Ancient Whodunit: scientists indict wee suspects in ancient deaths. (This week).

Unlike many modern murder mysteries, the one that occurred about 180,000 years ago didn't take place on a dark and stormy night. It probably began on a pleasant autumn afternoon, the fourth or fifth sunny day in a warm spell covering temperate northern Europe.

The unsuspecting victim: A healthy male fallow deer. After battling a series of competitors at the height of fall mating season, he strode chest-deep into a lake and drank deeply to slake his thirst. The next morning, however, the buck awoke in a state of confusion, his vitality gone. Bewildered and weak, he staggered into the lake for another drink, only to stumble, pitch forward, and drown. In so doing, he became just one more victim of a conscienceless killer.

The perpetrator? Scientists now pin the rap on a toxic microbe in the water.

In the past 15 years, coal mining in eastern Germany has exposed the sediments of what once was a 500-meter-diameter lake, says Andreas Braun, a paleontologist at the University of Bonn. The lake basin includes piles of skeletons of large mammals such as fallow deer, red deer, and aurochs, the ancestor of modern cattle. The remains of forest elephants, rhinos, and a cave lion indicate that the climate was generally warmer than it is there today.

Although archaeologists previously attributed the concentration of fossils in this basin to human butchery and dumping of the skeletons, there aren't any cuts on the bones, says Thekla Pfeiffer, a vertebrate paleontologist who was at the Museum of Natural History in Berlin when she studied the fossils and analyzed the lake's sediments. Also, the skeletons taken from the lake bottom are intact and the bones are in lifelike arrangements. That suggests a natural demise.

Scientists have analyzed the skeletons of more than 80 deer from the site. Most of the animals were strong young males, and their antler development indicates they died in the autumn. It was surprising that young males, the hardiest of the deer, would have been killed, says Pfeiffer.

When Braun and Pfeiffer analyzed the sediments, they found compounds that absorb the same wavelengths of light that pigments of the modern cyanobacteria Microcystis do. This genus manufactures potent chemicals that attack the liver and so are called hepatotoxins. The researchers also discovered fossilized bacterial cysts of microbes that often live in the same lakes as the toxin-making algae. Braun and Pfeiffer report their findings in the just-published winter issue of Paleobiology.

Present-day toxic cyanobacterial blooms often take place in lakes during sustained warm spells in autumn. Cyanobacterial hepatotoxins can kill an animal within 24 hours, says Michele A. Crayton, a biologist at Pacific Lutheran University in Tacoma, Wash. The toxins occasionally kill wildlife, cattle, and dogs in rural settings, he notes.

Hepatotoxins attack a protein in the membranes of liver cells, says Ken Frazier, a pathologist at the University of Georgia's Veterinary Diagnostic and Investigational Laboratory in Tifton. As a result, the cell membranes break down, and the animal dies from internal bleeding in just 12 to 48 hours. In the meantime, the animals are weak, lethargic, and bewildered.

Braun and Pfeiffer's research sets forth "a circumstantial case with good chemical support;' says Julie Bartley, a biogeochemist at the State University of West Georgia in Carrollton. "Whatever killed a lot of young and healthy male deer, something funny was going on," she notes.