[SydneyFreedberg]

And to continue, here are the races that dominate my setting in the "present," the Quartet -- four species with one intertwined history & society, and five different templates because one species has marked sexual dimorphism (and it takes two posts because the description is long):

The Quartet
Four closely allied species that live & work together: exuberant Triphibians, meditative Sessiles, stealthy Armstrongs, and above all the towering Striders.

Description

Spoiler:  

The four species known as the Quartet collectively dominate the galactic economy. A loose alliance rather than an empire, the Quartet protects and promotes peaceful trade. A web of interstellar treaties governs everything from the protection of intellectual property to the suppression of piracy, from contract law to first contact protocols. Many other species have signed on to these accords over the centuries, creating what is often called the Galactic Common Market. But the core of the system remains the Quartet races, which are widely respected, albeit often resented for their influence:
the Triphibians, small manta ray-like creatures that can swim, fly, and climb trees;
the Sessiles, huge soft-bodied sea-dwellers resembling an upside-down jellyfish;
the Armstrongs, lion-sized stealth predators with fluffy grey bodies; and
the Striders, tall armless bipeds with beaks and prehensile tongues.

Striders
A highly sociable species with a strong instinct for cooperation, the Striders founded the Quartet and remain its institutional backbone to this day. They are herd animals, evolved from tree-browsing herbivores with an ecological niche similar to Earth elephants. An adult female Strider stands nine feet tall — most of it leg — and weighs over a thousand pounds, with thick, hairless skin in various shades of pale red and orange; males are noticeably smaller and brighter. The head and torso form a single unit, with two eyes and a sharp beak at the front. The two pillar-like legs attach to the head/torso at a neck/hip joint directly behind the jaw. Striders have no arms, but they evolved long, prehensile tongues to grab leafy branches and pull them into their beaked mouths.
Striders never wear clothes for modesty and rarely need them for environmental protection — their hides are highly protective — but body art is common, especially tattoos but also piercings and body paint in some subcultures. Females generally wear tattoos signifying their herd and status, while males’ tattoos signify the herds and other groups they have frequent dealings with.
Females form 95 percent of the population and typically spend their whole life in the same herd, forging tight social bonds that make Strider society highly stable. The smaller and scarcer males move frequently from herd to herd, acting as ambassadors to ensure genetic and cultural mixing. Males are natural diplomats and rarely fight except when cornered; females are more combative but only resort to lethal force against creatures they see as dangerous to the herd — which they then stamp out with a grim thoroughness that can degenerate into genocide.
Both genders have evolved to be highly social and collaborative, in part because they have only one manipulative limb, so tasks that a two-armed creature can do easily alone require two Striders working closely together. The Striders’ greatest strength is that this instinct for cooperation extends beyond their own species. Males can make themselves welcome in a wide variety of cultures, while females often adopt intelligent aliens as part of their herd. The most commonly incorporated races are the other three members of the Quartet, but other species are not uncommon.
The Striders had invented steam engines and were vigorously industrializing when they first encountered alien life. Warships appeared in the sky, dropped shuttles full of troops, and told the baffled Striders they were now thralls of the Trinoc Empire. But the Empire was already in decay, having lost its monopoly on faster-than-light travel and grown dependent on thrall races. In particular, most of the ground force deployed to conquer the Striders were Armstrongs, a species conquered by the Trinocs just a few decades before.

Armstrongs
The Armstrongs are ambush predators who operate in packs, relying not on speed — they actually run slower than humans — but on stealth tactics, clever traps, and the extensible arms that give them their name. The average Armstrong is an inconspicuous grey-furred lump about a yard in diameter, rather like a living puffball, with eight eyes and a mouth on the front. They have four short legs and two arms: one human-sized, the other a massively muscled, multi-jointed affair capable of suddenly extending to snatch prey ten feet away.
Armstrongs are shy with strangers, even strangers from their own species, but they are brave, cunning fighters and fiercely loyal to their hunting pack. Like canines on Earth, they proved capable of extending this pack loyalty beyond their own species — first to the conquering Trinocs, who used them as slave soldiers, and then to the Striders, who treated them much better. The Striders, for their part, were initially leery of these alien carnivores but, once convinced the Armstrongs had no appetite for Strider flesh, soon adopted them into their herds.
The Armstrongs quietly helped the Striders steal Trinoc technology. The Striders helped the Armstrongs organize an underground liberation movement. 80 years after the conquest of the Striders, the Trinoc Empire erupted in one of its final civil wars, and the two races took the opportunity to ride together in revolt. They swiftly liberated the Strider homeworld and drove the Trinoc remnants from the surrounding stars, then launched a decade-long campaign to free the Armstrongs. Soon after, they explored beyond imperial space and discovered the planet shared by the Sessiles and Triphibians.

Sessiles
The Sessiles are a race of aquatic invertebrates that drift with the current as juveniles but attach to the sea-floor as adults. Adults are radially symmetrical. The main body is a soft disc about three yards in diameter with eight eyes, eight manipulative tentacles, and eight mouths evenly spaced around the rim. Tiny “roots” extend from the underside to ingest minerals from the sea-floor. Extending upwards from the body are dozens of bouyant tendrils, which sift the water to feed on plankton. (Their appearance has been likened to an upside-down jellyfish).
It is unusual for an underwater species to develop a technological civilization, and rarer still for a sessile species to evolve intelligence in the first place. But the Sessiles’ ancestors formed symbiotic relationships with a wide array of smaller, more mobile creatures, excreting a nutritious “milk” in return for protection from predators and parasites. Effectively, they began taming and selectively breeding other animals before they developed speech. These interactions stimulated the evolution of intelligence — not only in the Sessiles but in one of their symbiotes, the Triphibians.

Triphibians
The smartest creatures that swam around the ancestral Sessiles were the Triphibians. This race roughly resembles Earth manta rays, with a wingspan of 4-5’ feet, a long thin tail, and a cluster of protruding mandibles that act as manipulators. Triphibians are actually air-breathers, nesting on land, but they are capable of flying well out to sea and diving deep underwater in pursuit of prey, somewhat like an Earth pelican. A naturally sociable and curious species, they prefer to live in large colonies but are willing to explore great distances on their own in pursuit of new resources, living space, and trade partners.
Over time, the Triphibians came to rely on the Sessiles’ milk as a major source of nutrients not otherwise abundant in their diets and learned to protect this food source from threats, guarding juvenile Sessiles as they drifted, guiding them to choice settlement sites, and even transplanting mature rooted adults to better locations. In turn, the Sessiles rewarded the most helpful Triphibians with extra milk and grooming. The two species were both intelligent enough to, in effect, domesticate each other, stimulating each other to develop ever-greater intelligence and the ability to communicate.
The two species co-evolved into a Stone Age civilization, with the mobile Triphibians bringing stones, bones, and coral to the more dexterous Sessiles, who made them into tools for both races to use. Triphibian communities adopted Sessiles and treated them with religious reverence as a source of technology, nutrition, and wisdom, while the the Sessiles relied on the adventurous Triphibians to explore the wider world for them and bring word back.
It was at this stage in their development that the two races were discovered by the newborn Strider-Armstrong alliance. Already enthusiastic about inter-species cooperation, the Striders and Armstrongs saw the unique potential of this symbiotic relationship, made contact with the Sessile-Triphibian society, and began teaching them advanced technology.
[continued in next post]