The Bear Gulch Limestone.

The Bear Gulch Limestone is an amazing fossil bed that dates to the Mississippian epoch of the Carboniferous Period. Some of the most notable specimens are of fish. The Bear Gulch fish are extremely diverse. They include sharks, rhizodonts, petalodonts, and even a lamprey.

The Bear Gulch is a fresh and brackish water deposit, suggesting that the environment was similar to the Mazon Creek and was an estuary or a lagoon connected to the sea.

Since some of the fossils show signs of stress during death, and rarely show signs of decay and scavenging, scientists believe that they were buried rapidly, similar to how the Burgess Shale fauna were buried.

To me one of the most amazing fish skeletons from the Bear Gulch is a fossil of a pair of Falcatus that I believe are mating:

Lund, Richard, and Grogan, E.D., 2005 Bear Gulch website,
www.sju.edu/research/bear_gulch, accessed by lifebeforethedinosaurs.com on
11-27-2011, page referenced last updated 02-01-2006.

It's possible that one of the top predators in the Bear Gulch was a colossal rhizodont called Strepsodus. Scales of this fish found in the Bear Gulch Limestone were up to 6 cm in diameter. Huge, especially for fish scales, and would point to Strepsodus being a massive fish:

Lund, Richard, and Grogan, E.D., 2005 Bear Gulch website,
www.sju.edu/research/bear_gulch, accessed by lifebeforethedinosaurs.com on
11-27-2011, page referenced last updated 02-01-2006.

An amazing fossil from the Bear Gulch is Hardistiella, a very early lamprey. It is not the oldest, however. The oldest lamprey discovered so far is Priscomyzon, which was found in Africa and dates to the late Devonian Period, 360 million years ago. Hardistiella is quite strange compared to other lampreys because it has a body shaped more like a lancelet's, and no signs of a bloodsucking structure in the mouth. Scientists do not know if the Hardistiella fossil shown here is an adult or not, probably because it could possibly be the only known specimen:

Lund, Richard, and Grogan, E.D., 2005 Bear Gulch website,
www.sju.edu/research/bear_gulch, accessed by lifebeforethedinosaurs.com on
11-27-2011, page referenced last updated 02-01-2006.

Invertebrates in the Bear Gulch limestone are diverse, but not as diverse as fish, which are one of the most varied groups of organisms found in the Carboniferous Bear Gulch Limestone. This is similar to the Devonian Period, when fish were extremely diverse. But Bear Gulch fish probably weren't as diverse as Devonian fish, because many kinds of fish went extinct at the end of the Devonian, including placoderms, armored jawless fish, the so-called "fishapod" forms, and thelodonts.



References:

http://www.sju.edu/research/bear_gulch/

http://www.cas.umt.edu/paleontology/rc_beargulchlime.htm

http://www3.amherst.edu/~jwhagadorn/publications/Lag4.pdf

http://rstb.royalsocietypublishing.org/content/327/1242/595.short

http://www.scienceinafrica.co.za/2007/february/lamprey.htm

http://www.fossilmuseum.net/Fossil_Sites/beargulch.htm