Arandaspida

Arandaspida are a group of early, jawless prehistoric fish that lived during the Ordovician period and represent some of the oldest known vertebrates. They belong to the class Pteraspidomorphi and contain a single order, Arandaspidiformes. The oldest genus within this group is Sacabambaspis, found in South America.

Arandaspids have elongated head armor with a flat dorsal shield and a bulging ventral shield. Two closely set holes on the anterior part of the dorsal shield were initially thought to be pineal openings but are now considered external openings of the endolymphatic ducts. The eyes, surrounded by sclerotic rings, are located in notches at the front of the dorsal shield. The nostrils may have been situated between the eyes. The ventral lip of the mouth is equipped with long series of small oral plates, similar to those found in heterostracans. The gill openings are numerous (more than 15) and minute, located between diamond-shaped platelets that separate the dorsal and ventral shields.

The body is covered with rod-shaped scales arranged in chevrons, and the tail is likely pad-shaped and diphycercal. The dermal bones consist of aspidine (acellular bone) and are ornamented with oakleaf-shaped tubercles lacking dentine. Sensory lines are housed in narrow grooves between the tubercles.

Taxonomically, Arandaspida include families such as Astraspididae and Arandaspididae, with genera like Astraspis, Andinaspis, Apedolepis, Arandaspis, Areyongalepis, Pircanchaspis, Porophoraspis, Ritchieichthys, and Sacabambaspis.