The Cambrian Substrate Revolution
David J. Bottjer, Department of Earth
Sciences, University of Southern California, Los Angeles, CA 90089-0740
James W. Hagadorn, Division of Geological
and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
Stephen Q. Dornbos, Department of Earth
Sciences, University of Southern California, Los Angeles, CA 90089-0740
ABSTRACT
The broad marine ecological settings prevalent during the late Neoproterozoic-early Phanerozoic (600-500 Ma) interval of early metazoan body plan origination strongly impacted the subsequent evolution and development of benthic metazoans. Recent work demonstrates that late Neoproterozoic seafloor sediment had well-developed microbial mats and poorly developed, vertically oriented bioturbation, thus producing fairly stable, relatively low water content substrates and a sharp water-sediment interface. Later in the Cambrian, seafloors with microbial mats became increasingly scarce in shallow-marine environments, largely due to the evolution of burrowing organisms with an increasing vertically oriented component to their bioturbation. The evolutionary and ecological effects of these substrate changes on benthic metazoans, referred to as the Cambrian substrate revolution, are presented here for two major animal phyla, the Echinodermata and the Mollusca.
INTRODUCTION
Late Neoproterozoic and early Phanerozoic body and trace fossils commonly exhibit strange morphological adaptations and paleoenvironmental distributions (e.g., Fig. 1). At this time, the basic body plans of large metazoans were first evolving, and much research has been expended toward understanding the evolutionary relationships of these ancient animals. Of particular importance is that while this evolutionary play of metazoan body plan evolution was taking place, the ecological stage was shifting. Two changes in the biological dimensions of the marine ecological stage were especially important. First was the advent and development of predation, which, together with additional biological and geochemical factors, fostered the evolution of mineralized skeletons (e.g., Vermeij, 1989; Bengtson, 1994).
The second change in the biological dimensions of the ecological stage occurred in seafloor sediments, which act as the substrate on and in which benthic organisms live. This change was caused by increasing disturbance of sediments by bioturbation (e.g., Droser, 1987; Droser and Bottjer, 1989) (Fig. 2). Through analogy with the development of agriculture and its resulting effects upon soils, Seilacher and PflYger (1994) have termed this change the agronomic revolution. Late Neoproterozoic seafloors were typically characterized by well-developed microbial mats (e.g., Gehling, 1986, 1996, 1999; Schieber, 1986; Hagadorn and Bottjer, 1997, 1999) and poor development of sediment mixing by vertically oriented burrowing (e.g., Droser et al., 1999; McIlroy and Logan, 1999) (Fig. 2). Sediment layers on the seafloor thus had relatively low water content and were characterized by a sharp water-sediment interface. Work on carbonates (e.g., Awramik, 1991) and more recently on siliciclastics (e.g., Hagadorn and Bottjer, 1997, 1999) has shown that in the Cambrian shallow marine environments characterized by seafloors covered with microbial mats became increasingly scarce, largely due to increasing vertically oriented bioturbation (Fig. 2). This change to a more Phanerozoic-style seafloor resulted in relatively greater water content of seafloor sediment and a blurry water-sediment interface, which led to the first appearance of a mixed layer. Mixed layers constitute the soupy upper few centimeters of the substrate that are homogenized by bioturbation and are characteristic of later Phanerozoic fine-grained substrates (e.g., Ekdale et al., 1984). With near elimination of microbial mats in shallow-marine environments, microbial or mat-related food sources in sediment changed from being well layered to having a more homogeneously diffuse distribution in the sediment layers on the seafloor. Thus, this agronomic revolution led to the soft-sediment substrates we commonly see in shallow carbonate and siliciclastic marine environments today (Fig. 2). We term the effects this transition had on benthic organisms the Cambrian substrate revolution (Bottjer and Hagadorn, 1999). The Cambrian substrate revolution involved both evolutionary and ecological changes occurring at different time scales, including extinction, adaptation, and environmental restriction.
EVOLUTIONARY AND ECOLOGICAL IMPACTS
Paleobiologists have long been interested in the morphological features evolved by organisms that live on soft sediment seafloors (e.g., Thayer, 1975). Until recently such adaptations could only be adequately assessed for later Phanerozoic benthic organisms, due to an incomplete understanding of late Neoproterozoic and Cambrian paleobiology and paleoenvironments. New data from the Neoproterozoic-Phanerozoic transition have allowed paleobiologists to begin to address the adaptive morphology of these early animals. Environments of the Neoproterozoic-Phanerozoic transition were different from those today, requiring the use of nonuniformitarian approaches to analyze the paleobiology and paleoecology of animals living at this time (e.g., Bottjer, 1998). For example, Seilacher (e.g., 1999) has postulated that lifestyles of organisms that lived on late Neoproterozoic sediments characterized by microbial mats, or matgrounds, would include: (1) mat encrusters, which were permanently attached to the mat; (2) mat scratchers, which grazed the surface of the mat without destroying it; (3) mat stickers, which were suspension feeders that were partially embedded in the mat, and comprise a subset of adaptations resulting in organisms broadly termed sediment stickers; and (4) undermat miners, which burrowed underneath the mat and fed on decomposing mat material.
The presence of metazoan fossils perhaps as old as 570 Ma (e.g., Fig. 1 in Martin et al., 2000), and molecular data indicating a possibly earlier origin of metazoans (e.g., Wray et al., 1996), suggests that there was an early stage of evolution for most benthic metazoan groups before they evolved mineralized skeletons (e.g., Fortey et al., 1996, 1997). This early stage of evolution for benthic organisms was within the environmental context of a Neoproterozoic-style minimally bioturbated seafloor covered with microbial mats. Thus, how did this late Neoproterozoic-Phanerozoic transition to more Phanerozoic-style seafloor conditions affect the evolution, dispersal, and paleo-environmental distribution of metazoans, which were adapted to these Neoproterozoic seafloor sediments? Were there animals and perhaps entire communities that were adapted to these seafloor conditions, in the manner proposed by Seilacher (1999)?
We cannot yet fully answer these questions. However, mounting evidence suggests that many evolutionary and ecological changes, which took place during this time interval, were due to the transition in substrate style from the late Neoproterozoic marine environments and lifestyles described by Seilacher (e.g., 1999), to the bioturbated sedimentary environments and morphological adaptations documented for later Phanerozoic benthic organisms (e.g., Thayer, 1975). Early suspension-feeding echinoderms and grazing polyplacophoran and monoplacophoran mollusks (and their likely soft-bodied ancestors) provide two examples of the effects of this change in substrate character.
SESSILE SUSPENSION-FEEDING ECHINODERMS
Evolution of Cambrian suspension-feeding echinoderms that had an immobile, or sessile, lifestyle provides strong evidence for the short-term impact of the Cambrian substrate revolution. For example, the unusual Early Cambrian helicoplacoid echinoderms were well adapted for survival on Neoproterozoic-style substrates. These small (1–5 cm) suspension-feeding echinoderms (Fig. 3) lived as sediment stickers on a substrate that underwent only low-to-moderate levels of horizontally directed bioturbation and did not have a mixed layer (Dornbos and Bottjer, 2000a). Helicoplacoids lacked typical Phanerozoic soft-substrate adaptations, such as the ability to attach to available hard substrates or presence of a root-like holdfast. Significant increase in depth and intensity of bioturbation in shallow-water muds and sands through the Cambrian (e.g., Droser, 1987) destroyed the stable substrates that these small echinoderms required and likely led to their extinction (Dornbos and Bottjer, 2000a) (Fig. 3).
In contrast, both edrioasteroids and eocrinoids, the other groups of undisputed Cambrian sessile suspension-feeding echinoderms, were able to adapt to the change in substrates created by increased bioturbation. The earliest edrioasteroids lived unattached on the seafloor during the Early and Middle Cambrian, but by the Late Cambrian edrioasteroids lived attached to available hard substrates (e.g., Sprinkle and Guensburg, 1995) (Fig. 3). Similarly, several Early and Middle Cambrian eocrinoids were stemless and lived unattached on the seafloor (Ubaghs, 1967; Sprinkle, 1992) (Fig. 3). By the Late Cambrian, however, eocrinoids had evolved stems and also lived attached to available hard substrates (Fig. 3). Thus, by attaching to hard substrates or by developing stems, each of these Cambrian echinoderm groups avoided the detrimental effects of increased substrate instability caused by increasing bioturbation (Fig. 3), and they survived into the post-Cambrian Paleozoic. The remaining undisputed Cambrian echinoderms were all mobile deposit- or suspension-feeders (e.g., Sprinkle, 1992). Their mobility likely exacerbated the substrate changes occurring during this time, and, because they could adjust their position relative to the sediment-water interface, they would have been relatively immune to the effects of this change in substrate character.
EARLY GRAZING MOLLUSKS
Similarly, how did mobile organisms that grazed the sediment surface (a life habit likely typical of early mollusks) respond to this change in substrate character? If organisms crawled on top of the sediment surface in late Neoproterozoic marine environments and scratched or scraped microbial mats for food, disappearance of mats from these settings might have restricted them to marine hard substrate environments where mats still flourished, such as those typical of the nearshore (e.g., rocks or reefs), where scratching or scraping microbial layers and biofilms off hard substrates was still a viable strategy. In addition, if organisms depended upon the relatively sharp water-sediment interface that the combination of mats and minimal vertical bioturbation produced in marine soft sediments, then, in response to this widespread change in substrate character, they also could have become restricted to soft substrate environments where these conditions still prevailed. The most likely environments in which to find such conditions are in the deep sea, where: (1) mats built by chemoautotrophic and heterotrophic microbes occur (e.g., Hagadorn and Bottjer, 1999; Simonson and Carney, 1999; and references within); and (2) biogenic reworking, although highly variable, may be several orders of magnitude less than on the shelf (e.g., Thayer, 1983; Gage and Tyler, 1991).
The evolutionary relationships of the mollusks are still controversial (e.g., Runnegar, 1996). Aplacophorans, polyplacophorans, and monoplacophorans are the most primitive mollusks living today (e.g., Salvini-Plawen and Steiner, 1996). Aplacophorans are generally thought to be the most primitive, because of their wormlike body form and spiculate skeleton, but they have no fossil record (Pojeta et al., 1987). Polyplacophorans, or chitons, have a broad muscular foot covered by eight dorsal shell plates (Fig. 4), and living representatives graze surficial microbial mats and biofilms (Pojeta et al., 1987). The oldest known polyplacophorans lived in the Late Cambrian and grazed on shallow-water stromatolites (Runnegar et al., 1979) (Fig. 4). Living monoplacophorans have a broad foot and are also surface grazers, but unlike chitons they have a single continuous dorsal shell (Fig. 4). Fossil monoplacophorans have a broader variety of shell morphologies than living genera, (e.g., Pojeta et al., 1987), and this complexity is reflected in the variety of interpretations that exist concerning monoplacophoran evolutionary relationships (e.g., Pojeta et al., 1987; Salvini-Plawen and Steiner, 1996; Runnegar, 1996). The oldest known monoplacophorans are Early Cambrian, and include substrate grazers (e.g., Pojeta et al., 1987) (Fig. 4). The post-Cambrian fossil record of both polyplacophorans and monoplacophorans is poor, and little is known about how and where they lived (e.g., Pojeta et al., 1987; Squires and Goedart, 1995; Cherns, 1998).
However, the modern occurrence of chitons and monoplacophorans exhibits the type of environmental distribution that one would predict as a long-range consequence of the Cambrian substrate revolution. Modern polyplacophorans typically occur in rocky coastline environments but some live in the deep sea (Pojeta et al., 1987; Squires and Goedart,1995) (Fig. 4). Living monoplacophorans occur in the deep sea on soft substrates, although one genus lives on hard substrates at the shelf edge (Pojeta et al., 1987) (Fig. 4). Thus, although little currently is known about the ecology of soft-bodied late Neoproterozoic and Cambrian ancestors of polyplacophorans and monoplacophorans, they may have lived on soft as well as hard substrates in shallow marine environments and grazed microbial mats that covered the seafloor, a lifestyle that today is typically restricted to hard substrates and the deep sea.
Behavioral evidence, in the form of trace fossils, provides additional insight into the life habits of early metazoans that lived on these soft substrates. For example, Upper Cambrian bedding surfaces from Oman contain large scratch marks that are morphologically identical to traces made by the grazing of modern gastropods upon hard substrates. Because these grazing traces are associated with ovate traces most likely produced by a soft-footed organism, they suggest that early mollusks were grazing on soft seafloor sediments (Seilacher, 1977, 1995). Gehling (1996) has also documented grazing traces, together with flattened ovoid body impressions, in Vendian strata of Australia, suggesting association with a soft-footed mollusk. Similar traces occur in Lower Cambrian strata in Yunnan Province, China (Dornbos and Bottjer, 2000b) and Vendian strata of the White Sea area, Russia (Martin et al., 2000). All of these scratch-style traces are associated with diagnostic sedimentary structures indicative of the presence of microbial mats, and all except the White Sea traces are from medium- to coarse-grained arenites. Considered together, these occurrences suggest that early in their evolutionary history, mollusks in nearshore to shelf-edge environments grazed upon sands, which behaved in a semilithified manner due to the presence of microbial mats (Fig. 4).
ADDITIONAL IMPLICATIONS
Because adaptations to these mat-covered and more coherent Neoproterozoic-style
soft substrates required different morphologies and behaviors than soupier Phanerozoic-style
soft substrates, the Cambrian explosion is also characterized by a unique variety
of bedding-parallel trace fossils. For example, large meandering trace fossils
such as Plagiogmus and Taphrhelminthopsis (Fig.
1) were common in Early Cambrian shallow-marine environments, yet were likely
made by soft-bodied metazoans for which we have no body fossil record (McIlroy
and Heys, 1997; Hagadorn et al., 2000). Similarly, several other meandering trace
fossils as well as those exhibiting a network pattern, including Helminthoida
and Paleodictyon, also occur in Cambrian strata deposited in shallow-marine
environments (Crimes and Fedonkin, 1994). A number of these Cambrian trace fossil
genera, as well as ichnogenera with similar morphologies, are found only in deep-sea
strata after the Cambrian, and thus are united by a similar paleoenvironmental
history of onshore-offshore retreat (Bottjer et al., 1988; Crimes and Fedonkin,
1994; Hagadorn et al., 2000). This pattern of post-Cambrian restriction to the
deep sea by bedding-parallel trace fossils is mirrored by the record of microbial
structures produced in siliciclastic sediments (Hagadorn and Bottjer, 1999). Thus,
as for grazing mollusks, the environmental restriction shown by trace fossils
is likely also an effect of the Cambrian substrate revolution, caused by the broad
increase in vertically directed bioturbation and consequent decrease in development
of microbial mats, in shallow-marine environments.
Further analysis of the Cambrian substrate revolution may contribute to a better understanding of broader evolutionary phenomena. The Cambrian is characterized by a wide variety of metazoans, reflected in both body and trace fossils, many of which have morphologies that appear strange to the modern eye (e.g., Gould, 1989). Perhaps the co-occurrence during the Cambrian of benthic metazoans adapted more to Neoproterozoic-style soft substrates, with those more adapted to Phanerozoic-style substrates, contributed significantly to the high morphological disparity exhibited by animals of the Cambrian explosion.
ACKNOWLEDGMENTS
This contribution has benefited from numerous discussions over the years with M. Droser, A. Fischer, J. Gehling, A. Seilacher, W. Ausich, D. Gorsline, P. Myrow, F. PflYger, J. Schieber, and B. Waggoner. Helpful reviews were provided by R. Bambach, L. Babcock, and M. Miller. Bottjer thanks J.W. Schopf and the University of California at Los Angeles Center for the Study of Evolution and the Origin of Life for support of this research. Hagadorn is grateful for postdoctoral fellowship support from J. Kirschvink and the Caltech Division of Geological and Planetary Sciences. Dornbos thanks GSA, the Paleontological Society, and the University of Southern California Wrigley Institute for Environmental Studies and Department of Earth Sciences for support.
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Manuscript received May 9, 2000; accepted
June 30, 2000.
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