Cetaceans of the Calidocene

Art of the seasifter (top) and the river porphin (bottom) done by Tortoiseman on reddit

Cetaceans quickly bounce back after the extinction and fill many new niches in the recovering seas. Calidocene cetaceans all descend from the harbour porpoise, the only cetacean that survived into the calidocene. Most of the porpoise descendants are basically carbon copies of dolphins or, so lets look at two of the more derived ones.

The Seasifter

Seasifters (Genus: Harenaeos, sand mouth) are filter feeding, omnivorous porpoise descendants. They live in the arctic, pacific, and atlantic oceans and stay close to the coasts. The seasifters evolved to filter feed in response to a mass dieoff of fish during the extinction and feed off of benthic invertebrates and algae. In order to filter feed, they adapted to have a longer, duck-like snout with a lot of small teeth to strain water and particles out. The seasifter feeds by going to the bottom of the ocean floor and scooping up the substrate and straining it out.

A fully grown seasifter is about 7 feet in length and weigh about 200 pounds. 

Seasifters live in small pods of about 15 with males living on their own or closely following the females pod. Seasifters aren't aggressive but will fight each other for social status.

The River Porphin

Porphin's (family: delphinamimus) are the descendants of harbour porpoises that look exactly like dolphins, but things get way weirder with the freshwater and brackish varieties. River Porphins live in rivers and saltree forests of the northern hemisphere. River Porphins (Genus Cavumpore, hollow pore) are way smaller than their ancestors to navigate thick vegetation and measure at about 3 feet in length with a weight of 80 pounds. They feed mostly on small fish and have a long snout for doing so but they also eat snails, insects, and small land vertebrates. Their neck vertebrae aren't fused together and their flippers are way more flexible. Their eyesight is absolutely horrendous but instead of echolocation, they gained a better ability, electroreception. They are able to sense their surroundings through vibrissial crypts on their snouts, these are weak but are just powerful enough to detect their prey and get a vague map of their surroundings.

River Porphins have very similar behaviors to the boto that we gave today, but they are way less violent.