Lethaia Seminar Lerhaia, Vol. 26, pp. 184-185. Oslo, 1993 06 I5

Burgess Shale biotas: burrowed away? PETER A. ALLISON AND DEREK E.G. BRlGGS

The evidence of the Burgess Shale-type biotas is fundamental to any attempt to understand the nature of the Cambrian radiation. A large numberoflocalitiesofbwerand MiddleCambrianagearenowknown to yield Burgessshale taxa (Conway Moms 1989). Theamount ofevolution represented by the diversity of animals preserved has been exaggerated by some authors (e.g.. Could 1989) and the evidence suggests that disparity among Cambrian animals hardly exceeded that among the living biota (Briggs er a/. 1992a; but see Foote &Could I992 Briggs era/. 1992b). The stratigraphic ranges of some of the soft-bodied taxa far exceed those of most shelly organisms in coeval shallow-water habitats, suggesting an evolutionary conservatism (Conway Morris 1989). Analyses like these, of the stratigraphic ranges and extinction of Burgess Shale taxa, and rates of morphological evolution during theCambrian, rely on assumptions about the nature of the fossil record.

A number of extant soft-bodied groups, such as the polychaetes and priapulids. are represented in the Burgess S h a k .Where they are not represented in younger rocks this must be a taphonomic effect (Allison & Briggs 1991a. b). Thus it is clear that taphonomiccontrolsaccount at least partly for the rarity of Burgess Shale type faunas in late Cambrian and subsequent strata. Likely influences are those which are a consequence of biotic evolution, such as diversification of infaunal organisms (Sepkoski 1982; Plotnick 1986; Allison & Briggs 1991a, b. 1993). Aronson (1992) explored the roleofincreased bioturbation in thedisappearanceof Burgess Shale-type faunas. He considered the effect of increased sediment rework- ing, and of bioturbation to greater depths, and concluded that other taphonomic factors such as sedimentation or diagenesis are likely to have played a role.

Aronson ( 1992) compared the number of Burgess-type faunas found in a given stratigraphic interval with that expected where a random distribu- tion is assumed. He then calculated the amount of bioturbation required to explain departures from random. The amount of additional bioturbation required (a 42-fold increase in intensity or depth based on Cambrian vs. post-Cambrian, or a 16.5-fold increase based on Cambrian vs. Ordovician-Silurian) is not supported by evidence from the rock record. Thus Aronson ( 1992) rejected bioturbationasacompleteexplana- tion for the disappearance of Burgess Shale-type faunas.

It is likely that a variety of factors, taphonomic and evolutionary, contributed to the disappearance of the Burgess Shale biotas. Thus Aronson ( 1992) wascorrect, but aspectsofhisapproach and assumptions resulted in an underestimate of the importance of bioturbation. These are discussed here in order to encourage further investigation of the controls on the preservation of Burgess Shale-type biotas - a process critical to a complete understanding of their wider evolutionary significance.

Aronson’s (1992) estimates of the increase in bioturbation required to prevent the preservation of Burgess Shale-type faunas were based on the proportion of examples in the Cambrian compared to that in later Palaeozoic strata -a ratio of 352 (i.e. 0.950.05) (data of Conway Morris 1989). Thiscontrasts with an expected ratioof0. I 1:0.89, ifthedistribution of Burgess Shale-type faunas through the Palaeozoic were random. The Burgess Shale-type biotas are taxonomically defined, however; Aronson’s compilation ignores all other preservations of marine soft-bodied biotas, which are just as likely to have been affected by bioturbation. Other compilations of soft-bodied preservations reduce the dominance of the Cambrian, even though exceptional preservations are over-represented compared to the rest ofthe Phanerozoic (Allison & Briggs 1991a. 1993).

Allison & Briggs (1993) used the preservation of traces of the most labile tissues (e.g., muscle) as a threshold to ensure some comparability between soft-bodied biotas through the Phanerozoic; the ratio of numbers of Cambrian to later Palaeozoic marine examples isY:lO (0.470.53). and 9 3 (0.750.25) if the Cambrian is compared to the Ordovician and Silurian alone. Because it is taxonomicallydefined, the list of37 Burgess Shale-type faunas compiled by Conway Morris (1989) and used by Aronson ( 1992) includes many localities which yield only decay-resistant structural tissues (arthropod carapaces, for example). The equivalent number for the later Palaeozoic would far exceed the 10 included in the list ofbiotas displaying the highest gradeofpreservation (Allison & Briggs 1993). Plotnick (1983) listed 99 marine eurypterid localities in the Ordovician-Devonian, for example. Thus the increase in bioturbation required to explain the reduc- tion in soft-tissue preservation in post-Cambrian times would be much lower than that calculated by Aronson (1992).

Anadditionalproblem,asidentified by Aronson (lW2),isaIackofdata that would allow changes through time in specilicenvironmentals4rringsto be monitored. Most, but not all, Burgess Shale-type faunas occurred in deeper water (Conway Morris 1989). Major compilations on trace fossils are based on a variety of sedimentary environments and facies (Thayer 1983; Bottjer & Ausich 1986) as are our data on soft-bodied preservations (Allison & Briggs l99la, 1993). There is, however, some evidence (Bottjer er a/. 1988) of an onshore-offshore trend in trace-fossil distribution. Deeper burrowers appear to diversify first in shallower water and then migrate to deeper settings.

The disappearance of Burgess Shale-type faunas in the late Cambrian coincides with a striking reduction in the prevalence of soft-bodied pres- ervation. Bioturbation remains a possible candidate to explain this. Thus taphonomic changes artificially reduce the stratigraphic ranges of some Burgess Shale taxa. There are, however, no echoes of the Burgess Shale assemblage in thenext.albeit inadequate, sampling point, thelateOrdovi- cian kcher’s Trilobite Bed of New York State. It Seems likely that these Cambriananimalsgraduallybecameextinct during the lateCambrian and Ordovician (Conway Morris 1989).

Acknowledgemenrs. - We thank V.P. Wright, N. Hughes and M.L. Droser foradviceandcomments. Our research is funded by North AtlanticTreaty Organization grant CRC 910085 (to PAA) and Natural Environmental Research Council grant CR3/7235 (to DECB). University of Reading Postgraduate Research Institute for Sedimentology Contribution No. 279.

References Allison. P.A. & Briggs, D.E.G. 1991a: Taphonomy of non-mineralized

tissues. In Allison. P.A.. & Briggs, D.E.G. (eds.): Taphononty: Releasing rhe Data Locked in the Fossil Record 2540. Plenum, New York.

Allison. P.A. & Briggs, D.E.G. 1991b The taphonomy of soft-bodied animals. In Donovan, S.K. (ed.): The P r o c ~ r c e s o t j o n , 120-140. klhaven. New York.

Allison, P.A. & Briggs, D.E.C. 1993 Exceptional fossil record distribution of soft-tissue preservation through the Phanerozoic. Geology 21. in press.

Aronson, R.B. 1992: Decline of the Burgess Shale fauna - ecologic or taphonomic restriction? Lerhaia 25.225-229

L I T H A I A 26 ( 1993) Letlzaia Seminar 185

Ih)ttict-, l).l. & i\u\ich, ".I. 19x6: I'hanerivoic development of tiering in \olt-sitl)str&i \iis~~etision-feeding communities. Pdeobiology 12, 400- 420.

Ib)ttler, I ).I., I)roser, M.I.. 8( Jablonski, I). IYXX: Palaeoenvironmental trcnds i n the history oftrace fossils. N[tfitre 332, 252-255.

lhggs, l).l:,C;., I:ortcy, K.A. &Wills, M.A. 1992a: Morphological disparity i n [lie (hiI~r i . i r i . Siriviw25h. 1670-1673.

Ihiggs, l).l:..C;., Fortcy, K.A. & Wills, M.A. 1992b: Cambrian and Kecent t ~ i o r i ~ ~ i o ~ o g t c ~ i ~ dispdrity - Iksponse. Siivici~ 2%'. 181 7-18 18.

. 1989: The persistence of Burgess Shale type faunas: implic.itions tor the evolution ofderper-water faunas. Trmtsoctiom of r h . h p l .Sol.i,v}, o j E d i i h t r S h XD, 27 I-2x3.

Footc, hl . CY: C~o~tld, S.1. 1992: Cmibrian and Recent morphological

G(iukI, S.I. IYXY: \V~irider\Id l.i/i,. Uic I5rrrgtw Shrile orid thc Nrrttiri~ of disp,lt-ity. Si-icrrc.i,2.iH, 1816.

l l i s t ~ i r , ~ ~ . 347 pp. Norton, NKW York.

[ Plotnick, K.E. 1983: Patterns in the evolution o f the Eurypterids. 41 lpp. Unpublished Ph.D. thesis, University of Chicago. 1

Plotnick, R.E. 1986: Taphonomyofa modern shrimp: implic~itions for the arthropod fossil record. Pdriois I , 286-293.

Sepkoski, 1.1. Jr. 1982: Flat-pebble conglomerates, storm deposits and the Cambrian bottom fauna. I t 1 Einsele, G. & Seilacher, A. (eds.) Cyi-/k.ri!id Everrt Strntificntioti, 371-385. Springer, Berlin.

Thayer, C.W. 1983: Seditiient-mediated biological disturhance and the evolution of marine benthos. / , I Tevesz, M.I.S. & McOall, P.1.. (eds.): Biotic /!itenrctiotrs i r r Kecwrt orid Fossil /5rritlric ~ , . [ ) r t r r~ i i~rr ; f i [ , s , 479425. Plenum, New York.

Peter A . Allison, f'osfgrmhr/e Kescmdr IristitrtfcBfi?r . S ~ * ~ ~ J r r i i , r i t c i k ? ~ , 1'. 0. I5ox 227, The Uriiversity, W h i f e k i i i ~ h t s . Kecidirtg KGh LAR, U.K.; I)c*rck E.(;. Hrigqs, Uep . ofCeology, Hristol Utiiversity, Qitci*ti's Rotid, /<risfoI, IjS8 I ' I R , U.K.; received 2nd Apri l 1993.

Burgess Shale-type biotas were not just burrowed away: reply RICHARD B. AIiONSON

M y paper ( Arotisoii 1992) examined the temporal distribution of Burgess Shde-type launas. As Allison & I3riggs ( 1993) correctly state, these are taxonomically rather than taphonomically defined assemblages. One- tailed hinomial tests revealed that I3urgess Shale-type faunas are non- rdndomly distributed in the fossil record, declining precipitously after the Cambrisn. Wh.it .ire the processes behind this pattern? Based on current knowledge of how hioturbation changed through the Palaeozoic, I sug- gested (Aronson 1992) that increasing bioturbation alonecannot account for the observed decline.

Allisoil & Hriggs ( 1993) propose a taphononiic null hypothesis for testing increased bioturbation as a cause of the decline of Burgess Shale- type t'tunab after the Cambrian: compare the stratigraphic distribution of dll well-preserved, soft-bodied faunas to thedistribution ofthe (taxonomi- cally defined) Ihtrgess Shde-type faunas. Changes in the occurrence of well-presrrved, soft -bodied faunas should reflect maxinially the impact of shmging bioturb.ition. I calculated expected proportions, based on Alli- mi 8i Briggs' ( 1993) data on soft-body preservation, to make binomial cotiqurisons corresponding to those presented by me (Aronson 1992). The observed V ~ L I K S 'ire the satiie as before, but the expected proportions lor these coinprisons .ire more conservative than the expected propor- tlons of Aronscin ( 1992); there is now a gre,iter likelihood ofaccepting the null hypothesis thdt increasing bioturbation alone accounts for the ob- .\crvcci pattern. Although the one-tailed binomial probabilities, P, are higher than for the corresponding comparisons of Aronson ( 1992), they <ire still highly significant in both cases (P<<0.01; Table I ) . Therefore, increasing bioturb.ition was not the sole cause of the decline of Burgess Sh.ile-typc f'iiiunas 'ifter the Cambrian.

;I~Arriiivhr~~~.,rii.rifs. -This work was funded by the Institute of Marine and (:i)&istd Sciences, I<utgers University, with additional support from the Smith~otii'in Institution. Institute of Marine and Coastal Sciences Contri- hution No. 93-18,

?itble I . Expected proportions ( P r ) and one-tailed binomial prohabililies ( P ) for tests of decreasing occurrence of Uurgess Shde-type laun.is in adjacent stratigraphic intervals. The observed ratio is 3 5 2 in all compari- sons.

Comparison Pr I'

Tmottotiiic t i i t / / hjprhesis (Arorisoti 1992) A Cambrian 0.1 10

B Cambrian 0.239

Tnphonomic riull hypothesis (Allisori 8 !<rigs IYY3) A Cambrian 0.474

B Cambrian 0.750

Ordovician-Permian 0.890 1.49XlO

Ordovician-Silurian 0.761 6 .88~10 'I'

Ordovician-Permian 0.526 x.70x 10 I"

0.250 2.08X10 \ Ordovician-Silurian

References Allison, P.A. & Briggs, D.E.G. 1993: Burgess Shale biotds: burrowed away?

Aronson, R.B. 1992: Decline of the Burgess Shale fauna: ecologic o r Lethoia 26, 184-185.

taphonornic restriction? Lethnio 25, 225-229.

Richord B. Arorzsorz, Iristitirte of Mnriric mid Corrs/cil Scirriccs, Rrr/'qm Uriiversity, New Brutiswick, NJ 08903, USA; mid I l ip ir f t r totf o//,iver/ehrcifc Zoology, Nritioriril Mtrseurri of Nriturcil History, S r r t i f h w ~ r i ~ i r ~ lmfitufiotl, Wfishirigtori, DC 20560, USA; received .?/st Aprif I Y93.