Cladistics: Arachnopoda

There are few creatures as synonymous with the name Eryobis as the Arachnopods. These aptly named eight legged terrestrial vertebrates can be found in nearly every corner of Eryobis and are perhaps even more recognizable than the Anispondyls they share the land with.

Nearly all Arachnopods share a similar basic body plan as they all have vaguely tetrapod like heads on bodies supported by eight spider like legs. But they are no tetrapods, nor are they even closely related. Matter of fact, they're not even Sarcopterygians. They are ray-finned fish of the order Holostei, which on Earth were represented by bowfin fish and gars.>

Arachnopods look rather little like their terran counterparts however, so how did they come to be?

Arachnopoda are part of the Octopodichthyes, a member of the "spiderfish" clade. It is hard to determine if spiderfish are a natural group or if its various members evolved their legs independently since we only observe one other taxon of spiderfish alive today, and those have four legs instead of eight.

Spiderfish in general often fossilize poorly, so we do not know a lot of their early history compared Planosolincolans, which tend to fossilize a lot better.

It is theorized that spiderfish evolved their legs when they separated a number of pelvic finrays from the main fin in order to prod the sediment for sustenance. This would have later led to them moving around on their free finrays until they developed into proper ossified legs. 

At some point, probably in the late Narthian period, some Octopodichthyans made their way to land and would give rise to the Arachnopods we are familiar with today.

As their legs sprouted from the pelvic girdle, Arachnopods needed to hypertrophy their hips in order to support their weight on land. Their pectoral fins on the other hand atrophied since the no longer really served a use on land. 

Many Arachnopods heavily reduced them or lost them completely, while one group known as the Osteotarida modified their pectoral fins to aid in detecting sounds, essentially turning them into ear pinnae.

Because of their massive pelvic girdles, that sometimes heavily resemble the plastrons of turtles, Arachnopods usually have rather stiff torsos. Their eight sprawling legs somewhat negate this issue however, since they provide the animals with a lot of mobility in several directions.

A rather peculiar feature of Arachnopods are their skulls. More specifically, their jaws.

Many Arachnopods have a unique jaw mechanism wherein both the maxilla, premaxilla are separated from the cranium and a dentary that is not connected to the rest of the mandible.

The maxilla is connected to the cranium with a joint that allows it to swivel and is able to slide in a groove in the mandible where a number of muscles and tendons connect to it, creating a "deadbolt mechanism" where the jaw can be locked at a certain angle.

When the maxilla swivels, it usually also causes the premaxilla and dentary to move.

We do not completely understand why this strange jaw mechanism evolved, but we do know that it adds a lot of strength to their jaws opening and closing.

Modern Arachnopods can for the most part be categorized as belonging to one of three groups: Dactylourae, Euarachnopoda and Pulvinopoda.

While the first two likely descend from a single ancestor that survived the catastrophic mass extinction known as the World Scarring, Pulvinopoda appears to have come from two distinct survivors of that cataclysm. Of these two lineages, the Neopulvinopods are the most successful and widespread while Polytryposternians are quite restricted in where they occur nowadays. Hence why Arachnopods are typically placed in one of three categories instead of four.

Pulvinopoda are considered to be the most primitive of the living Arachnopods. They are all ectothermic, often scaleless, amphibian like creatures that lack ear pinnae. Calling them basal would be an insult however. Their skull morphology is highly divergent from the earliest Arachnopods and has close to zero resemblance to those of the other Arachnopod groups.

The Dactylourae are perhaps the oddest of the Arachnopods. These almost exclusively arboreal creatures are most easily recognized by the claw at the end of their tails. While a number of basal genera still exist, most Dactylourans can be placed either within Kremasmasauria or Aioratheria. 

Kremasmasaurs are reptilian or and serpentine in appearance and often completely lack ear pinnae. 

Aioratheres are homeothermic and are typically covered in filaments, which are actually hair like keratin projections of the scales underneath.

Where the other Arachnopods ruled the land, the Dactylourans adapted by specializing in the arboreal lifestyle, being among the very few Arachnopods with an affinity for climbing.

The Euarachnopods are what most explorers first image when they hear the word Eryobis.

These Arachnopods tend to grow much larger than their other relatives, live mostly on the ground and are not dependent on close proximity to water for survival, so they are often much more easy to spot.

While basal Euarachnopods look rather similar on the outside, one can quickly determine their origin by examining their skulls. Usually, they can either be placed in Pseudozygomia or Panhesparachnes.

Both these groups have a temporal fenestra in their skull, but they evolved them independently and from different structures.

Pseudozygomians evolved a fenestra by closing a section of the oral cavity in order to create more muscle attachments and to reinforce the skull. Pseudozygomians can mostly be found in the eastern hemisphere of Eryobis, existing in parts of Tlèëa, Lotharca and Rubiëra, with the subgroup Rubiarachnes being almost exclusively restricted to Rubiëra.

Panhesparachnes evolved their fenestra by forming a bony ring to give more support to the eye and thus essentially splitting the orbit in two. Basal Panhesparachnes often don't have a completely formed fenestra, but all members of Arachnotheria do.

Arachnotheres, the famous spider beasts of Eryobis, are fully endothermic and like the Aioratheres, are covered in hair derived from thin projections on their scales.

Arachnotheres can be found on every continent except Rubiëra and possesses a great number of families, but most of them can be placed in Bloëcatheria, Kentrotaria or Hesparachnes.