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

Ediacaran biota from Sonora, Mexico (Caborca region/Clemente Formation/early animals and protoctists/Johnnie oolite) MARK A. S. MCMENAMIN Department 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 diverse This cone is smooth and featureless except for a distinct community of macroscopic animals and protoctists. Body and wrinkle where it meets the relatively flat outer ring. The flat trace fossils in the Clemente Formation of northwestern outer ring bears thin, radially-oriented tubular structures. This Sonora extend downward the geologic range of Ediacaran fossil is most similar to Cyclomedusa plana Glaessner and forms. Taxa present in the Clemente Formation include cf. Wade from the Sekwi Brook of the Mackenzie Mountains of Cyclomedusa plana, Sekwia sp., an erniettid (bearing an air Canada (ref. 13; plate 1:4). Although the Canadian specimen mattress-like "pneu" body construction), and the trace fossils lacks the distinct wrinkle separating the central dome from the Lockeia ichnosp. and Palaeophycus tubularis. The trace fossils outer confirm the presence of sediment-dwelling animals in this flat ring (see arrowhead in Fig. 2A), in recognition of shallow marine community. The body fossils are headless, their similarities the Sonoran fossil is referred to here as cf. tailless, and appendageless. Some may be body fossils of Cyclomedusa plana Glaessner and Wade. animals but others may be fossils of large protoctists. These The specimen from the Clemente Formation illustrated in body and trace fossils, recovered from thinly bedded sand- Fig. 2B is a member of the Proterozoic genus Sekwia. This stones and siltstones, occur 75 meters lower in the Sonoran circular-to-elliptical fossil is 1 cm in greatest dimension with a stratigraphic section than a distinctive Clemente Formation preserved relief of 1.6 mm. The -fossil has the eccentric, oolite. The stratigraphic position ofthe fossils below this oolite seleniform-fold characteristic of the genus (13, 14). permits long-distance correlation between fossiliferous Pro- Sekwia is the fossil of a nonmineralized conical structure, terozoic strata of Mexico and the United States. Correlations perhaps the semi-rigid basal attachment structure of an early utilizing both the Clemente Formation oolite and a trace fossil diploblastic animal (13, 14). The conical object has been (Vermiforma antiqua) confirm the antiquity (600 million years laterally compressed and flattened by compaction of enclosing or more) of this body fossil-rich community of macroscopic sediment-this flattening imparts the crescentic fold diagnos- eukaryotes. The recently discovered body fossils are the oldest tic for the genus (14). The Mexican specimen most resembles known remains of the Ediacaran biota. the type species Sekwia excentrica (14), although it is of higher relief. The Ediacaran biota holds a special position in the history of An incomplete specimen of a quilted Ediacaran organism life; its cosmopolitan assemblages of fossils record the first from Mexico is shown in Fig. 2C. This fossil consists of an appearance of large and diverse eukaryotes. Pre-Paleozoic impression of six parallel, adjacent tubular cylinders from strata from the Altar Desert of the Caborca region (Sonora, 1.3-2.0 mm wide. The follow Mexico) have yielded a new locality for fossils of an early cylinders parallel straight paths shallow marine Ediacaran community. The fossils occur in the for 1.0-1.5 cm, then turn in the same direction making an arc Clemente Formation, a mixed siliciclastic-carbonate unit of approximately 350 over a distance of 7 mm. In their last whose type section is in the Sierra el Rajon (1). The fossils 1.5-2.0 mm each cylinder separates from the others, tapers described here were taken from that type section. distally, and comes to a point. These pointed ends of tubes The most distinctive rock unit in the Clemente Formation is form a serrate edge to the fossil. a pale tawny-weathering oolite (Fig. 1). This unit is also known Based on the length, curvature, and distal tapering of the as unit 5 of the Clemente Formation. It can be correlated to tubes, this fossil is assigned to the Erniettidae, a family of an oolite bed near the base of the Rainstorm Member of the Ediacaran body fossils best known from Namibia. The speci- Johnnie Formation in eastern California (1). men, insufficiently complete to permit more detailed diagno- The primary fossil locality (Appendix I) is a small (10 meters sis, is most similar to an undescribed erniettid from the lower x 20 meters) regolith patch near main rock exposures of the Wood Canyon Formation of Nevada (3). Sierra el Rajon. This patch is laterally continuous with the These Mexican body fossils cannot be assigned even to the main exposures. The primary fossil horizon is 5-10 meters highest taxa with certainty. The fact that both Cyclomedusa and downsection from an outcrop of the. Clemente Formation Sekwia resemble diploblastic animals, such as cnidarians, does oolite. Fossils are present at several other horizons within the not mean that they are necessarily animal fossils. Although no fine grained siliciclastic rocks below the oolite. One specimen (Fig. 2A) was collected approximately 75 meters below the unequivocal macroscopic fossil protoctists (15) have yet been oolite. recognized in the Ediacaran biota (primarily because no traits The fossils include three types of body fossils, two ichno- unique to Protoctista have been identified in the body fossils), fossils, and several enigmatic forms that have been reported it seems likely that protoctists constituted at least part of the elsewhere (2) and will be described later. community because protoctists are the evolutionary anteced- ents of animals. No traits unique to Animalia have been Body Fossils identified on the body fossils either. Assumptions that the Ediacaran organisms were animals are incautious (16), and The largest fossil (Fig. 2A) is a hyporelief with a high-relief caution is required when considering the phylogenetic affini- rounded central cone, 2.1 cm in diameter and 7 mm in height. ties of any Ediacaran organisms (17). Blanket references to body fossils of the Ediacaran biota as "multicellular animals" The publication costs of this article were defrayed in part by page charge are redundant since all animals are multicellular. Such usage payment. This article must therefore be hereby marked "advertisement" in also obscures the real scientific uncertainties as to the taxo- accordance with 18 U.S.C. §1734 solely to indicate this fact. nomic placement and somatic organization ofthese forms (18). 4990 Downloaded by guest on September 27, 2021 Geology: McMenamin Proc. Natl. Acad. Sci. USA 93 (1996) 4991

Windermere Supergroup Death Valley NW Sonora Carolina Slate Belt NW Canada California Mexico North Carolina

Backbone Ranges Carnbrian Formation Proterozoic lngta Buelna Formation Formation Caffraa Risky Formation_ _ Proveedora Qzt. ~~~~~Yadlken 54-7 bS Formation Zabriskie Qzt. Formation _ -540±7 RbSr

Wood Canyon Bluetfower Formation Puerto @0.. Formation @ Ediacaran fossils Eeanco ! x ! erniettid<>g;=2f\ Formation- McManus - Pterdinum Formation Gametra Stirling ;cloudinids Formation Quartzite La Cienega Formation cloudinids _ _000 Tecolote Qzt. t~Ediacaran fosasil Tietry as.,^V-zX7v~v~vPapalote _ Formation Sheepbed Formation Formation ~ Johnnie 586 ±10 U-Pb Formation X. el "Gamuza Formation Formation Pitiquito Qzi. ~Aaron Clemente _ Tepee Dotostone = Noonday Formation

r 3km Twitya m 2 Formation section Di F600 - -400 a00d conglomerate pi oolite ~(sections A, Cl F-200 F diamictite sandstone Lo 7,13.

g dolostone shale

limestone siltstone

Vermiforma VV-V unconformity

Ediacaran body 540 + 7 radiometic age data fossils 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 the new fossil find in the Mexican stratigraphic section. Radiometric age dates are in millions of years. Data were obtained from refs. 1-12.

Trace Fossils marine sediment type), and by the presence of small scale crossbedding in the thinly bedded siltstones and fine sand- Two types of trace fossils, ichnotaxa Lockeia and Palaeophycus stones of the fossiliferous interval. tubularis Hall, occur in the sedimentary rocks below the Clemente oolite. Specimens of Lockeia, small elongate ichno- Relative and Absolute Ages fossils, form low- relief teardrop-shaped convex hyporeliefs. In one example (Fig. 2D), three individual traces are oriented The Clemente Formation oolite is conglomeratic at its top, and with their long axes in approximately the same line. this oolite clast conglomerate is an expression of a major Palaeophycus tubularis is represented by eight specimens of unconformity. The stratigraphic horizon delineated by this straight to slightly sinuous horizontal cylindrical burrows from oolite conglomerate correlates to the transition between the 0.6-2.3 mm (mean = 1.4 mm) in diameter. Burrows are lined Johnnie oolite and overlying units in the Death Valley region by a very thin clay layer. Two examples show burrows crossing of California ("Proterozoic sequence boundary 3" of ref. 19). one another. Three specimens are preserved as concave hy- In addition to the major unconformity in the Mexican Pro- poreliefs, four specimens as convex hyporeliefs, and one terozoic sequence at the top of the Clemente oolite, two other specimen as a convex epirelief. The longest burrow (Fig. 2E) major unconformities also occur between the Clemente un- is a convex hyporelief that gives way to a concave hyporelief 7 conformity and the overlying Cambrian strata. cm long (measured as if straightened out). The Johnnie oolite and the Clemente oolite have been These trace fossils are the most ancient known in the linked by lithostratigraphic correlation, and may represent the Mexican Proterozoic sequence. The organisms represented by same once laterally-continuous rock unit (1). The Sonoran and these traces (as well as the associated body fossils) lived in a Mojave Proterozoic sections were at one time hundreds of shallow water marine habitat close to fair weather wave base. kilometers closer together before post-Proterozoic sinistral This is indicated both by the stratigraphic proximity of the displacement along the N 60° W-striking Mojave-Sonora fossils to the Clemente Formation oolite (certainly a shallow megashear (20). Downloaded by guest on September 27, 2021 4992 Geology: McMenamin Proc. Natl. Acad. Sci. USA 93 (1996)

FIG. 2. Body and trace fossils of the Clemente Formation (Sonora, Mexico). (A) cf. Cyclomedusa plana Glaessner and Wade. A discoid fossil preserved in hyporelief. Note annular ridge occurrence at the margin (arrowhead) of the central cone. Greatest dimension of rock specimen is 6.0 cm. 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; fossil occurrence is approximately 5-10 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. (C) Erniettid fossil. Note serrate 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 approximately 5-10 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. (D) Lockeia ichnosp. preserved as hyporelief. Eight specimens 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 5-10 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. (E) Palaeophycus tubularis Hall. This sinuous specimen occurs as 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 occurrence is approximately 15 meters below the Clemente Formation oolite, in unit 4 of the Clemente Formation. Regional correlations suggest (21) that both the top of the cloudinid biota [a lateral correlative of the La Cienega For- Johnnie and Clemente Formations and the oolite occurring mation cloudinid biota (4)] correlates to the lowest cloudinid below in both formations represent chronostratigraphic inter- occurrences in Namibia (5), it may be safely inferred that the vals (see Fig. 5 of ref. 21) earlier than the time of deposition new Clemente Formation Ediacaran biota is older than any of of strata containing the oldest convincing Ediacaran body the body fossils yet reported from Namibia, since the oldest fossils recorded in the Windermere Supergroup of northwest- cloudinids in Namibia are found below the oldest "soft- ern Canada (Sheepbed and Blueflower Formations; see ref. bodied" fossils in the Namibian section. This conclusion is 13). The new fossils from the Clemente Formation are thus the supported by carbon isotope data from carbonate rocks of the oldest convincing body fossils of Ediacaran forms and of trace Clemente Formation (6), which when compared to a global fossils of animals known from western North America. composite of isotopic variation (23) indicate a stratigraphic Cloudina is the oldest megafossil known in the Nama position well below the lowest fossil horizons in Namibia. sequence of Namibia, and occurs in the lower Kliphoek Reports of "metazoan" fossils in the Twitya Formation of Member of the Kuibis Subgroup (22). Since the Deep Spring the Mackenzie Mountains deposited below tillite (24) might be Downloaded by guest on September 27, 2021 Geology: McMenamin Proc. Natl. Acad. Sci. USA 93 (1996) 4993 derived from strata older than the Clemente Formation. These ref. 28)], lasting at least 80 million years. This stretch of putative body fossils, however, have been demoted to "possible geologic time exceeds the length of the Cretaceous, at 78 metazoan origin" (23). Because no convincing trace or body million years the longest geologic period. fossil has yet been recovered from the Twitya Formation, the fossils of the Clemente Formation provide the oldest direct Appendix I evidence for Ediacaran communities. The Clemente Formation fossils cannot be radiometrically Sample 4 of 3/16/95; Sekwia sp., Lockeia ichnosp.; 6 of 3/16/95; dated. However, an age estimate can be made owing to the Lockeia ichnosp., Palaeophycus tubularis; 7 of 3/16/95; Palaeo- presence of the trace fossil Vermiforma antiqua in siltstone of phycus tubularis. Coordinates of site are N30°24.041', the Clemente Formation deposited above the Clemente oolite. Wl 11057.141'. This fossil (see Fig. 10 of ref. 1), once interpreted as a Sample 5 of 3/16/95; member of Erniettidae. Coordinates pseudofossil (7), is biogenic and conspecific with Vermiforma of site are N30024.013', W111°57.196'. antiqua from strata of the Carolina Slate Belt (8). Originally Sample 1 of 3/17/95; cf. Cyclomedusaplana. Coordinates of described as a body fossil (8), Vermiforma antiqua has been site are N30023.978', W111°57.116' reinterpreted as a trace fossil (25). The parallelism of track- ways (1, 8, 18) exhibited by cohorts of this ichnospecies (on the I thank F. Corsetti, I. Dalziel, A. Dix, D. Evans, M. Godchaux, H. same bedding plane) may be due to the shared current- Hurtado, J. Kirschvink, L. Margulis, D. McMenamin, J. M. Morales- controlled orientation of the individual trace makers to con- Ramirez, S. Rowland, A. Seilacher, and J. H. Stewart for assistance stant (1, 18) or fluctuating (8) water current direction. with various aspects of this research. The age of Vermiforma antiqua in North Carolina is brack- eted by uranium-lead (U-Pb) radiometric dates. An age (in 1. Stewart, J. H., McMenamin, M. A. S. & Morales-Ramirez, J. M. 586 ± 10 U-Pb has been obtained from the (1984) U. S. Geol. Surv. Professional Paper 1309, 1-36. millions ofyears) of 2. McMenamin, M. A. S., Rowland, S. M., Nance, R. P. & Corsetti, Uwharrie Formation, which was deposited stratigraphically F. (1992) Abstracts of the 29th International Geological Congress above the Vermiforma antiqua occurrence in North Carolina. (Int. Geol. Congress, Kyoto), Vol. 2, p. 257 (abstr.). This 586 + 10 date from the upper part of the Uwharrie 3. Horodyski, R. J. (1991) Geol. Soc. Am. Abstr. Program 23, A163. Formation is a U-Pb concordia age based on zircon crystals 4. McMenamin, M. A. S. (1985) J. Paleontol. 59, 1414-1425. from felsic volcanic rocks (26). An age of 620 ± 20 U-Pb has 5. Corsetti, F. A. & Kaufman, A. J. (1994) Palaios 9, 211-219. been determined for rocks immediately below Vermiforma 6. Rowland, S. M., Corsetti, F. & McMenamin, M. A. S. (1993) antiqua (8, 9). This 620 + 20 date is also a concordia date for Geol. Soc. Am. Abstr. Program 25, 140. zircons from a felsic tuff-breccia at the top of map unit II in 7. McMenamin, M. A. S., Pittenger, S. L., Carson, M. R. & Larra- what is now mapped as the Hyco Formation (10). Higher in the bee, E. M. (1994) N.Y State Mus. Bull. 481, 213-227. stratigraphic section, a Rb-Sr (rubidium-strontium) whole- 8. Cloud, P., Wright, J. & Glover, L. (1976) Am. Sci. 64, 396-406. ± 9. Runnegar, B. R. & Fedonkin, M. A. (1992) in The Proterozoic rock isochron minimum age of 540 7 million years, has been Biosphere, eds. Schopf, J. W. & Klein, C. (Cambridge Univ. Press, reported from a lenticular mafic volcanic deposit in the Yadkin New York), pp. 369-388. Formation (11). A more recently determined U-Pb zircon date 10. Glover, L. & Sinha, A. K. (1973) Am. J. Sci. 273A, 234-251. of 575 ± 7.6 millions years (reported from the Cid Formation, 11. Harris, C. W. & Glover, L. (1988) Geol. Soc. Am. Bull. 100, 200-217. equivalent to the McManus Formation; ref. 27) fits neatly into 12. Gibson, G. G., Teeter, S. A. & Fedonkin, M. A. (1984) Geology the geochronologic framework established by the other three 12, 387-390. radiometric dates. 13. Narbonne, G. M. & Aitken, J. D. (1990) Palaeontology 33, 945-980. Accepting these dates and their error ranges as valid indi- 14. Hofmann, H. J. (1981) Lethaia 14, 303-310. cates an age for the North Carolina Vermiforma antiqua fossils 15. Margulis, L., Corliss, J. O., Melkonian, M. & Chapman, D. J., of between 576 (i.e., 586 - 10) and 640 (620 + 20) million eds. (1989) Handbook ofProtoctista (Jones and Bartlett, Boston). 16. Grotzinger, J. P., Bowring, S. A., Saylor, B. Z. & Kaufman, A. J. years. But since the fossil occurrence is stratigraphically much (1995) Science 270, 598-604. closer to volcanic rocks associated with the latter (more 17. Seilacher, A. (1995) Fossile Kunst: Alumblatter der Erdgeschichte ancient) date, it is likely that the age of the fossils is close to, (Goldschneck, Korb, Germany). or in excess of, 600 million years. 18. McMenamin, M. A. S. & McMenamin, D. L. S. (1990) The Emer- By biostratigraphic correlation to the North Carolina section gence ofAnimals: The Cambrian Breakthrough (Columbia Univ. (see Fig. 1), the Vermiforma-bearing strata of the Clemente Press, New York). Formation above the oolite may also be assigned an approx- 19. Levy, M. & Christie-Blick, N. (1991) Soc. Econ. Paleontol. imate age of 600 million years. The age of the body fossils Mineral., Pacific Sect., Publ. 67, 1, 371-386. occurring below the oolite (that itself occurs below the Mex- 20. Nourse, J. A., Anderson, T. H. & Silver, L. T. (1994) Tectonics 13, ican Vermiforma) could very well be some millions or tens of 1161-1182. older than 600 that 21. Elston, D. P., Link, P. K., Winston, D. & Horodyski, R. J. (1993) millions of years million years, considering in The Geology of North America, Precambrian: Conterminous there is a major unconformity truncating the top of the United States, eds. Reed, J. C., Jr., Bickford, M. E., Houston, Clemente oolite. Thus, the 600-million-year age for the body R. S., Link, P. K., Rankin, D. W., Sims, P. K. & Van Schmus, fossils of the Clemente Formation proposed here could well be W. R. (Geol. Soc. of Am., Boulder, CO), Vol. C-2, pp. 468-487. a conservative estimate. 22. Grant, S. W. F. (1990) Am. J. Sci. 290A, 261-294. The Mexican fossils found below the Clemente oolite are the 23. Narbonne, G. M., Kaufman, A. J. & Knoll, A. H. (1994) Geol. oldest known occurrence of body fossils of the Ediacaran biota. Soc. Am. Bull. 106, 1281-1292. The implications of this discovery are 3-fold. (i) The evolu- 24. Hofmann, H. J., Narbonne, G. M. & Aitken, J. D. (1990) Geology tionary innovations leading to the development of the Ediaca- 18, 1199-1202. ran biota occurred in shallow marine water. This accords with 25. Fedonkin, M. A. & Runnegar, B. R. (1992) in The Proterozoic inferences by others (28) that Ediacaran organisms originated Biosphere, eds. Schopf, J. W. & Klein, C. (Cambridge Univ. Press, New York), pp. 389-395. in shallow-water seas. (ii) The locus of origin of the Ediacaran 26. Wright, J. E. & Seiders, V. M. (1980) Geol. Soc. Am. Bull. 91, organisms was on, or near, the continental shelves of the North 287-294. American craton. This inference is supported by the fact that 27. Kozuch, M., Heatherington, A. L. & Mueller, P. A. (1992) Geol. the next oldest Ediacaran assemblage is in northwestern Soc. Am. Abstr. Program 24, 217. Canada. (iii) Fossils of the Ediacaran biota were deposited 28. Crimes, T. P., Insole, A. & Williams, B. P. J. (1995) Geol. J. 30, during a significant interval [600-520 million years ago (see 89. 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