Archangeliphausia SPINOSA 2004, SP. NOV.

ARCHANGELIPHAUSIA SPINOSA SP. NOV.

(FIGS 2–4, 5B)

Type horizon and locality: Dark-grey claystone from a depth of 4255.0– 4262.7 m, Lower Devonian (Lochkovian?). Borehole Medynskoye 1, Timan-Pechora region of polar Russia.

Material: Sixty more or less complete compressions.

Diagnosis: As for the genus.

Description: The largest specimen, PIN 4983/35, probably belongs to this species (Fig. 2B). It measures 12.5 mm in length from the carapace rostrum to the end of the telson. The smallest reasonably complete specimen is PIN 4983/37 (Fig. 2C) with an estimated length of c. 6.5 mm. The size of most specimens is close to the mean between these values. As the specimens are mostly complete skeletons, not exuvia, the dominance of larger individuals may reflect the structure of the original population at its repeated catastrophic extinctions.

Three basal segments of the 1st antenna are preserved in specimens PIN 4983/18 and 25. The proximal segment is approximately three times longer than the third one, while the second segment is intermediate in length (Fig. 5B). Of the 2nd antenna only the scaphocerite is preserved in a few specimens, the most complete being those of PIN 4983/2, 18, and 24 (Fig. 3A). The scaphocerite is oval, represented only by an organic film on the rock surface and its margins are not easy to trace.

The carapace has a sharp, relatively short rostrum, the ocular sinus being clearly visible in specimens PIN 4983/24A and B. The lower margin of the carapace, well preserved in holotype specimen PIN 4983/1a, has a very narrow band (Figs 2A, 4A). Being thicker, this band would have strengthened the cuticle. It is calcified and shows openings of pore canals; the posterior margin is hardly discernible except in the isolated dorsoventrally compressed carapace of specimen PIN 4983/43.

Laterally compressed sternites of thoracic segments are preserved in many specimens. The boundaries between the segments are discernible some distance dorsally of the sternites. All segments except for the first are recognizable in the specimen associated with the holotype (Figs 2A, 4B). They disappear at approximately half the height of the body, which probably corresponds to the limit of connection of the body with the carapace.

No remnants of the apparently weakly calcified thoracopods are preserved. Some faint marks may correspond to pleopods, but they are completely undefined morphologically.

The pleosomites increase gradually in length posteriorly, the 6th pleomere being very much longer than the preceding ones. Their pleura are somewhat expanded posteriorly to form oval lobes. The lobe of the 5th pleuron appears to extend almost to the midlength of the 6th pleosomite. All pleura bear sharp spines at their ventral tips, best preserved in specimens PIN 4983/31 and 24b (Fig. 3G).

The 6th pleosomite has almost parallel sides when, as in PIN 4983/36, compressed dorsoventrally (Fig. 3F). There is a kind of hinge connection with the basal segment of the uropods.

The telson is best preserved in PIN 4983/28 (Figs 2E, 4C), although its parts are recognizable in several other specimens. Its sides are gently convex but almost parallel to each other. The posterior margin bears about eight indistinct indentations probably corresponding to bristle bases. The surface, as preserved in some stronger sclerotized fragmentary specimens (e.g. PIN 4983/32; Fig. 3B), is externally smooth. The rami of the uropods were strongly sclerotized only along their external margins. This prevents delineation of their shapes, but they were probably rather wide.