Tripoda

Tripoda, commonly known as stinzers, are a clade of terrestrial and amphibious asterzoans. They are a subgroup of Osteoastera characterized by highly mobile lateral jaws, osteoderm-like teeth, and matching lateral and raptorial limb anatomy. They are all descended from Stinzerstar.

As the scientific name of the clade suggests, stinzers are ancestrally tripods. While they are not the only tripodal clade of Asterzoans, nor even the only ones which walk on the same specific set of limbs, they are the largest group with this body plan and, as terrestrial "vertebrates", they can be considered analogous to Terran tetrapods--which are similarly not technically the only quadrupeds on their planet. They have produced various forms in many environments, including land, water, and even the sky.

Anatomy
Stinzers, like other Osteoasterans, are "vertebrates" with bony endoskeletons, and a great portion of their skeleton is on some level made up of modified vertebrae or ribs. They breathe air through their four nostrils and have pinkish-purple or magenta iron-based blood.

Jaws and Teeth
Among the distinguishing characteristics of stinzers are their jaws and teeth. Their lateral jaws are highly mobile, set on a shoulder-like sliding joint that allows them to be pulled towards the end of the snout. As a hard bite with the lower jaw would put pressure on the eyeballs, many predatory species primarily use their lateral jaws for prey capture. The Eriken notably modified its lateral mandibles into a short arm-like appendage.

Stinzer teeth are keratinous with a bone core, somewhat like an osteoderm. The teeth almost completely cover the inside of the mouth rather than being in neat rows, and they chew by pressing their toothy tongue against the toothy roof of their mouth.

Limbs
The lateral and raptorial (front) legs of stinzers have 6 segments ending in a foot. The second segment, the poples, is not usually visible externally, as it is effectively only a "knee" segment and serves only to make the knee more flexible. In many species, the femur and 1st tibia are elongated to grant greater leverage for locomotion. As they are technically derived from vertebrae, rib and neural spine-like structures can appear on limb bones, including on the fingers. The anal arm, technically also a limb, is tail-like with segmented vertebrae.

The exception to the rule on limb anatomy is neotenous stinzers such as Monket. In lineages where the juveniles retain the pseudo-radial larval stage, neotenous species have highly segmented tail-like leg bones.

Internal Organs
Ancestrally, stinzers have the same internal organs as fish-grade Osteoasterans. This includes a digestive system, two hearts, and four lungs. They have a three-lobed reproductive organ, which is developmentally tied to the number of fingers on the anal arm; a polydactyl individual would have extra lobes. In males, this presents as three testes, and in females this is two ovaries and a womb.

In basal stinzers--effectively all species except those descended from Rivet--the digestive system is located behind the pelvic ring, inside the anal arm. In these species, this makes the anal arm their abdomen. However, the digestive system is in front of the pelvic ring in the larvae of many species. Rivet and its descendants are neotenous, keeping their digestive system in front of the pelvic ring in adulthood so that the anal arm can be flexible enough to be used as a grasper.

Locomotion
As tripods, stinzers do not have nearly as many locomotion options as creatures with an even number of limbs. Many are capable of hopping or bounding, and some solve the problem of stability at slow speeds by being facultative bipeds. Some species, such as the Corpse Spardi and the Azderoo, are obligate bipeds.

Evolution
Stinzers began their evolution in Darwin and Glicker starting in Generation 16, when the amphibious Stinzerstar first climbed onto land. In the time that followed, they exploded in diversity, producing various forms, including basal beach-dwellers such as Linzor, cursorial long-lipped herbivores such as Kugard, and amphibious frog-like creatures such as Rivet in Generation 17.

Descendants of the Rivet, distinguished by the neotenous characteristic of most of their internal organs being in front of their pelvic ring rather than behind it, underwent great diversification in Darwin throughout mid and late Week 3.

In Generation 19, the Corpse Spardi became the first stinzer--and, indeed, the first organism on Sagan IV (at least in this timeline)--to develop powered flight. That same generation, a neotenous stinzer, Monket, also evolved.