Description
The holotype of T. thomasi is 47.5 mm long without the antennae. The head is 4.6 mm long, a pair of large, functional eyes is located on the anterior half of the head. The antennae are long (10 mm) and consist of about 30 annuli. The mouthparts of T. thomasi were described as partially entognathous by Kukalova-Peck ; entognathy is a configuration in which the mouthparts are covered by lateral folds that create a pouch, it is typical of Entognatha. However, Kukalova-Peck noted that the mouthparts of T. thomasi are more exposed than in modern diplurans. In fact, the illustrations given by Kukalova-Peck do not show oral folds, which would lean toward a more ectognathic configuration for T. thomasi (opposite of entognathy : the mouthparts are exposed, which is usually observed in insects). The mouthparts include a pair of maxillary palps, a pair of labial palps, each consisting of eight segments and a pair of comb-like laciniae.
The prothorax, mesothorax and metathorax of T. thomasi have lateral lobes adjacent to the tergites. They are separated from the tergites by sutures. Kukalova-Peck suggested that these lateral lobes are of appendicular origin rather than expansions of the tergites. Her hypothesis is that the epicoxa, a proposed proximal segment of the legs of ancestral arthropods, would have fused with the terga to form the lateral surface where the leg is attached. According to Kukalova-Peck, exites (outgrowths) of the epicoxa would have evolved in a hypothetical structure, the protowing. Those protowings would have either evolved into wings or fused with epicoxa to form lateral lobes in wingless hexapods such as T. thomasi. The thoracic terga also have distinct sulci (grooves or furrows), which suggest a well-developed musculature. The six thoracic legs are well developed and articulated more ventrally than in modern diplurans. The leg is composed of eight articles: the coxa, the fused trochantero-prefemur, the femur, the patellotibia, the basitarsus, the tarsus (divided in two) and the posttarsus which bears two claws. The proximal articles of the thoracic legs bear conical exites.
The abdomen is 25 mm long and composed of eight-segments. As in the thorax, each segment bears lateral lobes. Each of the six anterior-most segments bears a pair of small, eight segmented, simple legs and large vesicles. The terminal segment bears a pair of cerci forming a forceps.
Affinities
Kukalova-Peck considered that Testajapyx was related to the dipluran family Japygidae based on the presence of grasping cerci and established its own family: Testajapygidae. However, it also shares similarities with primitive ectognathans. Testajapyx differs from Japygidae by numerous characteristics such as the eyes which are large and well developed in Testajapyx whereas japygids are blind, the mouthparts of Testajapyx are more exposed ventrally than those of japygids, the maxillary palps of Testajapyx are composed of eight segments while those of diplurans are reduced to a single segment, the sclerotized body of Testajapyx with well visible sulci and sutures contrasts with the soft cuticle of diplurans, the thoracic legs of Testajapyx are more similar to those of basal insects, they are strong and articulated more ventrally than in japygids, which suggest that they were adapted to move efficiently whereas the legs of japygids are short and weak.
She assumed that the highly entognathous condition was derived from more exposed configurations and that it evolved separately in different entognathan lineages. This was supported by the discovery of the Jurassic Sinojapyx giganteus which can be confidently attributed to Diplura while having an ectognathous configuration.
For her, Testajapyx provided evidence that diplurans are derived from a common ancestor with Ectognatha (insects) and evolved independently from other entognathan lineages (Protura and Collembola), which therefore means that Entognatha would not be a monophyletic group and that Diplura is the sister group of Ectognatha. However, later authors considered that the attribution of Testajapyx to Diplura, mainly based on the cercal forceps, was not convincing, some even suggested that Testajapyx was a female earwig. As of now, the position of Testajapyx within Hexapoda is still unresolved.
