Lower Cambrian marine animal and bacterial paleontology, and ancient endosymbiosis

Emre Unal, Purdue University

Abstract

The classical view of the early stages of the Sauk Transgression, deposition along the western margin of the Laurentia, was a gradual deepening reflected by a change from the cross-stratified quartz rich sands of the Wood Canyon and Zabriskie Quartzite to the fine silty shales of the Latham Shale and lastly to the oncoid rich Chambless Limestone in the Marble Mountains sequence. The presence of several local karst surfaces in Chambless Limestone indicates that the transgressing seas were shallower than previously believed and characterized by periodic increases of clastic influx during the early half of the Cambrian. Following the Cambrian Explosion, at the beginning of the Cambrian, fundamental changes began to appear in the life styles of grazer dominated benthic communities. These changes are believed to be in response to increased competition by new faunal elements and the appearance of new aggressive predators that have been well documented in the Middle Cambrian Burgess Shale lagerstätten. Predator avoidance strategies include the development of thicker shells, camouflage, chemical repellants or sheltering strategies. While thicker shells and the appearance of exoskeletons are easy to document, strategies base on stealth, chemical and sheltering strategies are nearly impossible to demonstrate. The remarkable well-preserved oncoid lagerstätten facies in the Chambless Limestone and Cadiz Formation in the Marble Mountains provide a rare opportunity both to document the earliest occurrences of bioclaustration (commensalism) and conchicolous strategies (sheltering predator avoidance) in the fossil record and to examine the initial stages of the diversification of benthic life strategies. Stromatolites (photosynthesizing cyanobacteria and algae) form the base of food web and are essential to understand the evolution of the structure of the benthic marine communities during the early part of the Cambrian. The role of stromatolites within the benthic community range from: (1) a source of food for grazers; (2) a substrate for shelters; (3) surface for epi-symbionts; and (4) host for infaunal symbionts. Approximately 80% of the oncoids in the Chambless and 95% of the Cadiz oncoid stromatolites display a variety of internal structures that formed either during the growth (syn-formational) or following the cessation of growth (post-formational). In the studied plane of sections of 20% of Chambless and 5% of Cadiz oncoids, these structures do not present. A large number of Marble Mountain oncoids display narrow tube-like structures distinct from borings which are believed to represent the earliest examples of bioclaustration, an endosymbiotic strategy of an organism living in the growing skeleton of a host organism. These structures within the Marble Mountain oncoids display a variety of shapes ranging from narrow tubes with slight inflations to broader internal passageways has been separated into 7 morphotypes. Differences in the wall structures suggest that more than one species may be responsible for the construction of the bioclaustrations. Although stratigraphic distribution of shape and tube characters is suggestive of more than a single species, it is not possible to document the presence of more than one species. Vermeij (1987) proposed the term conchicolous habit as the use of a discarded shell as shelter from potential predators. Species that use this avoidance strategies typically are soft-bodied and do not have any external shell of defensive structures. Conchicolous occurrences in the fossil record are difficult to document unless proxy evidence is present, such as aperture modification. The internal structure within the oncoids from the Marble Mountains clearly shows symbiotic activities ranging from burrows to the occupation of discarded hyolith shells by some unknown soft-bodied species for an extended period. While it is not possible to demonstrate that this species was used the shell as shelter from potential predator, it is a reasonable option. The numerous examples of paired passages extending from the aperture of hyolith within the oncoids suggest that the shell as occupied by a hyolith while the oncoid encrusted the shell or was occupied following the death of the hyolith prior to the onset of oncoid growth. The absence of any opercula associated with the hyolith conch suggests that shell was occupied some unknown soft-bodied species following the death of the hyolith. Three-dimensional modeling indicates that the shell was occupied for an extended period by the thickness of the oncoid cortex and the narrowing of the passageways. Under conditions where the shell was not enveloped by cyanobacteria growth, it would not be possible to demonstrate occupation of the shell unless if there was some bio-erosion features. The envelopment of the shell by the cyanobacteria provides indisputable evidence for an extended occupation of the hyolith and suggests that shelter avoidance appeared much earlier in the Phanerozoic than previously believed. The appearance of bioclaustration and sheltering in the oncoids from the Marble Mountains suggest that diversification and development of infaunal life strategies within the benthic community during Series 2 and 3 (520 to 500 ma) of the middle Cambrian began shortly after the Cambrian Explosion. The development of these new infaunal life-styles was in response to a combination of increased competition with new benthic elements and the rise of new and aggressive predators.

Degree

Ph.D.

Advisors

Zinsmeister, Purdue University.

Subject Area

Geology|Paleontology

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