Hydrocephalus, BARRANDE, 1846

Laibl, Lukáš, Esteve, Jorge & Fatka, Oldřich, 2016, Enrollment And Thoracic Morphology In Paradoxidid Trilobites From The Cambrian Of The Czech Republic, Fossil Imprint 72 (3 - 4), pp. 161-171 : 163

Hydrocephalus minor ( BOECK, 1827)

Text-fig. 3

Five partially enrolled specimens of H. minor were studied. The first specimen (NM-L 46543, Text-fig. 3a ) is a nearly complete, dorsally strongly arched exoskeleton, 135 mm long. The cephalon and the pygidium are inclined at an angle around of 80−90 degrees to each other. The left librigena is preserved in situ, the right librigena and part of the right fixigena are missing, due to splitting off of the rock. The facial suture is not opened. The exoskeleton is slightly compressed obliquely and laterally, suggesting that the specimen is preserved rather obliquely to perpendicularly to the lamination and main compaction vector. From the dorsal view, the flexed rear part of the trunk is slightly bent to the left. The articulating half-rings seem to be nearly completely exposed in the middle part of the thorax. The distal parts of pleurae in the posterior half of the thorax overlap each other. The three other specimens (NM-L 46544, NM-L 46545 and NM-L 46546, Text-fig. 3b–d ) are represented by incomplete dorsally arched exoskeletons (with 14, 16 and 16 preserved thoracic tergites, respectively). Sagittal lengths of preserved parts of these specimens range from 110 mm in NM-L 46545 to 130 mm in both NM-L 46544 and NM-L 46546. The main flexure of the thorax is located from the 10 th to the 14 th tergite, and in two specimens, NM-L 46545 and NM-L 46546, the thorax is upturned antero-ventrally. All three specimens have both librigenae preserved in situ, unopened facial sutures and in one specimen the hypostome is preserved in situ (NM-L 46545). Specimen NM-L 46545 shows distally overlapping pleurae. No posterior part of the trunk of any specimen is preserved, due to splitting of the rock samples.

The last specimen (NM-L 41157, Text-fig. 3e ) is represented by both internal and external molds of a large exoskeleton of a late holaspid stage, 145 mm long. This specimen has the last nine thoracic tergites and the pygidium flexed ventrally. The posterior part of trunk reaches the second thoracic tergite. Both librigenae are attached to the cranidium. The axis of the ventrally flexed posterior part of the trunk is not parallel to the axis of the anterior part of the trunk, but is slightly inclined (ca. 5 degrees) to the right.

Figures

Text-fig. 3. Hydrocephalus minor (BOECK, 1827); Jince Formation, Hypagnostus parvifrons to Paradoxides paradoxissimus gracilis Biozones, Příbram-Jince Basin: a – specimen no. NM-L 46543, from dorsal (a1), oblique lateral (a2) and posterior (a3) view; b – specimen no. NM-L 46544, from dorsal (b1), and posterior (b2) view; c – specimen no. NM-L 46545, from dorsal (c1) and posterior (c2) view; d – specimen no. NM-L 46546, from dorsal (d1) and ventral (d2) view; e – specimen no. NM-L 41157, external (e1) and internal (e2) mold. Abbreviations: op – overlapping pleurae.