Artistic License Biology / Serina

• The metamorph birds are easily one of the most controversial topics of this project due to how bizarre they are. Broadly speaking the chain of events leading to them (shrike-like birds laying eggs on meat piles for young to eat the go, said young become more independent via becoming more larval, said larvae being able to live elsewhere) is logical, but vertebrate development is infamous for being dictated by hyperspecific homeoboxes that constrain otherwise sound changes (i.e. mammals with more than seven neck vertebrae tend to die from cancer unless they have low metabolisms like sloths) which in birds do seem to correlate to things like an inflexible spine, hindlimb morphology and bone formation, which would make the oversimplification of all of these and other systems pretty hard to achieve. Likewise, in the given scenario the opposite would be more likely to happen: more independent and more well developed chicks, like those of other non-passerine birds.
• Several bird lineages become quadrupedal, most notably the serezelles and the bumblets. However, the avian wrist is not adapted for bearing weight, as well as their shoulder joints being located very far up on their backs, making quadrupedal avians highly unlikely.
  • This is explained in-universe; the bumblets in particular evolved weight-bearing limbs by replacing the entire forelimb with a robust hypertrophied hand adapted for burrowing, with penguins—which can also stand and even run on four legs—also serving as justification.
• The dogbeasts' tripedal gait is described as being superior to the quadrupedal gaits of the serezelles. In reality, it would be largely inferior — fewer limbs would need more energy to reach similar speeds and would be much more unstable when moving quickly.
  • There's also the issue of a vertebrate tail, an extension of the spine, being modified into a limb with knee and ankle analogues while containing the end portion of the spinal cord, as well as the lack of a hip joint or pelvis that is likely to compromise the movement or mobility of the limb.
  • And not to mention plain redundancy of having a "spare" limb: a quadruped that injures one of its hind legs can at least limp away from danger and have a chance to recover, while a tripedal tribbethere that breaks its sole back leg is hopelessly crippled (and this is indeed a hunting strategy of the bludgebirds, who target the single rear limb of their circuagodont^note ungulate-like mammal-analogue herbivore prey to incapacitate them).
  • This is actually brought up with the introduction of the squaves, a group of neotenic ornimorphs who remain in their quadrupedal lizard-like stage. Their gait is stated to be more efficient than that of the three-legged tribtiles, and they are now outcompeting them on the ground.
  • It's also heavily implied that it severely limits how large they can get due to the fact that the largest terrestrial tribbetheres are the omniphages which only get up to 1,500 pounds, much smaller than earth's quadrupedal megafauna despite the stronger gravity and also much smaller than all of Serina's bipedal and quadrupedal avian megafauna.
• Some of the carnivorous circuagodonts are said to occasionally adopt young of prey species and raise them to weaning ^note which is said to never end well for them as they end up associating with a predator species and are eaten by other packs who don't recognize thembut given the different behavioral and dietary requirements of the two species, even if separated by only a few years of evolution, the odds of them successfully raising a young herbivore to even that age are pretty nil (not to mention that the rest of the pack are likely to kill the "foreign" youngster, viewing it as competition.)
• It's mentioned that most molodonts have lost their tongues with only more basal species like omniphages retaining them. There are two problems with this, however.
  • The first is that tongues are necessary for land animals to swallow food; in fact, some paleontologists theorize that developing a tongue was what allowed early tetrapods to start colonizing the land.
  • The second is that most molodonts appear to be either omnivores or herbivores which generally have a more acute sense of taste so that they don't accidentally ingest something toxic.
• Many of these points are addressed here. Some examples include the quadrupedal metamorph birds being born at such an early embryonic stage that they were able to change the structures of their wings before developing the more restrictive wing muscles of other bird species, and the front limbs of the tribbet which are based off of the frogfish which does possess leg-like front fins for walking on the sea floor as such a limb would be most efficient for a large creature to walk on land.

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