting from higher stratigraphic levels,superimposed upon what may indeed have been anapproximation of community structure. From ourexperience of burrow densities in modern depositionalsetting, patchiness is a common and important phe-nomenon, and one that is inadequately incorporatedinto ichnological analysis (McIlroy 2007).

Identification of Skolithos andMonocraterion relating to a singlecolonization surface

The identification of trace fossils relating to a singlesedimentary surface is a significant challenge anddemands the presence of a distinctive characteristic ofthe junction between the burrow and the sediment–water interface on the preserved bedding plane. Thiscondition is met in the type material of Monocrateriontentaculum, which is expressed as a depressed clay-draped cone with tentacle-like surface grooves (cf.Jensen 1997, figs 38, 39) but is very rarely found inpipe-rock facies.

Sea floor erosion and re-colonization is a com-mon feature of pipe-rock facies. In many cases,

Fig. 2. Triangle diagram showing the interactions between sedimentation rate, and the contrast between continuous sedimentation and epi-sodic deposition, which can be used to infer the conditions experienced by the infauna in pipe-rock facies (modified from McIlroy & Garton2004).

422 Lethaia Discussion LETHAIA 43 (2010)

Monocraterion provides good evidence for the degreeof erosion experienced by a given burrow (Fig. 4). It isconsidered that the presence of a structure-less fill to aMonocraterion cone (Fig. 4C) is suggestive of castingof the colonization surface. By contrast, a concentriccircular cross-section is diagnostic of partially erodedMonocraterion (Fig. 4D). Deeply eroded Monocrateri-on only show the Skolithos expression (Fig 4E),although some Skolithos may not have developed theMonocraterion cone. In our experience, concentriccross-sections of the Monocraterion cone can be foundat least 30 cm below the former sediment–waterinterface.

We do not consider that concentric cross-sections of Monocraterion-type cones constitute evi-dence for a colonization surface (cf. Davies et al.2009, fig. 4D). Rather, the presence of concen-tric cross-sections conclusively demonstrates thaterosion has occurred, and that the surface shouldbe considered suspect as a suitable bedding planefor ecological study. This is due to the potentialfor both pre- and post-erosion communities beingrepresented on the same surface (Fig. 5C). This isnotwithstanding the potential for cross-cutting bydeep Skolithos from much higher stratigraphiclevels (Fig. 5E), which would have to be excluded

Fig. 3. The matrix of example ichnofabric icons shows the response of ichnofauna to a range of palaeoenvironmental conditions. Comparisonwith ichnofabric trends in the field can be used to infer palaeoenvironmental change, sedimentation style and help recognize hiatal surfaces.Complex tiering is not characteristic of the low-diversity pipe-rock ichnofacies, but is included here for completeness (from McIlroy 2004).

LETHAIA 43 (2010) Lethaia Discussion 423

by study of the overlying (eroded) bed in adjacentoutcrops.

At present, there are not any established objectivecriteria for distinguishing between uneroded anderoded Skolithos. Consequently, it is difficult to acceptthat Davies et al. (2009) can know that the Skolithos-dominated bedding plane represents the burrows of asingle community rather than the effect of compoundcolonization (sensu McIlroy 2004), cross-cutting rela-tionships, or a combination of both. We would, how-ever, greatly welcome publication of any such insight.

Demonstration of biologicalsynchroneity

The assertion that burrow-spacing reflects the distri-bution of organisms in a community is of fundamen-tal importance to an attempted study of meaningfulnearest neighbour analysis. The analysis, and the eco-logical or palaeobiological interpretations that arisefrom it, rely upon the confident recognition of syn-chroneity. This is considered to be a non-trivial task.Ichnologists will sometimes consider that burrowswith the same lithological fill are likely to be roughlycontemporaneous, and certainly if burrows with thesame fill can be directly related to a single surfacethere would be little contention. Burrows in the same

bed having a different passive fill are not generallyconsidered to be biologically contemporaneous.Unfortunately, the Pipe-Rock Member is comprisedof lithologically homogeneous quartz-rich sandstone;so, this approach cannot be used.

Our experience, from unpublished laboratoryexperiments is that: (1) mucous-supported burrowsin sand-rich substrates can remain open for monthsafter burrow abandonment; (2) a single burrowingorganism can cause more than one open burrow tobe present at any one time if the burrow is aban-doned; and (3) the presence of mobile sands inassociation with open (abandoned) burrows enablesrapid casting, thereby preserving multiple phases ofburrowing in association with the same coloniza-tion surface but would not produced by contempo-raneous organisms. Consideration of these factorsgives us considerable cause to treat with scepticismthe nearest neighbour analyses of Davies et al.(2009) which rely on all the burrows preserved ona single bedding plane being concurrently occupiedby the trace makers.

With these complications in mind, future field-basedstudies of nearest neighbour analysis require not onlysections through the studied bedding planes (includingstrata above and below the plane) to help establish thedegree of cross-cutting from higher stratigraphic levels(see Figs 4, 5) but also some means of determining the

AB

C

D

EF

Fig. 4. Block diagram showing the morphology of Monocraterion and its various expressions. A, Monocraterion cone as it would have beenseen on the ancient seafloor. B, same Monocraterion with the sediment made transparent to show the three-dimensional representation of thecone-in-cone structure, which is variable from a single cone to a stack of cones up to 30-cm deep with a central pipe which extends below thecone by up to tens of centimetres. C, the presence of a simple sand-filled Monocraterion surface depression, as in this diagram, is taken as rea-sonable evidence for the casting of an ancient sediment–water interface. D, cross-section through the upper (aggradational) level of theMonocraterion cone showing the concentric bedding plane expression that can be used to demonstrate that erosion has taken place (noticethe prominent central pipe, which is usually composed of clean, lithologically distinct sandstone). E, cross-section through the basal portionof the same Monocraterion cone showing closely spaced concentric cross-sections of the cone, much narrower than in the aggradational por-tion of the cone. F, deep erosion of a Monocraterion can lead to the preservation of a Skolithos-like bedding plane expression.

424 Lethaia Discussion LETHAIA 43 (2010)

presence of the sediment–water interface experiencedby the infaunal community (see Fig. 4).

Community structure of beddingplane assemblages and modernendobenthos

The density of vertical burrows recorded from Pipe-Rock Member bedding planes (7500 Skolithos shaftsper m2) is compared by Davies et al. (2009) with themuch lower densities of Diopatra cuprea in modernmarginal marine settings (up to 83 burrows per m2

Skoog et al. 1994). Diopatra is, however, a chimney-building polychaete and thus unlikely to be analogousto either Skolithos or Monocraterion (for which noevidence of a chimney has been recorded). It isdifficult to infer from the simple vertical tubularmorphology the behaviour and affinities of theSkolithos trace maker. It is interesting to note thatthe facultatively deposit- ⁄ detritus- ⁄ suspension-feedingpolychaete worm Nereis diversicolor (Herringshawet al. in press) exhibits very high population densities(2000–4000 individuals per m2; Koretsky et al.2002). This, however, is even exceeded by tube build-ing polychaete Spiophanes wigleyi, which can reach

densities of 98 000 per m2 (Featherstone & Risk1977). From this it is concluded that burrow densitiesand biomass in marginal marine settings can be veryhigh, certainly within the range of burrow densitiesrecorded from the Pipe-Rock Member of the EribollFormation, but that it is difficult to infer the biomassper unit area of ancient sedimentary units, particularlywhen considering burrows with simple verticalgeometries.

Conclusion

Palaeoecological analysis at the level of ecologicalcommunity is an extremely difficult task. There isconsiderable interest in the evolution of behaviour ofthe earliest Palaeozoic infaunal ecosystems and theirrole in modifying the biosphere (e.g. Droser & Bottjer1993; Brasier & McIlroy 1998; McIlroy & Logan1999). We agree with Davies et al. (2009) that adetailed palaeobiological and ecological understandingof the pipe-rock biotope is of great intellectual meritbut consider that there are flaws in the methodologycurrently used to determine the palaeoecologicalparameter of nearest neighbour in pipe-rock facies. Inparticular, their conclusion that cross-cutting fromhigher stratigraphic levels was not a significant

A

B

C

D

E

Fig. 5. A series of block diagrams to show the potential range of conditions that could lead to a bedding-plane assemblage of the type studiedby Davies et al. (2009), open burrows are represented with a white fill, filled burrows and cones are stippled, host sediment is grey. A, biologi-cal community with all burrows synchronously occupied by Skolithos (simple pipes) and Monocraterion (pipes with conical aperture), ifinstantaneously cast then this surface could be used for nearest neighbour analysis. B, progressive burial of the initially colonized surface byclimbing-ripple cross-lamination, with some tubes being cast (i), others showing escape behaviour producing Monocraterion cones (ii) andre-colonization from above by transported adult burrowers producing Skolithos (iii). C, Erosion of the climbing ripples in B producing a mix-ture of bedding plane expressions including: (i) Skolithos expression of a Monocraterion (cf. Fig. 4F); (ii) Monocraterion cone cross-sections;(iii) cast Skolithos from before burial; (iv) cast Monocraterion from before burial. D, with continuing deposition from (B) there is somecontinued equilibration (i), re-colonization by adult recruitment of Skolithos trace makers (ii), development of new Monocraterion cones (iii)E, erosion of (D) to a level equivalent to that in (A and C) results in a bedding plane assemblage similar to that in (C) but rich in palimpsestSkolithos-type bedding plane expressions of Monocraterion (i) and Skolithos (ii).

LETHAIA 43 (2010) Lethaia Discussion 425

ichnofabric-forming process in the Pipe-Rock Mem-ber cannot be securely deduced as: (1) the Monocrate-rion-bearing bed seems to have been subject toerosion and may include both pre- and post-erosionichnocoenoses represented in the ichnofabric; and (2)the Skolithos-rich bedding plane is similarly not dem-onstrated to be free of cross-cutting from higherstratigraphic levels. This work includes conceptualmodels to demonstrate the relative degree of erosionin ichnofabrics that include Monocraterion, but atpresent there is no means of confidently determiningwhether Skolithos burrows on any given bedding planewere biologically synchronous.

Our earlier work has proposed models to try toexplain the variety of ichnofabrics seen in pipe-rockfacies in terms of the interplay of a variety of palaeo-environmental stresses, especially sedimentation rate(McIlroy 2004; McIlroy & Garton 2004). Laboratorymethods have been developed for study of pipe-rockichnofabrics that avoid some of the flaws inherent instudying ichnofabrics in outcrop (Garton & McIlroy2006). It is considered that the ecological interpreta-tions based on nearest neighbour analysis of ancientbedding plane assemblages (Pemberton & Frey 1984;Davies et al. 2009) are overly ambitious and require ahigher level of analysis than is usually undertaken inancient successions. Inferring the detailed ecology ofpipe-rock facies requires careful palaeobiological anal-ysis integrated with detailed sedimentological under-standing.

In summary, the pipe-rock biotope encompasses adiverse range of habitats, and, while it is admittedlyrestricted in ichnodiversity, the ichnofabrics produceddemonstrate adaptation to a wide range of palaeoenvi-ronmental conditions. The low-diversity ichnofabricsof the Eriboll Sandstone are considered to reflect theactivity of a highly successful pioneer organism inthe early phase of ecological expansion into a non-uniformitarian marine environment.

Acknowledgements. – We thank two anonymous reviewers,N. Tonkin, A. Liu and C. Phillips for their comments on this work.Our ongoing work on the Eriboll Sandstone is supported in partby and NSERC Discovery Grant and award of a Canada ResearchChair to DMc.

Duncan McIlroy [dmcilroy@mun.ca], Department of Earth Sciences,Memorial University of Newfoundland, St John’s, Newfoundland,Canada NL A1B 3X5; Michael Garton, Department of Earth Sci-ences, University of Liverpool, Brownlow Street, Liverpool L69 3BQ,UK; Manuscript received on 7 ⁄ 4 ⁄ 2009; manuscript accepted on4 ⁄ 8 ⁄ 2009.

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