Late Cambrian hard substrate communities from Montana/ Wyoming: the oldest known hardground encrusters

CARLTON E. BRETT. W. D.4VID LIDDELL X S D KRAlG L. D E R m E R

Brerr. Carlton E.. LiddeU. W. David & Dersder. Krlie L. 198310 15. h t e Cmbnan hard substrate LETHAIA cammunitin imm ~onunwWyornmg. (he oldcsr *now hardpound encrusten irmaia, ib~. LC. pp.

a 31-3 0 5 1 0 . ISSE WS-1163.

W a r d p n d surfaces from rhe Late Cambnan S n o w Range Formatron In Moulonran~IVyomtng are the oldest known non-reefal hard substrat& exhtb~tlng encrusrlng losals. These surfaces range In age from Early Francon~an to early TrempenIeauan. Hardpunds were developed on slighrly hurnrnoclrr tv planzr. truncated surfaces of glauconire-nch. carbonarc. Bat pebble mnglornerates, which were dewsired dunng episodes of stom scouring In shallow subt~dd cnwmnments of the IviontanuWromtng shelf Soon7 Range hardgrounds are encrusted by a low dixcmty assemblage of fossiIs dornlnaterf by srmple &xo~Cal hoidfasrs of peImatozoans. probably cnno~&. and indudlog mati conlcal spon~ornorph &at ' and grobable stromatolites. M a a o w n n g (e g. Tcpm~res) are notably absent from aU nardground su&zes. although sharpwalled. vemcal. cylindrical holes (bonngs') o m In mlcnte <lasts [mbedded rn c e m n flat pebble conglomerata Yo endcnce of faunal succession or microecologc pacrttionlng ur ~rreguiuiar surfaces was obxrvcd on there Cambnan hardgrounds. O Hmdgromcls, epibionrr rracrohor ings. p h t o z o a n echmmdarrrr. palewcology, Cumbmn. .MonmdWyornrrrg.

Cafi~0.n E Bren, Depamnenr of Geologicul Sciences. E!e Universiq of Rochesrer, Rochester. .hew York , 1452.7: W. Davtd Liddell. Department of Geology. Utah Stute L'ruversq, Logan, Uruh 8133-7. Krarq L.

Render. Depamnenr of Geology. Unlvers i~ of .\'ew Orleans, New Drieanc. Louulona -OIL?. 9th Sep rmber, 1982.

Hardgrounds afford a uaique opportunity for pa- leoecological studies. Btcause of their predami- nantly encrusting and endoliihic life modes. hard substrate faunas are frequentiy preserved in siru on hardpunds, thereby permitting accurate re- consmction of the densirv, diversity, sparial pat- terns, and, occasionally. competitive interacrions of epifaunal organisms in ancient marine cornmu- nities (Palmer & Farsich 1974; Palmer & Palmer 1977; Brett & Liddelt 1978; Fksich 1939; Palmer 1982). Although e n m t i n g cavity-dwelling fau- nas (coelobites) have recently been described &om cryptic environments in Lower Cambrian archaeocyathid reefs (lames et al. 1977; Kobluk & James 19791, the oldest hardground-encrusting communities described ro date are from the Mid- dle Ordovician (Palmer & Palmer 1977; Brett & Liddell 1978). Recently, several hardgrounds in intraclastic limestones have been identified from the Snowy Range Formation (Upper Cambrian) of the western MontanaiWyoming border area (aIso recorded by Palmer 1982). Sedimentologic fcatures indicate that these hardground surfaces were formed by submarine lithifiation in near- shore shod areas.

Snowy Range hardgrounds lack macrobonngs but are enmsted by stromatolites, spongio-

motph algae? and pelrnatozoan echinoderm holdfasts. The latter provide examples of some of the tarliest known anached echinoderms.

Stratigraphic and geographic setting The Upper Cambrian Snowy Range formation comprises 30 to over 100 rn of interbedded silty shales, rhin sandstones and carbonates (Dorf & Luchman 1940; Lochman 1957; Grant 1965: Lochman-Balk 1472). This formation, the youn- gest Cambrian unit in the northern Rocky Moun- tains, crops out in the Horseshoe Hills. Bridger, Beartooth and Snowy ranges of south central Montana and northwestern Wyoming. Easrward in Montana, Wyoming and the Dako t s the Snowy Range passes into undifferentiated silty shales, sandstones and thin carbonate pebble conglomerates of the Upper Cambrian Dead- wood Formation. In northwestern Montana. the stratigraphid1y equivalent Red Lion Formation consists predominantly of carhnares, with an abundance of limestone pebble conglomerates. Thus, the Snowy Range represents a transitional facies, intermediate between nearshore ciastics and outer-sheif carbonates (Lochman-Balk

S- C. E. Brett, W. D. L idse i l and K. L. Dersde

Eq. I . Loatron map of Grave Cnxk (GCJ and Clark Fork f C n -ions in rhr: Beanmrh Mountains. Montana and Wyo- ming.

192). West of Yellowstone Park in Montana and Idaho equivalent age strata (Dresbachian- Trempealeauan) have been eroded prior to the Late Devonian (Lochman 1957; Grant 1965).

Snowy Range smra have been subdivided into three persistent members (Grant 1965). A lower Dry Creek Shale consists of abour 15 rn of black- ish-green to purpiish silty shales with thin silt- stones, sandstones and arenaceous carbonates. The middle Sage Member consists of a thick sequence (36-67 m) of greenish-gray, soft shale -5th abundant interbedded greenish carbonate, fiat pebble conglomerates (inttiisparmdites). cal- carenites fbiosparites), calcisiltites (biopelmi- crites and biopelsparites) and columnar to tabu- lar algal limestones (Grant 1965). The upper Grove Creek Member, which is locally partially or completely truncated by Ordovician through Devonian erosion, is a variable unit of C-12 m thickness, comprising sdey dolostones, greenish, dolomitic shales, and, near the base, rhick-bed- ded, pebble-cobble conglomerates (Grant 1965).

Biostratigraphic studia of Snowy Range trilo- birts (Grant 1%5) indiate that the age of this formation is medial to upper Late Cambrian (uppermost Dresbachiaa to lower Trempea- Ieauan). In the B ~ d g e r f inge , the matigraphic antact between the Sage and Grove Creek Members coincides with the FranconianRrem- pealeaurn Stage boundary, but at localities far- ther east, up to 30 m of the upper Sage beds may

also be incIuded in the basal Trempealeauan I!- lnenwzcs Zone (Grant 1965). The Snouy Range Formation is exposed be-

neath cliffs of the resistant Bighorn Dolomire (Lower Ordovician) on the southern periphery of the Beartooth Range dong the MontanaWpo- ming border. from just east of YelIowstone Park to near Red Lodge, Montana. tower portions of the formation are typically covered with talus but the Sage and Grove Creek members are weH exposed at several IocaIifies, In the presenr study, two sections of the Snowy Range Forma- tion were measured and studied in detail as fol- lows (Rg. 1): (1) Exposures above the steep northwest-facing sourh bank of the north fork of Grove Creek. about 8 km (5 mi) south of Red Lodge, Carbon Countv, Montana (3 112. XW 11 4 sec. 26. T 85, R ZOE; Red Lodge 15' Quadran- gie; see Dorf & Lochman 1940; Grant 1965). (7) Outcrops above a tributary gully along the north- em side of the canyon of Clark Fork or' the Yellowstone Ever , IQ krn west of h~ghway 170. arrd approximately 3 krn south of the Montana- Wyoming border and about 15 km south of Ccoke City. Montana; Park County. Wyoming (sea. 6 , J . T 56X, R lO3W; Deep Lake 15' Quad- rangle).

Sections. measured downward from the base of the Bighorn Dolomite, cornpraz the entire Grove Creek and the upper quarter to third of the Sage Member. Both localities expose a 3 to 4 rn upper interval of yellowish-weathering: silty doiostones. dolomitic shakes and flat pebble con- glomerates, (upper submember of the Grove Creek), underlain by 7 to 10 m of yellow~sh- green shale. dolomitic calcisiltites and yellow- weathering, rounded pebble-cobble conglorner- ates (lower Grove Creek submember). Following the usage of Dorf & L o c h a n (1940:54) and Grant (1965:14-16), the basal contact of the low- er Grove Creek submember is placed at the low- est bed of orange-stained conglomerate with well-rounded, red- to greenish-stained, perforat- ed (bored?) carbonate pebbles. In both sections this unit is followed by a long covered slope. presumably shale. Lawest measured intervals were a series of greenish-gray flat pebble con- glomerates, csilcisiltites and s e e n shales (Fig. 2 ) of the upper Sage Member. Encrusted hardgounds, described herein, oc-

cur primarily in the upper Sage Member at both sites. Rounded, pebble conglomerates with per- forated (bored?) carbonate clasts occur in the werfying Grove Creek Member, but no encrust-

ing fossils were observed on the upper surfaces of these beds. Both sequences belong to the lower Trempealeauan Stage.

A single example of a pebbIe conglomerate with encrusting &noid(?) holdfasts was also ob- rained from about 13 m above the base of the Snowy Range Formation at the Qark Fork local- ity- This unit occurs aboat a meter above a col- umnar alga1 iimestane ledge (Collenia m y n a ) marking tie base of the Sage Member, but beIow brachiopod coquinoid limestones ( E o o r r h ~ X e r - amera subzone; Grant 1965). This portion of the Snowy Range Formation belongs to the Eirinia trilobite zone of the lower Franconian Stage. Ths unit is the lowest observed Snowy Range hardground and the holdfasts on it appear to represent the oldest known hardground encrust- ing fauna.

Hardground Iithologies The Snowy Range hardgrounds are deveIoped on the upper surfaces of carbonate flat pebble con- glomerates (intraspamdites). They are typically overlain by a thin layer of green shale. The con- domerates consist of tabular Iaminated micritic pebbles up to 15 crn ah055 which are vicallv stained green with glauconite and which In some cases show reticulate pitting (submarine corm- sion?; Fig. 3A). Pebbles are rounded to subangu- lar and in many cases show imbrication (Fig. 33) . The matrix surrounding rhe carbonate peb- bles consists of fine skeletal debris. parriculady d o b i t e and echinoderm fragments, minor quartz silt and sparry calcite. The flat pebble conglomerates form lenticular to continuous bands up to 30 cm in thickness which generally e-xhibit sharp basal contam and may rruncate bedding in the underlying shales and siltstones. Upper surfaces of the conglomerate bands vary k o m planar to irregular and hummocky.

Approximately one-third of the flat pebble conglomerates examined in the upper porrion of the Sage Member exhibit evidence of early fithi- fication. A total of 10 hardground horizons were identified and studied in detail in the two out- crops. Evidence for early lithification of these mbonates include the following criteria: trunca- tion of intraclasts, pirting and etching of the up- per surfaces of clasts, and the presence of en- sting fossils on the surfaces.

Several hardgrounds observed in the Snowy Range Formation developed on planar abrasion

k d Uartl

H.G 1

H.G 2

H.G3

HG?

MG?

KG4

MG.5

Fig. 2. Stnriwphic column of upper Snowl; Rlnge furmarion at north Grove Creek near Red Lodee. Uontana. showln: positions of hardgrounds.

or erosion surfaces on the tops of flat pebble conglomerate beds; such surfaces truncate edgewise-imbi-icated flat pebbles (Fig. 38 ). Other h a r d ~ o u n d s occur on the non-truncated upper sides of flat pebble conglomerates which may be undulatory with pebbles standing out in relief on the surfaces. In certain instances the flat pebbles have been undercut and exhibit over- hanging surfaces (Fig, 3C) whlch have been en- crusted by pelmatotoan boldfasts. Burrowing does nor appear to have been an important factor in shaping any of the Cambrian hardgrounds ob- served, although Fsykman (1980) reports irregu- lar hardgrounds from the Middle Cambrian of Greenland associated with burrowing.

Finally. certain hardgrounds were developed on hummocky, mound-like masses of micritic

IS1 C. E. Brett. LV. D. Liddell and K. L. DesstIer LETHAIA 16 (19831

.Eip. 3. Features v i harcgroqrcr u r n [he Carnnridn Snowy Ranee Formation: all s ~ c i m c n r from norih Grorr C;cek, Red Lodge. 3lontana. 3 -4. U ~ p e r 5u:fzcc :i curbonarc pehnle conglomerate bed showing roundcd ;;iclsilrite pehblcs wtrh s=ptaria-like crscks 2nd finely-pltred suriace~ 13 Crus++ection nt fizt pebble ionqlumcrarc ~in:rzrpnritzi hardgmund: norc :early planar upper surface whtch truncares :-. :-2s:one pebbles. Scale in cm. Z C . Cross-sect~un oi 4ummoclc) hardground devciopcd on intramicnte (alpal?) mound: note 2:: . A;<: dnd slightly unaercxt knob: abo note infilling uf crcn:tces by skeletal dz%s (arrow-dl and s~romatoIitic? laminarion LI[ :::?onate s~lt. inri1Iing slight depression [arrow-bl. 1 3 Lppcr surface of hurnrnccxy hardground and In inrramimte mound. :nob*::.< mundant encrusting ~elrnatozoan boldfasrs. f E. C;os+recriun of hummoci +:ox-n in D. rhowin: nicrirrc sedimcnr wth jra;;. :>und intraciasts (on 5&t) juxtaposed lalg;lll? >ound'71 onto mound . ~ f i r~~ncared flat pebble :onplomrrste ( I t t r l : nurc ;c::.::ozodn ho1dfasts airached to hardground irn uFr?r suriacc of mtrarnicrire lerrcu;l.

sediment containmg 5i::tered small. rounded. gIauconite-coated 42: 22bhIes (Fig. 3C. D). These mounds or huir;cocks. which range up to 10 crn m height and arcrouimately 50 cm across, rest on hummock! io ;.snar beds of fl ar pebble congIomerate. They x z i ispresent non-[aminat- 2d algal beundstoncs. 2: ;ynsedimentary lithified carbonate mud mounds , lirhohermsf. However. these masses of staimcn; were evidently lithified on the sea floor as rht:, show features such as undercutting of crevrccs 2nd abundant encrusting pelmatotoan holdfasu ~ndlcative of firm to hard substrates (Fig. 3E).

Hardground biotas A11 or' th? recognizable Snowy Range hard- grounds ssniblt cncrustmg epibionts. The most abundant form consists of dlsco~dal holdfasts of pelmatozoans. These are conlcal c.xpansions of stereom rznging In diameter from I to 7 mm and with circular column facets U.5-2.0 rnm in diame- ter (Fig. LA. B) Three categories of holdfasrs were obscsced. including simple rounded disks with circular outlines (Fig. 4A. B). conical 'vol- cano shaped' holdfasts (Eg. -1C). and low. discoi- dal forms with slightly Iobate margins (Fig. 4D).

Oldest hardground encrusiers 285

;, r pslmskvzoan h o l d f ~ t s ~ U T Rdngc hardgrounds. : \ \ - r~ r \u r face of tvpicd -A-:rrt>unrl trom the Upper

\!ember 3t Clark Fork <-drt*n. 3huu,ng density and ;, ~ r t n b u t i v n of di~coldal k d 3 1 1 S : note overlapp~ng :m ,,I huldbsts (arrow a) and u n . ,lrnrcal spongiomorph d- 4' m u n d (arrow 6. cenrerl: . . I ~. = B. Overlapping. dis- ,d h a l d f ~ t morphotye: m slabshown in A; X 3.5. z C. Conical holdfast morpho- %*: * l.5. O D. Low discol- hi m>rphct!.~ uith slightly LWXC margins: x 1.0. C and D t.lm aummucky hardground in w r Srgc Xiember at north m 01 Grow Creek.

Sone of the rnorphotypes erhib~ts any tract of a many-plated integument. HoIdfasrs occurjn ag- q a t e s of up to 20 inditiduals per 100 CK-. with m avenge densit). for ober 6300 an' of sampled h rds round surface of about 10 pet 100 cm'. X h y holdfasts were attached to the surfzces of a r h n a t e intraclasts; however. others were fixed -ly to the intewen~ng matrix of the gebbie mdomerates, further indicating that t h a mate- - was Iithified at the time of colonizatron by w c o z o a n s . Holdfasts occur on the upper. lat- 4. and undercut crevice surfaces of ~ h s hard- W d hummocks.

fhc identity of these pelrnatozoan hoidfasts a not be definitely established. although a a n a b l e guess can be made. UnusuaI stem

,Ipcphology and rant? of the associated tocrin- ,-'-w fmCheiocrinus rexxeri (Sprinkle 1973 eiimi-

as the major cncruster. Other associated plater includs :hose oi cpiipire-

a d mamocysteilid eocrinoids. a Iblasto- 1 radiaily-braced plates. a cheiro- and a form (?hvbocrinoid) with

-surfaced plates. This smooth-plat-

ed form was common and it may have 5een the dominant zncruster. The simple discoidel hold- fasts closely resemble those of la tcr Oraovician hybocrinoidf Ho~bever, to date. no n!bocrinoids have been rtuortrd from rocks as old as the ~ re rn~ea1eauA.

A second fossil found encrusting on ;he Cam- brian hardground surfaces consists of smalI (1 to 2. crn diamztrr) conical mounds. typicail! w ~ t h depressed cznters (Fig. 5) . Cross-sec:ions of such mounds r tvsal a reticulate, fibrous acpzarlng internal structure resembling that ot' :he calafied Cambrian sponglomorph alsa R e n ~ k k ! Kobluk & James 19-9: J. K. Rigby. p e n . comm 1981).

Other biotrc dements found In caiczrenites associatrd r r ~ t h the hardgrounds inc!ude ~ Ia t e s of a large edrioasteroid. a phylIocysrrd st>iowhoran carpord and at least two other echinodtrrns of unknown aifiniry. These organisms may have lived on r h t hardgounds or on nearby muddy sea floors. Orthid brackopods. abundant in some pans of the upper Sage Member. may have been attached by -$edicles to hardground sur- faces.

- ,-:,:- r , --\::. 322gc hard- h.~$c *Iu-:-.: :t Clark

: - :,:-'.~rz~.nc-: : -aecirnen . - . -: .L.., :. ,:-:: .-" ->Lila[ .m-

,. ., ,j._... - .- . .... . x L . rhowinp . . -- .: , ; ; r . : ~ L t l .-.-.:i>: X 4.5.

. , - , , , -, - - ,.- .. .. !; .:,,\;,ne en- . -, - .::. . - -2L: , d a c e :

>-(: :<.:?k,:::. , - - . . ., ... , ,.. . ., , - . , . :Y.:F,,TS S U C ~ . .

. - . - .-. , -- . .- - - 3 ,.,: . > I ~ +:,:;,. 'i - - - . . & , .,,,I :L; -7achio- , , : -- - - : ;~.!;~Icrl!t:r;Z -~:rfaces. .qiw. : ; : . ~ < : < , i y ; > : : , : : . . - . !!-I i~cr:!ri:- . which - +3 ..I L , ; - i+~li ' i i ;~:<: ;*. <.- - - ..:-.: L ! r ~ i ~ > \ :<:-n hard-

. , :rouz;\ , i : ~ , ~ : ? i i - . ~ -. - . - : : !L!5; r~>~~; :'-,-.:I. were ?.otab:!, .L~.c:-,; ::- ,-- - - C.'+!;::!lr!;ir; h ~ i r i ~ r o u n d k ~ r f ~ i : , . T h < < l ~ , r i \ i r l ~ ! ~ d asso-

ciated with these surfaces were cylindrical, siight- ly irregular, straight-sided boringsl from 2 mrn to 3 mm in diameter which penetrate rnicriric intra- ctasts, but not the matrix (Fig. 5) .

Depositional environment and paleoecology Depositional environments of xhz Middle-Upper Cambrian in Montana and Wyornin~ have pre- viously been studied in detaiI ( Lochman 1 9 5 . Lochman-Balk 1972). Lare Cambrian Snowy Range sediments accumulated in the western portion of a stable, inner she!f sea. which ex- tended eastward to the upper blisjlsslppi Valley The MonranalWyorning sheif \?as bordered to the west by an actively subsiding outer carbonzre shelf, and on the east and s o u t h e s t by the Cana- dian Shield and the TranscontinenraI Arch. both emergent areas of exposed heCambr i an rock Upper Cambrian sections cxhiblt a northeast- ward increase in terrigenous ciasric 5ediments. presumably reflecting proximity 70 a major r h e r or rivers draining the Canadian Shiz!d (Loch- man-Balk 1972). Slight diastropnic uprvarp of this source region during the Dresbachian and Franconian ages resulted in an increase in detrr- tal sedimcntat~on, reflected in ihr shaies of the Dry Creek and S a g members. Lochman-Balk (1972: 138) visualizes a very broad -tidal flat' (800 km') extending from the eastern shoreline into central Montana and Wyoming during the late Franconian. wlth an inner littoral rand flat and an outer mud flat (Deadwood facies). bordered Seaward by a subtidal muddy shelf. or shelf la- goon in which limestone pebble conglomerates and shales accumulated (Snowy Range facies).

MantanwWyoming shelf seas were shallow (probably less than 30 rn) as evidenced by abun- dant storm deposits (flat pebbIe conglomerates). oolites, primary dolomites, and alga1 structures (Lochman 1957). Limonitic oxidation rinds on carbonate clasts within fresh samples o i certain flat pebble conglomerates in the Snow? Range Formation probably developed by submarine weathering; such rinds are common in totally marine hardgrounds of Mesozoic age (F. Fiirsich. pers. comm.). Conglomerates, including several on which hardgrounds were developed. yield no evidence of emergence. An abundance of carbonat? pebbie conglom-

erates characterizes severai MiddIe-Upper Cam- brian facies in the Rocky Mounrains (Sepkoski

OIdesr hardground encrusrers 287

1982). In part, this mav be due to the broad of the very shallow subtidal to intertidal

environments conducive to partial lithification of fine--pined carbonate sediments and subse- quent scouring by storm waves.

Discontinuous (nodular) early lithification of the carbonate pebble sediments rs indicated by their evident erodabiIitl; as large coherent clasts. often with sharp edges. and the presence of sharply-defined trace fossiIs (borings?) within the ciasts. Sepkoski gE Barnbach (1979) and Scpkoski f 1982) reasoned that exrensive b~oturbation in- hibits early Iithification by inmixing of fine clays and attributed the abundance of flat pebble con- ubmerates in the Canbrian to a scarcie of d e e p burrowing organisms at that time. The preserva- ;ion af primary Iamination in most Snowy Range iirnestone clasts appears to support this conten- don.

Formation of flat pebble conglomerates im- plies eplsodic scouring of the sea floor. prrsum- abIy during intense storms. Random to slightly imbricated orientation of flat pebbles and their packing in a matrix of coarse fossil debris with spam, calcite cement indicate high energy con- dirions which resulted in the w ~ n n o w i n of fine- grained sediments.

A variety of evidence suegests that such epi- sodes of disturbance punctuated a backgound of r)-pically slow sedimentation rates. Glauconite.

0s on which occurs both as ga ins and as coarin= many pebbles and hardground surfaces. is an excellent indicator of low rates of deposition in quiet water, slightly reducing envrronments (4tcRae 1972). The generally disarticularcd and fragmental nature of fossils as well as certain beds of well-rounded Iimestone pebbles suggest 'periodic reworking of sediments on the sea floor. Finally, evidence of synsedimentasy lirhification borh in the carbonate clasts and in hardgrounds on the upper surfaces of Snowy Range pebble conglomerate bands indicates periods of non- deposition, permitting cements to precipitate in grain interstices (cf. Bathurst 1976:39W6).

Following the deposirlon of lags of limestone pebbles and fossil debris this material was indur- ated by the formation of early cements. This lithification probably occurred slightly below the sediment-water interface. In certain instances. the initial binding of local hummocks of pebbles and debris also may have been aided by algal encrustation. Cross-sections of some hummocks show local clumps of edgewise pebbles surround- ed by irregular masses of micrite with vague

Fie. 6. 3 A. P-rfor~rcd flar. micrit~c ~ t b u i z :;om lnr;uspar[t: bed near hasc ai Grow Creek Mcrnbrr. Snoxv Range Forma- tion. at Sonh Grovc Creek. Montzna: nurc ! x g e size ot bor- ing? and :rrcular ourtine. IB. Phutomict~-.graph uE snother horinpl from r%c same ! w ~ l i t y ; nore spar Inijllinp dnd lieht (oxidzt~on !l halo aruund ~ h c hole: x i!.U

patchy or mottlcd textures. sugg5siive of aIgal boundstones (ci. Pratt 1982). E~;denrly. thrrc was IittIe or no burrowmg of the pebbly sedi- ments prior to Iithification, since original ~rnbr i - cate fabrics are preserved in most cases. This contrasts with most Ordovician and later hard- grounds and. agaln, supports the hyporhews of Sepkoski k Bambach (1979). However. hard- ground surfaces were typically shacrd by subma- rine erosion after lithification. >fan! surfaces were undercut somewhat and othc:s were ero- sionally -planerj' to nearIy flat surfaces which truncate cIasts.

Although holdfasts on Snow! Range hard- grounds arc generally well presened. none show articulated colcmns or thecar rtmarns. In all cases the pdmatozoans died and ivere disarticu- lated prior to the final burial of the hardgrounds. Furthermore, relatively little disarticulated skel- etal matenal was found on the surfaces of she hardgounds indicating that these surfaces were

288 C. E. Brett, W. D. Liddell and K. L. Derstler L E n t ~ 1 . 4 16 (1983)

swept clear of debris by currents. CoIumnaIs and plates are an important component of certain flat pebble conglomerates and calcatenites which are associated with the hardgrounds. A few holdfasts and several of the algal? mounds also exhibit slight abrasion andlor corrosion, but most speci- me= do not.

No evidence of faunal successionlprogression was observed en the surfaces, but the frequent ovtrgowth of the articular facets of holdfasts by other holdfasts ~ndicates encrustation of 'the sur- face by more than one seneration of pelrnaro- zaans.

Discussion The vpper Cambrian (Franconian to Trempea- leauan) discontinuity surfaces described herein are among the oldest known hardgounds and ate the oldest known surfaces to preserve tn- crusring faunas. Sepkoski (1977) made a detaiIed study of the stratigraphy and paleontology of slightly oIder Bresbachian rocks in Montana and

0 s or \-oming, but found no e\<dence of borin= encrcsting fossils on hardgrounds (J. J. Sepkoski, pers. comm. 1978). We surveyed flat pebble con- gIomrrates in the Middle Cambrian Upper Gros Venrsr Shale (= Park Shale) of northern WIJO- ming. inchding some beds with truncated upper surfaces suggesting synsedimentaq lithificanon, but also found no evidence for encrusting faunas.

Snowy Range hardgrounds show a Iow diversi- of skeIetonized encrusting biotas largely do-

rninarzd by one or two species of srnalI attached pe!rnatozoans. The occurrence of such holdfasts in thz lower Sage Member (Franconian) couId represent the oldest record of hybocrinoids and the oldest indirea evidence of crinoids, aside from the questionable Middle Cambrian Echma- iocsinw (Sprinkle 1973). These hardground en- crusting pelmatozoans, wharever their identiry . possessed simple discoidal holdfasts, which are solid masses of stereom with a well-developed articular facet for true colurnnals (such columnaIs are aIso known from biosparite beds of the S n o y Range Formation).

The presence of probable algal structures on hardpound surfaces contrasrs with most later ~aleozoic hardground biotas which lack algal crus6. Possibly, this is associated with an ab- sence of grazing organisms in the Cambrian.

from at ieast the Middle Ordovician onward (Brookfield & Brett in prep.).

The absence of macroborings on all tvpes of t a re Cambrian hardgrounds is both surprising and probably significant since borings are com- mon on Lower Ordovician discontinuin surfaces (Jaanusson 1961) and are the most ubiquitous and abundant fossil on later hard~rounds (Brett & Liddell 197S:346). Th~s observation is also enigmatic in view of the recenfdiscovery of ma- croborings (T~pani t e s ) in Lower Cambrian ar- chaeccyarhid reefs from Labrador (Kobluk et al. 1978; KobIuk & James 1979). However. this ob- servation is consistent with the hypothesis of ICobluk er al. (1978) that Trypanires-type bore- holes were produced by separate groups of or- ganisms: one which evohed in the Early Cambn- an, speciaIized in boring into archaeocyath~d reefs and presumablv became extinct with the extinction of archaeoqathids, and later forms which appeared in earliest Ordoblcian time and continued throughout much of the Phanerozo~c.

Finally, ir may be significant that cvlindrical. slightly tapering hies occur in certam of the rnicrite clasts within hardgrounds and other 1:. pes of f l at pebble conglomerates in the Snoxxv Range Formation. These holes (see above). m-hich are similar in size and shape to Ti7;panirex, were evidently produced in fine-grained. micritic sedl- rnenrs prior to their deposition as ciasrs based on their orientation in the conglomerates. Because these pebbles are entirely composed of fine- grained sediments. it is not possible to detcrrnine whether or not the holes are true bonngs (1.e. cut grains). We suggest that they are burrows or borings formed rn semilithified micrires. Howev- er, the organisms responsible for such ichnofos- sils evidently did not bore into the cemented surfaces of she hardgounds; no macroborings of similar type were found cutting gains or cements in the pebble conglomerates. Perhaps, these were the dwelling burrows of organisms which excavated firm but not entiref? lithified fine- grained sediments. The producers of these holes may represent precursors of organisms which were capable of excavating truly indurated sedi- ment.

Upper Cambrian hatdground biotas predate the appearance of most skeletonized colonial or- ganisms (e.g. bryozoans) which commonly occu- py or even dominate space in Iater hard substrate communities (cf. Jackson 1W7; Palmer 1982). Therefore, greater spatif dominance by solitary

Gastropods are associated with hardgrounds organisms (e.g. echinodezms) might be predict-

Oldest hardground encrusters 289

ed. Based on our admittedly smatl samples, this appears to be the case. Observed densities of echinoderm holdfasts (1-15/100 cm2. for four hardgrounds sampled) on several S n o v Range hardground slabs greatjy exceed maximal densi- ties of similar-sized holdfasts on later hard sub- stratc assemblages known to us.

In two cases, hummocky Snowy Range hard- grounds exhibit irregular topographies including flattened upper, steep lareral and slightly under- cut or overhanging surfaces. In both instances, we nottd that the-distribution of a single mor- photype of pelmatozoan holdfasts was essentially random with respect to hardground topography. T h e holdfasts were about equally abundant on sentry-sloped upper surfaces and steep lateral faces. Recent studies provide evidence for weak differentiation of exposed vs. cryptic biotas in Middle Ordovician hardgrounds (Brett 8z Liddell 1978). and somewhat more pronounced polariza- tion in the Silurian (Spjeldnaes 1974) and De- vonian (Koch & Sh-impIe 1968). It would appear that faunal partitioning of microenvironments by different organisms has increased considerabIy since the Cambrian.

,Icknowledgemem. - Rer~exed by Jeremy B . C. Jackson. .Ahsun R. Palmer. Fram FLITsIcS. and Cal Stevens. We thank XIarpr Gardner. Claire Sunazen. Robert Eaton. and Kay O'Comell for assistance in preparation of the manuscript and iIlustrarlons. I. K ~ i r h Rigby. Dan~el C. Fuher. and Matthew Xitecki examined section5 oi rhe spon@omosph aIgae? aoa offered suggstions as to their ~dentiry.

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