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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.

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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.