Ediacaran epifaunal tiering

M.E. Clapham   Department of Geological Sciences, Queen’s University, Kingston, Ontario K7L 3N6, Canada G.M. Narbonne 

ABSTRACT not light, is the limiting resource (Lane, 1963; Epifaunal tiering, the subdivision of vertical space within a community, is a fundamental Brett and Liddell, 1978; Watkins, 1991). attribute of Phanerozoic suspension-feeding communities. This paper documents tiering, Epifaunal tiering in ®lter-feeding commu- including the presence of meter-tall organisms, in Neoproterozoic Ediacaran communities. nities develops as a means of resource parti- Ediacaran tiering was studied from three exceptionally preserved deep-water communities tioning in an environment in which sessile or- at Mistaken Point, Newfoundland, which contain in situ census populations of hundreds ganisms receive nutrients from horizontally to thousands of organisms. Tiering consists of overlapping populations of dominant or- moving currents (Bottjer and Ausich, 1986). ganisms and is characterized by gradational, rather than abrupt, tier boundaries. At least A distinct velocity gradient forms within the three tiers are apparent: a lower level 0±8 cm above the sea¯oor, an intermediate level bottom current, so that epifaunal organisms between 8±22 cm above the sea¯oor, and an upper level that extends as high as 120 cm. can take advantage of faster water ¯ow, and Tier boundaries are relatively consistent between communities, but the constituent organ- therefore greater feeding capacity, by growing isms in each level are variable, suggesting that some Ediacaran taxa could ®ll different higher in this hydrodynamic boundary layer. tiers interchangeably. Development of a tiered epifaunal structure is consistent with sus- In addition, by becoming taller than its neigh- pension feeding or absorbing dissolved nutrients directly from seawater. Despite the com- bors, an organism can gain a distinct compet- mon occurrence of tall organisms, all communities share a similar population structure itive advantage by feeding from a previously in which biomass is concentrated in the basal 10 cm above the sea¯oor. Comparison with untapped nutrient source (Bottjer and Ausich, shallow-water Ediacaran localities suggests that the observed tiering structure is typical 1986). of Ediacaran communities. Ediacaran tierers also show the fundamental subdivision be- Tiering was best developed in the Paleozoic tween organisms and/or colonies that fed along their entire length and those that developed (post-Ordovician) and the early Mesozoic, a specialized feeding apparatus, implying that the features of Phanerozoic tiered skeletal when skeletal epifaunal communities were ecosystems were ®rst initiated in soft-bodied communities in the late Neoproterozoic. strati®ed into four levels: 0±5 cm, 5±20 cm, 20±50 cm, and 50±100 cm above the substrate Keywords: Ediacaran, Neoproterozoic, Newfoundland, paleoecology, tiering. (Ausich and Bottjer, 1982; Bottjer and Ausich, 1986). The relative abundance of organisms in INTRODUCTION 3 km, have been found in this area (Narbonne Epifaunal tiering, the development of a ver- et al., 2001). Taxonomic study of these assem- tically strati®ed community structure above the blages is ongoing; several important taxa that sea¯oor, is commonly adopted in extant marine have not yet been formally described are re- communities as a biological solution for re- ferred to in the literature by using consistent source partitioning. Tiering has been inferred but informal terms (e.g., spindles, pectinates). from numerous studies of Phanerozoic shelly Whereas most Ediacaran assemblages re¯ect fossil assemblages, and complex tiering of skel- shallow-water conditions (Narbonne, 1998), etal organisms is generally thought to have the turbiditic nature of the sediments at Mis- originated during the Ordovician with the evo- taken Point, containing abundant slumps and lution of large, stalked crinoids (Bottjer and debris ¯ows but no wave-generated structures, Ausich, 1986). Tiering in soft-bodied Ediacaran suggests that it is one of the few deep slope ecosystems has previously been discussed only localities, occurring well below both wave- from a theoretical perspective based on pub- base and the photic zone (Misra, 1971; Land- lished heights from the literature (Narbonne, ing et al., 1988; Myrow, 1995; Narbonne et 1998; Ausich and Bottjer, 2001). This paper is al., 2001). Fossil assemblages at Mistaken the ®rst to fully document tiering structure from Point were instantaneously buried beneath individual Ediacaran communities. volcanic-ash layers, preserving census popu- The Ediacara biota is an assemblage of soft- lations of in situ communities (Seilacher, 1992). bodied organisms that were globally distrib- The ash was dated as 565 Ϯ 3 Ma (U-Pb on uted during the late Neoproterozoic (Nar- zircons, Benus, in Landing et al., 1988), imply- bonne, 1998). The af®nities of the Ediacara ing that Mistaken Point assemblages are the biota are controversial (see Runnegar, 1995; oldest known complex animal communities. Narbonne, 1998, and references therein), but Tiering has long been recognized in terres- it is most commonly regarded as dominated trial forest communities, where height strati®- by cnidarian-grade animals and/or extinct cation results from competition for light (Can- groups possibly allied with the Cnidaria (Buss nell and Grace, 1993). Studies of subtidal kelp and Seilacher, 1994). Complex Ediacaran fos- forest communities suggested that strati®cation Figure 1. Occurrence of complexly tiered sils were ®rst described from the Mistaken in the marine realm could also result from com- Ediacaran communities at Mistaken Point, Point area in southeastern Newfoundland (Fig. petition for light (Foster, 1975). Epifaunal Newfoundland. A: Location map (Avalon ter- 1) by Anderson and Misra (1968). Subse- tiering was also observed in paleontological rane is shaded). B: Composite stratigraphic section, showing position of Lower Mistaken quently, hundreds of fossil-bearing surfaces, studies, mainly of Paleozoic suspension-feed- Point (LMP), D, and E fossil surfaces and .spanning a stratigraphic interval of more than ing, skeletal organisms, where particulate food, ash dated as 565 ؎ 3 Ma

᭧ 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; July 2002; v. 30; no. 7; p. 627±630; 3 ®gures. 627 each tier was not presented, although studies from well-preserved Silurian communities show that Ͻ10% of suspension-feeding spe- cies were taller than 10 cm (Watkins, 1991; Taylor and Brett, 1996). Two food-gathering strategies were adopted by Phanerozoic pri- mary tiering organisms: colonial tierers (e.g., tabulate corals and bryozoans) gathered food along their entire length, whereas solitary tier- ers (e.g., crinoids) developed a specialized feeding apparatus suspended above the sea- ¯oor (Bottjer and Ausich, 1986). Tiering was not well developed in Cambri- an and Early Ordovician skeletal epifaunal communities (Ausich and Bottjer, 1982, 2001). Studies of tiering in Cambrian Lager- staÈtten have not been carried out, but the pres- ence of tall ®lter-feeding organisms in the Burgess Shale (Conway Morris, 1986, 1993) and Chengjiang (Chen and Zhou, 1997) biotas suggests that detailed studies of tiering among soft-bodied ®lter-feeders could be fruitful.

EPIFAUNAL TIERING AT MISTAKEN POINT Ediacaran assemblages at Mistaken Point (Fig. 1) provide a superb natural laboratory to investigate the ecology of early animals. The three largest, most densely populated, and di- verse fossil surfaces in the Mistaken Point Formation, the D, E, and Lower Mistaken Point surfaces, were selected for detailed study of tiering relationships. All identi®able Figure 2. Cumulative-percentage species composition of each tier and total-population size- complete fossils on each surface (250±2800 frequency distribution for each surface studied at Mistaken Point. A and B: D surface. C and D: E surface. E and F: Lower Mistaken Point surface. fossils) were measured. Height was deter- mined by measuring the combined frond and stem length of frondose taxa and the total overlapping regions characterized by one or E Surface length of other upright organisms (see Sei- several dominant organisms. Dominance was The E surface displays a similar, but more lacher, 1999; Narbonne et al., 2001). Because quanti®ed by the percentage abundance of complex, pattern of overlapping species (Fig. the region has undergone tectonic deforma- complete fossils in each 2 cm height class 2C) to that observed on the D surface. Species tion, all measured fossil heights were mathe- (Fig. 2). Representative examples of each of that occupied a single level on the D surface matically retrodeformed to remove the effects the main fossils in the Mistaken Point For- may occupy the same level, a different level, of cleavage-related shortening and restore the mation are illustrated in Figure 3. or multiple levels on the E surface. In addi- original height of the living organism (see Sei- tion, more unique forms are present on the E lacher, 1992). Spindles were dominantly hor- D Surface surface. Spindles again dominate the lowest izontal on the sea¯oor; however, taphonomic The D surface species composition (Fig. level from 0 to 8 cm height. Intermediate lev- evidence suggests that they had a height above 2A) displays the gradation of forms above the els are composed of gradational populations the substrate approximately equal to half of sea¯oor that is typical of Ediacaran commu- of Bradgatia (6±14 cm), Charnia (8±22 cm), their width (J. Gehling, 2001, personal com- nities at Mistaken Point. There is a progres- and triangles (G in Fig. 3; 10±20 cm). Duster mun.). As with Phanerozoic and living ben- sive domination of speci®c taxa with increas- forms (H in Fig. 3) occur at all heights up to thos (Okada and Ohta, 1993), it is probable ing height, beginning with the lowest level of 16 cm and Charniodiscus (C in Fig. 3) dom- that at least some of the frondose taxa of the spindles (A in Fig. 3) at 0±8 cm above the inates most height classes, especially above 20 Mistaken Point Formation were oblique to the sea¯oor. The next epifaunal level is dominated cm. As with the D surface, organism lengths sea¯oor during life (see also Seilacher, 1992). by pectinates (E in Fig. 3) between 4 and 14 are strongly skewed to short heights, with a If so, the measured organism length does not cm, and Bradgatia (F in Fig. 3), which occurs peak at 4 cm and 95% of specimens Ͻ10 cm provide an absolute value for height above the at heights of 8±22 cm. The upper epifaunal tall. sea¯oor, but does provide a measure that can level, between 22 and 30 cm, is exclusively be directly compared with Phanerozoic com- populated by Charnia (D in Fig. 3). The total- Lower Mistaken Point Surface munities in which tiering height was also mea- population height distribution (Fig. 2B) has a The Lower Mistaken Point surface contains sured from the maximum length of the fossil. maximum at 4 cm and is strongly left skewed, the simplest tiering structure, although the Tiering is well developed on all three of the so that, although heights to 30 cm are ob- general pattern is the same as that observed fossil surfaces (Fig. 2). On these bedding served, nearly 95% of all specimens are Ͻ10 on the previously described surfaces. The planes, epifaunal space is subdivided into cm tall. community is composed almost entirely of

628 GEOLOGY, July 2002 currents and a thicker boundary layer. Com- parison of recorded fossil heights from shallow- water Ediacaran localities (cf. Ausich and Bo- ttjer, 2001) provides an indication that the tiering structure of Mistaken Point communi- ties may be representative of Ediacaran com- munities in general. For example, lower levels in Australian communities may have included Tribrachidium and Arkurua, with frondose taxa to 73 cm in height occupying interme- diate and upper levels (Jenkins and Gehling, 1978; Jenkins, 1992; Ausich and Bottjer, 2001). Similarly, lower feeding levels in Na- mibia were dominated by Beltanelliformis, Er- nietta, and small fronds (e.g., Rangea); larger fronds (e.g., Swartpuntia) to 25 cm in length were also present (Jenkins, 1992; Narbonne et al., 1997). These published measurements suggest that the tiering at Mistaken Point was comparable to that of shallow-water Ediacaran communities and was not merely a response to deep-water hydrodynamic conditions.

TIER COMPOSITION Figure 3. Diorama of composite Mistaken Point community, showing tiered epifaunal struc- Although the height of tier boundaries ap- ture. A: Spindle. B: Ostrich feather. C: Charniodiscus. D: Duster. E: Charnia.F:Bradgatia. pears relatively consistent between communi- G: Pectinate. H: Triangle. I: Xmas Tree. ties, the individual tier constituents display a considerable amount of plasticity. Most organ- frondose taxa; only three spindles are present per taxon, as seen in the division between pec- isms may occur in different tiers at different at the lowest levels. Charnia dominates most tinates and Bradgatia on the D surface. Ex- sites or in multiple tiers at a single site. For levels up to 35 cm. The non-Charnia popu- tremely tall specimens are also present on the example, Charnia is characteristic of the up- lation is subdivided between ``ostrich feath- Lower Mistaken Point surface, suggesting that per level on the D surface, but occurs mainly ers'' (B in Fig. 3), which are present at low there may be another suite of taxa that dom- at intermediate heights in the E surface com- heights (0±10 cm), and Charniodiscus, which inates heights exceeding 1 m. A composite di- munity, and is found at all heights at Lower is more abundant at higher levels (14±22 cm). orama of tiered Mistaken Point communities Mistaken Point, dominating at heights where The population structure has a peak at 4 cm is given in Figure 3. more specialized tierers at that locality were height and is dominated by small specimens Ediacaran tier boundaries (6±8 cm, 22 cm, not present. The fact that tier composition (80% are Ͻ10 cm tall). However, this surface 35 cm, and possibly ϳ100 cm) are similar to varies signi®cantly between communities sug- differs from the D and E surfaces in the pres- the levels recognized by Bottjer and Ausich gests that some organisms were ®lling the ence of three rare forms (such as the ``Xmas (1986) for Phanerozoic communities of skel- same ecological niche and could occupy dif- tree''; I in Fig. 3) at heights of 90±120 cm etal organisms (5 cm, 20 cm, 50 cm, and 100 ferent tiers interchangeably, depending on above the sea¯oor. cm). The only signi®cant difference occurs in their population structure (governed by chance the upper boundary of the third tier (35 cm factors such as time of recruitment). Only Tiering Structure versus 50 cm); however, the rarity of pre- spindles are invariably con®ned to a single tier All three surfaces display a consistent pat- served specimens Ͼ35 cm at Mistaken Point (lowest level), likely for constructional rea- tern of overlapping dominant organisms, al- obscures the exact tier boundary. The maxi- sons. Spindles allocate most of their biomass though the faunal composition differs consid- mum height in tiered Ediacaran communities in horizontal growth, whereas all other organ- erably. Boundaries between each of the levels (ϳ90±120 cm) is the same as the upper size isms are dominantly upward growing, so that are typically gradational (e.g., Fig. 2B), but limit in Paleozoic skeletal communities (Bo- even the oldest and largest spindles would be occur at similar heights as de®ned by faunal ttjer and Ausich, 1986). In addition, the dom- shorter than most frondose taxa. changes on a species-height plot (e.g., Fig. inance of small (Ͻ10 cm tall) specimens is 2A). There is a signi®cant faunal change be- consistent with Phanerozoic population size EDIACARAN FEEDING STRATEGIES tween spindle-dominated lower levels and distributions (Watkins, 1991; Taylor and Brett, Various feeding strategies have been pro- frond-dominated intermediate levels at 6±8 1996). In contrast to Phanerozoic communi- posed for Ediacaran taxa, including photosyn- cm on the D and E surfaces. This shift is ties, infaunal tiering is not developed in Edi- thetic or photosymbiotic (McMenamin, 1986), marked by the transition between ostrich acaran communities, and de®nite burrows are absorbing dissolved nutrients directly from feathers and Charniodiscus on the Lower Mis- absent from Mistaken Point. seawater (Seilacher, 1989), and suspension taken Point surface, also at ϳ8 cm height. The feeding (Jenkins and Gehling, 1978). Photo- intermediate level fauna is replaced abruptly DEEP-WATER VERSUS SHALLOW- autotrophic methods can be discounted for at 22 cm on all surfaces with a monospeci®c WATER COMMUNITIES Mistaken Point communities because the or- upper suite of tall fronds (either Charnia or Bottjer and Ausich (1986) explicitly ad- ganisms were living in deep water well below Charniodiscus). Subdivisions within this 8±22 dressed tiering only in shallow-water com- the photic zone (Seilacher, 1992; Narbonne et cm range can be complex, as on the E surface, munities, because deep-water organisms tend al., 2001); however, both absorption of dis- or simply partitioned between a lower and up- to be taller to compensate for slower bottom solved matter and ®lter feeding are consistent

GEOLOGY, July 2002 629 with the development of a tiered epifaunal cambrian extinctions of Ediacara taxa and/or tis pyrifera forest: Marine Biology, v. 32, structure. In both cases nutrients are derived cropping of upper tier organisms by evolving p. 313±329. Jenkins, R.J.F., 1992, Functional and ecological as- from horizontally ¯owing bottom currents, fa- Cambrian macropredators. Although there are pects of Ediacaran assemblages, in Lipps, voring epifaunal tiering to increase feeding ef- no organisms in common between Mistaken J.H., and Signor, P.W., eds., Origin and early ®ciency by partitioning nutrient supplies. Point communities and Phanerozoic skeletal evolution of the Metazoa: New York, Plenum, Analogues of colonial tierers and solitary assemblages, many key properties of tiered p. 131±176. Jenkins, R.J.F., and Gehling, J.G., 1978, A review tierers (Bottjer and Ausich, 1986) are also Phanerozoic ecosystemsÐsuch as approxi- of the frond-like fossils of the Ediacara assem- found in the Ediacaran ecosystems at Mistak- mate tier boundaries, maximum organism blage: South Australian Museum Records, en Point. Although it is not possible to deter- height, and dominance of low-level organ- v. 17, p. 347±359. mine whether Ediacaran organisms were co- ismsÐwere initiated in soft-bodied commu- Landing, E., Narbonne, G.M., and Myrow, P., 1988, Trace fossils, small shelly fossils and the Pre- lonial or solitary, they have adopted the same nities in the late Neoproterozoic. cambrian-Cambrian boundary: New York feeding strategies as Phanerozoic colonial and State Museum and Geological Survey Bulle- solitary tierers. Ediacaran organisms such as ACKNOWLEDGMENTS tin, v. 463, 81 p. Bradgatia, Charnia, and spindles collected Funding for the project was provided by a Nat- Lane, N.G., 1963, The Berkeley crinoid collection nutrients along their entire length, exploiting ural Sciences and Engineering Research Council of from Crawfordsville, Indiana: Journal of Pa- leontology, v. 37, p. 1001±1008. a similar strategy to Phanerozoic colonial cor- Canada (NSERC) grant (to Narbonne) and by an NSERC postgraduate scholarship (to Clapham). McMenamin, M.A.S., 1986, The garden of Edi- als and bryozoans. However, Charniodiscus, Rodrigo Sala and Marc La¯amme provided helpful acara: Palaios, v. 1, p. 178±182. dusters, and ostrich feathers were more similar assistance in the ®eld. Fieldwork in the Mistaken Misra, S.B., 1971, Stratigraphy and depositional to Phanerozoic crinoids in having a special- Point Ecological Reserve was carried out under Sci- history of late Precambrian coelenterate-bear- ing rocks, southeastern Newfoundland: Geo- ized frond suspended above the sea¯oor. Both enti®c Research Permits granted by the Parks and Natural Areas Division, Department of Tourism, logical Society of America Bulletin, v. 82, of these feeding strategies are also well rep- Culture, and Recreation, Government of Newfound- p. 979±988. resented in shallow-water Ediacaran ecosys- land and Labrador. This paper was greatly improved Myrow, P.M., 1995, Neoproterozoic rocks of the tems in Australia, Namibia, and the White by reviews by David Bottjer, Carlton Brett, and Jim Newfoundland Avalon zone: Precambrian Re- Gehling. search, v. 73, p. 123±136. Sea. Okada, H., and Ohta, S., 1993, Photographic evi- dence of variable bottom-current activity in CONCLUSIONS REFERENCES CITED the Suruga and Sagami bays, central Japan: Anderson, M.M., and Misra, S.B., 1968, Fossils Epifaunal tiering is well developed in Edi- Sedimentary Geology, v. 82, p. 221±237. found in the Precambrian Conception Group Narbonne, G.M., 1998, The Ediacara biota: A ter- acaran communities at Mistaken Point. 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630 GEOLOGY, July 2002