BRYOZOAN PALEONTOLOGY IN JAPAN
Japan is rich in fossiliferous Pleistocene deposits, and with the works of Sakakura, Hayami, Kataoka, Arakawa, and Fukuda there is a long history of study of the bryozoans in these deposits. In 2005, our laboratory and colleagues recently discovered perhaps the best deposit of Pleistocene bryozoans in the world, in the Lower Pleistocene Setana Formation at Kuromatsunai, Hokkaido, Japan. What makes this locality special is the abundance and diversity of fossil bryozoans, and their excellent preservation. This site has come to be known as “KOKEMUSHI PARADISE“ (kokemushi is the Japanese word for bryozoan). Study of this deposit is underway in collaboration with Paul Taylor at the Natural History Museum, London. To date we have documented with SEM images of around 120 species of cheilostome and cyclostome bryozoans in this deposit.
Members of the 2005 expedition that discovered Kokemushi Paradise. Left to right, Luella Taranto (my daughter), myself, Andrei Grischenko, Masato Hirose, Shunsuke F. Mawatari, and Reishi Takashima
A curious phenomenon, whereby I (2nd from left in top-most photo) miraculously transform into expedition member Andrew Ostrovsky
Prof. Shunsuke Mawatari examining the strata at Kokemushi Paradise in 2004. In 2005, the site was still an active quarry being mined for fossil stylasterid corals, which the miners ground up (along with the fossil bryozoans attached to them) for use as a special plant nutrient sold to farmers and greenhouses.
Same view of the quarry as above, taken in 2007. The quarrying operation had temporarily stopped and the walls had been terraced for safety
Fossil stylasterid corals, the product of the quarry. These are sorted from the background soil and rubble, ground to powder, and sold in bulk bags.
At Kokemushi Paradise, virtually every shell fragment and pebble is covered with a diversity of bryozoans. The white patches on all the pebbles seen here are the fossilized skeletons of bryozoan colonies
Stratigraphic sampling at Kokemushi Paradise in 2006. Left, Prof. Reishi Takashima, Geology Department, Hokkaido University; right, Masato Hirose
My daughter Shione Yamamoto collecting fossil bryozoans at Kokemushi Paradise in 2006
SEM images of representative cheilostome bryozoan species from Kokemushi Paradise, showing the excellent preservation typical of the locality
In addition to Kokemushi Paradise (Utasai Section), there are several other Setana Formation deposits of various ages in the vicinity of Kuromatsunai that contain abundant fossil bryozoans
From Takashima et al. ( 2008)
Setana Formation at Soebetsu River, Kuromatsunai, Hokkaido
Layer of fossil bivalves at Soebetsu River; bryozoans are also diverse and abundant here
Reishi Takashima measuring the thickness of the Nakasato Conglomerate at Kuromatsunai. This is yet another site rich in bryozoans
Kuromatsunai is only one of many promising localities for Pleistocene bryozoans in Japan. Some areas have a long history of study, while others have scarcely been explored.
From Dick et al. (2008b)
What good are fossil bryozoans? Since the climate underwent cyclical oscillations throughout the Pleistocene, and in many time intervals was different than it is now, information on the continuity of bryozoan species between the Pleistocene and now can shed light on the responses of assemblages of benthic marine animals to climate change. In addition, the species composition of fossil bryozoan assemblages and various other techniques can provide information on the paleoenvironment at the time the fossils were deposited.
MART analysis is a technique that uses variation in zooid size within bryozoan colonies to estimate the mean annual temperature range (MART) of the environment at the time the bryozoan colonies were alive. We have used this technique to estimate the MART of the paleoenvironment at Kokemushi Paradise (Dick et al., 2008b).
Results in the following table suggest indicate that the mean annual temperature range decreased from the Lower Setana Formation (1.0-1.2 million years ago) to the Upper Setana Formation (0.6-1.0 mya). This indicates a change in climate or environmental conditions between the two ages, though the nature of the change is not clear from the MART analysis alone. A higher mean annual temperature range in the Lower Setana could mean more temperate conditions relative to the Upper Setana.
Until a few years ago, there was only one record of a Cretaceous bryozoan from all of eastern Asia, including Japan. This changed in 2005, when Andrew Ostrovsky, a visiting scientist from St. Petersburg University, suggested to our geologist colleague Reishi Takashima that we look for fossil bryozoans at Cretaceous sites in Hokkaido that Reishi was familiar with. This led to a trip to Mikasa, a couple of hours north of Sapporo by car. In a few hours, we found many specimens of a single bryozoan species. Ostrovsky et al. (2006) reported this find as the second record of a Cretaceous bryozoan from Japan and eastern Asia.
Members of the trip to collect fossil Cretaceous bryozoans at Mikasa in 2005
Andrew Ostrovsky at Mikasa
Light micrographs showing the undescribed species of malacostegan bryozoan of lower Middle Cenomanian age discovered at Mikasa in 2005 (from Ostrovsky et al., 2006)
After the Mikasa discovery, Reishi contacted his friend and colleague Prof. Toshi Komatsu at Kumamoto University. Komastsu had done extensive work on the paleontology of Cretaceous bivalves at Goshoura Island, Kyushu. It turned out that he had already found Cretaceous bryozoans at Goshoura, but had not done anything with them, as he was a bivalve specialist. Komatsu organized several collecting trips to Goshoura in which he, Takashima, Ostrovsky, and I participated.
Quarry on Goshoura Island, Kyushu, containing relatively abundant Cretaceous cheilostome bryozoan fossils. The main collecting site is high in the quarry, toward the far upper left
Komatsu and Takashima having lunch at Goshoura quarry, 2006
Trilp to collect bryozoan fossils at Goshoura quarry in October 2007; left to right, Komatsu, Takashima, Ostrovsky, and three students of Komatsu
Collecting at another Cretaceous site on Goshoura Island.
Yet another site ...
Goshoura is an important locality for investigating the taxonomy and paleoecology of Cretaceous bryozoans. Most previous information on Cretaceous bryozoans has come from localities representing the Eastern Tethys Sea (mostly Europe and the USA); research at Goshoura provides the first information from the Western Tethys. Through his research, Komatsu has worked out in great detail the paleoenvironments and distributions of bivalves in the Goshoura deposits, and bivalves are the primary substrate on which bryozoans are found.
Fossil bivalve (Pterotrigonia sp.) and bryozoan colony (left) collected on Goshoura Island
Paleoenvironmental distribution of Cretaceous bivalves at Goshoura (from Komatsu and Maeda, 2005)
Substrates of Cretaceous bryozoans at Goshoura, based on the 2006 collections. G, gastropod; B, bivalve.
Distribution of zooid size for fossil bryozoan colonies collected in 2006 at Goshoura, indicating several species based on zooid size alone
The preservation of Cretaceous bryozoan fossils in Japan is not as good as in other parts of the world, such as Europe and the USA. Colonies are rarely preserved with the original frontal surface exposed (below, left). More often they are preserved with the base of the colony exposed, having peeled away from the substrate, with the vertical walls and frontal surface buried in the matrix (below, center). Often the calcareous walls of the colony have dissolved naturally, leaving a mold of he colony (below, right). In some cases, the walls of colonies like that in the center can be dissolved away with HCl, leaving a mold. This is possible for Goshoura specimens because the matrix is not limestone; at Mikasa, the matrix is limestone, prohibiting this approach.
Dental technologists at the Hokkaido University School of Dentistry collaborated with me in applying dental casting techniques to make silicone casts of Cretaceous bryozoan colony molds, as shown below.
The silicone casts can be coated with a heavy metal and examined by SEM. Below are SEM images of three Cretaceous bryozoan species from Goshoura. The left and center images are of undescribed neocheilostome (?) and malacostegan species, respectively; the image to the right is a cyclostome, the form-genus Berenicia.
Ostrovsky AN, Takashima R, Dick MH, Grischenko AV, Nishi H, Mawatari SF (2006) First record of a Cretaceous cheilostome bryozoan from Hokkaido, Japan. Cretaceous Research 27: 859–862.
Dick MH, Osawa T, Nodasaka Y (2009) Method for making detailed, SEM-suitable
VPS silicone casts of colony molds from fossil bryozoans. Paleontological Research 13
Dick MH, Takashima R, Komatsu T, Kaneko N, Mawatari SF (2008a) Overview of Pleistocene bryozoans in Japan. In H Okada, SF Mawatari, N Suzuki, P Gautam (Eds.) Origin and Evolution of Natural Diversity, Proc. Int. Symp. “The Origin and Evolution of Natural Diversity”, 1–5 October 2007, Hokkaido University, Sapporo, Japan, pp. 83–91.
Dick MH, Hirose M, Takashima R, Ishimura T, Nishi H, Mawatari SF (2008b) Application of MART analysis to infer paleoseasonality in a Pleistocene shallow marine benthic environment. In H Okada, SF Mawatari, N Suzuki, P Gautam (Eds.) Origin and Evolution of Natural Diversity, Proc. Int. Symp. “The Origin and Evolution of Natural Diversity”, 1–5 October 2007, Hokkaido University, Sapporo, Japan, pp. 93–99.
Takashima R, Dick MH, Nishi H, Mawatari SF, Nojo A, Hirose M, Gautam P, Keiichi N, Tanaka T (2008) Geology and sedimentary environments of the Pleistocene Setana Formation in the Kuromatsunai district, southwestern Hokkaido, Japan. In H Okada, SF Mawatari, N Suzuki, P Gautam (Eds.) Origin and Evolution of Natural Diversity, Proc. Int. Symp. “The Origin and Evolution of Natural Diversity”, 1–5 October 2007, Hokkaido University, Sapporo, Japan, pp. 75–82.
Ishimura T, Dick MH, Takashima R, Hirose M, Gautam P, Nishi H, Tsunogai U (2008) Experimental study of the use of the stable isotopic composition of calcareous microfossils in shallow marine sediments for reconstructing paleoenvironment, and a comparison with the MART index. In H Okada, SF Mawatari, N Suzuki, P Gautam (Eds.) Origin and Evolution of Natural Diversity, Proc. Int. Symp. “The Origin and Evolution of Natural Diversity”, 1–5 October 2007, Hokkaido University, Sapporo, Japan, pp. 109–114.
Ostrovsky AN, Taylor PD, Dick MH, Mawatari SF (2008) Pre-Cenomanian cheilostome Bryozoa: current state of knowledge. In H Okada, SF Mawatari, N Suzuki, P Gautam (Eds.) Origin and Evolution of Natural Diversity, Proc. Int. Symp. “The Origin and Evolution of Natural Diversity”, 1–5 October 2007, Hokkaido University, Sapporo, Japan, pp. 69–74.