Mammals

"This is the Second World," said the shaman, standing beside her, "the World of Water."

"Does anything live here?" asked Meredith, fanning the neck of her robe. She peered through the humid murk.

"See for yourself," said her guide. Meredith stepped forward, avoiding the limestone stalagmites that studded the cave floor. Ahead, the mist rose from the still dark pool enclosed by the steep sides of the cave. The far side was lost in darkness.

World of Water

Meredith emerged from the sipapu into a damp cave which was not as dark as the first cave. Gray light filtered through the mist in the warm and humid air. The cave walls enclosed a steamy pool and large drops of water dripped from the ceiling, playing a staccato beat on the smooth wet stones.  The air smelled strongly of dampness and decay.

Algae

Creeping cautiously over the slippery stones, Meredith approached the muddy edge of the dark pool and peered in. An unpleasant odor made her draw back. Resolutely she looked again and made out long chains of blue green algae twined into filaments and tangled together into mats. They lay across each other forming thick restraining layers.  

Algae are one of the earliest life forms, their mats among the oldest of fossils, she remembered, and they release oxygen into the air! That means that animals could breathe here if there are any, and yes, bacteria grow inside the algae chain and fix nitrogen in the water to fertilize the plants. That explains the odor. 

She reached into the water and tugged at a strand of algae, causing the entire mass to drift in her direction. Breaking off a piece, she shook it dry and examined it before placing it in her pouch. 

A Gaper Clam

What time period is this? she wondered. Algae grow during any epoch, so they're inconclusive.

She could ask the shaman but, as a scientist, she preferred to find the evidence herself. Without an isotropic microscope, it was impossible to date the mats. What other clues were there?

Searching about, she noticed delicate grooves crisscrossing the water's edge.  The trails ended in small gray bubbles in the mud, which vibrated with her footsteps as she walked. 

Just then a small jet of liquid squirted up beside her. She turned in time to see a tiny grayish pink creature disappear into one of the mud holes. A mollusk. Eagerly she dug out the spot, scratching the skin on the back of her hands. At last she felt it and drew it forth. 

YES!  A soft-shelled clam. A gaper. The clam quickly withdrew its pinkish wedge-shaped foot into its soft white shell.

Tertiary

A filter-feeding mollusk. Two retractable siphons. Definitely a bivalve, Meredith thought, examining the clam. Here's the mantle; let's see, there's a pocket for it in the shell.  Fairly highly evolved. Probably the Tertiary. The Tertiary epoch followed the Jurassic, so apparently in terms of time, she was headed toward home rather than away from it, and this felt reassuring.fs

Hmmm, there could be mammals here, if we're in the Tertiary. She was excited at the prospect. But where? She deposited the clam in her pocket and scanned the pond intently.