basilisks are omnivorous non-avian saurischian dinosaurs, averaging at around 1.5-1.6 m at the shoulder, with a highly variable appearance. a dactylopodian, they are unusual quadrupeds with winged forelimbs, the name hinting at the nature of said limbs; that being, that the "foot" is composed of two highly derived fingers, one supporting the limb ("foot") while the other serves as a webbed wing. as mentioned, their variable appearance makes them difficult to describe with absolute consistancy, but their general body and facial structure can be. basilisks integuement is varied, consisting of feathers of different structure ranging from plume-like to hair-like and thin (the latter found covering the leathery, "bare" areas, except for the bare face). their faces and hindlegs are scaled, the former in a very delicate manner. there is little basal sexual dimorphism in basilisks, with the exception of size (females being larger on average) and a feather "fan" present near the base of the tail only in males, however, exceptions exist for basilisks which resemble sexually dimorphic species (see shapeshifting section). males also possess two pouches on their sides which are usually hard to see, but expand internally after having mated.

SHAPESHIFTING

something unique to their genus, their variable appearance is correlated with their ability to shapeshift. basilisks possess a complex organ (the metamorfonum) connected to their digestive system, which utilises anda to drive other processes to store cells with intact genetic material and anda from animals that the basilisk eats. this organ is incredibly sophisticated, having several chambers, one to store genetic material and another where anda-rich fat is allowed to accumulate. the genetic storage will only store material that it cannot recognise from prior, instructing the rest of the digestive system to digest a material from a familiar species that they have consumed before as normal for energy efficiency. it is through this organ that the basilisk is able to "learn" the forms of other animals that it is then able to shapeshift into.

the metamorfonum is connected to a network of nodes distributed within the body, and the shapeshifting process itself can be likened to a sort of metamorphosis. these nodes are used to distribute a corrosive liquid produced by the metamorfonum itself during the process which is "gently" corrosive, in that it disassembles cells without destroying them. these nodes, along with the other vital organs of the animal, have a tough and flexible outer tissue resistant to the acid, and is instead made more malleable and able to be reshaped to a degree by it. a basilisk shift occurs in these stages, and the whole process takes about five minutes;

1. the basilisk enters a quick sort of "torpor", anda extraction from dedicated anda-fat stores begins and the body utilises it to nullify the basilisk's senses until they lose consciousness.
2. the hide toughens and forms a hard shell. the metamorfonum starts the dissolving process, and the animal's tissues are liquified into a "malleable" state.
3. anda allows the metamorfonum to communicate with the cells to reconstruct into the shape of one of the basilisk's stored forms, using the genetic material "blueprint" in its possession.
4. this stage differs depending on the size of the target form. if smaller than the current form, the basilisk reforms inside of the hard shell and bursts out of it after having fully formed. if larger, additional energy is expended to build pressure until the basilisk bursts out before fully forming, quickly reshaping as the liquid comes in contact with the air. this latter variety of shift is much more disorienting and energy-intensive than the former.
5. the basilisk's senses slowly return to them over about ten minutes after they successfully reform.

what shapeshifting has to do with their unusually variable appearence is suspected to be tied to its complexity. when the basilisk is developing, the metamorfonum and anda-instructed shifting system takes time to fully form in the egg, however, there is an uneven nature to how the development occurs at the same time, resulting in the embryo becoming especially susceptible to anda exuded by other animals in a decently large range at one point in development, and as a byproduct, the unformed shifting system mistakenly allows said anda from other organisms to influence the development of the chick. there is a disproportionate distribution of what animals tend to contribute to this influence, namely, animals more closely related are more likely to than others, and so the most common influences come from other chirosaurians or the basilisk's next closest (extant) related order, birds. the influence is mostly limited to coat colour, but it also influences build and the shape of the skull, as well as sexual dimorphism if applicable.

FORMS

basilisk forms are very specific, but also limited. a basilisk will not just learn the generic shape of the species it has stored, but of the very individual it stores itself, and will therefore resemble that individual quite closely when it comes to the basic structure (such as facial structure, sex, etc.). however, the limitations mainly are in how precise the integuement is to the target. a basilisk cannot completely change its integuement, so for example, if shifting into a human, the resulting human shape will only possess integuement found on the original basilisk form, such as feathers, scales and pseudofeathers, as well as the pigmentation. the basilisk makes up for this through anda. another, very fatty, organ in their body metabolises anda-rich fat when they are shifted to constantly exude anda which will muddle the perception of anyone looking at the form, preventing them from recognising what would register as "unusual" by an observer. anyone looking at a shifted basilisk will find it hard to describe what/who they're seeing, and if reflecting on what/who they saw post-hoc, they'll simply recall something which resembles the individual the form was from.

the illusion is nullified, however, if an observer has a) seen the original form and b) has genuinely made the mental connection between the form and the individual basilisk. the constant anda exertion also means that the metabolic rate of a basilisk in a form will be higher relative to its body size than a basilisk in its base form.

REPRODUCTION

as mentioned prior, basilisks don't (often) have very obvious sexual dimorphism, and what sexual dimorphism that does occur by chance actually plays very little into how sexual selection played into their evolution. it is also just less relevant as sexual selection nowadays is very culturally influenced. basilisks undergo an annual fertility period during spring, after which the female will lay 1-2 eggs during summer, unfertile or fertile. the male's pouches mentioned earlier, a trait basal to dactylopodinae, are then usually utilised for an intriguing form of pseudo-ovovivipary. the eggs are placed inside of the pouches where the chicks develop over the following year until hatching during the next spring. of course though there are surely cultures where this practice might have changed or are carried out in different ways. the chicks are altricial, and instinctively are inclined to cling to its father, staying in the pouch until growing too large to where it will climb onto his back hump.

something to note about basilisk sexes is that all basilisks possess both male and female gonads. however, the female gonads in a male will be somewhat "atrophied" and nonfunctional in a typical situation, and vice versa. however, it is possible for a basilisk to change this if they shapeshift into a form whose individual they learnt it from was of the other sex, as the sex of that individual is information that is preserved in the metamorfonum. it is, however, only the reproductive system which is changed over to the other corresponding basilisk sex, and not (basilisk) secondary sex characteristics like the male pouch.