Proc. Natl. Acad. Sci. USAVol. 93, pp. 4990-4993, May 1996Geology

Ediacaran biota from Sonora, Mexico(Caborca region/Clemente Formation/early animals and protoctists/Johnnie oolite)

MARK A. S. MCMENAMINDepartment of Geography and Geology, Mount Holyoke College, South Hadley, MA 01075

Communicated by Lynn Margulis, University of Massachusetts, Amherst, MA, January 19, 1996 (received for review May 15, 1995)

ABSTRACT The Ediacaran biota is the earliest diversecommunity of macroscopic animals and protoctists. Body andtrace fossils in the Clemente Formation of northwesternSonora extend downward the geologic range of Ediacaranforms. Taxa present in the Clemente Formation include cf.Cyclomedusa plana, Sekwia sp., an erniettid (bearing an airmattress-like "pneu" body construction), and the trace fossilsLockeia ichnosp. and Palaeophycus tubularis. The trace fossilsconfirm the presence of sediment-dwelling animals in thisshallow marine community. The body fossils are headless,tailless, and appendageless. Some may be body fossils ofanimals but others may be fossils of large protoctists. Thesebody and trace fossils, recovered from thinly bedded sand-stones and siltstones, occur 75 meters lower in the Sonoranstratigraphic section than a distinctive Clemente Formationoolite. The stratigraphic position of the fossils below this oolitepermits long-distance correlation between fossiliferous Pro-terozoic strata of Mexico and the United States. Correlationsutilizing both the Clemente Formation oolite and a trace fossil(Vermiforma antiqua) confirm the antiquity (600 million yearsor more) of this body fossil-rich community of macroscopiceukaryotes. The recently discovered body fossils are the oldestknown remains of the Ediacaran biota.

The Ediacaran biota holds a special position in the history oflife; its cosmopolitan assemblages of fossils record the firstappearance of large and diverse eukaryotes. Pre-Paleozoicstrata from the Altar Desert of the Caborca region (Sonora,Mexico) have yielded a new locality for fossils of an earlyshallow marine Ediacaran community. The fossils occur in theClemente Formation, a mixed siliciclastic-carbonate unitwhose type section is in the Sierra el Rajon (1). The fossilsdescribed here were taken from that type section.The most distinctive rock unit in the Clemente Formation is

a pale tawny-weathering oolite (Fig. 1). This unit is also knownas unit 5 of the Clemente Formation. It can be correlated toan oolite bed near the base of the Rainstorm Member of theJohnnie Formation in eastern California (1).The primary fossil locality (Appendix I) is a small (10 meters

x 20 meters) regolith patch near main rock exposures of theSierra el Rajon. This patch is laterally continuous with themain exposures. The primary fossil horizon is 5-10 metersdownsection from an outcrop of the. Clemente Formationoolite. Fossils are present at several other horizons within thefine grained siliciclastic rocks below the oolite. One specimen(Fig. 2A) was collected approximately 75 meters below theoolite.The fossils include three types of body fossils, two ichno-

fossils, and several enigmatic forms that have been reportedelsewhere (2) and will be described later.

Body Fossils

The largest fossil (Fig. 2A) is a hyporelief with a high-reliefrounded central cone, 2.1 cm in diameter and 7 mm in height.

This cone is smooth and featureless except for a distinctwrinkle where it meets the relatively flat outer ring. The flatouter ring bears thin, radially-oriented tubular structures. Thisfossil is most similar to Cyclomedusa plana Glaessner andWade from the Sekwi Brook of the Mackenzie Mountains ofCanada (ref. 13; plate 1:4). Although the Canadian specimenlacks the distinct wrinkle separating the central dome from theflat outer ring (see arrowhead in Fig. 2A), in recognition oftheir similarities the Sonoran fossil is referred to here as cf.Cyclomedusa plana Glaessner and Wade.The specimen from the Clemente Formation illustrated in

Fig. 2B is a member of the Proterozoic genus Sekwia. Thiscircular-to-elliptical fossil is 1 cm in greatest dimension with apreserved relief of 1.6 mm. The -fossil has the eccentric,seleniform-fold characteristic of the genus (13, 14).

Sekwia is the fossil of a nonmineralized conical structure,perhaps the semi-rigid basal attachment structure of an earlydiploblastic animal (13, 14). The conical object has beenlaterally compressed and flattened by compaction of enclosingsediment-this flattening imparts the crescentic fold diagnos-tic for the genus (14). The Mexican specimen most resemblesthe type species Sekwia excentrica (14), although it is of higherrelief.An incomplete specimen of a quilted Ediacaran organism

from Mexico is shown in Fig. 2C. This fossil consists of animpression of six parallel, adjacent tubular cylinders from1.3-2.0 mm wide. The cylinders follow parallel straight pathsfor 1.0-1.5 cm, then turn in the same direction making an arcof approximately 350 over a distance of 7 mm. In their last1.5-2.0 mm each cylinder separates from the others, tapersdistally, and comes to a point. These pointed ends of tubesform a serrate edge to the fossil.Based on the length, curvature, and distal tapering of the

tubes, this fossil is assigned to the Erniettidae, a family ofEdiacaran body fossils best known from Namibia. The speci-men, insufficiently complete to permit more detailed diagno-sis, is most similar to an undescribed erniettid from the lowerWood Canyon Formation of Nevada (3).These Mexican body fossils cannot be assigned even to the

highest taxa with certainty. The fact that both Cyclomedusa andSekwia resemble diploblastic animals, such as cnidarians, doesnot mean that they are necessarily animal fossils. Although nounequivocal macroscopic fossil protoctists (15) have yet beenrecognized in the Ediacaran biota (primarily because no traitsunique to Protoctista have been identified in the body fossils),it seems likely that protoctists constituted at least part of thecommunity because protoctists are the evolutionary anteced-ents of animals. No traits unique to Animalia have beenidentified on the body fossils either. Assumptions that theEdiacaran organisms were animals are incautious (16), andcaution is required when considering the phylogenetic affini-ties of any Ediacaran organisms (17). Blanket references tobody fossils of the Ediacaran biota as "multicellular animals"are redundant since all animals are multicellular. Such usagealso obscures the real scientific uncertainties as to the taxo-nomic placement and somatic organization of these forms (18).

The publication costs of this article were defrayed in part by page chargepayment. This article must therefore be hereby marked "advertisement" inaccordance with 18 U.S.C. §1734 solely to indicate this fact.

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Proc. Natl. Acad. Sci. USA 93 (1996) 4991

Windermere SupergroupNW Canada

Death ValleyCalifornia

NW SonoraMexico

Carolina Slate BeltNorth Carolina

CarnbrianProterozoic

CaffraaFormation_

Zabriskie Qzt.

Wood CanyonFormation

@ Ediacaran fossils

StirlingQuartzite

t~Ediacaran fosasil

~ Johnnie

Formation

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~(sections A, Cl

Buelna Formation

_ Proveedora Qzt.

PuertoEeanco

! x ! <>g;=2f\ Formation-erniettid

;cloudinidsLa CienegaFormation

Tecolote Qzt.

as.,^V-zX7v~v~vPapaloteFormation

X. el "Gamuza Formation

Pitiquito Qzi.

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Caborca Formation0 Arpa Formation

F600 m-400

F-200

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_ _000

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F7,13. diamictite

g dolostone

limestone

~~~~~Yadlken 54-7 bSFormation _ -540±7 RbSr

McManus - Pterdinum

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cloudinids

TietryFormation

586 ±10 U-Pb

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sandstone

shale

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Vermiforma

Ediacaran bodyfossils

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540 + 7 radiometic age data

FIG. 1. Correlations between the Proterozoic strata of the Windermere Supergroup, northwestern Canada (section A); Death Valley, California(section B); northwestern Sonora, Mexico (section C); and the Carolina Slate Belt, North Carolina (section D). Arrow indicates location of thenew fossil find in the Mexican stratigraphic section. Radiometric age dates are in millions of years. Data were obtained from refs. 1-12.

Trace Fossils

Two types of trace fossils, ichnotaxa Lockeia and Palaeophycustubularis Hall, occur in the sedimentary rocks below theClemente oolite. Specimens of Lockeia, small elongate ichno-fossils, form low- relief teardrop-shaped convex hyporeliefs. Inone example (Fig. 2D), three individual traces are orientedwith their long axes in approximately the same line.

Palaeophycus tubularis is represented by eight specimens ofstraight to slightly sinuous horizontal cylindrical burrows from0.6-2.3 mm (mean = 1.4 mm) in diameter. Burrows are linedby a very thin clay layer. Two examples show burrows crossingone another. Three specimens are preserved as concave hy-poreliefs, four specimens as convex hyporeliefs, and onespecimen as a convex epirelief. The longest burrow (Fig. 2E)is a convex hyporelief that gives way to a concave hyporelief 7cm long (measured as if straightened out).These trace fossils are the most ancient known in the

Mexican Proterozoic sequence. The organisms represented bythese traces (as well as the associated body fossils) lived in a

shallow water marine habitat close to fair weather wave base.This is indicated both by the stratigraphic proximity of thefossils to the Clemente Formation oolite (certainly a shallow

marine sediment type), and by the presence of small scalecrossbedding in the thinly bedded siltstones and fine sand-stones of the fossiliferous interval.

Relative and Absolute Ages

The Clemente Formation oolite is conglomeratic at its top, andthis oolite clast conglomerate is an expression of a majorunconformity. The stratigraphic horizon delineated by thisoolite conglomerate correlates to the transition between theJohnnie oolite and overlying units in the Death Valley regionof California ("Proterozoic sequence boundary 3" of ref. 19).In addition to the major unconformity in the Mexican Pro-terozoic sequence at the top of the Clemente oolite, two othermajor unconformities also occur between the Clemente un-conformity and the overlying Cambrian strata.The Johnnie oolite and the Clemente oolite have been

linked by lithostratigraphic correlation, and may represent thesame once laterally-continuous rock unit (1). The Sonoran andMojave Proterozoic sections were at one time hundreds ofkilometers closer together before post-Proterozoic sinistraldisplacement along the N 60° W-striking Mojave-Sonoramegashear (20).

Backbone RangesFormation

lngtaFormation

RiskyFormation

BluetfowerFormation

GametraFormation

SheepbedFormation

Tepee Dotostone=

loe BookFormation/

Keele Formation

TwityaFormation

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FIG. 2. Body and trace fossils of the Clemente Formation (Sonora, Mexico). (A) cf. Cyclomedusa plana Glaessner and Wade. A discoid fossilpreserved in hyporelief. Note annular ridge occurrence at the margin (arrowhead) of the central cone. Greatest dimension of rock specimen is 6.0cm. Sample 1 of 3/17/95; fossil occurrence is approximately 75 meters below the Clemente Formation oolite, in unit 1 of the Clemente Formation.(B) Sekwia sp. Greatest dimension of circular fossil is 1 cm. Note crescentic indentation on the right side of the fossil. Sample 4 of 3/16/95; fossiloccurrence is approximately 5-10 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. (C) Erniettid fossil. Noteserrate margin on the right edge of the fossil. Width of rock specimen in view is 3 cm. Sample 5 of 3/16/95; fossil occurrence is approximately5-10 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. (D) Lockeia ichnosp. preserved as hyporelief. Eightspecimens are visible in this photograph. Width of rock sample in view is 2.5 cm. Sample 6 of 3/16/95; fossil occurrence is approximately 5-10meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. (E) Palaeophycus tubularis Hall. This sinuous specimen occursas both a convex hyporelief (lower) and a concave hyporelief (upper). Width of rock specimen in view is 6 cm. Sample 7 of 3/16/95; fossil occurrenceis approximately 15 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation.

Regional correlations suggest (21) that both the top of theJohnnie and Clemente Formations and the oolite occurringbelow in both formations represent chronostratigraphic inter-vals (see Fig. 5 of ref. 21) earlier than the time of depositionof strata containing the oldest convincing Ediacaran bodyfossils recorded in the Windermere Supergroup of northwest-ern Canada (Sheepbed and Blueflower Formations; see ref.13). The new fossils from the Clemente Formation are thus theoldest convincing body fossils of Ediacaran forms and of tracefossils of animals known from western North America.

Cloudina is the oldest megafossil known in the Namasequence of Namibia, and occurs in the lower KliphoekMember of the Kuibis Subgroup (22). Since the Deep Spring

cloudinid biota [a lateral correlative of the La Cienega For-mation cloudinid biota (4)] correlates to the lowest cloudinidoccurrences in Namibia (5), it may be safely inferred that thenew Clemente Formation Ediacaran biota is older than any ofthe body fossils yet reported from Namibia, since the oldestcloudinids in Namibia are found below the oldest "soft-bodied" fossils in the Namibian section. This conclusion issupported by carbon isotope data from carbonate rocks of theClemente Formation (6), which when compared to a globalcomposite of isotopic variation (23) indicate a stratigraphicposition well below the lowest fossil horizons in Namibia.

Reports of "metazoan" fossils in the Twitya Formation ofthe Mackenzie Mountains deposited below tillite (24) might be

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derived from strata older than the Clemente Formation. Theseputative body fossils, however, have been demoted to "possiblemetazoan origin" (23). Because no convincing trace or bodyfossil has yet been recovered from the Twitya Formation, thefossils of the Clemente Formation provide the oldest directevidence for Ediacaran communities.The Clemente Formation fossils cannot be radiometrically

dated. However, an age estimate can be made owing to thepresence of the trace fossil Vermiforma antiqua in siltstone ofthe Clemente Formation deposited above the Clemente oolite.This fossil (see Fig. 10 of ref. 1), once interpreted as apseudofossil (7), is biogenic and conspecific with Vermiformaantiqua from strata of the Carolina Slate Belt (8). Originallydescribed as a body fossil (8), Vermiforma antiqua has beenreinterpreted as a trace fossil (25). The parallelism of track-ways (1, 8, 18) exhibited by cohorts of this ichnospecies (on thesame bedding plane) may be due to the shared current-controlled orientation of the individual trace makers to con-stant (1, 18) or fluctuating (8) water current direction.The age of Vermiforma antiqua in North Carolina is brack-

eted by uranium-lead (U-Pb) radiometric dates. An age (inmillions of years) of 586 ± 10 U-Pb has been obtained from theUwharrie Formation, which was deposited stratigraphicallyabove the Vermiforma antiqua occurrence in North Carolina.This 586 + 10 date from the upper part of the UwharrieFormation is a U-Pb concordia age based on zircon crystalsfrom felsic volcanic rocks (26). An age of 620 ± 20 U-Pb hasbeen determined for rocks immediately below Vermiformaantiqua (8, 9). This 620 + 20 date is also a concordia date forzircons from a felsic tuff-breccia at the top of map unit II inwhat is now mapped as the Hyco Formation (10). Higher in thestratigraphic section, a Rb-Sr (rubidium-strontium) whole-rock isochron minimum age of 540 ± 7 million years, has beenreported from a lenticular mafic volcanic deposit in the YadkinFormation (11). A more recently determined U-Pb zircon dateof 575 ± 7.6 millions years (reported from the Cid Formation,equivalent to the McManus Formation; ref. 27) fits neatly intothe geochronologic framework established by the other threeradiometric dates.

Accepting these dates and their error ranges as valid indi-cates an age for the North Carolina Vermiforma antiqua fossilsof between 576 (i.e., 586 - 10) and 640 (620 + 20) millionyears. But since the fossil occurrence is stratigraphically muchcloser to volcanic rocks associated with the latter (moreancient) date, it is likely that the age of the fossils is close to,or in excess of, 600 million years.By biostratigraphic correlation to the North Carolina section

(see Fig. 1), the Vermiforma-bearing strata of the ClementeFormation above the oolite may also be assigned an approx-imate age of 600 million years. The age of the body fossilsoccurring below the oolite (that itself occurs below the Mex-ican Vermiforma) could very well be some millions or tens ofmillions of years older than 600 million years, considering thatthere is a major unconformity truncating the top of theClemente oolite. Thus, the 600-million-year age for the bodyfossils of the Clemente Formation proposed here could well bea conservative estimate.The Mexican fossils found below the Clemente oolite are the

oldest known occurrence of body fossils of the Ediacaran biota.The implications of this discovery are 3-fold. (i) The evolu-tionary innovations leading to the development of the Ediaca-ran biota occurred in shallow marine water. This accords withinferences by others (28) that Ediacaran organisms originatedin shallow-water seas. (ii) The locus of origin of the Ediacaranorganisms was on, or near, the continental shelves of the NorthAmerican craton. This inference is supported by the fact thatthe next oldest Ediacaran assemblage is in northwesternCanada. (iii) Fossils of the Ediacaran biota were depositedduring a significant interval [600-520 million years ago (see

ref. 28)], lasting at least 80 million years. This stretch ofgeologic time exceeds the length of the Cretaceous, at 78million years the longest geologic period.

Appendix I

Sample 4 of 3/16/95; Sekwia sp., Lockeia ichnosp.; 6 of 3/16/95;Lockeia ichnosp., Palaeophycus tubularis; 7 of 3/16/95; Palaeo-phycus tubularis. Coordinates of site are N30°24.041',Wl 1 1057.141'.Sample 5 of 3/16/95; member of Erniettidae. Coordinates

of site are N30024.013', W111°57.196'.Sample 1 of 3/17/95; cf. Cyclomedusaplana. Coordinates of

site are N30023.978', W111°57.116'

I thank F. Corsetti, I. Dalziel, A. Dix, D. Evans, M. Godchaux, H.Hurtado, J. Kirschvink, L. Margulis, D. McMenamin, J. M. Morales-Ramirez, S. Rowland, A. Seilacher, and J. H. Stewart for assistancewith various aspects of this research.

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