Whenever Wagner Fischer drives, he notices the roadkill.

As a graduate student in the 1990s, Dr. Fischer, now a biologist with the Federal University of Mato Grosso do Sul, traveled through Brazil’s Pantanal, a tropical wetland the size of Wisconsin, and the largest freshwater wetland in the world. From his motorcycle, he saw monkeys swinging from roadside trees; capybaras slept on the shoulder. He was looking for fishing bats, the subject of his graduate research. But he was fascinated and appalled by the roadside carnage: caimans, anacondas, giant black-necked storks called jabirus and, once, a dead giant anteater with her cub, still alive, clutching her back.

The region’s main road, the BR-262, is a long thread of tarmac through the carpet of green, connecting the growing cities of Campo Grande and Corumbá, 430 miles apart. Dr. Fischer began taking photographs, thousands of them, and tallying the species along the road. He shared his unpublished results with other researchers and government officials.

“Everyone from the scientific community kept asking me, ‘When are you going to publish that?’” Mr. Fischer recalled recently.

Two decades later, he finally has. His paper, published on Oct. 19 in the online biodiversity journal Check List, is a grim tally. From 1996 to 2000, Dr. Fischer counted dead 930 animals representing 29 reptile species and 47 bird species. A separate tally of mammals, to be published soon, includes more than 2,200 specimens. But even in its unpublished phase, his study inspired others like it, all of them confirming Dr. Fischer’s initial conclusion: that for wildlife, BR-262 is the deadliest road in Brazil and one of the deadliest in the world.