I would love some help on this. Not sure if I read it in the 80s, 90s, or early 00s, so I don't know how old it is, but I don't think it was written before the mid-80s. I have vague recollections of it, but certain imagery stuck with me.
The story centers on a planet with a dying sun, not sure if it was earth or not; and a race of beings that are bird-like or possibly dolphin-like, but I've always pictured them more penguin-like, and all black. IIRC, there is a family - just a couple with 1 offspring - who are the last survivors on this world, which is becoming dark and cold. They are telepathic, do not communicate vocally, and can access memories of other beings who once lived on the planet but are now extinct, so the novel switches back and forth between the stories of those other beings (humanoid, I think?) and these bird-like creatures who are the last ones left and dying also.
There was something important about the water on the planet, and a rectangular or square sort of pool they lived in, with straight vertical walls outlining its perimeter, and also a sort of bluish-greenish irridescent watery crystal kind of thing, maybe diamond-shaped, that they focused on to access the memories/images in their minds' eyes and/or to communicate. I think they wore it on a pendant. The ending was very sad and melancholy, as they knew they would die as the sun was dying, but also the memories of all who had lived there would die with them, or maybe their offspring/child dies before they do (not sure about that). There may have been something about these beings having come from another world and now they knew they'd never make it back home (again, not sure), or maybe it ended with one lonely survivor left and about to die.
I'm pretty sure it was a novel with several story lines and not an anthology of short stories, but I do remember that there were stories of other beings in surroundings distinctly different from the dark, cold, dying planet at the end. I think the book's cover was mostly black with an outline of that pool and a glimmer of the dying sun. That's all I remember. I searched the questions but didn't find anything that sounded like it. Been trying for years to find it again. Ring any bells?