The urban bedrock of a low-relief landscape beneath a crowded city seems like an unusual place for a significant fossil discovery. However, a new fossil find at four sites amid the bustling expressways and vast expanses of homes and commercial buildings of metropolitan Miami, Florida, indicates that these sites were once a unique marine habitat. In a report published in the May 2007 issue of Geology, scientists from the U.S. Geological Survey (USGS), Brigham Young University, and Smith College describe the newly discovered fossils: large, dense aggregations of hefty vase- and barrel-shaped sponges as much as 2 m high and 1.8 m in diameter. The fossilized sponge communities occur as biostromesextensive, blanket-like masses of rock built and composed mainly of sedentary organisms, in this case, sponges. More simply, the aggregations may be thought of as sponge reefs. Within the limestone bedrock at the discovery sites, the upright fossil sponges are commonly packed so closely that they resemble a dense forest of wide, stubby tree trunks. The sponges are part of the Miami Limestone, which formed in southeastern Florida during the last interglacial period, approximately 125,000 years ago, when sea level was somewhat higher than today. This geologic unit forms the foundation of the natural terrain in present-day Miami.
These fossil sponges belong to a class of sponge called the demosponge, which includes the familiar variety of natural sponge used for bathing. Relations between the newly discovered sponge reefs and surrounding rock types indicate that the sponges thrived mostly within tidal channels where seawater was cyclically exchanged at relatively high rates of flow between the ancient Atlantic Ocean and a landward shallow lagoon in an area that includes part of the present-day and predevelopment Everglades. The fossil sponge reefs appear to be constrained to ancestral tidal channels, with bends and straight stretches of their reef structures conforming to the courses of the original tidal channels. These ancient tidal channels cut northeast to southwest through a region of large submarine sand dunes that were several meters above the surrounding sea floor, creating a mosaic submarine landscape that separated the lagoonal waters from the open waters of the paleo-Atlantic Ocean. The largest of the sponge reefs follows the axis of one relict tidal channel for a distance of 3.5 kilometers.
USGS hydrogeologist Kevin Cunningham discovered the first two of four reefs during the summer of 2004. On a routine drive through suburban Miami, he noticed something unusual in the wall of one of the numerous drainage canals constructed throughout Miami more than 40 years ago. "I stopped and took a quick look at a few of the sponges. At first, it wasn't clear to me just what I'd found, but after more careful examination, I realized I'd lucked upon a real beauty of a sponge reef. Because researching the sponges was outside the normal scope of my responsibilities with the USGS, I sent off samples to fossil-sponge expert Keith Rigby at Brigham Young University, who verified that the sponges are a single species of demosponge." Rigby has identified the sponges as a new, extinct species, which he and Cunningham have named Miamiamplia vasiforma in a newly published paper on the sponge's taxonomy (see Journal of Paleontology, July 2007, URL http://jpaleontol.geoscienceworld.org/cgi/content/extract/81/4/788).
"Once I realized that I was on to something with potential substantial impact on the geologic community," continued Cunningham, "I began to work on the primary discovery site on weekends and even used vacation time." According to Cunningham, "These are the only sponge reefs currently known, within both the modern and fossil record, that were constructed within moderate- to high-energy tidal channels. Scientists have generally assumed that large barrel sponges live in rather low-energy submarine settings."
The new findings raise more questions: What contributed to the ability of these particular sponges to inhabit the ancient tidal channels with such robust individual sizes, and to form long, kilometer-scale reefs? Some of the sponges have clearly developed "holdfasts" that extend outward from the lower parts of the sponges and can look like the roots of a tree. These holdfasts may have played a role in keeping the sponges upright in the swift currents and shifting sand dunes within the tidal channels. Cunningham speculates that "high nutrient levels, flowing from a more northern part of the ancient flooded Floridan peninsula during the last interglacial period, gave the sponges an advantage over competing organisms in the stressed channel environments. This would allow the sponges to virtually dominate some areas of the channel landscape."
Scientists use comparisons between the modern environment and the fossil record to improve our understanding of Earth history. The occurrence of the Florida fossil sponges in relict tidal channels provides a new example of an environmental niche where sponges can construct large-scale reefs. Today, similar large sponge reefs, produced by siliceous sponges, are known from a few places, notably off the west coast of British Columbia, but in cold, deep water. Calcified demosponges are the primary frame-building organisms within some modern coral reefs; however, they occur in the much deeper parts of the reefs, below the zone of active coral growth near the sea surface. Cunningham commented that "compared to these other types of modern deep-water sponge reefs, the geologically young Miami sponge reefs are unique. They are the only known large-scale sponge reefs I'm aware of that have formed in shallow-marine, tropical seas since many millions of years ago." But that doesn't mean there may not be others. Not far away to the east of Miami, on the Great Bahama Bank, modern examples of similar mosaics of tidal channels and submarine sand dunes are common. Like the ancient sand dunes where the fossil sponges were discovered, the sand dunes on the Great Bahama Bank consist largely of ooids: smooth, round grains formed by concentric layers of calcium carbonate precipitated around a nucleus. Ooids are primarily found where strong bottom currents exist. Not long ago, in the 1980s, geologists were amazed by the discovery of large, erect columns of algal-bound sediment called stromatolites growing in association with oolitic sand bodies in Bahamian tidal channels. Up to then, modern stromatolites were thought to be confined to hypersaline waters (for example, Hamelin Pool in Shark Bay, Australia). The discovery of numerous stromatolites growing in normal marine waters led to a revolution in how Earth scientists interpret the marine environments in which fossil stromatolites had formed.
After seeing the spectacular Miami sponge reefs, some of Cunningham's geologic colleagues interested in comparing modern and ancient limestone environments have begun to discuss exploring the tidal channels of the Great Bahama Bank in search of modern shallow-water sponge reefs. Perhaps one day a Bahamian tidal channel will be found to contain the modern counterpart to the fossil sponge reefs discovered by Cunningham in the Miami Limestone.
According to Cunningham, "It's amazing to me that no one had noticed the astonishing reefs before. The engineers and construction teams that built these canals must have known they were digging up something very unusual but obviously didn't realize what they were or their significance. Additionally, the Miami area has been mapped by a number of prestigious geologists who have left great legacies of understanding the geologic history of South Florida. Somehow these sponges have kept their existence secret until now."
Details about the newly discovered sponges are available in the recent articles in Geology and Journal of Paleontology:
Cunningham, K.J., Rigby, J.K., Wacker, M.A., and Curran, H.A., 2007, First documentation of tidal-channel sponge biostromes (upper Pleistocene, southeastern Florida): Geology, v. 35, no. 5, p. 475-478, doi:10.1130/G23402A.1 [URL http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2FG23402A.1].
Rigby, J.K., and Cunningham, K.J., 2007, A new, large, late Pleistocene demosponge from southeastern Florida: Journal of Paleontology, v. 81, no. 4, p. 788-793 [URL http://jpaleontol.geoscienceworld.org/cgi/content/extract/81/4/788.]
in this issue: