Eriken
The Eriken split from its ancestor. As a defense against predators, it has developed spikes derived from its scales. These spikes are hollow and each consist of 5 elongated scales twisted together. To warn predators of its sharp spikes and make itself look bigger, it can curl its back into an arch to show them off—wait, it can do what!?
By default, all Astergnathiformes have a single bone called a “pelvic ring” which all their limbs are attached to. This offers limited flexibility, especially in the Eriken’s ancestor, the Sagmalix, which moved one of its limbs forward by warping this bone. Ancestrally, the entire torso was almost completely rigid due to this. This caused its bounding locomotion to be inherently unstable, as the torso cannot twist or bend. By all accounts, this should also mean the back cannot be arched. So how on Sagan 4 did Eriken gain this ability? Well, the answer is by basically permanently dislocating its raptorial limb.
The same ribs and muscles which originally served to prevent the raptorial limb from being dislocated and the pelvic ring from being driven into its organ-filled body incidentally also made the pelvic ring not entirely necessary to keep the raptorial limb in place. With a slight modification, the ribs that stabilized the ring if its clavicle ribs were broken now form a semi-flexible track which the raptorial part of the pelvic ring can slide through. The raptorial sternum, once attached rigidly to the pelvic ring, is now attached only to the clavicle ribs and serves to prevent the pelvic ring from jumping the track. This all allows the pelvic ring to move independently from the raptorial arm, being able to slide significantly in front of or behind it. While twisting and left-right bending of the back is still impossible, this anatomical innovation allows the back to arch. In addition to allowing the Eriken to make itself look much bigger and more intimidating, this also increases its stride length and therefore the general speed and stability of its locomotion.
With this innovation, the Eriken was also able to get larger than its ancestor. It has a semi-erect posture, and its anal arm is a lot longer and thicker to allow it to easily move on two legs as needed. With its jaws being less efficient than those of arthrotheres of similar size, its dietary habits are largely opportunistic with a preference for meat and other fleshy material over crystals. Its lateral jaws are stronger and more flexible, with the lower bone of the mandibles being modified to function more as an extra segment. As developing its jaws like this takes extra time, its offspring have significantly more developed jaws at birth so that as much of it can happen during the fetal stage as possible. It does not use its “normal” jaws to grab or tear food at all, only to mash up and swallow what it pulls into its mouth with its stronger lateral mandibles.
Like its ancestor, the Eriken has a “saddle” on its upper back made of one large scale. It guards this relatively unprotected part of its body by pressing its neck back against it when threatened. A “second saddle” on the back of its neck, where there are only flat scales and no spikes, serves to prevent it from stabbing itself or any babies it has riding on its back. With all its spikes, it has lost the flushed mating display its ancestor had; instead, it has a pair of elongated spikes on its face which serve as more general health indicators based on their length. As spikes make mounting out of the question, Erikens mate belly-to-belly like many spiky animals on Earth.
As mentioned before, the changes to the Eriken’s jaws necessitated that some of their jaw development occur before birth. Indeed, the heads of newborns are now completely bilateral and almost resemble the adult without any spikes, while everything behind the head is still externally radial. While the amount of energy needed to complete metamorphosis is ever-increasing, the Eriken’s highly efficient parental care—now with extra predator deterrence—has continued to prevent strong pressure to do it before birth, somewhat like the dilemma that early mammals on Earth also found themselves in. Thus, baby Erikens continue to ride upon their parents’ backs, totally dependent on them for survival.